Kenichi Yoshino

Representation of tooth pulp in the mesencephalic trigeminal nucleus and the trigeminal ganglion in the cat, as revealed by retrogradely transported horseradish peroxidase

Horseradish peroxidase (HRP) injection into the tooth pulp of the left lower canine in 4 adult cats resulted in labeling a maximal number of nerve cell bodies of 45 and 149 ipsilaterally in the mesencephalic trigeminal nucleus (MTN) and trigeminal ganglion (TG), respectively. In a cat subjected to HRP application to the same canine and subsequent 3 posterior teeth, the respective numbers of labeled neurons in MTN and TG were increased to 92 and 933, respectively.

Innervation of the tooth pulp by the mesencephalic trigeminal nucleus in the cat: a retrograde horseradish peroxidase study.

HRP was applied to the tooth pulp of 8 cats. Six were subjected to postoperative administration of the anti-inflammatory drug, prednisolone, whereas the remaining two were not. In all prednisolone-treated cats, labeled neurons were found in both the mesencephalic trigeminal nucleus and trigeminal ganglion, ipsilaterally. On the other hand, no labeled neurons were observed in the mesencephalic nucleus in cats receiving no steroid.

Control of oro-facio-lingual movements by the substantia nigra pars reticulata: High-frequency electrical microstimulation and GABA microinjection findings in rats

To provide direct evidence for substantia nigra pars reticulata (SNr) control of oro-facio-lingual muscle activity, high-frequency electrical microstimulation (mainly trains of 20, 333-Hz cathodal pulses at 40-60 microA) and GABA microinjection (1-5 microl of 10 mM GABA in saline) were carried out using a three-barreled microelectrode at the same SNr site in lightly anesthetized, chronically decorticated rats (n=39). Decortication eliminated the possibility that SNr microstimulation might activate corticofugal fibers descending in the adjoining cerebral peduncle. When the most ventral layer of the SNr was approached, high-amplitude electromyographic (EMG) activity of up to 6 mV with a distinctive waveform appeared synchronously with electrical stimuli in the anterior digastric, masseter, genioglossus, and levator labii superioris muscles. This EMG activity was evoked bilaterally, with an ipsilateral predominance. Eye movements, mostly rotation of the eyeball vertically down in the orbit, were noted. Infrequent blinking was also noted. Histologic examination localized the effector site to the middle third of the mediolateral extent of the caudal SNr corresponding to between 5.8 mm (level of the oculomotor nerve) and 6.5mm (caudal end of the SNr) caudal to bregma; and to the ventralmost peripeduncular region of the SNr corresponding to 7.7 mm to 8.0 mm beneath the cortical surface. We referred to this site as the substantia nigra pars reticulata oro-facio-lingual (SNr-ofl) region. GABA injection produced tonic EMG discharge with consistent amplitude in all of the four muscles studied. The GABA effect was negated by a preceding microinjection of the GABA-A receptor antagonist bicuculline, whereas saline control injection had no effect. Changes in amplitude of evoked EMG activity according to location of the stimulating microelectrode reflected somatotopic organization of the SNr-ofl region. This extremely localized electrical and receptor microstimulation in the SNr produced synchronized powerful contraction of jaw, tongue, and facial muscles with different neural innervation. These findings advance our understanding of the mechanisms of the SNr concerning oro-facio-lingual movements.

DOPAMINE RECEPTOR ANTAGONISTS INCREASE MARKEDLY THE QUANTITY OF RETROGRADE TRANSPORT OF HRP IN THE RAT MASSETERIC MOTONEURON

Horseradish peroxidase (HRP) was injected, bilaterally, into the rat masseter muscle, subsequent to an intramuscular or intraperitoneal injection of one of five dopamine antagonists (chlorpromazine and haloperidol as the D1 and D2 receptor antagonist, SCH 23390 as the specific D1 receptor antagonist, sulpiride and domperidone as the specific D2 receptor antagonist). Control rats received an injection of a corresponding vehicle solution. After a survival period of 16 h, the brainstem was cut into 60 microns cryosections and processed with the TMB technique. The amount of retrogradely transported HRP was quantitatively measured in terms of the amount of HRP reaction product present in the motoneuron by the method which we have developed using an image processing system combined with a light microscope and a TV camera. Chlorpromazine, haloperidol, SCH 23390 and sulpiride significantly raised the quantity of retrograde transport of HRP. On the contrary, domperidone which can not penetrate the blood-brain barrier showed no significant change in the amount of the retrograde transport. In addition, an intravenous injection of chlorpromazine (8 mg/kg) was found to increase the amplitude of monosynaptic masseteric reflex EMG activity evoked by stimulations of the mesencephalic trigeminal nucleus. These results suggest that a possible regulatory system involving the dopamine receptor in the uptake and retrograde transport of HRP from axon terminals to cell bodies of the masseteric motoneuron exists in higher order neurons which make synaptic contact with the motoneuron.

Neostriatal stimulation activates tongue-protruder muscle, but not tongue-retractor or facial muscles: an electrical and chemical microstimulation study in rats

Whether electrical microstimulation of a rats striatal jaw region (SJR) in fact induced tongue or facial muscle activity in addition to jaw muscle activity was tested. Microstimulation of SJR-evoked EMG activity in a jaw-opener (anterior digastricus) and tongue-protruder (genioglossus). No activity was evoked in jaw-closers (temporalis or masseter), tongue-retractor (hyoglossus) or in facial muscles. In addition, the EMG effect could still be induced after extensive ablation of the neocortex; it was reproduced by microinjection of 50 mM kainic acid into SJR. The effective sites were histologically localized to a small central striatal region adjoining the anterior commissure. These findings may be of considerable value in understanding the striatal mechanism of orolingual dyskinesia involving involuntary jaw and tongue movements.

Influences of entopeduncular nucleus stimulation upon electromyogram activity of masticatory muscles.

Influences of stimulation of the entopeduncular nucleus (Ep) upon electromyogram (EMG) activity of masticatory muscles were examined. In the rat lightly anesthetized with halothane, high frequency (HF) microstimulation (trains of 20, 333-Hz cathodal pulses at 30-60 microA) and GABA microinjection (0.2-0.6 microl of 10 mM GABA dissolved in physiological saline) were performed in the Ep by using a three-barreled microelectrode. EMG activity was recorded from the anterior digastrics and the anterior superficial masseter muscles by using two fine enamel-insulated copper wires. The EMG activity was also evoked by the GABA microinjection. The effect of the GABA microinjection was negated by the microinjection of bicuculline prior to the GABA microinjection. The EMG activity was classified into the tonic spike-type, burst-type, or mixed type on the basis of the waveform. In each rat, the location of the microelectrode tip was estimated by observing a series of serial frontal sections through the whole rostrocaudal extent of the Ep. The present data suggested that Ep neurons involved in elicitation of tonic spike-type activity in the jaw muscles might be located mainly in the rostral third of the Ep, and that Ep neurons implicated in provocation of burst-type activity in jaw muscles might be located in the caudal third of the Ep. Possible neuronal pathways from the Ep to motoneurons innervating the masticatory muscles were discussed. The present data shed new light on the control mechanisms of the basal ganglia upon jaw movements.