SENSORY RESPONSIVENESS OF BROAD-SPIKE NEURONS IN THE LATERODORSAL TEGMENTAL NUCLEUS, LOCUS-COERULEUS AND DORSAL RAPHE OF AWAKE RATS - IMPLICATIONS FOR CHOLINERGIC AND MONOAMINERGIC NEURON-SPECIFIC RESPONSES

Although cholinergic neurons in the laterodorsal and pedunculopontine tegmental nuclei have been shown to have a pivotal role in neural mech anisms of paradoxical sleep, their function during wakefulness is less understood. To examine the latter, we have recorded from ''broad-spik e neurons'', which were distinguished by their long spike duration, in the laterodorsal tegmental nucleus of undrugged, head-restrained rats , and examined their response properties to sensory stimuli such as li ght touch to the tail, air puff to the face, 2 kHz pure tone and flash es of light. Broad-spike neurons from the locus coeruleus and dorsal r aphe nucleus were studied for comparison; these neurons have been demo nstrated to be noradenergic and serotonergic, respectively. The broad- spike neurons in the laterodorsal tegmental nucleus have also been sug gested to be cholinergic. There were two kinds of responses: (1) a sim ple increase or decrease in firing, reflecting an elevated level of vi gilance; and (2) a phasic response composed of a single spike or brief , high frequency burst, usually diminishing or disappearing upon repet ition of the stimulus. When two or more types of stimuli were effectiv e in a neuron, they evoked responses of the same quality. Most of the dorsal raphe neurons displayed only the simple increase of firing, whe reas the locus coeruleus neurons gave a phasic response with rather we ak attenuation upon repetition. Compared with these, the laterodorsal tegmental neurons were heterogeneous: about one-quarter showing only a simple change of firing (half increasing, half decreasing); and two-t hirds displaying phasic responses. The latter response of many neurons attenuated strongly upon repetition. The laterodorsal tegmental neuro ns were classified into several groups according to their spontaneous firing behavior during sleep and wakefulness, but every neuron in a gr oup did not show the same type of response. For example, some of the n eurons which were most active during paradoxical sleep and essentially silent during wakefulness decreased or stopped firing upon sensory st imulation, while others in this group had strong phasic responses. The se results suggest that putative cholinergic neurons in the laterodors al tegmental nucleus have heterogenous properties not only with respec t to their spontaneous activity during sleep and wakefulness but also with respect to their response to sensory stimulation. Some of these n eurons may function to induce a global attentive state in response to a novel stimulus,