MORPHINE, 5-HT2 AND 5-HT3 RECEPTOR ANTAGONISTS REDUCE C-FOS EXPRESSION IN THE TRIGEMINAL NUCLEAR-COMPLEX FOLLOWING NOXIOUS CHEMICAL-STIMULATION OF THE RAT NASAL-MUCOSA

Noxious chemical stimulation of the rat nasal mucosa induces the expre ssion of the immediate early gene c-fos in trigeminal brainstem neuron s. In the present study, we applied the irritant mustard oil (1%) into the left nostril of urethane anesthetized rats. Immunohistochemical m ethods were used to evaluate the expression of Fos protein in the trig eminal subnuclei interpolaris and caudalis and to test the effects of putative analgesics that might depress synaptic transmission in neuron s related to nociception. For this purpose, morphine (3 mg/kg and 10 m g/kg), the 5-HT2 antagonist ketanserin (0.5 mg/kg and 5 mg/kg) and the 5-HT3 antagonist ICS 205-930 (0.1 mg/kg and 1 mg/kg) were administere d intravenously prior to noxious stimulation. Pretreatment with any of the three compounds reduced Fos-like immunoreactivity. The effect of morphine was reversible with naloxone. The reduction of the expression of Fos-like immunoreactivity by exogenous morphine speaks in favour o f an opioidergic link in the modulation of orofacial pain in the trige minal nuclei. The effects of the 5-HT receptor antagonists are most li kely mediated via 5-HT2 and 5-HT3 receptors located on primary afferen t fibres.