Cortical stimulation and tooth pulp evoked potentials in rats: a model of direct anti-nociception.

While the effect of cortex stimulation on pain control is widely accepted, its physiological basis remains poorly understood. We chose an animal model of pain to study the influence of sensorimotor cortex stimulation on tooth pulp stimulation evoked potentials (TPEPs). Fifteen awake rats implanted with tooth pulp, cerebral cortex, and digastric muscle electrodes were divided into three groups, receiving 60 Hz, 40 Hz and no cortical stimulation, respectively. TPEPs were recorded before, one, three and five hours after continuous stimulation. We observed an inverse relationship between TPEP amplitude and latency with increasing tooth pulp stimulation. The amplitudes of the early components of TPEPs increased and their latency decreased with increasing tooth pulp stimulation intensity. Cortical stimulation decreased the amplitude of TPEPs; however, neither the latencies of TPEPs nor the jaw-opening reflex were changed after cortical stimulation. The decrease in amplitude of TPEPs after cortical stimulation may reflect its anti-nociceptive effect.

Somatosensory evoked response and jaw opening reflex elicited by tooth pulp stimulation in awake freely moving rats.

OBJECTIVE: Investigation of pain and nociception refers to different models. Depending upon the intensity of stimulation, unmyelinated pulpal fibers or periodontal A-fibers can be stimulated producing a short or a long latency jaw opening reflex of the digastric muscle. This paper investigates the different components of the jaw opening reflex in addition to the correlation between afferent fibers involved in the cortical evoked response. DESIGN AND SETTING: Fifteen awake male rats were implanted with tooth pulp stimulation electrodes, digastric and cortical recording electrodes. Ten rats were submitted to recordings after a single tooth pulp stimulation, while five rats were using conditioning and test stimulation. Tooth pulp evoked potentials and digastric EMG were simultaneously recorded. A multiresolution denoising method was used for signal processing. RESULTS: Following tooth pulp stimulation, a cortical response was produced including the following peaks: P6.5 +/- 1.1, N11 +/- 1.2, P17 +/- 1.2, P27 +/- 2.9, N53 +/- 7.5, P69 +/- 5.8, P88 +/- 13, N160 +/- 9.7, P204 +/- 14.2. The distribution and amplitude of these peaks are correlated to the stimulation intensity (r=0.96, p<0.01). An interaction between the different components of the jaw opening reflex was identified on EMG, following a conditioning shock, where a cortical evoked response showed a P30 +/- 2.7 peak which was observed concurrently with the jaw opening long latency reflex. CONCLUSION: Our results identify the interaction between the different components of the jaw opening reflex and the correlation to the cortical evoked response.