Repeated administration of desipramine and a GABAB receptor antagonist, CGP 36742, discretely up-regulates GABAB receptor binding sites in rat frontal cortex.

1. GABAB receptor binding site densities within laminar regions of the rat frontal cortex were examined autoradiographically following repeated administration (21 days) of the antidepressants desipramine, paroxetine and amitriptyline in addition to the GABAB receptor antagonists, CGP 35348 and CGP 36742. beta 1-Adrenoceptor autoradiography was studied in parallel with that for GABAB receptor sites. 2. The effects of these compounds were examined concomitantly on the GABAB receptor-mediated inhibition of forskolin- and enhancement of noradrenaline-stimulated cyclic AMP production. 3. GABAB receptor binding was increased by both desipramine (20 mg kg-1, p.o. and 10 mg kg-1, i.p.) and CGP 36742 (100 mg kg-1, i.p.) in the outer laminar region of the frontal cortex by around 50% above control levels. Conversely, no significant changes were mediated by paroxetine, amitriptyline, CGP 35348 or the GABAB receptor agonist, baclofen. 4. With the exception of paroxetine, all compounds down-regulated the total beta-adrenoceptor population throughout frontal cortical laminae which was attributable to the beta 1-adrenoceptor subtype. In contrast, the reduction in beta-adrenoceptors mediated by CGP 35348 and CGP 36742 did not occur as a consequence of reduced beta 1-adrenoceptor numbers. 5. Protracted treatment with CGP 35348, failed to influence forskolin-stimulated cyclic AMP production; however, a significant increase in the accumulation of cyclic AMP produced in response to forskolin was seen after treatment with CGP 36742. 6. Such discretely localized changes in GABAB receptor densities induced by desipramine and CGP 36742 may provide an explanation for the discrepancies reported in membrane binding studies and possibly implicate a role for GABAB receptor antagonists in antidepressant therapy.