Locus coeruleus neurons and sympathetic nerves: activation by cutaneous sensory afferents

The effects of mechanical and thermal cutaneous sensory stimulation on the activity of central norepinephrine (NE) neurons in the locus coeruleus (LC) and on peripheral sympathetic nerve activity (NE-SNA) in a renal branch of the splanchnic nerve were studied, using electrophysiological techniques in the anesthetized rat. Noxious, mechanical sensory stimulation caused a parallel and virtually identical change in central NE-LC activity and peripheral NE-SNA and both neuronal systems showed a biphasic excitation-inhibition response. Both non-noxious and noxious thermal sensory stimulation above 36 degrees C caused a parallel increase in NE-LC firing rate and NE-SNA, changes that were accompanied by increases in arterial blood pressure and heart rate. The increases in central and peripheral NE neuronal activity were linearly correlated. However, the two neuronal systems differed in their ability to adapt during prolonged noxious thermal stimulation. NE-LC neurons adapted completely and returned to baseline firing rate within 5 min during ongoing noxious thermal stimulation. In contrast, an increase in NE-SNA remained throughout the stimulation period. In recent studies in this laboratory, blood volume depletion was found associated with a marked NE-LC activation. In contrast to the present results during prolonged noxious stimulation, there was no attenuation of the NE-LC activation during prolonged hemorrhage. Taken in conjunction, these data indicate a discriminatory capacity of the NE-LC system in monitoring sensory or autonomic stimuli. Thus, only those stimuli of imperative importance for the animal were found to be associated with continuing robust NE-LC discharge.