Neurochemical studies on the existence, origin and characteristics of the serotonergic innervation of small pial vessels

Substantial concentrations of serotonin (5-HT) and 5-hydroxyindoleacetic acid (5-HIAA), comparable to those found in brain tissue, were measured in the small pial vessels of the rat, rabbit and cat. Both rat and rabbit pial vessels exhibited a high affinity uptake process with kinetic parameters similar to those identified for the cerebral cortex. Labelled 5-HT, taken up by isolated rabbit pial vessels was released, in a calcium-dependent manner, by potassium-induced depolarization. Various pharmacological manipulations were carried out in the rat. Systemic administration of the 5-HT precursor, 5-hydroxytryptophan and the monoamine oxidase inhibitor, pargyline, significantly increased the concentration of 5-HT in the pial vessels; in contrast, two depleting agents (p-chloroamphetamine and reserpine) and the tryptophan hydroxylase inhibitor, p-chlorophenylalanine, all decreased the perivascular 5-HT levels. A serotonergic antagonist (methysergide) and a 5-HT receptor agonist (MK 212) respectively increased and decreased the concentrations of 5-HIAA in the pial vessels. These pharmacologically induced changes observed in pial vessels were not dissimilar from those noted for cortical tissue. Electrolytic lesions of the nuclei raphes medianus and/or dorsalis markedly decreased the levels of 5-HT and 5-HIAA in these small cerebral arterioles. Electrical stimulation of these nuclei decreased 5-HT although 5-HIAA concentrations tended to increase. A number of conclusions may be drawn from these studies. Thus, there is a serotonergic innervation of the cerebral circulation in several laboratory species which unequivocally originates in the raphénuclei. Furthermore, these perivascular fibres possess synthetic, storage, release, inactivation and autoregulatory processes for 5-HT which, when further elucidated, may offer some rationale for the treatment of those cerebrovascular diseases in which this neurotransmitter and vasoactive agent is believed to be of pathological importance.