Neural substrates for processing chemosensory information in snakes.

Snakes interact with their chemical environment through their olfactory and vomeronasal systems. The present report summarizes advances on neural substrates for processing chemosensory information. First, the efferent and centrifugal afferent connections of the main and accessory olfactory bulbs were reinvestigated. Second, the afferent and efferent connections of the nucleus sphericus, the main target of the accessory olfactory bulb, were characterized. The nucleus sphericus gives rise to a very small projection to the hypothalamus, but it does project to other telencephalic structures where olfactory and vomeronasal information could converge. Third, the intra-amygdaloid circuitry and the amygdalo-hypothalamic projections were described. The medial amygdala, for instance, receives both vomeronasal and olfactory inputs and projects to the hypothalamus, namely, to the lateral posterior hypothalamic nucleus. Fourth, because the lateral posterior hypothalamic nucleus projects to the hypoglossal nucleus, the motor center controlling the tongue musculature, this projection could constitute a pathway for chemosensory information to influence tongue-flicking behavior. In summary, vomeronasal information is mostly relayed to the hypothalamus not via the nucleus sphericus but through other telencephalic structures. Convergence of olfactory and vomeronasal information appears to occur at different levels in the telencephalon. A neural substrate for the chemosensory control of tongue-flicking behavior is provided.