Effects of chronic desipramine treatment on rat brain noradrenergic responses to α-adrenergic drugs

It has been previously reported that long-term tricyclic antidepressant treatment in the rat causes a subsensitivity of central β-receptor-stimulated adenylate cyclase along with alterations of brain norepinephrine (NE) content and metabolism. We have confirmed earlier findings that after one week of desipramine treatment (5.0 mg/kg b.i.d.) brain NE levels decline while NE metabolism is similar to control animals, but is above control after 12 days of treatment. Single cell recordings from noradrenergic neurons of the locus coeruleus (LC) show that after one week of desipramine treatment, neuronal firing rate is lower than in control rats but greater than that seen in response to acutely administered drug. Furthermore, desipramine injection in a dose which profoundly altered LC impulse flow in control rats produced little or no effect on impulse flow in chronically treated rats. Of 25 or 250 μg/kg doses of clonidine, which are equieffective for decreasing brain NE metabolism in control animals, only the larger dose decreased NE metabolism in 12 day desipramine-treated rats. The postsynaptic α-antagonist prazosin (5.0 mg/kg) increased NE metabolism in both groups. These results suggest that presynaptic (α₂) adrenoreceptors become subsensitive during long-term desipramine treatment, thus allowing recovery of noradrenergic impulse flow in the presence of NE uptake inhibition.