Neuroscience
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A major ascending nociceptive pathway from spinal lamina I to the mesencephalon has previously been reported in the cat, rat and monkey. In the present paper, we have used single and double retrograde labeling techniques to describe this projection system and its collateralization to the thalamus in the rat. Injections of wheat germ agglutinin-horseradish peroxidase into the pontomesencephalic parabrachial area labeled cell bodies bilaterally in lamina I and deeper laminae of the spinal cord. ⋯ The significance of these findings rest on previous work in this and other laboratories and concerns the understanding of spinal nociceptive mechanisms. Lamina I projection neurons are primarily nociceptive-specific in their response properties and have been shown to project to both the midbrain and thalamus via the dorsolateral funiculus in a number of species. The role of this projection system in nociceptive transmission may lie in its ability to distribute precise information to multiple brain stem sites which in turn activate autonomic or affective responses or descending pain modulatory mechanisms.
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N-(2-chloroethyl)-N-ethyl-2-bromobenzylamine (DSP-4) is a potent and highly selective neurotoxin which induces degeneration of noradrenergic axons. The effects of DSP-4 vary considerably in different brain regions: the drug produces nearly complete depletion of noradrenaline in neocortex, hippocampus, cerebellum and spinal cord, but only partial depletion in hypothalamus and brainstem. In this study we have employed an immunohistochemical method to assess the neurotoxic effects of DSP-4 on the structural integrity of central noradrenergic neurons in the rat, and to identify those noradrenergic axons that remain in the central nervous system 2-4 weeks after DSP-4 treatment. ⋯ This study provides the first direct evidence that DSP-4 destroys noradrenergic axon terminals from the locus coeruleus, but not those from non-locus coeruleus neurons. This profound differential sensitivity of noradrenergic axons to DSP-4 is matched by distinct differences in their morphology and their topographic projections. The results support the view that locus coeruleus and non-locus coeruleus noradrenergic neurons constitute two separate subsystems, which differ not only in their projections but also with respect to the pharmacological properties of their axon terminals.
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The isolated Muller glial cell of the neotenous tiger salamander retina is used as an experimental model for studying the effects of non-uniform membrane conductance on the shape of charging curves in response to step current inputs. A simple cable model of the Muller cell is formulated and used to interpret the experimental data. The Muller cell model is completely described by three parameters: (a) electrotonic length L; (b) the membrane time constant tau m; and (c) the percentage of the total membrane conductance accounted for by the endfoot S. ⋯ The error that results from misinterpreting the first equalizing time constant tau l as the membrane time constant tau m can have a significant effect on estimates of specific membrane resistance and capacitance. The algorithm described in this paper provides a means for obtaining direct estimates of the membrane time constant and will make possible more accurate estimates of specific membrane resistance and capacitance in Muller glial cells. The fact that the estimation procedure is based on a simple electrophysiological measurement suggests that it may be useful for studying asymmetry of membrane conductance in glial and neural elements of the intact nervous system.
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Resiniferatoxin is an extremely irritant diterpene present in the latex of several members of the genus Euphorbia. Its mechanism of action has been shown to be clearly distinct from that of the structurally related phorbol esters. Since resiniferatoxin possesses a 4-hydroxy-3-methoxyphenyl substituent, a key feature of capsaicin, the major pungent ingredient of plants of the genus Capsicum, we examined the ability of resiniferatoxin to induce typical capsaicin responses. ⋯ Resiniferatoxin was 3 4 orders of magnitude more potent than capsaicin for the effects on thermoregulation and neurogenic inflammation. Resiniferatoxin was only comparable in potency to capsaicin, however, in the assay for induction of acute pain, and the desensitization to acute pain appeared to require less resiniferatoxin than did desensitization for the other responses. We conclude that resiniferatoxin acts as an ultrapotent capsaicin analog and hypothesize that it may distinguish between subclasses of capsaicin response.
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Review of the normally occurring neuronal patterns of the hippocampus suggests that the two principal cell types of the hippocampus, the pyramidal neurons and granule cells, are maximally active during different behaviors. Granule cells reach their highest discharge rates during theta-concurrent exploratory activities, while population synchrony of pyramidal cells is maximum during immobility, consummatory behaviors, and slow wave sleep associated with field sharp waves. Sharp waves reflect the summed postsynaptic depolarization of large numbers of pyramidal cells in the CA1 and subiculum as a consequence of synchronous discharge of bursting CA3 pyramidal neurons. ⋯ It is assumed that recurrent excitation during the population burst is strongest on those cells which initiated the population event. It is suggested that the strong excitatory drive brought about by the sharp wave-concurrent population bursts during consummatory behaviors, immobility, and slow wave sleep may be sufficient for the induction of long-term synaptic modification in the initiator neurons of the CA3 region and in their targets in CA1. In this two-stage model both exploratory (theta) and sharp wave states of the hippocampus are essential and any interference that might modify the structure of the population bursts (e.g. epileptic spikes) is detrimental to memory trace formation.