8-OH-DPAT-INDUCED HYPOTENSIVE ACTION AND SYMPATHOEXCITATORY NEURONS IN THE ROSTRAL VENTROLATERAL MEDULLA OF THE RAT

We examined whether the selective 5-hydroxytryptamine 1A (5-HT1A) rece ptor agonist 8-hydroxy-2-(di-n-propylamino)-tetralin (8-OH-DPAT) injec ted systemically can act directly on sympathoexcitatory neurons locate d in the rostral ventrolateral medulla (RVLM) to cause the hypotensive effect of this agent in rats. Microinjections of 8-OH-DPAT and buspir one into the RVLM produced a dose-dependent decrease in blood pressure . Microinjections of spiperone and pindolol, 5-HT1A antagonists, into the RVLM inhibited the depressor response to 8-OH-DPAT intravenously i njected or injected into the RVLM. Microiontophoretic application of 8 -OH-DPAT onto RVLM sympathoexcitatory neurons inhibited the firing of RVLM sympathoexcitatory neurons and the inhibition of unit activity by 8-OH-DPAT was blocked by microiontophoretic spiperone. Intravenous ad ministration of 8-OH-DPAT also inhibited the firing of these neurons. Microiontophoretic application of spiperone onto the RVLM sympathoexci tatory neurons reversed the inhibitory response to intravenous 8-OH-DP AT. These results are consistent with the hypothesis that 8-OH-DPAT ma y exert a portion of its hypotensive effect through a direct inhibitio n of RVLM sympathoexcitatory neurons in rats. The receptor involved is probably the 5-HT1A type.