Intrapulpal nerve stimulation in the rat.
Electrical stimulation of the rat incisor pulp has been frequently used as a method to study the mechanisms of pain. However, several of the techniques used to stimulate nociceptive nerves in these teeth also tend to excite non-nociceptive nerves in adjacent tissues. This report deals with the development and verification of a procedure which selectively stimulates pulpal nerves over a wide range of stimulus intensities. The methods consisted of placing electrodes in labial cavities of each of the two lower rat incisors, 1 mm distal to the level of the crest of the interdental papilla. Each cavity had a diameter of 0.5 mm and a depth of about three-quarters fo the crown diameter. Each electrode consisted of a Ag/AgCl filling compacted onto the end of a 0.1 mm silver wire. Constant current stimuli were passed between the two electrodes. EMG was monitored to detect reflex activation of the digastric muscle in response to intradental or extradental afferent excitation. The response consisted of two components. One was a long latency potential (LLP) (37 ms latency) (89 microA ave TH); the other was a short latency potential (SLP) (12 ms latency) (650 microA min TH). In all animals that LLP was abolished by bilateral sectioning of the tooth pulp 7 mm below the level of the labial gingival margin. The SLP was not abolished. This would indicate that the LLP was due to stimulation of pulpal fibers, and the SLP to excitation of extrapulpal fibers. This may prove to be a helpful method for nociceptive researchers to easily verify that they are actually stimulating pulpal afferents.