Effects of long-term treatment of rats with antidepressants on adrenergic-receptor sensitivity in cerebral cortex: Structure activity study

The effects of 20 tricyclic and 12 chemically unrelated 'atypical' antidepressant drugs on the noradrenaline (NA) receptor coupled Adenylate Cyclase system were investigated in slices of the rat cerebral cortex. While no changes occurred after a single dose, 14 tricyclic compounds down-regulated the receptor system when administered for 9 days. Six tricyclic antidepressants (trimipramine, butriptyline, noxiptyline, doxepine, dosulepine, propizepine) failed to desensitize the NA sensitive Adenylate Cyclase although some were potent inhibitors of the neuronal uptake of NA. Using the two optically active enantiomers of oxaprotiline inhibition of NA uptake was observed with the (+)-enantiomer while the (?)-enantiomer had only weak inhibitory potency. However, in contrast to published data, both enantiomers and the racemate administered at 30 mg/kg for 9 days did down-regulate the NA receptor coupled Adenylate Cyclase. Therefore, the experiments were repeated with Sprague-Dawley rats from a different supplier. Now, data published earlier were reproducible, only the racemate and the (+)-enantiomer of oxaprotiline being significantly active on the desensitization of the NA sensitive Adenylate Cyclase. Using F-344, Long Evans and Wistar rats significant differences were found in the response of the adrenoceptor coupled Adenylate Cyclase to a 9 day treatment with 30 mg/kg imipramine. Although some of the atypical antidepressants are potent inhibitors of the biogenic amine uptake systems none of these compounds lead to statistically significant changes of the NA stimulated cAMP formation after a 9-day treatment period. Only with the NA uptake inhibitor tandamine and with the serotonin uptake inhibitors zimelidine and fluoxetine a trend toward adrenergic down-regulation was recognized. Using enantiomers of mianserin only the (?)-isomer which is a poor NA uptake inhibitor, was slightly active. It thus appears that the therapeutic action of antidepressant drugs cannot generally be related to postsynaptic adaptive changes in the sensitivity of the noradrenergic Adenylate Cyclase receptor system. Variabilities in pharmacokinetics and in NA sensitivity of the cAMP generating system in various rat strains and possibly in different animal species may be important factors determining whether ?-receptor down-regulation will occur during chronic treatment with antidepressant drugs.