Substance P: its distribution, pharmacological actions and possible physiological role in sensory neurons

The undecapeptide substance P (SP) is a normal constituent of most tissues, particularly the central and peripheral nervous system. SP-immunoreactive cell bodies and terminal networks are distributed in most areas of the brain. Particular interest has been focused on the presence and function of SP in the primary sensory neuron. SP is released from both the central and peripheral part of the sensory neuron following stimulation of the dorsal roots and peripheral sensory nerves. Evidence is given for a transmitter role of SP in the sensory neurons and for the hypothesis that SP is involved in the vascular effects induced by sensory stimulation, referred to as the axon reflex. SP is present in large amounts in the gastro-intestinal tract. By immunohistochemistry SP is demonstrated within nerve cell bodies and terminals of the intramular plexuses but also in extrinsic neurons. It is released both from intrinsic sensory neurons of the gut and from the gastro-intestinal lumen in response to vagal stimulation. SP stimulates motor activity and is ascribed a regulatory role for both the intestinal motility and blood flow. SP has numerous pharmacological effects, the most obvious being excitation of spinal motoneurons, vasodilation and stimulation of salivary and pancreatic secretion. It is a normal constituent of blood plasma but there is no evidence that SP acts as a circulating hormone.