Neuroscience
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We have examined the distribution of microglia in the normal adult mouse brain using immunocytochemical detection of the macrophage specific plasma membrane glycoprotein F4/80. We were interested to learn whether the distribution of microglia in the adult brain is related to regional variation in the magnitude of cell death during development and resulting monocyte recruitment, or whether the adult distribution is influenced by other local microenvironmental cues. We further investigated the possibility that microglia are sensitive to their microenvironment by studying their morphology in different brain regions. ⋯ They can be extremely elaborate and there is wide variation in the length and complexity of branching of the processes. There was no evidence of monocyte-like cells in the adult CNS. The systematic variation in microglial morphology provides further evidence that these cells are sensitive to their microenvironment.
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[3H]GABA quantitative autoradiography was used to examine the binding kinetics and regional distribution of GABAB receptors in rat brain. The regional distribution was compared to that of GABAA receptors. At 4 degrees C, [3H]GABA binding to GABAB receptors reached equilibrium within 45 min. ⋯ Areas high in GABAB binding included the medial and lateral geniculates, the superior colliculus and certain amygdaloid nuclei. Binding to white matter tracts and ventricles was negligible. The distribution of GABAB receptors was in agreement with previously postulated sites of action of baclofen.
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Effects of senna on the myenteric plexus of the colon were investigated in view of earlier reports that this anthraquinone cathartic depletes the plexus of its intrinsic neurons. Rats and mice were given purgative doses of sennosides in their drinking water for 4 and 5 months, respectively. Body growth was reduced, and the weight of the colon with its contents was increased relative to the weight of the whole body in the treated animals. ⋯ Treatment with senna was not associated with absence of neuronal somata or fibres stainable with any of the antisera in either species. Thus, there was no evidence of toxic destruction of any identifiable population of neurons that might have been too small to affect the total counts. We conclude that senna does not kill myenteric neurons in the colon of the rat or mouse.
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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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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.