Bulbospinal serotonin neurons and hypotensive effects of methyldopa in the spontaneously hypertensive rat

Systemic methyldopa administration in genetically hypertensive rats evoked a hypotension which was attenuated after prior treatment with the neurotoxin 5,7-dihydroxytryptamine (5,7-DHT) given either intracerebroventricularly, to produce a generalized ablation of serotonin nerves in brain and spinal cord, or by injection into the cervical spinal cord, to cause a selective destruction of descending serotonin pathways. Direct microinjection of methyldopa into the ventrolateral medulla in the area of the B₁ and B₃ serotonin cells was also effective in lowering arterial pressure. The hypotensive response to this medullary administration of methyldopa was again attenuated by 5,7-DHT given either intracerebroventricularly or by intraspinal injection. On the other hand, prior treatment intracerebroventricularly with 6-hydroxydo-pamine did not affect the hypotensive effect of methyldopa injected into the region of the ventrolateral B¹ and B₃ cells, supporting the suggestion that this effect of methyldopa is mediated by bulbospinal serotonin neurons and not by descending catecholamine nerves. The fact that methyldopa injection into the ventrolateral medulla in a region coinciding with the B₁ and B₃ cells lowers blood pressure is consistent with previous studies demonstrating that these neurons serve to maintain or elevate arterial pressure.