NMDA receptor blockade in the periaqueductal grey prevents stress-induced analgesia in attacked mice

Interactive Mechanisms of Supraspinal Sites of Opioid Analgesic Action: A Festschrift to Dr. Gavril W. Pasternak

Almost a half century of research has elaborated the discoveries of the central mechanisms governing the analgesic responses of opiates, including their receptors, endogenous peptides, genes and their putative spinal and supraspinal sites of action. One of the central tenets of "gate-control theories of pain" was the activation of descending supraspinal sites by opiate drugs and opioid peptides thereby controlling further noxious input. This review in the Special Issue dedicated to the research of Dr. Gavril Pasternak indicates his contributions to the understanding of supraspinal mediation of opioid analgesic action within the context of the large body of work over this period. This review will examine (a) the relevant supraspinal sites mediating opioid analgesia, (b) the opioid receptor subtypes and opioid peptides involved, (c) supraspinal site analgesic interactions and their underlying neurophysiology, (d) molecular (particularly AS) tools identifying opioid receptor actions, and (e) relevant physiological variables affecting site-specific opioid analgesia. This review will build on classic initial studies, specify the contributions that Gavril Pasternak and his colleagues did in this specific area, and follow through with studies up to the present.

Mu-opioid and CB1 cannabinoid receptors of the dorsal periaqueductal gray interplay in the regulation of fear response, but not antinociception

Evidence indicates that periaqueductal gray matter (PAG) plays an important role in defensive responses and pain control. The activation of cannabinoid type-1 (CB1) or mu-opioid (MOR) receptors in the dorsal region of this structure (dPAG) inhibits fear and facilitates antinociception induced by different aversive stimuli. However, it is still unknown whether these two receptors work cooperatively in order to achieve these inhibitory actions. This study investigated the involvement and a likely interplay between CB1 and MOR receptors localized into the dPAG on the regulation of fear-like defensive responses and antinociception (evaluated in tail-flick test) evoked by dPAG chemical stimulation with N-methyl-d-aspartate (NMDA). Before the administration of NMDA, animals were first intra-dPAG injected with the CB1 agonist ACEA (0.5 pmol), or with the MOR agonist DAMGO (0.5 pmol) in combination with the respective antagonists AM251 (CB1 antagonist, 100 pmol) or CTOP (MOR antagonist, 1 nmol). To investigate the interplay between these receptors, microinjection of CTOP was combined with ACEA, or microinjection of AM251 was combined with DAMGO. Our results showed that both the intra-PAG treatments with ACEA or DAMGO inhibited NMDA-induced freezing expression, whereas only the treatment with DAMGO increased antinociception induced with NMDA, which are completely blocked by its respective antagonists. Interestingly, the inhibitory effects of ACEA or DAMGO on freezing was blocked by CTOP and AM251, respectively, indicating a functional interaction between these two receptors in the mediation of defensive behaviors. However, this cooperative interaction was not observed during the NMDA-induced antinociception. Our findings indicate that there is a cooperative action between the MOR and CB1 receptors within the dPAG and it is involved in the mediation of NMDA-induced defensive responses. Additionally, the MORs into the dPAG are involved in the modulation of the antinociceptive effects that follow a fear-like defense-reaction induced by dPAG chemical stimulation with NMDA.

Ventrolateral Periaqueductal Gray Matter Neurochemical Lesion Facilitates Epileptogenesis and Enhances Pain Sensitivity in Epileptic Rats

The ventrolateral periaqueductal gray matter (vlPAG) plays a critical role in the pathogenesis of migraine and few studies have shown that vlPAG might be involved in the pathophysiology of epilepsy. But its roles in epileptogenesis and comorbid relationship between migraine and epilepsy have never been reported. In this study, the impairments of vlPAG neuronal network during spontaneous recurrent seizure (SRS) development after status epilepticus (SE) were investigated, and the pain sensitivity as well as the SRS investigated after neurochemical lesion to vlPAG to determine the role of vlPAG in epileptogenesis and in migraine comorbidity with epilepsy. Neuronal loss and alterations of excitatory and inhibitory neural transmission within vlPAG accompanied the development of epileptogenesis induced by SE. On the other hand, neurochemical lesion to vlPAG enhanced frequency and duration of spontaneous seizure event and frequency of epileptiform inter-ictal spike discharges in electroencephalography (EEG), but decreased pain threshold in epileptic rats. This indicates an involvement of the pain regulating structure, vlPAG, in the pathogenesis of epilepsy. This may imply that vlPAG network alterations could be a possible underlying mechanism of the interactive comorbid relationship between epilepsy and migraine.

Inhibition of GABAergic Neurons and Excitation of Glutamatergic Neurons in the Ventrolateral Periaqueductal Gray Participate in Electroacupuncture Analgesia Mediated by Cannabinoid Receptor

Although electroacupuncture (EA) has become a worldwide practice, little is understood about its precise target in the central nervous system (CNS) and the cell type-specific analgesia mechanism. In the present study, we found that EA has significant antinociceptive effects both in inflammatory and neuropathic pain models. Chemogenetic inhibition of GABAergic neurons in the ventrolateral periaqueductal gray (vlPAG) replicated the effects of EA, whereas the combination of chemogenetic activation of GABAergic neurons and chemogenetic inhibition of glutamatergic neurons in the vlPAG was needed to reverse the effects of EA. Specifically knocking out CB1 receptors on GABAergic neurons in the vlPAG abolished the EA effect on pain hypersensitivity, while specifically knocking out CB1 receptors on glutamatergic neurons attenuated only a small portion of the EA effect. EA synchronously inhibits GABAergic neurons and activates glutamatergic neurons in the vlPAG through CB1 receptors to produce EA-induced analgesia. The CB1 receptors on GABAergic neurons localized in the vlPAG was the basis of the EA effect on pain hypersensitivity. This study provides new experimental evidence that EA can bidirectionally regulate GABAergic neurons and glutamatergic neurons via the CB1 receptors of the vlPAG to produce analgesia effects.

Divergent Modulation of Nociception by Glutamatergic and GABAergic Neuronal Subpopulations in the Periaqueductal Gray

The ventrolateral periaqueductal gray (vlPAG) constitutes a major descending pain modulatory system and is a crucial site for opioid-induced analgesia. A number of previous studies have demonstrated that glutamate and GABA play critical opposing roles in nociceptive processing in the vlPAG. It has been suggested that glutamatergic neurotransmission exerts antinociceptive effects, whereas GABAergic neurotransmission exert pronociceptive effects on pain transmission, through descending pathways. The inability to exclusively manipulate subpopulations of neurons in the PAG has prevented direct testing of this hypothesis. Here, we demonstrate the different contributions of genetically defined glutamatergic and GABAergic vlPAG neurons in nociceptive processing by employing cell type-specific chemogenetic approaches in mice. Global chemogenetic manipulation of vlPAG neuronal activity suggests that vlPAG neural circuits exert tonic suppression of nociception, consistent with previous pharmacological and electrophysiological studies. However, selective modulation of GABAergic or glutamatergic neurons demonstrates an inverse regulation of nociceptive behaviors by these cell populations. Selective chemogenetic activation of glutamatergic neurons, or inhibition of GABAergic neurons, in vlPAG suppresses nociception. In contrast, inhibition of glutamatergic neurons, or activation of GABAergic neurons, in vlPAG facilitates nociception. Our findings provide direct experimental support for a model in which excitatory and inhibitory neurons in the PAG bidirectionally modulate nociception.

Repetitive transcranial magnetic stimulation in chronic pain: a review of the literature

OBJECTIVE

To review the literature on the analgesic effects of repetitive transcranial magnetic stimulation (rTMS) in chronic pain according to different pain syndromes and stimulation parameters.

DATA SOURCES

Publications on rTMS and chronic pain were searched in PubMed and Google Scholar using the following key words: chronic pain, analgesia, transcranial magnetic stimulation, neuropathic pain, fibromyalgia, and complex regional pain syndrome.

STUDY SELECTION

This review only included double-blind, controlled studies with >10 participants in each arm that were published from 1996 to 2014 and written in English. Studies with relevant information for the understanding of the effects of rTMS were also cited.

DATA EXTRACTION

The following data were retained: type of pain syndrome, type of study, coil type, target, stimulation intensity, frequency, number of pulses, orientation of induced current, number of session, and a brief summary of intervention outcomes.

DATA SYNTHESIS

A total of 33 randomized trials were found. Many studies reported significant pain relief by rTMS, especially high-frequency stimulation over the primary motor cortex performed in consecutive treatment sessions. Pain relief was frequently >30% compared with control treatment. Neuropathic pain, fibromyalgia, and complex regional pain syndrome were the pain syndromes more frequently studied. However, among all published studies, only a few performed repetitive sessions of rTMS.

CONCLUSIONS

rTMS has potential utility in the management of chronic pain; however, studies using maintenance sessions of rTMS and assessing the effects of rTMS on the different aspects of chronic pain are needed to provide a more solid basis for its clinical application for pain relief.

Motor cortex stimulation inhibits thalamic sensory neurons and enhances activity of PAG neurons: possible pathways for antinociception

Motor cortex stimulation is generally suggested as a therapy for patients with chronic and refractory neuropathic pain. However, the mechanisms underlying its analgesic effects are still unknown. In a previous study, we demonstrated that cortical stimulation increases the nociceptive threshold of naive conscious rats with opioid participation. In the present study, we investigated the neurocircuitry involved during the antinociception induced by transdural stimulation of motor cortex in naive rats considering that little is known about the relation between motor cortex and analgesia. The neuronal activation patterns were evaluated in the thalamic nuclei and midbrain periaqueductal gray. Neuronal inactivation in response to motor cortex stimulation was detected in thalamic sites both in terms of immunolabeling (Zif268/Fos) and in the neuronal firing rates in ventral posterolateral nuclei and centromedian-parafascicular thalamic complex. This effect was particularly visible for neurons responsive to nociceptive peripheral stimulation. Furthermore, motor cortex stimulation enhanced neuronal firing rate and Fos immunoreactivity in the ipsilateral periaqueductal gray. We have also observed a decreased Zif268, δ-aminobutyric acid (GABA), and glutamic acid decarboxylase expression within the same region, suggesting an inhibition of GABAergic interneurons of the midbrain periaqueductal gray, consequently activating neurons responsible for the descending pain inhibitory control system. Taken together, the present findings suggest that inhibition of thalamic sensory neurons and disinhibition of the neurons in periaqueductal gray are at least in part responsible for the motor cortex stimulation-induced antinociception.

Involvement of glutamatergic receptors in the nucleus cuneiformis in modulating morphine-induced antinociception in rats

The nucleus cuneiformis (CnF), located just ventrolateral to the periaqueductal gray, is part of the descending pain modulatory system. Neurons in the CnF project to medullary nucleus raphe magnus (NRM), which plays an important role on pain modulation. In this study, we investigated the effect of microinjection of the non-competitive NMDA receptor antagonist MK-801, the competitive NMDA receptor antagonist AP-7, and the kainate/AMPA receptor antagonist DNQX, alone or in combination with morphine into the nucleus cuneiformis on morphine-induced analgesia to understand the role of glutamatergic receptors in the modulating activity of morphine. Antinociception was assessed with the tail-flick test. Morphine (10, 20, 40 microg in 0.5 microl saline) had an antinociceptive effect, increasing tail-flick latency in a dose-dependent manner. Microinjection of MK-801 (10 microg/0.5 microl saline) and AP7 (3 microg/0.5 microl saline) prior to morphine microinjection (10 microg/0.5 microl saline) attenuated the antinociceptive effects of morphine, whereas DNQX (0.5 microg/0.5 microl saline) showed a partial antinociceptive effect and potentiated the analgesic effect of morphine. These results indicated that the NMDA receptor partially potentiates the antinociceptive effect of morphine. Our results suggest that NMDA but not non-NMDA receptors are involved in the antinociception produced by morphine in the CnF. The non-NMDA receptors in this area may have a facilitatory effect on nociceptive transmission. The fact that morphine's effect was potentiated by NMDA receptor suggests that projection neurons within the CnF are under tonic, glutamatergic input and when the influence of this input is blocked, the descending inhibitory system is inactivated.

N-methyl-d-aspartate receptors in the medial septal area have a role in spatial and emotional learning in the rat

Cholinergic and GABAergic neurons in the medial septal/vertical limb of the diagonal band of Broca (MS/vDB) area project to the hippocampus and constitute the septohippocampal pathway, which has been implicated in learning and memory. There is also evidence for extrinsic and intrinsic glutamatergic neurons in the MS/vDB, which by regulating septohippocampal neurons can influence hippocampal functions. The potential role of glutamatergic N-methyl-D-aspartate (NMDA) receptors within the MS/vDB for spatial and emotional learning was studied using the water maze and step-through passive avoidance (PA) tasks, which are both hippocampal-dependent. Blockade of septal NMDA receptors by infusion of the competitive NMDA receptor antagonist D-(-)-2-amino-5-phosphonopentanoic acid (D-AP5) (0.3-5 microg/rat), infused 15 min prior to training, impaired spatial learning and memory at the 5 microg dose of D-AP5, while doses of 0.3 and 1 microg per rat had no effect. The impairment in spatial learning appears not to be caused by sensorimotor or motivational disturbances, or anxiogenic-like behavior. Thus, d-AP5-treated rats were not impaired in swim performance or visuospatial abilities and spent more time in the open arms of the elevated plus-maze. In the PA task, intraseptal D-AP5 infused 15 min before training impaired retention as examined 24 h after training. This impairment was observed already at the 0.3 microg dose, suggesting that NMDA receptors within the MS/vDB may be more important for emotional than spatial memory. In summary, the present data indicate that changes in septal glutamate transmission and NMDA receptor activity can influence activity-dependent synaptic plasticity in the hippocampus and thereby learning and memory.

Defensive-like behaviors and antinociception induced by NMDA injection into the periaqueductal gray of mice depend on nitric oxide synthesis

Glutamate NMDA receptor activation within the periaqueductal gray (PAG) leads to antinociceptive, autonomic and behavioral responses characterized as the fear reaction. Considering that NMDA receptor triggers activation of neuronal nitric oxide synthase (nNOS), enzyme that produces nitric oxide (NO), this study investigated the effects of intra-PAG infusions of NPLA (Nomega-propyl-L-arginine), an nNOS inhibitor, on behavioral and antinociceptive responses induced by local injection of NMDA receptor agonist in mice. The behaviors measured were frequency of jumping and rearing as well as duration (in seconds) of running and freezing. Nociception was assessed during the second phase of the formalin test (injection of 50 microl of formalin 2.5% into the dorsal surface of the right hind paw). Five to seven days after stereotaxic surgery for intracerebral cannula implantation, mice were injected with formalin into the paw, and 10 min later, they received intra-dPAG injection of NPLA (0, 0.2, or 0.4 nmol/0.1 microl). Ten minutes later, they were injected with NMDA (N-methyl-D-aspartate: 0 or 0.04 nmol/0.1 microl) into the same midbrain site and were immediately placed in glass holding cage for recording the defensive behavior and the time spent on licking the injected paw with formalin during a period of 10 min. Microinjections of NMDA significantly decreased nociception response and produced jumping, running, and freezing reactions. Intra-dPAG injections of NPLA (0.4 nmol) completely blocked the NMDA effects without affecting either behavioral or nociceptive responses in intra-dPAG saline-injected animals, except for the rearing frequency that was increased by the nNOS inhibitor. These results strongly suggest the involvement of NO within the PAG in the antinociceptive and defensive reactions induced by local glutamate NMDA receptor activation in this midbrain structure.

Genomics and variation of ionotropic glutamate receptors

Sequencing of the human, mouse, and rat genomes has enabled a comprehensive informatics approach to gene families. This approach is informative for identification of new members of gene families, for cross-species sequence conservation related to functional conservation, for within-species diversity related to functional variation, and for historical effects of selection. This genome informatics approach also focuses our attention on genes whose genomic locations coincide with linkages to phenotypes. We are identifying ionotropic glutamate receptor (IGR) sequence variation by resequencing technologies, including denaturing high-performance liquid chromoatography (dHPLC), for screening and direct sequencing, and by information mining of public (e.g., dbSNP and ENSEMBL) and private (i.e., Celera Discovery System) sequence databases. Each of the 16 known IGRs is represented in these databases, their positions on a canonical physical map (for example, the Celera map) are established, and comparison to mouse and rat sequences has been performed, revealing substantial conservation of these genes, which are located on different chromosomes but found within syntenic groups of genes. A collection of 38 missense variants were identified by the informatics and resequencing approaches in several of these receptor genes, including GRIN2B, GRIN3B, GRIA2, GRIA3, and GRIK1. This represents only a fraction of the sequence variation across these genes, but, in fact, these may constitute a large fraction of the common polymorphisms at these genes, and these polymorphisms are a starting point for understanding the role of these receptors in neurogenetic variation. Genetically influenced human neurobehavioral phenotypes that are likely to be linked to IGR genetic variants include addictions, anxiety/dysphoria disorders, post-brain injury behavioral disorders, schizophrenia, epilepsy, pain perception, learning, and cognition. Thus, the effects of glutamate receptor variation may be protean, and the task of relating variation to behavior difficult. However, functional variants of (1) catechol-O-methyltransferase, (2) serotonin transporter, and (3) brain-derived neurotrophic factor have recently been linked both to behavioral differences and to intermediate phenotypes, suggesting a pathway by which functional variation at IGRs can be tied to an etiologically complex phenotype.

Modulation of defensive behavior by periaqueductal gray NMDA/glycine-B receptor

Glutamate (GLU) associated with glycine, act as co-transmitter at the N-methyl-D-aspartate/glycine-B (NMDA/GLY(B)) receptor. Dorsal periaqueductal gray (dPAG) neurons express NMDA/GLY(B) receptors suggesting a GLU physiological role in mediating the responses elicited by stimulation of this area. Immunohistochemical data provided evidence of a possible correlation among elevated plus-maze (EPM), fear-like defensive behavior, and dPAG activity. The present data show that whereas the NMDA/GLY(B) receptor agonists increased the open-arm avoidance responses in the EPM, the antagonists had the opposite effects. Microinjection of NMDA/GLY(B) receptor agonists within the dPAG during test sessions in the EPM resulted in an enduring learned fear response detected in the retest. Therefore, in addition to the proposed role for the dPAG in panic attacks (escape), these findings suggest that the dPAG can also participate in more subtle anxiety-like behaviors.

Regulation of acute nociceptive responses by the NMDA receptor GluRepsilon2 subunit

Heterozygous mice mutant for the NMDA-type glutamate receptor (GluR) epsilon2 subunit with a highly homogeneous genetic background showed exaggerated responses to various acute noxious stimuli in the footshock, tail-flick, hot-plate and tail-pinch tests. Because the noxious stimuli in these behavioral tests were electrical, thermal and mechanical, the reduction of GluRepsilon2 proteins exerted stimulatory effects on acute nociceptive responses across modalities. Previous studies showed that GluRepsilon1 and GluRepsilon4 subunit mutant mice exhibited no alteration in the responses to acute noxious stimuli. Thus, among NMDA receptor subunits, the GluRepsilon2 subunit specifically plays an important role in the regulation of the acute nociceptive responses.

Characterization of synaptic transmission in the ventrolateral periaqueductal gray of rat brain slices

Synaptic transmission evoked by focal stimulation in the ventrolateral periaqueductal gray was characterized using the whole-cell recording technique in rat brain slices. At resting membrane potential (-62+/-1 mV), focal stimulation (0.05-0.1 ms, 0.03 Hz) usually evoked a 6-cyano-7-nitroquinoxaline-2, 3-dione-sensitive fast excitatory postsynaptic potential and a DL-2-amino-5-phosphonopentanoic acid-sensitive slow excitatory postsynaptic potential with a bicuculline-sensitive inhibitory postsynaptic potential in between. In the presence of kynurenic acid, bicuculline-sensitive inhibitory postsynaptic currents recorded in the voltage-clamp mode displayed a reversal potential of -68+/-3 mV, resembling GABA(A) receptor-mediated inhibitory postsynaptic currents. However, no GABA(B) receptor-mediated inhibitory postsynaptic current was evoked, even at stronger stimulating intensity. 6-Cyano-7-nitroquinoxaline-2,3-dione-sensitive fast excitatory postsynaptic currents were isolated by DL-2-amino-5-phosphonopentanoic acid plus bicuculline and DL-2-amino-5-phosphonopentanoic acid-sensitive slow fast excitatory postsynaptic currents by bicuculline plus 6-cyano-7-nitroquinoxaline-2,3-dione. Both types of excitatory postsynaptic current reversed at potentials near 0 mV. The I-V curve of slow fast excitatory postsynaptic currents or N-methyl-D-aspartate currents displayed a negative slope at potentials more negative than -30 mV in an Mg(2+)-sensitive manner. The control postsynaptic currents reversed at potentials between -50 and -35 mV, inclined to the reversal potential of GABA(A), but not glutamate, receptor channels. It is concluded that, in the ventrolateral periaqueductal gray, focal stimulation elicits both inhibitory and excitatory transmission, while the former is dominant. The inhibitory transmission is mediated by GABA(A) but not GABA(B) receptors. The excitatory transmission is mediated by glutamate acting on alpha-amino-3-hydroxy-5-methylisoxazole-4-propionate/kainate as well as N-methyl-D-aspartate receptors.

Blockade of tolerance to stress-induced analgesia by MK-801 in mice

The N-methyl-D-aspartate (NMDA) receptor has been implicated in mechanisms of tolerance to morphine-induced analgesia. The present study examined the role of the NMDA receptor in the development of tolerance to stress-induced analgesia (SIA). In the first experiment, mice were exposed to a stressor (a 3-min forced swim in water maintained at 32 degrees C) once daily for 15 consecutive days. Analgesia was measured 2 min after stress on the first and last day using the hot-plate test. To examine the role of the NMDA receptor in the development of tolerance to SIA mice were treated daily with the non-competitive NMDA receptor antagonist, MK-801, 15 min before swimming. Pretreatment with MK-801 was found to block both analgesia and tolerance. In a second experiment, to examine whether SIA and tolerance to SIA are mediated by similar or different mechanisms, mice were injected daily with MK-801 after analgesia had dissipated (1 h following swim). Tolerance to SIA was blocked by delayed injections of MK-801. These results suggest that the NMDA receptor is involved in mechanisms of tolerance to SIA, independent of its role in analgesia.

Excitatory amino acid antagonists in the rostral ventromedial medulla inhibit mesencephalic morphine analgesia in rats

Supraspinal opioid analgesia is mediated in part by connections between the periaqueductal gray (PAG) and rostral ventromedial medulla (RVM). Morphine analgesia elicited from the PAG is respectively decreased by selective serotonergic and opioid receptor antagonists administered into the RVM, and increased by RVM neurotensin antagonists. Since glutamate and excitatory amino acid (EAA) receptors are also active in the RVM, the present study evaluated whether either competitive (AP7) or non-competitive (MK-801) N-methyl-D-aspartate (NMDA) antagonists or a kainate/AMPA (CNQX) antagonist microinjected into the RVM altered morphine (2.5 micrograms) analgesia elicited from the PAG as measured by the tail-flick and jump tests. Mesencephalic morphine analgesia was markedly reduced on both tests after RVM pretreatment with either AP7 (0.01-1 microgram, 0.08-7.8 nmol) or MK-801 (0.03-3 micrograms, 0.04-4.4 nmol). In contrast, small but significant reductions in mesencephalic morphine analgesia occurred on the jump test following CNQX (0.5 microgram, 2.2 nmol) in the RVM. NMDA antagonists did not markedly alter either basal nociceptive thresholds following RVM administration, or mesencephalic morphine analgesia following administration into medullary placements lateral or dorsal to the RVM. These data implicate EAA and particularly NMDA receptors in the RVM in modulating the transmission of opioid pain-inhibitory signals from the PAG.

Functional characteristics of the midbrain periaqueductal gray

The major functions of the midbrain periaqueductal gray (PAG), including pain and analgesia, fear and anxiety, vocalization, lordosis and cardiovascular control are considered in this review article. The PAG is an important site in ascending pain transmission. It receives afferents from nociceptive neurons in the spinal cord and sends projections to thalamic nuclei that process nociception. The PAG is also a major component of a descending pain inhibitory system. Activation of this system inhibits nociceptive neurons in the dorsal horn of the sinal cord. The dorsal PAG is a major site for processing of fear and anxiety. It interacts with the amygdala and its lesion alters fear and anxiety produced by stimulation of amygdala. Stimulation of PAG produces vocalization and its lesion produces mutism. The firing of many cells within the PAG correlates with vocalization. The PAG is a major site for lordosis and this role of PAG is mediated by a pathway connecting the medial preoptic with the PAG. The cardiovascular controlling network within the PAG are organized in columns. The dorsal column is involved in pressor and the ventrolateral column mediates depressor responses. The major intrinsic circuit within the PAG is a tonically-active GABAergic network and inhibition of this network is an important mechanism for activation of outputs of the PAG. The various functions of the PAG are interrelated and there is a significant interaction between different functional components of the PAG. Using the current information about the anatomy, physiology, and pharmacology of the PAG, a model is proposed to account for the interactions between these different functional components.

Non-NMDA receptor antagonist attenuates antinociception induced by morphine but not beta-endorphin, D-Pen2-D-Pen5-enkephalin, and U50, 488H administered intracerebroventricularly in mice

The antinociception induced by morphine but not beta-endorphin, D-Pen2-D-Pen5-enkephalin (DPDPE), or U50, 488H (trans-3,4-dichloro-N-methyl-N-[2-(1-pyrrolidinyl) cyclohexyl] benzeocetamide) administered intracerebroventricularly (i.c.v.) has been previously demonstrated to be mediated by the N-Methyl-D-Aspartic Acid (NMDA) receptor. The present study was designed to determine if non-NMDA receptors are involved in opioid-induced antinociception. Antinociception was assessed by the tail-flick test in male ICR mice. Various doses of CNQX (6-Cyano-7-nitroquinoxaline-2,3-dione), a competitive non-NMDA receptor antagonist, (0.001 to 0.5 microgram) injected intracerebroventricularly (i.c.v.) alone did not show any antinociceptive effect. CNQX (0.01 to 1 micrograms, i.c.v.) dose-dependently attenuated the inhibition of the tail-flick response induced by morphine (1 microgram). However, inhibition of the tail-flick response induced by i.c.v. administered beta-endorphin (1 microgram), DPDPE (10 micrograms), or U50, 488H was not affected by i.c.v. administered CNQX. It is concluded that non-NMDA receptors are involved in i.c.v. morphine-induced antinociception. However, non-NMDA receptors are not involved in i.c.v. administered beta-endorphin-, DPDPE-, and U50, 488H-induced antinociception at the supraspinal level.

The NMDA antagonist 3-(2-carboxypiperazin-4-yl)propyl-1-phosphonic acid (CPP) has antinociceptive effect after intrathecal injection in the rat

This behavioral study was performed in order to delineate the antinociceptive effects of and the influence on motor function of a highly potent, competitive NMDA receptor antagonist 3-(2-carboxypiperazin-4-yl)propyl-1-phosphonic acid (CPP). After intrathecal (i.t.) administration of CPP to chronically catheterized rats, antinociception was studied in 3 different nociceptive tests: the tail-flick test, the hot-plate test, and the formalin test. The lowest dose producing visible motor dysfunction was 1 nmol, with 2 of 8 animals showing slight ataxia. Dose-related motor dysfunction and apparent sedation was present after 5 and 10 nmol. Dose-related antinociception was evident in the thermal tests following doses that produced little or no motor dysfunction. In the tail-flick test, the antinociceptive effect was attenuated at higher doses, resulting in a bell-shaped dose-response relationship. Dose-related antinociception was found in both the first and second phase of the formalin test following doses from 0.25 up to 1 nmol. The present study shows that the competitive NMDA antagonist CPP has an antinociceptive effect in doses that do not affect motor function. Furthermore, antinociception was evident in both phasic and tonic nociceptive tests. Finally, the dose-response relationship in the tail-flick test was bell-shaped. As discussed this indicates that NMDA receptors may be involved in functionally divergent nociceptive systems.

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.

Opioid footshock-induced analgesia in mice acutely falls by stress prolongation

The application of 80 footshocks (S-80) to mice induces a decrease in nociceptive responses as measured by the tail-flick test, which is opioid mediated as well as prevented by naloxone (10 mg/kg, SC). When the stress is prolonged up to 240 shocks (S-240) (i.e., from 6 min 40 s to 20 min), no analgesia can be seen immediately after the stress. We have examined the two most obvious possibilities, but they do not seem to be responsible for this fact. When morphine (1-5 mg/kg IP) is injected in the S-240 situation, a potentiation of its analgesic effects is seen, so that a desensitization of mu opioid receptors is unlikely. On the other hand, although cortisol (3-30 mg/kg IP) inhibits the analgesic response to S-80, metyrapone (40 and 80 mg/kg IP) and cortexolone (3-18 mg/kg IP) do not cause S-240 to be analgesic. Thus, an increase of endogenous glucocorticoids released during the long-duration stress does not seem responsible for the lack of analgesia after S-240.

Effects of CGS 19755 and dizocilpine (MK 801) on delayed time discrimination performance

The effects of the competitive NMDA (N-methyl-D-aspartate) receptor antagonist CGS 19755 and the non-competitive NMDA receptor antagonist dizocilpine (MK 801) on time discrimination and short-term memory were investigated in rats trained on a delayed time discrimination task. In a two-lever operant chamber, pressing one lever was correct and reinforced with a food pellet after presentation of a stimulus light for 2 s (SD(short)); pressing the opposite lever was correct after presentation of a stimulus light for 8 s (SD(long)). CGS 19755 (3.0 mg/kg) attenuated performance, decreased nose-pokes (an activity necessary to trigger the presentation of the discriminative stimulus and the presentation of the response levers), and increased response latencies (time from 'opportunity to leverpress' to 'actual leverpress'). The highest dose of dizocilpine (0.2 mg/kg) tested also attenuated performance. Further, the number of nose-pokes and response latencies were not altered by any dose of dizocilpine. With increasing delays, saline-injected animals developed a bias towards reporting an occurrence of an SD(long), independent of the actual stimulus presented. This bias was attenuated or even reversed by CGS 19755 (3.0 mg/kg) and (0.2 mg/kg). Our results suggest that NMDA receptors are directly or indirectly involved in time discrimination performance.

NMDA receptor antagonist MK-801 blocks non-opioid stress-induced analgesia in the formalin test

The analgesic effect of a 3-min swim stress was assessed using the formalin test. Male Swiss mice were injected i.p. with naloxone (0.1 or 1.0 mg/kg), MK-801 (0.075 mg/kg) or saline 15 min prior to swimming in water maintained at 20 degrees C or 32 degrees C. The mice were then injected with 20 microliters of 5% formalin into the plantar surface of 1 hind paw and pain behaviour (time spent licking the injected paw) was continuously monitored during the subsequent 10 min. Swim stress produced a significant reduction in pain behaviour at both 20 degrees C and 32 degrees C. MK-801 completely blocked the analgesia produced by both the 20 degrees C and 32 degrees C swim. At a dose of 0.1 mg/kg, naloxone partially antagonized the analgesia produced by the 32 degrees C swim but did not affect the analgesia produced by the 20 degrees C swim. Naloxone at a dose of 1.0 mg/kg had no effect on swim stress-induced analgesia. Neither MK-801 nor 0.1 mg/kg naloxone altered baseline pain behaviour, although 1.0 mg/kg naloxone did significantly reduce it. It is unlikely that the effect of MK-801 on swim stress-induced analgesia is due to an interaction with an opioid mechanism, as MK-801 had no effect on morphine analgesia. These results suggest that the analgesia produced by the 20 degrees C swim stress in the formalin test is non-opioid in nature and mediated via the NMDA receptor, whereas the 32 degrees C swim stress-induced analgesia has both an opioid and non-opioid component.

N-methyl-D-aspartic acid (NMDA) receptor antagonist MK-801 blocks non-opioid stress-induced analgesia. II. Comparison across three swim-stress paradigms in selectively bred mice

The effects of the specific N-methyl-D-aspartic acid (NMDA) receptor antagonist MK-801 (dizocilpine, 0.075 mg/kg, i.p.) on swim-stress-induced analgesia (SSIA) were studied in control (C) mice and in mice selectively bred for high (HA) or low (LA) SSIA. In three consecutive experiments, animals were subjected to forced swimming at water temperature of 20 degrees C, 32 degrees C and 15 degrees C and the resulting analgesia (hot-plate test) was found to be mixed opioid/non-opioid, opioid and non-opioid, respectively, as a function of the degree of antagonism by naloxone (10 mg/kg, i.p.). The major finding of this study is that MK-801 attenuated 15 degrees C SSIA, against which naloxone was ineffective, but had no effect on 32 degrees C SSIA, which naloxone blocked completely. A combination of naloxone and MK-801 significantly attenuated 20 degrees C SSIA in C and HA mice and in HA mice this attenuation was significantly larger than that produced by either drug alone. Morphine analgesia (10 mg/kg, i.p.) was unaffected by MK-801. It is concluded that low doses of MK-801 selectively block non-opioid mechanisms of SSIA.

Intracerebroventricular administration of kappa-agonists induces convulsions in mice

Intracerebroventricular (ICV) administration of kappa-agonists (PD 117302, U-50488H and U-69593) induced convulsions in a dose-related manner in mice. The dose at which 50% of animals convulsed (CD50) was in nmol ranges for all opioids. Among the opioids used, PD 117302 was the most potent convulsant. ICV administration of either vehicle alone or U-53445E, a non-kappa-opioid (+) enantiomer of U-50488H did not induce convulsions. The convulsive response of kappa-agonists was differentially susceptible for antagonism by naloxone and/or MR 2266. Collectively, these findings support the view that convulsions induced by kappa-agonists in mice involve stereospecific opioid receptor mechanisms. Furthermore, the convulsant effect of kappa-agonists could not be modified by pretreatment with MK-801, ketamine, muscimol or baclofen. It is concluded that kappa-opioid but not NMDA or GABA receptor mechanisms are involved in convulsions induced by kappa-agonists. These results are the first experimental evidence implicating stereospecific kappa-receptor mechanisms in opioid-induced convulsions in mice.

N-methyl-D-aspartic acid (NMDA) receptor antagonist MK-801 blocks non-opioid stress-induced analgesia. I. Comparison of opiate receptor-deficient and opiate receptor-rich strains of mice

The effects of the specific N-methyl-D-aspartic acid (NMDA) receptor antagonist MK-801 (0.075 mg/kg), and the specific opiate receptor antagonist naloxone (10 mg/kg), on swim stress-induced analgesia (SSIA) were studied in opiate receptor-deficient (CXBK) and opiate receptor-rich (CXBH) mice. Animals were subjected to forced swimming, and analgesia was assessed using the hot-plate test. In CXBK mice SSIA was blocked by MK-801 but was completely insensitive to naloxone. In CXBH mice SSIA was partially attenuated both by naloxone and MK-801, and it was nearly abolished by a combination of these drugs. Morphine analgesia (10 mg/kg) was abolished by naloxone but completely unaffected by MK-801 in CXBH mice. These findings suggest that the NMDA receptor is critically involved in the non-opioid component of SSIA.

Excitatory amino acid antagonists (kynurenic acid and MK-801) attenuate the development of morphine tolerance in the rat

To investigate the possible role of excitatory amino acids (EAAs) in the mechanisms of morphine tolerance, rats were treated either with the wide-spectrum EAA antagonist, kynurenic acid (150 mg/kg), or the specific N-methyl-D-aspartic acid (NMDA) receptor antagonist. MK-801 (0.05 mg/kg), during a four-day induction period of morphine tolerance. Morphine was given once daily at a dose of 15 mg kg. On the fifth day rats were injected only with morphine (15 mg/kg), and analgesia was assessed using the hot-plate test. Morphine tolerance was significantly reduced by both EAA antagonists. Control experiments showed that at the same doses neither acute nor chronic administration of these antagonists affected morphine analgesia itself in a manner that can explain these findings. The possible involvement of EAAs in the mechanisms of morphine tolerance is discussed.

An ethological model for the study of activation and interaction of pain, memory and defensive systems in the attacked mouse. Role of endogenous opioids

The present work reviews neurochemical, physiological and behavioral data recorded from the attacked mouse and integrates them into a model of coping mechanisms during social conflict. More specifically, the possible relationships between systems of pain, memory and defense are presented, with special emphasis on the role of endogenous opioid peptides (EOPs). In recipients of attack, decreased beta-endorphin-like immunoreactivity and changes in opiate and benzodiazepine binding characteristics are found in structures of the brain defensive system. EOPs mediate the social conflict-induced increase of dopamine synthesis in the periaqueductal grey and frontal cortex. Social conflict analgesia in attacked mice is under the control of central opioid and nonopioid (e.g., benzodiazepine, glutamate) mechanisms, and is modified by experience (e.g., long-term analgesic reaction; tolerance). EOPs and pain-inhibitory mechanisms participate in the organization of behavioral defense, recuperative behavior and the memory of attack experience. The data are considered in relation to the perceptual-defensive-recuperative model of fear and pain, forwarded by Bolles and Fanselow.

Sex differences in the inhibitory effects of the NMDA antagonist, MK-801, on morphine and stress-induced analgesia

We examined the effects of intraperitoneal administrations of the noncompetitive NMDA receptor antagonist, (+) MK-801, its inactive enantiomer, (-) MK-801, and the prototypic opiate antagonist, naloxone, on restraint- and morphine-induced analgesia in male and female deer mice, Peromyscus maniculatus. Both restraint (30 min) and morphine (1.0 mg/kg) induced significant analgesic responses with male mice displaying significantly greater levels of opioid-induced analgesia than female animals. These analgesic responses were completely blocked by, naloxone (1.0 mg/kg), significantly reduced by (+) MK-801 (0.25 mg/kg) and unaffected by (-) MK-801 (0.25 mg/kg) pretreatments. There were significant male-female differences in the inhibitory effects of (+) MK-801; the higher levels of morphine- and restraint-induced analgesia of the males were completely blocked, while the lower level analgesic responses of the females were significantly reduced, but not blocked, by (+) MK-801. These observations provide further evidence that NMDA receptors are involved in the mediation of endogenous and exogenous opioid analgesia and show that there are significant male-female differences in the inhibitory effects of (+) MK-801 on opioid-mediated analgesia.