Tandospirone and its metabolite, 1-(2-pyrimidinyl)-piperazine--I. Effects of acute and long-term administration of tandospirone on serotonin neurotransmission.
The acute and long-term effects of the antidepressant/anxiolytic selective 5-HT1A receptor ligand, tandospirone (SM-3997) on 5-HT neurotransmission were assessed using single-cell extracellular recording in chloral hydrate-anaesthetized rats. The acute intravenous administration of tandospirone decreased the firing rate of 5-HT neurones of the dorsal raphe (ED50 = 9.1 +/- 1.1 micrograms/kg). A treatment with tandospirone for 2 days (10 mg/kg/day, s.c.), markedly reduced the firing activity of 5-HT neurones of the dorsal raphe; this was followed by a partial recovery after 7 days and by complete recovery after 14 days of administration of tandospirone. After treatment with tandospirone for 14 days (10 mg/kg/day, s.c.), the responsiveness of 5-HT neurones to the intravenous administration of LSD was reduced, suggesting that somatodendritic 5-HT autoreceptors had desensitized. The depressant effects of microiontophoretically-applied tandospirone and 5-HT, on the firing activity of CA3 pyramidal neurones in the hippocampus were blocked by the intravenous injection of the 5-HT1A receptor antagonist, BMY-7378. The depressant effect of microiontophoretically-applied 5-HT onto these same neurones was markedly reduced during concurrent background application of tandospirone, suggesting that the latter acted as a partial agonist at postsynaptic 5-HT1A receptors. The sustained administration of tandospirone for 14 days (10 mg/kg/day, s.c.) altered neither the effectiveness of microiontophoretically-applied 5-HT and tandospirone nor that of endogenous 5-HT, released by the electrical simulation of the afferent 5-HT pathway, in suppressing the firing activity of pyramidal neurones in the hippocampus, suggesting that postsynaptic 5-HT1A receptors had not desensitized. Furthermore, long-term treatment with tandospirone did not alter the sensitivity of the terminal 5-HT autoreceptor. It is thus concluded that desensitization of somatodendritic 5-HT autoreceptors permits 5-HT neurones to regain their physiological rate of firing during long-term treatment with tandospirone and, consequently, to release a normal amount of 5-HT into the synaptic cleft. This, combined with the sustained activation of normosensitive postsynaptic 5-HT1A receptors by tandospirone, during such a treatment, should result in an enhanced tonic activation of postsynaptic 5-HT1A receptors.
Godbout
R
R
Department of Psychiatry, McGill University, Montréal, Québec, Canada.
Chaput
Y
Y
Blier
P
P
de Montigny
C
C
D000818:Animals; D014151:Anti-Anxiety Agents / Q000494:pharmacology*; D001921:Brain / Q000187:drug effects / Q000378:metabolism; D004594:Electrophysiology; D007275:Injections, Intravenous; D054833:Isoindoles; D008297:Male; D009474:Neurons / Q000187:drug effects; D009638:Norepinephrine / Q000494:pharmacology; D010879:Piperazines / Q000494:pharmacology*; D011743:Pyrimidines / Q000494:pharmacology*; D051381:Rats; D011919:Rats, Inbred Strains; D012701:Serotonin / Q000502:physiology*
D014151:Anti-Anxiety Agents; D054833:Isoindoles; D010879:Piperazines; D011743:Pyrimidines; C055267:tandospirone; D012701:Serotonin; C053428:BMY 7378; D009638:Norepinephrine