Simultaneous-recordings of Periodontal Neurons in Rat-trigeminal ganglion and Mesencephalic Nucleus

Objectives:Periodontal mechanoreceptors (PM) are dually innervated by trigeminal ganglion (TG) and mesencephalic nucleus (MS). To precisely compare response characteristics of primary PM-neurons in TG and MS, single-unit responses to controlled forces applied to a single tooth were recorded simultaneously from TG and MS of the same rat. Methods:Rats weighing 440-560g were anesthetized with intraperitoneal pentobarbitone-Na 50mg/kg. Each head was fixed in a stereotaxic apparatus. Following widespread craniotomy, brain tissues overlying right TG were removed and exposed TG covered with mineral oil was kept warm by radiant heat. Left upper incisor and both lower incisors were cut off. Small notches were cut in incisal edge of right upper incisor so that mechanical pressure could be applied easily in a labial or lingual direction using electromechanical force generator incorporating a vibrator. For simultaneous single-unit recordings, two vanish-coated elgiloy-microelectrodes were inserted into caudal part of MS and maxillary division of TG on right side. Recording sites were confirmed histologically. Results:Successful simultaneous-recordings were attained in 33 pairs of TG- and MS-single-units from 33 rats. Primary PM-units in TG showed spontaneous sustained-responses (30.7±3.1Hz, M±SD; n=14); whereas those in MS exhibited substantially no spontaneous-activity. By changing displacement amplitude (200-500µm) and direction of trapezoidal (rise 0.1s, plateau 0.2s, fall 0.1s) or rectangular(duration 0.4s) pressure stimuli, it was found that lingo-labial pressure produced essentially very slowly-adapting spikes (max.220Hz) and transient spikes (on or on-off) in TG- and MS-units, respectively; labio-lingual pressure provoked substantially silent period and transient spikes (off or on-off) in TG- and MS- units, respectively. Conclusions:These results indicate that PM afferent pathway projecting to MS might be involved in production of transient periodontal-masseteric reflex caused by brisk tapping of upper incisor; whereas PM pathway arising from TG might be participated in elicitation of tonic periodontal- masseteric reflex induced by lingolabial slow push of upper incisor.