Reduced brain serotonergic activity after repeated treatment with beta-adrenoceptor antagonists

Rats were treated subchronically (14 days) or acutely (single dose) with the beta 1-selective adrenoceptor antagonist metoprolol or the beta 3-selective adrenoceptor antagonist ICI 118,551. Metoprolol (350 mg/kg/day for 14 days, orally) significantly reduced the 5-hydroxytryptophan (5-HTP) accumulation when measured 30 min after inhibition of L-amino acid decarboxylase by NSD 1015 (100 mg/kg IP) in the limbic forebrain, the corpus striatum, the cerebral cortex, the brain stem, and in the cerebellum. ICI 118,551 (0.5 mg/kg, twice daily for 14 days, SC) also significantly reduced the 5-HTP accumulation in the same brain regions except in the corpus striatum and the brain stem Simultaneously assayed tryptophan levels were largely unaffected. Thus sustained beta-adrenoceptor blockade causes a decrease in the in vivo rate of tryptophan hydroxylation in various rat brain regions. The subchronic treatments with metoprolol or ICI 118,551 also significantly reduced the endogenous levels of 5-hydroxytryptamine (5-HT) in the various rat brain regions studied. Acute treatment with either metoprolol (2 mg/kg SC) or ICI 118,551 (0.5 mg/kg SC) did not affect the 5-HTP accumulation or the endogenous 5-HT levels in the brain regions studied. This inhibitory effect on brain 5-HT systems produced by sustained beta-adrenoceptor blockade may be of significance both for the long-tem cardiovascular action and for occasional neuropsychiatric side effects during beta-blocking therapy.