Hypothalamic neurohormonal peptides

The occurrence of a variety of neuropeptides in the brain has been discussed in Section A. The first of these to be discovered811 was the 11-residue substance P (Table 30-4), which was isolated in 1931. Like other neuropeptides it may function either as a transmitter or neuromodulator or perhaps both. Substance P, as well as many other neuropeptides, has been localized to specific neurons. Along with somatostatin, CCK, and enkephalins, it is found in high concentrations in the basal ganglia. Enkephalin and substance P are also found in specific neural elements in the visual system of lobsters.812 In some cases a neuron contains both synaptic vesicles containing a major neurotransmitter and also vesicles containing a peptide or other cotrans-mittor. The peptide pituitary hormones ACTH, MSH, and vasopressin as well as the hypothalamic neurohormones may have effects on learning and behavior.813... 

Peptide-secreting cells of the hypothalamic-hypophyseal circuits originally were described as neurosecretory cells, receiving synaptic information from other central neurons, yet secreting transmitters in a hormone-like fashion into the circulation. The transmitter released from such neurons was termed a neurohormone, i.e., a substance secreted into the blood by a neuron. These hypothalamic neurons also may form traditional synapses with central neurons, and cytochemi-cal evidence indicates that the same substances that are secreted as hormones from the posterior pituitary (oxytocin, arginine-vasopressin see Chapters 29 and 55) mediate transmission at these sites. Thus, the designation hormone relates to the release at the posterior pituitary and does not necessarily describe all actions of the peptide. 

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