B. C. Shyu

Endorphin mediated increase in pain threshold induced by long-lasting exercise in rats

Rats were trained to run spontaneously, without stress, in running wheels. The running activity increased gradually and could reach a plateau of 7 km/night after 3-4 weeks. During the first hour of running in the dark phase the squeak threshold increased significantly and remained high in the morning. The degree of increased threshold was correlated to the amount of running activity. The squeak threshold declined during the following 6 hours of inactivity. A rapid decrease in threshold occurred after naloxone (1-2 mg/kg i.p.). It is suggested that long-lasting muscle exercise (e.g. jogging), acupuncture, and low frequency electrical stimulation of afferent nerve fibres produce discharges in muscle afferents which influence central endorphin mechanisms giving analgetic effects.

Is a selective stimulation of the rat incisor tooth pulp possible

Abstract The purpose of the present study was to investigate whether electrical stimulation via bipolar intradental or extrapulpal electrodes can selectively activate intrapulpal nerve fibres of the rat lower incisor. The compound action potential was recorded from the mandibular nerve following stimulation. This potential had the same threshold and latency before and after apical pulp extirpation. Transection of the inferior alveolar nerve distal to the connexion between the tooth pulp afferents and this nerve abolished the response. It was also possible to record action potentials of the same shape and amplitude both from the mandibular and the inferior alveolar nerve following the same intrapulpal stimulus. It is concluded that the major part of the compound action potential obtained by stimulating the rat incisor tooth pulp is not produced by pulp afferents but by activity in nerve fibres outside the pulp. The results suggest that electrical stimulation of the rat incisor tooth is not a useful method for physiological studies of nociceptive mechanisms.

Projection from the thalamic intralaminar nuclei on the isocortex of the rat: a surface potential study

SummaryCortical surface potentials evoked from thalamic intralaminar nuclei have been studied in rats anaesthetized with chloralose. Stimulation with low current intensity in central lateral nucleus (CL), evoked potentials in large areas of the rat isocortex. In the posterior parietal cortex responses with a short latency negativity were evoked which followed high frequency repetitive stimulation. Its latency and ability to follow high frequency stimulation indicated a monosynaptic connection from CL to this part of the cortex. The short latency potential was followed by a second negativity with longer latency and varying amplitude. This second negativity did not follow repetitive stimulation exceeding 10 Hz, and was also reduced by supplementary doses of anaesthetics, indicating a polysynaptic origin. Stimulation at different CL sites elicited cortical potentials with short latency in a topographical pattern. Laminar analysis in the parietal and motor cortex suggested both a superficial and a deep layer termination of afferents from CL. Similar topografical relations and afferent layer distributions have previously been found in cats. The role of the thalamocortical projection from CL to parietal cortex in arousal, attention and pain mechanisms is discussed.

Localized responses in the midsuprasylvian gyrus of the cat following stimulation of the central lateral nucleus in thalamus.

SummaryEvoked responses were mapped in the cerebral cortex following low intensity electrical stimulation in serial penetrations of the medial and intralaminar nuclei of the thalamus of the cat. A projection was found from one of the intralaminar nuclei, the central lateral nucleus (CL) to the midsuprasylvian gyrus, mainly areas 5 and 7. The projection is suggested to be direct, since the evoked responses had a short latency initial positivity. The most characteristic type of response consisted of this early positivity followed by two successive negativities. The earlier, so called first negativity followed high frequency stimulation and was recorded in a smaller area of the cortex than the later, so called second negativity. The first negativity is suggested to depend on monosynaptic depolarization and activation of cortical cells. The second negativity failed at frequencies higher than 10 Hz and was strongly depressed by the administration of barbiturates; it is suggested to depend on polysynaptic depolarization and cellular activity. In electrode penetrations of the cortex both negativities reversed at the border between cortical layers II and III, indicating a superficial termination of thalamic afferents in the cortex. The cortical evoked response to CL stimulation was facilitated by light mechanical and low intensity electrical stimulation of the periphery, as well as by electrical stimulation of the tooth pulp. The possible significance and function of this projection is discussed in relation to arousal, attention and pain.

Influence of changes of tooth temperature on reflex and central activity evoked by stimulation of tooth pulp afferents

The importance of the temperature of the dentine was studied in teeth prepared for electrical stimulation. During experiments with the mouth open, the temperature of teeth covered by cement was normal. The digastric EMG and the brainstem--evoked response following electrical stimulation of the tooth pulp as well as the threshold for eliciting a jaw-opening response remained constant throughout prolonged experiments. However, heat produced by the cement used to fixate the tooth electrodes could have damaged the tooth if the dentine temperature had exceeded 45 degrees C. A careful preparation of the tooth pulp by repeated application of thin layers of cement allowed an adequate preparation without damage to tooth pulp afferents.

Effects of acute and chronic nerve compression on compound action potentials

OF ACUTE AND ~~ONIC~NFRVE ~NPR@SICN ON COMPOUND ACTION FWENTI~. B.C. mvu , L. pahlin , B. Olaysson and S. Andersson , Departments of Physiology and Anatomy, University of Gateborg, S-400 33 Gateborg, Sweden. Aim of Investigation: To compare the relative charges in the depression and recovery of mvelinated (A) and unmvelinated (C) nerve fibre trsnsmission caused by compression and to assess” the effect of sympathetic trunk stimulation on chronically injured nerves. Methods: The ccmmon peroneal nerve of rabbit was exposed and canpressed for a length of 10 mn in a chamber consisting of symmetrical halves to which thin rubber membranes were glued. Via a compressed air system a preset pressure was applied to the nerve. Results: In acut experiments, a pressure of 200 mnRg blocked the shortest latency component (Al) of A fibres after 20,min. The second ccxnponent of the A fibre potential (A2) was reduced to 20% of the control level after compression for 2 h. The C fibre potential did not charge. A pressure of 400 III+@ during 50 min blocked the A fibres completely, and the C fibre potential was reduced to 30% of the control value. There was only partial restitution of the potentials during 2 h of recovery. In chronic experiments, the effect of sympathetic trunk sttilation was examined 2 weeks after nerve compression of 400 mn~g during 30 min. In 60% of the animails, the C Mbre potential was sugmented during and after sympathetic stlmulation. Only in 20% of the preparations, sympathetic sttilation caused a suppression of the C fibre potential, an effect normally found in intact nerves. Conclusion: The C fibres are less sensitive to compression compared to A fibres. The msdu.latory effect of the sympathetic system on the C fibres changes after injury. These findings could be of relevance in the understanding of sympathetically related pain after nerve injury.

Midsuprasylvian cell activity evoked by stimulation of peripheral afferents and from N. Centralis Lateralis

Dept. of Physiolqg, Univ. of G&teborg,Box 33031, S-400 33 Gateborg, Sweden. Aim of Investigation: Cell activity in cortical areas 5 and 7 was recorded to study the convergence from tooth pulps, low threshold afferents, the visual svstem snd from cells in n. centralis lateralis (CL). Methods: “&rgsten electrodes in CL were used for stimulation (50 uA). Micropipettes recorded activity extraor intracellularly in cortical cells. In some experiments cell response evoked by CL stimuJ.atibn was conditioned with tooth pulp, low threshold afferents or with Jight flash stimulation. Results: Cells activated by CL (n=70) were found in Jaminae II-III and in Jsminae V-VI. At both locations two groups of cells with different latenties were separated. The first group showed short latency EPSPs (4-8 ms) and action potentials. The other group showed lor-ger latencies to the EPSPs (10-15 ms). Some cells activated by CL received input frcxn canine tooth pulps (35%). Lsminae II-III tooth pulp projection gave EPSPs (28 ms) and spikes of shorter latency than the deeper projection (42 ms). Low intensity stimulation of bilateral afferents in face snd forelimb evoked EPSPs of lorg latency (>lO ms) and action potentials in about half of the CL activated cells. In some cells also IPSPs were found. A similar activation pattern occurred after light flash stimulation. A preceding tooth pulp stimulation facilitated the CL response but a preceding stimulation of low threshold somatic afferents or a visual stirmli inhibited the CL response. Discussion: The results indicate convegent input from low and high threshold afferents to CL activated cells in cortical areas 5 and 7. Electrical stimulation in CL evoked a response pattern similar to that after tooth pulp stimulation although the latency was different. These findings suggest that the tooth pulp projection pathway to the cortex cay involve CL.

Sympathetic effects on C-fibre kesponses

COMPARISON OF NOCICEPTORS AND PROPRIOCEPTORS IN PULP AND 10 Slide PERIODONTAL LIGAMENT OF RAT INCISORS USING AUTORADIOGRAPHY HRP-CYTOCHEMISTRY AND ELECTRON MICROSCOPY. M.R.Byers and K.Y.Chan*, Depts. of Anesthesiology, Biol. Structure, I,y Sat 3:00 G. B. I Ophtha mo ogy, an Ctr. Res. Oral Biology, University of Washington, Seattle, WA, USA. Aim: In order to learn more about the structure of nociceptors, we cmpared &al receptors of rat incisors with specialized proprioceptors in adjacent ligament. Rat incisors are advantageous because they lackdentinal innervation and only contain slow conducting nociceptive axons; also their ligament has numerous proprioceptors. Methods: We injected 3H-amino acids or horseradish peroxidase (HRP or WGA-HRP) into the R. trigeminal ganglion of adult rats and allowed 20-24h axonal transport to the receptors. After aldehyde fixation, incisors plus adjacent ligament and bone were decalcified and prepared forautoradiography (AR), HRP-cytochemistry or electron microscopy (EN). Results: Light microscopic AR and HRP studies showed sensory neurite distribution: pulpal axons ended among blood vessels or near (but usually not in) the odontoblast layer in the incisal third of pulp; ligament axons formed large and small branched endings with distinctive patterns in avascular ligament regions. EM studies found bundles of small axons or single axons in pulpal receptor zones, but no specialized structures; in ligament we found large and small Ruffini-like receptors, bundles of small axons, but no encapsulated receptors. The large Ruffini endings had neural fingers that contacted ligament collagen. Conclusions: Nociceptors in rat incisor pulp branch from small axonsand form unmyelinated bundles or individual neurites that are enclosed by a Schwann cell. They differ from ligament proprioceptors by their simplicity, their unspecialized Schwann cells and their lack of neural fingers contacting connective tissue. (Supported by Grants DE05159, DE02600, DE00099).

Stomatology Is a selective stimulation of the rat incisor tooth pulp possible?: Pain, 15 (1983) 27–34

Studies on the in~ath~a1 effect of endorphin in primate. T.L. Yaksh, K.E. Gross and Choh Hao Li (Departments of Neurosurgery and Pharmacology. Mayo Foundation, Rochester, Minn. 55905, U.S.A.), Brain Res., 241 (1982) 261-269. The intrathecal administration of /3-endorphin in the primate through an indweliing spinal catheter, produced a significant elevation in the nociceptive threshold as measured by the discrete trial shock titration task. The time of onset, duration of effect and magnitude of effect were all dose dependent over a range of 150750 pg. The effects were antagonized in a dose-dependent fashion by the systemic administration of naioxone. Aside from the elevations in the shock titration threshold produced by intrathecal ,&endorphin, no untoward effect on the animal’s motor function or behavioral reactivity was noted. There was no change in mean blood p0, and pCO,, heart rate and blood pressure measured immediately before and at 1, 3 and 6 h after intrathecal injection of /?-endorphin. The intrathecal administration was unaccompanied by any signs of irritation, agitation, retching or changes in pupil size. Significantly, unlike morphine, intrathecal ~-endo~hin failed to produce any signs of scratching behavior at the doses used in these experiments. Once daily administration of intrathecal /3-endorphin (500 pg) showed a significant progressive decline in the ~tinociceptive effect over an 8 day period. Animals made tolerant to /3-endorphin in this fashion showed a significantly reduced response to an otherwise active dose of intrathecal morphine, indicating evidence for cross-tolerance.