Tooth-pulp-evoked rostral spinal trigeminal nucleus neuron activity is inhibited by conditioning sciatic nerve stimulation in the rat: possible role of 5-HT3 receptor mediated GABAergic inhibition
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Abstract
The purpose of the present study was to determine whether modulation of the trigeminal
spinal nucleus oralis (TSNO) neurons related to tooth-pulp (TP)-evoked jaw-opening
reflex (JOR) after electrical stimulation of the sciatic nerve (SN) is mediated by
the descending serotonergic (5-HT(3)) inhibitory system activated by inhibitory GABAergic
interneurons. In 30 anesthetized rats, the activity of TSNO neurons (87.5%, 35/40)
and all digastric muscle electromyograms (dEMG, n=30) in response to TP stimulation
(at an intensity of 3.5 times the threshold for JOR) were inhibited by conditioning
stimulation of the SN (5.0 mA x 0.5 ms, 1 Hz, conditioning-test intervals; 50 ms).
The inhibitory effects were significantly attenuated after intravenous administration
of the 5-HT(3) receptor antagonist ICS 205-930 (n=6). Using multibarrel electrodes,
iontophoretic application of ICS 205-930 into the TSNO significantly reduced the SN
stimulation-induced inhibition of TP-evoked TSNO neuronal excitation (n=6), and in
the same neurons, iontophoretic application of the GABA(A) receptor antagonist bicuculline
into the TSNO greatly inhibited their effect. On the other hand, we found the expression
of 5-HT(3) receptor immunoreactive neurons in the TSNO. These results suggest that
SN stimulation may activate the descending serotonergic (5-HT(3)) inhibitory system
through activation of inhibitory GABAergic interneurons, which inhibit excitatory
responses of the TSNO neurons to TP stimulation.