The influence of serotoninergic drugs on dopaminergic neurotransmission in rat substantia nigra, striatum and limbic forebrain in vivo.

The effects of serotoninergic drugs on dopaminergic neurotransmission in the substantia nigra, the striatum and the limbic forebrain of rat have been investigated. The accumulation of 3-methoxytyramine (3-MT) following inhibition of monoamine oxidase with pargyline was used as an indirect measure of dopamine (DA) activity in vivo. The effects of the following serotoninergic drugs were tested: the 5-HT1A receptor agonist 8-OH-DPAT, the 5-HT1B receptor agonist trifluoromethyl-phenylpiperazine (TFMPP), CGS 12066 B and RU 24969, the 5-HT1A/1B antagonist (+/-)pindolol, the 5-HT2/1C receptor antagonist ritanserin, the 5-HT2/1C receptor agonist DL-1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane (DOI), the 5-HT3 receptor antagonist BRL 43694, the unselective 5-HT receptor antagonist methiothepin, and carbidopa + L-5-hydroxytryptophan (L-5-HTP) to achieve a general, unselective stimulation of multiple 5-HT receptors. In the substantia nigra, carbidopa + 5-HTP treatment increased the 3-MT accumulation by 26% and decreased the DA concentration to 67% of controls, tentatively suggesting a 5-HTP-induced displacement of nigral DA. A minor, non dose-related reduction in nigral 3-MT was seen after the 5-HT1A receptor agonist 8-OH-DPAT. None of the other serotonin receptor acting drugs induced any pronounced effect on the nigral 3-MT accumulation. Taken together, the findings provide little support for the idea that one single 5-HT receptor subtype serves a modulatory function on DA activity in the substantia nigra. In the striatum and the limbic forebrain, trifluoromethyl-phenylpiperazine dose-dependently increased the 3-MT accumulation to maximally 200%-220% of controls.(ABSTRACT TRUNCATED AT 250 WORDS)