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Newborn, unfed garter snakes (Thamnophis spp.) respond preferentially to aqueous extracts of natural prey items, and these responses are mediated by the vomeronasal system (VNS). Since the VNS, and possibly the olfactory system (OS), are functional at birth, we examined the ontogeny of VNS and OS structures in four embryonic stages and two postnatal ages in garter snakes. The results of this study show 1) significant changes in thickness of the receptor epithelia for both systems; 2) temporal differences in the innervation of the telencephalon for each system; and 3) concurrent development of primary and secondary projection sites in both systems. Possible interactions between different cell populations and their significance for morphogenesis are discussed.

Previous studies have shown that the vomeronasal and olfactory epithelia of adult vertebrates provide good models for studying normal neuronal turnover and regeneration in response to axotomy. However, little is known about the cell dynamics in the embryonic vomeronasal and olfactory epithelia or the origins of different cell types in these structures. By using 3H-thymidine autoradiography, both in vivo and in vitro, the origins of receptor and supporting cells and the survival of labelled cells in the embryonic vomeronasal and olfactory epithelial of garter snakes were examined. The results of this study suggest that the receptor and supporting cells of both epithelial arise from separate stem cells and that two subpopulations of stem cells exist for receptor cells in the embryonic vomeronasal epithelium. One subpopulation generates cells that migrate through the receptor cell columns, while another subpopulation remains at the base of the epithelium for approximately 50 days. Although it is unclear how long receptor cells in the embryonic olfactory epithelium survive, the results of this study suggest that they survive at least 37 days and may survive over 56 days. In addition, the development of these sensory epithelia appears different in early versus late embryos, and regeneration in the vomeronasal and olfactory epithelia of adult garter snakes appears similar to development during late gestation. Cells in the developing receptor cell layer of the olfactory epithelium lose their ability to incorporate 3H-thymidine before those in the vomeronasal epithelium, suggesting that the onset of neuronal maturation occurs earlier in the olfactory epithelium than in the vomeronasal epithelium.

During avian vocal learning, birds memorize conspecific song patterns and then use auditory feedback to match their vocal output to this acquired template. Some models of song learning posit that during tutoring, conspecific visual, social, and/or auditory cues activate neuromodulatory systems that encourage acquisition of the tutor's song and attach incentive value to that specific acoustic pattern. This hypothesis predicts that stimuli experienced during social tutoring activate cell populations capable of signaling reward. Using immunocytochemistry for the protein product of the immediate early gene c-Fos, we found that brief exposure of juvenile male zebra finches to a live familiar male tutor increased the density of Fos+ cells within two brain regions implicated in reward processing; the ventral tegmental area (VTA) and substantia nigra pars compacta (SNc). This activation of Fos appears to involve both dopaminergic and nondopaminergic VTA/SNc neurons. Intriguingly, a familiar tutor was more effective than a novel tutor in stimulating Fos expression within these regions. In the periaqueductal gray (PAG), a dopamine-enriched cell population that has been implicated in emotional processing, Fos labeling also was increased after tutoring, with a familiar tutor once again being more effective than a novel conspecific. Since several neural regions implicated in song acquisition receive strong dopaminergic projections from these midbrain nuclei, their activation in conjunction with hearing the tutor's song could help establish sensory representations that later guide motor sequence learning.