Journal of neurophysiology
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Comparative Study
Sympathetic modulation of activity in Adelta- and C-primary nociceptive afferents after intradermal injection of capsaicin in rats.
Neuropathic and inflammatory pain can be modulated by the sympathetic nervous system. In some pain models, sympathetic postganglionic efferents are involved in the modulation of nociceptive transmission in the periphery. The purpose of this study is to examine the sensitization of Adelta- and C-primary afferent nociceptors induced by intradermal injection of capsaicin (CAP) to see whether the presence of sympathetic efferents is essential for the sensitization. ⋯ In sympathetically intact rats, pretreatment with an alpha(1)-adrenergic receptor antagonist (terazosin) blocked completely the enhanced responses of C-fibers after CAP injection in sympathetically intact rats without significantly affecting the enhanced responses of Adelta-fibers. In contrast, a blockade of alpha(2)-adrenergic receptors by yohimbine only slightly reduced the CAP-evoked enhancement of responses. We conclude that the presence of sympathetic efferents is essential for the CAP-induced sensitization of Adelta- and C-primary afferent fibers to mechanical stimuli and that alpha(1)-adrenergic receptors play a major role in the sympathetic modulation of C-nociceptor sensitivity in the periphery.
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Comparative Study
Cortical injury affects short-term plasticity of evoked excitatory synaptic currents.
The hypothesis that plastic changes in the efficacy of excitatory neurotransmission occur in areas of chronic cortical injury was tested by assessing short-term plasticity of evoked excitatory synaptic currents (EPSCs) in neurons of partially isolated neocortical islands (undercut cortex). Whole cell recordings were obtained from layer V pyramidal neurons of sensorimotor cortical slices prepared from P36-P43 control and undercut rats. AMPA/kainate receptor-mediated EPSCs elicited by stimuli delivered at 40 to 66.7 Hz exhibited more paired-pulse depression (PPD) in undercut cortex than control, the time constant of depression evoked by trains of 20- to 66.7-Hz stimuli was faster, and the steady-state amplitude of EPSCs reached after five to seven EPSCs was lower. ⋯ Increasing [Ca(2+)](o) from 2 to 4 mM increased PPD, with a smaller effect in neurons of the undercut. The I-V relationship of AMPA/kainate receptor-mediated EPSCs was close to linear in both control and undercut neurons, and spermine had no significant effect on the EPSCs, suggesting that decreases in postsynaptic glutamate receptors containing the GluR2 subunit were not involved in the alterations in short-term plasticity. Results are compatible with an increase in the probability of transmitter release at excitatory synapses in undercut cortex due to functional changes in presynaptic terminals.