Reduced brain monoamine synthesis by systemic treatment with terbutaline, a beta 2-receptor agonist

The effects of acutely (5 mg/kg s.c.) or subchronically (2.5 mg/kg s.c., twice daily, 4 days) administered terbutaline, a beta 2-receptor agonist, on the in vivo rate of tryptophan and tyrosine hydroxylation in various rat brain parts were studied. The accumulation of 5-hydroxytryptophan (5-HTP) during 30 min following treatment with NSD 1015, 100 mg/kg i.p., an inhibitor of L-aromatic amino acid decarboxylase, appeared reduced in several brain parts by the terbutaline treatments, the effect being significant in the limbic forebrain and the hemispheres after subchronic administration. This treatment also reduced the simultaneously measured accumulation of 3,4-dihydroxyphenylalanine (DOPA) in the same brain parts as well as in corpus striatum, where the effect was seen also after acutely administered terbutaline. The concentration of tryptophan in the various brain parts was not significantly affected by the terbutaline treatments and the tyrosine levels were only reduced in some brain parts (the hemispheres and the brain stem). The central effects obtained by terbutaline treatment may be mediated indirectly via peripheral inputs to e.g. the monoamine carrying neurons and/or via putative changes in cerebral blood flow.