THE FUNCTION OF NORADRENERGIC NEURONS IN MEDIATING ANTINOCICEPTION INDUCED BY ELECTRICAL-STIMULATION OF THE LOCUS-CERULEUS IN 2 DIFFERENT SOURCES OF SPRAGUE-DAWLEY RATS

Although noradrenergic neurons in the nucleus locus coeruleus are known to project to the spinal cord, these neurons appear to innervate different regions of the spinal cord in Sprague-Dawley rats obtained from two different vendors. Recent anatomical studies demonstrated that the noradrenergic neurons in the locus coeruleus in Sasco Sprague-Dawley rats primarily innervate the ventral horn, whereas Harlan Sprague-Dawley rats have coeruleospinal projections that terminate in the dorsal horn of the spinal cord. This report describes the results of behavioral experiments that were designed to determine the functional significance of these anatomical differences. Electrical stimulation of neurons in the locus coeruleus produced antinociception in both Harlan and Sasco rats. The antinociception in Harlan rats was readily reversed by intrathecal injection of yohimbine, a selective alpha2-adrenoceptor antagonist, or by phentolamine, a non-selective alpha2-adrenoceptor antagonist. In contrast, these antagonists did not alter the antinociception produced by locus coeruleus stimulation in Sasco rats. Finally, the alpha2-antagonist, idazoxan, did not alter the antinociceptive effect of locus coeruleus stimulation in either group of rats. These observations indicate that coeruleospinal noradrenergic neurons in Harlan and Sasco Sprague-Dawley rats have different physiological functions. Thus, electrical stimulation of noradrenergic neurons in the locus coeruleus that innervate the spinal cord dorsal horn (Harlan rats) produces antinociception, but stimulation of coeruleospinal noradrenergic neurons that project to the ventral horn (Sasco rats) does not produce antinociception. It is likely that genetic differences between these outbred stocks of rats account for the fundamental differences in the projections of coeruleospinal neurons and their function in controlling nociception.