Functional properties of thalamic relay cells in VPM and PO activated by electrical stimulation of tooth pulp, and effect of morphine on these cells were investigated electrophysiologically in rats.
1) Unitary responses evoked by electrical stimulation of tooth pulp were recorded from VPM and PO cells.
2) Functional properties of VPM cells differed from those of PO cells in regard to receptive field, latency, spike number and threshold. Cells belonging to VPM generally responded to specific peripheral field, but not so with those belonging to PO. Latency distribution of the first spikes in VPM gathered at 5, 10, 15 msec and that in PO extended over 5 to 25 cosec. In bilaterally responsive cells, latencies after the stimulation of contralateral tooth pulp (CTP) were shorter than those after the stimulation of ipsilateral tooth pulp (ITP) both in VPM and PO. Number of the spikes evoked by the stimulation of pulp was usually single in VPM, but numerous in PO. Intensity of stimulation in VPM showing the maximal spike number was greater than that in VPM.
3) Forty percent of VPM cells and 80 percent of PO cells were depressed by morphine (10mg/kg). This depression, though antagonized by levallorphan (0.1mg/kg), was recognized as a reduction of spike number and a prolongation of latency.
4) Morphine (20mg/kg) did not depressed the spikes in VPM evoked by tactile stimulation, however those in PO evoked by pin pricking were depressed by 5_??_10mg/kg.
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