Effects of cardiac vagal afferent electrostimulation on the responses of trigeminal and trigeminothalamic neurons to noxious orofacial stimulation.

We have previously reported that electrical stimulation of cardiac vagal afferents produces an inhibition of the feline's digastric reflex evoked by tooth-pulp stimulation. In the present study, we evaluated whether cardiac vagal afferent stimulation (CVAS) alters the responses of trigeminal sensory neurons to noxious orofacial stimulation in alpha-chloralose-anesthetized cats. A total of 37 trigeminal and trigeminothalamic neurons were recorded from trigeminal nucleus caudalis and trigeminal nucleus oralis. Thirty-five of these 37 neurons were classified as wide-dynamic-range (WDR) neurons because they had cutaneous receptive fields and responded to both noxious heat and non-noxious tactile stimuli. The effects of continuous CVAS (5 Hz, 3 msec, 2 mA) on heat-evoked responses (6 sec 50 degrees C heat pulse) were examined on 32 WDR neurons. CVAS inhibited (21 of 32 neurons), facilitated (5 of 32 neurons) or did not affect neuronal responses (6 of 32 neurons) to noxious heat. The effects of CVAS on heat-evoked responses of trigeminal and trigeminothalamic neurons were equivalent. The effects of intermittent CVAS (7 pulses at 333 Hz, 5 mA, delivered 200 msec prior to the test stimulus) on the responses to electrical test stimuli delivered to the center of a neuron's cutaneous receptive field or to the tooth pulp were also examined. Intermittent CVAS inhibited (15 of 24 neurons), facilitated (4 of 24 neurons) or had no effect (5 of 24 neurons) on A delta-mediated responses evoked by the electrical stimulation of facial skin. Intermittent CVAS either inhibited (8 of 12 neurons) or had no effect (4 of 12 neurons) on C-fiber-mediated responses evoked by electrical stimulation of the facial skin. Eight cells were recorded that received tooth-pulp input. Six of these 8 cells also received afferent input from facial skin, the remaining 2 cells responded only to tooth-pulp stimulation. Intermittent CVAS either inhibited (7 of 8 neurons) or had no effect (1 of 8 neurons) on A delta-mediated responses evoked by electrical stimulation of the tooth pulp. The modulatory actions of intermittent CVAS on trigeminal and trigeminothalamic neuronal responses to convergent afferent input from both skin and tooth pulp were equivalent. The outcomes of this study provide additional evidence that cardiopulmonary vagal afferent stimulation modulates neuronal responses to noxious stimulation and suggest that alterations in cardiopulmonary dynamics may modulate nociception.

Effects of naloxone and electroacupuncture treatment on plasma concentrations of LH in sheep.

Mature ewes were injected intravenously with the opioid antagonist naloxone (1.1 mg/kg) during the breeding season. Ewes with luteal phase concentrations of plasma progesterone responded with a significant (P less than 0.05) increase in plasma LH 14-23 min after naloxone injection. In contrast, non-luteal ewes with low plasma progesterone did not respond to injection of naloxone with an LH increase. Similar treatment of castrated males (wethers) with this dosage of naloxone failed to increase plasma LH. Electroacupuncture (EA) treatment of luteal phase ewes prevented the ability of exogenous naloxone to increase plasma LH. Treatment of wethers by EA decreased significantly (P less than 0.01) their high basal concentrations of plasma LH, but similar EA treatment of intact ewes did not change their low basal concentrations of LH.

Plasma cortisol and beta-endorphin in horses subjected to electro-acupuncture for cutaneous analgesia.

Electro-acupuncture (EA) treatment of horses to induce cutaneous analgesia also increased plasma concentrations of beta-endorphin (beta-EP) and cortisol. The magnitude of these increases did not relate consistently to the degree of EA-induced analgesia. Respiration and heart rates were also markedly increased during EA treatment. Intact female horses had higher packed cell volume and plasma beta-EP as well as lower plasma total protein than castrated male horses. Plasma cortisol, heart rate, and respiration rate did not differ significantly between sexes. None of the parameters measured before or during EA treatment provided an explanation for the differential cutaneous analgesia which depended on sex of subject and locus of stimulation as reported elsewhere.

Electroacupuncture analgesia in rats: naltrexone antagonism is dependent on previous exposure.

Inhibition of pain by low frequency electroacupuncture (EA) has been thought to be mediated by endogenous opioids. We reported in a previous paper, however, that naloxone (NAL) and naltrexone (NTX) either potentiated or had no effect on analgesia in EA-naive rats, independent of origin (American or Chinese), sex, weight, geographic location (the U.S.A. or China), or numerous variations of experimental methodology. In the present study, we hypothesized that the number of exposures to EA treatment may be responsible for the discrepant effect of opiate antagonists. We found, as previously demonstrated, analgesia in EA-naive rats was potentiated by NTX. After two pre-exposures to EA, however, NTX antagonized analgesia. These results indicate that, in rats: (1) pre-exposure is a key variable for opiate antagonists to produce antagonism or potentiation of analgesia; (2) environmental cues paired with the initial analgesic manipulation may be responsible for converting analgesia from non-opioid to opioid, as already demonstrated with classically conditioned and learned helplessness induced analgesia; and (3) EA analgesia in rats is a multidimensional manipulation which can be influenced by subtle environmental changes.

Electroacupuncture analgesia in naive rats: effects of brainstem and spinal cord lesions, and role of pituitary-adrenal axis.

Recent studies have shown that analgesia is potentiated by naltrexone (NTX) and naloxone (NAL) pretreatment in rats exposed for the first time to electroacupuncture (EA). In the present study, we have investigated the role of the pituitary-adrenal axis and of brainstem and spinal cord structures in EA analgesia and its potentiation by NTX. The pituitary and adrenal glands do not participate in the production of EA analgesia, but may produce a non-opioid substance which interferes with the development of EA analgesia. Spinalization or dorsolateral funiculi lesions blocked EA analgesia, and intrathecal NTX had no effect. These results indicate that supraspinal structures are necessary to produce and potentiate EA analgesia. Contrary to their critical role in morphine and other models of environmentally produced analgesia nucleus raphe alatus and raphe structures dorsal to it are not necessary for the development of EA analgesia. These structures, however, may contain opiate synapses on which NTX may act as an agonist to potentiate analgesia. The various components which appear to participate in the production of EA analgesia imply a complex circuit of pain modulation systems and indicate that an organism can adapt to distinct environmental conditions with versatile means to avoid pain.

Evaluation of vagal afferent modulation of the digastric reflex in cats.

In the present study, we have examined the relative ability of cervical, thoracic, cardiac and diaphragmatic vagal stimulation to modulate the digastric reflex produced by tooth-pulp stimulation in anesthetized cats. The right maxillary tooth pulp was stimulated and the digastric reflex was recorded from the right digastric muscle. Cervical vagal stimulation produced a biphasic effect on the digastric reflex. The reflex was facilitated at conditioning test intervals less than 20 ms and inhibited at conditioning test intervals between 100 ms and 500 ms. Cardiac and thoracic vagal stimulation did not significantly facilitate the digastric reflex but inhibited the reflex at conditioning test intervals between 50 ms and 500 ms with maximum inhibition observed at 200 ms. In contrast, diaphragmatic vagal stimulation produced a weaker inhibition of the digastric reflex. The relative ability of different vagal segments to inhibit the digastric reflex was: thoracic = cardiac = cervical greater than diaphragmatic. The inhibitory effects were not related to cardiovascular responses to vagal afferent stimulation. These findings suggest cardiopulmonary vagal afferents represent an important source of vagal afferents which modulate the digastric reflex in the cat.

A parametric analysis of the effects of cardiopulmonary vagal electrostimulation on the digastric reflex in cats.

The thoracic trunk and the cardiac branch of the vagus were stimulated electrically in chloralose-anesthetized cats. The experiments were conducted to determine the parameters of vagal afferent stimulation (VAS) capable of producing an inhibition of the digastric reflex (DGR), to assess the duration of this inhibition, and to test whether endogenous opioids mediate the inhibitory effects. In experiments using intermittent trains of pulses, the effects of pulse number (1, 2, 7 or 35 pulses), frequency (13, 66 or 333 Hz), intensity (0.1, 0.5, 1, 2, 3, 4 or 5 mA), and duration (1 or 3 ms) were evaluated. A 7 pulse train (3 mA) was sufficient to produce maximal inhibition (77 +/- 7%) of the tooth-pulp stimulation-evoked DGR regardless of the pulse duration or frequency. These effects were mediated by vagal afferents since stimulation of the central end produced as much inhibition as stimulation of the intact nerve. VAS also significantly reduced the DGR when elicited by tooth-pulp intensities at 1x -5x threshold. In experiments using 90 s of continuous VAS, 16 combinations of frequency and intensity yielded a threshold intensity for DGR inhibition between 0.1 and 0.5 mA and a threshold frequency at 2 Hz. Maximal DGR inhibition was produced at 5 Hz-0.5 mA by VAS. Reflex inhibition occurred within 10 s and outlasted VAS for longer than 60 s. Opiate-receptor blockade did not alter VAS inhibition of the DGR and, thus, opioids are not likely to mediate VAS-induced digastric inhibition.(ABSTRACT TRUNCATED AT 250 WORDS)

Electroacupuncture in rats: evidence for naloxone and naltrexone potentiation of analgesia.

Low frequency electroacupuncture (EA) analgesia has been thought to be mediated by endogenous opioids. Among other lines of evidence, it has been reported that EA stimulation delivered at 2 and 2-15 Hz in rats could be blocked or partially antagonized by naloxone (NAL) and naltrexone (NTX). In contrast, experiments in one of our laboratories (D.J.M.) showed that NAL did not inhibit 2 Hz, and even potentiated 125 Hz EA analgesia. In an attempt to resolve these discrepancies, we conducted joint experiments in the U.S.A. and in China using the methods which previously yielded NAL reversibility of EA analgesia. In no experiment did opiate antagonists block or reduce EA analgesia. On the contrary, we found that, in most experiments, NAL and NTX potentiated 2 and 2-15 Hz EA analgesia respectively. The potentiation occurred independently of laboratory methods, geographic location of the experiment, strain (Chinese or American), tail temperature, sex, and weight of rats. This potentiation suggests the existence of an opioid anti-analgesic system or that NAL and NTX acquired analgesic properties following EA. These results indicate that EA analgesia in rats is a variable phenomenon even when laboratory methods are rigorously replicated. The EA stimulation may activate multiple conflicting neural circuits which interact and ultimately modulate the analgesic outcome.

Hormonal mediation of the analgesia produced by food deprivation.

Research has demonstrated that a wide variety of environmental conditions are capable of producing analgesia. In the present experiment, the analgesia produced by 24 hr of food deprivation was examined following adrenalectomy, hypophysectomy, naltrexone (7 mg/kg), dexamethasone (0.4 mg/kg), or saline treatment. Results revealed that 24 hr of starvation elicited an analgesic response in the saline-treated and sham-operated groups. Naltrexone, dexamethasone, adrenalectomy, and hypophysectomy blocked the analgesia produced by food deprivation. The results demonstrate that 24 hr of food deprivation induced an opiate-mediated analgesic system that involves hormonal factors.

Opioid inhibition of kainic acid-induced scratching: mediation by mu and sigma but not delta and kappa receptors.

Scratching induced by intrathecal (IT) administration of kainic acid (0.5 nmol) to rats was inhibited by IT pretreatment with the selective mu agonists levorphanol (30 and 90 nmol), [D-Ala2,N-Met-Phe4,Gly5-ol]-enkephalin (DAGO, 0.4 and 1.1 nmol), or morphine (90 nmol), the mixed mu-delta agonist [D-Ala2,D-Leu5]-enkephalinamide (DADLE, 10 and 30 nmol), or the sigma/phenycyclidine (PCP) agonists dextrorphan (90 nmol) or (+)-N-allyl-N-normetazocine ([+]-NAM, 90 nmol). The kappa agonists dynorphin (1.1 nmol) and ethylketocyclazocine (EKC, 90 nmol) had no significant effect, nor did the selective delta agonist [D-Pen2,D-Pen5]-enkephalinamide (DPDPE, 90 nmol). The nonopioids (+)-3-(3-hydroxyphenyl)-N-(1-propyl)piperidine ([+]-3-PPP, 90 nmol) and PCP (90 nmol), selective for sigma and PCP sites, respectively, both antagonized kainic-induced scratching. Levorphanol- and DADLE-induced attenuation of scratching was partially antagonized by naltrexone. These findings suggest that opioid inhibition of kainic acid-induced scratching is mediated by classical mu receptors as well as sigma and PCP sites.

Electroacupuncture-induced analgesia in sheep: measurement of cutaneous pain thresholds and plasma concentrations of prolactin and beta-endorphin immunoreactivity.

Eleven male and 15 female sheep were subjected to electroacupuncture (EA) treatment, using 2 needle loci described in the Chinese veterinary literature as Yao Pang (lumbar region) and San Yang Lu (1 thoracic limb). Noninjurious cutaneous stimuli were applied, using a calibrated pin-prick probe, a clamp, and a contact heat (75 to 95 C) probe. Cutaneous pain thresholds (PT) were quantified in 7 body areas during control (no acupuncture needles and no electrostimulation) and EA experiments, before and after IV injection of naloxone. Using each animal as its own control, each EA experiment was classified as inducing either good or poor anagelsia on the basis of whole-body PT values. Plasma concentrations of immunoreactive beta-endorphin (beta E) and prolactin were quantified in sequential plasma samples collected at 9-minute intervals throughout all experiments. Electroacupuncture at each locus increased (P less than 0.01) PT (ie, caused cutaneous analgesia) in 6 of 7 body areas, and increased (P less than 0.05) plasma concentrations of immunoreactive beta E and prolactin. In EA experiments in which good analgesia was induced, plasma beta E was increased more (P less than 0.05) than in EA experiments in which poor analgesia was induced. This difference was more evident for the Yao Pang locus. Generally, plasma prolactin concentrations were increased more with good analgesia than with poor analgesia for the Yao Pang locus. Electroacupuncture stimulation of the San Yang Lu locus was associated with higher plasma beta E concentrations than that associated with the Yao Pang locus. Increases in plasma prolactin concentrations were comparable between loci.(ABSTRACT TRUNCATED AT 250 WORDS)

Production of cutaneous analgesia by electroacupuncture in horses: variations dependent on sex of subject and locus of stimulation.

Cutaneous pain thresholds to pinprick, pinch, and heat stimuli were quantified during control and electroacupuncture trials in 23 horses. Pain thresholds for 8 areas of the body during control trials (no needles) were statistically compared with pain thresholds measured in the same areas of the same horse when given electroacupuncture treatment. Statistically significant increases of pain threshold were interpreted as induced analgesia and occurred mainly in 5 areas of the trunk, but not in the head or extremities. Analgesic efficacy varied between sexes and among 3 groups of points chosen from Chinese traditional veterinary literature. Analgesia was induced equally well in both castrated males and intact females by the electrostimulation of 5 needles inserted on the gluteal (rump) and lumbar (loins) regions. However, stimulation of 2 needles located only in the gluteal region caused a significant analgesia in females only. In contrast, stimulation of 2 needles located in the thoracic limb was analgesic in males, but infrequently so in females. Therefore, we observed differential analgesia due to an interaction between needle location and sex of subject.

Effects of nerve injury on sympathetic excitation of A delta mechanical nociceptors.

1. The effects of sympathetic stimulation and close arterial injection of norepinephrine were tested on cutaneous myelinated-fiber (A delta) mechanical nociceptors [high-threshold mechanoreceptors-(MyHTMs)] from normal and from partially transsected nerves. 2. Neither sympathetic stimulation nor close arterial injection of norepinephrine (200 ng) excited MyHTMs (18) recorded from the uninjured great auricular nerve of adult rabbits. 3. MyHTMs (58) conducting across the site of partial cut lesions, made 2 to 28 days previously, had threshold and responsiveness to mechanical stimuli, receptive field organization, and absence of background discharge typical of such elements in normal nerve. 4. Four MyHTMs recorded from the injured nerves were excited by sympathetic stimulation and/or norepinephrine injection but only one gave more than two impulses within 60 s to either form of stimulation. 5. The meagerness of the sympathetic and adrenergic excitation of MyHTMs after nerve injury contrasts with that observed under similar conditions for C-fiber polymodal nociceptors. Therefore, induction of adrenergic responsiveness in nociceptors after partial denervation in cutaneous MyHTMs appears to be less important for mechanisms related to pathogenic pain than alterations in certain C-fiber nociceptors.

Sympathectomy induces adrenergic excitability of cutaneous C-fiber nociceptors.

1. The effects of ipsilateral removal of the superior cervical ganglion on the subsequent responsiveness of C-fiber polymodal nociceptors (CPMs) of the ear to close-arterial injections of norepinephrine (NE) were evaluated in adult, anesthetized rabbits. 2. In normal unanesthetized rabbits, the two ears were usually at the same temperature. Immediately after the ganglionectomy, the ipsilateral ear was warmer; however, at the time of electrophysiological recordings (4-23 days) the majority of animals had the ipsilateral ear cooler by > or = 1 degree C, suggestive of denervation supersensitivity. 3. NE (50 ng) did not activate any CPMs (n = 28) from intact animals. 4. Seven of 22 CPMs recorded from sympathectomized ears were activated by NE (50 ng). The responses varied considerably but typically consisted of 2-4 impulses in the 60 s after the NE injection. In some instances, repetitive activity continued for many minutes. Such prolonged discharge differs from the adrenergic responses seen after partial nerve damage. 5. The induction of adrenergic excitability in CPMs by sympathectomy is suggested to be a counterpart to postsympathectomy neuralgia in human beings and a possible part of the mechanism leading to sympathetically related pain states.

Enhancement of whole cell synaptic currents by low osmolarity and by low [NaCl] in rat hippocampal slices.

We recorded whole cell currents of patch-clamped neurons in stratum pyramidale of CA1 region of rat hippocampal tissue slices. Synaptic currents were evoked by orthodromic stimulation while holding potential of the neuron was varied from hyperpolarized to depolarized levels. Extracellular osmolarity (pi(o)) was lowered by superfusion with artificial cerebrospinal fluid in which NaCl concentration ([NaCl]) was reduced. The effect of low extracellular NaCl was tested in additional trials in which NaCl was substituted by isosmolar fructose. Both lowering of pi(o) and isosmotic lowering of extracellular [NaCl] ([NaCl]o) caused reversible increase of excitatory postsynaptic currents. The effect of lowering pi(o) was concentration dependent, and it was significantly stronger than the effect of equivalent isosmotic lowering of [NaCl]o. Inhibitory postsynaptic currents also increased in many but not in all cases. Lowering of pi(o) caused a prolongation of the time constant of relaxation of the capacitive charging current induced by small hyperpolarizing voltage steps. A virtual input capacitance, calculated by dividing this time constant by the input resistance, increased during hypotonic exposure. Isosmotic lowering of [NaCl]o had no effect on time constant or input capacitance. Depolarizing voltage commands evoked spikelike inward currents presumably representing Na+-dependent action potentials generated outside the voltage-clamped region of the cell. These current spikes became smaller in low pi(o) and in low [NaCl]o. Broader, voltage-dependent, presumably Ca2+-mediated inward currents became more prominent during hypotonic exposure. We conclude that lowering of [NaCl]o causes enhancement of excitatory synaptic transmission. Transmission may be facilitated by the uptake of Ca2+ into presynaptic terminals as well as into postsynaptic target neurons, induced by the low [NaCl]o. Lowering of pi(o) enhances synaptic transmission more than does a corresponding isosmotic lowering of [NaCl]. The excess increase recorded from the cell soma in low pi(o) may in part be due to changing electrotonic length caused by the swelling of dendrites.

Radioimmunological measurement of beta-endorphin in equine plasma.

Radioimmunoassay procedures were developed and validated for the quantification of beta-endorphin (beta-EP)-like immunoreactivity in equine plasma. beta-EP could be quantitatively extracted from plasma with silicic acid powder and subsequently assayed, however, valid estimates of this hormone could also be obtained on unextracted plasma. Although beta-lipotropin (beta-LPH) cross-reacted in the assay, it was not necessary to correct for beta-LPH activity when assaying unextracted plasma because chromatographic analyses showed that 92% of the immunoreactivity in plasma extracts was similar in molecular weight to authentic beta-EP (1-31). In addition, electroacupuncture treatment did not alter the relative proportion of immunoreactivity among different molecular weight fractions.