In vivo catechol activity in the rat locus coeruleus following different nociceptive stimuli and naloxone

The nucleus locus coeruleus (LC) has been implicated in the processing of spinal reflexes following noxious stimuli. It has been demonstrated that noxious stimuli activate LC neuronal firing, but little is known about the neurochemical changes that might occur following such activation. To determine the effects of different noxious stimuli on LC neuronal activity, anaesthetized rats were exposed to mechanical (tail pinch), thermal (55 °C water), and chemical (5% Formalin injected in the hind paw) stimuli; the catechol oxidation current (CA·OC), an index of noradrenergic neuronal activity, in the locus coeruleus was monitored using differential normal pulse voltammetry. In addition, the effect of the opioid antagonist naloxone on the CA·OC in the LC was examined. Exposure to both mechanical and chemical stimuli significantly increased CA·OC indicating an increase in LC noradrenergic neuronal activity, while the thermal stimulus had no effect. Treatment with naloxone (1 mg/kg i.v.) had no effect on CA·OC in the LC. The results show a differential responsiveness of LC noradrenergic neurons to different modes of noxious stimuli and fail to demonstrate a tonic opioid regulation of these neurons in the anaesthetized rat.