Nucleus locus coeruleus: Evidence for α1-adrenoceptor mediated hypotension in the cat

Microinjection of noradrenaline or phenylephrine into the nucleus locus coeruleus of cats induced a dose dependent and long lasting hypotension. Clonidine was required in a dose of 1 μg for eliciting a significant hypotension while its lower doses (up to 500 ng) failed to elicit any significant cardiovascular alteration. The effects on heart rate evoked by these agents were insignificant. Microinjection of α-adrenoceptor antagonists prazosin, piperoxan and RX 781094 per se did not evoke any significant cardiovascular effects and only prazosin pretreatment showed dose dependent antagonism of the hypotensivee effect of clonidine. Piperoxan was required in four times higher dose (20 μg) to partially antagonize the clonidine induced hypotension. RX 781094, a selective α2-adrenoceptor antagonist, however, even up to a dose of 20 μg (four times that of prazosin) did not alter the effect of clonidine. Similar pattern of antagonism was also seen for noradrenaline and phenylephrine. The results demonstrate the presence of α1-adrenoceptors in the nucleus locus coeruleus, the activation of which leads to a fall in blood pressure.