CO 2 Laser Pulse - Evoked Nocifensive Behavior Mediated by C - Fibers

Most tests that assess pain in animals involve motor responses to noxious stimuli. This practice depends on the implicit premise of a strong relationship between nociception and motor activity [Le Bars et al., 2001, Smith, 1995]. Therefore, a behavior can and should be analyzed as thoroughly as any brain structure and function involving motor activity in neurological investigations. The importance of analyzing behavior has been demonstrated by several studies on the mechanisms of nociception. For example, Rousseaux et al. [Rousseaux et al., 1999] showed that patients with spinothalamic tract injury had a higher temperature threshold than normal in response to nociceptive stimuli and have pain but not heat sensations. Thermal and electrical stimulation of the affected side in these patients elicited a withdrawal reaction. Another example, demonstrated by Shyu et al. [2003], was conditioned CO2 laser-evoked behavior in rats to distinguish emotional components from sensory conductive pathways [Kung et al., 2003]. The ideal model of pain stimulation should be nociceptive-specific, controllable, safe, and reproducible. Pain pathways are a part of the somatosensory system. Therefore, neurophysiological investigation of pain pathways is related to the recording of somatosensory-evoked responses. To test the integrity of nociceptive pathways with evoked nociceptive responses, the timing of the activation of nociceptive afferents must be precise for stimulus-locked averaging. Many experimental pain models that fulfill this requirement have been successfully used in pharmacological studies. Nevertheless, most of these models have limited clinical application for precise diagnosis. For example, pain has been elicited by cutaneous intense electrical stimulation, which activates peripheral fibers that respond to both noxious and innocuous stimulation. The drawback of electrical stimulation may be partially overcome by using various blocking techniques. Another approach is to induce nociception by activating tooth pulp, which is innervated mostly by Aδand C-fibers [Roos et al, 1982, Van Hassel, 1972]. Stowell [1974] used an electromechanically driven pin to produce cutaneous pain, and Mitchell and Hellon [1977] used gradual warming to induce cutaneous thermal stimulation. However, both methods can excite thermoor mechanoreceptors in addition to nociceptors [Burgess & Perl, 1973]. Non-contact thermostimulation is required for the investigation of the neural mechanisms underlying pain sensation without concurrent activation of mechanoreceptors. To record thermal-evoked potentials, thermostimulation must be very brief to permit the time-locking of potential averaging to the sensory event. Many repetitions of the thermal stimulation