Response properties of nucleus reticularis lateralis neurons after electroacupuncture stimulation in rats.

A descending inhibitory mechanism from the periaqueductal gray (PAG) to the spinal cord through the nucleus raphe magnus (NRM) is strongly involved in endogenous analgesic system produced by acupuncture stimulation. In addition to the PAG to NRM system which descends in the medial pathway of the brain stem, the nucleus reticularis lateralis (NRL) situated in the lateral part of the brain stem is reported to play an important role in modulating centrifugal antinociceptive action. In the present study, to clarify the role of NRL in acupuncture analgesia, we investigated the response properties of NRL neurons to acupuncture stimulation. The majority of NRM-projecting NRL neurons were inhibited by electroacupuncture stimulation. This effect was antagonized by ionophoretic application of naloxone, indicating that endogenous opioids act directly onto these NRL neurons. By contrast, about half of spinal projecting NRL neurons were excited by electroacupuncture stimulation, suggesting that part of the NRL neurons may modulate pain transmission directly at the spinal level.

Disinhibitory involvement of the anterior cingulate cortex in the descending antinociceptive effect induced by electroacupuncture stimulation in rats.

The present study was conducted to clarify the role of the anterior cingulate cortex (ACCX) in acupuncture analgesia. Experiments were performed on 35 female Wistar albino rats weighing about 300 g. Single unit recordings were made from ACCX neurons with a tungsten microelectrode. Descending ACCX neurons were identified by antidromic activation from electrical shocks applied to the ventral part of the ipsilateral PAG through a concentric needle electrode. Cathodal electroacupuncture stimulation of Ho-Ku (0.1 ms in duration, 45 Hz) for 15 min was done by inserting stainless steel needles bilaterally. An anodal silver-plate electrode (30 mm x 30 mm) was placed on the center of the abdomen. Naloxone (1.0 mg/kg, i.v.) was used to test whether changes of ACCX activities were induced by the endogenous opioid system. Data were collected from a total of 73 ACCX neurons. Forty-seven neurons had descending projection to the PAG, and the other 26 had no projections to the PAG. A majority of descending ACCX neurons were inhibited by electroacupuncture stimulation. By contrast, non-projection ACCX neurons were mainly unaffected by electroacupuncture. Naloxone did not reverse acupuncture effects on the changes of ACCX neuronal activities. Acupuncture stimulation had predominantly inhibitory effects on the activities of descending ACCX neurons. Since the functional connection between ACCX and PAG is inhibitory, electroacupuncture caused disinhibition of PAG neurons, whose activity is closely related to descending antinociception to the spinal cord. This disinhibitory effect elicited by acupuncture stimulation is thought to play a significant role in acupuncture analgesia.

Characteristics of nociceptors in the periodontium--an in vitro study in rats.

Recent studies have indicated that nociceptors can be classified into various types according to their physiological properties. These studies have clarified that the frequency distribution of various nociceptor types is different among body sites and animal species. In the present study, we investigated the physiological properties of rat's periodontal nociceptors in an in vitro jaw-nerve preparation. Responses were recorded from functional single filaments in the inferior alveolar nerve. To determine the nociceptor type, calibrated von Frey filaments, heat, and bradykinin (BK) stimuli were used. We found five subtypes of nociceptors in the periodontal ligaments of the lower incisor: Adelta-high threshold mechanonociceptors (Adelta-HTM, n=28), Adelta-mechanoheat nociceptors (Adelta-MH, n=6), Adelta-polymodal nociceptors (Adelta-POLY, n=26), C-high threshold mechanonociceptors (C-HTM, n=3) and C-polymodal nociceptors (C-POLY, n=4). Most nociceptors were Adelta-innervated, while only a small number of C-innervated nociceptors were found. The present results suggest that periodontal nociceptors transmit mainly fast pain, and may thus play a role in rapid detection of injure-related stimuli during mastication.