Peptidergic regulation in neuroendocrine and autonomic systems

Neuropeptides are found in dense networks of neuronal perikarya, fibers and terminals within numerous brain regions. Among the more striking of these collections are sites within the central nervous system that are presumed to regulate either endocrine or autonomic function. A recent example of a neuropeptide which is likely to play a significant role in endocrine regulation is cortocotropin releasing factor (CRF). Immunohistochemical studies revealed that CRF immunoreactivity was found in many brain regions, including the paraventriculo-infundibular pathway. CRF released from nerve terminals belonging to this pathway presumably regulates ACTH release. Treatment of rats with reserpine depletes CRF as well as vasopressin from the external layer of the median eminence, suggesting tonic, monoaminergic inhibition of CRF and vasopressin containing neurons. CRF antisera were found which stain urotensin I immunoreactivity within the caudal neurosecretory system of fish. Numerous putative neurotransmitters impinge upon preganglionic sympathetic neurons within the intermediolateral cell column of the spinal cord. Preganglionic sympathetic neurons which innervate the adrenal medulla appear to have a specific input from somatostatin immunoreactive fibers. In addition, binding sites for serotonin and alpha-2 adrenergic ligands are more highly concentrated over sympathoadrenal neurons. Finally, the pancreatic islet contains peptide producing endocrine cells which possess several neuron-like properties. Some of these properties are reviewed, especially the finding that the insulin producing cells contain glutamate decarboxylase immunoreactivity, the biosynthetic enzyme for GABA. Further studies revealed that GABA agonists inhibit somatostatin release from islet cells.