Distribution of neuropeptides in the limbic system of the rat: The amygdaloid complex

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The distribution of six neuropeptides (vasoactive intestinal polypeptide, cholecystokinin octapeptide, substance P, neurotensin, methionine-enkephalin and somatostatin) has been mapped in the amygdala using immunocytochemical methods. Cell bodies containing each peptide showed a differential distribution throughout the various subnuclei. Large numbers of vasoactive intestinal polypeptide and cholecystokinin-octapeptide-containing cell bodies were located in the lateral and cortical nuclei respectively, neurotensin- and methionine enkephalin-containing cell bodies in the central nucleus, and substance P-containing cell bodies primarily in the medial nucleus. Somatostatin-containing cell bodies were found in all nuclei. Neuropeptide-containing fibres were also differentially distributed. Substance P and cholecystokinin fibres formed dense plexuses in the medial nucleus whilst the greatest concentration of vasoactive intestinal polypeptide, neurotensin and methionine enkephalin fibres were seen in the central nucleus. Close observation of serial sections showed that all the neuropeptides studied had extensive intra-amygdaloid pathways and connections with other brain areas. The central nucleus and stria terminalis have particular importance in the organisation of peptides within the amygdala. The central nucleus acts as a focus for a number of converging/diverging peptide pathways and incoming catecholaminergic afferents. The stria terminalis contains all six peptides and represents the major efferent peptidergic system. The amygdala is thought to control a number of endocrine reponses and to regulate complex behavioural functions. The abundance of neuropeptides within the amygdala and their complex pattern of pathways imply that they may act to regulate endocrine responses to external events (e.g. stress) or alter emotional tone, functions thought to be controlled by the amygdala.