effects of lesions in central thermosensitive areas on thermoregulatory responses in rat

9
Physiology & Behavior, Vol. 19, pp. 503--511. Pergamon Press and Brain Research Publ., 1977. Printed in the U.S.A. Effects of Lesions in Central Thermosensitive Areas on Thermoregulatory Responses in Rat I WARREN W. ROBERTS AND JAMES R. MARTIN Department of Psychology, University of Minnesota, Minneapolis, MI 55455 (Received 13 May 1977) ROBERTS, W. W. AND J. R. MARTIN. Effects of lesions in central thermosensitive areason thermoregulatory responses in rat. PHYSIOL. BEHAV. 19(4) 503-511, 1977. - Behavioral and autonomic responses elicited by ambient heat were tested before and after lesioning five thermosensitive areas previously localized in the rat brain. Lesions in the PO/AH area, where local warming elicits prone body extension, markedly reduced or abolished extension but had no effect on locomotion, grooming, or tail vasodilation. Lesions in the posterior hypothalamus, where local warming induces grooming, and lesions in the septum, medial midbrain, and dorsomedial medulla, where local warming evokes locomotion, had no significant effects on thermoregulatory responses except for reduced body extension following midbrain lesions, which may have interrupted the efferent path from PO/AH thermosensors. It was concluded that extension is predominantly dependent on PO/AH thermosensors and/or integrative interneurons, while locomotion, grooming, and vasodflation are controlled by input from peripheral and central sensors. The present and earlier data from the rat are less consistent with a relatively localized general integrating mechanism producing a unitary output signal to thermoregulatory effectors than with a spatially distributed network of specific paths from sensors to separate response mechanisms, where summation of inputs may occur. Thermoregulation Thermosensors Preoptic area Hypothalamus Septal area Midbrain Medulla Grooming Locomotor activity Vasodilation WHEN rats are exposed to elevated ambient temperatures, they display four principal behavioral and autonomic responses: (1) increased relaxation in a prone extended position [30,36], which reduces muscle heat production and increases heat dissipation through surface exposure to air and substrate; (2) increased grooming and salivation [ 13, 15, 30, 36], which enhance heat loss through evaporation of saliva; (3) increased locomotion [13,30], which raises the probability of escape to a cooler area, and (4) peripheral vasodilation [26,35], especially in the tail, which increases heat loss by raising surface temperature. In a search for central thermosensors controlling these respon- ses, Roberts and Mooney [29] found five differentiated areas in the brain of the rat where they could be elicited by localized diathermic warming while the ambient tempera- ture was at thermoneutrality, and therefore other central and peripheral sensors were minimally activated. Relaxed body extension was obtained from the preoptic and anterior hypothalamic region (PO/AH), grooming from the posterior hypothalamus, locomotor activity principally from the septal area, medial midbrain, and dorsomedial medulla, and tail vasodilation from all thermosensitive areas. In a study of peripheral thermosensor control of these responses, Roberts and Martin [28] found that local warming of a variety of superficial and deep sites in the skin, respiratory passage, and viscera using implanted heaters or a warm floor elicited grooming, locomotion, and tail vasodilation, but never the body extension component. This suggested that extension may be predominantly dependent on central sensors, while the other three components reflect the summed input of both peripheral and central sensors. A persistent question in the central organization of thermoregulation concerns where and to what extent the inputs from diverse peripheral and central thermosensors are integrated. One widely held view is that most integra- tion of central and peripheral thermal signals (i.e., summa- tion of similar inputs and mutual inhibition of opposite inputs) takes place in a relatively specific general integrating mechanism located in the preoptic area and extending posteriorly into the hypothalamus and possibly the mid- brain [1, 3, 4, 16, 17]. The outcome of this signal integration is presumed to be a generalized thermoregula- tory control signal that activates similarly all components of the appropriate thermoregulatory system, sometimes subject to different response thresholds but undifferentia- ted with respect to input from different thermosensor sites. Other investigators, impressed by the capacity of thermo- sensors in the spinal cord and medulla to induce thermore- gulatory responses following destruction or surgical discon- nection of the forebrain mechanism have expanded the integrating system to include local networks in the lower brainstem and spinal cord, but have continued to assume that the areas are normally coordinated with each other by t This research was supported by Grant MH 06901 from the National Institute of Mental Health to W. W. Roberts. Grateful acknowledgement is made to Eunice K. Roberts for assistance. 503

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Physiology & Behavior, Vol. 19, pp. 503--511. Pergamon Press and Brain Research Publ., 1977. Printed in the U.S.A.

Effects of Lesions in Central Thermosensitive Areas on Thermoregulatory Responses in Rat I

WARREN W. ROBERTS AND JAMES R. MARTIN

Department o f Psychology, University o f Minnesota, Minneapolis, MI 55455

(Received 13 May 1977)

ROBERTS, W. W. AND J. R. MARTIN. Effects of lesions in central thermosensitive areas on thermoregulatory responses in rat. PHYSIOL. BEHAV. 19(4) 503-511, 1977. - Behavioral and autonomic responses elicited by ambient heat were tested before and after lesioning five thermosensitive areas previously localized in the rat brain. Lesions in the PO/AH area, where local warming elicits prone body extension, markedly reduced or abolished extension but had no effect on locomotion, grooming, or tail vasodilation. Lesions in the posterior hypothalamus, where local warming induces grooming, and lesions in the septum, medial midbrain, and dorsomedial medulla, where local warming evokes locomotion, had no significant effects on thermoregulatory responses except for reduced body extension following midbrain lesions, which may have interrupted the efferent path from PO/AH thermosensors. It was concluded that extension is predominantly dependent on PO/AH thermosensors and/or integrative interneurons, while locomotion, grooming, and vasodflation are controlled by input from peripheral and central sensors. The present and earlier data from the rat are less consistent with a relatively localized general integrating mechanism producing a unitary output signal to thermoregulatory effectors than with a spatially distributed network of specific paths from sensors to separate response mechanisms, where summation of inputs may occur.

Thermoregulation Thermosensors Preoptic area Hypothalamus Septal area Midbrain Medulla Grooming Locomotor activity Vasodilation

WHEN rats are exposed to elevated ambient temperatures, they display four principal behavioral and autonomic responses: (1) increased relaxation in a prone extended position [30,36], which reduces muscle heat production and increases heat dissipation through surface exposure to air and substrate; (2) increased grooming and salivation [ 13, 15, 30, 36] , which enhance heat loss through evaporation of saliva; (3) increased locomotion [13,30], which raises the probability of escape to a cooler area, and (4) peripheral vasodilation [26,35], especially in the tail, which increases heat loss by raising surface temperature. In a search for central thermosensors controlling these respon- ses, Roberts and Mooney [29] found five differentiated areas in the brain of the rat where they could be elicited by localized diathermic warming while the ambient tempera- ture was at thermoneutrality, and therefore other central and peripheral sensors were minimally activated. Relaxed body extension was obtained from the preoptic and anterior hypothalamic region (PO/AH), grooming from the posterior hypothalamus, locomotor activity principally from the septal area, medial midbrain, and dorsomedial medulla, and tail vasodilation from all thermosensitive areas. In a study of peripheral thermosensor control of these responses, Roberts and Martin [28] found that local warming of a variety of superficial and deep sites in the skin, respiratory passage, and viscera using implanted heaters or a warm floor elicited grooming, locomotion, and

tail vasodilation, but never the body extension component. This suggested that extension may be predominantly dependent on central sensors, while the other three components reflect the summed input of both peripheral and central sensors.

A persistent question in the central organization of thermoregulation concerns where and to what extent the inputs from diverse peripheral and central thermosensors are integrated. One widely held view is that most integra- tion of central and peripheral thermal signals (i.e., summa- tion of similar inputs and mutual inhibition of opposite inputs) takes place in a relatively specific general integrating mechanism located in the preoptic area and extending posteriorly into the hypothalamus and possibly the mid- brain [1, 3, 4, 16, 17]. The outcome of this signal integration is presumed to be a generalized thermoregula- tory control signal that activates similarly all components of the appropriate thermoregulatory system, sometimes subject to different response thresholds but undifferentia- ted with respect to input from different thermosensor sites. Other investigators, impressed by the capacity of thermo- sensors in the spinal cord and medulla to induce thermore- gulatory responses following destruction or surgical discon- nection of the forebrain mechanism have expanded the integrating system to include local networks in the lower brainstem and spinal cord, but have continued to assume that the areas are normally coordinated with each other by

t This research was supported by Grant MH 06901 from the National Institute of Mental Health to W. W. Roberts. Grateful acknowledgement is made to Eunice K. Roberts for assistance.

503

504 ROBERTS \NI) MA[~IJN

neural interconnections and produce a generalized activa- tion of thermoregulatory responses [33]. Recently, howe- ver, these concepts of relatively unitary functional integra- tion have been questioned by a number of studies in which localized thermal stimulation of different central and peripheral thermosensory sites in rats, pigs, monkeys and pigeons elicited different responses or partially different combinations of responses [1, 6, 10, 22, 28, 29, 31, 33]. These findings are inconsistent with a generalized control signal and the high degree of functional integration required to produce it. Instead, it appears that there may often exist relatively separate channels from different thermosensors to specific response mechanisms, and signal integration, when it occurs, may take place within or close to response mechanisms.

The general purpose of the present experiment was to gather further evidence concerning the specificity and independence of the different central thermosensitive areas in the rat brain. Following placement of lesions in the areas, measures were taken of activity, grooming, postural exten- sion, and tail vasodilation induced by ambient heat to determine (1) whether destruction of the PO/AH, medial posterior hypothalamus, or medial midbrain produces the generalized reduction of all four responses that would be expected if a general integrating mechanism were damaged, (2) whether PO/AH lesions produce the specific loss of extension that would be expected if extension alone is predominantly dependent on PO/AH thermosensors (and/ or local integrative neurons), and (3) whether destruction of the other thermosensitive areas weakens grooming, activity, or vasodilation, as would be expected if the influence of peripheral thermosensors is mediated through them [16], or they provide tonic facilitation essential to the effectiveness of peripheral input elsewhere in the brain.

METHOD

A nimals

Pigmented Long-Evans male Sats aged 6 - 8 months and weighing 4 2 5 - 6 2 0 g were procured from Simonsen Labora- tories, Gilroy, CA. They were housed at 22 -24° C in a light-cycled room (15 hr light, 9 hr dark), and were tested during the light phase.

Procedure

Before and after the lesions were made, the animals were tested under conditions of ambient heat stress for behavio- ral and vascular thermoregulatory responses. The behavioral tests were given in a cylindrical mesh stabilimeter cage 270 mm in diameter and height having a solid metal floor that was mounted on a central pivot surrounded by four microswitches that were activated by locomotion and climbing activity in the cage. The animal was adapted to the activity cage for 45 min before each session, which began when the cage was placed inside a convection-type labora- tory oven (Thelco Model 17, Precision Scientific Co.) having an observation window in the door and a single small light. Two 45 min sessions at 25°C and two at 37°C were administered in alternating order before the lesions were made. Although male rats of this strain and age will tolerate temperatures up to 4 0 - 4 1 ° C for 45 rain, an appreciably lower degree of heat stress was used to avoid a possible response ceiling effect that might mask partial deficits in thermoregulatory responses. During the tests, the experi-

menter recorded oven temperatures, number ol act~w~} counts, and durations of grooming, body extension, and resting the head on the floor (a more advanced stage oi' relaxation in the prone extension posture) by 5 rain blocks. Rectal temperatures were obtained within 2 -3 rain ol removal from the home cage, 15 min after the start of the adaptation period preceding the behavioral tests~ and at the end of the tests.

Tail vasodilation responses elicited by radiant warming of the body were measured while the rat was confined in a close-fitting wire mesh restraint cage that allowed the tail to extend from one end. The body was warmed by two 250 W heat lamps supplied with a reduced voltage of 60 V from a transformer. The lamps were located on each side of the rat at a distance of 30 cm and an angle of 45 ° from horizontal. The tail was shaded from the lamps by a cardboard shield, and vasodilation was measured as an increase in skin surface temperature using a thermistor thermometer (Yellow Springs Instrument Co., Model 44TG) and a disc thermistor (YSI Model 427) taped to the ventral surface of the tail. Deep body temperature was also monitored with a rectal thermistor (YSI 402). On the day before testing, the rats received 30 min adaptation experience in the restraint cage. On the test day, they were placed in the restraint cage for pretest adaptation for 40 min, or longer if the tail surface temperature remained above 29°C. The test consisted of 8 min of radiant heating of the body, during which the surface temperature of the tail was recorded at 30 sec intervals, and rectal temperature at 60 sec intervals. In a few cases, tail temperature rose only slightly during the 8 rain, and the test was extended up to 15 min. Following the heat test, the animals were allowed 60 rain for recovery, or longer if the tail temperature was still above 29"C, and a second 8 rain heat test was given.

Because of the possibility that the lesions might affect thermoregulatory grooming, cleanliness grooming was also tested. After 10 min adaptation in the stabilimeter cage, five drops of mineral oil were placed on the fur of the snout just in front of and between the eyes, and the duration of grooming was recorded for the subsequent 10 min.

Following completion of the prelesion tests, five groups of animals received lesions in the five areas where localized warming elicited grooming, postural extension, and loco- motion in the study of Roberts and Mooney [29]: the PO/AH region, septal area, posterior hypothalamus, ventral midbrain, and dorsomedial medulla. Sodium pentobarbital (60 mg/kg) was used for anesthesia. To minimize possible local irritative effects, the lesions were made with a Grass LM-2 radio-frequency (2 MHz) lesion maker. The electrodes consisted of four insulated stainless steel insect pins that were exposed for 2 mm at the tip and held in a 2 × 2 mm square array so their uninsulated tips enclosed a 2 × 2 x 2 mm cube. A minimum of 15 days was allowed for postoperative convalescence, and this was extended up to 28 days for animals whose body weight was appreciably below the preoperative level. Animals that ate poorly were given wet mash, chocolate chip cookies, or intragastric injections of Sustagen diet, depending on the severity of their hypophagia. All rats returned to near-preoperative weight levels by time of testing except for four having medullary lesions who averaged 21% below preoperative weights. The early fatality rate following anterior hypotha- lamic-preoptic lesions was high (71%), as others have reported [18, 19, 24, 32], probably mainly due to pulmonary edema [ l 1 ]. In an attempt to improve survival,

CENTRAL THERMOSENSOR LESIONS IN RATS 505

a few rats received successive unilateral lesions on each side of the brain separated by an 8 - 2 1 day interval, or large unilateral lesions that sometimes crossed the midline. There were also a few fatalities among animals having lesions in the posterior hypothalamus, midbrain, or medulla. The survivors numbered 10 with anterior hypothalamic-preoptic lesions, 7 with septal lesions, 9 with posterior hypothalamic lesions, 8 with ventromedial midbrain lesions, 8 with dorsomedial medulla lesions, and 11 operated control animals having no or very small unilateral lesions.

The postlesion tests followed the same procedure as the prelesion tests. After completion of testing, the animals were perfused with Formalin, and the brains were embed- ded in albumin, sectioned on a freezing microtome, and stained alternately with cresyl violet and Sudan black. The lesions were localized on frontal diagrams at 0.4 mm intervals in the atlas of Pellegrino and Cushman [25] or in a locally constructed caudal extension through the posterior medulla. Histological sections were projected directly on the atlas at matched magnification. In so far as possible, the borders of surviving tissue were plotted, rather than the simple outline of the puckered acellular area. Sagittal projections of the lesions were reconstructed on a sagittal stereotaxic diagram by plotting the maximal bilateral dorsal and ventral extent of the lesions in each frontal plane. The relative size of lesions was measured with a polar planimeter on the sagittal diagram. Histology was available for all animals except for one PO/AH lesion case.

Statistical Analysis

Because of the number of lesion groups to be compared, all data were subjected first to overall tests of significance before individual comparisons were made. The effects of the lesions on baseline measures at 25°C were assessed in 2-way analyses of variance having prelesion and postlesion measures as one dimension and the six lesion groups as the other dimension. Lesion effects on heat-induced increments in the measures were evaluated in similar analyses of 37°C measures and the differences between 37°C and 25°C measures. When significant interactions were obtained, separate one-way anovas were performed on prelesion and postlesion data to determine whether intergroup differences were present before or after surgery. If the one-way anovas yielded significant F's, individual lesion groups were com- pared to the control group using Dunnett 's test. Differences

between pre- and postlesion measures for individual groups were evaluated with 2-tailed t-tests.

RESULTS

Prelesion Tests

As in earlier studies [13, 15, 26, 30, 35, 36] , ambient heat at 37°C induced increased body extension, grooming, locomotor activity, and rectal temperature as compared to thermally neutral 25°C. Table 1 presents overall means for the prelesion measures. All increases were significant beyond the .001 level in overall tests and beyond the .05 level in separate tests of lesion subgroups, whose data is shown in the upper graphs of Figs. 1, 2 and 3. Radiant warming of the body consistently elicited tail vasodilation, resulting in an increase in the mean surface temperature of the tail from 26.8°C to 34.1°C with a mean latency of 2.1 min for the first 0.5 ° increase.

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CONTROL

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PO/AH SEPTUM 'POST. HYP.'MIDBRAIN ' MEDULLA '

FIG. 1. Mean duration of body extension in 45 min tests at thermally neutral and hyperthermic temperatures before and after creation of lesions in thermosensitive regions of rat brain. Open bars on left of each pair represent tests at 25°C and hatched bars on right

represent tests at 37°C.

TABLE 1

PRELESION MEASURES OF THERMOREGULATORY RESPONSES IN AC- TIVITY CAGE AT NEUTRAL AND WARM AMBIENT TEMPERATURES

Measure 25°C 37°C

Body extension 4.2 sec 816.9 sec*

Body extension, head down 1.6 sec 512.7 sec*

Grooming 319.8 sec 559.8 sec*

Locomotion 55.3t 171.0*t Pretest rectal temp.~: 38.25°C 38.19°C

Posttest rectal temp. 37.91°C 39.68°C *

*p<0.001. t Activity counts. *Taken 15 min after start of 45 min habituation period preceding test.

506 ROBERTS ANI) MAR rlN

The postural extension response consisted of relaxed elongation of the body in a prone or near-prone position, with the normally curved spine straightened horizontally, forelegs placed under the neck, and hindlegs flexed outward and toward the rear. At first the head usually remained erect, but later was rested on the floor with the eyes often partially closed and retracted as during natural drowsiness. The increased locomotion at the elevated temperature was intermittent, and consisted mainly of short bursts of a few steps with the body usually retaining the low extended posture. The grooming at 37°C typically began with licking the forepaws and rubbing them over the face, but during longer bouts, licking was also directed toward the hind paws, flanks, scrotum, and belly. As in a previous study [30], the temporal trends in the three behaviors differed across the nine five-min blocks of the test session. The thermal increment in grooming (37°C-25°C scores) appea- red first and reached a maximum early in the test session; postural extension emerged last and increased gradually to a maximum near the end of the session; and activity was intermediate.

Rectal temperature was lowest immediately after remo- val from the home cage, averaging 37.38°C. During the first 15 min of adaptation in the stabilimeter cage, rectal temperature rose 0.84°C to 38.22°C (p<0.001), probably because of arousal and increased investigatory activity. At the end of the 25°C test session, rectal temperature was 0.34°C below the midadaptation level (p<0.001), while after the 37°C session it was 1.49°C higher (p<0.001), as is typical of rats exposed to ambient heat [ 13 ].

Postlesion Tests

Measures at Thermoneutrality. The lesions had no significant effects on baseline measures of extension, grooming, or locomotion at thermally neutral 25°C, with the single exception of a 210% increase in locomotion following lesions in the dorsomedial medulla (p<0.05). Nor did the lesions produce significant changes in rectal temperatures taken immediately after removal from the home cage, in the middle of the pretest habituation period, or after the test at 25°C, except for a decrease of 0.64°C in the medullary lesion group at the end of the 25°C session.

Responses to heat. Lesions in the PO/AH region, where local warming elicits postural extension, reduced the duration of heat-induced body extension (i.e., 37°C-25°C scores) by an average of 60%, as shown in Fig. 1. The two lesions that destroyed the greatest proportion of the effective area for local warming reduced extension by 100% and 99%. The decrease in extension was significant beyond the 0.01 level in an analysis of variance over all groups and in a Dunnett test comparing the PO/AH group with the control group. Measures of the more advanced stage of extension with the head resting on the floor yielded similar findings at a lower level of significance. No obvious postural or motor symptoms were observed that might have interfe- red with body extension. There was no reduction in the amount of heat-induced grooming or locomotion, which actually increased slightly though not significantly after the PO/AH lesions were made. The septal, posterior hypotha- lamic, and medullary lesions had no effect on body extension, but midbrain lesions produced a similar decrease in extension of about 60% (p<0.01), presumably by damaging pathways or control mechanisms efferent to the PO/AH thermosensors.

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CONTROL ' PO/AH ' SEPTUM 'POST. HYP. 'MIDBRAIN 'MEDULLA

FIG. 3. Mean duration of grooming in 45 min tests at 25°C (open bars) and 37°C (hatched bars) before and after creation of lesions.

As shown in Fig. 2, locomotor activity at 37°C and the increase in activity from 25 ° to 37°C did not diminish significantly in any group after placement of lesions, including those in the septal, midbrain, and medullary areas where local warming elicits locomotion [29]. The medulla- ry lesion group was significantly more active at both temperatures after lesioning, but the difference remained the same.

Figure 3 shows a general postoperative decline in grooming at 37°C in the control group and all lesion groups except the PO/AH rats (overall p<0.01). The difference in the amount of grooming at 25°C and 37°C also decreased postoperatively (overall p = 0.001), but there was a signifi- cant interaction with lesions (p<0.01), with the PO/AH group continuing to show significantly more grooming at 37 ° (p< 0.01). The decrease in thermoregutatory grooming

CENTRAL THERMOSENSOR LESIONS IN RATS 507

PO/AH 8 MIDBRAIN LESIONS S ~ M ~ U L L A R Y LESIONS

CENTRAL THERMOSENSITIVE ZONES LATERAL 0.6

FIG. 4. Localization of areas destroyed bilaterally as projected on sagittal diagrams located .6 mm lateral to midline. Lower left diagram presents the central zones of the five thermosensitive areas as roughly defined by a boundary interconnecting the centers of the most peripheral diathermic electrode arrays that elicited responses of at least moderate strength at medium or low thresholds in the study of Roberts and Mooney [28]. Lower right diagram shows the location of some prominent brain structures, with those outside of the Lat. 0.6 plane represented by dashes and dots. Vertical lines in upper diagrams indicate planes of frontal diagrams in Fig. 5. Diagram constructed from atlases of DeGroot [9] and PeUegrino and Cushman [24] with the aid of a sagittal histological series. A, commissura anteriox; AS, aqueductus Sylvius; C, corpus callosum; CH, chiasma opticum; DF, funicuU dorsalis; DPT, decussatio pyramidum; F, fomix; G, genu nervi facialis; GR, n. gracilis; H, habenula; HC, commissura hippocampi ventralis; HI, hippoeampus; HP, traetus habenulo-interpeduncularis; M, tractus mimiUothalamicus; MM, N. mamillaris; O, n. olivaris inferior; P, commissura posterior; PA, n. paraventricularis; PN, pons; PO, area preoptica; PR, n. praepositus hypoglossi; PT, tractus pyramidalis; R, n. ruber; S, area septalis; T, tectum; TH, thalamus; TR, corpus trapezoideum; VM,

n. ventromedialis hypothalami; VII, nervi facialis descendens.

was not paralleled by cleanliness grooming induced by oil on the snout, which increased from 155 sec preoperatively to 239 sec postoperatively (p < 0.001 ).

Rectal temperatures at the end of the 45 min sessions at 37°C did not differ significantly between groups, although the PO/AH group had the highest mean temperature (PO/AH 40.55°C, controls 40.28°C, posterior hypothala- mus 40.28°C, medulla 40.16°C, septal area 39.87°C, midbrain 39.85°C). Temperatures after heat stress were slightly higher postoperatively than preoperatively in all groups, including controls (mean = 0.5 I°C, p<0.001 ).

Tail vasodilation elicited by radiant heating of the body was unimpaired by any lesions, and the latency of the initial 0.5°C increase in surface temperature of the tail

actually declined about 20% between the pre- and postle- sion tests. Although this difference was significant in an overall analysis of variance (p<0.001), the lesion groups and control group did not differ significantly from each other. Tail vasodilation began before there was any change in rectal temperature, and the increase in rectal temperature during the 8 min tests averaged only 0.17 ° C.

Lesions

Sagittal reconstructions of the lesions are presented in Fig. 4, where they may be compared with a diagram of the central areas most effective for local diathermic elicitation of extension, grooming, and locomotion. The lesions were

508 ROBERTS ;\NI) MARLIN

PO/AH

SEPTAL AREA POST. HYPOTHALAMUS

FIG. 5. Cross-sections of lesions in frontal planes of PeUegrino and Cushman atlas [24] located near middle of lesion arrays and central areas most effective for elicitation of responses by local warming.

Locations of planes are indicated on sagittal diagrams of Fig. 4 by vertical lines.

fairly symmetrical on the two sides of the brain, except for 3 PO/AH lesions, and extended 0 .7 -1 .9 mm lateral to the midline. Figure 5 presents frontal cross-sections through the approximate center of each group of lesions, except for the PO/AH lesions, which are shown in two planes. The PO/AH lesions were more variable than other lesions in size, location, and bilateral symmetry, partly because the high mortality rate tended to select against large preoptic lesions, and partly because seriatim or unilateral lesions were made in a few cases to reduce mortality.

The magnitude of loss of body extension in the PO/AH lesion group was significantly correlated with lesion size and percentage destruction of the central area most effective for diathermic induction of extension as measured in the sagittal plane (Pearson r's = .87 and .85, p 's<0.01). One of the two lesions that abolished extension was a large

unilateral lesion that crossed the midline by a maximum of 0.7 mm, suggesting that the critical receptors or their efferents may be close to the midline. Only one of the PO/AH lesions reached the internal capsule, and in that case the damage was slight and unilateral. Loss of body extension in the midbrain lesion group was much less correlated with lesion size (r = .39, p> 0.30), and a graphical analysis of effective and ineffective lesions suggested that the critical area for loss of extension was probably in 'or closely adjacent to the ventral half of the central grey.

DISCUSSION

Lesions in the PO/AH region where local warming elicits relaxed postural extension ~29] markedly reduced or, if sufficiently large, eliminated the postural extension respon-

CENTRAL THERMOSENSOR LESIONS IN RATS 509

se to natural ambient heat. Thus, the thermosensors and other neural elements in the PO/AH region are not only sufficient to elicit extension when locally warmed, but are also necessary for evocation of extension by ambient heat. Postural relaxation has also been obtained from the spinal cord and dorsal medulla with thermode warming [20,22] , although the greater variability of the medullary response [22] and the failure to elicit it with diathermy electrodes that were consistently effective in the PO/AH [29] suggest that medullary thermosensors have a weaker influence than PO/AH thermosensors. The marked loss of extension after large PO/AH lesions and the lack of effect of meduUary lesions also argue for the predominance of the PO/AH region in the control of extension. Since postural extension cannot be elicited by local warming of a large number of deep and superficial sites in the body [28,29] , it appears that the extension response is predominant ly if not entirely dependent on central sensors. This is consistent with evidence in the present and a preceding study [30] that postural extension is slower to emerge than grooming or locomotion in the presence of ambient heat, which would prompt ly excite superficial sensors eliciting grooming and locomotion [28] but would only slowly raise the tempera- ture of deep central sensors controlling extension.

The loss of heat-induced postural extension produced by PO/AH lesions was response-specific, since heat-induced locomotion and tail vasodilation were unimpaired, and the thermal increment in grooming (37° -25°C scores) was larger postoperatively in the PO/AH group than in any other group. This is consistent with the considerable specificity of PO/AH warming, which does not elicit grooming and induces locomotion only in a minori ty of cases and then weakly and at high thresholds [29] , although tail vasodilation is readily evoked, as from other central and peripheral thermosensitive areas. Considered together, the lesion and local warming data indicate that PO/AH thermosensors and interneurons do not exert an essential general control over the rat 's heat stress repertoire as a whole, but instead have a relatively specialized primary role with respect to postural extension, a secondary role shared with most other thermosensors in the control of vasodilation, possibly a still weaker effect on locomotion, and no significant influence on grooming behavior, which Hainsworth and Stricker [ 14] found crucially important at high temperatures when escape and nonsalivary evaporative cooling were not available. Lipton [21] has shown that learned heat-escape bar-pressing also survives PO/AH le- sions. Salivation, which is essential to the heat-dissipating effectiveness of grooming, is reduced by preoptic lesions [37] but this is a generalized loss evidenced in feeding as well as heat stress, and may be due to impairment of salivary motor mechanisms rather than loss of thermosen- sory input.

The findings that the postural extension response can also be eliminated with medial midbrain lesions involving the ventral central grey suggests that the efferent path from PO/AH thermosensors producing extension may pass through or terminate in this region. Direct project ion~from the PO/AH region to the central grey have been demonstra- ted recently [7,8] . The failure of the medial posterior hypothalamic lesions to produce similar loss of extension indicates that the critical connections do not traverse the medial posterior hypothalamus, but may pass laterally through the lateral hypothalamus or dorsally through the thalamus.

The increase in locomotion induced by ambient heat was not affected by lesions in the septal, midbrain, or medullary areas where local warming elicits locomotor activity, indicating that input from peripheral and surviving central thermosensors was able to compensate fully for the loss of any one central thermosensitive area controlling locomo- tion. Vasodilation of the tail, which can be elicited by local warming of all central and peripheral thermosensors thus far studied [28,29] , was likewise unimpaired by any lesions. Since the latency was short, and rectal temperature was unchanged at the onset of vasodilation, it was probably mainly elicited by superficial peripheral sensors, whose connections with central vasomotor mechanisms are evi- dently not critically dependent on any one of the lesioned areas.

Unexpectedly, the increase in grooming at 37°C over that at 25°C, which was highly significant in preoperative tests, declined postoperatively to nonsignificant or slightly negative levels in the control group and all lesioned groups except the PO/AH animals. About 40% of this decline was due to increased grooming at 25°C, and about 60% to decreased grooming at 37°C. Before surgery, spontaneous grooming at 25°C increased from the first to the second preoperative session (p<0.05), probably because of habitua- tion to the test situation, and the further increase after surgery may have been a continuation of this trend. Since the postoperative decrease in grooming at 37°C was evidenced by the controls as well as lesioned animals, it does not appear to be a lesion effect, but whether it resulted from some general aspect of the surgery, further habituation, or other factors is unclear. In any event, it cannot be concluded that all thermal facilitation of grooming was lost postoperatively, since grooming at 37°C had to compete with increased locomotion and postural extension induced by the heat, and as a consequence the measure of grooming obtained at 25°C probably overesti- mated the amount of spontaneous grooming produced by nonthermal factors at 37 ° C.

The hypothesis of a relatively localized general integra- ting mechanism for thermoregulation is not confirmed by the present findings in the rat. Even the largest lesions in the PO/AH and posterior hypothalamus, where such a mechanism is most often presumed to be located, failed to produce the generalized reduction in thermoregulatory responses that would be expected from damage to a general integrating mechanism. The losses that did ensue were limited to a single response category, postural extension. The possibility that the loss of extension was due to interruption of a specific motor efferent path from a general integrating mechanism is highly unlikely in the case of PO/AH lesions, which were located in the principal thermosensitive brain region capable of inducing extension and therefore were on the afferent side of any integration processes. The present findings, together with earlier evidence for specificity in the responses controlled by different central and peripheral sensors [28,29] , suggest that the thermoregulatory system of the rat may be viewed as a spatially distributed set of specific connections between localized sensors and particular response mechanisms. In this model, the interaction of thermosensory inputs, invol- ving summation of inputs of similar sign and mutual inhibition of inputs of opposite sign, which is one of the main reasons for hypothesizing a general integrating mecha- nism, would occur separately in each specific response mechanism or in specific pathways to each mechanism

510 R O B E R T S ANI) MARLIN

r a t h e r t h a n in a un i t a r y in teg ra t ing sys tem p roduc ing a general ized t h e r m o r e g u l a t o r y con t ro l signal. Add i t i ona l s u p p o r t for this a l te rna t ive concep t of the cent ra l organiza- t ion of t h e r m o r e g u l a t i o n is f o u n d in o t h e r lesion and the rma l s t imu la t i on s tudies t h a t have f o u n d apprec iab le d issoc ia t ion of learned and largely u n l e a r n e d the rmoregu la - to ry responses in the rat [5, 10, 21, 22, 23, 32, 3 7 ] , squirrel m o n k e y [ 1 ] , pig [6] and p igeon [331. Part ial d i ssoc ia t ion of g rooming and pos tu ra l r e l axa t ion has also been o b t a i n e d in the opos s um [ 2 7 ] .

The decrease in pos tu ra l e x t e n s i o n in the P O / A H and m i d b r a i n les ion groups did no t resul t in s igni f icant increases in rec ta l t e m p e r a t u r e above the con t ro l s or o t h e r lesion groups dur ing the m o d e r a t e hea t stress of the p resen t e x p e r i m e n t , ind ica t ing t ha t o t h e r t h e r m o r e g u l a t o r y respon- ses were able to c o m p e n s a t e for the decreased re l axa t ion and b o d y surface exposu re t ha t resu l ted f rom the ex tens ion

loss. Since o the r s tudies have found h y p e r t h e r m i a ~h younger P O / A H les ioned rats sub jec ted to longer or warme~ tes t sessions [2, 12, 2 1 , 2 3 , 371, it is possible tha t greater heal stress or younge r animals might have yie lded to hyper- t he rmia fo l lowing P O / A H and midb ra in lesions m the p resen t s tudy . All groups , inc luding cont ro l s , had slightly h igher rectal t e m p e r a t u r e s (0.5~C) af ter pos topera t ive heat tests t han af ter p reopera t ive hea t tests. A similar s ignif icant increase in con t ro l ra ts ' t e m p e r a t u r e s is ev ident in the data of Han and Brobeck [ 1 2 ] , who used a similar pre- and pos t les ion tes t design. Since the con t ro l animals in the p resen t s t udy and t h a t of Han and Brobeck [12] exper ien- ced all or mos t of the surgical p rocedures up to and inc lud ing lower ing of e lec t rodes in to the bra in , it is unc lear w h e t h e r the effect was due to repea ted tes t ing or nonspeci - fic e f fec ts of the general ope ra t ion procedures .

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