Striatal Inhibition of Nociceptive Responses Evoked in Trigeminal Sensory Neurons by Tooth Pulp Stimulation

Abstract

The noxious evoked response in trigeminal sensory neurons was studied to address the role of striatum in the control of nociceptive inputs. In urethane-anesthetized rats, the jaw opening reflex (JOR) was produced by suprathreshold stimulation of the tooth pulp and measured as electromyographic response in the digastric muscle, with simultaneous recording of noxious responses in single unit neurons of the spinal trigeminal nucleus pars caudalis (Sp5c). The microinjection of glutamate (80 mol/0.5 l) into striatal JOR inhibitory sites significantly decreased the A␦ and C fiber-mediatedevoked response (53 Ϯ 4.2 and 43.6 Ϯ 6.4% of control value, P Ͻ 0.0001) in 92% (31/34) of nociceptive Sp5c neurons. The microinjection of the solvent was ineffective, as was microinjection of glutamate in sites out of the JOR inhibitory ones. In another series of experiments, simultaneous single unit recordings were performed in the motor trigeminal nucleus (Mo5) and the Sp5c nucleus. Microinjection of glutamate decreased the noxious-evoked response in Sp5c and Mo5 neurons in parallel with the JOR, without modifying spontaneous neuronal activity of trigeminal motoneurons (n ϭ 8 pairs). These results indicate that the striatum could be involved in the modulation of nociceptive inputs and confirm the role of the basal ganglia in the processing of nociceptive information.

The Striatum and Pain Modulation

“…For these reasons the JOR evoked by high intensities tooth pulp stimulation was considered to be a nociceptive response and a good measurement of analgesic effects. In addition, the latency of the JOR 7.9 ± 0.25 ms (range 6-10 ms, n = 60) (Saunier-Rébori and Pazo 2006), is compatible with conduction velocity of tooth pulp Ad nociceptors (range 10-20 m/s) and it is in agreement with previous articles from our and other laboratories (Belforte and Pazo 2005;Gear et al 1999).…”

“…6a). Belforte and Pazo (2005) Similar results were observed in the responses of the C fibers input (Fig. 6a).…”

“…The lesion of this pathway in animals and in Belforte and Pazo (2005) Parkinson patients leads to a reduction of movements as well as other motor deficits. However, different evidences have also involved striatal dopamine in pain control.…”

“…Horizontal bar indicates duration of intrastriatal microinjection. Modified from Belforte and Pazo (2005) Cell Mol Neurobiol (2012) 32:1-12 5 JOR and on the evoked responses of the neurons of the trigeminal NC to tooth pulp stimulation. The microinjection of unspecific agonist of the dopamine receptors apomorphine (APO) (0.5 ll) into the striatum, at sites that produced inhibition of the JOR (effective sites; Fig.…”

Nociceptive behavioral responses to chemical, thermal and mechanical stimulation after unilateral, intrastriatal administration of 6-hydroxydopamine

“…The majority of cutaneous nociceptive CPu and GP neurons have large trigeminal receptive fields, sometimes incorporating the ipsilateral side of the face or much of the entire body (Chudler et al, 1993;Chudler, 1998). Electrical stimulation of the CPu can inhibit nociceptive neurons in the spinal trigeminal nucleus (Belforte and Pazo, 2005) and reduce the jaw opening reflex in rats (Belforte et al, 2001). However, the effects of dopamine depletion on tonic persistent trigeminal pain have not been performed.…”