Reciprocal pre- and postsynaptic actions of cocaine at a central noradrenergic synapse

Single-unit microelectrode studies were conducted to test the effects of systemic cocaine HCl on spontaneously firing single noradrenergic locus ceruleus (presynaptic) and cerebellar Purkinje (postsynaptic) neurons in rats in vivo. The spontaneous neuronal activity of all locus ceruleus neurons was inhibited by cocaine in a dose-dependent manner (0.5 to 2 mg/kg). These doses of cocaine elicited a predominant activation of postsynaptic Purkinje neurons. No effect of cocaine on neuronal action potential amplitude or slope was observed. Similar doses of the local anesthetic agent, procaine, did not affect action potential amplitudes or slopes of either locus ceruleus or Purkinje neurons. In addition, although cocaine elicited a significantly greater absolute change in the discharge rate of locus ceruleus neurons than of Purkinje neurons, the effects of procaine on those neurons were not significantly different from each other. The inhibition of locus ceruleus neurons by cocaine was significantly attenuated by pretreatment either with the alpha 2-adrenoceptor antagonist, yohimbine, or with reserpine. The activation of Purkinje neurons by cocaine was also significantly attenuated by reserpine pretreatment. Systemic cocaine administration (1 mg/kg, i.v.) did not potentiate the inhibitory effects of either locus ceruleus stimulation or local iontophoretic application of norepinephrine on Purkinje neuron discharge rate. We conclude that cocaine potently inhibits locus ceruleus neurons and this effect probably elicits Purkinje cell activation through disinhibition.