Neuroscience
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The effect of capsaicin on voltage-activated calcium currents was investigated in voltage-clamped somata of cultured adult rat dorsal root ganglion neurons. About half the neurons studied were sensitive to capsaicin, which induced an inward current at negative membrane potentials accompanied by an increase in membrane conductance. In the sensitive neurons capsaicin inhibited voltage-activated calcium current to an extent that depended on the size and duration of the capsaicin-induced inward current. ⋯ Substituting Ca with Co did not prevent the prolonged block of calcium channels. It is concluded that the inhibition of voltage-activated calcium currents by capsaicin is secondary to increased intracellular Ca levels due to calcium entry through capsaicin-activated cation-specific ion channels in the plasma membrane. Long-lasting inhibition of voltage-activated calcium channels may contribute to the mechanism of the analgesic and anti-inflammatory effects of capsaicin through inhibition of neurotransmitter release from central and peripheral terminals of primary afferent nociceptive neurons.
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The topographical organization of amygdaloid projections to the caudatoputamen, nucleus accumbens, and lateral portions of the bed nucleus of the stria terminalis and central amygdaloid nucleus was investigated, in the rat, using the retrograde transport of wheat germ agglutinin-conjugated horseradish peroxidase. Although the caudatoputamen and nucleus accumbens are the principal components of the striatum, there is evidence that lateral portions of the bed nucleus of the stria terminalis and central amygdaloid nucleus may be striatal-like structures. The basolateral nucleus was the main source of amygdaloid fibers to all of these structures. ⋯ The principal striatal projection of the caudal basolateral nucleus was to the medial nucleus accumbens. Amygdaloid labeling produced by injections into the medial nucleus accumbens was very similar to that seen with injections into the lateral portions of the bed nucleus of the stria terminalis and central amygdaloid nucleus. The retrograde amygdaloid labeling seen in this investigation, when compared to labeling seen with cortical injections in previous studies, suggests that specific amygdaloid domains project to particular cortical areas as well as to the principal striatal targets of the same areas.
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Following a set of studies concerning the intrinsic electrophysiology of mammalian central neurons in relation to global brain function, we reach the following conclusions: (i) the main difference between wakefulness and paradoxical sleep lies in the weight given to sensory afferents in cognitive images; (ii) otherwise, wakefulness and paradoxical sleep are fundamentally equivalent brain states probably subserved by an intrinsic thalamo-cortical loop. From this assumption, we conclude that wakefulness is an intrinsic functional realm, modulated by sensory parameters. In support of this hypothesis, we review morphological studies of the thalamocortical system, which indicate that only a minor part of its connectivity is devoted to the transfer of direct sensory input. ⋯ These considerations lead us to challenge the traditional Jamesian view of brain function according to which consciousness is generated as an exclusive by-product of sensory input. Instead, we argue that consciousness is fundamentally a closed-loop property, in which the ability of cells to be intrinsically active plays a central role. We further discuss the importance of spatial and temporal mapping in the elaboration of cognitive and perceptual constructs.
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Recordings were made from neurons in subnucleus reticularis dorsalis of the rat. Two populations of neurons could be distinguished: those with total nociceptive convergence which were driven by activating A delta- and C-fibers from any part of the body and those with partial nociceptive convergence which were driven by activating A delta-fibers from any part of the body or C-fibers from some, mainly contralateral, regions. The effects on subnucleus reticularis dorsalis neurons of manual acupuncture, performed by a traditional Chinese acupuncturist at the "Renzhong", "Sousanli", "Changqiang", and "Zusanli" acupoints and at a non-acupoint next to "Zusanli", were studied. ⋯ No differences were found between the capacities to activate subnucleus reticularis dorsalis neurons of the "Zusanli" point and the adjacent non-acupoint, no matter whether these were stimulated ipsi- or contralaterally; this suggests a lack of topographical specificity in the activation of these neurons. Since subnucleus reticularis dorsalis neurons are activated exclusively or preferentially by noxious inputs, it is concluded that the signals elicited by manual acupuncture travel through pathways responsible for the transmission of nociceptive information. Since acupuncture, a manoeuvre which is known to elicit widespread extrasegmental antinociceptive effects, activates subnucleus reticularis dorsalis neurons which, anatomically, send dense projections to the dorsal horn at all levels of the spinal cord, we would suggest that this structure may be involved not only in signalling pain but also in modulating pain by means of spino-reticulo-spinal feed-back mechanisms.
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Comparative Study
Aspartate-like immunoreactivity in primary afferent neurons.
There is now good evidence that amino acids act as neurotransmitters in primary afferent neurons of dorsal root ganglia. Glutamate is the primary candidate for such a role, and there are reasons to believe that release of glutamate may be accompanied by the release of other neuroactive substances. Using immunocytochemical techniques, we have tested the hypothesis that some dorsal root ganglion neurons contain elevated levels of aspartate as well as glutamate. ⋯ The presence of high levels of aspartate in terminals located in the superficial laminae of the dorsal horn was verified by pre- and post-embedding immunocytochemistry with the electron microscope. Aspartate was demonstrated in scalloped terminals, including dark scalloped terminals believed to be associated with unmyelinated fibers of nociceptors. This evidence supports the hypothesis that aspartate as well as glutamate is present in the cell bodies and terminals of nociceptive primary afferents, and may be released by the terminals of these afferents to activate neurons in the superficial laminae of the dorsal horn.