Neuroscience
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A long line of studies emphasizes the contribution of serotonergic fibres descending from the rostral ventromedial medulla in the control of spinal nociceptive information processing. A growing body of evidence, however, suggests that the relative contribution of serotonin to the mediation of spinal neuronal activity from the rostral ventromedial medulla may require re-evaluation. It has recently been substantiated that, in addition to the serotonergic fibres, the spinal dorsal horn receives an abundant non-serotonergic projection from the rostral ventromedial medulla. ⋯ We show that the majority of the labelled raphe-spinal terminals in laminae I-IIo and IV-V contain GABA and some of the GABA-immunoreactive terminals are also immunoreactive for glycine. We also disclose that GABA-immunoreactive raphe-spinal terminals establish synaptic contacts primarily with GABA- and glycine-negative, presumably excitatory, spinal neurons, including Calbindin-D28k- as well as parvalbumin-immunoreactive cells in both laminae I-IIo and IV-V. The results suggest that volleys in fibres descending from the rostral ventromedial medulla may evoke GABA release from raphe-spinal terminals, and the released GABA, in some cases probably acting together with glycine, might play a crucial, as yet mostly unidentified, role in the inhibition of nociceptive information processing in the dorsal horn of the spinal cord.
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The regional distribution of the serotonin uptake system was studied in rat brain using a specific polyclonal antibody raised against the putative extracellular loop between transmembrane domains 7 and 8 of the cloned rat serotonin transporter. Light microscope analysis with fluorescence and avidin-biotin-peroxidase techniques revealed a punctate staining as well as numerous labelled thin fibres, which exhibited accumulation of reaction end-product deposit over varicosities. These immunopositive processes were widely and heterogeneously distributed in the rat brain. ⋯ In addition, some immunoreactive fibres were present in the molecular and granular layers of the cerebellum as well as in the cochlear and olivary nuclei. In none of the regions analysed was evidence for glial staining obtained. The present immunocytochemical data reveal a widespread and heterogeneous distribution of the serotonin transporter in rat brain and suggest that serotoni transporter is preferentially sorted into axons, where it appears concentrated at varicosities and terminal boutons.
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Regions of the prefrontal cortex that project to the nucleus accumbens in the rat receive input from midline thalamic and basal amygdaloid nuclei which also project to the same striatal region as their prefrontal cortical target. For example, the prelimbic cortex projects to the medial nucleus accumbens, and receives input from the paraventricular thalamic nucleus and the parvicellular basal amygdala. These latter two areas also project to the medial nucleus accumbens. ⋯ In the matrix of the core and the ventral part of the caudate-putamen, fibers from the superficial layers of the dorsal agranular insular area overlap precisely with afferents from the intermediodorsal nucleus. In the patches, projections from the deep layers of the dorsal agranular insular cortex coincide with those from the magnocellular basal amygdala. The present findings have implications for the compartmental structure of the nucleus accumbens and provide novel insights into the organizational principles of prefrontal corticostriatal circuits.
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The ventrobasal thalamus is the principal somatosensory thalamic relay nucleus, and it receives two major sources of excitatory input: firstly an input from ascending sensory afferents, and secondly a descending projection from the primary somatosensory cortex. There is considerable anatomical evidence to suggest that both of these projections utilise the excitatory amino acid L-glutamate as their neurotransmitter. Previous work from this laboratory has shown that the sensory input to the rat ventrobasal thalamus in vivo is mediated by ionotropic excitatory amino acid receptors of both the N-methyl-D-aspartate and non-N-methyl-D-aspartate type. ⋯ These data indicate that both N-methyl-D-aspartate receptors and Group I (possibly metabotropic glutamate receptors type I) metabotropic receptors are involved in the mediation of corticothalamic transmission. Such a transmitter mechanism would allow a modulatory system that could selectively enhance other excitatory inputs. Some of these data have been reported in abstract form.
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The differentiation of neurons involves the establishment of distinct molecular compartments which regulate neuronal shape and function. This requires targeting of specific gene products to growth-associated regions of the neuron. We have investigated the temporal and spatial regulation of SCG10 gene expression during neuronal differentiation. ⋯ In neurons which had developed polarity, the 2 kb SCG10 messenger RNA was consistently found in the cell body and axon. This study demonstrates both temporal and spatial post-transcriptional regulation of SCG10 expression which is associated with neurite outgrowth. The directed transport and positional translation of SCG10 messenger RNA provide a potential mechanism for protein targeting and the creation of molecular compartments during neuronal differentiation.