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Nitric oxide (NO) is an unconventional transmitter molecule in the nervous system, synthesized by nitric oxide synthase (NOS) following activation of the N-methyl-D-aspartate (NMDA) receptor. Several in vivo studies have demonstrated that NO modulates the extracellular levels of various neurotransmitters in the central nervous system, while serotonin (5-HT) re-uptake may be influenced by the NO pathway. Moreover, inhibitors of NOS exhibit antidepressant-like and anxiolytic-like properties in various animal models. Therefore, the aims of the present study were to clarify the involvement of distinct antidepressants acting on the serotonin re-uptake site in the regulation of the activity of hippocampal NOS in vitro, in vivo and ex vivo. We found that citalopram, paroxetine, imipramine and N(G)-nitro-L-arginine dose dependently decreased the hippocampal NOS activity in vitro. Moreover, local administration of citalopram, paroxetine, tianeptine, imipramine and N(G)-nitro-L-arginine significantly decreased the hippocampal NOS activity in vivo at a concentration significantly lower than in vitro. No effect on NOS activity following retrodialysis with 5-HT was observed. Acute (5 mg/kg, s.c.) and chronic (3 weeks, 20 mg/kg/24 h) systemic administration of citalopram did not influence NOS activity ex vivo. The effects on NOS represent a response to structurally dissimilar serotonergic antidepressants. However, since these data reflect effects on basal NOS activity, we believe that serotonergic antidepressants do not directly affect NOS at dosages used clinically, but the findings may reflect a secondary action of antidepressants on the glutamate NMDA receptor following their primary inhibitory action at the 5-HT transporter.

In the present study, we have investigated the effects of prolonged inhibition of nitric oxide synthase (NOS) by infusion of NOS inhibitor, L-nitroarginine, to examine the pentobarbital-induced sleep, modulation of GABA(A) receptor binding, and GABA(A) receptor subunit mRNA level in rat brain. Pre-treatment with L-nitroarginine 30 min before pentobarbital treatment (60 mg/kg, i.p.) significantly increased the duration of sleep in rats. However, the duration of pentobarbital-induced sleep was shortened by the prolonged infusion of L-nitroarginine into ventricle. We have investigated the effect of NOS inhibitor on GABA(A) receptor binding characteristics in discrete areas of brain regions by using autoradiographic and in situ hybridization techniques. Rats were infused with L-nitroarginine (10, 100 pmol/10 microl/h, i.c.v.) for 7 days, through pre-implanted cannula by osmotic minipumps. The levels of [(3)H]muscimol and [(3)H]flunitrazepam binding were markedly elevated in almost all of brain regions including cortex, caudate putamen, thalamus, hippocampus, and cerebellum. However, there was no change in the level of [(35)S]TBPS binding. The levels of beta2-subunit were elevated in the cortex, brainstem, and cerebellar granule layers. By contrast, the levels of beta3-subunit were significantly decreased in the cortex, hippocampus, and cerebellar granule layers in L-nitroarginine-infused rats. Following L-nitroarginine treatment, the levels of alpha6- and delta-subunits which were strictly localized to the cerebellum, were not changed in the cerebellar granule layer. These results show that the prolonged inhibition of NOS by L-nitroarginine-infusion markedly elevates [(3)H]muscimol and [(3)H]flunitrazepam binding throughout the brain, and alters GABA(A) receptor subunit mRNA levels in different directions. Chronic inhibition of NO generation has differential effects on the various expressions of GABA(A) receptor subunits. These suggest the involvement of different regulatory mechanisms for the NO-induced expression of GABA(A) receptor.