Alpha(1)-adrenoceptor-mediated excitation of substantia nigra pars reticulata neurons

The effect of noradrenaline was studied in principal neurons of the substan tia nigra pars reticulata in rat brain slices using patch clamp recordings. Perfusion of noradrenaline or the alpha(1)-adrenoceptor agonist phenylephr ine increased the spontaneous firing activity of reticulata cells. The alph a(1)-adrenoceptor antagonist prazosin counteracted the effects of noradrena line. In contrast, the beta-adrenoceptor agonist isoproterenol did not affe ct the activity of reticulata cells and the beta-adrenoceptor antagonist pi ndolol did not prevent noradrenaline's effect. In whole-cell recordings, at -60 mV holding potential, noradrenaline caused a tetrodotoxin-resistant in ward current with a time-course similar to the increase in firing activity. Analysis of the reversal potential of this current did not give homogeneou s results. The net noradrenaline current could be associated with a conduct ance decrease or increase, or in some cases it did not reverse over a range from -120 to -30 mV. It is suggested that noradrenaline increases the exci tability of substantia nigra reticulata cells through alpha(1)-adrenoceptor s. Both a reduction and an increase in membrane conductance may mediate thi s effect. The increase in the tonic firing of principal reticulata cells caused by no radrenaline may have significant consequences in regulating the final outpu t of the basal ganglia and consequently in motor-related behaviours. (C) 20 00 IBRO. Published by Elsevier Science Ltd. All rights reserved.