Effects of activating spinal alpha-adrenoreceptors on sympathetic nerve activity in the rat.

The effects of several alpha-adrenoreceptor agonists and antagonists administered intrathecally at T10 level on renal sympathetic nerve activity (RSNA) were examined, in chloralose-urethane anaesthetised rats. Intrathecal noradrenaline (NA, 0.5-500 micrograms) produced one of 3 responses depending on dose, an inhibition of RSNA at low doses, an excitation of RSNA at high doses, or a biphasic effect. Intrathecal adrenaline (5-200 micrograms) was inhibitory in the main but some doses elicited poorly repeatable brief excitation followed by prolonged inhibition. Intrathecal methoxamine (ME; 2.5 ng-25 micrograms) caused a dose-dependent increase in RSNA (mean maximum response 27 +/- 0.5%). The excitatory effects of NA and ME were blocked (72% +/- 12%) by pretreatment with intrathecal prazosin (PRA, 20-200 ng) but not by yohimbine (YOH, 200 ng). Intrathecal guanabenz (GUA 3-15 micrograms) caused a dose-dependent inhibitory effect on RSNA (mean maximum 32% +/- 5%). The inhibitory effects of NA, adrenaline and GUA were blocked by pretreatment with intrathecal YOH (200 ng-2 micrograms). Intrathecal PRA (200 ng) had no effect on the inhibitory effects of NA and GUA. Intravenous administration of each of the adrenoreceptor agonists (apart from adrenaline), at similar doses to those given intrathecally, in most cases had no significant effect on RSNA; in a few cases the opposite effects to those produced by intrathecal administration were seen.