Potentiation of GABA_A receptor-mediated inhibitory neurotransmission contributes to the anesthetic action of thiopental. However, the inhibiting action of general anesthetic on excitatory neurotransmission also purportedly underlies its effects. The aim of the study was to elucidate the role of glutamate receptors (NMDA and AMPA receptors) in thiopental-induced anesthesia. Intracerebroventricular (i.c.v.) NMDA (50 ng) significantly increased the induction time of loss of righting reflex and decreased sleep time induced by intraperitoneal injection (i.p.) of thiopental (50 mg/kg). Furthermore, NMDA at 50 ng i.c.v. increased the 50% effective dose values for thiopental to produce loss of righting reflex and immobility in response to noxious tail clamp by 25% and 21% (p < 0.05), respectively. However, intrathecal (IT) administration of NMDA or both of i.c.v. or IT administration of AMPA did not show such antagonizing effects on thiopental action at subconvulsive dose. Finally, thiopental (25 mg/kg i.p.) inhibited convulsions induced by NMDA (0.4 µg i.c.v.) or bicuculline (0.6 µg i.c.v.). However, i.p. muscimol (1 mg/kg) blocked the convulsions induced by bicuculline, but not those induced by NMDA at 3 mg/kg. Similarly, i.p. MK-801 (0.1 mg/kg) antagonized NMDA-induced convulsions, but not bicuculline-induced convulsions at 0.3 mg/kg. Therefore, we suggest that the effects of the selective GABA_A and NMDA receptors on convulsive behavior are special to their sites of action, and that the inhibitory action of thiopental on NMDA receptors is possibly not mediated by secondary effects of its GABA_A receptors agonism. These results above indicate the involvement of NMDA receptors in thiopental-induced anesthesia in mice.

The present study evaluated the role of ventrolateral periaqueductal gray (vlPAG)-located orphanin-FQ (OFQ) in the opioid tolerance induced by repeated microinjections of morphine (MOR) into vlPAG. Microinjection of MOR (5 µg/0.5 µl) into vlPAG caused antinociception as quantified with the tail flick and the hot plate tests. When MOR microinjection was repeated twice daily, the antinociceptive effect disappeared within 2 days (tolerance). However, if MOR microinjection was preceded by the OFQ receptor antagonist nocistatin (NST; 1 ng/0.5 µl), the microinjections of MOR did not induce tolerance. If NST microinjections were suspended, subsequent MOR microinjections induced tolerance. In MOR-tolerant rats, a single NST microinjection into vlPAG was enough to restore the antinociceptive effect of MOR. Furthermore, if OFQ (1 ng/0.5 µl) was microinjected into vlPAG, then a MOR microinjection administered 15 min later into vlPAG did not elicit antinociception. Finally, opioid tolerance induced by repeated systemic MOR injections (5 mg/kg, i.p.) was reversed by a single microinjection of NST into vlPAG. This emphasizes the central importance of vlPAG-located OFQ in the MOR tolerance.

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