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 know n to project to the spinal cord, these neurons appear to innervate dif ferent regions of the spinal cord in Sprague-Dawley rats obtained from two different vendors. Recent anatomical studies demonstrated that th e noradrenergic neurons in the locus coeruleus in Sasco Sprague-Dawley rats primarily innervate the ventral horn, whereas Harlan Sprague-Daw ley rats have coeruleospinal projections that terminate in the dorsal horn of the spinal cord. This report describes the results of behavior al experiments that were designed to determine the functional signific ance of these anatomical differences. Electrical stimulation of neuron s in the locus coeruleus produced antinociception in both Harlan and S asco rats. The antinociception in Harlan rats was readily reversed by intrathecal injection of yohimbine, a selective alpha2-adrenoceptor an tagonist, or by phentolamine, a non-selective alpha2-adrenoceptor anta gonist. In contrast, these antagonists did not alter the antinocicepti on 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 observa tions indicate that coeruleospinal noradrenergic neurons in Harlan and Sasco Sprague-Dawley rats have different physiological functions. Thu s, electrical stimulation of noradrenergic neurons in the locus coerul eus that innervate the spinal cord dorsal horn (Harlan rats) produces antinociception, but stimulation of coeruleospinal noradrenergic neuro ns that project to the ventral horn (Sasco rats) does not produce anti nociception. It is likely that genetic differences between these outbr ed stocks of rats account for the fundamental differences in the proje ctions of coeruleospinal neurons and their function in controlling noc iception.