Neuroscience
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The effects of elevated CO2 (i.e. hypercapnia) on neurons in the nucleus tractus solitarii were studied using extracellular (n = 82) and intracellular (n = 33) recording techniques in transverse brain slices prepared from rat. Synaptic connections from putative chemosensitive neurons in the ventrolateral medulla were removed by bisecting each transverse slice and discarding the ventral half. In addition, the response to hypercapnia in 20 neurons was studied during high magnesium-low calcium synaptic blockade. ⋯ These neurons were not driven synaptically by putative chemosensitive neurons of the ventrolateral medulla since this region was removed from the slice. Furthermore, because chemosensitivity persisted in most neurons tested during synaptic blockade, we conclude that some neurons in the nucleus tractus solitarii are inherently CO2-chemosensitive. Although the function of dorsal medullary chemosensitive neurons cannot be determined in vitro, their location and their inherent chemosensitivity suggest a role in cardiorespiratory central chemoreception.
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Intracellular recordings from the intermediolateral cell nucleus of the neonate rat thoracolumbar spinal cord slice preparation revealed a population of neurons which displayed three types of spontaneous rhythmic activity: burst firing, tonic beating and membrane oscillations. Most neurons displayed more than one of these types of activity. Neurons had mean resting potentials of -59 mV and input resistances ranging from 10 to 48 m omega. ⋯ Burst firing was abolished by cobalt and membrane hyperpolarization but not by barium, low calcium or tetraethylammonium chloride. The switch from tonic beating to burst firing may, in part, involve activation of a voltage- and calcium-dependent afterdepolarization potential. We conclude that a population of neurons in the lateral horn of the spinal cord are capable of rhythmic activity with underlying spontaneous pacemaker-like oscillations.
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Local treatment of rat peripheral nerves with capsaicin induces permanent impairment of afferent C-fiber functions and changes in the response properties of spinal dorsal horn neurons. In this study a new experimental approach, the "capsaicin gap" technique, has been utilized in an attempt to unravel pathomorphological alterations which commence in the domain of primary sensory neurons as a consequence of perineural treatment with capsaicin. The technique relies on the facts that peripheral nerves in the spinal dorsal horn are represented in a strict somatotopic manner, and on the observation that in the adult rat systemic injection of appropriate doses of capsaicin results in a selective degeneration of primary afferent fibers within Rexed's laminae I and II of the spinal cord. ⋯ It is concluded that the central terminals of capsaicin-sensitive sciatic afferents underwent transganglionic degeneration as a result of prior perineural treatment with capsaicin, and a subsequent systemic injection of this neurotoxin therefore failed to cause axon terminal degeneration in somatotopic areas served by the treated nerve. Comparative quantitative morphometric analysis of cell populations of dorsal root ganglia related to capsaicin- or vehicle-treated nerves disclosed (1) a marked reduction in the proportion of small-sized neurons, (2) a fall of about 80% in the percentage of neurons which undergo degeneration after the systemic injection of capsaicin, and (3) a marked decrease in the total number of neurons in ganglia ipsilateral to the capsaicin-treated nerves. Quantitative electron microscopic studies on saphenous nerves treated perineurally with capsaicin revealed a 32% reduction in the number of unmyelinated axons as compared with the controls, whereas the number of myelinated fibers was unchanged.(ABSTRACT TRUNCATED AT 400 WORDS)
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Synaptosomal-associated protein, 25 kD, (SNAP-25) is a novel protein containing a possible transition metal binding site and encoded by a neuronal-specific mRNA. We examined the distribution of SNAP-25 mRNA and protein in the hippocampal formation of the adult rat following kainic acid, colchicine, and entorhinal lesions. The results show that destruction of granule cells of the dentate gyrus and CA3 pyramidal cells did not diminish SNAP-25 immunoreactivity in the dendritic fields of these cells. ⋯ These results support the identification of SNAP-25 as a novel presynaptic protein. In addition, SNAP-25 immunoreactivity was increased in afferent fibers which project to areas adjacent to the deafferented region, and expression of SNAP-25 mRNA was increased in neurons deafferented by the lesion. Examination of SNAP-25 immunoreactivity and mRNA expression may provide a useful marker of major hippocampal pathways and of axonal plasticity in neurological disorders such as Alzheimer's disease and temporal lobe epilepsy.
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The study was designed to obtain information on the spinal processing of input from receptors in deep somatic tissues (muscle, tendon, joint). In anaesthetized rats, the impulse activity of single dorsal horn cells was recorded extracellularly. In a pilot series, the proportion of neurons responding to mechanical stimulation of deep tissues was determined: 46.7% had receptive fields in the skin only, 35.5% could only be driven from deep tissues (deep cells), and 17.7% possessed a convergent input from both skin and deep tissues (cutaneous-deep cells). ⋯ In these presumably nociceptive cells the descending inhibition had a differential action in that the input from deep tissues was more strongly affected than was the cutaneous input to the same neuron. The recording sites of the neurons with deep input were located in the superficial dorsal horn and in and around lamina V. The results suggest that in the rat a considerable proportion of dorsal horn cells receives input from deep nociceptors and that this input is controlled by descending pathways in a rather selective way.