Masseteric facilitation induced by electrical stimulation of rat orofacial tissues

Descriptive and functional neuroanatomy of locus coeruleus-noradrenaline-containing neurons involvement in bradykinin-induced antinociception on principal sensory trigeminal nucleus

The present study was carried out in Wistar rats, using the jaw-opening reflex and dental pulp stimulation, to investigate noradrenaline- and serotonin-mediated antinociceptive circuits. The effects of microinjections of bradykinin into the principal sensory trigeminal nucleus (PSTN) before and after neurochemical lesions of the locus coeruleus noradrenergic neurons were studied. Neuroanatomical experiments showed evidence for reciprocal neuronal pathways connecting the locus coeruleus (LC) to trigeminal sensory nuclei and linking monoaminergic nuclei of the pain inhibitory system to spinal trigeminal nucleus (STN). Fast blue (FB) injections in the locus coeruleus/subcoeruleus region retrogradely labeled neurons in the contralateral PSTN and LC. Microinjections of FB into the STN showed neurons labeled in both ipsilateral and contralateral LC, as well as in the ipsilateral Barrington's nucleus and subcoeruleus area. Retrograde tract-tracing with FB also showed that the mesencephalic trigeminal nucleus sends neural pathways towards the ipsilateral PSTN, with outputs from cranial and caudal aspects of the brainstem. In addition, neurons from the lateral and dorsolateral columns of periaqueductal gray matter also send outputs to the ipsilateral PSTN. Microinjections of FB in the interpolar and caudal divisions of the STN labeled neurons in the caudal subdivision of STN. Microinjections in the STN interpolar and caudal divisions also retrogradely labeled serotonin- and noradrenaline-containing nucleus of the brainstem pain inhibitory system. Finally, the gigantocellularis complex (nucleus reticularis gigantocellularis/paragigantocellularis), nucleus raphe magnus and nucleus raphe pallidus also projected to the caudal divisions of the STN. Microinjections of bradykinin in the PSTN caused a statistically significant long-lasting antinociception, antagonized by the damage of locus coeruleus-noradrenergic neuronal fibres with (N-(2-chloroethyl)-N-ethyl-2-bromobenzylamine) (DSP4), a neurotoxin that specifically depleted noradrenaline from locus coeruleus terminal fields. These data suggest that serotonin- and noradrenaline-containing nuclei of the endogenous pain inhibitory system exert a key-role in the antinociceptive mechanisms of bradykinin and the locus coeruleus is crucially involved in this effect.

The masseteric jaw-jerk reflex in older dentate subjects and edentulous denture wearers

The aim of the present study was to investigate any variations in the jaw-jerk reflex in edentulous subjects wearing complete dentures, compared to an age and sex-matched dentate group. The reflex was elicited by chin taps in 22 older dentate subjects with mean age 61.3 years and in 22 denture wearers with mean age 63.1 years. Surface electromyographic recordings were obtained from the masseter muscle of the preferred chewing side during mandibular rest and at moderate clenching (40% of the individual maximum clenching masseteric EMG activity). A jaw-jerk reflex was recorded in all subjects at least once, and its occurrence during clenching was reduced compared to rest. The occurrence of the reflex was however increased in the denture wearers in both experimental conditions, while minor differences were observed in the values for latency, duration and amplitude between the two dental status groups. These results suggest that under the present experimental conditions the periodontal ligament receptors might inhibit reflex activity. Multiple sensory interactions are expected in denture wearing. However a particular source of sensory feedback is provided by the stimulation of mucosal receptors from the acrylic denture base. Since the occurrence of the jaw-jerk at clench in the denture wearers was also reduced compared to the rest experiments, a potential inhibitory effect of the mucosal receptors can be speculated. According to the findings in the present study the loss of teeth and the rehabilitation with complete dentures do not severely disrupt the reflex activity investigated.