Kazuo Toda

Dopamine modulates a voltage-gated calcium channel in rat olfactory receptor neurons

Dopamine D2 receptors exist in the soma of rat olfactory receptor neurons. Actions of dopamine on the voltage-gated Ca(2+) channels in the neurons were investigated using the perforated whole-cell voltage-clamp. In 10 mM Ba(2+) solution, rat olfactory receptor neurons displayed the inward currents elicited by the voltage ramp (167 mV/s) and depolarizing step pulses from a holding potential of -91 mV. The inward Ba(2+) currents were greatly reduced by 10 microM nifedipine (L-type Ca(2+) channel blocker). The Ba(2+) currents were inhibited by the external application of dopamine. The IC(50) for the inhibition was about 1 microM. Quinpirole (10 microM, a D2 dopamine agonist) also inhibited the Ba(2+) currents. Quinpirole did not affect the activation and inactivation kinetics of the Ba(2+) currents. The results suggest that dopamine modulates the L-type Ca(2+) channels in rat olfactory receptor neurons via the mechanism independent of voltage.

Subtypes of nociceptive units in the rat temporomandibular joint

Response properties of nociceptors in the rats temporomandibular joint (TMJ) were investigated using an in vitro TMJ-nerve preparation. Recordings were obtained from 33 nociceptive units that responded to mechanical, chemical, and/or thermal stimuli. According to both characteristics of nociceptors and afferent fibers, nociceptive units in the TMJ area were classified into the following four subtypes: Adelta-high-threshold mechanonociceptor (HTM) (12.1%), Adelta-polymodal nociceptor (POLY) (36.4%), C-HTM (12.1%), and C-POLY (39.4%). The mean mechanical threshold of the Adelta units was significantly lower than that of the C units. Bradykinin increased the discharge of Adelta- and C-POLY units. No significant differences of thermal thresholds between Adelta and C units were found. The percentage of Adelta units was 47.2% and of C units was 52.8%, respectively. In the TMJ area, POLY units were predominant (75.8%), suggesting that inflammatory reactions can easily evoke pain sensation.

Changes in response properties of periodontal mechanoreceptors after experimental orthodontic tooth movement in rats.

Using an in vitro preparation, we investigated chronological changes in response properties of periodontal mechanoreceptors (PMRs) in the rat right mandibular first molar (M1) after experimental orthodontic tooth movement. Orthodontic force was applied to M1 for 14 days by activating 24.5 mN superelastic titanium-nickel alloy closed coil springs anchored to the mandibular incisors. Experiments were performed on days 3, 7, 10, and 14 during application of orthodontic force and on days 7, 14, 21, and 28 after removal of orthodontic force. The rats without application of orthodontic force were used as control group. In each group, direct mechanical stimulation using von Frey hairs and electrical stimulation was applied to the distal root of M1. Results showed that compared with controls (1) the mechanical thresholds were significantly lower during application of orthodontic force; however, no significant difference was found after removal of force application and (2) conduction velocities were significantly lower from day 7 during application of orthodontic force to day 14 after removal of orthodontic force; however, no significant difference was found on days 21 and 28 after removal of orthodontic force. These results suggest that the PMRs, although having some of their response properties altered during orthodontic force application, were able to recover and adapt to the newly acquired intraoral condition after removal of the orthodontic force.

Serotonin differentially modulates the electrical properties of different subsets of taste receptor cells in bullfrog

Serotonin (5‐hydroxytryptamin, 5‐HT) is localized in taste bud cells of vertebrates. Effects of the external application of 5‐HT on the membrane currents of frog taste receptor cells (TRCs) were investigated using patch‐clamp technique in whole‐cell configuration. The 5‐HT (0.1–1u2003µm) and 5‐HT1A receptor agonist (+/–)‐8‐OH‐2‐(D1‐n‐propyl‐amino)tetralin (8‐OH‐DPAT) (1–20u2003µm) inhibited both voltage‐gated sodium current (INa) and voltage‐gated potassium current (IK) in 50% of TRCs, but potentiated IK without any significant effect on INa in another subset of 18% of TRCs. Voltage‐gated currents in the residual TRCs were not affected by 5‐HT or 8‐OH‐DPAT. External application of 10u2003µm forskolin and 300u2003µm 8‐cpt cAMP [8‐(4‐chlorophenylthio)adenosine 3′:5′‐cyclic monophosphate] mimicked the inhibitory effect of 5‐HT and 8‐OH‐DPAT on IK and INa while internal dialysis with 50u2003µm protein kinase A inhibitor prevented the 5‐HT‐mediated inhibitory effects on IK and INa in TRCs. Internal dialysis of TRCs with high Ca2+‐pipette solution (1u2003µm) increased the IK in 58% of TRCs. The 5‐HT reversibly increased the [Ca2+]i in 17% of TRCs when measured by Ca2+‐imaging using a Ca2+‐sensitive dye (fura‐2 AM). These results suggest that 5‐HT differentially modulates the voltage‐gated membrane currents in different subsets of TRCs.

Response properties of periodontal mechanoreceptors in rats, in vitro.

Single unit activities of the inferior alveolar nerve evoked by calibrated von Frey stimuli (1.1, 2.9, 7.8, 11.8, and 17.2mN) on the periodontal ligaments of the mandibular molars or incisors were recorded in an in vitro jaw-nerve preparation of Wistar albino rats. The data of 55 (lower incisor) and 100 (lower molars) units were collected in the present study. Both rapidly (RA) and slowly adapting (SA) type units were found in the incisors, and most of these units were innervated by Abeta fibers. While all the units of the molars were of RA types, the innervated fibers of two-thirds (67/100) of the units have been identified as Adelta fibers. The response patterns of the RA type were subdivided into three types (ON, OFF or ON-OFF type) both in the incisors and the molars. While von Frey thresholds of all incisor units were 11.8 mN except one unit that was 7.8 mN, those of the molars varied from 2.9 to 11.8 mN. In the molars, a majority of afferents innervated the periodontal ligaments of more than one tooth. This study suggests that response properties of periodontal mechanoreceptors are different between the incisors and the molars in rats, suggesting that these receptors have different functions in the regulation of mastication.

Nature of Neck Motor Unit Activities Evoked by Different Trigeminal Inputs in Rats

Recent reports have suggested functional interactions between the masticatory and head motor systems. However, the neurophysiological mechanisms behind these phenomena have not been thoroughly investigated. In this study, the motor unit activity was recorded bilaterally from the dorsal neck splenius muscles (SPL) while the cell bodies of trigeminal afferent fibers were stimulated at either the trigeminal mesencephalic nucleus (Me5) or the Gasserian ganglion (5Gn) in 13 barbiturate-anesthetized Wistar rats. After Me5 electrical stimulation, SPL motor unit responses were recorded ipsilaterally (n = 24), with an onset latency distribution between 1.3 and 1.8 ms. After 5Gn stimulation, SPL responses were recorded bilaterally from 18 single units, with latencies ranging from 2.2 to 2.4 ms. Our results suggest that there are at least two distinct pathways from the trigeminal afferents to the neck muscles, with significant functional differences.

Functional maturation of periodontal mechanoreceptors during development in rats

The influence of development on periodontal mechanoreceptors (PMRs) was investigated in four groups of male Wistar albino rats aged 1, 3, 5 weeks and 6 months using an in vitro jaw-nerve preparation. The mean values of conduction velocities of the nerve innervating PMRs in 5-week and 6-month groups were significantly higher than those in the other two groups. All fiber types obtained in the 5-week and 6-month groups were Abeta. The mechanical thresholds of 5-week and 6-month groups were significantly higher than those of 1- and 3-week groups. These data suggest that the response properties of rats PMRs are matured by 5-week after birth, when functional molar occlusion and transition of dietary contents from liquid to hard-diet can be achieved.

Afferent nerve characteristics during acupuncture stimulation

Abstract A number of studies have provided evidence that acupuncture or acupuncture-like stimulation increases pain threshold in humans. According to traditional Chinese theory, there is a specific point called meridian point to treat a specific symptom. For example, if a needle was inserted into Ho-Ku point, and this point was stimulated electrically or manually, oro-facial pain, especially tooth pain, is fairly diminished. To clarify peripheral mechanisms of acupuncture effects and to reveal the nature of meridian points scientifically, characteristics of afferent nerve information evoked by acupuncture stimulation were investigated in Wistar albino rats. The magnitude of the jaw opening reflex induced by stimulation of the lower incisor tooth pulp was estimated as an indicator of the acupuncture effect. Acupuncture needling was given at one meridian point of Ho-Ku and two nonmeridian points in the forepaw. Compound action potentials of ulnar, median and radial nerves elicited by electroacupuncture stimulation (0.1 ms duration at 45 Hz for 15 min) were recorded from the brachial plexus. Electroacupuncture stimulation applied to the Ho-Ku point showed a greater suppressive effect than that to the two nonmeridian points. The threshold current intensity for producing the compound action potentials was the lowest at Ho-Ku. There was a significant correlation between the amplitude of A-beta fibers in the compound action potentials elicited by electroacupuncture stimulation and the degree of suppression of the jaw opening reflex. The results of the present study are in good agreement with the traditional view that stimulation of the meridian point is an important factor for obtaining excellent acupuncture effects. Moreover, we propose that A-beta component plays a leading role in producing acupuncture effects (A-beta theory). Activation of A-delta or C components may play a supplemental role in detecting the magnitude of the A-beta activities, and relate to the “Tokki”, which is reported to be dull, warm or tingling sensation. At the central terminal site of the primary nociceptive afferent, three inhibitory mechanisms were observed in the spinal trigeminal nucleus.

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 rats 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.

Hypergravity modulates behavioral nociceptive responses in rats.

Hypergravity (2G) exposure elevated the nociceptive threshold (pain suppression) concomitantly with evoked neuronal activity in the hypothalamus. Young Wistar male rats were exposed to 2G by centrifugal rotation for 10 min. Before and after 2G exposure, the nociceptive threshold was measured as the withdrawal reflex by using the von Frey type needle at a total of 8 sites of each rat (nose, four quarters, upper and lower back, tail), and then rats were sacrificed. Fos expression was examined immunohistochemically in the hypothalamic slices of the 2G-treated rats. When rats were exposed to 2G hypergravity, the nociceptive threshold was significantly elevated to approximately 150 to 250% of the 1G baseline control levels in all the examination sites. The 2G hypergravity remarkably induced Fos expression in the paraventricular and arcuate nuclei of the hypothalamus. The analgesic effects of 2G hypergravity were attenuated by naloxone pretreatment. Data indicate that hypergravity induces analgesic effects in rats, mediated through hypothalamic neuronal activity in the endogenous opioid system and hypothalamo-pituitary-adrenal axis.