An inability to antagonize with naloxone the elevated nociceptive thresholds resulting from electrical stimulation of the mesencephalic central gray

Alternative pain management via endocannabinoids in the time of the opioid epidemic: Peripheral neuromodulation and pharmacological interventions

The use of opioids in pain management is hampered by the emergence of analgesic tolerance, which leads to increased dosing and side effects, both of which have contributed to the opioid epidemic. One promising potential approach to limit opioid analgesic tolerance is activating the endocannabinoid system in the CNS, via activation of CB1 receptors in the descending pain inhibitory pathway. In this review, we first discuss preclinical and clinical evidence revealing the potential of pharmacological activation of CB1 receptors in modulating opioid tolerance, including activation by phytocannabinoids, synthetic CB1 receptor agonists, endocannabinoid degradation enzyme inhibitors, and recently discovered positive allosteric modulators of CB1 receptors. On the other hand, as non-pharmacological pain relief is advocated by the US-NIH to combat the opioid epidemic, we also discuss contributions of peripheral neuromodulation, involving the electrostimulation of peripheral nerves, in addressing chronic pain and opioid tolerance. The involvement of supraspinal endocannabinoid systems in peripheral neuromodulation-induced analgesia is also discussed. LINKED ARTICLES: This article is part of a themed issue on Advances in Opioid Pharmacology at the Time of the Opioid Epidemic. To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v180.7/issuetoc.

Brain stimulation for neurological and psychiatric disorders, current status and future direction

Interest in brain stimulation therapies has been rejuvenated over the last decade and brain stimulation therapy has become an alternative treatment for many neurological and psychiatric disorders, including Parkinson's disease (PD), dystonia, pain, epilepsy, depression, and schizophrenia. The effects of brain stimulation on PD are well described, and this treatment has been widely used for such conditions worldwide. Treatments for other conditions are still in experimental stages and large-scale, well controlled studies are needed to refine the treatment procedures. In the treatment of intractable brain disorders, brain stimulation, especially transcranial magnetic stimulation (TMS), is an attractive alternative to surgical lesioning as it is relatively safe, reversible, and flexible. Brain stimulation, delivered either via deeply implanted electrodes or from a surface-mounted transcranial magnetic device, can alter abnormal neural circuits underlying brain disorders. The neural mechanisms mediating the beneficial effects of brain stimulation, however, are poorly understood. Conflicting theories and experimental data have been presented. It seems that the action of stimulation on brain circuitry is not limited to simple excitation or inhibition. Alterations of neural firing patterns and long-term effects on neurotransmitter and receptor systems may also play important roles in the therapeutic effects of brain stimulation. Future research on both the basic and clinical fronts will deepen our understanding of how brain stimulation works. Real-time computation of neural activity allows for integration of brain stimulation signals into ongoing neural processing. In this way abnormal circuit activity can be adjusted by optimal therapeutic brain stimulation paradigms.

Effect of cerebral ventricles perfusion with naloxone on trigemino-hypoglossal reflex in rats

The goal of this study was to determine whether opioid receptor antagonist naloxone abolishes the influence of periaqueductal central gray (PAG) on nociceptive evoked tongue jerks (ETJ) -- a trigemino-hypoglossal reflex induced by tooth pulp stimulation. In rats under chloralose anesthesia three series of experiments were performed. In the first two groups perfusions of lateral ventricles-cerebellomedullary cistern with McIlwain-Rodnight's solution and naloxone were carried out. In group 3 naloxone was infused through a catheter through the jugular vein. The amplitudes of tongue jerks induced by tooth pulp stimulation were recorded during subsequent 10 min perfusions. Mean amplitude of tongue movements induced by tooth pulp stimulation was regarded as the indicator of the magnitude of trigemino-hypoglossal reflex. We observed that perfusion of the cerebral ventricles with naloxone (100 nmol/ml) increased the trigemino-hypoglossal reflex up to 143%. The amplitude of ETJ was significantly reduced during PAG stimulation with a train of electrical impulses. After obtaining a significant -- 93% -- inhibition of ETJ (7% of the control), naloxone (100 nmol/ml) was added to the perfusion fluid. This led to a significant increase of the reflex up to 68%. Infusion of naloxone through the jugular vein did not affect the reflex. The above results suggest that the inhibition of ETJ due to PAG stimulation is partially reversed by naloxone and mediated via interactions with endogenous opioid systems involved in modulation of nociception.

Endogenous opioids acting at a medullary mu-opioid receptor contribute to the behavioral antinociception produced by GABA antagonism in the midbrain periaqueductal gray

This study examined the contribution of endogenous opioids to the antinociception produced by microinjection of the GABAA receptor antagonist, bicuculline, into the rat midbrain ventrolateral periaqueductal gray region. Microinjection of bicuculline (40 ng/0.4 microliter) into the periaqueductal gray produced robust antinociception as measured by the tail-flick latency to noxious heat. This antinociception was partially reversed by intravenous administration of the non-selective opioid antagonist naloxone hydrochloride (1 and 5 mg/kg), indicating that endogenous opioid release is necessary for this effect. To determine whether opioid release in the rostral ventromedial medulla, a major projection target of the periaqueductal gray, contributes to this effect, we microinjected another opioid antagonist, naltrexone, into the rostral ventromedial medulla. Naltrexone in the rostral ventromedial medulla (5 and 10 micrograms/microliter) significantly attenuated bicuculline antinociception elicited from the periaqueductal gray. Cys2, tyr3, orn5, pen7-amide (26.5 nmol), a selective mu-opioid receptor antagonist, also reversed the antinociception when microinjected into the rostral ventromedial medulla. Microinjections of naltrexone (10 micrograms/microliter) or cys2, tyr3, orn5, pen7-amide at sites in the medulla dorsal to the rostral ventromedial medulla were ineffective. None of the antagonists altered baseline tail-flick latencies. These results support the hypothesis that a population of periaqueductal gray neurons produces antinociception through a mu-opioid receptor-mediated action of endogenous opioids in the rostral ventromedial medulla. Thus, two opioid-sensitive pain-modulating brainstem sites are linked by an endogenous opioid synapse in the rostral ventromedial medulla.

Release into ventriculo-cisternal perfusate of beta-endorphin- and Met-enkephalin-immunoreactivity: effects of electrical stimulation in the arcuate nucleus and periaqueductal gray of the rat

To examine the resting and evoked release of the endogenous opioid peptides beta-endorphin and Met-enkephalin from brain, we examined the levels of the respective immunoreactivities in the lateral ventricle-cisterna magna perfusate of the halothane-anesthetized rat. Ten Hz but not 100 Hz stimulation in the arcuate nucleus (ARC) of the hypothalamus released beta-endorphin immunoreactivity (beta-EPir) to the perfusate, whereas 100 Hz but not 10 Hz stimulation in the periaqueductal gray (PAG) of the mid brain released Met-enkephalin immunoreactivity (MEir). MEir was not released by stimulation in ARC and beta-EPir was not released by stimulation in PAG. Characterization of the released beta-EPir and MEir by high performance liquid chromatography showed that authentic beta-endorphin and Met-enkephalin were the major constituents of beta-EPir and MEir, respectively. Systemic administration of the dopaminergic antagonist haloperidol increased plasma, but not perfusate levels of beta-EPir. Both the opioid antagonist naloxone and the NMDA antagonist MK-801 failed to affect beta-EPir or MEir release. ARC and PAG stimulated inhibited a nociceptive reflex (tail-dip in 52.5 degrees C water), and naloxone did not reliably reverse this inhibition. These data support the previously suggested possibility of opioid mediation of stimulation induced analgesia, although we were unable to confirm the theory by naloxone reversibility in this study. Furthermore, the data support the assumption that measurement of opioid peptides in cerebrospinal fluid is a relevant approach in research aimed at elucidating the physiological and pathophysiological roles of endogenous opioid peptides.

Analgesia from the periaqueductal gray in the developing rat: focal injections of morphine or glutamate and effects of intrathecal injection of methysergide or phentolamine

The aim of these experiments was to examine the changes in antinociception elicited by morphine or glutamate stimulation of the periaqueductal gray of the midbrain (PAG) during the postnatal development of the rat. Pups, aged 3, 10, and 14 days, were implanted with cannulas aimed at either the dorsal or the ventral aspect of the PAG, and glutamate (vehicle, 60 mM or 180 mM) or morphine (vehicle, 2 micrograms or 6 micrograms) was microinjected into one of those two sites. Pups were tested for analgesia against noxious thermal and mechanical stimuli. Morphine produced analgesia at 3 and 10 days of age only when administered to the ventral part of the PAG and the thermal noxious stimulus was tested. Conversely, analgesia induced by glutamate was seen at 3 and 10 days of age only when glutamate was given to the dorsal aspect of the PAG and the mechanical stimulus was used. In 14-day-old pups, both drugs produced analgesia against both types of noxious stimuli regardless of their site of administration within the PAG. Systemically administered naloxone attenuated the analgesic effects of both drugs when they were administered to the ventral PAG, but did not consistently attenuate the analgesic effect of either compound given to the dorsal aspect of the PAG. When either morphine or glutamate was injected into the ventral PAG, intrathecal injections of methysergide attenuated analgesia against the thermal stimulus to a significantly greater degree than the mechanical stimulus and intraspinal injection of phentolamine attenuated analgesia against the mechanical stimulus more potently. When glutamate was given to the dorsal PAG, analgesia against both stimulus types was significantly attenuated. These results indicate that the morphine- and glutamate-induced analgesia mediated by the PAG are developmentally differentiated. These ontogenetic differences most likely reflect differences in the mechanism of action by which these drugs produce analgesia when administered to the PAG, as well as neuroanatomical differences within the dorsal and the ventral regions of the PAG.

The development of stimulation-produced analgesia (SPA) in the rat

The present study studied the development of stimulation produced analgesia (SPA) from the periaqueductal gray (PAG) in rats. A monopolar stimulating electrode was lowered into the dorsal or ventral PAG of animals aged 7, 14, 21, or 90-120 days. Constant current cathodal pulses (100 Hz, 100 microseconds) were delivered, starting 10 s before analgesia was tested by the tail-flick (TF) test and continuing throughout each TF trial or until cut-off (7 s). Current intensity was increased stepwise (3-200 microA). It was found that SPA can be elicited starting at 21 days, but not earlier. However, supraspinal modulation of nociception is still immature at 3 weeks after birth. First, stimulation intensities needed to produce SPA are higher in 21-day-old pups than in adult animals. Second, in 21-day-old pups, but not in adults effective current intensities in the dorsal PAG are higher than in the ventral PAG. Third, naltrexone decreases SPA from the ventral PAG in 21-day-old pups, but not in adult animals. These findings indicate that supraspinal modulation of nociception develops only 3 weeks after birth, with the ventral PAG maturing prior to the dorsal PAG, and that the contribution of endogenous opioids to SPA does not remain constant throughout the ontogeny of rats.

Stimulation-produced analgesia in the mouse: evidence for laterality of opioid mediation

The effect of naloxone on periaqueductal gray matter (PAG) stimulation-produced analgesia (SPA) was examined in pentobarbital anesthetized Swiss-Webster mice. Electrodes were placed either in dorsolateral or ventrolateral PAG, and SPA threshold was assessed using the hind paw-flick test (paw withdrawal from radiant heat). SPA threshold did not differ between dorsal and ventral PAG, and naloxone equally attenuated SPA from both areas. SPA threshold for the paw contralateral to the stimulation site was half that for the ipsilateral paw. Elevation of SPA threshold by naloxone was greater for the contralateral than ipsilateral paw. Exposure to analgesic neck pinch prior to SPA almost completely abolished the antinociceptive effect of contralateral PAG stimulation without affecting SPA on the ipsilateral paw. This effect of pinch was itself reversed by prior naloxone administration. We suggest that the substrate of opioid mediated SPA from PAG in the mouse has principally a contralateral organization.

Electrical stimulation of the rat ventral midbrain elicits antinociception via the dorsolateral funiculus

The pain-suppressive effects of focal electrical stimulation of sites throughout the ventral midbrain were examined in awake rats. Chronic bipolar electrodes were implanted in medial and lateral regions of the midbrain. Current thresholds for suppression of the tail-flick reflex in response to noxious heat were determined for both a biphasic and a monophasic stimulation parameter at each site. Stimulation of areas throughout the ventral midbrain produced tail-flick suppression (TFS), but no one area was consistently effective in all animals. Monophasic and biphasic stimulation were qualitatively equal in the duration of TFS and the distribution of effective sites. The production of TFS was not correlated with other behavioral reactions to brain stimulation. TFS appeared to be mediated by non-opiate pathways since naloxone administration (10 mg/kg) had no discernible effect on the production of TFS. The current threshold for producing TFS was extremely variable over both short (one half hour) and long (one week) intervals. The incidence of TFS from previously effective sites was significantly less following bilateral dorsolateral funiculus (DLF) lesions, indicating that the antinociceptive effects of ventral midbrain stimulation are mediated by this spinal pathway.

Opiate and serotonergic mechanisms of stimulation-produced analgesia within the periaqueductal gray

These studies investigated the distribution of analgesia-producing sites within the periaqueductal gray (PAG), and their potential reversal by naloxone and methysergide. The PAG is not differentiable in its ability to elicit stimulation-produced analgesia (SPA) until the point of stimulation is caudal to the dorsal raphe nucleus, where analgesia was not obtained. Naloxone, however, was found to have a differential effect at specific loci, significantly reducing SPA from ventral but not dorsal sites. In contrast, methysergide was effective in reversing analgesia both at ventral and dorsal sites. The site of stimulation was critical to whether motor effects were elicited: Motor effects accompanied by analgesia were most often produced rostrally, while motor effects without analgesia were most frequently produced in the middle PAG. Null effects for both motor activity and analgesia were obtained from caudal PAG sites.

Organismic variables and pain inhibition: roles of gender and aging

Multiple pain-inhibitory systems dependent upon both opioid and nonopioid mechanisms of action have been identified, particularly in the rodent. The experimental subject has typically been the young, adult male rat, and generalizations concerning these systems have been made from this subject pool. This review focuses upon the roles of two organismic factors, aging and gender, in the modulation of analgesic processes. Using an array of age cohorts (4, 9, 14, 19, 24 months), these data illustrate that aging produces differential decrements in the analgesic responses following morphine, different parameters of footshock, continuous cold-water swims (CCWS: a nonopioid stressor), intermittent cold-water swims (ICWS: an opioid stressor) and 2-deoxy-D-glucose (a mixed opioid/nonopioid stressor). In contrast, neither beta-endorphin nor food deprivation analgesia is affected by aging. This review identifies that CCWS and ICWS analgesia are sensitive to gender differences, gonadectomy differences and steroid replacement differences such that females display less analgesia than males, gonadectomy reduces both analgesic responses, and that testosterone is most effective in reinstating gonadectomy-induced analgesic deficits. These data are considered in terms of therapeutic implications for the organismic variables under study as well as for the conceptual and methodological modifications that must be made in studying intrinsic pain inhibition.

Site specificity in the development of tolerance to stimulation-produced analgesia from the periaqueductal gray matter of the rat

Pentobarbital-anesthetized rats were subjected to 21 min of continuous electrical stimulation of the caudal periaqueductal gray matter (PAG) at the current threshold for analgesia. Stimulation at ventral PAG sites supported analgesia for only 1 or 2 min in most animals. Stimulation at more dorsal PAG sites supported analgesia for the entire 21 min of stimulation. This demonstration of acute tolerance with continuous ventral, but not more dorsal, PAG stimulation corresponds well with previous evidence suggesting opioid mediation of analgesia from this brain region.

Stimulation-produced descending inhibition from the periaqueductal gray and nucleus raphe magnus in the rat: mediation by spinal monoamines but not opioids

Focal electrical stimulation in the midbrain periaqueductal gray (PAG) or medullary nucleus raphe magnus (NRM) inhibits spinal nociceptive transmission and nociceptive reflexes. The purpose of this study was to evaluate, in lightly pentobarbital-anesthetized rats, the spinal neurotransmitter(s) mediating descending inhibition of the nociceptive tail-flick (TF) reflex produced by focal electrical stimulation in the PAG or NRM. To characterize the neurotransmitter(s) mediating inhibition of the TF reflex, selective pharmacologic antagonists were administered into the lumbar intrathecal space. Stimulation thresholds in the PAG or NRM for inhibition of the TF reflex were established and the effects of intrathecally administered phentolamine, yohimbine, prazosin, methysergide (15 micrograms initially, 30 micrograms cumulative) or naloxone (10 micrograms initially, 20 micrograms cumulative) on TF inhibitory thresholds determined. Phentolamine, yohimbine and methysergide increased the intensity of stimulation in the PAG and the NRM for inhibition of the TF reflex; prazosin and naloxone had no effect. Descending inhibition produced by focal electrical stimulation in the PAG or NRM is mediated in part by spinal serotonergic and/or alpha 2-adrenergic receptors. Naloxone was administered both intrathecally and intravenously; however, a role for opioid receptors in descending inhibition from the midbrain or medulla was not found.

Stimulation-produced analgesia under repeated morphine treatment in rats

Components of complex emotional reaction to nociceptive stimulation as well as antinociceptive effect of periaqueductal gray matter (PAG) electrical stimulation were determined in rats. Animals were treated with morphine hydrochloride or saline for 10 days. Morphine analgesic effect during subchronic dosage (50 mg/kg a day) was gradually decreased. The same was true for stimulation-produced analgesia (SPA). Naloxone (2 mg/kg) exerted a partially antagonistic effect in relation to SPA in saline-treated and failed to abolish SPA in morphine-treated rats. It is concluded that the opiate component of the antinociceptive system is of importance for the tolerance development to morphine-like drugs.

Depletion of central beta-endorphin blocks midbrain stimulation produced analgesia in the freely-moving rat

The present study examines the role of central beta-endorphin in the generation of stimulation-induced analgesia from the ventral midbrain periaqueductal gray of freely-moving rats. Electrical stimulation of the ventral midbrain periaqueductal gray led to an antinociception against noxious heat which gradually subsided post-stimulation over a period of about 15 min. Locomotor effects (ipsilateral rotation) were also seen which were not correlated in intensity with the analgesia and which disappeared immediately with termination of stimulation. There was no indication of any aversive effects. Application of the opioid antagonist, naloxone, 10 min pre-stimulation, strongly attenuated the antinociception without changing basal thresholds. It did not influence the locomotor changes. Bilateral, radiofrequency lesions of the mediobasal arcuate hypothalamus greatly depleted immunoreactive beta-endorphin from brain tissues without affecting its levels in plasma. Lesioned rats showed a pronounced reduction of stimulation-produced antinociception in the absence of any change in basal thresholds; the locomotor effects of stimulation were not influenced. The degree of depletion of immunoreactive-beta-endorphin significantly correlated with the degree of attenuation of antinociception. These data suggest: stimulation of the ventral midbrain periaqueductal gray leads both to an antinociception and locomotor effects in freely-moving rats: these can be clearly dissociated from each other; the antinociception (but not locomotor effects) are mediated by an endogenous opioid sensitive to blockade by naloxone; and central beta-endorphin may be the endogenous opioid mediating stimulation-produced antinociception from the ventral midbrain periaqueductal gray in the rat.

Stimulation-produced and stress-induced analgesia: cross-tolerance between opioid forms

Electrical stimulation of medial brainstem sites produces potent analgesia in rats that is either opioid- or non-opioid-mediated depending on the specific brain region stimulated. Footshock stress also causes opioid and non-opioid forms of analgesia in rats depending on the exact parameters of footshock administered. We now report that opioid, but not non-opioid, stress analgesia demonstrates cross-tolerance with opioid, but not non-opioid, stimulation-produced analgesia. This finding suggests that opioid forms of stimulation-produced and stress-induced analgesia share a common substrate.

Intrinsic mechanisms of pain inhibition: activation by stress

Portions of the brain stem seem normally to inhibit pain. In man and laboratory animals these brain areas and pathways from them to spinal sensory circuits can be activated by focal stimulation. Endogenous opioids appear to be implicated although separate nonopioid mechanisms are also evident. Stress seems to be a natural stimulus triggering pain suppression. Properties of electric footshock have been shown to determine the opioid or nonopioid basis of stress-induced analgesia. Two different opioid systems can be activated by different footshock paradigms. This dissection of stress analgesia has begun to integrate divergent findings concerning pain inhibition and also to account for some of the variance that has obscured the reliable measurement of the effects of stress on tumor growth and immune function.

Opiate withdrawal behavior after focal brain stimulation

Electrical stimulation of the brainstem abolishes pain, while continued stimulation induces tolerance to the analgesic effect. Analgesic drugs producing tolerance also induce physical dependence, suggesting that the phenomenon of tolerance is associated with addiction. There is evidence that the neural mechanism for stimulation-produced analgesia is related to the release of opiate substances within the brain. We therefore propose that repeated or protracted brain stimulation elicits dependence upon the endorphins released by electrical stimulation of the neurons themselves. To investigate this possibility, rats were given repetitive bursts of analgesic electrical brain stimulation for two hours. Immediately thereafter, they were injected with the opiate antagonist, naloxone. Behaviors associated with low grade opiate withdrawal were observed. These data suggest that prolonged analgesic stimulation can result in naloxone-precipitated behaviors similar to the behaviors exhibited during opiate withdrawal.

A reinvestigation of the analgesic effects induced by stimulation of the periaqueductal gray matter in the rat. I. The production of behavioral side effects together with analgesia

It has been shown that stimulation-produced-analgesia (SPA) in the cat elicited from the periaqueductal gray matter (PAG) is obtained from sites located in the ventral part, particularly the dorsal raphé nucleus (DRN). These data contrast with the numerous studies performed in the rat in which efficient sites seem widely distributed throughout the PAG. These discrepancies led us to reinvestigate SPA from PAG and adjacent structures in the rat. Central stimulation was delivered through bipolar concentric electrodes (one for each animal). Analgesia was evaluated (before and during central stimulation) by measuring the modification in the vocalization threshold induced by electrical tail shocks or by considering the reaction of the animal to pinch. In contrast with the majority of previous studies, these experiments were performed on the totally freely-moving rat. The most striking result was that, in order to obtain analgesia from all regions of the PAG, it was necessary to apply intensities of central stimulation which also triggered other strong behavioral reactions. With intensities of PAG stimulation which did not induce such side effects, very few effective analgesic sites were found (21/129 sites of which 14/83 were strictly located in the PAG). However, it was possible to define two 'pure analgesic regions', both located in the ventral PAG: one centered on the dorsomedial part of the DRN and the other one situated in the ventrolateral PAG. No modification of nociceptive thresholds was observed when stimulating the dorsal and dorsolateral parts of the PAG as well as structures adjacent to these regions; in some rats, an increase in pain reactivity was even noted. When the intensity of central stimulation (applied to the various parts of the PAG) was increased, some stereotyped 'behavioral responses' occurred depending on the location of the stimulation site: motor effects (gnawing, rotation or tremor) in the ventral PAG and aversive effects (flight, jumping and on occasions, distress vocalizations) in the dorsal, dorsolateral PAG and in the ventral region just surrounding the cerebral aqueduct. Under these conditions, analgesia was obtained from practically the entire PAG, the vocalization threshold being increased dramatically on occasions. It must be emphasized that antinociceptive effects associated with other obvious behavioral manifestations (aversive ones) were also obtained from sites located outside the PAG (colliculi and tectum adjacent to the dorsal and dorsolateral PAG).(ABSTRACT TRUNCATED AT 400 WORDS)

Basic and regulatory mechanisms of in vitro release of Met-enkephalin from the dorsal zone of the rat spinal cord

Under control conditions, superfused slices of the dorsal half of the lumbar enlargement from adult rats released Met-enkephalin-like material (MELM) that behaved as authentic Met-enkephalin under two different chromatographic procedures (Bio-gel filtration, HPLC). MELM release increased markedly on exposure of slices to batrachotoxin (0.5 microM) or to an excess of K+ (28 and 56 mM instead of 5.6 mM). The K+-evoked release was totally dependent on the presence of Ca2+ in the superfusing fluid whereas the spontaneous efflux of MELM was only partially Ca2+-dependent. Further experiments performed with tissues of polyarthritic rats indicated that the increase in their MELM levels was associated with a lower fractional rate constant of MELM release, therefore suggesting that spinal Met-enkephalin turnover might be reduced in chronically suffering animals. Examination of the possible modulation of MELM release by various neuroactive compounds present within the dorsal horn revealed that cholecystokinin (10 microM), but not its desulphated derivative, substance P-sulphoxide (10 microM), and to a lesser extent substance P, enhanced the K+-evoked MELM release. In contrast, gamma-aminobutyric acid (10 microM) and (-)-baclofen (1 microM) partially prevented the stimulatory effect of K+ on MELM release. Other compounds such as serotonin, somatostatin, and neurotensin altered neither the spontaneous nor the K+-evoked release of MELM.

Interactions between cardiovascular and pain regulatory systems

A review of pharmacological, neuroanatomical, electrophysiological, and behavioral data indicates that systems controlling cardiovascular function are closely coupled to systems modulating the perception of pain. This view is directly supported by experiments from our laboratory showing that activation of either the cardiopulmonary baroreceptor reflex arc or the sinoaortic baroreceptor reflex arc induces antinociception. The outcomes of studies using pharmacological treatments, peripheral nerve stimulation, peripheral nerve resection, and CNS lesions are also presented as a preliminary means of characterizing cardiovascular input to pain regulatory systems. The network formed by these systems is proposed to participate in the elaboration of adaptive responses to physical and psychological stressors at various levels of the neuroaxis, and possibly to participate in "diseases of adaptation." In particular, the present analysis suggests that the inhibition of pain brought about by elevations in either arterial or venous blood pressure may provide a form of psychophysiological relief under situations of stress and contribute to the development of essential hypertension in humans.

A reinvestigation of the analgesic effects induced by stimulation of the periaqueductal gray matter in the rat. II. Differential characteristics of the analgesia induced by ventral and dorsal PAG stimulation

This study consists of a detailed analysis of the analgesic effects induced by stimulation of the various parts of the periaqueductal gray matter (PAG) in the freely moving rat. In order to characterize the analgesia, two criteria are considered: (1) the evaluation of the degree of analgesia and behavioral side effects evoked during central stimulation; and (2) the presence of post-effects. Central stimulation (50 Hz sine waves) was delivered via bipolar concentric electrodes and analgesia was quantified by the change in the vocalization threshold induced by electrical stimulation of the tail. Within the ventral PAG, the vocalization threshold increased gradually with the intensity of the central stimulation, the degree of analgesia generally being powerful. There was no relationship between the strength of the analgesic effects and the motor disturbances also produced by stimulation of this region. Antinociceptive effects generally disappeared when the stimulation ceased. Only when the intensity of the stimulation was strong enough to induce very powerful analgesic effects were post-stimulation analgesic effects noticed. Within the dorsal and dorsolateral PAG as well as in the ventral region just surrounding the aqueduct, analgesia appeared suddenly, was generally less pronounced and was always concomitant with strong aversive reactions. In contrast with the analgesia from the ventral PAG, marked post-effects were observed. These latter characteristics were also obtained from stimulation of regions located outside the PAG (colliculi, intercollicular commissure and tectum adjacent to the dorsolateral PAG) although these zones were not extensively studied. By consideration of various data in the literature, it is concluded from this study, which clearly distinguishes stimulation-produced-analgesia (SPA) from ventral PAG versus dorsal PAG, that analgesia induced from this midbrain area involves at least two different neuronal substrates. Whilst the ventral PAG seems to be more preferentially involved in pain modulation, the authenticity of 'analgesia' triggered by stimulation of aversive regions (which are widely spread over the PAG) is questioned and proposals to explain the simultaneous appearance of analgesic effects and aversion are considered.

Non-opiate analgesia induced by carbachol microinjection into the pontine parabrachial region of the cat

These studies investigated the effect of microinjection of the cholinergic agonist carbamylcholine (carbachol) into various sites of the dorsolateral pontine tegmentum of the cat. Carbachol microinjection into an area surrounding the lateral half of the brachium conjunctivum (parabrachial region, PBR) produced profound suppression of nociceptive responses. In the dorsal part of PBR, carbachol microinjection produced no generalized sensory, emotional or motor deficits, indicating that nociceptive transmission was primarily affected. Carbachol microinjection into the ventral part of PBR resulted in slight suppression of motor responses in addition to profound nociceptive suppression. Carbachol-produced analgesia (CPA) observed within PBR blocked supraspinally as well as spinally integrated responses normally elicited by either phasic or tonic noxious stimuli. Atropine sulfate, but not mecamylamine hydrochloride, significantly antagonized CPA, indicating that muscarinic receptors mediate this phenomenon. The opiate antagonist naloxone, systemically administered either prior to or after carbachol microinjection, did not reliably attenuate CPA. Microinjection of morphine into the sites from which CPA had previously been obtained did not produce significant effects on nociceptive responses. Thus, opiate mechanisms appear not to be necessary either for the activation of this system or for the production of the resultant analgesia. These findings indicate that the neural population examined in the present study is anatomically and pharmacologically distinct from previously identified opiate-mediated pain inhibitory systems. Results are discussed in light of other recent evidence indicating the existence of endogenous non-opiate pain inhibitory systems.

Projections from the periaqueductal gray matter to the B3 cellular area (nucleus raphe magnus and nucleus reticularis paragigantocellularis) as revealed by the retrograde transport of horseradish peroxidase in the rat

Afferent projections from the periaqueductal gray matter (PAG) to the B3 region (nucleus raphe magnus, NRM; and nucleus reticularis paragigantocellularis, NPG) were examined by means of the horseradish peroxidase (HRP) method revealed by using the tetramethylbenzidine (TMB) procedure. Following iontophoretic injection of HRP into the B3 cellular area, numerous labeled neurons were found between the third oculomotor complex and the rostral part of the tegmental nucleus dorsalis of Gudden. The most densely labeled regions were the lateral parts of the dorsal raphe nucleus (wings of the DRN) and Hamilton's nucleus dorsalis of the PAG. Very few neurons were found within the medial part of the DRN. In contrast, results with control injections performed outside the B3 area gave a different distribution of labeled neurons. The functional significance of these connections, particularly those emanating from the wings of the DRN which represent new information, is briefly discussed with regard to mechanisms operative in the control of pain. Although the major PAG-B3 direct projections could underlie a role for the B3 area in PAG-induced analgesia, the comparison of our anatomical and behavioral results points out some problems. Indeed, there is no strict correlation between the location of stimulation sites which induce analgesia in the freely moving rats and the distribution within the PAG of B3 projections. These observations lead us to question the role of direct PAG-B3 connections in the antinociception induced by PAG stimulation and alternative hypotheses are proposed.

Analgesia produced by electrical stimulation of the brain

Electrical stimulation of the brain can produce a selective and potent modulation of responding to noxious stimuli in animals and man. The influence of various stimulation parameters is discussed. Brain stimulation at numerous loci results in analgesia. The most well characterized regions are the mesencephalic periaqueductal gray matter and the medullary raphe nuclei. One pain inhibitory system activated by brain stimulation involves a neural circuit from the PAG to the medullary raphe nuclei. Output from there descends via the DLF to modulate pain transmission in the dorsal horn of the spinal cord. Other analgesia systems are also activated by brain stimulation. Compelling evidence implicates endogenous opiates in SPA. Monoaminergic neurotransmitters are also involved in SPA. Brain stimulation has proven to be useful for the management of some forms of intractable pain in man.

Does naloxone have functional significant activity on medial thalamic neurons? Microiontophoretical study

Local administration (microiontophoretically) of naloxone was tested in 57 parafascicularis thalamic (PF) neurons of morphine-naive and morphine-dependent rats. In morphine-naive rats microiontophoretic applications of naloxone induced changes in 52% of the PF neurons. Reduction in neuronal activity was observed in the majority of them; this reduction phenomena exhibited dose response characteristics, i.e., each incremental naloxone dose caused further decrease of the neuronal discharges. In morphine-dependent animals, 64% of the PF neurons were affected. The changes seen after naloxone were mainly increases of electrical discharges (i.e. the opposite effects obtained in morphine-naive animals).

Naloxone reversible inhibition of reticular neurones in the rat caudal medulla produced by electrical stimulation of the periaqueductal grey matter

Chronic dorsal periaqueductal grey matter electrodes were implanted into adult rats under pentobarbitone anaesthesia. Stimulating these electrodes (25-300 microA) produced behavioural analgesia in 23 of 44 rats tested. In rats given the opiate antagonist naloxone attenuation of this analgesia was seen. In 14 rats displaying behavioural analgesia to periaqueductal grey matter stimulation acute electrophysiological experiments were performed under urethane anaesthesia. Microelectrode recordings were made from neurones, excited by noxious heat or pinch applied to the limbs and tail, and located in the reticular formation of the caudal medulla. Stimulation of the periaqueductal grey matter at an intensity sufficient to produce analgesia in the conscious animal produced direct inhibition of the firing of 62% of neurones tested, excited 23%, had no effect on 14% and attenuated the nociceptive responses of 66%. The inhibitions were characteristically long. Local application of naloxone by microiontophoresis attenuated these long inhibitions in 11 out of 16 neurons tested. Immunohistochemical localization of beta-endorphin containing structures in the vicinity of stimulating and recording sites suggested that the naloxone sensitive inhibition of nociceptive neuronal responses in caudal medulla reticular formation may be due to activation of beta-endorphin fibres descending through the periaqueductal area to the caudal medulla.

Cross-tolerance between two brainstem sites supporting stimulation-produced analgesia

Male albino Holtzman rats were stereotaxically implanted with two bipolar stimulating electrodes, aimed at the periaqueductal gray matter of the brainstem. Focal brain stimulation-produced analgesia was assessed by the tail-flick method. After establishing that focal brain stimulation elicited analgesia at both sites, behavioral tolerance (i.e., reduced analgesia) was induced at one site through repeated stimulation. Upon elicitation of tolerance at one site, stimulation was immediately switched to the other site (which had not been previously rendered tolerant) and analgesia was assessed. Tail-flick latencies revealed transfer of behavioral tolerance from the site given repeated stimulation to the site not given repeated stimulation. While the mechanism involved in this cross-tolerance is not known, a neurochemical substrate may be involved.

Evidence for opioid and non-opioid forms of stimulation-produced analgesia in the rat

This study compares stimulation-produced analgesia (SPA) elicited from two different midline regions of the midbrain of the rat. Dorsal electrode placements were in the caudal periaqueductal gray matter; ventral placements lay within or subjacent to the dorsal raphe n. SPA thresholds were measured by the tail-flick method both during and immediately after the period of brain stimulation. Thresholds were consistently higher in the post-stimulation test. SPA from dorsal and ventral regions differed in the following ways: (1) Post-stimulation analgesia was significantly more difficult to obtain in ventral than in dorsal regions, whereas during-stimulation analgesia did not vary as a function of electrode location; (2) Although a continuous distribution of thresholds was seen for ventral placements, thresholds for dorsal placements tended to be either high or low on both during- and post-stimulation tests; (3) Naloxone (0.01--10 mg/kg) reliably elevated SPA thresholds for ventral but not dorsal stimulation placements. We conclude that different substrates of SPA lie in close proximity to one another in the medial midbrain of the rat. This portion of the midbrain appears to mediate both opioid and non-opioid mechanisms of analgesia.

Electroconvulsive shock (ECS) and H-endorphin-induced analgesia: unconventional interactions with naloxone

Acute administration of electroconvulsive shock (ECS) has been shown previously to produce potent analgesia which is only partially reversed by naloxone but shows almost complete tolerance after both repeated ECS and chronic morphine administration. In an attempt to elucidate the underlying basis of ECS analgesia it was recently compared with the analgesic effect of a newly identified opioid, humoral (H)-endorphin. Intracerebroventricular (i.c.v.) injection of H-endorphin to rats produces a dose-related analgesic effect as measured by the tail flick method. Furthermore, 4 days of daily i.c.v. injections of 40 microgram of morphine resulted in complete tolerance to the analgesic effect of H-endorphin. However, naloxone only caused a partial reversal of H-endorphin analgesia. Surprisingly only the lower dose of 1 mg/kg exerted a significant antagonistic effect while a higher dose of 10 mg/kg of the antagonist was without effect. A similar unconventional profile of the effect of naloxone could be seen with ECS analgesia. Here, pretreatment with 1 mg/kg of naloxone significantly attenuated analgesia, whereas administration of 10 mg/kg of the antagonist was without effect. In contrast, catalepsy measured in the same animals was not affected by 1 mg/kg of naloxone but increasing the dose to 10 mg/kg produced a significant attenuation of ECS catalepsy. The opioid nature of H-endorphin analgesia on one hand and the unconventional dose relation with naloxone of both H-endorphin and ECS analgesia on the other hand, suggests the involvement of this opioid in analgesia induced by ECS. Furthermore, it is possible that other behavioral manipulations which display only partial opiate characteristics may be mediated by H-endorphin or similar endogenous substances.

Caerulein and morphine in a model of visceral pain. Effects on the hypotensive response to renal pelvis distension in the rat

In pentobarbital-anaesthetized rats (60 mg/kg, i.p.) renal pelvis distension with a pressure of 80 cm H2O caused a decline in mean arterial blood pressure. This pressure response, which disappeared rapidly after cessation of the distension, was used to study the effects of analgesic drugs known to be effective in renal colic pain in man. Morphine (0.75 and 1 mg/kg, s.c.) and the decapeptide caerulein (1.6, 4 and 8 microgram/kg, s.c.) abolished the pressure response. The effects of the largest doses lasted for at least 30 min. Ineffective in this respect were (a) desulphated caerulein (40 microgram/kg, s.c.) and (b) additional doses of pentobarbital (20 and 40 mg/kg, s.c.). This shows (a) the importance of the sulphated tyrosine (known from previous studies on central effects) and (b) the missing influence of the depth of anaesthesia. Naloxone (0.5 mg/kg, s.c.) abolished the effect of morphine (1 mg/kg, s.c.) but failed to influence that of caerulein (8 microgram/kg, s.c.). Even a fourfold dose of naloxone (2 mg/kg, s.c.) did not weaken the effect of caerulein. Naloxone, per se, was ineffective. These results suggest different mechanisms of the present effects of morphine and caerulein. It appears that renal pelvis distension in the anaesthetized rat can serve as a model of renal colic.

Neural and neurochemical mechanisms of pain inhibition

Recent studies which suggest the existence of an endogenous neural substrate of pain inhibition are reviewed. Electrical stimulation of some areas of the medial brain stem in laboratory rats has produced a dramatic degree of analgesia. Such stimulation-produced analgesia is partially blocked by an opiate antagonist drug. This observation has proved seminal to those searching for endogenous opiate-like neurochemicals, the opioid peptides (enkephalins and endorphins). Particular attention is focused on studies of stimulation-produced analgesia, the underlying anatomy and physiology of endogenous analgesia systems elucidated by such work, and the relation of these findings to discoveries about opiate receptors and opioid peptides. Very recent work suggesting that certain forms of stress are natural triggers for activating endogenous analgesia mechanisms is described. Although certain stressors cause analgesia mediated by opioids, other stressors cause analgesia by neurochemically different means. Thus, multiple analgesia substrates, opioid and nonopioid, appear to exist.

Naloxone does not antagonize the anesthetic-induced depression of nociceptor-driven spinal cord response in spinal cats

The effects of several anaesthetics on spinal cord nociceptive neural mechanisms and their interactions with the opiate antagonist, naloxone, were studied in acute, spinal cord transected cats. Intra-arterial injection of bradykinin was used as the noxious test stimulus. Spontaneous activity and the neural response induced by bradykinin were recorded by the multi-unit activity technique in the lateral funiculus of the spinal cord. Naloxone, 0.1 or 2.0 mg/kg i.v. had little effect on the bradykinin-induced response, but enhanced the spontaneous firing of the lateral funiculus significantly. Fentanyl, 30 micrograms/kg i.v., depressed both the bradykinin-induced response and spontaneous firing. These effects of fentanyl were antagonized completely by naloxone, 0.1 mg/kg i.v. Nitrous oxide, thiamylal, halothane and ether depressed the bradykinin-induced response considerably, but it was not antagonized by naloxone, 0.1-2.0 mg/kg i.v. Enflurane had little effect on the bradykinin-induced response. The effects of these anesthetics on spontaneous firing were divergent: nitrous oxide enhanced it while other drugs depressed it, to various degrees. All these data suggest that the neural and/or neurochemical mechanisms of anesthetic-induced analgesia differ from mechanisms related to opioids.

The effect of systemic morphine upon diffuse noxious inhibitory controls (DNIC) in the rat: evidence for a lifting of certain descending inhibitory controls of dorsal horn convergent neurones

The effects of exogenous opiates upon diffuse noxious inhibitory controls (DNIC) was investigated in intact anaesthetized rats. 58 convergent neurones, responding to both noxious and innocuous stimuli applied to their cutaneous receptive fields, were recorded at the lumbar level. These cells received A- and C-peripheral fibre inputs as shown by electrical stimulation of their receptive fields and were mainly located in the medial part of the dorsal horn. The immersion of the distal two-thirds of the tail in hot water (52 degrees C) induced strong inhibition of the responses to both A-(23%) and C-(69%) fibres. Post-effects of long duration were commonly observed after cessation of the conditioning stimulus. While systemic injection of morphine at a low dose-range (0.1-1 mg/kg) did not significantly affect the unconditioned responses, the DNIC-mediated inhibitions were profoundly altered. (a) DNIC of responses to C fibres were dose-dependently (P less than 0.01) lifted by morphine: (b) the post-effects observed after cessation of conditioning stimuli were dose-dependently (P less than 0.01) diminished; (c) DNIC of responses to A-fibre were similarly altered but this effect was less significant (P less than 0.05); (d) DNIC of responses to sustained moderate pressure were greatly diminished by morphine (P less than 0.01); and (e) these effects were specific since they were antagonized by the opiate antagonist, naloxone. In addition, they were shown to be stereospecific since while the dextrogyre stereoisomer, dextrorphan, was ineffective the levogyre derivative, levorphanol, induced a significant lifting of DNIC. It is concluded that morphine decreases the supraspinal inhibitory controls of dorsal horn convergent neurones, at least when these controls are triggered by noxious stimuli. Assuming that a basic somatosensory background activity (noise) is transmitted to higher centres by dorsal horn convergent neurones, and that the pain-signalling message is the contrast between the activity of the segmental pool of neurones induced by the noxious stimulus and the DNIC-mediated silence of the remaining neuronal population, it is proposed that, by a reduction in DNIC, low-dose morphine could restore the initial level of background activity, the final result being analgesia.

Effect of naloxone upon diffuse noxious inhibitory controls (DNIC) in the rat

Twenty-eight convergent neurones, responding to both noxious and innocuous stimuli applied to their cutaneous receptive fields were recorded at the lumbar level in anaesthetized intact rats. These cells received Aa and C fibre inputs as shown by electrical stimulation of their receptive fields, and were located in the medial part of the dorsal horn. (a) For 15 units, Diffuse Noxious Inhibitory Controls (DNCI)67,68 were investigated by applying noxious thermal stimuli (52 degrees C) to the distal two-thirds of the tail. This conditioning stimulus induced strong inhibition of the responses to both Aa (28%) and C (71%) fibres. Post-effects of long duration were commonly observed after cessation of the conditioning stimulus. The systemic injection of naloxone (0.3 mg/kg, i.v.) resulted in a partial reduction in these inhibitory effects with a decrease of about 50% for both Aa and C fibre response 10 min after naloxone administration. This was followed by a progressive recovery lasting 30 min. (b) 28 convergent units, including the 15 reported above, were recorded to investigate the effect of naloxone upon the unconditioned response. Responses to Aa fibre were unaffected, whereas the responses to C fibre were slightly (17%) but significantly increased by naloxone.

Effects of focal electrical stimulation and morphine microinjection in the periaqueductal gray of the rat mesencephalon on neuronal activity in the medullary reticular formation

Neurons in the medullary reticular formation (MRF; nucleus reticularis gigantocellularis and nucleus reticularis paragigantocellularis) were evaluated for their involvement in the analgesia produced by focal electrical stimulation and microinjection of morphine into the periaqueductal gray region (PAG) of the rat mesencephalon. Analgesia-producing PAG stimulation altered the spontaneous activity of 80% of the neurons in the MRF (both excitation and inhibition were observed) and inhibited the noxious-evoked excitation of 75% of MRF neurons. Microinjection of morphine into the PAG also increased (50%) and decreased (17%) the spontaneous activity of MRF units and inhibited the noxious-evoked excitation of 47% of MRF neurons. These effects were specific for analgesia produced by the PAG manipulations and were partially reversed by naloxone. The role of the MRF in PAG-induced analgesias and the degree of overlap in neuronal systems influenced by intracranial morphine and electrical stimulation is discussed.