Inhibition of neuronal uptake reduces the presynaptic effects of clonidine but not of α-methylnoradrenaline on the stimulation-evoked release of ³H-noradrenaline from rat occipital cortex slices

The iminoimidazolidine clonidine reduced concentration-dependently the release of ³H-noradrenaline evoked by electrical stimulation from the rat cerebral cortex. Exposure to the neuronal uptake inhibitors cocaine (10 μM), desipramine (0.1 to 1 μM) and amphetamine (1 μM) significantly increased the stimulation-evoked overflow of tritium. These uptake inhibitors antagonized the effects of clonidine on stimulation-evoked ³H-noradrenaline release but failed to modify the inhibition induced by the catecholamine α-methylnoradrenaline. Inhibition of monoamine oxidase by preincubation of cerebral cortex slices with 0.5 mM pargyline significantly increased the stimulation-evoked overflow of tritium, but clonidine was as effective as in the controls in inhibiting ³H-noradrenaline overflow. The antagonism by desipramine of the clonidine-induced inhibition of neurotransmission could not be attributed to a blockade of presynaptic α-adrenoceptors because: (1) the facilitating effect of phentolamine on ³H-noradrenaline overflow was not modified in the presence of desipramine; (2) the magnitude of the inhibition of the stimulation-evoked ³H-noradrenaline release elicited by α-methylnoradrenaline was the same in the presence of cocaine or desipramine; (3) exposure to desipramine in the presence of cocaine did not further increase the stimulation-evoked release of ³H-transmitter. Since the catecholamine α-methylnoradrenaline inhibited neurotransmission in the presence of desipramine or cocaine, we can conclude that inhibition of neuronal uptake of noradrenaline antagonized selectively the presynaptic inhibitory effects of imidazolines on α₂-adrenoceptors. The influence of the inhibition of neuronal uptake on the presynaptic effects of imidazolines and catecholamines should be taken into account when the relative order of potencies of various α₂-adrenoceptors agonists is determined.