AN ASCENDING SEROTONERGIC PAIN MODULATION SYSTEM

This dissertation describes an ascending serotonergic pain modulation system projecting from the dorsal raphe (DR) nucleus of the midbrain to the parafascicularis (PF) nucleus of the thalamus. Previous studies by other investigators have led to the hypothesis that the DR would modulate responses to noxious stimuli in the PF by using 5HT. These other studies have shown that the DR contains serotonergic (5HT) cell bodies which project to many areas of the forebrain including the PF, that the PF is involved in pain perception, that electrical stimulation of the DR causes analgesia, and 5HT is necessary for this type of analgesia. One theory of the mechanisms of an endogenous pain modulation system is that brainstem nuclei have a decsending projection to the spinal cord to inhibit responses to noxious input at this level. The present study tests the hypothesis that there is also an ascending pain modulation pathway from the brainstem to the thalamus.^ To test this hypothesis, several types of experiments were performed on anesthetised rats. The major results of the experiments are as follows: (1) Three types of spontaneously active PF neurons were found: slow units firing at 1-10 spikes/sec, bursting units firing 2-5 times in 10-20 msec, pattern repeating every 1-2 sec, and fast units firing at 15-40 spikes/sec. The first two groups showed similar results to the treatments and were analysed together. The fast firing units did not respond to any of the treatments. (2) Noxious stimuli primarily increased neuronal firing rates in the PF, where as DR stimulation primarily decreased neuronal activity. DR stimulation applied simultaneously with noxious stimuli decreased the responses to the noxious stimuli as recorded in the PF units. (3) Microiontophoretically applied 5HT in the PF decreased spontaneous activity in the PF in a dose dependent manner and decreases responses to noxious stimuli in the PF. (4) Reduction of brain 5HT by 5,7 dihydroxytryptamine, a potent 5HT neurotoxin, caused PF units to be hypersensitive to both noxious and non noxious stimuli, reversed the effects of DR stimulation so that DR stimulation increased single units activity in the PF, and prolonged and intensified the depressant action of microiontophoretically applied 5HT. The results of this study are consistent with the hypothesis that the DR uses 5HT in a direct ascending pathway to the PF to modulate pain in the thalamus. ^

Quantitation of contacts among sensory, motor, and serotonergic neurons in the pedal ganglion of aplysia.

Present models of long-term sensitization in Aplysia californica indicate that the enhanced behavioral response is due, at least in part, to outgrowth of sensory neurons mediating defensive withdrawal reflexes. Presumably, this outgrowth strengthens pre-existing connections by formation of new synapses with follower neurons. However, the relationship between the number of sensorimotor contacts and the physiological strength of the connection has never been examined in intact ganglia. As a first step in addressing this issue, we used confocal microscopy to examine sites of contact between sensory and motor neurons in naive animals. Our results revealed relatively few contacts between physiologically connected cells. In addition, the number of contact sites was proportional to the amplitude of the EPSP elicited in the follower motor neuron by direct stimulation of the sensory neuron. This is the first time such a correlation has been observed in the central nervous system. Serotonin is the neurotransmitter most closely examined for its role in modulating synaptic strength at the sensorimotor synapse. However, the structural relationship of serotonergic processes and sensorimotor synapses has never been examined. Surprisingly, serotonergic processes usually made contact with sensory and motor neurons at sites located relatively distant from the sensorimotor synapse. This result implies that heterosynaptic regulation is due to nondirected release of serotonin into the neuropil.

ANALYSIS OF SEROTONERGIC MODULATION OF NEURONS INVOLVED IN TWO DEFENSIVE BEHAVIORS IN APLYSIA CALIFORNICA (CYCLIC-AMP, AROUSAL, CALCIUM, POTASSIUM)

Sensitization is a simple form of learning which refers to an enhancement of a behavioral response resulting from an exposure to a novel stimulus. While sensitization is found throughout the animal world, little is known regarding the underlying neural mechanisms. By taking advantage of the simple nervous system of the marine mollusc Aplysia, I have begun to examine the cellular and molecular mechanisms underlying this simple form of learning. In an attempt to determine the generality of the mechanisms of neuromodulation underlying sensitization, I have investigated and compared the modulation of neurons involved in two defensive behaviors in Aplysia, the defensive inking response and defensive tail withdrawal.^ The motor neurons that produce the defensive release of ink receive a slow decreased conductance excitatory postsynaptic potential (EPSP) in response to sensitizing stimuli. Using electrophysiological techniques, it was found that serotonin (5-HT) mimicked the physiologically produced slow EPSP. 5-HT produced its response through a reduction in a voltage-independent conductance to K('+). The 5-HT sensitive K('+) conductance of the ink motor neurons was separate from the fast K('+), delayed K('+), and Ca('2+)-activated K('+) conductances found in these and other molluscan neurons. 5-HT was shown to produce a decrease in K('+) conductance in the ink motor neurons through an elevation of cellular cAMP.^ The mechanosensory neurons that participate in the defensive tail withdrawal response are also modulated by sensitizing stimuli through the action of 5-HT. Using electrophysiological techniques, it was found that 5-HT modulated the tail sensory neurons through a reduction in a voltage-dependent conductance to K('+). The serotonin-sensitive K('+) conductance was found to be largely a Ca('2+)-activated K('+) conductance. Much like the ink motor neurons, 5-HT produced its modulation through an elevation of cellular cAMP. While the actual K('+) conductance modulated by 5-HT in these two classes of neurons differs, the following generalizations can be made: (1) the effects of sensitizing stimuli are mimicked by 5-HT, (2) 5-HT produces its effect through an elevation of cellular cAMP, and (3) the conductance to K('+) is modulated by 5-HT. ^