Neural Adaptations for Brood Parasitism

Using Substance P to define the borders ofthe Cowbird Hippocampus

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Brood Parasites are Brainy Birds

Brood Parasitism

A form of breeding in which the parasite lays its eggs in thenests of another species, the host.

Special Demands on Spatial Memory

� patrol the home range, identify nests and remember thelocation and status of each potential site.

Do these spatial memory demands give rise to specificadaptations in the brains of brood parasites ?

� update this information after each egg is laid - book-keeping.

Parasites

6 species inhabit the American continent

5 species are OBLIGATE BROOD PARASITES

The Bay-winged Cowbird non-parasitic

The Screaming Cowbird specialist

single host: the Bay-winged Cowbirdboth males and females target host nests

The Shiny Cowbirdgeneralist

over 200 different host speciesSexually dimorphic

Females alone perform the parasitic behaviour

The Cowbirds

coronal section

� a brain structure involved in the processing of spatialmemory.

� lesions disrupt the learning of locations and landmarks.

avian brain - dorsal view

hippocampal region

The Avian Hippocampus

� essential for tasks such as spatial navigation and food-hoarding.

� the parasitic cowbirds have larger hippocampal formationsthan the non-parasitic species.

Seasonal hippocampal enlargement in parasites

* in collaboration with JC Reboreda and NS Clayton

Our neuroanatomical studies* comparing hippocampal volumesin the three cowbird species show that:

During the Breeding Season

� in the dimorphic species where only the female performs the parasitic behaviour only the female has the larger hippocampus .

These differences are not seen outside the breeding season

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The Cresyl Violet stain gives only an approximatedemarcation for the lateral boundary of the hippocampus

Substance P (SP)

� a neuropeptide found in the central nervous system of manyvertebrates (mammals, reptiles, fish,birds)

SP is thought to play a role in promoting and facilitatingmemory and learning processes in vivo:

� administration of SP has been shown to affect memory andlearning in mammals and fish :

• within 5 hours post-trial SP promotes memory consolidationand longer retention of learning.

• during conditioning trials SP acts as a reinforcer.

� chronic SP administration reverses age-related memorydeficits in rats

Gould et al. (2001) investigated the distribution of SP in 4species of song bird:

A food-hoarder (Black-capped chickadee) was compared tothree non-storers (blue tit, great tit, dark eyed junco).

The relative volume of Spm was larger in the storer than in thethree other species.

SP-rich regions in the Songbird hippocampal region

SP and the hippocampus in food-storing birds

Hippocampus

“Lateral SP field”

“Medial SP field”

“Medial SP field”

“Lateral SP field”

Hippocampus

SP areas around the Cowbird hippocampus

Is the medial SP field larger in brood parasites?

A Substance P-rich region forms a defined lateral boundary forthe avian hippocampus

What are these punctate particleswithin this novel, well-defined regionof the cowbird hippocampus?

cells? fibres? terminals?

A population of SP+ particles in the cowbird hippocampus

Illustrating the outline of neurones within the hippocampus

Nomarski differential interference microscopy

The SP rich terminals densely encapsulate neuronal cell bodies

SP +ve pre-synaptic boutons contacting neurones

Synaptic Boutons

Synapses• the electro-chemical connections between neurones

• plasticity- a mechanism for encodinginformation in memory and learningprocesses?

A qualitative difference in the density and size of SPterminals

Brain Observations

16

74 r

92 r

68 v

70 v

64 v

95 r

69 r

91 r

terminals are faint and small

Medium intensity of signal

terminals are faint and small

terminals are darkly stained and large

terminals are darkly stained and large

terminals are darkly stained and large

terminals are faint and small

terminals are small and widespread

terminals are faint and small

Are these differences related to Brood Parasitism?

Conclusions and Future Work

• Description of a novel regional subdivision within the avianhippocampus.

• Differences in density and size of Substance P terminals couldbe related to specific adaptations for Brood Parasitism.

• What is the significance of Substance P circuits within theavian hippocampus in the context of learning and memory?

Bridging the gap between ecological, behavioural andneuro-anatomical approaches to memory and learning.

• Future work: Immunohistochemistry, Golgi Staining andElectron Microscopy to investigate the structure of neuronalconnections and neural circuits within the cowbird hippocampus.

AcknowledgementsThanks to:

Juan Carlos Reboreda (University of Buenos Aires) Jon Erichsen (Cardiff University)

Jenny Corrigan (Oxford).

Funding: The Wellcome Trust and Magdalen College Oxford