Organization of the ophidian amygdala: chemosensory pathways to the hypothalamus

Although recent studies in squamate reptiles have importantly clarified how chemical information is processed in the reptilian brain, how the amygdala relays chemosensory inputs to the hypothalamus to influence chemically guided behaviors is still poorly documented. To identify these chemosensory pathways, the amygdalo-hypothalamic projections, intra-amygdaloid circuitry and afferents from the lateral cortex (LC) to the amygdala were investigated by injecting conjugated dextran-amines into the hypothalamus, amygdala, and LC of garter snakes. The amygdala was divided into olfactory recipient (ventral anterior and external amygdalae), vomeronasal recipient (nucleus sphericus, NS, and medial amygdala, MA), and nonchemosensory (e.g., posterior dorsal ventricular ridge, PDVR, and dorsolateral amygdaloid nucleus, DLA) subdivisions. Rostroventral (LCrv) and dorsocaudal subdivisions of the LC were distinguished. In addition to receiving afferents from the main olfactory bulb, the olfactory amygdala receives afferents from NS and projects to the NS, PDVR, and dorsal hypothalamus. The NS has only a minor projection to the lateral hypothalamus, whereas the MA, which receives afferents from the LCrv and NS, has projections to the ventromedial hypothalamic (VMH) and lateral posterior hypothalamic nuclei. Among the nonchemosensory amygdaloid structures, the PDVR receives afferents from the LCrv and the olfactory amygdala and projects to the VMH, whereas DLA receives afferents from the LCrv and NS, and projects to the periventricular hypothalamus. These results substantially clarify the olfactory and vomeronasal tertiary connections and demonstrate that parts of the nonchemosensory amygdala play a major role in relaying chemosensory information to the hypothalamus.