Alpha 2-adrenoceptor modulation of A1 noradrenergic neuron input to supraoptic vasopressin cells

The A1 noradrenaline (NA) cell group of the caudal medulla provides a direct excitatory input to supraoptic nucleus (SON) vasopressin (VP) cells. We have now investigated the possibility that NA released from A1 terminals acts presynaptically to modulate A1 input to VP cells. Initial experiments done in pentobarbitone anaesthetized rats established that SON application of NA excited VP cells but also depressed their response to activation of the A1 input. Moreover, the latter effect was mimicked by the alpha 2-adrenoceptor agonist clonidine and antagonised by intravenous administration of the alpha 2-adrenoceptor antagonist yohimbine. Further studies showed that yohimbine also prevented the relative decline in synaptic excitation of VP cells normally observed as A1 activation frequency increases. These data are consistent with the proposal that NA, released from the SON terminals of A1 NA cells, acts via alpha 2-adrenoceptors to depress A1 transmitter release and thus A1 influence on VP cells.