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

Relative contribution of the dorsal raphe nucleus and ventrolateral periaqueductal gray to morphine antinociception and tolerance in the rat

The dorsal raphe nucleus (DRN) is embedded in the ventral part of the caudal periaqueductal gray (PAG). Electrical or chemical activation of neurons throughout this region produces antinociception. The objective of this manuscript is to determine whether the ventrolateral PAG and DRN are distinct antinociceptive systems. This hypothesis was tested by determining the antinociceptive potency of microinjecting morphine into each structure (Experiment 1), creating a map of effective microinjection sites that produce antinociception (Experiment 2) and comparing the development of antinociceptive tolerance to repeated microinjections of morphine into the ventrolateral PAG and DRN (Experiment 3). Morphine was more potent following cumulative injections (1.0, 2.2, 4.6 & 10 μg/0.2 μL) into the ventrolateral PAG (D₅₀  = 3.3 μg) compared to the lateral (4.3 μg) or medial DRN (5.8 μg). Antinociception occurred following 94% of the morphine injections into the ventrolateral PAG, whereas only 68.3% and 78.3% of the injections into the lateral and medial aspects of the DRN produced antinociception. Repeated microinjections of morphine into the ventrolateral PAG produced tolerance as indicated by a 528% difference in potency between morphine and saline pretreated rats. In contrast, relatively small changes in potency occurred following repeated microinjections of morphine into the lateral and medial aspects of the DRN (107% and 49%, respectively). These data indicate that the ventrolateral PAG and DRN are distinct antinociceptive structures. Antinociception is greater with injections into the ventrolateral PAG compared to the DRN, but this antinociception disappears rapidly because of the development of tolerance.

Analgesia induced by localized injection of opiate peptides into the brain of infant rats

BACKGROUND

Stimulation of a variety of brain sites electrically or by opiates activates descending inhibitory pathways to attenuate noxious input to the spinal cord dorsal horn and produce analgesia. Analgesia induced by electrical stimulation of the periaqueductal grey (PAG) of the midbrain or medial rostral ventral medulla (RVM) matures late, towards the end or past the pre-weaning period. Descending facilitation takes precedence over inhibition. Yet opiates injected intracerebroventricularly or directly into the PAG induce analgesia relatively early in development. Our goal was to re-examine the role of opiates specific to individual receptor types in analgesia at several supraspinal sites.

METHODS

Antinociception was tested following microinjection of DAMGO (μ-opiate agonist), DPDPE (∂-opiate agonist) or U50,488 (κ-opiate agonist) into the PAG, RVM or dorsal lateral pons (DLP) in 3-, 10- and 14-day-old rats.

RESULTS

DAMGO produced analgesia at 3 days of age at each brain area; the RVM was the most effective and the dorsal PAG was the least effective site. DPDPE produced modest analgesia at 10 and 14 days of age at the ventral PAG, RVM or DLP, but not the dorsal PAG. U50,488H was ineffective at all sites and all ages.

CONCLUSIONS

Antinociception could be elicited at all three sites by DAMGO as early as 3 days of age and DPDPE at 10 and 14 days of age. The degree of analgesia increased gradually during the first 2 weeks of life, and likely reflects the maturation of connections within the brain and of descending inhibitory paths from these sites.

Activation of reciprocal pathways between arcuate nucleus and ventrolateral periaqueductal gray during electroacupuncture: involvement of VGLUT3

Electroacupuncture (EA) at the Jianshi-Neiguan acupoints (P5-P6, overlying the median nerve) attenuates sympathoexcitatory responses through activation of the arcuate nucleus (ARC) and ventrolateral periaqueductal gray (vlPAG). Activation of the ARC or vlPAG respectively leads to neuronal excitation of the both nuclei during EA. However, direct projections between these two nuclei that could participate in central neural processing during EA have not been identified. The vesicular glutamate transporter 3 (VGLUT3) marks glutamatergic neurons. Thus, the present study evaluated direct neuronal projections between the ARC and vlPAG during EA, focusing on neurons containing VGLUT3. Seven to ten days after unilateral microinjection of a rodamine-conjugated microsphere retrograde tracer (100nl) into the vlPAG or ARC, rats were subjected to EA or served as a sham-operated control. Low frequency (2Hz) EA was performed bilaterally for 30min at the P5-P6 acupoints. Perikarya containing the microsphere tracer were found in the ARC and vlPAG of both groups. Compared to controls (needle placement without electrical stimulation), c-Fos immunoreactivity and neurons double-labeled with c-Fos, an immediate early gene and the tracer were increased significantly in the ARC and vlPAG of EA-treated rats (both P<0.01). Moreover, some neurons were triple-labeled with c-Fos, the retrograde tracer and VGLUT3 in the two nuclei following EA stimulation (P<0.01, both nuclei). These results suggest that direct reciprocal projections between the ARC and vlPAG are available to participate in prolonged modulation by EA of sympathetic activity and that VGLUT3-containing neurons are an important neuronal phenotype involved in this process.

Intraperiaqueductal gray glycine and D-serine exert dual effects on rostral ventromedial medulla ON- and OFF-cell activity and thermoceptive threshold in the rat

We have studied the involvement of the N-methyl-D-aspartate receptor (NMDAR) glycine site and the strychnine-sensitive glycine receptor (GlyR) in the ventrolateral periaqueductal gray (VL-PAG) on nociceptive behavior (tail flick) and pain-related changes on neuronal activity in the rostral ventromedial medulla (RVM). Glycine or D-serine increased the tail-flick latency, reduced OFF-cell pause, and delayed its onset and increased the time between the onset of the OFF-cell pause and the tail withdrawal. Conversely, they decreased the ongoing activity of the ON cell, the tail-flick-induced ON-cell firing, whereas they delayed the onset of increased tail-flick-induced ON-cell firing. Also, glycine or D-serine reduced the interval between the onset of the increased ON-cell firing and tail withdrawal. Whereas 7-Cl-kynurenic acid (7-Cl-KYN) prevented such effects, strychnine did not do so. A higher dose of 7-Cl-KYN or strychnine was per se able to reduce or increase tail-flick latency and increase or reduce ON-cell activities, respectively. A higher dose of glycine was hyperalgesic in the presence of 7-Cl-KYN, whereas such an effect was prevented by strychnine. These data suggest 1) a dual role of glycine in producing hyperalgesia or analgesia by stimulating the GlyR or the NMDARs within the VL-PAG, respectively; 2) consistently that RVM ON and OFF cells display opposite firing patterns to the stimulation of the VL-PAG NMDAR glycine site and GlyR activation; and 3) a tonic role of these receptors within the VL-PAG-RVM antinociceptive descending pathway.

Intermittent dosing prolongs tolerance to the antinociceptive effect of morphine microinjection into the periaqueductal gray

Tolerance to the antinociceptive effect of microinjecting morphine into the ventrolateral periaqueductal gray (vPAG) develops with repeated administration. The objective of the present experiment was to determine whether the magnitude and duration of tolerance differ depending on the interval between injections. Rats were injected with morphine or saline into the vPAG either once daily for 4 days or twice daily for 2 days. All rats were injected with morphine into the vPAG for the fifth injection to determine whether tolerance had developed. Morphine microinjection produced tolerance in both morphine-pretreated groups regardless of inter-dose interval. One and two weeks later, microinjection of morphine produced an increase in hot plate latency in all groups except rats pretreated with daily morphine microinjections. That is, tolerance was evident 2 weeks following the induction of tolerance produced by daily, but not twice daily injections of morphine. Although a long inter-dose interval has been shown to prolong the duration of tolerance after systemic morphine administration, this is the first report showing a similar effect with direct administration of morphine into the brain. Given that associative learning underlies prolonged tolerance with systemic morphine administration, the present data suggest that associative mechanisms of tolerance are also engaged when morphine administration is restricted to the PAG.

Differential susceptibility of the PAG and RVM to tolerance to the antinociceptive effect of morphine in the rat

The periaqueductal gray (PAG) and rostral ventromedial medulla (RVM) are part of a nociceptive modulatory system. Microinjection of morphine into either structure produces antinociception. Tolerance develops to ventrolateral PAG mediated antinociception with repeated microinjection of morphine. In contrast, there are no published reports of tolerance to morphine administration into the RVM. Three experiments were conducted to determine whether tolerance develops to morphine microinjections into the RVM. Experiment 1 compared tolerance to the antinociceptive effect of microinjecting morphine (5 microg/0.5 microl) into the PAG and RVM following daily injections for four consecutive days. Experiment 2 assessed tolerance to a range of morphine doses (2.5-20 microg) after injecting morphine into the RVM twice a day for two consecutive days. Experiment 3 followed a similar procedure except twice as many RVM injections were made (8 microinjections in 4 days). The degree to which tolerance developed to the antinociceptive effect of morphine was much greater with microinjections into the PAG compared to the RVM. There was a 64% drop in hot plate latency from the first to the fifth injection of morphine into the PAG, but only a 36% drop in latency following RVM microinjections. Reducing the interdose interval to two injections a day or increasing the total number of injections from 4 to 8 did not enhance the development of tolerance to RVM morphine administration. These data demonstrate that opioid-sensitive neurons in the RVM are relatively resistant to the development of tolerance compared to PAG neurons.

Electroacupuncture induces c-Fos expression in the rostral ventrolateral medulla and periaqueductal gray in cats: relation to opioid containing neurons

Our previous studies have shown that electroacupuncture (EA) at the Neiguan-Jianshi (P5-P6) acupoints inhibits sympathetic outflow and attenuates excitatory visceral cardiovascular reflexes through enkephalin- or beta-endorphin-related opioid receptors in the rostral ventrolateral medulla (rVLM). It is not known whether EA at these acupoints activates neurons containing enkephalin or beta-endorphin in the rVLM as well as in the periaqueductal gray (PAG) that are involved in EA-mediated central neural regulation of sympathetic activity. The present study evaluated activated neurons in the rVLM and PAG by detecting c-Fos immunoreactivity, and identified the relationship between c-Fos nuclei and neuronal structures containing enkephalin or beta-endorphin in these regions. To enhance the detection of cell bodies containing enkephalin or beta-endorphin, colchicine (90-100 microg/kg) was injected into the subarachnoid space in anesthetized cats 28-30 h prior to EA or the sham-operated control for EA. Following bilateral barodenervation and cervical vagotomy, EA (1-4 mA, 2 Hz, 0.5 ms) was performed at the P5-P6 acupoints (overlying median nerve; n=7) for 30 min. Identical procedures, with the exception of electrical stimulation, were carried out in five control animals. EA decreased blood pressure (BP) in four of seven cats (5-15 mm Hg) while the sham procedure for EA produced no responses. Perikarya containing enkephalin were found in the rVLM and rarely in the PAG, while no cell bodies labeled with beta-endorphin were identified in either region. Compared to animals in the control group, more c-Fos immunoreactivity, located principally in close proximity to fibers containing enkephalin or beta-endorphin, was observed in the rVLM and ventrolateral PAG (vlPAG) in EA-treated cats. Moreover, neurons double-labeled with c-Fos and enkephalin in the rVLM were significantly increased in cats following EA stimulation (P<0.05). These data indicate that EA at the P5-P6 acupoints activates neurons in the rVLM and vlPAG. These activated neurons contain enkephalin in the rVLM, and most likely interact with nerve fibers containing enkephalin or beta-endorphin in both the rVLM and vlPAG. The results from this study provide the first anatomical evidence showing that EA at the P5-P6 acupoints has the potential to influence neuronal structures (perikarya, axons and/or dendrites) containing enkephalin or beta-endorphin in specific regions of the brain stem. These neurons likely form the substrate for EA's influence on sympathoexcitatory cardiovascular reflexes.

Hypoalgesia induced by elbow manipulation in lateral epicondylalgia does not exhibit tolerance

Previous studies have demonstrated that the initial hypoalgesic effect of spinal manipulative therapy was not antagonized by naloxone and did not exhibit tolerance with repeated applications. The implication is that endogenous opioid mechanisms of pain relief are probably not at play in spinal manipulative therapy. The role of endogenous opioid peptides in manipulation of the peripheral joints has not been investigated. The aim of this study was to evaluate whether the initial hypoalgesic effect of a peripheral manipulative technique (mobilization-with-movement treatment for the elbow) demonstrated a tolerance to repeated applications (ie, reduction in magnitude of effect over repeated applications). Twenty-four participants with unilateral chronic lateral epicondylalgia participated in the study. A repeated measures study was conducted to examine the effect of repeated applications of the mobilization-with-movement treatment for the elbow on 6 separate treatment occasions at least 2 days apart. Pain-free grip strength and pressure pain threshold were chosen as the pain-related outcome measures. Changes in the percent maximum possible effect scores of measures of hypoalgesia were evaluated across the 6 treatment sessions by using linear trend analysis. The results showed no significant difference for the hypoalgesic effect of the treatment technique between sessions (P >.05). This peripheral manipulative therapy treatment technique appeared to have a similar effect profile to previously studied spinal manipulative therapy techniques, thereby contributing to the body of knowledge that indicates that manipulative therapy most likely induces a predominant non-opioid form of analgesia.

Behavioral evidence linking opioid-sensitive GABAergic neurons in the ventrolateral periaqueductal gray to morphine tolerance

Tolerance develops to the antinociceptive effects of morphine with repeated microinjections into the ventrolateral periaqueductal gray (PAG). This tolerance could be caused by adaptations within the PAG or anywhere along the descending pathway (rostral ventromedial medulla to spinal cord). If tolerance is caused by a change along the descending pathway, then tolerance should develop to direct activation of PAG output neurons. However, if tolerance is caused by a change to neurons within the PAG, then tolerance will not occur with repeated direct activation of PAG output neurons. This hypothesis was tested by assessing antinociception following repeated microinjections of the GABA antagonist bicuculline and the excitatory amino acid kainate into the ventrolateral PAG. Microinjection of bicuculline and kainate produces antinociception by disinhibition and direct excitation of ventrolateral PAG output neurons, respectively. Repeated administration of these drugs into the ventrolateral PAG produced antinociception with no evidence of tolerance. That is, the hot-plate latency and responsiveness to intraplantar formalin administration was comparable whether rats received the drug for the first or fifth time. Moreover, microinjection of bicuculline or kainate produced comparable antinociception in rats pretreated with these drugs and saline-treated control rats. These data demonstrate that repeated activation of ventrolateral PAG output neurons is not sufficient to produce tolerance. Thus, tolerance must be caused by a change in neurons preceding output neurons in this circuit, presumably opioid-sensitive GABAergic neurons.

Comparison of morphine and kainic acid microinjections into identical PAG sites on the activity of RVM neurons

The rostral ventromedial medulla (RVM) modulates nociception through changes in the activity of two classes of neuron, ON- and OFF-cells. The activity of these neurons is regulated, in part, by input from the periaqueductal gray (PAG). The objective of this study was to determine whether PAG-mediated antinociception is associated with excitation of both ON- and OFF-cells in the RVM. Microinjection of morphine into the ventrolateral PAG produced antinociception at 50% of the injection sites. This antinociception was associated with continuous activation of RVM OFF-cells and inhibition of both the spontaneous and reflex-related activity of RVM ON-cells. Microinjection of kainic acid into the same injection sites produced antinociception 92% (37/40) of the time. Although kainic acid directly excites PAG output neurons, the changes in ON- and OFF-cell activity associated with microinjection of kainic acid into the ventrolateral PAG were the same as when morphine was injected. That is, ON-cells were inhibited and OFF-cells were activated. These data indicate that the excitatory connection between the PAG and RVM is directed at RVM OFF-cells specifically. In addition, these data suggest that direct activation of PAG output neurons, as occurs with kainic acid, is much more likely to produce antinociception than disinhibition of output neurons as occurs following morphine administration.

Influence of dorsolateral periaqueductal grey (dlPAG) lesions on contextual conditioning with massed and distributed shock

Three experiments investigated the conditions under which electrolytic lesions of the dorsolateral periaqueductal grey (dlPAG) facilitate conditioned defensive freezing in the rat (Rattus norvegicus). Experiment 1 found that dlPAG lesions placed before context-shock pairings facilitated conditioned defensive freezing with massed but not distributed shock. No such effect was found in Experiment 2, when the lesions were placed after context-shock pairings. Experiment 3 found that dlPAG lesions facilitated subsequent conditioning with massed but not a single shock. In addition, no differences in sensitivity to thermal or shock pain were evident in lesioned and unlesioned rats. Taken together, these results are consistent with the suggestion that dlPAG activation interferes with the processing of contextual cues during association formation.

Activation of the ventrolateral periaqueductal gray reduces locomotion but not mean arterial pressure in awake, freely moving rats

Activation of the ventrolateral periaqueductal gray produces immobility and antinociception. It has been argued that these behaviors are part of either a defensive fear response to threat or a recuperative quiescence response to deep tissue injury. Data collected in anesthetized animals showing that activation of the ventrolateral periaqueductal gray has a hypotensive effect supports the quiescence hypothesis. Our objective was to determine whether activation of the ventrolateral periaqueductal gray in awake, freely moving rats results in a decrease in blood pressure as it does in anesthetized animals. Changes in blood pressure produced by microinjection of the neuroexcitant D,L-homocysteic acid were measured using radio telemetry while rats were awake and while anesthetized with pentobarbital. Consistent with earlier reports, microinjection of D,L-homocysteic acid into the ventrolateral periaqueductal gray caused a decrease in blood pressure in anesthetized rats. In contrast, microinjection at the same ventrolateral periaqueductal gray sites while rats were awake had no effect on blood pressure, even though the animals became immobile and heart rate decreased. Thus, the immobility evoked from ventrolateral periaqueductal gray is not associated with a fall in mean arterial pressure. Two conclusions can be drawn from these data. (1) Caution must be used in generalizing from data collected in anesthetized animals. (2) The ventrolateral periaqueductal gray is as likely to contribute to defensive fear as to recuperative quiescence.

Immobility accompanies the antinociception mediated by the rostral ventromedial medulla of the rat

The rostral ventromedial medulla (RVM) is part of a descending pain modulatory system that runs from the periaqueductal gray (PAG) to the spinal cord. The objective of the present study was to determine whether the antinociception mediated by the RVM is associated with locomotor changes as has been reported for the PAG [42]. Kainate (4, 20, or 40 pmol), morphine (1, 5, or 10 microg), or saline (0.2 or 0. 5 microl) was injected into the RVM and locomotion and nociception assessed. Microinjections of kainate and morphine that produced antinociception almost invariably inhibited locomotor activity. In some rats this immobility consisted of no movements when placed in the center of the open field chamber. These data are consistent with the immobility and antinociception produced by activation of the ventrolateral PAG and indicate that the descending ventrolateral PAG/RVM system integrates a behavioral response of which antinociception is only one component.

Pharmacological evidence for a periaqueductal gray-nucleus raphe magnus connection mediating the antinociception induced by microinjecting carbachol into the dorsal periaqueductal gray of rats

A previous study demonstrated that microinjection of carbachol (CCh) into the dorsal periaqueductal gray matter (dPAG) of rats increases the latency for the tail flick reflex. Several other studies have implicated the raphe magnus (NRM) and the reticularis paragigantocellularis (NRPG) nuclei as relay stations through which descending pathways from the PAG project to the spinal cord via the dorsolateral funiculus (DLF). In the present study, the effects of microinjecting CCh into the dPAG on the tail flick test were examined in rats in which the ipsilateral DLF was previously lesioned, or saline or lidocaine (2%) was microinjected into the NRM or ipsilateral NRPG. The DLF lesion did not change the baseline threshold of the animals in the test, but abolished the CCh-induced increase in the tail flick latency from the dPAG. The neural block of the NRM or NRPG with lidocaine also did not change significantly the latency for the tail flick reflex. The increase in the tail flick latency produced by CCh from the dPAG was not changed by the neural block of the NRPG, but was significantly reduced by the neural block of the NRM. These results are interpreted as indicative that the central antinociceptive mechanisms activated by CCh from the dPAG depend on a descending pathway that projects to the spinal cord via DLF utilizing at least the NRM, but not the NRPG, as an intermediary relay station.

Immobility and flight associated with antinociception produced by activation of the ventral and lateral/dorsal regions of the rat periaqueductal gray

It has long been known that the periaqueductal gray (PAG) plays an important role in the modulation of nociception. Given that activation of the lateral PAG also produces wild running and tachycardia, it has been suggested that PAG mediated antinociception is part of an integrated defensive reaction. However, an alternative hypothesis is that these effects are merely a secondary response to aversive brain stimulation. If antinociception and flight reactions are caused by aversive brain stimulation, then these effects should always occur together. The objective of the present study was to determine whether antinociception and locomotion could be dissociated by microinjecting morphine and kainic acid into various subdivisions of the caudal PAG. Non-selective activation of lateral and dorsal regions of the PAG by microinjection of kainic acid produced wild running, while injections into the ventrolateral PAG produced immobility. Microinjection of morphine evoked similar locomotor effects, although the onset to effect was slower with morphine (approximately 5 min vs. 1 min for kainic acid), and the antinociceptive efficacy of microinjecting 0.2 microl of morphine was less than with kainic acid injections. In fact, microinjection of morphine evoked locomotor effects in the absence of antinociception on 39% of the tests. Increasing the injection volume to 0.4 microl (dose remained at 5 microg) greatly enhanced the likelihood that antinociception and locomotor effects (e.g. running, freezing, circling) occurred simultaneously (79%). These findings indicate that, although distinct locomotor effects are associated with antinociception from the ventral and more dorsal regions of the PAG, antinociceptive and locomotor effects can occur independently. This finding is consistent with the hypothesis that ventral and dorsal regions of the PAG integrate defensive freezing and flight reactions, respectively.

Endothelin-1 in periaqueductal gray area of mice induces analgesia via glutamatergic receptors

The aim of the study was to examine whether endothelin-1 (ET-1) injected into dorsolateral periaqueductal gray (PAG) area of mice produces antinociception. ET-1, from 1 to 4 pmol/mouse, induced antinociceptive effect in a dose-dependent manner. This antinociceptive effect was prevented by NMDA receptor antagonists (2-APV and MK-801) injected in the same area (2-APV) or by intraperitoneal route (MK-801). CNQX, a non-NMDA receptor antagonist, did not inhibit the ET-1 effects. Prazosin, an alpha 1-adrenergic blocking agent, also prevented the ET-1 antinociceptive effect. We suggest that the activation of NMDA glutamatergic receptors in the PAG area may be a necessary step for ET-1 induced antinociception.

Antinociceptive effects of carbachol microinjected into different portions of the mesencephalic periaqueductal gray matter of the rat

The changes in tail-flick latency (TFL) to noxious heating of the skin produced by the microinjection of carbachol (CCh) into the dorsal (dPAG), lateral (lPAG), and ventral (vPAG) portions of the mesencephalic periaqueductal gray matter (PAG) were studied in the rat. A significant increase in TFL was produced by CCh (0.2 microgram/0.5 microliter) microinjected into sites widely distributed within the PAG. The effect of CCh was stronger in the most caudal portion of the DPAG. Smaller effects were obtained after injection of CCh into the aqueduct, indicating that drug diffusion from the injection sites to the aqueduct lumen is unlikely to cause the antinociceptive effect of CCh. Dimethyl-phenyl-piperazinium (0.35 microgram/0.5 microliter), but not bethanechol (0.22 and 0.44 microgram/0.5 microliter), produced effects similar to CCh (0.2 microgram/0.5 microliter), when injected into the dPAG. The effects of CCh were inhibited by the previous administration of mecamylamine (1 microgram/0.5 microliter), but not atropine (1 microgram/0.5 microliter) or naloxone (1 microgram/0.5 microliter), into the dPAG. These results are indicative that antinociception produced by CCh from the dPAG depends on nicotinic, but not muscarinic or opioid mechanisms within the dPAG. The intraperitoneal administration of phenoxybenzamine (1 mg/kg) or mecamylamine (1 mg/kg), but not naloxone (1 mg/kg), methysergide (1 mg/kg), or atropine (1 mg/kg), inhibited the effects of CCh injected into the dPAG. In contrast, a higher dose of intraperitoneal phenoxybenzamine (5 mg/kg) was ineffective against the antinociception evoked by CCh when injected into the vPAG. Therefore, the effects of CCh from the dPAG may depend on the activation of centrifugal pathways involving both nicotinic and alpha-adrenergic mechanisms. In addition, the results indicate that different cholinergic substrates in the PAG may mediate both alpha-adrenergic and non-alpha-adrenergic descending pain mechanisms activated by the dPAG and vPAG, respectively.

Blockade of morphine-induced analgesia and tolerance in mice by MK-801

The effect of MK-801 on morphine-induced analgesia, tolerance and opioid binding sites was examined in mice. In analgesia studies, mice received either naloxone or MK-801. Controls were injected with saline. Mice were then injected with morphine 10 or 30 min following naloxone or MK-801, respectively, and tested for analgesia (tail flick assay) 45 min later. Pretreatment with naloxone or MK-801 blocked morphine-induced analgesia. In tolerance studies, mice were pretreated with either saline or MK-801. Thirty minutes later, mice were injected with either saline or morphine (acutely or chronically) and tested for analgesia 24 h later. Pretreatment with MK-801 partially or completely blocked the development of acute and chronic tolerance, respectively. In binding studies, MK-801 displaced [3H]naloxone poorly compared to naloxone or morphine. Together, these data suggest a role for NMDA receptors in morphine-induced analgesia and tolerance. The poor inhibition of the [3H]naloxone binding sites by MK-801 supports the possibility that MK-801 might not act directly on the opioid receptors, but rather, inhibits morphine-induced analgesia and tolerance by some other mechanisms.

Nitric oxide synthase immunoreactive neurons anatomically define a longitudinal dorsolateral column within the midbrain periaqueductal gray of the rat: analysis using laser confocal microscopy

Nitric oxide has recently been proposed as a neuronal messenger in both the central and peripheral nervous system. Antibodies against nitric oxide synthase (NOS), the synthesizing enzyme for nitric oxide, were used in combination with immunocytochemistry and confocal laser microscopy to analyze the distribution of this enzyme in the midbrain periaqueductal gray (PAG) of the rat. NOS immunoreactive neurons were localized predominantly in a longitudinally oriented column in the dorsolateral PAG. NOS immunoreactive fibers and processes were scattered throughout the PAG but were most prevalent in the dorsolateral column and in the juxta-aqueductal column. This study provides neurochemical support for the existence of longitudinal columns in the PAG which are postulated to underlie the functional organization of this complex brainstem region.

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 of the periaqueductal gray matter of the rat produces a preferential ipsilateral antinociception

The few studies analyzing somatotopic organization of stimulation-produced antinociception (SPA) from the periaqueductal gray matter (PAG) have reported contradictory results. In the present study, the distribution of SPA on the hindquarters was assessed by measuring the threshold for inhibition of withdrawal reflexes to noxious heat applied to the hindpaws and tail in pentobarbital-anesthetized rats. Of the 3 body regions tested, the hindpaw contralateral to the stimulating electrode required the highest level of PAG stimulation to inhibit withdrawal. Reducing the intensity of the heat stimulus applied to the hindpaws caused a concomitant reduction in SPA threshold. As before, a higher stimulation current was needed to inhibit the withdrawal reflex in the contralateral than in the ipsilateral paw. These data indicate the antinociception from PAG stimulation is not equally distributed throughout the body, and that the intensity of the noxious stimulus influences the threshold for SPA.

Differential effects of selective opioid peptide antagonists on the acquisition of pavlovian fear conditioning

Pretreatment with opioid antagonists enhances acquisition of Pavlovian fear conditioning. The present experiments attempted to characterize the type of opioid receptor responsible for this effect using a procedure that assessed the fear of rats to a chamber previously associated with electric shock (1 mA, 0.75 s). Freezing, a species-typical immobility, was employed as an index of fear. Two mu opioid antagonists, CTOP (40 ng) and naloxonazine (10 micrograms), enhanced conditioning. On the other hand, the kappa antagonist nor-binaltorphimine reduced conditioning. Two delta antagonist treatments (16-methyl cyprenorphine and naltrindole) had no reliable effect on acquisition. Thus the enhancement of conditioning appears to be mediated by mu receptors. Previous research has shown that the conditional fear produced by these procedures caused an analgesia that is also mediated by mu receptors. It is argued that the enhancement effect occurs because of an antagonism of this analgesia and that the analgesia normally acts to regulate the level of fear conditioning.

Subregions of the periaqueductal gray topographically innervate the rostral ventral medulla in the rat

Previous anatomical and physiological studies have revealed a substantial projection from the periaqueductal gray (PAG) to the nucleus paragigantocellularis (PGi). In addition, physiological studies have indicated that the PAG is composed of functionally distinct subregions. However, projections from PAG subregions to PGi have not been comprehensively examined. In the present study, we sought to examine possible topographic specificity for projections from subregions of the PAG to PGi. Pressure or iontophoretic injections of wheat germ agglutinin-conjugated horseradish peroxidase, or of Fluoro-Gold, placed into the PGi of the rat retrogradely labeled a substantial number of neurons in the PAG from the level of the Edinger-Westphal nucleus to the caudal midbrain. Retrogradely labeled neurons were preferentially aggregated in distinct subregions of the PAG. Rostrally, at the level of the oculomotor nucleus, labeled neurons were i) compactly aggregated in the ventromedial portion of the PAG corresponding closely to the supraoculomotor nucleus of the central gray, ii) in the lateral and ventrolateral PAG, and iii) in medial dorsal PAG. More caudally, retrogradely labeled neurons became less numerous in the dorsomedial PAG but were more widely scattered throughout the lateral and ventrolateral parts of the PAG. Only few retrogradely labeled neurons were found in the ventromedial part of the PAG at caudal levels. Injections of retrograde tracers restricted to subregions of the PGi suggested topography for afferents from the PAG. Injections into the lateral portion of the PGi yielded the greatest number of labeled neurons within the rostral ventromedial PAG. Medially placed injections yielded numerous retrogradely labeled neurons in the lateral and ventrolateral PAG. Injections placed in the rostral pole of the PGi (medial to the facial nucleus) produced the greatest number of retrogradely labeled neurons in the dorsal PAG. To examine the pathways taken by fibers projecting from PAG neurons to the medulla, and to further specify the topography for the terminations of these afferents in the PGi, the anterograde tracer Phaseolus vulgaris-leucoagglutinin was iontophoretically deposited into subregions of the PAG that contained retrogradely labeled neurons in the above experiments. These results revealed distinct fiber pathways to the rostral medulla that arise from the dorsal, lateral/ventrolateral, and ventromedial parts of the PAG. These injections also showed that there are differential but overlapping innervation patterns within the PGi. Consistent with the retrograde tracing results, injections into the rostral ventromedial PAG near the supraoculomotor nucleus yielded anterograde labeling immediately ventral to the nucleus ambiguus in the ventrolateral medulla, within the retrofacial portion of the PGi.(ABSTRACT TRUNCATED AT 400 WORDS)

Periaqueductal gray stimulation produces a spinally mediated, opioid antinociception for the inflamed hindpaw of the rat

The objective of the present study was to characterize stimulation-produced antinociception from the periaqueductal gray matter (PAG) in rats with unilateral hindlimb inflammation induced by an intraplantar injection of Freund's complete adjuvant. Rats were chronically implanted with a bipolar stimulating electrode in the PAG. Nociception was assessed using a paw pressure test. Prior to inflammation, PAG stimulation significantly increased paw pressure threshold in both paws compared to non-stimulated controls. Following inflammation, PAG stimulation inhibited nociception in the inflamed, but not the non-inflamed paw. Systemic administration of naloxone blocked antinociception from ventral, but not dorsal PAG stimulation sites. Intrathecal, but not subcutaneous, administration of quaternary naltrexone completely blocked stimulation-produced antinociception from the PAG. The known increased levels of endogenous opioids occurring in the spinal cord ipsilateral to the site of inflammation suggest a mechanism for the selective antinociceptive effect of ventral PAG stimulation seen for the inflamed paw.

Arcuate nucleus projections to brainstem regions which modulate nociception

Anterograde tracing studies were conducted in order to identify efferents from the arcuate nucleus, which contains the hypothalamic opiocortin neuronal pool. Phaseolus vulgaris leucoagglutinin (PHA-L) was stereotaxically iontophoresed into the arcuate nucleus and the terminal fields emanating from the labelled perikarya were identified immunocytochemically. PHA-L-immunoreactive (-ir) fibers were identified in nucleus accumbens, lateral septal nucleus, bed nucleus of the stria terminalis, medial and lateral preoptic areas, anterior hypothalamus, amygdaloid complex, lateral hypothalamus, paraventricular nucleus, zona incerta, dorsal hypothalamus, periventricular gray, medial thalamus and medial habenula. In the brainstem, arcuate terminals were identified in the periaqueductal gray (PAG), dorsal raphe nucleus (DRN), nucleus raphe magnus (NRM), nucleus raphe pallidus, locus coeruleus, parabrachial nucleus, nucleus reticularis gigantocellularis pars alpha, nucleus tractus solitarius and dorsal motor nucleus of the vagus nerve. Dual immunostaining was used to identify the neurochemical content of neurons in arcuate terminal fields in the brainstem. Arcuate fiber terminals established putative contacts with serotonergic neurons in the ventrolateral PAG, DRN and NRM and with noradrenergic neurons in periventricular gray, PAG and locus coeruleus. In the PAG, arcuate fibers terminated in areas with neurons immunoreactive to substance P, neurotensin, enkephalin and cholecystokinin (CCK) and putative contacts were identified with CCK-ir cells. This study provides neuroanatomical evidence that putative opiocortin neurons in the arcuate nucleus influence a descending system which modulates nociception.

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.

The role of glutamate in opiate descending inhibition of nociceptive spinal reflexes

The present experiment examined descending inhibition of the nociceptive tail-flick reflex produced by microinjection of morphine and glutamate into the periaqueductal gray (PAG) matter and the neurotransmitters mediating the inhibition at the level of the nucleus raphe magnus (NRM). The longlasting opiate analgesia was significantly reduced by microinjection of excitatory amino acid antagonists 1-(p-chlorobenzoyl)-piperazine-2,3-dicarboxylate (PCB, 3.25 mumol) or DL-2-amino-5-phosphono-valerate (APV, 25.38 mumol) into the NRM, whereas the short-lived glutamate analgesia was not. This indicates that although both opiate and non-opiate analgesia may originate in the PAG, the former is relayed through the NRM, whereas the latter is relayed by additional or different nuclei in the medulla. Two observations shed light on the question which receptors mediate the above effect in the NRM. First, PCB blocked morphine analgesia at doses that were 8 times lower than doses of APV that were effective. Second, analgesia produced by injection of glutamate into the NRM was antagonized by PCB (3.25 mumol), whereas APV (25.38 mumol) failed to do so. Together these results indicate that kainate/quisqualate, but not N-methyl-D-aspartate (NMDA), receptors are implicated in the NRM as a relay station in opiate descending inhibition.

Stimulation of the periaqueductal gray matter inhibits nociception at the supraspinal as well as spinal level

Stimulation of the periaqueductal gray matter (PAG) is known to modulate nociception at the spinal level. Several studies have suggested that nociception may also be modulated via ascending projections from the PAG. To study this hypothesis, the descending pathway was selectively disrupted immediately caudal to the PAG in 28 rats. Twenty-eight additional rats served as non-lesioned controls. All animals were chronically implanted with a stimulating electrode in the PAG, and antinociception was assessed using tests involving spinally and supraspinally mediated responses (tail-flick and hot-plate tests, respectively). Significantly fewer lesioned than non-lesioned rats showed stimulation-produced analgesia (SPA) in the tail-flick test (4 of 28 vs 14 of 28, respectively). In contrast, no significant difference in the incidence of SPA occurred between lesioned and non-lesioned rats in the hot-plate test. These findings demonstrate that nociception can be modulated at the supraspinal, as well as spinal, level.