Midline B3 serotonin nerves in rat medulla are involved in hypotensive effect of methyldopa

Previous experiments in this laboratory have shown that microinjection of methyldopa onto the ventrolateral cells of the B3 serotonin neurons in the medulla elicits a hypotensive response mediated by a projection descending into the spinal cord. The present experiments were designed to investigate the role of the midline cells of the B3 serotonin neurons in the medulla, coinciding with the raphe magnus. In spontaneously hypertensive, stroke-prone rats, microinjection of methyldopa into the area of the midline B3 serotonin cell group in the ventral medulla caused a potent hypotension of 30-40 mm Hg, which was maximal 2-3 h after administration and was abolished by the serotonin neurotoxin 5,7-dihydroxytryptamine (5,7-DHT) injected intracerebroventricularly. However, intraspinal injection of 5,7-DHT to produce a more selective lesion of only descending serotonin projections in the spinal cord did not affect this hypotension. Further, 5,7-DHT lesion of serotonin nerves travelling in the median forebrain bundle, one of the main ascending pathways from the B3 serotonin cells, did not affect the fall in blood pressure associated with a midline B3 serotonin methyldopa injection. It is concluded therefore that, unlike the ventrolateral B3 cells which mediate a methyldopa-induced hypotension via descending projections, the midline serotonin B3 cells in the medulla contribute to the hypotensive action of methyldopa, either by way of an ascending projection which does not pass through the median forebrain bundle, or through a projection restricted to the caudal brainstem.