Short Communication

GABAA Receptor-MediatedNeurotransmission in the Dentate Gyrus of

the Rhesus Monkey:Comparison With the Rat

SUSAN O’BRIEN,1 DOUGLAS L. ROSENE,1,3,4 AND JENNIFER I. LUEBKE1,2,3*1Center for Behavioral Development, Boston University School of Medicine, Boston, Massachusetts 02118

2Department of Psychiatry, Boston University School of Medicine, Boston, Massachusetts 021183Department of Anatomy & Neurobiology, Boston University School of Medicine, Boston, Massachusetts 02118

4Yerkes Regional Primate Research Center, Emory University, Atlanta, Georgia 30322

The validity of the rodent model to an understandingof neural processes in higher mammals is an issue thatis widely assumed but has received little systematicinvestigation. Indeed, little is known of the structuraland functional differences between areas of the rodentand primate brain, even those as well studied as thedentate gyrus of the hippocampal formation. This is animportant issue in light of the large number of electro-physiological studies that have been performed on rathippocampal slices and assumed to model processes inthe primate nervous system. While it is known thatthere is a significant similarity of the cytoarchitecturein the dentate gyrus of rat, monkey, and human (Blattand Rosene, 1998; Saunders and Rosene, 1988), it ispossible that overall information processing throughthe dentate gyrus of the monkey and human may differin a number of important ways from that in the rat.Recent studies have shown that rodent and monkeyhippocampal principal cells are remarkably similar interms of their basic electrophysiological and morpho-logical properties (Urban et al., 1996; St. John et al.,1997; Buckmaster and Amaral, 2001). Whether there isa similar preservation of synaptic signaling propertiesis not known. Synaptic inhibition has been examined insurgically resected human hippocampal (Schwartz-kroin, 1986; Williamson et al., 1993, 1995) and neocor-tical tissue (Dudek et al., 1995); however, because ofthe inherent pathological condition of human surgicaltissue, it is not known whether basic GABAergic mech-anisms in the primate are similar to those described inthe rodent. This is an important question becauseGABAergic inhibitory processes play an essential rolein the modulation of normal neuronal signaling and ofcertain pathophysiological states in the mammaliannervous system (Mehta and Ticku, 1999; Rabow et al.,1995). The present study expands our knowledge ofdifferences and similarities in the electrophysiological

properties of rodent and primate neurons by directlycomparing fast GABAA receptor-mediated transmis-sion in the dentate gyrus of the monkey vs. the rat.

Studies were performed on hippocampal slices pre-pared from five healthy young (10 � 2.5 years old)adult male rhesus monkeys that were perfused as apart of other ongoing projects, and from 10 young (90–140 day old) adult male Sprague-Dawley rats (CharlesRiver Laboratories, Wilmington, MA). Monkey sliceswere prepared as described in detail previously (St.John et al., 1997). While under deep anesthesia (intra-venous sodium pentobarbital to effect �15 mg/kg),monkeys were perfused through the ascending aortawith ice-cold Krebs-Henseleit buffer. After perfusionthe brain was removed and a 5-mm thick block of themidbody of the hippocampus was dissected from thebrain, glued against an agar slab in a tissue holder, andcut into 400 �-thick transverse sections with a vi-bratome. Rat slices were prepared as described in de-tail previously (Luebke et al., 2000; Mokler et al.,2001). All animals were housed at the Boston Univer-sity Laboratory Animal Science Center in strict accor-dance with animal care guidelines as outlined in theNIH Guide for the Care and Use of Laboratory Ani-mals. Whole-cell patch-clamp recordings were per-formed as previously described (Luebke et al., 2000;Mokler et al., 2001). The extracellular medium wasoxygenated Ringer’s solution, comprised of (in mM): 26

Contract grant sponsor: NIH; Contract grant numbers: P01AG00001,P01HD22539, RR-00165.

*Correspondence to: Jennifer I. Luebke, Center for Behavioral Development,M923, Boston University School of Medicine, 85 E. Newton St., Boston, MA02118. E-mail: jluebke@bu.edu

Received 20 March 2003; Accepted 19 April 2003

DOI 10.1002/syn.10237

SYNAPSE 49:287–289 (2003)

© 2003 WILEY-LISS, INC.

NaHCO3, 124 NaCl, 2 KCl, 3 KH2 PO4, 10 Glucose, 2.5CaCl2, 1.3 MgCl2; pH 7.4, and the internal solution wascomprised of (in mM): 140 cesium chloride, 2 MgCl2, 0.5EGTA, and 10 Na-HEPES. All chemicals were pur-chased from Fluka (Ronkonkoma, NY). Only those cellsthat maintained stable access resistance (less than10% change over the course of data acquisition) wereused for experiments. Electrophysiological data werestored on hard disk and subsequently analyzed usingthe “MiniAnalysis” program from Synaptosoft (Deca-tur, GA) as described previously (Luebke et al., 2000;Mokler et al., 2001). Electrophysiological data wereincorporated in a database (“Excel,” Microsoft Corp.)and analyzed for statistical significance using Stu-dent’s t-test (two-tailed), with significance defined atP � 0.05. Data are given as the mean � standard errorof the mean.

Spontaneous IPSCs were recorded in the presence of(�)-2-amino-5-phosponopentanoic acid (APV, 40 �M, toblock glutamatergic NMDA responses) and 6,7-dinitro-quinoxaline-2,3-dione (DNQX, 10 �M, to block gluta-matergic AMPA responses) at a holding potential of -80mV for a period of 5 min in each cell, such that eachsample contained several hundred events. The applica-tion of the GABAA receptor antagonist bicuculline me-thiodide (10 �M) completely eliminated spontaneousIPSCs in all cells examined (not shown). The propertiesof spontaneous IPSCs were fully characterized in 22cells from 10 rats and 20 cells from 5 monkeys. Asshown in Figure 1A and Table I, spontaneous IPSCs incells from rats vs. monkeys were essentially indistin-guishable in terms of frequency, amplitude, rise time,and decay time. Following collection of spontaneousIPSC data, the perfusate was switched to control Ring-er’s solution containing tetrodotoxin (TTx, 600 nM, toblock action potentials), plus APV and DNQX. GABAA

receptor-mediated miniature IPSCs were examined ata holding potential of –80 mV in 16 cells from 10 ratsand 15 cells from 5 monkeys. Representative traces ofminiature IPSCs are shown in Figure 1B and meanvalues for frequency, amplitude, rise, and decay timeare given in Table I. As was the case with spontaneousIPSCs, miniature IPSCs in cells from the two specieswere not significantly different in terms of frequency,amplitude, or kinetics. Following acquisition of base-line miniature IPSC data, the benzodiazepine agonistchlordiazepoxide (CDP, 0.1, 0.5, 1 �M) was bath-ap-plied to the slice. Recording of miniature IPSCs com-menced approximately 4 min after drug applicationand continued for an additional 10 min. At each con-centration CDP elicited an increase in the decay time ofminiature IPSCs in both rat (n � 16) and monkey (n �15) cells (Fig. 1C). There was no significant differencein the degree of response to the different concentra-tions of CDP (not shown). The magnitude of the re-sponse was not different in cells from the two species,with the mean percent increase in the decay time of

events in response to 1 �M CDP being 58.1 � 8.2% (ratcells) and 53.8 � 6.7% (monkey cells).

In summary, the characteristics of spontaneousIPSCs and miniature IPSCs in monkey dentate gran-ule cells were found to be remarkably similar tothose of the rat. The lack of a difference in thefrequency of spontaneous IPSCs in cells from rats vs.monkeys is consistent with the idea that the densityof innervation of dentate granule cells by local inter-neurons is similar in the two species. In addition, itsuggests that interneurons impinging upon dentategranule cells likely exhibit similar levels of excita-tion and hence spontaneous, action potential-depen-dent GABA release. The finding that miniature IPSCfrequency was also the same in cells from the twospecies supports this view and further indicates thatthe overall level of quantal (action potential-indepen-

Fig. 1. Characteristics of spontaneous and miniature IPSCs in ratvs. monkey dentate granule cells. A: Traces of spontaneous IPSCsfrom representative rat (left panel), and monkey (right panel) dentategranule cells. B: Traces of miniature IPSCs from representative rat(left panel), and monkey (right panel) dentate granule cells. C: Su-perimposed, averaged traces of miniature IPSCs in representative rat(left) and monkey (right) dentate granule cells under control condi-tions (gray) and in the presence of CDP (black). Traces representaverages of a minimum of 100 events for each condition.

288 S. O’BRIEN ET AL.

dent) GABA release from interneurons innervatingdentate granule cells is very similar. The amplitudeand kinetics of the miniature IPSCs did not differ incells from rats compared to monkeys, indicating thatthe subunit composition of the GABAA receptorspresent on dentate granule cells may be similar, ifnot identical, as these subunits determine the con-ductance of the channel (reflected in amplitude ofminiature IPSCs) as well as the gating of the channelwhich is reflected in the speed of rise (channel open-ing) and rate of decay (channel closing) of the min-iature IPSCs (Rabow et al., 1995). It should be noted,however, that the apparent similarity in event am-plitude could also result from a decrease in the con-ductance of individual channels together with anincrease in the total number of open channels. Fi-nally, the lack of a difference in the benzodiazepinemodulation of GABAA receptor-mediated miniatureIPSCs indicates further that the postsynapticGABAA receptor subunits which confer benzodiaz-epine sensitivity are not substantially different withregard to sensitivity in dentate granule cells fromthe two species. These data provide important sup-port for the view that data from in vitro slice studiesof rat neurons are useful in understanding GABAer-gic neuronal function in nonhuman primates andperhaps even in the human brain.

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TABLE I. Spontaneous and miniature IPSCs in rat vs. monkey dentate granule cells

Frequency (Hz) Amplitude (pA) 10–90% rise time (ms) 10–90% decay time (ms)

sIPSCsRat 3.7 � 0.6 42.7 � 3.8 2.4 � 0.1 16.8 � 1.2Monkey 3.6 � 0.6 43.5 � 2.8 2.1 � 0.4 15.9 � 0.5

mIPSCsRat 1.5 � 0.4 32.1 � 2.4 2.7 � 0.1 15.2 � 1.0Monkey 1.6 � 0.3 28.0 � 1.4 2.6 � 0.2 14.4 � 0.8

N � 22 cells from 10 rats and 20 cells from 5 monkeys for sIPSCs. N � 16 cells from 10 rats and 15 cells from 5 monkeys for sIPSCs.

INHIBITORY TRANSMISSION IN THE MONKEY VS. RAT 289