Lesioning of serotoninergic and noradrenergic nerve fibres of the rat brain does not decrease binding of 3H-clonidine and 3H-rauwolscine to cortical membranes

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Abstract
α2-adrenoceptors located presynaptically on nerve terminals are known to modulate the release of neurotransmitters from noradrenergic and serotoninergic neurons. The pre- and/or postsynaptic localization of binding sites for α2-adrenergic radioligands, the agonist 3H-clonidine and the antagonist 3H-rauwolscine, was investigated in the rat cerebral cortex by the use of specific neurotoxins. Intracerebroventricular ijections of 6-hydroxydopamine (6-OH-DA) and 5,7-dihydroxytryptamine (5,7-DHT) were used to destroy the noradrenergic and serotoninergic neurons, respectively, and the success of the treatment was controlled by measurement of tritium accumulation in cortex slices incubated with 3H-noradrenaline or 3H-serotonin. In cortical membranes, 3H-rauwolscine bound to a single site (KD about 5 nmol/l; Bmax 217–247 fmoles/mg protein), whereas 3H-clonidine bound to a high affinity site (KD 0.6–1.4 nmol/l) and a low affinity site (KD 6–10 nmol/l). The total number of high plus low affinity 3H-clonidine binding sites was about two thirds of the number of 3H-rauwolscine binding sites. 6-OH-DA treatment significantly increased the number of high affintiy 3H-clonidine binding sites without reducing the number of high plus low affinity binding sites, indicating a denervation supersensitivity. KD- as well as Bmax-values for 3H-rauwolscine remained unaltered after 6-OH-DA-treatment. Since an increase in postsynaptic α2-adrenoceptors due to 6-OH-DA-administration might have masked a loss of presynaptic α2-adrenergic binding sites, rats were chronically treated with high doses of clonidine in order to prevent a possible supersensitivity of postsynaptic receptors. Even under these conditions 6-OH-DA did not reduce the number of 3H-clonidine and 3H-rauwolscine binding sites. Injection of 5,7-DHT had no influence on binding parameters of 3H-clonidine and 3H-rauwolscine. It is concluded that in the cerebral cortex the number of postsynaptic α2-adrenoceptors is much greater than that of presynaptic α2-adrenoceptors. Therefore, the changes in the number of presynaptic α2-adrenoceptors due to destruction of noradrenergic or/and serotoninergic neurons cannot be detected by equilibrium binding studies with 3H-clonidine or 3H-rauwolscine.