Endogenous opioid-mediated inhibition of putative pain-modulating neurons in rat rostral ventromedial medulla

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.

Endogenous opioid peptides in the descending pain modulatory circuit

The opioid epidemic has led to a serious examination of the use of opioids for the treatment of pain. Opioid drugs are effective due to the expression of opioid receptors throughout the body. These receptors respond to endogenous opioid peptides that are expressed as polypeptide hormones that are processed by proteolytic cleavage. Endogenous opioids are expressed throughout the peripheral and central nervous system and regulate many different neuronal circuits and functions. One of the key functions of endogenous opioid peptides is to modulate our responses to pain. This review will focus on the descending pain modulatory circuit which consists of the ventrolateral periaqueductal gray (PAG) projections to the rostral ventromedial medulla (RVM). RVM projections modulate incoming nociceptive afferents at the level of the spinal cord. Stimulation within either the PAG or RVM results in analgesia and this circuit has been studied in detail in terms of the actions of exogenous opioids, such as morphine and fentanyl. Further emphasis on understanding the complex regulation of endogenous opioids will help to make rational decisions with regard to the use of opioids for pain. We also include a discussion of the actions of endogenous opioids in the amygdala, an upstream brain structure that has reciprocal connections to the PAG that contribute to the brain's response to pain.

NOP-Related Mechanisms in Pain and Analgesia

Since the discovery of the NOP receptor and N/OFQ as the endogenous ligand, evidence has appeared demonstrating the involvement of this receptor system in pain. This was not surprising for members of the opioid receptor and peptide families, particularly since both the receptor and N/OFQ are highly expressed in brain regions involved in pain, spinal cord, and dorsal root ganglia. What has been surprising is the complicated picture that has emerged from 25 years of research. The original finding that N/OFQ decreased tail flick and hotplate latency, when administered i.c.v., led to the hypothesis that NOP receptor antagonists could have analgesic activity without abuse liability. However, as data accumulated, it became clear that not only the potency but the activity per se was different when N/OFQ or small molecule NOP agonists were administered in the brain versus the spinal cord and it also depended upon the pain assay used. When administered systemically, NOP receptor agonists are generally ineffective in attenuating heat pain but are antinociceptive in an acute inflammatory pain model. Most antagonists administered systemically have no antinociceptive activity of their own, even though selective peptide NOP antagonists have potent antinociceptive activity when administered i.c.v. Chronic pain models provide different results as well, as small molecule NOP receptor agonists have potent anti-allodynic and anti-hyperalgesic activity after systemic administration. A considerable number of electrophysiological and anatomical experiments, in particular with NOP-eGFP mice, have been conducted in an attempt to explain the complicated profile resulting from NOP receptor modulation, to examine receptor plasticity, and to elucidate mechanisms by which selective NOP agonists, bifunctional NOP/mu agonists, or NOP receptor antagonists modulate acute and chronic pain.

Action of β-endorphin and nonsteroidal anti-inflammatory drugs, and the possible effects of nonsteroidal anti-inflammatory drugs on β-endorphin

This study aimed to review research on the effects of nonsteroidal anti-inflammatory drugs (NSAIDs) on β-endorphin. NSAIDs are commonly used as anti-inflammatory and analgesic drugs. They are well known for inducing peripheral analgesia by inhibiting cyclooxygenase (COX). However, an increasing number of studies have shown that NSAIDs have an analgesic effect not only in the periphery but also at the center. It means that a central analgesic mechanism of the action of NSAIDs exists besides the peripheral mechanism, and the central mechanism likely involves β-endorphin. β-Endorphin is one of the most prominent endogenous peptides, existing in the hypophysis cerebri and hypothalamus. It plays an irreplaceable role in the central and peripheral analgesia in the human body mainly through three mechanisms including three parts, the spinal cord, the supraspinal cord, and peripheries. β-Endorphin plays an important role in the development of hyperalgesia. However, the specific signal transduction pathways between prostaglandin E₂ or NSAIDs and β-endorphin are still not quite clear. Whether NSAIDs can lead to the increased content of β-endorphin in all patients after any operation needs further investigation. Further studies should determine the optimal dose when NSAIDs and opioid drugs are used together, and also explore the existence of one NSAID that has the potential to replace the traditional opioid drugs and can achieve adequate analgesia.

Activity correlations between on-like and off-like cells of the rostral ventromedial medulla and simultaneously recorded wide-dynamic-range neurons of the spinal dorsal horn in rats

Considerable evidence supports the notion that on- and off-cells of the rostral ventromedial medulla (RVM) facilitate and depress, respectively, spinal nociceptive transmission. This notion stems from a covariation of on- or off-cell activities and spinal nocifensive reflexes. Such covariation could theoretically be due to their independently responding to a common source, or to an RVM-derived modulation of ventral horn neurons. Here, we tested whether on- and off-cells indeed modulate spinal nociceptive neurons. In deeply anesthetized rats, unitary recordings were simultaneously made from an RVM on-like or off-like cell and a spinal nociceptive neuron that shared a receptive field (RF) at a hind paw. Action potential firing in RVM/spinal neuron pairs was highly correlated, positively for on-like cells and negatively for off-like cells, both during ongoing activity and during application of calibrated noxious pressure to the RF. Microinjection of morphine into RVM induced a correlated decrease in on-like cell/spinal neuron ongoing activity and response to noxious stimulation. RVM morphine induced changes in off-like cell activity that were not correlated with spinal neuronal activity. These results suggest that on-cells exert a positive modulation upon spinal nociceptive neurons, upstream to ventral horn circuits and plausibly at the origin of nociceptive information that eventually reaches the cerebral cortex. On-cells may in this manner contribute to inflammation- and neuropathy-induced increases in withdrawal reflexes. Most significantly, on-cell modulation of nociceptive neurons may be a key factor in clinical pain conditions such as hyperalgesia and allodynia.

Peripheral Nerve Injury Reduces Analgesic Effectsof Systemic Morphine via Spinal 5-Hydroxytryptamine 3 Receptors

Background:

Morphine produces powerful analgesic effects against acute pain, but it is not effective against neuropathic pain, and the mechanisms underlying this reduced efficacy remain unclear. Here, the authors compared the efficacy of systemic morphine between normal rats and rats with peripheral nerve injury, with a specific focus on descending serotonergic mechanisms.

Methods:

After L5 spinal nerve ligation injury, male Sprague–Dawley rats were subjected to behavioral testing, in vivo microdialysis of the spinal dorsal horn to determine serotonin (5-hydroxytryptamine [5-HT]) and noradrenaline release, and immunohistochemistry (n = 6 in each group).

Results:

Intraperitoneal administration of morphine (1, 3, or 10 mg/kg) produced analgesic effects in normal and spinal nerve ligation rats, but the effects were greater in normal rats (P < 0.001). Morphine increased 5-HT release (450 to 500% of the baseline), but not noradrenaline release, in the spinal dorsal horn via activation of serotonergic neurons in the rostral ventromedial medulla. Intrathecal pretreatment with ondansetron (3 μg), a 5-HT3 receptor antagonist, or 5,7-dihydroxytryptamine creatinine sulfate (100 μg), a selective neurotoxin for serotonergic terminals, attenuated the analgesic effect of morphine (10 mg/kg) in normal rats but increased the analgesic effect of morphine in spinal nerve ligation rats (both P < 0.05).

Conclusions:

Systemic administration of morphine increases 5-HT levels in the spinal cord, and the increase in 5-HT contributes to morphine-induced analgesia in the normal state but attenuates that in neuropathic pain through spinal 5-HT3 receptors. The plasticity of the descending serotonergic system may contribute to the reduced efficacy of systemic morphine in neuropathic pain.

Xylazine induced central antinociception mediated by endogenous opioids and μ-opioid receptor, but not δ-or κ-opioid receptors

Endogenous opioids have been implicated in compound-induced antinociception, and our group previously suggested that xylazine induces peripheral antinociception by releasing endogenous opioids that act on their respective receptors. In this study, we investigated the involvement of endogenous opioids in α2-adrenoceptor agonist xylazine-induced central antinociception. The nociceptive threshold for thermal stimulation was measured in Swiss mice using the tail-flick test. The drugs were administered via the intracerebroventricular route. Probabilities less than 5% (p<0.05) were considered to be statistically significant (ANOVA/Bonferroni's test). Our results demonstrated that opioid receptor antagonist naloxone and μ-opioid receptor antagonist clocinnamox, but not δ-opioid receptor antagonist naltrindole and κ-opioid receptor antagonist nor-binaltorphimine, antagonized xylazine-induced central antinociception. These data provide evidence for the involvement of endogenous opioids and μ-opioid receptors in xylazine-induced central antinociception. In contrast, δ- and κ-opioid receptors do not appear to be involved in this effect. The results contribute to a greater understanding of the central antinociceptive mechanisms of a drug widely used in veterinary therapy.

CC12, a P450/epoxygenase inhibitor, acts in the rat rostral, ventromedial medulla to attenuate morphine antinociception

Brain cytochrome P450 epoxygenases were recently shown to play an essential role in mediating the pain-relieving properties of morphine. To identify the CNS sites containing the morphine-relevant P450s, the effects of intracerebral (ic) microinjections of the P450 inhibitor CC12 were determined on morphine antinociception in rats. CC12 inhibited morphine antinociception when both drugs were injected into the rostral ventromedial medulla (RVM), but not following co-injections into the periaqueductal gray (PAG) or into the spinal subarachnoid space. In addition, intra-RVM CC12 pretreatment nearly completely blocked the effects of morphine following intracerebroventricular (icv) administration. Although morphine is thought to act in both the PAG and RVM by pre-synaptic inhibition of inhibitory GABAergic transmission, the present findings show that 1) the mechanism of morphine action differs between these two brainstem areas, and 2) P450 activity within the RVM is important for supraspinal morphine antinociception. Characterization of morphine-P450 interactions within RVM circuits will further enhance the understanding of the biochemistry of pain relief.

Selective ablation of mu-opioid receptor expressing neurons in the rostral ventromedial medulla attenuates stress-induced mechanical hypersensitivity

AIMS

Chronic stress-related conditions are often associated with stress-induced hyperalgesia. However, the neural circuitry responsible for producing stress-induced hyperalgesia is not well characterized. The aim of this study was to determine the contribution of mu-opioid expressing brainstem neurons to the expression of stress-induced hyperalgesia.

MAIN METHODS

The present study utilized a model of stress-induced mechanical hypersensitivity that involved application of repeated, light tactile whisker pad stimulation (WPS) in rats. Repeated WPS (10 applications/session, 4 sessions/h in 1 day, sessions on days 1-5 and 8-12) increased defensive-aggressive and hypervigilant behaviors, and produced hypersensitivity to tactile stimulation of the hind paw. In order to test the possible involvement of mu-opioid receptor expressing neurons in the rostral ventral medulla (RVM) to this response, rats received RVM microinjections of the toxin conjugate dermorphin-saporin or its control, saporin. Fourteen days later rats underwent either WPS or sham conditioning.

KEY FINDINGS

Repeated WPS produced defensive-aggressive behaviors directed towards the stimulus and mechanical hypersensitivity of the hind paw that persisted for up to 2 weeks after the final WPS session. Dermorphin-saporin, but not saporin, microinjections prevented the development of hind paw mechanical hypersensitivity, but did not affect the defensive-aggressive behaviors.

SIGNIFICANCE

The finding that chronic stress produces mechanical hypersensitivity through circuitry that involves the RVM provides a potential neurobiological basis for the complex interaction between chronic stress and pain.

Interictal dysfunction of a brainstem descending modulatory center in migraine patients

Background

The brainstem contains descending circuitry that can modulate nociceptive processing (neural signals associated with pain) in the dorsal horn of the spinal cord and the medullary dorsal horn. In migraineurs, abnormal brainstem function during attacks suggest that dysfunction of descending modulation may facilitate migraine attacks, either by reducing descending inhibition or increasing facilitation. To determine whether a brainstem dysfunction could play a role in facilitating migraine attacks, we measured brainstem function in migraineurs when they were not having an attack (i.e. the interictal phase).

Methods and Findings

Using fMRI (functional magnetic resonance imaging), we mapped brainstem activity to heat stimuli in 12 episodic migraine patients during the interictal phase. Separate scans were collected to measure responses to 41°C and noxious heat (pain threshold+1°C). Stimuli were either applied to the forehead on the affected side (as reported during an attack) or the dorsum of the hand. This was repeated in 12 age-gender-matched control subjects, and the side tested corresponded to that in the matched migraine patients. Nucleus cuneiformis (NCF), a component of brainstem pain modulatory circuits, appears to be hypofunctional in migraineurs. 3 out of the 4 thermal stimulus conditions showed significantly greater NCF activation in control subjects than the migraine patients.

Conclusions

Altered descending modulation has been postulated to contribute to migraine, leading to loss of inhibition or enhanced facilitation resulting in hyperexcitability of trigeminovascular neurons. NCF function could potentially serve as a diagnostic measure in migraine patients, even when not experiencing an attack. This has important implications for the evaluation of therapies for migraine.

Non-opioid antinociception produced by brain stem injections of improgan: significance of local, but not cross-regional, cannabinoid mechanisms

Improgan, a cimetidine derivative which lacks activity at known histamine, opioid or cannabinoid receptors, acts by an unknown mechanism in the periaqueductal gray (PAG) and raphe magnus (RM) to stimulate descending, analgesic circuits. These circuits may utilize cannabinoid mechanisms. To characterize further the nature of these circuits, the effects of intracerebral (i.c.) microinjections of rimonabant (a CB(1) receptor inverse agonist) were studied on antinociceptive responses following i.c. microinjections of improgan and the cannabinoid agonist WIN 55,212 (WIN) in rats. Separate intra-RM injections of improgan (30 microg) and WIN (8 microg) produced near-maximal antinociception on both the hot plate (HP) and tail flick (TF) nociceptive tests. Pretreatment with intra-RM rimonabant (20 microg) antagonized the antinociception produced by both intra-RM improgan and intra-RM WIN, but had no effects when given alone. Similar studies with improgan demonstrated rimonabant-sensitive sites within the dorsal and ventrolateral PAG. However, intra-RM pretreatment with rimonabant had no effect on antinociceptive responses following intra-PAG improgan. These studies show that improgan activates pain-relieving mechanisms in the PAG and the RM, both of which may utilize local cannabinoid mechanisms.

Role of opioidergic and GABAergic neurotransmission of the nucleus raphe magnus in the modulation of tonic immobility in guinea pigs

Tonic immobility (TI) is an inborn defensive behavior characterized by a temporary state of profound and reversible motor inhibition elicited by some forms of physical restraint. Previous results from our laboratory have demonstrated that nucleus raphe magnus (NRM) is also a structure involved in the modulation of TI behavior, as chemical stimulation through carbachol decreases the duration of TI in guinea pigs. In view of the fact that GABAergic and opioidergic circuits participate in the regulation of neuronal activity in the NRM and since these neurotransmitters are also involved in the modulation of TI, the objective of the present study was to evaluate the role of these circuits of the NRM in the modulation of the behavioral TI response. Microinjection of morphine (4.4 nmol/0.2 microl) or bicuculline (0.4 nmol/0.2 microl) into the NRM increased the duration of TI episodes while muscimol (0.5 nmol/0.2 microl) decreased it. The effect of morphine injection into the NRM was blocked by previous microinjection of naloxone (2.7 nmol/0.2 microl). Muscimol at 0.25 nmol did not produce any change in TI duration; however, it blocked the increased response induced by morphine. Our results indicate a facilitatory role of opioidergic neurotransmission in the modulation of the TI response within the NRM, whereas GABAergic activity plays an inhibitory role. In addition, in the present study the modulation of TI in the NRM possibly occurred via an interaction between opioidergic and GABAergic systems, where the opioidergic effect might be due to inhibition of tonically active GABAergic interneurons.

Thalamic nucleus submedius receives GABAergic projection from thalamic reticular nucleus in the rat

GABAergic projection from thalamic reticular nucleus to thalamic nucleus submedius in the medial thalamus of the rat was studied by using immunohistochemistry for GABA, retrograde labeling with Fluoro-Gold combined with immunohistochemistry for GABA, and anterograde labeling with biotinylated dextranamine. Immunohistochemistry displayed that only GABA immunoreactive terminals were observed in the thalamic nucleus submedius, while GABA immunoreactive neuronal cell bodies were located in the thalamic reticular nucleus and lateral geniculate nucleus. Injection of Fluoro-Gold into the thalamic nucleus submedius resulted in massive retrogradely labeled neuronal cell bodies in the rostroventral portion of the ipsilateral thalamic reticular nucleus and a few in the contralateral thalamic reticular nucleus, and most of these cell bodies showed GABA immunopositive staining. Many biotinylated dextranamine anterogradely labeled fibers and terminals in the thalamic nucleus submedius were observed after injection of biotinylated dextranamine into the thalamic reticular nucleus. The present results provide a morphological evidence for a hypothesis that a disinhibitory effect on output neurons elicited by opioid or 5-hydroxytryptamine inhibiting a GABAergic terminal in the thalamic nucleus submedius may lead to activation of the descending inhibitory system and depression of the nociceptive inputs at the spinal cord level.

Activation of mu-opioid receptors inhibits synaptic inputs to spinally projecting rostral ventromedial medulla neurons

The rostral ventromedial medulla (RVM) is a major locus for the descending control of nociception and opioid analgesia. However, it is not clear how opioids affect synaptic inputs to RVM neurons. In this study, we determined the effect of mu-opioid receptor activation on excitatory and inhibitory synaptic transmission in spinally projecting RVM neurons. RVM neurons were retrogradely labeled with a fluorescent tracer injected into the dorsal horn of the spinal cord in rats. Whole-cell voltage-clamp recordings were performed on labeled RVM neurons in brain slices in vitro. The mu-receptor agonist [D-Ala(2),N-Me-Phe(4),Gly(5)-ol]-enkephalin (DAMGO, 1 microM) significantly decreased the amplitude of evoked excitatory postsynaptic currents (EPSCs) in 52% (9 of 17) of labeled cells. DAMGO also significantly reduced the amplitude of evoked inhibitory postsynaptic currents (IPSCs) in 69% (11 of 16) of cells examined. Furthermore, DAMGO significantly decreased the frequency of miniature EPSCs in 55% (15 of 27) of cells and significantly decreased the frequency of miniature IPSCs in all 12 cells studied. Although most EPSCs and IPSCs were mediated by glutamate and GABA, the nicotinic and glycine receptor antagonists attenuated EPSCs and IPSCs, respectively, in some labeled RVM neurons. Immunocytochemical labeling revealed that only 35% of recorded RVM neurons were tryptophan hydroxylase-positive, and 15% cells had GABA immunoreactivity. Thus, this study provides important functional evidence that activation of mu-opioid receptors decreases the release of both excitatory and inhibitory neurotransmitters onto most spinally projecting RVM neurons.

Involvement of GABAergic modulation of the nucleus submedius (Sm) morphine-induced antinociception

Previous studies have shown that microinjection of morphine into the nucleus submedius (Sm) of the thalamus produces antinociception. The aim of the current study was to examine whether gamma-aminobutyric acid (GABA)ergic terminals in the Sm were involved in this antinociception. Under light anesthesia, the GABA(A) receptor antagonist bicuculline or agonist muscimol was microinjected into the Sm of the thalamus in Sm non-morphine-treated (control) or Sm morphine-treated (microinjection into the Sm in the thalamus) rats. Tail flick latencies (TFL) were measured in each of these groups of rats every 5 min. Bicuculline (100, 200, 500 ng in 0.5 microL) depressed the TF reflex in a dose-dependent fashion, and this effect was blocked by microinjection of the opioid receptor antagonist naloxone (0.5 microg) into the same Sm site. A small dose (100 ng) of bicuculline microinjected into Sm significantly enhanced the morphine-evoked inhibition of TF reflex. In contrast, administration of muscimol (250 ng) did not significantly influence the TF reflex in Sm non-morphine-treated rats, but it significantly attenuated the morphine-induced antinociception in the Sm morphine-treated rats. These results suggest that locally released GABA acting at GABA(A) receptors is involved in the modulation of Sm morphine-induced antinociception, and support the hypothesis that a disinhibitory effect elicited by morphine on GABAergic terminals in Sm may lead to activation of the Sm-ventrolateral orbital cortex (VLO)-perioqueductal gray (PAG) brainstem descending inhibitory system and depression of the nociceptive inputs at the spinal cord level.

Effects of acetylsalicylic acid and morphine on neurons of the rostral ventromedial medulla in rat

Morphine exerts its analgesic effect via the endogenous pain control system consisting of the periaqueductal grey (PAG) and the rostral ventromedial medulla (RVM). Acetylsalicylic acid (ASA) may also act via this system, but so far this has only been demonstrated for the inhibitory effect on the tail-flick reflex with extremely high doses (200-300 mg/kg). Both drugs show synergistic effects on PAG neurons in vitro. It is unclear whether this mechanism accounts for the well-known analgesic synergism of these drugs in vivo. Thus, the effects of ASA (30 mg/kg) and morphine on off- and on-cells in the RVM and the jaw-opening reflex (JOR) were investigated in anesthetized rats. Under morphine, off-cell activity increased (+34%), on-cell activity decreased (-98%) and the reflex was suppressed (-53%). ASA increased off-cell activity (+20%) and decreased the activity of on-cells (-52%). After preceding ASA administration, the effects of morphine on off- and on-cells and on the reflex did not alter statistically. The experiments document the modulatory effect of a clinically relevant dose of ASA on RVM cells. This effect resembles that of morphine. The results do not support the hypothesis of a mediation of the analgesic synergism of morphine and ASA by the PAG-RVM-network in vivo.

Systemic morphine-induced release of serotonin in the rostroventral medulla is not mimicked by morphine microinjection into the periaqueductal gray

We used in vivo microdialysis in awake rats to test the hypothesis that intravenous morphine increases serotonin (5-HT) release within the rostral ventromedial medulla (RVM). We also injected morphine into various sites along the rostrocaudal extent of the periaqueductal gray (PAG), and examined the extent of its diffusion to the RVM. Intravenous morphine (3.0 mg/kg) produced thermal antinociception and increased RVM dialysate 5-HT, 5-hydroxyindole acetic acid (5-HIAA), and homovanillic acid (HVA) in a naloxone-reversible manner. As neither PAG microinjection of morphine (5 micro g/0.5 micro L) nor RVM administration of fentanyl or d-Ala(2),NMePhe(4),Gly-ol(5)]enkephalin (DAMGO) increased RVM 5-HT, we were unable to determine the precise site of action of morphine. Surprisingly, peak morphine levels in the RVM were higher after microinjection into the caudal PAG as compared to either intravenous injection or microinjection into more rostral sites within the PAG. Naloxone-precipitated withdrawal in morphine-tolerant rats not only increased extracellular 5-HT in the RVM, but also dopamine (DA) and HVA. We conclude that substantial amounts of morphine diffuse from the PAG to the RVM, and speculate that opioid receptor interactions at multiple brain sites mediate the analgesic effects of PAG morphine. Further studies will be required to elucidate the contribution of 5-HT and DA release in the RVM to opioid analgesia and opioid withdrawal.

Mu- and delta-opioid receptor mRNAs are expressed in periaqueductal gray neurons projecting to the rostral ventromedial medulla

Opioid antinociception appears to be mediated at least in part by a pathway that projects from the periaqueductal gray (PAG) to the rostral ventromedial medulla (RVM), but the relationship between opioid receptors and PAG-RVM projection neurons is unclear. Previous electrophysiological studies have suggested that opioids act directly on some PAG neurons projecting to the RVM. However, immunoreactivity for neither the cloned mu-opioid receptor (MOR1) nor the cloned delta-opioid receptor (DOR1) has been observed in PAG cells retrogradely labeled from the RVM. In the present study, we examined the expression of DOR1 and MOR1 mRNAs in PAG neurons projecting to RVM using quantitative in situ hybridization and retrograde tract-tracing. Mesencephalic neurons were labeled in three male Sprague-Dawley rats by microinjection of Fluoro-Gold into the RVM. Five micrometer cryostat sections were cut and in situ hybridization was performed using full-length cRNA probes labeled with 35S-UTP. Retrogradely labeled neurons that were also labeled for MOR1 or DOR1 mRNA were observed in the dorsomedial, lateral, and ventrolateral portions of the PAG. Quantification was performed in the dorsomedial and ventrolateral PAG using the physical disector. We found that of 219 retrogradely labeled neurons, 50 +/- 14% expressed DOR1 mRNA. In a second set of 120 Fluoro-Gold-labeled neurons, 27 +/- 8% expressed MOR1 mRNA. Significantly more PAG-RVM projection neurons were labeled for MOR1 mRNA in the ventrolateral subregion of the PAG than in the dorsomedial subregion. However, no significant difference was observed in the proportions of retrogradely labeled neurons labeled for DOR1 mRNA in the ventrolateral subregion compared to the dorsomedial subregion. We conclude that opioids are likely to exert direct effects on PAG-RVM projection neurons through both delta- and mu-opioid receptors. In addition, direct effects on PAG-RVM projection neurons from activation of MOR1 appear more likely to be exerted in the ventrolateral PAG than in the dorsomedial PAG.

Physiological characteristics of the projection pathway from the medial preoptic to the nucleus raphe magnus of the rat and its modulation by the periaqueductal gray

Anatomical studies have shown a strong projection from the medial preoptic nucleus of the hypothalamus (MPO) to both the periaqueductal gray (PAG) and nucleus raphe magnus (NRM). In this study, we examined the physiological characteristics of MPO to NRM connections and examined how blockade of neuronal transmission and of the glutamatergic system within the PAG modifies this pathway. In deeply anesthetized rats, recordings were made from NRM neurons that were identified by their response to peripheral mechanical stimulation and designated as "E", "I", or "N" if they were excited, inhibited, or not activated by noxious stimulation. In addition, cells were identified as spinally projecting if they could be antidromically activated by stimulation of the dorsolateral funiculus at the thoracic level. The responses of 204 NRM neurons to electrical and 87 cells to both chemical and electrical stimulation of MPO were recorded. The response of NRM neurons to MPO stimulation was highly dependent on the sensory class of these cells. Chemical stimulation of MPO inhibited 50% (16/32) and excited 16% (5/32) of the I-cells. In contrast, 23% (9/39) of the E-cells were inhibited and 49% (19/39) were excited by chemical stimulation of MPO. Electrical stimulation at intensities below 80 microA at 100Hz had similar effects on the two classes of cells; 62% (24/39) of the E-cells and 31% (10/32) of the I cells were excited, and 31% (12/39) of the E-cells and 59% (19/32) of the I-cells were inhibited. The excitatory response to chemical stimulation lasted for an average of 136.8+/-73.2s and inhibitory response lasted for an average of 143.8+/-102.1s. Electrical stimulation of MPO at 1Hz excited 27%, inhibited 3%, and had no effect on 70% of NRM cells. The mean latency to peak excitation was 9.6+/-6.6ms. Antidromic activation of MPO neurons by NRM stimulation showed an average latency of 6.3+/-3.4ms. Blocking the glutamatergic transmission within the PAG (by injecting kynurenic acid (KYN) into the PAG) blocked the inhibitory response of 40% (6/15) of the I-cells and inhibitory response of 43% (3/7) of the E-cells. The excitatory response of 27% (3/11) of the I-cells and the excitatory response of 14% (1/7) of the E-cells were blocked by kynurenic injection into the PAG. It is concluded that: (1) in response to chemical stimulation of MPO, the number of I-cells that were inhibited was more than three times the number of I-cells that were excited; in contrast, the number of E-cells that were excited was more than twice the number of E-cells that were inhibited. (2) The interaction between MPO and NRM can be modulated by blockade of the neuronal transmission or blockade of the glutamatergic system in the PAG. (3) Simultaneous activity of many synapses is required for activation of the MPO-NRM pathway. (4) MPO to NRM interaction is mediated by fibers with a conduction velocity of less than 1m/s.

Occurrence of morphine tolerance and dependence in the nucleus paragigantocellularis neurons

The occurrence of morphine tolerance and dependence in the nucleus paragigantocellularis neurons was investigated. The spontaneous activity was recorded from the nucleus paragigantocellularis neurons of urethane-anesthetized rats, using single unit recording. Morphine microinjected (20 mg/ml, 120-200 nl) into the nucleus paragigantocellularis of control rats had both excitatory and inhibitory effects. These effects were reversed by microinjection of naloxone, revealing the possible involvement of mu receptors. Morphine microinjected into morphine-dependent rats failed to change the spontaneous activity of the nucleus paragigantocellularis neurons that accounts for the occurrence of tolerance to morphine in these neurons. Microinjection of naloxone (25 mg/ml, 120-200 nl) in control rats had no effect on the spontaneous firing rate of the nucleus paragigantocellularis neurons but in morphine-dependent rats, either alone or after morphine microinjection, naloxone increased neuronal activity significantly, indicating the occurrence of dependence on morphine in the nucleus paragigantocellularis neurons. These data show that the nucleus paragigantocellularis neurons may play a role in physical dependence on morphine. This conclusion is consistent with the finding, that activation of the nucleus paragigantocellularis by electrical stimulation in morphine-naive rats can elicit behaviors similar to those observed during naloxone-precipitated morphine withdrawal.

Stage-dependent changes in the modulation of spinal nociceptive neuronal activity during the course of inflammation

Spinal and supraspinal controls can tonically or phasically modulate the output of spinal nociceptive neurons. Alterations of these modulatory systems have been described during the acute stage of inflammation. In the present study in the rat, tonic descending controls were assessed during acute (24--48 h) and chronic (3--4 weeks) stages of monoarthritis of the ankle. The electrophysiological properties of spinal convergent neurons with ankle input were compared before and after spinalization. In a parallel series of experiments, spinal convergent neurons were recorded from the normal side in order to assess the propriospinal and supraspinal inhibitory controls triggered by nociceptive stimulation of the inflamed ankle. Tonic descending inhibition of convergent neurons with input from the inflamed ankle was enhanced during the acute stage and then decreased during the chronic stage of monoarthritis. Contralateral-induced inhibitions exhibited a similar temporal evolution. Time-dependent changes in the spinal transmission of nociceptive signals were shown by removing descending modulation in animals with monoarthritis; sensitization of spinal neurons with input from the inflamed ankle was demonstrated during the acute stage of monoarthritis, whereas a crossed transmission between inflamed and normal sides was observed during the chronic stage of the disease. These results show that dynamic and stage-dependent modifications of descending controls tend to dampen the central changes associated with inflammation.

Bidirectional modulation of nociception by GABA neurons in the dorsolateral pontine tegmentum that tonically inhibit spinally projecting noradrenergic A7 neurons

The A7 catecholamine cell group in the dorsolateral pontine tegmentum constitutes an important part of the descending pathways that modulate nociception. Evidence from immunocytochemical studies demonstrate that noradrenergic A7 neurons are densely innervated by GABA terminals arising from GABA neurons that are located in the dorsolateral pontine tegmentum medial to the A7 cell group. GABA(A) receptors are also located on the somata and dendrites of noradrenergic A7 neurons. These findings suggest that noradrenergic neurons in the A7 cell group may be under tonic inhibitory control by GABA neurons. To test this hypothesis, the GABA(A) antagonist bicuculline methiodide in doses of 0.2 or 1.0nmol was microinjected into sites located dorsal to the A7 cell group and the resulting effects on tail flick and nociceptive foot withdrawal responses were measured. Both doses of bicuculline produced significant increases in tail flick latencies and small, but significant, increases in foot withdrawal latencies. Intrathecal injection of the alpha(2)-adrenoceptor antagonist yohimbine, in a dose of 76.7nmol (30microg), attenuated the antinociceptive effect of bicuculline on both the tail and the feet. In contrast, the alpha(1)-adrenoceptor antagonist WB4101, in a nearly equimolar dose of 78.6nmol (30microg), increased the antinociceptive effect of bicuculline on both the tail and the feet. Intrathecal injection of the antagonists alone did not consistently alter nociceptive responses of either the feet or the tail. These findings suggest that noradrenergic neurons in the A7 cell group are tonically inhibited by local GABA neurons. Furthermore, these findings suggest that inhibition of GABA(A) receptors located on spinally-projecting A7 noradrenergic neurons disinhibits, or activates, two populations of A7 neurons that have opposing effects on nociception. One of these populations facilitates nociception by an action mediated by alpha(1)-adrenoceptors in the spinal cord dorsal horn and the other population inhibits nociception by an action mediated by alpha(2)-adrenoceptors.

Supraspinal circuitry mediating opioid antinociception: antagonist and synergy studies in multiple sites

Supraspinal opioid antinociception is mediated by sensitive brain sites capable of supporting this response following microinjection of opioid agonists. These sites include the ventrolateral periaqueductal gray (vIPAG), the rostral ventromedial medulla (RVM), the locus coeruleus and the amygdala. Each of these sites comprise an interconnected anatomical and physiologically relevant system mediating antinociceptive responses through regional interactions. Such interactions have been identified using two pharmacological approaches: (1) the ability of selective antagonists delivered to one site to block antinociception elicited by opioid agonists in a second site, and (2) the presence of synergistic antinociceptive interactions following simultaneous administration of subthreshold doses of opioid agonists into pairs of sites. Thus, the RVM has essential serotonergic, opioid, cholinergic and NMDA synapses that are necessary for the full expression of morphine antinociception elicited from the vIPAG, and the vIPAG has essential opioid synapses that are necessary for the full expression of opioid antinociception elicited from the amygdala. Further, the vIPAG, RVM, locus coeruleus and amygdala interact with each other in synergistically supporting opioid antinociception.

Delta opioid receptor mediated actions in the rostral ventromedial medulla on tail flick latency and nociceptive modulatory neurons

The rostral ventromedial medulla (RVM) is critical for the modulation of dorsal horn nociceptive transmission. Three classes of RVM neurons (ON, OFF, and NEUTRAL) have been described that have distinct responses to noxious stimuli and mu opioid receptor (MOR) agonists. The present study in barbiturate anesthetized rats investigated the effects of the delta 2 opioid receptor (DOR2) agonist, [D-Ala2]deltorphin II (DELT), microinfused into the RVM on the tail flick reflex and activity of RVM neurons. Tail flick latencies increased dose-dependently after administration of DELT (0.6 nmol and 1.2 nmol). Furthermore, DELT inhibited the tail flick related increase in ON cell activity and shortened the tail flick related pause in OFF cell activity. The activity of NEUTRAL cells was not affected. The antinociceptive effects and corresponding changes in ON and OFF cell activity produced by DELT were antagonized by the DOR2 antagonist, naltriben methanesulfonate, administered at the same site. These DOR2 mediated effects on noxious stimulation-evoked changes in RVM neuronal activity are similar to those reported for MOR agonists and suggest that both DOR2 and MOR produce analgesia through activation of OFF cells.

Selective opioid delta agonists elicit antinociceptive supraspinal/spinal synergy in the rat

A multiplicative antinociceptive interaction of morphine activity at supraspinal and spinal sites has been clearly established and is thought to be responsible, in part, for the clinical utility of this compound in normal dose-ranges. While synergistic actions of mu-opioid receptor agonists have been shown, it is unclear whether a similar interaction exists for opioid agonists acting via delta-opioid receptors. Responses to acute nociception were determined with the 52 degrees C hot plate, 52 degrees C warm-water tail-flick and the Hargreaves paw-withdrawal tests. The peptidic opioid delta(1) agonist [D-Pen(2),D-Pen(5)]enkephalin (DPDPE) or delta(2) agonist [D-Ala(2),Glu(4)]deltorphin (DELT) were given into the rostral-ventral medulla (RVM), intrathecally (i.th.) or simultaneously into both the RVM and i.th. (1:1 fixed ratio). Both of the opioid delta agonists produced dose-dependent antinociception in all tests. With the exception of DPDPE in the hot plate test, isobolographic analysis revealed that the supraspinal/spinal antinociceptive interaction for both DPDPE and DELT were synergistic in all nociceptive tests. These data suggest that opioid delta agonists exert a multiplicative antinociceptive interaction between supraspinal and spinal sites to acute noxious stimuli and suggest possibility that compounds acting through delta-opioid receptors may have sufficient potency for eventual clinical application.

Central mechanisms of pain modulation

Bulbospinal serotonergic neurons and two physiological classes of bulbospinal nonserotonergic cells interact to modulate pain transmission. Recent studies have begun to elaborate targets of descending pain modulation other than the well-studied flexion withdrawal pathways. Site-specific, naloxone-sensitive placebo analgesia, which is hard to reconcile with current models of descending pain modulation, presents an exciting challenge to the field.

Physiological properties of raphe magnus neurons during sleep and waking

Neurons in the medullary raphe magnus (RM) that are important in the descending modulation of nociceptive transmission are classified by their response to noxious tail heat as ON, OFF, or NEUTRAL cells. Experiments in anesthetized animals demonstrate that RM ON cells facilitate and OFF cells inhibit nociceptive transmission. Yet little is known of the physiology of these cells in the unanesthetized animal. The first aim of the present experiments was to determine whether cells with ON- and OFF-like responses to noxious heat exist in the unanesthetized rat. Second, to determine if RM cells have state-dependent discharge, the activity of RM neurons was recorded during waking and sleeping states. Noxious heat applied during waking and slow wave sleep excited one group of cells (ON-U) in unanesthetized rats. Other cells were inhibited by noxious heat (OFF-U) applied during waking and slow wave sleep states in unanesthetized rats. NEUTRAL-U cells did not respond to noxious thermal stimulation applied during either slow wave sleep or waking. ON-U and OFF-U cells were more likely to respond to noxious heat during slow wave sleep than during waking and were least likely to respond when the animal was eating or drinking. Although RM cells rarely respond to innocuous stimulation applied during anesthesia, ON-U and OFF-U cells were excited and inhibited, respectively, by innocuous somatosensory stimulation in the unanesthetized rat. The spontaneous activity of >90% of the RM neurons recorded in the unanesthetized rat was influenced by behavioral state. OFF-U cells discharged sporadically during waking but were continuously active during slow wave sleep. By contrast, ON-U and NEUTRAL-U cells discharged in bursts during waking and either ceased to discharge entirely or discharged at a low rate during slow wave sleep. We suggest that OFF cell discharge functions to suppress pain-evoked reactions during sleep, whereas ON cell discharge facilitates pain-evoked responses during waking.

An analgesia circuit activated by cannabinoids

Although many anecdotal reports indicate that marijuana and its active constituent, delta-9-tetrahydrocannabinol (delta-9-THC), may reduce pain sensation, studies of humans have produced inconsistent results. In animal studies, the apparent pain-suppressing effects of delta-9-THC and other cannabinoid drugs are confounded by motor deficits. Here we show that a brainstem circuit that contributes to the pain-suppressing effects of morphine is also required for the analgesic effects of cannabinoids. Inactivation of the rostral ventromedial medulla (RVM) prevents the analgesia but not the motor deficits produced by systemically administered cannabinoids. Furthermore, cannabinoids produce analgesia by modulating RVM neuronal activity in a manner similar to, but pharmacologically dissociable from, that of morphine. We also show that endogenous cannabinoids tonically regulate pain thresholds in part through the modulation of RVM neuronal activity. These results show that analgesia produced by cannabinoids and opioids involves similar brainstem circuitry and that cannabinoids are indeed centrally acting analgesics with a new mechanism of action.

Does intravenous administration of GABA(A) receptor antagonists induce both descending antinociception and touch-evoked allodynia?

Effects of intravenous administration of picrotoxin (PTX), a GABA(A) receptor antagonist, upon activities of wide dynamic range (WDR) neurons in the lumbar spinal cord were studied in urethane-chloralose anesthetized cats. Intravenous PTX augmented tactile evoked responses of WDR neurons, but reduced nociceptive responses dose-dependently. Spinal transection reversed the suppression of nociceptive responses. In the spinal cat, intravenous PTX enhanced the tactile evoked response. Intravenous PTX enhanced the spontaneous firing of nucleus raphe dorsalis (NRD) and/or ventral periaqueductal gray (PAG) neurons projecting to nucleus raphe magnus. Lidocaine injected into NRD/PAG reversed the antinociceptive action of intravenous PTX. PTX injected into NRD/PAG reduced heat-evoked responses of WDR units. These data suggest that antinociceptive effects of intravenous PTX is primarily due to disinhibitory activation of the descending antinociceptive system originating from NRD and PAG, and that PTX reinforces touch-evoked responses in the spinal cord.

Mu and kappa opioid receptors in periaqueductal gray and rostral ventromedial medulla

The periaqueductal gray (PAG) and rostral ventromedial medulla (RVM) are important brain stem pain modulating regions. Recent evidence suggests that kappa opioids antagonize the effects of mu opioids in the RVM. However, the anatomical relationship between mu and kappa opioid receptors in PAG and RVM is not well characterized. This study examined relationships between mu and kappa opioid receptor immunoreactivity (IR) and mRNA in PAG and RVM. Brain slices were processed for either immunocytochemistry or in situ hybridization. We found considerable anatomical overlap of mu and kappa opioid IR and mRNA in the RVM and PAG. These results provide an anatomical basis for recent behavioral and electrophysiological findings in RVM, and suggest modulatory interactions between mu and kappa opioids in PAG.

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.