Development of and recovery from subsensitivity of the noradrenergic cyclic AMP generating system in brain. Effect of amphetamine following inhibition of its aromatic hydroxylation by iprindole.

Amphetamine given intraperitoneally (10 mg/kg b.i.d.) for 2 days did not alter either the basal level of cyclic AMP or the neurohormonal response of the cylcic AMP generating system to noradrenaline (NA). The same doses of amphetamine caused a significant reduction in the responsiveness to NA and the beta-adrenergic agonist isoprenaline following the inhibition of the aromatic hydroxylation by iprindole. The EC50 values (concentration of NA which causes half maximal cyclic AMP stimulation) were not significantly changed: 9.5 micro M (controls) and 11 micro M (iprindole + amphetamine). Following discontinuation of the drugs, the recovery from adrenergic subsensitivity to NA was complete within 1 week in the limbic forebrain while the adrenergic responsiveness in the cortex was still only 65% of its control value 3 weeks following discontinuation of the drugs. The subsensitivity in both limbic forebrain and cortex was linked to a decreased Bmax value of specific 3H-dihydroalprenolol binding without changes in the Kd values. The different rates of recovery from noradrenergic subsensitivity in limbic forebrain and cortex following withdrawal of the drugs were reflected in the density of beta-adrenergic receptors in the two brain regions. Since inhibition of the aromatic hydroxylation of amphetamine markedly prolongs the half life of the drug and prevents the accumulation of p-hydroxynorephedrine (a potential NA antagonist), the results support the view that homospecific down-regulation of the NA receptor coupled adenylate cyclase system in brain depends on a sustained and unhindered NA receptor interaction.