Optogenetic stimulation in the medial prefrontal cortex modulates stimulus valence from rewarding and aversive to neutral states

Introduction

Understanding the modulations of the medial prefrontal cortex (mPFC) in the valence of the stimulus from rewarding and aversive status to neutral status is crucial for the development of novel treatments for drug addiction. This study addressed this issue and examined whether optogenetic ChR2 photostimulation in the cingulate, prelimbic, and infralimbic cortices of the mPFC regulated the valence of saccharin solution consumption from the rewarding property, the aversive property induced by morphine's conditioning, and the neutral states via saccharin extinction processes after morphine's conditioning.

Methods

All rats received virus infection, buried optical fiber, optical stimulation, water deprivation, and saccharin solution consumption phases. In Experiment 1, rats were given ChR2 virus infection into the cingulate cortex (Cg1), prelimbic cortex (PrL), and infralimbic cortex (IL) to influence the rewarding saccharin solution consumption under photostimulation. In Experiment 2, rats were given ChR2 or EYFP virus infection into the Cg1, PrL, and IL to alter the saccharin solution consumption in the morphine-induced aversively conditioned taste aversion (CTA) and the saccharin solution consumption in the neutral state following the extinction process under photostimulation. Later, the immunohistochemical staining with c-Fos protein was performed for the Cg1, IL, PrL, nucleus accumbens core, nucleus accumbens shell, central amygdala, basolateral amygdala, ventral tegmental area, and dentate gyrus.

Results

The results showed that optogenetic PrL stimulation decreased the rewarding valence of saccharin solution consumption and increased the morphine-induced, aversive valence of saccharin solution consumption. PrL stimulation decreased the neutral valence of saccharin solution consumption via the extinction process. Cg1 optogenetic stimulation increased the rewarding valence of saccharin solution consumption and the aversive valence of saccharin solution consumption induced by morphine in conditioning. Optogenetic IL stimulation increased the aversive valence of saccharin solution consumption induced by morphine via conditioning.

Conclusion

Altogether, optogenetic stimulation in the subareas of the mPFC modulated the reward, aversion, and neutral valences of the stimulus and altered neuronal activity in the mPFC, amygdala, nucleus accumbens, and hippocampus. Notably, the change of valence was temporary alternation during light-on related to the light-off periods. However, the findings may provide insights in the development of novel treatments for addictive symptoms.

Modulation of melatonin to the thalamic lesion-induced pain and comorbid sleep disturbance in the animal model of the central post-stroke hemorrhage

The devastating chronic central post stroke pain is associated with variety of comorbidities. Disrupted sleep is a severe comorbidity, causing an increase in the suicide rate, due to CPSP’s pain symptom. Melatonin is a well-known jet-lag compound, which helps in entrainment of sleep cycle. Accordingly, whether melatonin as a therapeutic measurement for the regulation of sleep disturbance related to central post stroke pain remains unclear. Exogenous melatonin administration entrained the disrupted 24 h circadian cycle, more effectively after 2 and 3 week of administration. The effect of melatonin was persisted on 4th week too, when melatonin administration was discontinued. Also, melatonin ameliorated the pain due to distorted sleep-activity behavior after melatonin administration for 3 weeks. The low levels of melatonin in blood plasma due to CPSP were restored after 3 weeks of melatonin administration. After 30 mg/kg melatonin administrations for 3 weeks, all the disrupted resting and activity behaviors were reduced during light and dark periods. The results suggested that melatonin significantly ameliorated CPSP’s pain symptoms and comorbid sleep disturbance showing in activity behavior.

Tricyclic antidepressants and selective serotonin reuptake inhibitors but not anticonvulsants ameliorate pain, anxiety, and depression symptoms in an animal model of central post-stroke pain

Background

Central post-stroke pain (CPSP) is a type of neuropathic pain caused by dysfunction in the spinothalamocortical pathway. However, no animal studies have examined comorbid anxiety and depression symptoms. Whether the typical pharmacological treatments for CPSP, which include antidepressants, selective serotonin reuptake inhibitors (SSRIs), and anticonvulsants, can treat comorbid anxiety and depression symptoms in addition to pain remains unclear? The present study ablated the ventrobasal complex of the thalamus (VBC) to cause various CPSP symptoms. The effects of the tricyclic antidepressants amitriptyline and imipramine, the SSRI fluoxetine, and the anticonvulsant carbamazepine on pain, anxiety, and depression were examined.

Results

The results showed that VBC lesions induced sensitivity to thermal pain, measured using a hot water bath; mechanical pain, assessed by von Frey test; anxiety behavior, determined by the open-field test, elevated plus-maze test, and zero-maze test; and depression behavior, assessed by the forced swim test. No effect on motor activity in the open-field test was observed. Amitriptyline reduced thermal and mechanical pain sensitivity and anxiety but not depression. Imipramine suppressed thermal and mechanical pain sensitivity, anxiety, and depression. Fluoxetine blocked mechanical but not thermal pain sensitivity, anxiety, and depression. However, carbamazepine did not affect pain, anxiety, or depression.

Conclusion

In summary, antidepressants and SSRIs but not anticonvulsants can effectively ameliorate pain and comorbid anxiety and depression in CPSP. The present findings, including discrepancies in the effects observed following treatment with anticonvulsants, antidepressants, and SSRIs in this CPSP animal model, can be applied in the clinical setting to guide the pharmacological treatment of CPSP symptoms.

Involvement of the ventral tegmental area but not periaqueductal gray matter in the paradoxical rewarding and aversive effects of morphine

The paradoxical effects of reward and aversion with abused drugs may interact to produce drug addiction, which is the so-called paradoxical effect hypothesis of abused drugs. However, there is no research examining how the ventral tegmental area (VTA) or periaqueductal gray matter (PAG) regulates morphine's paradoxical effect of reward and aversion. The present study addresses this issue, utilizing a high concentration of N-methyl-D-aspartic acid (NMDA) via injections to destroy the VTA or the PAG. Moreover, the study employed the new "pre- and postassociation" experimental paradigm (2010) to test whether the simultaneous rewarding and aversive effects of morphine can be affected by an NMDA lesion in the VTA or the PAG. The results indicated that the NMDA lesion of the VTA simultaneously reduced morphine-induced conditioned suppression of saccharin solution intake in conditioned taste aversion (CTA) and morphine-induced spent time in the preference compartment in conditioned place preference (CPP), whereas the PAG lesion did not change either measure. Thus, the VTA, but not the PAG, appears to contribute to the paradoxical effect reward in CPP and aversion in CTA induced by morphine. The VTA's involvement in morphine-induced CTA aversion and CPP reward supports the paradoxical effect hypothesis of abused drugs. (PsycInfo Database Record (c) 2021 APA, all rights reserved).

A Wireless Magnetic Resonance Device for Optogenetic Applications in an Animal Model

The currents of optical stimulation devices with tethered or untethered systems have various disadvantages, including optical fiber breakage, disrupted animal movements, heavy batteries carried on heads, and high-frequency electromagnetic impacts. Our novel wireless remote control was developed to address these issues. The novel wireless device uses a magnetic resonance technique to modify the deficits of the conventional magnetic induction or radio-frequency power sources. The present device emits a strong and steady electromagnetic power. It is cheaper than previous versions, and the receiver coil on its head is very light (≦ 1 g). For the present wireless remote-controlled device, the electromagnetic field's range (i.e., +5 cm and -5 cm of the outside coil) is larger than the range for the magnetic induction and radio-frequency power sources. The present device controls animals' behavior by the electromagnetic field's effective range via photostimulation. The novel wireless remote-controlled device with a magnetic resonance technique can be applied in many behavioral tasks in mice and rats. To avoid the adverse effects of high radio frequency and to extend the electromagnetic field's range, this novel technique serves as a helpful tool to modulate the neuronal activity of target neurons in specific brain areas for optogenetic experiments.

Abstracts of the 7th Asian Pain Symposium

The Asian Pain Symposium (APS) is a main pain research meeting in Asia. Since established in 2000 in Kyoto, five other APSs have been held in different Asian regions including Seoul of Korea in 2004, Fukuoka of Japan in 2008, Shanghai of China in 2011, Okazaki of Japan in 2013, and Suzhou of China in 2015. The 7th Asian Pain Symposium (APS 2017) was held in Taipei of Taiwan during October 26th to October 29th, 2017. The APS 2017 was sponsored by The Ministry of Science and Technology of Taiwan and Institute of Biomedical Science and Neuroscience Program of Academia Sinica and Taiwan Pain Society. The president of the APS 2017 was Dr. Bai Chuang Shyu, Institute of Biomedical Sciences, Academia Sinica, Taiwan. Local organizing committee also include Dr. Jen-Chuen Hsieh, Institute of Brain Science, National Yang-Ming University and Veteran General Hospital, Taiwan, Dr. Wei-Zen Sun, Department of Anesthesiology, National Taiwan University Hospital, Taiwan, and Dr. Chih-Cheng Chen, Institute of Biomedical Sciences, Academia Sinica, Taiwan. Main topics of the APS 2017 included the latest progress of pain research and novel strategies of pain treatments. Symposium attendees presented their interesting and exciting research findings in the areas of 1) basic sensory and nociceptive functions, 2) ion channels and their functions in somatosensory physiology and pain, 3) brain functions and regulations in pain, 4) spinal cord mechanisms of nociception and pain, 5) analgesia and pain regulations, 6) chronic pain mechanisms and treatment, and 7) brain circuits underlying the physiological and pathological pain. There were a total of 29 oral presentations and 23 poster presentations at the 7th APS. A council meeting was held during the 7th APS, and at this council meeting Dr. Seog Bae OH (Seoul National University) was elected as the president of 8th Asian Pain Symposium to organize the next symposium in Seoul, Korea in 2019. In order to keep a permanent record and to help promote pain research in Asia, we have collected abstracts of oral presentations and posted them below in the order when the presentations were given at the 7th Asian Pain Symposium.

Lhx2 Expression in Postmitotic Cortical Neurons Initiates Assembly of the Thalamocortical Somatosensory Circuit

Cortical neurons must be specified and make the correct connections during development. Here, we examine a mechanism initiating neuronal circuit formation in the barrel cortex, a circuit comprising thalamocortical axons (TCAs) and layer 4 (L4) neurons. When Lhx2 is selectively deleted in postmitotic cortical neurons using conditional knockout (cKO) mice, L4 neurons in the barrel cortex are initially specified but fail to form cellular barrels or develop polarized dendrites. In Lhx2 cKO mice, TCAs from the thalamic ventral posterior nucleus reach the barrel cortex but fail to further arborize to form barrels. Several activity-regulated genes and genes involved in regulating barrel formation are downregulated in the Lhx2 cKO somatosensory cortex. Among them, Btbd3, an activity-regulated gene controlling dendritic development, is a direct downstream target of Lhx2. We find that Lhx2 confers neuronal competency for activity-dependent dendritic development in L4 neurons by inducing the expression of Btbd3.

Suppression of cortical seizures by optic stimulation of the reticular thalamus in PV-mhChR2-YFP BAC transgenic mice

Deep brain stimulation in thalamic regions has been proposed as a treatment for epilepsy. The electrical current excites thalamocortical activity which is controlled by γ-aminobutyric acid (GABA)ergic interneurons in the reticular thalamic nucleus (nRT). Previous studies showed that enhancing GABAergic inhibitory strength in the nRT reduces the duration and power of seizures, indicating that the thalamus plays an important role in modulating cortical seizures. The aim of the present study was to apply optogenetics to study the role of the nRT in modulating cortical seizures. We used PV-ChR2-EYFP transgenic mice from Jackson Laboratories, in which only Channelrhodopsin-2 (ChR2) is expressed in parvalbumin-expressing interneurons. Cortical seizure-like activity was induced by electrical stimulation of the corpus callosum after applying 4-aminopyridine. ChR2 expression was abundant in the nRT and cerebellum in PV-ChR2-EYFP transgenic mice. Light stimulation in the nRT caused burst firing in regions of the thalamus and nRT in vitro. Multi-unit activity increased during high-frequency (100 and 50 Hz) light stimulation in the S1 region and thalamus in vivo. Corpus callosum stimulation-induced seizure-like activity was effectively suppressed by high-frequency (100 Hz) and long-duration (10 s) light stimulation. The suppressive effects were reversed by applying a GABAB receptor antagonist but not a GABAA receptor antagonist in the cortex. The results indicated that light stimulation affected thalamocortical relay neurons by activating ChR2-expression neurons in the nRT. High-frequency and long-duration light stimulation was more effective in suppressing cortical seizure-like activity. GABAB receptors may participate in suppressing seizure-like activity.

Autoradiographic Measurements of [14C]-Iodoantipyrine in Rat Brain Following Central Post-Stroke Pain

Approximately 8% of stroke patients present symptoms of central post-stroke pain (CPSP). CPSP is associated with allodynia and hypersensitivity to nociceptive stimuli. Although some studies have shown that neuropathic pain may involve the dorsolateral prefrontal cortex, rostral anterior cingulate cortex, amygdala, hippocampus, periaqueductal gray, rostral ventromedial medulla, and medial thalamus, the neural substrates and their connections that mediate CPSP remain unclear. [(14)C]-Iodoantipyrine (IAP) uptake can be measured to evaluate spontaneously active pain. It can be used to assess the activation of neural substrates that may be involved in CPSP in an animal model. The [(14)C]-IAP method in rats is less expensive to perform compared with other brain mapping techniques. The present [(14)C]-IAP protocol is used to measure the activation of neural substrates that are involved in CPSP that is induced by lesions of the ventral basal nucleus (VB) of the thalamus in a rodent model.

A [14C]iodoantipyrine study of inter-regional correlations of neural substrates following central post-stroke pain in rats

BACKGROUND

Central pain syndrome is characterized by a combination of abnormal pain sensations, and pain medications often provide little or no relief. Accumulating animal and clinical studies have shown that impairments of the spinothalamic tract (STT) and thalamocingulate pathway causes somatosensory dysfunction in central post-stroke pain (CPSP), but the involvement of other neuronal circuitries in CPSP has not yet been systematically examined. The aim of the present study was to evaluate changes in brain activity and neuronal circuitry using [(14)C]iodoantipyrine (IAP) in an animal model of CPSP.

RESULTS

Rats were subjected to lateral thalamic hemorrhage to investigate the characteristics of CPSP. Thermal and mechanical hyperalgesia developed in rats that were subjected to thalamic hemorrhagic lesion. The medial prefrontal cortex (mPFC), anterior cingulate cortex (ACC), striatum, thalamus, hypothalamus, and amygdala were more active in the CPSP group compared with rats that were not subjected to lateral thalamic hemorrhage. The inter-regional correlation analysis showed that regional cerebral blood flow in the mPFC was highly correlated with the amygdala in the right brain, and the right brain showed complex connections among subregions of the ACC. Rats with CPSP exhibited strong activation of the thalamocingulate and mPFC-amygdala pathways.

CONCLUSIONS

These results corroborate previous findings that the STT and thalamocingulate pathway are involved in the pathophysiological mechanisms of CPSP symptoms. The mPFC, amygdala, and periaqueductal gray emerged as having important correlations in pain processing in CPSP. The present data provide a basis for a neural correlation hypothesis of CPSP, with implications for CPSP treatment.

Cingulate seizure-like activity reveals neuronal avalanche regulated by network excitability and thalamic inputs

BACKGROUND

Cortical neurons display network-level dynamics with unique spatiotemporal patterns that construct the backbone of processing information signals and contribute to higher functions. Recent years have seen a wealth of research on the characteristics of neuronal networks that are sufficient conditions to activate or cease network functions. Local field potentials (LFPs) exhibit a scale-free and unique event size distribution (i.e., a neuronal avalanche) that has been proven in the cortex across species, including mice, rats, and humans, and may be used as an index of cortical excitability. In the present study, we induced seizure activity in the anterior cingulate cortex (ACC) with medial thalamic inputs and evaluated the impact of cortical excitability and thalamic inputs on network-level dynamics. We measured LFPs from multi-electrode recordings in mouse cortical slices and isoflurane-anesthetized rats.

RESULTS

The ACC activity exhibited a neuronal avalanche with regard to avalanche size distribution, and the slope of the power-law distribution of the neuronal avalanche reflected network excitability in vitro and in vivo. We found that the slope of the neuronal avalanche in seizure-like activity significantly correlated with cortical excitability induced by γ-aminobutyric acid system manipulation. The thalamic inputs desynchronized cingulate seizures and affected the level of cortical excitability, the modulation of which could be determined by the slope of the avalanche size.

CONCLUSIONS

We propose that the neuronal avalanche may be a tool for analyzing cortical activity through LFPs to determine alterations in network dynamics.

Current source density analysis of laser heat-evoked intra-cortical field potentials in the primary somatosensory cortex of rats

The role of the primary somatosensory cortex in thermal pain perception has been established. However, the cortical circuitry that mediates the thermo-nociceptive information processing has not been elucidated. The aim of present study was to investigate the intracortical synaptic currents in primary somatosensory cortex evoked by short laser pulses and to determine their transmission pathway. Noxious CO2 laser pulse stimuli or innocuous electrical and mechanical stimuli were delivered to the hind paw of halothane-anesthetized rats. Multi-channel field potentials were recorded simultaneously in primary somatosensory cortex and laminar-specific transmembrane currents were analyzed using a current source density method. A distinct spatial-temporal pattern of intra-cortical sink source currents was evoked by laser pulse stimuli. The amplitude of the early component was graded by laser energy output and influenced by contralateral signals, whereas the late components were not intensity-dependent and exhibited bilateral excitation. Intra-cortical current flows revealed that synaptic activation occurred initially at layers IV and VI separately and then was relayed transynaptically to the more superficial and the deeper layers. Latency, amplitude and intracortical distributions of the activated intra-cortical currents evoked by noxious stimuli differed significantly from those evoked by innocuous stimuli. Conduction velocity data together with the results of tetrodotoxin, capsaicin and morphine treatments indicated that the early and late components were mediated separately by A-delta and C fibers. Our results suggest that large and small diameter thermal nociceptive afferents generated laminar-specific intracortical synaptic currents in primary somatosensory cortex and that these excitatory synaptic currents were conveyed separately by lateral and medial thalamic nuclei.

Differential projections from the mediodorsal and centrolateral thalamic nuclei to the frontal cortex in rats

The aim of the present study was to investigate afferent projections from the medial thalamic nuclei (MT) to the frontal cortical areas using a single small iontophoretic injection of biotinylated dextran amine (BDA) and analysis of the anterogradely labeled fibers and varicosities. Projections from the mediodorsal (MD) nuclei were found primarily and extensively in the anterior cingulate cortex (ACC), whereas those from the centrolateral (CL) thalamic nucleus were found in the frontal motor cortex. The density of terminals in the ACC was high in layers II and III and sparse in layer I. The majority of projected fibers from the CL were found at a high density in layer V, with a moderate density in the superficial layers. The differential projection patterns were topographically organized in the medial prefrontal cortex and sensory motor cortex. These findings support the results of our previous electrophysiological studies suggesting that neurons in the medial thalamic nuclei relay nociceptive information to the limbic or sensory motor cortical areas. The present results agree with the current notion that the medial thalamo-frontal cortical network circuitry plays an important role in processing the emotional aspect of nociception.

A fMRI study of brain activations during non-noxious and noxious electrical stimulation of the sciatic nerve of rats

An acute pain animal model for fMRI study would provide useful spatial and temporal information for studying the supraspinal nociceptive neuronal responses. The aim of the present study was to investigate whether the nociceptive responses in different brain areas can be differentiated by using functional magnetic resonance imaging (fMRI) in anesthetized rats. Functional changes in brain regions activated by noxious or non-noxious stimuli of the sciatic nerve were investigated using fMRI in a 4.7 T MR system in alpha-chloralose anaesthetized rats. To determine the electrical intensity for noxious and non-noxious stimuli, compound action potential recording was employed to reveal the type of fibers activated by graded electrical stimulation of sciatic nerve. It showed that innocuous A-beta fibers were excited by two times the muscle twitch threshold and nociceptive A-delta and C fibers were recruited and excited by 10 and 20 times threshold, respectively. A series of four-slice gradient echo images were acquired during innocuous (two times threshold) and noxious (10 and 20 times threshold) stimuli in a 4.7 T MR system. Contralateral somatosensory cortex was the most prominent brain area activated by innocuous stimuli. Both signal intensity and activated areas were significantly increased in the somatosensory cortex, cingulate cortex, medial thalamus and hypothalamus during noxious stimuli. These four brain areas activated by noxious stimuli were significantly suppressed by prior intravenous injection of morphine (5 mg/kg). The present findings demonstrated that the difference of the innocuous and nociceptive responses in the brain could be detected and localized by an in vivo spatial map using fMRI. Results suggest that fMRI may be an invaluable tool for studying pain in anesthetized animals.

Evoked responses of the anterior cingulate cortex to stimulation of the medial thalamus

In the present study we characterized the field potentials in the anterior cingulate cortex (ACC) evoked by electrical stimulation of the medial thalamus (MT), and elucidated the synaptic organization of the ACC. Male Sprague Dawley rats were maintained in general anesthesia by alpha-chloralose (50 mg/kg, i.v.). Tungsten micro-electrodes were used for electric stimulation and recordings. The field potentials and multiple unit activities in the ACC were evoked by electric stimulation of the MT where the nociceptive responses were identified. A MT-evoked positive-negative potential was recorded on the medial frontal surface. The polarity of the surface negative potential was reversed between 0.5 to 1.0 mm in the deep layer of the ACC. Maximum evoked negative potential appeared at about 4 mm anterior to the bregma and 1 mm lateral to the midline. The maximum evoked positive potential occurred at about 3 mm anterior to the bregma and 1 mm lateral to the midline. The evoked multiple unit activities coincided with the deep negative field potential at a latency between 16 ms and 24 ms at a depth between 0.5 mm and 1.5 mm in the ACC. These electrophysiological findings confirmed that nociceptive information in the MT is transmitted to the ACC and trans-synaptically activates deeper and more superficial layers of cortical neurons.

Electrophysiological study of the connection between medial thalamus and anterior cingulate cortex in the rat

We characterized the neuronal properties of the anterior cingulate cortex (ACC) evoked by electrical stimulation of the medial thalamus (MT). MT stimulation sites were found by their neuronal responses to noxious stimuli. Of 487 units identified histologically in the rat ACC, 94% were activated trans-synaptically at different areas of the ACC. Six percent of MT-evoked ACC units were activated antidromically and all of these units projected to a specific nucleus of MT. We suggest that MT nuclei mediate different aspects of nociceptive information to specific ACC areas, and that nociceptive information in the MT is modulated reciprocally by activities from the ACC.

The suppressive effect of electrical stimulation on nociceptive responses in the rat

BACKGROUND AND PURPOSE

The aim of this investigation was to study the effect of electrical stimulation on nociceptive responses within the lumbar levels of the rat spinal cord.

METHODS

A single high-energy thermal pulse produced by a surgical laser stimulator (5 W, 30 milliseconds) was applied on the plantar surface of the hind paws of male Sprague-Dawley rats. The spinal cord field potential evoked by the laser pulse was used as an indicator of thermosensitive nociceptive responses. Low-intensity single stimulation, high-intensity single stimulation, low-intensity train stimulation, and high-intensity train stimulation were applied on the common peroneal nerve with protected cuff electrodes in different trials.

RESULTS

Neither low-intensity nor high-intensity single stimulation suppressed field potentials. In contrast, low-intensity train stimulation elicited partial inhibition of field potentials. Furthermore, high-intensity train stimulation elicited biphasic inhibition at a wider range of intervals lasting for 20 seconds.

CONCLUSION AND DISCUSSION

The results demonstrate that two modes of train electrical stimulation can produce two patterns of fast-onset (within milliseconds), short-duration (within 20 seconds) inhibition of field potentials in the spinal cord. These results provide evidence that noxious heat-related impulses are modulated by the presence of specific electrical stimulation. The clinical application of transcutaneous electrical nerve stimulation to block pain is supported.

Intrathecally administered c-fos antisense oligodeoxynucleotide decreases formalin-induced nociceptive behavior in adult rats

c-fos antisense strategy was applied as a pharmacological approach to characterize its dose-dependent role and reversibility in the reduction of formalin-induced hyperalgesia. Nociceptive behavioral responses (weighted score, flinching response, licking/biting) following formalin (50 microl 5%) injection were assessed in adult Wistar rats receiving different doses (50 nM, 250 nM) of intrathecally administered c-fos antisense oligodeoxynucleotides at different times prior to formalin injections. The treatments dose dependently decreased both Fos immunoreactivity expression in dorsal horn of rat lumbar spinal cord and all nociceptive measures in the tonic phase of the formalin test. c-Fos correlated well with weighted pain score and/or flinching responses, but not with licking/biting behavior. With the exception of a 48-120 h period required for licking/biting behavior to be restored to its normal status, the suppressive effect on c-fos expression and other nociceptive behaviors disappeared 48 h following c-fos antisense oligodeoxynucleotide treatment. The results suggest a pharmacological potential of c-fos antisense oligodeoxynucleotides in the central nervous system to block immediate-early genes and their resulting physiological consequence following noxious stimulus.

Protein synthesis inhibitor cycloheximide dose-dependently decreases formalin-induced c-Fos protein and behavioral hyperalgesia in rats

We had previously demonstrated that c-fos antisense oligodeoxynucleotides dose-dependently suppressed formalin-induced c-Fos protein and behavioral hyperalgesia. To test whether de novo protein synthesis is required for the development of persistent pain after peripheral inflammation, we observed formalin-induced spinal c-Fos protein and nociceptive behaviors following pretreatment with cycloheximide, a protein synthesis inhibitor. Cycloheximide dose-dependently inhibited formalin-induced spinal c-Fos protein and tonic nociceptive responses. The possible non-specific effects other than protein synthesis inhibition on nociceptive behavior were carefully discussed and excluded. These results provide further support to the hypothesis that de novo protein synthesis is essential for the development of behavioral hyperalgesia.

Nitrous oxide or halothane, or both, fail to suppress c-fos expression in rat spinal cord dorsal horn neurones after subcutaneous formalin

In rats injected s.c. with formalin, behavioural correlates of the amount and pattern of Fos-like immunoreactivity (Fos-Ll) (molecular responses to pain) were studied to test if early phase treatment with 75% nitrous oxide or 2% halothane, or both, suppressed subsequent spinal sensitization. Rats were allocated to four treatment groups: (1) 100% oxygen (control, n = 15), (2) 75% nitrous oxide (0.5 MAC, n = 12), (3) 2% halothane (1 MAC, n = 12), and (4) 75% nitrous oxide with 2% halothane (1.5 MAC, n = 18) for 20 min. Each rat then received a s.c. injection of 1% formalin 50 microliters into the left hindpaw and anaesthesia was maintained for another 5 min (early phase). A fifth group of rats receiving fentanyl 100 micrograms kg-1 (n = 12) 10 min before formalin injection were studied simultaneously as a positive control. Rats in all groups were killed 60 min after formalin injection and maximal counts of Fos-Ll labelled neurones in the dorsal horn of the rat spinal cord were compared according to laminar distribution. Formalin-induced behavioural hyperalgesia during the early phase was suppressed completely by fentanyl, 75% nitrous oxide, or 2% halothane, or both. The late phase response was attenuated by all four anaesthetic regimens within 20 min after injection, whereas behavioural scores for the nitrous oxide, halothane, or both, groups were nearly identical to the control 20 min later. Fentanyl suppressed the late phase response until 30 min after formalin injection but failed to reduce it thereafter. The numbers of Fos-Ll labelled neurones for groups given nitrous oxide, or halothane, or both, were identical to the control, whereas numbers for fentanyl were 47.2% less (P < 0.01). The decrease occurred predominantly in the neck of the dorsal horn (44.9% of control, P < 0.01) and also in the nucleus proprius and superficial laminae (54.4% and 56.2% of control, P < 0.05). In summary, we found that nitrous oxide, or halothane, or both, did not suppress subsequent spinal sensitization to noxious stimulation. This result supports the previous hypothesis that inhalation anaesthesia lacks pre-emptive analgesic action. Inhalation anaesthetic agents, unlike fentanyl, suppress the early and late phase response because of anaesthetic but not analgesic effects. Thus, we suggest that measuring the genetic product of c-fos proto-oncogene is a useful adjunct to pharmacological tests whenever behavioural hyperalgesia is questionable or unobtainable.

Analysis of nocifensive behavior induced in rats by CO2 laser pulse stimulation

To characterize nocifensive behavior, a laser beam was applied to the hind footpad of nonanesthetized and unrestrained rats and the reaction pattern was analyzed. Fifty-four rats were divided into nine groups of six animals, and each group was given one of nine combinations of laser stimuli: intensity of 4, 8 or 12 W and duration of 10, 30, or 50 ms. A single pulse was applied to a 0.13 cm2 area of right or left footpad and the trial was repeated 20 times with 3 min between trials. The behavior was videotaped and reviewed for a period of 2 min following each stimulation. It seemed to consist of eight discrete responses, and each response was scored for whether it occurred and for its summed duration per trial. The component responses and the behavior as a whole were characterized by their sensitivity in terms of the level of energy required to attain 50% of the maximum response, and their linear or quadratic trends with increasing stimulus energy. The most sensitive index of pain stimulation was the composite score, followed by foot jumping, foot elevation, body movements, licking, and then foot movements. As stimulus energy increased, rats exhibited a greater number of different responses and a greater frequency of each component response. The results suggest that a pool of hierarchically organized responses in the nocifensive motor system are recruited partially or wholly by nociceptive stimuli of varying intensity.

Dorsal column inhibition of nociceptive thalamic cells mediated by gamma-aminobutyric acid mechanisms in the cat

Cells in posterior parts of the cat thalamus were investigated. Responses in single units excited by electrical stimulation in the lateral funiculus (LF), the dorsal column nucleus (DCN) or the canine tooth pulp (TP) were analysed. All cells had a spontaneous resting activity which could be increased by extracellular iontophoretic application of DL-homocysteic acid (DLH) and decreased by gamma-aminobutyric acid (GABA). No effect on the spontaneous firing rate was observed following iontophoresis of the selective GABA-antagonists, picrotoxin (GABA-A receptor antagonist) or saclofen (GABA-B receptor antagonist). However, the decreased firing following GABA application was partially blocked by picrotoxin but not by saclofen. A phasic inhibition induced by DCN stimulation in nociceptive thalamic cells is indicated since simultaneous administration of picrotoxin increased the evoked response. This type of inhibitory mechanism could not be detected following LF or TP stimulation. The extracellular activity evoked by electrical stimulation of LF or TP was significantly depressed by preceding electrical stimulation in the DCN. This inhibition was reversed by simultaneous administration of picrotoxin, indicating an involvement of GABA-A receptors. The reversal of the DCN-induced depression of the late responses following LF stimulation occurred after application of saclofen. It is suggested that this effect is partly mediated via GABA-B receptors. Results from the present study indicate an interaction in the thalamus between presumed low-threshold (DCN) and presumed nociceptive afferents (LF and TP) similar to that previously described in the spinal cord.

Spinal antinociception mediated by a cocaine-sensitive dopaminergic supraspinal mechanism

The role of dopaminergic descending supraspinal processes in mediating the antinociceptive action of cocaine was studied in the rat using a combination of extracellular neuronal recording and behavioral techniques. Neurons in the superficial laminae (I-II) of the spinal dorsal horn with receptive fields on the tail were recorded in anesthetized rats using insulated metal microelectrodes. Stimulation of the receptive field with either high intensity transcutaneous electrical pulses or with an infrared CO2 laser beam produced a biphasic increase in dorsal horn unit discharge. Conduction velocity estimates indicated that the early discharge corresponded to activity in A delta whereas the late response corresponded to activity in C afferent fibers. Cumulative doses of cocaine (0.1-3.1 mg/kg i.v.) inhibited the late response to either electrical or laser stimulation in a dose-related manner. The early response to laser, but not electrical, stimulation was also suppressed by cocaine. Neurons in the spinal dorsal horn with receptive fields on the ipsilateral hindpaw were activated by natural noxious (pinch) or innocuous (tap) somatic stimulation. Cocaine selectively suppressed nociceptively evoked dorsal horn unit discharge. This antinociceptive effect was dose-related (0.3-3.1 mg/kg, i.v.) and antagonized by eticlopride (0.05-0.1 mg/kg, i.v.), a selective D2 dopamine receptor blocker. The same doses of cocaine failed to inhibit the responses of dorsal horn neurons to low threshold innocuous stimulation. Complete thoracic spinal cord transection eliminated the antinociceptive effect of cocaine on dorsal horn neurons and also eliminated the cocaine-induced attenuation of the tail-flick reflex.(ABSTRACT TRUNCATED AT 250 WORDS)

Tooth pulp deafferentation is not associated with changes in primary afferent depolarization of facial afferent endings in the brain stem

Previous studies have demonstrated that tooth pulp deafferentation is associated with statistically significant alterations in the low-threshold facial mechanoreceptive field properties of brain stem neurons in trigeminal (V) subnucleus oralis. A loss of spinal afferent-induced presynaptic inhibition as a consequence of a decrease in primary afferent depolarization (PAD) following spinal nerve deafferentation has been invoked as a mechanism underlying deafferentation-induced somatosensory neuroplasticity. Therefore, this study was initiated to determine if these pulp deafferentation-induced neuroplastic changes could be accounted for by an alteration in PAD of low-threshold facial afferent endings in subnucleus oralis of anesthetized rats. In control (unoperated) rats (n = 7) and rats (n = 7) that had undergone mandibular pulp deafferentation 6-10 days previously, antidromic compound action potentials evoked by test stimulation in V subnucleus oralis were recorded in branches of the infraorbital (IO) and supraorbital (SO) nerves, and conditioning stimuli were applied to some of the same nerves. PAD of the afferent endings in oralis of these nerve branches was documented in all animals, and there was no significant difference between the two groups in the incidence or any of the other features of PAD. The features of the PAD were consistent with those described in several previous studies of normal animals. These findings indicate that the reported deafferentation-induced loss of spinal presynaptic regulatory mechanisms cannot be entrapolated to all forms of deafferentation injury and that the mechanoreceptive field changes that can occur in central V somatosensory neurons as a result of tooth pulp deafferentation may not reflect an alteration in PAD.

Thalamic nociceptive mechanisms in cats, influenced by central conditioning stimuli

Field potentials and single cell activity evoked by tooth pulp (TP) stimulation were studied in the ventrobasal (VB) complex of the cat. The experiments were performed using a conditioning-test paradigm. Evoked cell activity or field potentials following TP stimulation was used as a test. Conditioning stimulus was given to different regions of the thalamic central lateral nucleus (CL). Conditioning electrical stimulation in medial (ML 2.8-3.6 mm) parts of CL induced a depression of the TP evoked response in 10 cells. Stimulation sites in lateral CL (ML 3.6-4.2 mm) induced facilitation in eight cells and decreased activity in seven cells. Tooth pulp evoked field potentials in thalamus were facilitated by a preceding stimulation in lateral CL. Cells in the lateral parts of CL are suggested to induce an increased activity in cells in the VB complex which mediate nociceptive information. This effect is suggested to be mediated via a CL induced disinhibition at a reticular thalamic (RE) or at a VB complex level. The medial parts of CL seem to give a traditional feedback inhibition on VB cells. Such an effect is also suggested to be mediated via the RE complex. The importance of these findings are discussed with relation to changes in the thalamus that may occur following long lasting nociceptive stimulation.

Neurophysiological, pharmacological and behavioral evidence for medial thalamic mediation of cocaine-induced dopaminergic analgesia

These studies examined the effects of cocaine on thalamic neurons that respond maximally either to noxious or to innocuous somatic stimulation. Cocaine attenuated high intensity electrically-evoked nociceptive responses of all 25 units studied in the parafascicular and central lateral nuclei of the medial thalamus. A dose of 1 mg/kg intravenously (i.v.) suppressed medial thalamic unit discharge evoked by both noxious somatic stimulation (49.4 +/- 8.7% of control response) and spinal cord stimulation (76.2 +/- 6.6% of control response). The effect of cocaine on unit responses to noxious somatic stimulation was dose-related in the range of 0.3-3.5 mg/kg i.v. and was attenuated by eticlopride, a D-2 selective dopamine receptor antagonist. Morphine also suppressed noxious somatic evoked responses of medial thalamic units in a dose-dependent manner. Units in the lateral (ventrobasal) thalamus (n = 4) that responded only to innocuous stimuli were not affected by cocaine at doses up to 3.5 mg/kg i.v. Ibotenic acid lesions in the parafascicular nucleus of the medial thalamus attenuated the analgesic effect of cocaine in the formalin test. These results suggest that both cocaine and the parafascicular nucleus interact with dopaminergic mechanisms that attenuate nociceptive spinal projections to the medial thalamus.

Effects of sympathetic stimulation on C-fibre response after peripheral nerve compression: an experimental study in the rabbit common peroneal nerve

Non-myelinated C-fibre responses during sympathetic trunk stimulation were studied in rabbit common peroneal nerve 2 weeks after the nerve had been subjected to compression at 400 mmHg for 30 min. Our previous studies have demonstrated that during sympathetic trunk stimulation the compound action potential of uninjured somatic C-fibres is characterized by a reduced amplitude and an increased latency. In the present study, nerve compression changed the C-fibre response to sympathetic stimulation. Three out of eight nerves reacted to nerve compression by increased C-fibre compound action potential amplitude in response to sympathetic stimulation. In three other rabbits with compressed nerves the C-fibre action potential amplitude was unchanged, and in the remaining two rabbits the action potential amplitude was decreased during sympathetic stimulation. The action potential latency increased in all tested compressed C-fibres. The phenomenon of increased C-fibre amplitude during sympathetic activation has not been observed in uninjured nerves. As in uninjured nerves, noradrenaline infusion produced an increased C-fibre action potential amplitude and latency in six animals. Sympathetic stimulation did not affect the A-fibre response. These results indicate that sympathetic activity influences the conduction properties in C-fibres of somatic origin and that the response can be changed after a nerve injury. The findings may be of importance for the understanding of pain aggravation in different types of nerve injuries during increased sympathetic activity.

Properties of single neurons in the cat midsuprasylvian gyrus

Responses of cells in the midsuprasylvian gyrus (MSSG) of cats were investigated following electrical stimulation of the central lateral nucleus (CL) of the thalamus and tooth pulp, low-threshold cutaneous or visual afferents. Electrical stimulation in CL induced excitation in many cells located in cortical areas 5 and 7. Cells in these areas also received input from somato-sensory and visual afferents. Cells in MSSG showed a wide convergence from tooth pulp, low-threshold cutaneous afferents and from the CL. The majority of wide convergent cells in area 5 were found in layers IV and V, while cells excited by CL and tooth pulp were found in layers II and III. Similarities were found between CL and tooth pulp evoked responses with regard to the excitation-inhibition pattern. The excitation evoked from CL and tooth pulp was less often followed by a hyperpolarizing potential compared to that seen after low-threshold lip, paw and visual afferent stimulation. Stimulation sites in the lateral parts of CL-evoked responses with the shortest latencies in area 5. In this part of the cortex, short latency synaptic potentials were found in cells in superficial layers. In the same area, synaptic potentials of short latency were also evoked by electrical stimulation of tooth pulp, lip and paw. Light-flash stimulation evoked responses with the shortest latencies in area 7. The results of this study demonstrate that putative nociceptive information reaches the parietal association cortex and that part of this input may be relayed via CL. We suggest that the excitatory influences of nociceptive and CL stimulation is related to behavioral arousal and attention mechanisms.

Field potential analysis of the cortical projection of the central lateral nucleus in the cat

A previous field potential study has indicated a monosynaptic projection of fibres from the central lateral nucleus (CL) to the mid-suprasylvian gyrus (MSSG). The present study, which is based on an analysis of current source density (CSD), aims to investigate further the sites of major localized synaptic activities in different layers of the MSSG after electrical stimulation in the CL. An initial positive surface potential was evoked in the MSSG with a latency of 3-5 ms and followed by a large negative potential with a peak latency of 8-15 ms. The initial positivity was only found in the rostral part of the MSSG, which corresponds to area 5. The positivity reversed in deeper layers. The CSD analysis showed a sink at a depth from 650 to 1050 microns. A corresponding source was found more superficially at 400-600 microns. This indicates that CL fibres have an excitatory synaptic termination on the soma or proximal dendrites of neurons in layers III and IV. The surface negative potential reversed at the border between layers II and III, suggesting a superficial CL projection. The CSD analysis of potentials in superficial layers showed a sink appearing between the pial surface and a depth of 350 microns, and a source lying in layers below. This indicates a depolarization of apical dendrites of cells in layers II and III. The superficial sink appeared in a large part of the MSSG. Application of a solution of 0.5% gamma-aminobutyric acid (GABA) on the surface of the cortex blocked the superficial sink and source and revealed a prominent sink current in layers III and IV in agreement with a deep termination of CL fibres. Application of a solution of 25 mM DL-2-amino-5-phosphono-valeric acid (APV) abolished CL-evoked cortical responses indicating that N-methyl-D-aspartate (NMDA) receptors are involved in the cortical activation. The CSD analysis confirms that CL has a wide superficial projection to the MSSG. It also confirms a deeper monosynaptic projection from CL to area 5.

Sympathetic and noradrenaline effects on C-fibre transmission: single-unit analysis

Single afferent unmyelinated fibres were dissected from the otherwise intact sural nerve in anesthetized rabbits. The sympathetic trunk could be stimulated via electrodes implanted through the abdomen. The response in single C fibres was elicited by electrical stimulation in the cutaneous innervation area of the fibre. Sympathetic stimulation (8 Hz, 1 ms pulses, 5 mA for 60 s) increased the latency in all tested C fibres (2.0% +/- 0.8%, mean +/- SD, n = 17). In 48% of the units the amplitude of the action potential decreased (26.4% +/- 12.3%) during sympathetic stimulation. Infusion of noradrenaline (5 micrograms min-1) increased (7.7% +/- 4.1%) the latency in all units and increased (36.9% +/- 29.8%) the amplitude of 25% of the units. The effects of sympathetic stimulation and noradrenaline infusion were blocked by pre-treatment with phentolamine (3 mg kg-1 i.v.). The results suggest that catecholamines change the membrane properties of unmyelinated fibres.

Effects of sympathetic stimulation on C-fibre responses in rabbit

Unmyelinated C-fibre responses to electrical stimulation were recorded in common peroneal, sural and tibial nerves of rabbits. Three distinct C elevations, here called C1, C2 and C3, were recorded. C2 is probably of somatic origin because it was depressed due to collision by peripheral stimulation of cutaneous receptors. The conduction velocity of C3 corresponded to that of sympathetic post-ganglionic fibres. During sympathetic trunk stimulation the A-fibre responses were not significantly changed while C responses, especially C2, were reduced in amplitude and slightly delayed. The C-fibre responses were also influenced by intra-arterial infusion of noradrenaline. In most cases, the latency of the response was increased. The effect of sympathetic stimulation was completely blocked by hexamethonium, and partly blocked by phentolamine, an adrenergic alpha-receptor blocking agent which also blocked the effect of noradrenaline. The findings suggest that there are adrenergic receptors distributed along unmyelinated somatic afferent fibres. Sympathetic activity may release noradrenaline in the peripheral nerve, resulting in changed conductive properties in unmyelinated fibres transmitting sensory information.

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

Cortical 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.

Effects of nerve compression or ischaemia on conduction properties of myelinated and non-myelinated nerve fibres. An experimental study in the rabbit common peroneal nerve

Compound action potentials of both myelinated (A) and non-myelinated (C) fibres in the common peroneal nerve of rabbits were studied during and after acute, graded compression of the nerve at 200 or 400 mmHg applied for 2 h or during ischaemia created by nitrogen inhalation or aortic occlusion. Compression of the nerve at 200 mmHg blocked the AI component (large myelinated fibres) after about 23 min, while compression at 400 mmHg shortened this time to 11 min. The A2 component (thinner myelinated fibres) had a lower conduction velocity and a higher resistance to compression. There was just a slight decrease in conduction velocity of the non-myelinated fibres when the nerves were compressed at 200 mmHg for 2 h. However, compression at 400 mmHg for 2 h induced a marked deterioration of amplitude and conduction velocity of the C-fibres. There was an incomplete restitution of function of A- and C-fibres during 2 h of recovery. The thinner myelinated fibres were more susceptible to deprivation of oxygen than the thicker ones, while non-myelinated fibres differed in response according to method of ischaemia induction. It is concluded that non-myelinated fibres are very resistant to compression and a very high pressure (greater than 400 mmHg) is needed to affect these fibres.

Isometric muscle tension generated by masseter stimulation after prolonged alteration of the consistency of the diet fed to growing rats

The forces developed by the masseter muscle and the diet-related changes were studied in situ. The active tetanic tension was measured indirectly by recording the forces applied to the mandible in four different predetermined interincisal distances. The highest tension developed in all the experimental groups was with an interincisal distance of 11 mm. The tetanic tension was significantly lower in rats fed a soft diet than in those fed a normal diet. This difference might be due to the smaller muscle fibres in rats with decreased functional demand. The changes in the masticatory muscles may influence the tension applied to the facial skeleton and cause the alteration in the craniofacial growth previously found in rats fed a soft diet.

Course and mode of action of descending system conveying nucleus raphe magnus induced inhibition of flexion reflex in rats

In lightly anaesthetized rats, the latencies and magnitudes of heat-evoked withdrawal reflexes from the hindlimb were measured electromyographically. Low-intensity (20-50 microA) stimulation of the nucleus raphe magnus (NRM) resulted in an inhibition of the flexion reflex (commonly referred to as analgesia) as evidenced by increased latency and decreased amplitude. The effect lasted for several minutes after the stimulation was terminated. Following lesions of the dorsolateral funiculus (DLF) at the neural thoracic levels 7-8, baseline latencies were reduced and the effect of the NRM stimulation was abolished. Lesions of the DLF at lumbar level 1 resulted in unaltered baseline latencies with persistence of the inhibitory effect of NRM stimulation. The results of the present experiment show that pathways exerting a tonic inhibition of the withdrawal reflex, and mediating the effect of electrical stimulation from the NRM, descend in the DLF at the thoracic level of the spinal cord. At the lumbar level, there is a shift away from the DLF. The antinociceptive effect of 20 microA NRM stimulation was partly reduced by pretreatment with the serotonin synthesis inhibitor para-chlorophenylalanine (PCPA) or the opiate naloxone (1 mg kg-1 i.v.). In animals pretreated with PCPA, naloxone (1 mg kg-1 i.v.) completely abolished the analgesic effect of the stimulation. Thus, both serotonergic and opioid systems may be implicated in mediating the analgesia. With 50 microA stimulation the same treatment only partly attenuated the NRM-induced analgesia, indicating an additional non-serotonergic and non-opioid mechanism which requires a higher current intensity for its activation.

Differential stress effects on responses to noxious stimuli as measured by tail-flick latency and squeak threshold in rats

In the present study tail-flick latency (TFL) and squeak threshold (ST) were investigated in different environmental conditions in normotensive Wistar Kyoto rats (WKY) and spontaneously hypertensive rats (SHR). The TFL did not vary significantly during the testing period (120 min). The ST increased gradually and significantly with time when measured in rats restrained in the TFL test tube. Freely moving rats transferred to the TFL test tube showed a marked decrease of the ST. During the stay in the TFL test tube, the ST value increased gradually. Also after transfer from the test tube to the home cage the threshold increased to the same level as freely moving rats. There were no significant differences in these effects between WKY and SHR. Naloxone (1 mg kg-1 i.v.) decreased the ST in both freely moving and restrained animals of either strain. Intravenous injection of morphine (mg kg-1) gave a pronounced threshold increase of ST in both groups. It is suggested that animals restrained in the TFL test tube become hyperalgesic. The adaptation to stress is reflected in the ST but not in the TFL. Squeak threshold appears to be influenced by a tonically active endorphin system.

Circulatory events following spontaneous muscle exercise in normotensive and hypertensive rats

In previous studies we have shown that spontaneously hypertensive rats (SHR) develop a running behaviour and, secondary to the running behaviour, develop an endorphin-mediated analgesic effect. In the present study the role of the central endorphin system in the cardiovascular responses to spontaneous exercise in normotensive Wistar Kyoto rats (WKY) and SHR was investigated. The experimental design allowed us to record mean arterial pressure (MAP) and heart rate (HR) continuously for more than 1 week without interfering with the daily activities of the animals. They were active in running wheels during the dark period (19.00-07.00 h) and the activity was accompanied by a marked rise in HR. In SHR, a clear depression of blood pressure lasting for about for about 50 min was noted following each running period. The MAP during the post-running depression was 131.4 +/- 1.6 mmHg which was significantly lower than the pre-running control value (145.2 +/- 2.3 mmHg, P less than 0.01). In contrast, MAP in the post-running period in WKY was not significantly different from the pre-running values. In addition, the depression period of SHR had a mean post-running length of 49.7 +/- 3.4 min, which is significantly longer than in the WKYs (37.8 +/- 3.5 min, P less than 0.05). In control rats, naloxone infusion had no effect on blood pressure but a marked bradycardia was observed. In nine running SHR receiving a naloxone infusion, their MAP during the depression period was not different from the control pressure. Our study indicates that endorphin systems are involved in the regulating of blood pressure and HR during muscle exercise in SHR. These systems trigger the transient depression of blood pressure observed immediately after a running period in the SHR.

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.

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

Evoked 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.

Effects of naloxone on dental pain threshold following muscle exercise and low frequency transcutaneous nerve stimulation: a comparative study in man

Previous studies have shown that muscle exercise and low frequency transcutaneous nerve stimulation (TNS) give rise to an analgesic effect in humans and animals. Endorphin has been proposed to mediate this analgesia. In this investigation, the effect of muscle exercise and low frequency TNS, on dental pain thresholds was studied and the possible involvement of endorphinergic mechanisms was investigated using naloxone as an antagonist. Dental pain thresholds were measured in 11 volunteers following leg or arm exercise and after low frequency TNS of the hands or face. After exercise (20 min) or stimulation (30 min) either 0.8 mg naloxone (2 ml) or saline (2 ml) was injected i.v. in a double-blind fashion. Pain thresholds were measured repetitively before and after exercise or stimulation. Both leg and arm exercise increased pain threshold. Stimulation of the hands also increased pain threshold, but less than arm exercise. A marked increase in pain threshold was seen after face stimulation. These changes in pain threshold were unaffected following injections of either naloxone or saline, except for an early and short-lasting reduction when naloxone was injected following arm exercise. The increases in pain threshold following muscle exercise and after low frequency TNS, showed similarities suggesting that a common mechanism might be involved. The pain threshold increase after arm exercise could only be partially mediated by endorphinergic mechanisms.

Spontaneous running in wheels. A microprocessor assisted method for measuring physiological parameters during exercise in rodents

When examining physiological parameters during exercise in rodents most techniques use forced exercise on treadmills or forced swimming which undoubtedly will employ stress to the animals. This paper describes a convenient method to measure physiological parameters during spontaneous exercise in rodents. Hamsters and rats develop a spontaneous running pattern in wheels. The hamsters are reaching maximal activity almost immediately. In contrast, rats increase their running activity with time and need about 4 weeks to reach an average of about 7 km/day. The amount of running is measured by a microprocessor system. With this technique pain threshold, blood pressure and heart rate for example could be measured for extended periods of time, without changing the environment of the animals.

Circulatory depression following low frequency stimulation of the sciatic nerve in anesthetized rats

Earlier experiments have shown that afferent electrical stimulation of the sciatic nerve for 30 min induces a long-lasting post-stimulatory endorphin-dependent decrease in blood pressure in awake spontaneously hypertensive rats (SHR). In the present study we have examined whether this depressor response can be observed also in anesthetized SHR. The sciatic nerve was stimulated for 30 min with low-frequent (3 Hz) trains of impulses and the changes in blood pressure, heart rate and renal nerve activity were observed during the stimulation and in the post-stimulatory period. Animals anesthetized with Nembutal, Althesin and N2O did not show any post-stimulatory depression. In contrast, during chloralose anesthesia combined with muscle paralysis with Flaxedil, sciatic nerve stimulation induced a long-lasting post-stimulatory decrease in blood pressure due to central inhibition of sympathetic activity. The decrease in blood pressure could be prevented by naloxone and was therefore likely to be mediated via activation of central endorphin systems.

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.