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

Summary ₂-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 ₂-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 (K_D about 5 nmol/l; B_max 217–247 fmoles/mg protein), whereas ³H-clonidine bound to a high affinity site (K_D 0.6–1.4 nmol/l) and a low affinity site (K_D 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. K_D- as well as B_max-values for ³H-rauwolscine remained unaltered after 6-OH-DA-treatment. Since an increase in postsynaptic ₂-adrenoceptors due to 6-OH-DA-administration might have masked a loss of presynaptic ₂-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 ₂-adrenoceptors is much greater than that of presynaptic ₂-adrenoceptors. Therefore, the changes in the number of presynaptic ₂-adrenoceptors due to destruction of noradrenergic or/and serotoninergic neurons cannot be detected by equilibrium binding studies with ³H-clonidine or ³H-rauwolscine.