Analgesia from electrical stimulation of the hypothalamic arcuate nucleus in pentobarbital-anesthetized rats

Inhibition of noxious heat-induced tail flick by electrical stimulation of the arcuate nucleus of the hypothalamus (ARH) was examined and characterized in pentobarbital-anesthetized rats. Systematic mapping studies revealed that inhibition of the tail flick reflex could be induced by stimulating widespread areas in the ventromedial parts of the hypothalamus, which include the paraventricular nucleus, ventromedial nucleus, dorsomedial nucleus, anterior hypothalamic area as well as the ARH areas. The ARH stimulation-produced tail flick suppression could be completely blocked by systemic naloxone (2 mg/kg) which shows the involvement of an opiate mechanism in this effect. Although the tail flick reflex in the lightly anesthetized state is of significantly shorter latency than in the unanesthetized state, thresholds of the ARH stimulation for suppressing spinal nociceptive reflexes in the lightly anesthetized state were not significantly different from the thresholds at the same ARH sites in the awake state.

Antinociception vs motor effects of intrathecal vasopressin as measured by four pain tests

The effects of intrathecal (i.t.) vasopressin (VP) on nociception were quantitatively tested in rats using 4 pain tests: tail flick, tail shock vocalization, hot plate, and formalin. In addition, motor effects of VP were examined qualitatively. I.t. VP produced a prolonged antinociception lasting at least 40 min on the tail flick (2.5 and 25 ng) and formalin (25 ng) tests, and a brief antinociception lasting less than 20 min on the tail shock (25 ng) and hot plate (25 ng) tests. Those rats not responding to the pain tests showed no signs of perceiving the pain stimulus, such as orientation to the stimulus or vocalization. In addition, i.t. VP produced scratching bouts (2.5 and 25 ng) and suppressed hindbody motor function (25 ng). The motor inhibitory effects of VP, although severe in some rats, were brief, lasting less than 15 min. In conclusion, i.t. VP produces antinociception in addition to its motor effects, and these properties appear to be due to separate mechanisms.

Serotonin and/or an excitatory amino acid in the medial medulla mediates stimulation-produced antinociception from the lateral hypothalamus in the rat

Several lines of evidence have demonstrated a role for the lateral hypothalamus (LH) in an endogenous system of descending inhibition. Descending inhibition from the LH relies, at least in part, on a relay(s) in the midbrain and/or medulla. The medullary nucleus raphe magnus (NRM) serves as one such relay. The present study, in rats lightly anesthetized with pentobarbital, was undertaken to systematically examine the transmitter(s) in the medial medulla mediating descending inhibition of the nociceptive tail flick (TF) reflex produced by focal electrical stimulation in the LH. The microinjection of pharmacologic receptor antagonists (5 micrograms) into the NRM revealed that the glutamate receptor antagonists, gamma-D-glutamylglycine and 2-amino-5-phosphonovalerate produced the largest increases in stimulation thresholds in the LH for inhibition of the TF reflex (107.6% and 102.6%, respectively). Methysergide, a serotonin receptor antagonist, also produced a significant increase (81.5%) in the stimulation threshold in the LH for inhibition of the TF reflex. The opioid receptor antagonist, naloxone, however, was without effect, producing only a 4.0% increase in the LH stimulation threshold. These results suggest that serotonin and/or an excitatory amino acid are transmitters at the bulbar relay in the medial medulla mediating descending inhibition of the TF reflex produced by focal electrical stimulation in the LH.

Medial hypothalamic stimulation suppresses nociceptive spinal dorsal horn neurons but not the tail-flick reflex in the rat

This study investigated the potential analgesic effects of medial hypothalamic stimulation (HS) on a measure of nocifensive behavior (tail-flick test (TF] in awake rats, and potential inhibitory effects of identical HS on spinal dorsal horn neuronal responses to noxious skin heating in the same animals anesthetized with sodium pentobarbital. Sixty-five male Sprague-Dawley rats implanted with a bipolar stimulation electrode in histologically verified medial hypothalamic sites were tested behaviorally for TF suppression during HS (100 ms trains at 100 Hz, 3/s, 100-1100 microA) in 2-4 consecutive weekly test sessions. Thirty-three of these rats were then used in electrophysiological experiments to record responses of 36 dorsal horn units to noxious skin heating (48-54 degrees C, 10 s/2 min) of the hindfoot pad in the absence of and during HS. Behaviorally, 31/65 rats had no TF suppression at the highest HS intensity tested (1100 microA), 24/65 rats exhibited aversive behavior or motor activity which disallowed reliable TF testing, and only 10/65 rats showed TF suppression in at least one test session. In electrophysiological experiments, the heat-evoked responses of 25/36 dorsal horn units were inhibited to at least 50% of control during HS. The responses of 11 units remained at 65-100% of the control responses during HS of up to 1100 microA. In rats demonstrating TF suppression, 4/7 units were inhibited. In rats with no TF suppression, 10/15 units were inhibited, and in rats showing aversive behavior, 11/14 units were inhibited by HS. These data indicate that although HS suppresses spinal nociceptive neurons, it does not cause reliable TF suppression in unanesthetized rats and bring into question the often-held assumption that stimulation-evoked descending inhibition of spinal nociceptive neurons implies behavioral analgesia.

Spinal monoamine mediation of stimulation-produced antinociception from the lateral hypothalamus

Stimulation-produced antinociception can be evoked from a wide variety of sites in the brain, including the lateral hypothalamus (LH). The present study, in rats lightly anesthetized with pentobarbital, examined descending inhibition of the nociceptive tail flick (TF) reflex produced by focal electrical stimulation in the LH and the neurotransmitter(s), at the level of the lumbar enlargement, mediating the inhibition. Systematic tracking studies demonstrated that stimulation in the diencephalon dorsal to the hypothalamus did not reliably inhibit the TF reflex. Inhibition of the TF reflex was produced, however, throughout the hypothalamus at intensities of stimulation typically between 50 and 200 microA. The area requiring low intensities of stimulation (50-100 microA) to inhibit the TF reflex was a diffuse region of the LH, inferior to the mammillothalamic tract and internal capsule, medial to the supraoptic decussation and including the medial forebrain bundle. Microinjections of S-glutamate (100 mM, 0.5 microliter) in the LH did not inhibit the TF reflex, suggesting that activation of fibers of passage by stimulation was responsible for inhibition of the TF reflex produced from the LH. The intrathecal administration of pharmacologic antagonists (15-30 micrograms; naloxone, methysergide, phentolamine, prazosin or yohimbine) revealed that the alpha-adrenoceptor antagonists phentolamine and yohimbine produced the greatest increases in stimulation thresholds in the LH for inhibition of the TF reflex (83.7% and 89.8%, respectively). The intrathecal administration of methysergide produced a lesser, but statistically significant 11% increase in the stimulation threshold for inhibition of the TF reflex. These results indicate that spinal alpha 2-adrenoceptors primarily mediate the descending inhibition of the TF reflex produced by electrical stimulation in the LH.

Hypothalamic inhibition of rat dorsal horn neuronal responses to noxious skin heating

The responses of single lumbar dorsal horn neurons to noxious radiant heat stimuli (42-54 degrees C, 10 sec, 1/2 min) applied to glabrous hind paw skin were recorded in rats anesthetized with sodium pentobarbital. Unit responses to 50 or 52 degrees C stimuli were constant over time and were consistently and powerfully inhibited during bipolar stimulation (three 100 msec trains/sec at 100 Hz, 200 microA) in the medial hypothalamus. Inhibition was also evoked by stimulation in medial and ventrobasal thalamic nuclei, lateral hypothalamus and adjacent cerebral peduncle, and amygdala. Inhibition increased with graded increases in intensity of hypothalamic stimulation, with a mean inhibitory threshold of 71 +/- 43 (S.D.) microA for 13 units. The responses of dorsal horn units to graded increases in the temperature of noxious heat stimuli were inhibited during hypothalamic stimulation, such that slopes of the linear temperature-response functions were reduced with no change in response threshold (mean: approximately 44 degrees C). Inhibition was blocked or reduced in 4/7 units following systemic administration of the 5-hydroxytryptamine (5-HT) antagonist methysergide. The results confirm and extend previous work in the cat and are discussed in relation to analgesic mechanisms.

D-phenylalanine: a putative enkephalinase inhibitor studied in a primate acute pain model

D-Phenylalanine, along with morphine, acetylsalicylic acid and zomepirac sodium were evaluated for their antinociceptive actions in monkeys (M. fascicularis) trained to autoregulate nociceptive stimulation using a discrete-trials, aversive-threshold paradigm. Morphine sulfate produced dose-related increases in aversive threshold which were reversible after administration of naloxone (12.5 or 25 micrograms/kg i.m.). D-Phenylalanine (500 mg/kg p.o.) produced a small increase in aversive threshold which was not statistically significant and not naloxone reversible. Acetylsalicylic acid (200 mg/kg p.o.) but not zomepirac sodium (200 mg/kg p.o.) in combination with D-phenylalanine (500 mg/kg) produced a small statistically significant increase in aversive threshold. Our results argue against the hypothesis that D-phenylalanine is responsible for increasing aversive thresholds via opiate receptor mechanisms involving increased activity of enkephalins at synaptic loci. Previous studies by others in rats and mice showed that D-phenylalanine and acetylsalicylic acid produced increases in nociceptive thresholds which were naloxone reversible. Our failure to find opiate receptor mediated analgesia in a primate model with demonstrated opiate receptor selectivity and sensitivity is discussed in terms of previous basic and clinical research indicating an analgesic role for D-phenylalanine. Possible species difference in drug action is discussed in terms of inhibition by D-phenylalanine of carboxy-peptidase-like enkephalin processing enzymes as well as inhibition of carboxypeptidase-like enkephalin degrading enzymes.

An assessment of the antinociceptive and aversive effects of stimulating identified sites in the rat brain

At many sites in the brain electrical stimulation with low current intensity is both aversive and causes antinociception. In view of the well documented antinociception caused by various types of stress and pain it is possible that in some parts of the brain the antinociception is secondary to the stress of the stimulation. At 114 sites in the rat brain the intensity of stimulation required to evoke an aversive response has been compared with the antinociceptive current intensity. Only stimulation in the dorsal hippocampus and pretectal area caused antinociception without significant aversion. Strong aversion resulted from stimulation of 46% of the sites including the central gray and nucleus raphe magnus. Antinociception was significantly correlated with the aversiveness of the stimulation although in 15% of the stimulation sites strong aversion was seen with no antinociception. It is concluded that there can be little justification in assigning a primary antinociceptive role to a brain area which evokes strong escape reactions when stimulated.

Raphe magnus inhibition of primate T1-T4 spinothalamic cells with cardiopulmonary visceral input

Effects of stimulation of nucleus raphe magnus on upper thoracic spinothalamic tract neurons were determined. Experiments were performed on 15 monkeys (Macaca fascicularis) anesthetized with alpha-chloralose. Forty-two T1-T4 spinothalamic tract neurons with viscerosomatic inputs were studied. Stimulation of nucleus raphe magnus inhibited activity of all 42 neurons. Thirty-two of these cells had background activity. The magnitude of the inhibition of background activity was related to the raphe magnus stimulus current. Current strengths as low as 300 microA (100 Hz, 0.2 msec duration) completely inhibited most cells. Current thresholds averaged 80 +/- 10 microA and were unrelated to the type of somatic or visceral input the cell received, or to the cell location. Conditioning stimuli applied to nucleus raphe magnus inhibited cell responses to electrical stimulation of cardiopulmonary sympathetic A delta and C afferent fibers. However, in order to demonstrate preferential inhibition of responses to C fiber input it was necessary to use 200 msec trains of raphe stimuli which were concurrent with the cell response to sympathetic afferent stimuli. Twenty-five spinothalamic neurons were tested for responses to intracardiac injections of bradykinin and 17 cells increased their discharge rate. Stimulation of nucleus raphe magnus (280 +/- 25 microA) near the peak of the response reduced activity of all 17 cells from 26 +/- 3 to 4 +/- 1 spikes/sec (P less than 0.001). Raphe stimulation inhibited responses of 41 of 41 cells to noxious pinch and responses of 15 of 15 wide dynamic range and the 1 low threshold cell to blowing hair. The results establish the capacity of the raphe-spinal pathway to modulate activity of upper thoracic spinothalamic tract neurons including their response to potentially noxious cardiac stimuli. It is therefore possible that descending inhibitory systems may modulate ascending information related to cardiac pain and perhaps account for myocardial ischemic attacks which occur without pain.

Effects of periaqueductal gray stimulation of diencephalic neural activity

The effects of ipsilateral mesencephalic periaqueductal gray (PAG) stimulation on lateral hypothalamic (LH), lateral preoptic area (LPA), and ventral and subthalamic activity were determined in anesthetized rats. Recordings from 119 diencephalic neurons indicate that the PAG provides a predominantly inhibitory input to diencephalic neurons. Excitatory input occurred infrequently in the hypothalamus and was not observed in the thalamus. Following single rectangular pulse stimulation, 0.5 msec, 0-500 micro A, short latency decreases in activity occurred. Longer latency increases in discharge frequency were also observed. Dose response relations were established for 74% of the LH neurons, 68% of the LPA neurons, and for 72% of the ventral and subthalamic neurons following VTA stimulation. Decreases and, in a few hypothalamic neurons, increases in activity seemed to involve only one or two synapses. The effects of contralateral PAG stimulation on LPA-LH neuronal activity were alos determined. Dose response relations were established for 66% of the LPA-LH neurons following contralateral stimulation. However, results were different in that many more cells were increased with a shorter latency and at a lower threshold following contralateral stimulation. Antidromic responses verified PAG and diencephalic interconnections and revealed relatively slow conduction velocities, less than 1.0 m/sec. Results were discussed in terms of the anatomy of known PAG pathways, PAG neuronal activation vs. PAG fibers of passage, and the functions of midbrain-hypothalamic interconnections in the integration of somatic, visceral and nociceptive sensory inputs.

Elevation of pain threshold to tooth shock by brain stimulation in primates

Electrical stimulation of the brain (ESB) for modulation of pain has been previously demonstrated in primates, but many of the sites which yield stimulation-produced analgesia (SPA) also elicit aversive side effects. In order to examine the aversive as well as analgesic effects of brain stimulation, nine rhesus monkeys (Macaca mulatta) were first trained to press a lever to escape or titrate noxious tooth shock. Stimulating electrodes were placed under the frontal cortex in 4 monkeys and were implanted in the diencephalon, brain stem, and cerebellum of five remaining monkeys. Diencephalic stimulation sites resulted in marked elevations of tooth shock threshold at ESB intensities which did not elicit aversive behaviors. The analgesic effects lasted up to 2 h past ESB offset. Moderate elevations of tooth shock threshold were also observed with orbital cortex stimulation. The midbrain central gray and the nucleus raphe magnus, however, did not greatly alter tooth shock level and typically resulted in aversive reactions. The diencephalic sites which elicited SPA also led to self-stimulation behavior, whereas stimulation of the brain stem or cerebellum usually resulted in escape responses. These findings thus indicate that, in primates, more effective relief of pain can be achieved with electrical activation of the medial diencephalon than with brain stem stimulation.

Chemical hypophysectomy for relief of bone pain in carcinoma of the prostate

A series is presented of 10 patients with intractable pain secondary to diffuse metastatic prostatic carcinoma. These patients underwent pituitary ablation by a new method of chemical hypophysectomy, which consisted of multiple injections of absolute alcohol into the pituitary gland under stereotaxic control. Of the 10 patients 9 had good to excellent pain relief. There was no operative mortality. The sequelae are those associated with pituitary destruction, the most significant being diabetes insipidus. Preoperative and postoperative hormonal studies failed to reveal any significant hormonal changes despite good and almost immediate pain relief.

Attenuation of pain reactivity by caudate nucleus stimulation in monkeys

The effect of caudate nucleus stimulation on reactivity to painful stimuli was investigated in Macaca speciosa monkeys chronically implanted with electrodes in the right caudate nucleus. The force with which subjects escaped from electrocutaneous leg shock was used as a measure of pain reactivity and was decreased by caudate stimulation. Escape threshold and latencies were not influenced by the brain stimulation. Decreased escape force was obtained only when 50 msec trains of caudate stimulation preceded 20 msec trains of leg shock by 0-100 msec. Pain reactivity was not affected if brain stimulation followed leg shock or if leg shock followed brain stimulation by more than 100 msec. Intershock response distributions indcated that direct motor inhibition was not responsible for the depression of escape force, and the effectiveness of a restricted range of caudate-leg stimulation intervals ruled out generalized effects on arousal. The results indicate that the effect of caudate stimulation is to reduce the affective components of pain elicited by noxious electrocutaneous stimuli. The time course of this caudate effect parallels that previously reported for the caudate-induced depression of evoked activity in the non-specific somatosensory projections of the reticular formation and thalamus.

Pain reduction by focal electrical stimulation of the brain: an anatomical and behavioral analysis

These experiments have examined several aspects of analgesia produced by focal electrical stimulation of the brain. (1) Anatomical locus of analgesic effects: only stimulation of the mesencephalic central gray matter and periventricular gray matter greatly reduced or totally abolished responsiveness to all noxious stimuli employed. Stimulation of other brain areas increased jump threshold to electric shock (septal nuclei, dorsomedial thalamic nucleus) and sometimes even abolished responsiveness to tissue destructive pinch (ventral tegmentum, dorsomedial thalamic nucleus), but never eliminated resonding to radiant heat applied to the tail. Stimulation of the ventrobasal complex of the thalamus and the lateral hypothalamus produced little or no analgesia at all. (2) Magnitude of analgesia: stimulation of the central and periventricular gray matter produced analgesia equal to or greater than 10 mg/kg morphine on all tests. (3) Relationship to reward: stimulation-produced analgesia was found not to be casually related to the rewarding properties of the stimulation. Analgesia was often produced by stimulation at electrode sites which did not support self-stimulation behavior; and many animals self-stimulated at high rates but were not analgesic. (4) Relationship to seizure activity: electrographic or overt motor seizure activity was not related to stimulation-produced analgesia. (5) By analogy with the site and mechanism of morphine action, it is proposed that focal electrical stimulation activates a pain suppressive system concentrated in periventricular and periaqueductal regions and its activation reduces responsiveness to noxious stimuli, at least in part, by blocking transmission of nociceptive information through the spinal cord.