Mechanism underlying increased neuronal activity in the rat ventrolateral periaqueductal grey by a mu-opioid

Cannabinoids and Opioids Differentially Target Extrinsic and Intrinsic GABAergic Inputs onto the Periaqueductal Grey Descending Pathway

The midbrain periaqueductal gray (PAG) plays a central role in pain modulation via descending pathways. Opioids and cannabinoids are thought to activate these descending pathways by relieving intrinsic GABAergic inhibition of PAG neurons which project to the rostroventromedial medulla (RVM), a process known as disinhibition. However, the PAG also receives descending extrinsic GABAergic inputs from the central nucleus of the amygdala (CeA) which are thought to inhibit PAG GABAergic interneurons. It remains unclear how opioids and cannabinoids act at these different synapses to control descending analgesic pathways. We used optogenetics, tract tracing and electrophysiology to identify the circuitry underlying opioid and cannabinoid actions within the PAG of male and female rats. It was observed that both RVM-projection and nonprojection PAG neurons received intrinsic-PAG and extrinsic-CeA synaptic inputs, which were predominantly GABAergic. Opioids acted via presynaptic µ-receptors to suppress both intrinsic and extrinsic GABAergic inputs onto all PAG neurons, although this inhibition was greater in RVM-projection neurons. By contrast, cannabinoids acted via presynaptic CB1 receptors to exclusively suppress the direct descending GABAergic input from the CeA onto RVM-projection PAG neurons. These findings indicate the CeA controls PAG output neurons which project to the RVM via parallel direct and indirect GABAergic pathways. While µ-opioids indiscriminately inhibit GABAergic inputs onto all PAG neurons, cannabinoids selectively inhibit a direct extrinsic GABAergic input from the amygdala onto PAG projection neurons. These differential actions of opioids and cannabinoids provide a flexible system to gate the descending control of analgesia from the PAG.SIGNIFICANCE STATEMENT The disinhibition hypothesis of analgesia states that opioids activate the midbrain periaqueductal gray (PAG) descending pathway by relieving the tonic inhibition of projection neurons from GABAergic interneurons. However, the PAG also receives extrinsic GABAergic inputs and is the locus of action of cannabinoid analgesics. Here, we show the relative sensitivity of GABAergic synapses to opioids and cannabinoids within the PAG depends on both the origin of presynaptic inputs and their postsynaptic targets. While opioids indiscriminately inhibit all GABAergic inputs onto all PAG neurons, cannabinoids selectively inhibit a direct extrinsic GABAergic input from the amygdala onto PAG descending projection neurons. These differential actions of opioids and cannabinoids provide a flexible system to gate PAG descending outputs.

Opioid Receptor-Mediated Regulation of Neurotransmission in the Brain

Opioids mediate their effects via opioid receptors: mu, delta, and kappa. At the neuronal level, opioid receptors are generally inhibitory, presynaptically reducing neurotransmitter release and postsynaptically hyperpolarizing neurons. However, opioid receptor-mediated regulation of neuronal function and synaptic transmission is not uniform in expression pattern and mechanism across the brain. The localization of receptors within specific cell types and neurocircuits determine the effects that endogenous and exogenous opioids have on brain function. In this review we will explore the similarities and differences in opioid receptor-mediated regulation of neurotransmission across different brain regions. We discuss how future studies can consider potential cell-type, regional, and neural pathway-specific effects of opioid receptors in order to better understand how opioid receptors modulate brain function.

Addressing opioid tolerance and opioid-induced hypersensitivity: Recent developments and future therapeutic strategies

Opioids are a commonly prescribed and efficacious medication for the treatment of chronic pain but major side effects such as addiction, respiratory depression, analgesic tolerance, and paradoxical pain hypersensitivity make them inadequate and unsafe for patients requiring long-term pain management. This review summarizes recent advances in our understanding of the outcomes of chronic opioid administration to lay the foundation for the development of novel pharmacological strategies that attenuate opioid tolerance and hypersensitivity; the two main physiological mechanisms underlying the inadequacies of current therapeutic strategies. We also explore mechanistic similarities between the development of neuropathic pain states, opioid tolerance, and hypersensitivity which may explain opioids' lack of efficacy in certain patients. The findings challenge the current direction of analgesic research in developing non-opioid alternatives and we suggest that improving opioids, rather than replacing them, will be a fruitful avenue for future research.

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.

Opioid presynaptic disinhibition of the midbrain periaqueductal grey descending analgesic pathway

BACKGROUND AND PURPOSE

The midbrain periaqueductal grey (PAG) plays a central role in modulating pain through a descending pathway that projects indirectly to the spinal cord via the rostroventral medial medulla (RVM). While opioids are potent analgesics that target the PAG, their cellular actions on descending projection neurons are unclear.

EXPERIMENTAL APPROACH

Patch clamp recordings in voltage- and current-clamp mode were made from acutely prepared PAG slices from animals that received retrograde tracer injections into the RVM.

KEY RESULTS

The μ-agonist DAMGO reduced GABAergic evoked inhibitory postsynaptic currents (IPSCs) in retro-labelled, RVM-projecting neurons to a greater extent than in unlabelled neurons. The κ-opioid agonist U69593 reduced evoked IPSCs to a similar extent in both neuronal groups, while the δ-opioid agonist deltorphin-II was without effect. DAMGO and U69593 both produced a reduction in the rate, but not amplitude of spontaneous miniature IPSCs and asynchronous evoked IPSCs in retro-labelled neurons. DAMGO and U69593 also suppressed glutamatergic EPSCs in retro-labelled and unlabelled neurons. The DAMGO inhibition of evoked EPSCs, however, was less than that for evoked IPSCs in retro-labelled, but not unlabelled neurons. In current clamp, DAMGO produced a depolarizing increase in evoked postsynaptic potentials in retro-labelled neurons, but directly inhibited unlabelled neurons.

CONCLUSION AND IMPLICATIONS

These findings suggest that μ-opioids activate the descending analgesic pathway from the midbrain PAG by a combination of presynaptic disinhibition of RVM-projecting neurons and postsynaptic inhibition of presumptive interneurons.

μ-Opioid receptors in primary sensory neurons are involved in supraspinal opioid analgesia

Both inhibiting ascending nociceptive transmission and activating descending inhibition are involved in the opioid analgesic effect. The spinal dorsal horn is a critical site for modulating nociceptive transmission by descending pathways elicited by opioids in the brain. μ-Opioid receptors (MORs, encoded by Oprm1) are highly expressed in primary sensory neurons and their central terminals in the spinal cord. In the present study, we tested the hypothesis that MORs expressed in primary sensory neurons contribute to the descending inhibition and supraspinal analgesic effect induced by centrally administered opioids. We generated Oprm1 conditional knockout (Oprm1-cKO) mice by crossing Advillin^Cre/+ mice with Oprm1^flox/flox mice. Immunocytochemical labeling in Oprm1-cKO mice showed that MORs are completely ablated from primary sensory neurons and are profoundly reduced in the superficial spinal dorsal horn. Intracerebroventricular injection of morphine or fentanyl produced a potent analgesic effect in wild-type mice, but such an effect was significantly attenuated in Oprm1-cKO mice. Furthermore, the analgesic effect produced by morphine or fentanyl microinjected into the periaqueductal gray was significantly greater in wild-type mice than in Oprm1-cKO mice. Blocking MORs at the spinal cord level diminished the analgesic effect of morphine and fentanyl microinjected into the periaqueductal gray in both groups of mice. Our findings indicate that MORs expressed at primary afferent terminals in the spinal cord contribute to the supraspinal opioid analgesic effect. These presynaptic MORs in the spinal cord may serve as an interface between ascending inhibition and descending modulation that are involved in opioid analgesia.

Parkinson's disease and pain: Modulation of nociceptive circuitry in a rat model of nigrostriatal lesion

Parkinson's disease (PD) is a neurodegenerative disorder that causes progressive dysfunction of dopaminergic and non-dopaminergic neurons, generating motor and nonmotor signs and symptoms. Pain is reported as the most bothersome nonmotor symptom in PD; however, pain remains overlooked and poorly understood. In this study, we evaluated the nociceptive behavior and the descending analgesia circuitry in a rat model of PD. Three independent experiments were performed to investigate: i) thermal nociceptive behavior; ii) mechanical nociceptive behavior and dopaminergic repositioning; and iii) modulation of the pain control circuitry. The rat model of PD, induced by unilateral striatal 6-hydroxydopamine (6-OHDA), did not interfere with thermal nociceptive responses; however, the mechanical nociceptive threshold was decreased bilaterally compared to that of naive or striatal saline-injected rats. This response was reversed by apomorphine or levodopa treatment. Striatal 6-OHDA induced motor impairments and reduced dopaminergic neuron immunolabeling as well as the pattern of neuronal activation (c-Fos) in the substantia nigra ipsilateral (IPL) to the lesion. In the midbrain periaqueductal gray (PAG), 6-OHDA-induced lesion increased IPL and decreased contralateral PAG GABAergic labeling compared to control. In the dorsal horn of the spinal cord, lesioned rats showed bilateral inhibition of enkephalin and μ-opioid receptor labeling. Taken together, we demonstrated that the unilateral 6-OHDA-induced PD model induces bilateral mechanical hypernociception, which is reversed by dopamine restoration, changes in the PAG circuitry, and inhibition of spinal opioidergic regulation, probably due to impaired descending analgesic control. A better understanding of pain mechanisms in PD patients is critical for developing better therapeutic strategies to improve their quality of life.

Mechanisms of placebo analgesia: A dual-process model informed by insights from cross-species comparisons

Placebo treatments are pharmacologically inert, but are known to alleviate symptoms across a variety of clinical conditions. Associative learning and cognitive expectations both play important roles in placebo responses, however we are just beginning to understand how interactions between these processes lead to powerful effects. Here, we review the psychological principles underlying placebo effects and our current understanding of their brain bases, focusing on studies demonstrating both the importance of cognitive expectations and those that demonstrate expectancy-independent associative learning. To account for both forms of placebo analgesia, we propose a dual-process model in which flexible, contextually driven cognitive schemas and attributions guide associative learning processes that produce stable, long-term placebo effects. According to this model, the placebo-induction paradigms with the most powerful effects are those that combine reinforcement (e.g., the experience of reduced pain after placebo treatment) with suggestions and context cues that disambiguate learning by attributing perceived benefit to the placebo. Using this model as a conceptual scaffold, we review and compare neurobiological systems identified in both human studies of placebo analgesia and behavioral pain modulation in rodents. We identify substantial overlap between the circuits involved in human placebo analgesia and those that mediate multiple forms of context-based modulation of pain behavior in rodents, including forebrain-brainstem pathways and opioid and cannabinoid systems in particular. This overlap suggests that placebo effects are part of a set of adaptive mechanisms for shaping nociceptive signaling based on its information value and anticipated optimal response in a given behavioral context.

Endogenous opioidergic dysregulation of pain in fibromyalgia: a PET and fMRI study

Endogenous opioid system dysfunction potentially contributes to chronic pain in fibromyalgia (FM), but it is unknown if this dysfunction is related to established neurobiological markers of hyperalgesia. We previously reported that µ-opioid receptor (MOR) availability was reduced in patients with FM as compared with healthy controls in several pain-processing brain regions. In the present study, we compared pain-evoked functional magnetic resonance imaging with endogenous MOR binding and clinical pain ratings in female opioid-naive patients with FM (n = 18) using whole-brain analyses and regions of interest from our previous research. Within antinociceptive brain regions, including the dorsolateral prefrontal cortex (r = 0.81, P < 0.001) and multiple regions of the anterior cingulate cortex (all r > 0.67; all P < 0.02), reduced MOR availability was associated with decreased pain-evoked neural activity. Additionally, reduced MOR availability was associated with lower brain activation in the nucleus accumbens (r = 0.47, P = 0.050). In many of these regions, pain-evoked activity and MOR binding potential were also associated with lower clinical affective pain ratings. These findings are the first to link endogenous opioid system tone to regional pain-evoked brain activity in a clinical pain population. Our data suggest that dysregulation of the endogenous opioid system in FM could lead to less excitation in antinociceptive brain regions by incoming noxious stimulation, resulting in the hyperalgesia and allodynia commonly observed in this population. We propose a conceptual model of affective pain dysregulation in FM.

Characteristics of local field potentials correlate with pain relief by deep brain stimulation

OBJECTIVE

To investigate the link between neuronal activity recorded from the sensory thalamus and periventricular gray/periaqueductal gray (PVAG) and pain relief by deep brain stimulation (DBS).

METHODS

Local field potentials (LFPs) were recorded from the sensory thalamus and PVAG post-operatively from ten patients with neuropathic pain. The LFPs were quantified using spectral and time-frequency analysis, the relationship between the LFPs and pain relief was quantified with nonlinear correlation analysis.

RESULTS

The theta oscillations of both sensory thalamus and PVAG correlated inversely with pain relief. The high beta oscillations in the sensory thalamus and the alpha oscillations in the PVAG correlated positively with pain relief. Moreover, the ratio of high-power duration to low-power duration of theta band activity in the sensory thalamus and PVAG correlated inversely with pain relief. The duration ratio at the high beta band in the sensory thalamus correlated positively with pain relief.

CONCLUSIONS

Our results reveal distinct neuronal oscillations at the theta, alpha, and beta frequencies correlating with pain relief by DBS.

SIGNIFICANCE

The study provides quantitative measures for predicting the outcomes of neuropathic pain relief by DBS as well as potential biomarkers for developing adaptive stimulation strategies.

Hypofunction of glutamatergic neurotransmission in the periaqueductal gray contributes to nerve-injury-induced neuropathic pain

Neuropathic pain, a chronic pain due to neuronal lesion, remains unaltered even after the injury-induced spinal afferent discharges have declined, suggesting an involvement of supraspinal dysfunction. The midbrain ventrolateral periaqueductal gray (vlPAG) is known to be a crucial supraspinal region for initiating descending pain inhibition, but its role in neuropathic pain remains unclear. Therefore, here we examined neuroplastic changes in the vlPAG of midbrain slices isolated from neuropathic rats induced by L5/L6 spinal nerve ligation (SNL) via electrophysiological and neurochemical approaches. Significant mechanical hypersensitivity was induced in rats 2 d after SNL and lasted for >14 d. Compared with the sham-operated group, vlPAG slices from neuropathic rats 3 and 10 days after SNL displayed smaller EPSCs with prolonged latency, less frequent and smaller miniature EPSCs, higher paired-pulse ratio of EPSCs, smaller AMPAR-mediated EPSCs, smaller AMPA currents, greater NMDAR-mediated EPSCs, greater NMDA currents, lower AMPAR-mediated/NMDAR-mediated ratios, and upregulation of the NR1 and NR2B subunits, but not the NR2A, GluR1, or GluR2 subunits, of glutamate receptors. There were no significant differences between day 3 and day 10 neuropathic groups. These results suggest that SNL leads to hypoglutamatergic neurotransmission in the vlPAG resulting from both presynaptic and postsynaptic mechanisms. Upregulation of NMDARs might contribute to hypofunction of AMPARs via subcellular redistribution. Long-term hypoglutamatergic function in the vlPAG may lead to persistent reduction of descending pain inhibition, resulting in chronic neuropathic pain.

Serotonergic modulation of neuronal activity in rat midbrain periaqueductal gray

Serotonin (5-HT) modulates pain and anxiety from within the midbrain periaqueductal gray (PAG). In the present study, the effects of 5-HT- and 5-HT1/2 subtype-selective ligands on rat PAG neurons were examined using whole cell patch-clamp recordings in brain slices. In voltage clamp, 5-HT produced outward and inward currents in distinct subpopulations of neurons that varied throughout different subregions of the PAG. The 5-HT1A agonist R(+)-8-OH-DPAT (1 μM) produced outward currents in subpopulations of PAG neurons. By contrast, sumatriptan (1 μM) and other 5-HT1B, -D, and -F subtype agonists had little or no postsynaptic activity. The 5-HT2A/C agonists DOI (3 μM) and TCB-2 (1 μM) produced inward currents in subpopulations of PAG neurons, and DOI enhanced evoked inhibitory postsynaptic currents via a presynaptic mechanism. In current clamp, both R(+)-8-OH-DPAT and sumatriptan produced an excitatory increase in evoked mixed postsynaptic potentials (PSPs). In addition, R(+)-8-OH-DPAT, but not sumatriptan, directly hyperpolarized PAG neurons. By contrast, the 5-HT2 agonist DOI depolarized subpopulations of neurons and produced an inhibitory decrease in evoked mixed PSPs. These findings indicate that 5-HT1A and 5-HT1B/D ligands have partly overlapping inhibitory effects on membrane excitability and synaptic transmission within the PAG, which are functionally opposed by 5-HT2A/C actions in specific PAG subregions.

Activation of orexin 1 receptors in the periaqueductal gray of male rats leads to antinociception via retrograde endocannabinoid (2-arachidonoylglycerol)-induced disinhibition

Orexin A and B are hypothalamic peptides known to modulate arousal, feeding, and reward via OX1 and OX2 receptors. Orexins are also antinociceptive in the brain, but their mechanism(s) of action remain unclear. Here, we investigated the antinociceptive mechanism of orexin A in the rat ventrolateral periaqueductal gray (vlPAG), a midbrain region crucial for initiating descending pain inhibition. In vlPAG slices, orexin A (30-300 nm) depressed GABAergic evoked IPSCs. This effect was blocked by an OX1 [1-(2-methylbenzoxazol-6-yl)-3-[1,5]naphthyridin-4-yl urea (SB 334867)], but not OX2 [N-acyl 6,7-dimethoxy-1,2,3,4-tetrahydroisoquinoline hydrochloride (compound 29)], antagonist. Orexin A increased the paired-pulse ratio of paired IPSCs and decreased the frequency, but not amplitude, of miniature IPSCs. Orexin A-induced IPSC depression was mimicked by (R)-(+)-[2,3-dihydro-5-methyl-3-(4-morpholinylmethyl)pyrrolo[1,2,3-de]-1,4-benzoxazin-6-yl]-1-napthalenylmethanone (WIN 55,212-2), a cannabinoid 1 (CB1) receptor agonist. 1-(2,4-Dichlorophenyl)-5-(4-iodophenyl)-4-methyl-N-(1-piperidyl)pyrazole-3-carboxamide (AM 251), a CB1 antagonist, reversed depressant effects by both agonists. Orexin A-induced IPSC depression was prevented by 1-[6-[[(17β)-3-methoxyestra-1,3,5(10)-trien-17-yl]amino]hexyl]-1H-pyrrole-2,5-dione (U73122) and tetrahydrolipstatin, inhibitors of phospholipase C (PLC) and diacylglycerol lipase (DAGL), respectively, and enhanced by cyclohexyl[1,1'-biphenyl]-3-ylcarbamate (URB602), which inhibits enzymatic degradation of 2-arachidonoylglycerol (2-AG). Moderate DAGLα, but not DAGLβ, immunoreactivity was observed in the vlPAG. Orexin A produced an overall excitatory effect on evoked postsynaptic potentials and hence increased vlPAG neuronal activity. Intra-vlPAG microinjection of orexin A reduced hot-plate nociceptive responses in rats in a manner blocked by SB 334867 and AM 251. Therefore, orexin A may produce antinociception by activating postsynaptic OX1 receptors, stimulating synthesis of 2-AG, an endocannabinoid, through a Gq-protein-mediated PLC-DAGLα enzymatic cascade culminating in retrograde inhibition of GABA release (disinhibition) in the vlPAG.

Capsaicin in the periaqueductal gray induces analgesia via metabotropic glutamate receptor-mediated endocannabinoid retrograde disinhibition

BACKGROUND AND PURPOSE

Capsaicin, an agonist of transient receptor potential vanilloid 1 (TRPV1) channels, is pro-nociceptive in the periphery but is anti-nociceptive when administered into the ventrolateral periaqueductal gray (vlPAG), a midbrain region for initiating descending pain inhibition. Here, we investigated how activation of TRPV1 channels in the vlPAG leads to anti-nociception.

EXPERIMENTAL APPROACH

We examined synaptic transmission and neuronal activity using whole-cell recordings in vlPAG slices in vitro and hot-plate nociceptive responses in rats after drug microinjection into the vlPAG in vivo.

KEY RESULTS

Capsaicin (1-10 µM) depressed evoked GABAergic inhibitory postsynaptic currents (eIPSCs) in vlPAG slices presynaptically, while increasing miniature excitatory PSC frequency. Capsaicin-induced eIPSC depression was antagonized by cannabinoid CB₁ and metabotropic glutamate (mGlu₅) receptor antagonists, and prevented by inhibiting diacylglycerol lipase (DAGL), which converts DAG into 2-arachidonoylglycerol (2-AG), an endocannabinoid. Capsaicin induced membrane depolarization in 2/3 neurons recorded but, overall, increased neuronal firings by increasing evoked postsynaptic potentials. Intra-vlPAG capsaicin reduced hot-plate responses in rats, effects blocked by CB₁ and mGlu receptor antagonists. Effects of capsaicin were antagonized by SB 366791, a TRPV1 channel antagonist.

CONCLUSIONS AND IMPLICATIONS

Capsaicin activated TRPV1s on glutamatergic terminals to release glutamate which activated postsynaptic mGlu₅ receptors, yielding 2-AG from DAG by DAGL hydrolysis. 2-AG induces retrograde inhibition (disinhibition) of GABA release via presynaptic CB₁ receptors. This disinhibition in the vlPAG leads to anti-nociception by activating the descending pain inhibitory pathway. This is a novel TRPV1 channel-mediated anti-nociceptive mechanism in the brain and a new interaction between vanilloid and endocannabinoid systems.

Blockade of GABA(B) receptors facilitates evoked neurotransmitter release at spinal dorsal horn synapse

Metabotropic GABA type B (GABA(B)) receptors are abundantly expressed in the rat spinal dorsal horn. Activation of GABA(B) receptors by exogenous agonists inhibits synaptic transmission, which is believed to underlie the GABA(B) receptor-mediated analgesia. However, little effort has been made to test whether endogenous GABA might also mediate inhibition by acting on GABA(B) receptors. In this study, whole-cell recording techniques were employed to study the effect of endogenous GABA on GABA(B) receptors in substantia gelatinosa (SG) neurons in adult rat spinal cord slices. In current-clamp mode, blockade of GABA(B) receptors by their selective antagonist 3-[[[(3,4-dichlorophenyl)methyl]amino]propyl] (diethoxy-methyl) phosphinic acid (CGP 52432) facilitated presynaptic stimulation-induced action potential discharge and increased amplitude of postsynaptic potentials (PSPs), meaning a GABA(B) receptor-mediated inhibition of SG neuron excitability. In voltage-clamp mode, blockade of GABA(B) receptors increased the amplitude of evoked excitatory postsynaptic currents (eEPSCs) and decreased paired-pulse ratio, indicating a presynaptic CGP 52432 action. Primary afferent Aδ or C fiber-evoked EPSCs were also facilitated by CGP 52432 application. Amplitudes of evoked GABAergic and glycinergic inhibitory postsynaptic currents (eIPSCs) were enhanced by GABA(B) receptor blockade. The facilitation of amplitude persisted in the presence of a specific GABA transporter 1 (GAT-1) blocker, tiagabine, or GAT-2/3 blocker SNAP5114. However, blockade of GABA(B) receptors had no effect on action potential-independent miniature EPSCs (mEPSCs), miniature IPSCs (mIPSCs), or membrane conductance. Taken together, these results suggest that endogenous GABA modulates evoked synaptic transmission in SG neurons by acting on GABA(B) receptors. This GABA(B) receptor-mediated homeostatic regulation of neuronal excitability and neurotransmitter release might contribute to modulation of nociception in spinal dorsal horn.

GABAergic synaptic response and its opioidergic modulation in periaqueductal gray neurons of rats with neuropathic pain

Background

Neuropathic pain is a chronic and intractable symptom associated with nerve injury. The periaqueductal gray (PAG) is important in the endogenous pain control system and is the main site of the opioidergic analgesia. To investigate whether neuropathic pain affects the endogenous pain control system, we examined the effect of neuropathic pain induced by sacral nerve transection on presynaptic GABA release, the kinetics of postsynaptic GABA-activated Cl^- currents, and the modulatory effect of μ-opioid receptor (MOR) activation in mechanically isolated PAG neurons with functioning synaptic boutons.

Results

In normal rats, MOR activation inhibited the frequency of GABAergic miniature inhibitory postsynaptic currents (mIPSCs) to 81.3% of the control without any alteration in their amplitude. In neuropathic rats, the inhibition of mIPSC frequency by MOR activation was 82.4%. The frequency of GABAergic mIPSCs in neuropathic rats was 151.8% of normal rats without any difference in the mIPSC amplitude. Analysis of mIPSC kinetics showed that the fast decay time constant and synaptic charge transfer of mIPSCs in neuropathic rats were 76.0% and 73.2% of normal rats, respectively.

Conclusions

These results indicate that although the inhibitory effect of MOR activation on presynaptic GABA release is similar in both neuropathic and normal rats, neuropathic pain may inhibit endogenous analgesia in the PAG through an increase in presynaptic GABA release.

Quantitative study of the antagonistic effect of (-)-cis-1-Methyl-7-[[4-(2,6-dichlorophenyl)piperidin-1-yl]methyl]-6,7,8,9-tetrahydro-5H-benzocyclohepten-5-ol (SB-612111) on nociceptin/orphanin FQ-mediated potassium channel activation in rat periaqueductal gray slices

Nociceptin/orphanin FQ (N/OFQ) peptide (NOP) receptor, a non-opioid branch of the opioid receptor family, shows structural similarities to traditional opioid receptors but binds opioid with very poor affinity. This receptor has been implicated in many physiological functions including pain regulation. This study quantitatively investigated the effect of (-)-cis-1-Methyl-7-[[4-(2,6-dichlorophenyl)piperidin-1 -yl]methyl]-6,7,8,9-tetrahydro-5H-benzocyclohepten-5-ol (SB-612111), a novel non-peptide ligand of NOP receptor, on the native NOP receptors in the midbrain ventrolateral periaqueductal gray (vlPAG), a crucial region for pain regulation. SB-612111 concentration-dependently antagonized N/OFQ-induced G-protein coupled inwardly rectifying K(+) (GIRK) current in vlPAG neurons. The IC(50) value of SB-612111 estimated from dose-response curves is 87.7±1.2nM. SB-612111 had no intrinsic agonistic activity and did not affect the GIRK current induced by [D-Ala(2), N-Me-Phe(4), Gly(5)-ol]-enkephalin, a mu-opioid receptor agonist, when tested at concentrations of up to 1μM. It is concluded that SB-612111 is a pure, potent and selective antagonist of NOP receptors that mediate GIRK channel activation in the vlPAG neurons.

Functional Interaction Between TRPV1 and μ-Opioid Receptors in the Descending Antinociceptive Pathway Activates Glutamate Transmission and Induces Analgesia

The transient receptor potential vanilloid-1 (TRPV1) receptor is involved in peripheral and spinal nociceptive processing and is a therapeutic target for pain. We have shown previously that TRPV1 in the ventrolateral periaqueductal gray (VL-PAG) tonically contributes to brain stem descending antinociception by stimulating glutamate release into the rostral ventromedial medulla and off neuron activity. Because both opioid and vanilloid systems integrate and transduce pain sensation in these pathways, we studied the potential interaction between TRPV1 and μ-opioid receptors in the VL-PAG–rostral ventromedial medulla (RVM) system. We found that the TRPV1 agonist, capsaicin, and the μ-receptor agonist [D-Ala(2),N-Me-Phe(4),Gly(5)-ol]enkephalin, when coadministered into the ventrolateral-PAG at doses nonanalgesic per se, produce 1) antinociception in tests of thermal nociception; 2) stimulation of glutamate release into the RVM; and 3) inhibition of on neuron activity in the RVM. These effects were all antagonized by the TRPV1 and opioid receptor antagonists 5′-iodo-resiniferatoxin and naloxone, respectively, thus suggesting the existence of a TRPV1–μ-opioid interaction in the VL-PAG–RVM system. By using double immunofluorescence techniques, we found that TRPV1 and μ-opioid receptors are coexpressed in several neurons of the VL-PAG. These findings suggest that μ-receptor activation not only acts on inhibitory neurons to disinhibit PAG output neurons but also interacts with TRPV1 activation at increasing glutamate release into the RVM, possibly by acting directly on PAG output neurons projecting to the RVM.

1-Benzyl-N-[3-[spiroisobenzofuran-1(3H),4'-piperidin-1-yl]propyl]pyrrolidine-2-carboxamide (Compound 24) antagonizes NOP receptor-mediated potassium channel activation in rat periaqueductal gray slices

Nociceptin/orphanin FQ (N/OFQ) peptide (NOP) receptor, the fourth member of opioid receptor family, shares 60-70% homology with traditional opioid receptors but displays little affinity for opioids. This receptor was implicated in many neurological functions and its functional heterogeneity has been proposed. Therefore, it is imperative to develop and characterize new ligands for NOP receptors. 1-Benzyl-N-[3-[spiroisobenzofuran-1(3H),4'-piperidin-1-yl]propyl]pyrrolidine-2-carboxamide (Compound 24) is a new non-peptide ligand of NOP receptor having antagonistic actions in cloned and peripheral NOP receptors. In this study, we quantitatively characterized its effect on the native NOP receptors in the midbrain slices containing ventrolateral periaqueductal gray (vlPAG), a region with dense NOP receptors and involved in pain regulation. In vlPAG neurons, N/OFQ induced G-protein-coupled inwardly rectifying potassium (GIRK) current through NOP receptors. Compound 24, at 0.3-10 microM, attenuated N/OFQ-induced GIRK current concentration-dependently. The antagonistic potency of Compound 24 in vlPAG neurons (IC(50): 2.6+/-0.6 microM) was, however, lower than that obtained in mouse vas deferens preparations or expressed human NOP receptors. The action kinetic of Compound 24 was slower than [Nphe(1), Arg(14), Lys(15)]N/OFQ-NH(2) (UFP-101), a peptide antagonist, in the same preparation. Compound 24 had no intrinsic agonistic activity at NOP receptors at the concentration up to 10 microM. However, at concentrations higher than 3 microM, it also attenuated the GIRK current induced by [D-Ala(2), N-Me-Phe(4), Gly(5)-ol]-enkephalin, a mu-opioid receptor agonist. It is concluded that Compound 24 acts as a pure antagonist at the native NOP receptors in the vlPAG with moderate potency and selectivity.

Sumatriptan inhibits synaptic transmission in the rat midbrain periaqueductal grey

Background

There is evidence to suggest that the midbrain periaqueductal grey (PAG) has a role in migraine and the actions of the anti-migraine drug sumatriptan. In the present study we examined the serotonergic modulation of GABAergic and glutamatergic synaptic transmission in rat midbrain PAG slices in vitro.

Results

Serotonin (5-hydroxytriptamine, 5-HT, IC₅₀ = 142 nM) and the selective serotonin reuptake inhibitor fluoxetine (30 μM) produced a reduction in the amplitude of GABA_A-mediated evoked inhibitory postsynaptic currents (IPSCs) in all PAG neurons which was associated with an increase in the paired-pulse ratio of evoked IPSCs. Real time PCR revealed that 5-HT1A, 5-HT1B, 5-HT1D and 5-HT1F receptor mRNA was present in the PAG. The 5-HT1A, 5-HT1B and 5-HT1D receptor agonists 8-OH-DPAT (3 μM), CP93129 (3 μM) and L694247 (3 μM), but not the 5-HT1F receptor agonist LY344864 (1 – 3 μM) inhibited evoked IPSCs. The 5-HT (1 μM) induced inhibition of evoked IPSCs was abolished by the 5-HT1B antagonist NAS181 (10 μM), but not by the 5-HT1A and 5-HT1D antagonists WAY100135 (3 μM) and BRL15572 (10 μM). Sumatriptan also inhibited evoked IPSCs with an IC₅₀ of 261 nM, and reduced the rate, but not the amplitude of spontaneous miniature IPSCs. The sumatriptan (1 μM) induced inhibition of evoked IPSCs was abolished by NAS181 (10 μM) and BRL15572 (10 μM), together, but not separately. 5-HT (10 μM) and sumatriptan (3 μM) also reduced the amplitude of non-NMDA mediated evoked excitatory postsynaptic currents (EPSCs) in all PAG neurons tested.

Conclusion

These results indicate that sumatriptan inhibits GABAergic and glutamatergic synaptic transmission within the PAG via a 5-HT1B/D receptor mediated reduction in the probability of neurotransmitter release from nerve terminals. These actions overlap those of other analgesics, such as opioids, and provide a mechanism by which centrally acting 5-HT1B and 5-HT1D ligands might lead to novel anti-migraine pharmacotherapies.

Differential regulation of morphine antinociceptive effects by endogenous enkephalinergic system in the forebrain of mice

BACKGROUND

Mice lacking the preproenkephalin (ppENK) gene are hyperalgesic and show more anxiety and aggression than wild-type (WT) mice. The marked behavioral changes in ppENK knock-out (KO) mice appeared to occur in supraspinal response to painful stimuli. However the functional role of enkephalins in the supraspinal nociceptive processing and their underlying mechanism is not clear. The aim of present study was to compare supraspinal nociceptive and morphine antinociceptive responses between WT and ppENK KO mice.

RESULTS

The genotypes of bred KO mice were confirmed by PCR. Met-enkephalin immunoreactive neurons were labeled in the caudate-putamen, intermediated part of lateral septum, lateral globus pallidus, intermediated part of lateral septum, hypothalamus, and amygdala of WT mice. Met-enkephalin immunoreactive neurons were not found in the same brain areas in KO mice. Tail withdrawal and von Frey test results did not differ between WT and KO mice. KO mice had shorter latency to start paw licking than WT mice in the hot plate test. The maximal percent effect of morphine treatments (5 mg/kg and 10 mg/kg, i.p.) differed between WT and KO mice in hot plate test. The current source density (CSD) profiles evoked by peripheral noxious stimuli in the primary somatosenstory cortex (S1) and anterior cingulate cortex (ACC) were similar in WT and KO mice. After morphine injection, the amplitude of the laser-evoked sink currents was decreased in S1 while the amplitude of electrical-evoked sink currents was increased in the ACC. These differential morphine effects in S1 and ACC were enhanced in KO mice. Facilitation of synaptic currents in the ACC is mediated by GABA inhibitory interneurons in the local circuitry. Percent increases in opioid receptor binding in S1 and ACC were 5.1% and 5.8%, respectively.

CONCLUSION

The present results indicate that the endogenous enkephalin system is not involved in acute nociceptive transmission in the spinal cord, S1, and ACC. However, morphine preferentially suppressed supraspinal related nociceptive behavior in KO mice. This effect was reflected in the potentiated differential effects of morphine in the S1 and ACC in KO mice. This potentiation may be due to an up-regulation of opioid receptors. Thus these findings strongly suggest an antagonistic interaction between the endogenous enkephalinergic system and exogenous opioid analgesic actions in the supraspinal brain structures.

Quantitative study of [(pF)Phe4,Arg14,Lys15]nociceptin/orphanin FQ-NH2 (UFP-102) at NOP receptors in rat periaqueductal gray slices

The nociceptin/orphanin FQ (N/OFQ) peptide (NOP) receptor is a novel member of the opioid receptor family with little affinity for traditional opioids. This receptor and its endogenous ligand, N/OFQ, are widely distributed in the brain and are implicated in many physiological functions including pain regulation. [(pF)Phe(4),Arg(14),Lys(15)]N/OFQ-NH(2) (UFP-102) is a newly developed peptide agonist of NOP receptors. In this study, we quantitatively investigated the effect of UFP-102 at native NOP receptors of the ventrolateral periaqueductal gray (PAG), a crucial midbrain area involved in pain regulation and enriched with NOP receptors, using blind patch-clamp whole-cell recording technique in rat brain slices. UFP-102, like N/OFQ, induced an outward current in ventrolateral PAG neurons and increased the membrane current elicited by a hyperpolarization ramp from -60 to -140 mV. The current induced by UFP-102 was characterized with inward rectification and had a reversal potential near the equilibrium potential of K(+) ions, indicating that UFP-102 activates G-protein coupled inwardly rectifying K(+) channels. The effect of UFP-102 was concentration-dependent with the maximal effect similar to that of N/OFQ. The EC(50) value was 11+/-2 nM, which is 5 fold lower than that of N/OFQ. The effect of UFP-102 was not affected by naloxone while competitively antagonized by UFP-101 ([Nphe(1),Arg(14),Lys(15)]N/OFQ-NH(2)), a potent NOP receptor antagonist, with a pA(2) value of 6.7. These results suggest that UFP-102 is a full agonist at the postsynaptic NOP receptors of the midbrain of rats and is 5 fold more potent than N/OFQ.

Disruption of maternal behaviour by acute conspecific interaction induces selective activation of the lateral periaqueductal grey

Maternal behaviour is sensitive to stress and opioidergic activation. The periaqueductal grey (PAG) is involved in coping strategies to stress, whereas morphine inhibition of maternal behaviour depends on the activation of the PAG. The aim of this study was to investigate whether the PAG is activated by disrupting maternal behaviour. Lactating Wistar rats were assigned to four groups: C (control); E1 (acute exposure to a male rat); E2 (daily 2-h exposure to another lactating female and a male rat from Day 3 to 6 of lactation); and E1 + 2 (treated first as E2 and, on Day 9, as E1). Maternal behaviour was recorded on Day 9 of lactation and analysed for 1 h. The E1 group spent more time retrieving their pups, took longer to initiate nursing, had shorter nursing bouts and spent more time in non-maternal activities compared with control. Rats submitted to E2 or E1 + 2 did not differ from the control. In another experiment, lactating rats were treated as above, except that 90 min after the end of the observation period the rats were killed and their brains were processed for immunohistochemical detection of Fos protein in the PAG. Fos increased in the lateral PAG only in the E1 group. We also observed that neurons activated by acute conspecific interaction in the PAG could be responsible for an opioid-dependent decrease in maternal behaviour as this effect was reversed by a microinjection of naltrexone, nor-binaltorphimine or naloxonazine into the lateral PAG. Chronic conspecific interaction alters the way this circuitry responds to acute conspecific interaction.

Intracortical circuits in rat anterior cingulate cortex are activated by nociceptive inputs mediated by medial thalamus

We investigated the afferents and intracortical synaptic organization of the anterior cingulate cortex (ACC) during noxious electrical stimulation. Extracellular field potentials were recorded simultaneously from 16 electrodes spanning all layers of the ACC in male Sprague-Dawley rats anesthetized by halothane inhalation. Laminar-specific transmembrane currents were calculated with the current source density analysis method. Two major groups of evoked sink currents were identified: an early group (latency = 54.04 +/- 2.12 ms; 0.63 +/- 0.07 mV/mm(2)) in layers V-VI and a more intense late group (latency = 80.07 +/- 4.85 ms; 2.16 +/- 0.22 mV/mm(2)) in layer II/III and layer V. Multiunit activities were evoked mainly in layer V and deep layer II/III with latencies similar to that of the early and late sink groups. The evoked EPSP latencies of pyramidal neurons in layers II/III and V related closely with the sink currents. The sink currents were inhibited by intracortical injection of CNQX (1 mM, 1 microl), a glutaminergic receptor antagonist, and enhanced by intraperitoneal (5 mg/kg) and intracortical (10 microg/microl, 1 microl) injection of morphine, a mu-opioid receptor agonist. Paired-pulse depression was observed with interpulse intervals of 50 to 1,000 ms. High-frequency stimulation (100 Hz, 11 pulses) enhanced evoked responses in the ACC and evoked medial thalamic (MT) unit activities. MT lesions blocked evoked responses in the ACC. Our results demonstrated that two distinct synaptic circuits in the ACC were activated by noxious stimuli and that the MT is the major thalamic relay that transmits nociceptive information to the ACC.

GABA-immunoreactive neurons and terminals in the cat periaqueductal gray matter: a light and electron microscopic study

Immunocytochemical and electron microscopic methods were used to study the GABAergic innervation in adult cat periaqueductal gray matter (PAG). A mouse monoclonal antibody against gamma -aminobutyric acid (GABA) was used to visualize the inhibitory neuronal system of PAG. At light microscopy, GABA-immunopositive (GABA(IP)) neurons formed two longitudinally oriented columns in the dorsolateral and ventrolateral PAG that accounted for 36% of the neuronal population of both PAG columns; their perikaryal cross-sectional area was smaller than that of unlabeled (UNL) neurons found in the same PAG subdivisions. At electron microscopic level, patches of GABA immunoreactivity were readily detected in neuronal cell bodies, proximal and distal dendrites, axons and axon terminals. Approximately 35-36% of all terminals were GABA(IP); they established symmetric synapses with dendrites (84.72% of the sample in the dorsolateral PAG and 86.09% of the sample in the ventrolateral PAG) or with cell bodies (7-10% of the sample). Moreover, 49.15% of GABA(IP) axon terminals in the dorsolateral and 52.16% in the ventrolateral PAG established symmetric synapses with GABA(IP) dendrites. Immunopositive axon terminals and unlabeled terminals were also involved in the formation of a complex synaptic arrangment, i.e. clusters of synaptic terminals in close contact between them that were often observed in the PAG neuropil. Moreover, a fair number of axo-axonic synapses between GABA(IP) and/or UNL axon terminals were present in both PAG subdivisions. Several dendro-dendritic synapses between labeled and unlabeled dendrites were also observed in both PAG subdivisions. These results suggest that in the cat PAG there exist at least two classes of GABArgic neurons. The first class could exert a tonic control on PAG projecting neurons, the second could act on those GABAergic neurons that in turn keep PAG projecting neurons under tonic inhibition. The functional implications of this type of GABAergic synapse organization are discussed in relation to the dishinibitory processes that take place in the PAG.

[Nphe1,Arg14,Lys15]N/OFQ-NH2 is a competitive antagonist of NOP receptors in the periaqueductal gray

Nociceptin/orphanin FQ (N/OFQ) and N/OFQ peptide (NOP) receptors are implicated in many physiological functions including pain regulation. This study quantitatively investigated the interaction of a novel NOP receptor antagonist, UFP-101 ([Nphe1,Arg14,Lys15]N/OFQ-NH2), with N/OFQ in the ventrolateral periaqueductal gray, a crucial midbrain area for pain regulation. N/OFQ concentration-dependently activated G-protein coupled inwardly rectifying K+ (GIRK) channels in ventrolateral neurons of periaqueductal gray slices. UFP-101 antagonized N/OFQ-induced GIRK channel activation in a concentration-dependent manner and produced a parallel shift of the concentration-response curve of N/OFQ. The pA2 value estimated from Schild plot is 6.92+/-0.06. At concentrations up to 1 microM, UFP-101 had no effect on membrane current per se and did not affect the GIRK current activated by [d-Ala2, N-Me-Phe4, Gly-ol5]-enkephalin, a mu-opioid receptor agonist. It is concluded that UFP-101 is a potent and competitive peptide antagonist of NOP receptors that mediate GIRK channel activation in ventrolateral periaqueductal gray neurons.

From inhibition to excitation: Functional effects of interaction between opioid receptors

Opioids have excitatory effects in multiple regions of the nervous system. Excitation by opioids is generally attributed to inhibition of inhibitory pathways (disinhibition). However, recent studies indicate that opioids can directly excite individual cells. These effects may occur when opioid receptors interact with other G protein coupled receptors, when different subtypes of opioid receptors interact, or when opioids transactivate other receptors such as receptor tyrosine kinases. Changes in the relative level of expression of different receptors in an individual cell may therefore determine its functional response to a given ligand. This phenomenon could represent an adaptive mechanism involved in tolerance, dependence and subsequent withdrawal.

Opioid inhibition of GABAergic neurotransmission in mechanically isolated rat periaqueductal gray neurons

The descending pain control system is activated by opioid peptides mainly at the midbrain periaqueductal gray (PAG). Although activation of presynaptic opioid receptors has been reported to inhibit gamma-aminobutyric acid (GABA) release, the exact electrophysiological mechanisms are controversial. To elucidate the mechanisms involved in the opioid modulation of presynaptic GABA release, we isolated single PAG neurons with functionally intact synaptic terminals by a mechanical dissociation in the absence of enzyme. With the conventional whole-cell recording mode under the voltage-clamp conditions, the spontaneous miniature inhibitory postsynaptic currents (mIPSCs) were recorded. Bicuculline completely and reversibly blocked mIPSCs. A specific mu-opioid agonist, [d-Ala(2),N-Me-Phe(4),Gly(5)-ol]-enkephalin (DAMGO), reversibly reduced the frequency of mIPSCs without any alteration of amplitude. The inhibitory effect of DAMGO was blocked by N-ethylmaleimide. Blockade of presynaptic Ca(2+) influx by cadmium or depletion of extracellular Ca(2+) did not alter the DAMGO inhibition. In addition, K(+) channels blockers, Ba(2+) or 4-aminopyridine, did not affect the DAMGO effect. The present study indicates that activation of presynaptic mu-opioid receptors coupled to G-proteins inhibits GABA release through unknown intracellular mechanisms downstream of Ca(2+) influx.

Does gabapentin act as an agonist at native GABAB receptors?

Gabapentin, a novel anticonvulsant and analgesic, is a gamma-aminobutyric acid (GABA) analogue but was shown initially to have little affinity at GABA(A) or GABA(B) receptors. It was recently reported to be a selective agonist at GABA(B) receptors containing GABA(B1a)-GABA(B2) heterodimers, although several subsequent studies disproved that conclusion. In the present study, we examined whether gabapentin is an agonist at native GABA(B) receptors using a rat model of postoperative pain in vivo and periaqueductal gray (PAG) slices in vitro; PAG contains GABA(B) receptors, and their activation results in antinociception. An intrathecal injection of gabapentin or baclofen, a GABA(B) receptor agonist, induced antiallodynia in this postoperative pain model. Intrathecal injection of GABA(B) receptor antagonists CGP 35348 and CGP 55845 antagonized baclofen- but not gabapentin-induced antiallodynia. In ventrolateral PAG neurons, baclofen activated G-protein-coupled inwardly rectifying K(+) (GIRK) channels in a manner blocked by CGP 35348 or CGP 55845. However, gabapentin displayed no effect on the membrane current. In neurons unaffected by gabapentin, baclofen activated GIRK channels through GABA(B) receptors. It is concluded that gabapentin is not an agonist at GABA(B) receptors that are functional in baclofen-induced antiallodynia in the postoperative pain model in vivo and in GIRK channel activation in ventrolateral PAG neurons in vitro.

Activation of μ-opioid receptors excites a population of locus coeruleus-spinal neurons through presynaptic disinhibition

The nucleus locus coeruleus (LC) plays an important role in analgesia produced by opioids and by modulation of the descending noradrenergic pathway. The functional role of micro-opioid receptors (muOR) in regulation of the excitability of spinally projecting LC neurons has not been investigated. In the present study, we tested the hypothesis that activation of presynaptic mu-opioid receptors excites a population of spinally projecting LC neurons through attenuation of gamma-aminobutyric acid (GABA)-ergic synaptic inputs. Spinally projecting LC neurons were retrogradely labeled by a fluorescent dye injected into the spinal dorsal horn of rats. Whole-cell current- and voltage-clamp recordings were performed on labeled LC neurons in brain slices. All labeled LC noradrenergic neurons were demonstrated by dopamine-beta-hydroxylase (DbetaH) immunofluorescence. In 37 labeled LC neurons, (D-Ala(2),N-Me-Phe(4),Gly-ol(5))-enkephalin (DAMGO) significantly increased the discharge activity of 17 (45.9%) neurons, but significantly inhibited the firing activity of another 15 (40.5%) cells. The excitatory effect of DAMGO on seven labeled LC neurons was diminished in the presence of bicuculline. DAMGO significantly decreased the frequency of GABA-mediated miniature inhibitory postsynaptic currents (mIPSCs) in all nine labeled LC neurons. However, DAMGO had no effect on glutamate-mediated miniature excitatory postsynaptic currents (mEPSCs) in 12 of 15 neurons. Furthermore, DAMGO significantly inhibited the peak amplitude of evoked inhibitory postsynaptic currents (eIPSCs) in all 11 labeled neurons, but had no significant effect on the evoked excitatory postsynaptic currents (eEPSCs) in 10 of these 11 neurons. Thus, data from this study suggest that activation of micro-opioid receptors excites a population of spinally projecting LC neurons by preferential inhibition of GABAergic synaptic inputs. These findings provide important new information about the descending noradrenergic modulation and analgesic mechanisms of opioids.

Pre- and postsynaptic inhibition mediated by GABA(B) receptors in rat ventrolateral periaqueductal gray neurons

The present study examined the actions of a GABA(B)-receptor agonist, baclofen, on synaptic transmission in rat ventrolateral periaqueductal gray (PAG) neurons of brainstem slices by using whole-cell voltage-clamp recordings. Baclofen (10 microM) induced a slow outward current (peak amplitude: 30.1+/-3.1pA, n=13) at -70mV, which persisted in the presence of tetrodotoxin (0.5 microM) and was diminished in the presence of postsynaptic intracellular K(+)-channel blockers (Cs(+) and TEA) and GDP-beta-S, indicating a direct postsynaptic depression mediated by K(+) channels and G proteins. Baclofen (10 microM) also decreased the frequency of both glutamatergic spontaneous EPSC (by 36+/-7%, n=11) and GABAergic spontaneous IPSC (by 37+/-12%, n=6) without changes in their amplitudes, indicating its presynaptic inhibitions. Taken together, the activation of postsynaptic GABA(B) receptors inhibits ventrolateral PAG neurons directly. At the same time, activating presynaptic GABA(B) receptors on glutamatergic and GABAergic nerve terminals inhibits glutamate and GABA release, respectively. The overall effects might influence an output of ventrolateral PAG neurons that build up the descending pain control system to the spinal dorsal horn.

Prenatal morphine exposure decreases analgesia but not K+ channel activation

The present study has investigated the possible supraspinal adaptive changes induced by prenatal administration of morphine, including morphine-induced supraspinal antinociception in vivo, the density and binding affinity of mu-opioid receptors in the brain and the cellular action of morphine in brain slices in vitro. The cellular action of morphine was assessed by its activation of K+ channels in the ventrolateral periaqueductal gray (PAG), a crucial area for the supraspinal analgesic effect of morphine. Female rats were treated with morphine 7 days before mating at 2 mg/kg. The treatment was continued during pregnancy and after delivery at doses which increased by 1 mg/kg every 2 weeks. Experiments were conducted in the offspring at p14 days. Prenatal morphine exposure induced tolerance to supraspinal morphine-induced tail-flick response. The binding affinity and maximal binding of [(3)H]DAMGO in whole brain were not significant different between the morphine- or saline-treated dams. Autoradiographic analysis shows that the mu-opioid receptor density was decreased in the striatum, thalamus and amygdala but not in the midbrain, nucleus accumbens, hippocampus or cortex in morphine offspring. In ventrolateral PAG neurons, morphine activated inwardly rectifying K+ channels in 59% of recorded neurons of morphine offspring. Neither the magnitude of K channel activation nor the percentage of sensitive neurons was different between the saline- and morphine-treated offspring. It is concluded that prenatal morphine exposure induces tolerance to supraspinal analgesia and this tolerance is not attributed to a change in the mu-opioid receptor density or the receptor-function coupling efficiency in the midbrain periaqueductal gray.

Endomorphin-2 inhibits GABAergic inputs to cardiac parasympathetic neurons in the nucleus ambiguus

The nucleus ambiguus is an area containing cardiac vagal neurons, from which originates most of the parasympathetic control regulating heart rate and cardiac function. GABAergic pathways to these neurons have recently been described, yet modulation of this GABAergic input and its impact upon cardiac vagal neurons is unknown. The nucleus ambiguus has been shown to contain mu-opioid receptors and endomorphin-1 and endomorphin-2, the endogenous peptide ligands for the mu-receptor, whilst microinjections of opioids in the ambiguus area evoke bradycardia. The present study therefore examined the effects of endomorphin-1, endomorphin-2 and DAMGO (a synthetic, mu-selective agonist) on spontaneous GABAergic IPSCs in cardiac parasympathetic neurons. Only endomorphin-2 (100 microM) produced a significant inhibition, of both the frequency (-22.8%) and the amplitude (-30.5%) of the spontaneous IPSCs in cardiac vagal neurons. The inhibitory effects of endomorphin-2 were blocked by naloxonazine (10 microM), a selective mu(1) receptor antagonist. Naloxonazine alone (10 microM) had a potentiating effect on the frequency of the GABAergic IPSCs (+161.43%) but not on the amplitude, indicating that GABA release to cardiac vagal neurons may be under tonic control of opioids acting at the mu(1) receptor. Endomorphin-2 did not reduce the responses evoked by exogenous application of GABA. These results indicate that endomorphin-2 acts on mu(1) receptors located on precedent neurons to decrease GABAergic input to cardiac vagal neurons located in the nucleus ambiguus. The subsequent increase in parasympathetic outflow to the heart may be one mechanism by which mu-selective opioids act to induce bradycardia.

Activation of delta-opioid receptors excites spinally projecting locus coeruleus neurons through inhibition of GABAergic inputs

Stimulation of the noradrenergic nucleus locus coeruleus (LC) releases norepinephrine in the spinal cord, which inhibits dorsal horn neurons and produces analgesia. Activation of this descending noradrenergic pathway also contributes to the analgesic action produced by systemic opioids. The delta-opioid receptors are present presynaptically in the LC. However, their functional role in the control of the activity of spinally projecting LC neurons remains uncertain. In this study, we tested the hypothesis that activation of presynaptic delta-opioid receptors excites spinally projecting LC neurons through inhibition of GABA release. Spinally projecting LC neurons were retrogradely labeled by a fluorescent dye, DiI, injected into the spinal dorsal horn of rats. Whole cell voltage- and current-clamp recordings were performed on DiI-labeled LC neurons in brain slices in vitro. Retrogradely labeled LC noradrenergic neurons were demonstrated by dopamine-beta-hydroxylase immunofluorescence. [D-Pen(2), D-Pen(5)]-enkephalin (DPDPE, 1 microM) significantly decreased the frequency of GABA-mediated miniature inhibitory postsynaptic currents (IPSCs) of nine DiI-labeled LC neurons from 2.1 +/- 0.5 to 0.7 +/- 0.2 Hz without altering their amplitude and the kinetics. On the other hand, the miniature excitatory postsynaptic currents (EPSC) of nine DiI-labeled LC neurons were not significantly altered by DPDPE. Furthermore, DPDPE significantly inhibited the amplitude of evoked IPSC but not EPSC in eight DiI-labeled LC neurons. Under the current-clamp condition, the firing activity in 9 of 11 DiI-labeled LC neurons was significantly increased by 1 microM DPDPE from 4.6 +/- 0.7 to 6.2 +/- 1.0 Hz. Bicuculline (20 microM) also significantly increased the firing frequency in 13 of 20 neurons from 1.8 +/- 0.5 to 2.8 +/- 0.6 Hz. Additionally, the excitatory effect of DPDPE on LC neurons was diminished in the presence of bicuculline. Collectively, these data strongly suggest that activation of presynaptic delta-opioid receptors by DPDPE excites a population of spinally projecting LC neurons by preferential inhibition of GABA release. Thus presynaptic delta-opioid receptors likely play an important role in the regulation of the excitability of spinally projecting LC neurons and the descending noradrenergic inhibitory system.

Nitric oxide inhibits spinally projecting paraventricular neurons through potentiation of presynaptic GABA release

Nitric oxide (NO) in the paraventricular nucleus (PVN) is involved in the regulation of the excitability of PVN neurons. However, the effect of NO on the inhibitory GABAergic and excitatory glutamatergic inputs to spinally projecting PVN neurons has not been studied specifically. In the present study, we determined the role of the inhibitory GABAergic and excitatory glutamatergic inputs in the inhibitory action of NO on spinally projecting PVN neurons. Spinally projecting PVN neurons were retrogradely labeled by a fluorescent dye, 1,1'-dioctadecyl-3,3,3',3'-tetramethylindocasbocyane (DiI), injected into the spinal cord of rats. Whole cell voltage- and current-clamp recordings were performed on DiI-labeled PVN neurons in the hypothalamic slice. The spontaneous miniature inhibitory postsynaptic currents (mIPSCs) recorded in DiI-labeled neurons were abolished by 20 microM bicuculline, whereas the miniature excitatory postsynaptic currents (mEPSCs) were eliminated by 20 microM 6-cyano-7-nitroquinoxaline-2,3-dione. Bath application of an NO donor, 100 microM S-nitroso-N-acetyl-penicillamine (SNAP), or the NO precursor, 100 microM L-arginine, both significantly increased the frequency of mIPSCs of DiI-labeled PVN neurons, without altering the amplitude and the decay time constant of mIPSCs. The effect of SNAP and L-arginine on the frequency of mIPSCs was eliminated by an NO scavenger, 2-(4-carboxypheny)-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide, and an NO synthase inhibitor, 1-(2-trifluoromethylphenyl) imidazole, respectively. Neither SNAP nor L-arginine significantly altered the frequency and the amplitude of mEPSCs. Under current-clamp conditions, 100 microM SNAP or 100 microM L-arginine significantly decreased the discharge rate of the DiI-labeled PVN neurons, without significantly affecting the resting membrane potential. On the other hand, 20 microM bicuculline significantly increased the impulse activity of PVN neurons. In the presence of bicuculline, SNAP or L-arginine both failed to inhibit the firing activity of PVN neurons. This electrophysiological study provides substantial new evidence that NO suppresses the activity of spinally projecting PVN neurons through potentiation of the GABAergic synaptic input.

CompB (J-113397), selectively and competitively antagonizes nociceptin activation of inwardly rectifying K(+) channels in rat periaqueductal gray slices

A novel opioid receptor family, the nociceptin/orphanin FQ (N/OFQ) peptide (NOP) receptors, has been identified to be involved in many physiological functions including pain regulation. CompB (also known as J-113397) is the first non-peptide antagonist of NOP receptors. Using the patch-clamp recording technique in brain slices, we have quantitatively studied the interactions of CompB with N/OFQ at native NOP receptors of ventrolateral neurons of the midbrain periaqueductal gray (PAG), a crucial region for N/OFQ-induced reversal of opioid analgesia. N/OFQ concentration-dependently activated inwardly rectifying K(+) channels in response to hyperpolarization ramps from -60 to -140 mV. CompB attenuated the magnitude but not the reversal potential of the K(+) current activated by N/OFQ in a concentration-dependent manner. The presence of CompB produced a parallel right-shift of the concentration-response curve to N/OFQ. The Schild plot analysis yielded a pA(2) value of 8.37. At concentrations up to 1 microM, CompB affected neither the membrane current per se nor the inwardly rectifying K(+) current activated by [D-Ala(2), N-Me-Phe(4),Gly-ol(5)]-enkephalin or baclofen, a mu-opioid and GABA(B) receptor agonist, respectively. It appears that CompB, at nanomolar concentrations, is a pure, selective and competitive antagonist of postsynaptic NOP receptors that mediate inwardly rectifying K(+) channel activation in ventrolateral PAG neurons.

[Nphe(1)]N/OFQ-(1-13)-NH(2) is a competitive and selective antagonist at nociceptin/orphanin FQ receptors mediating K(+) channel activation in rat periaqueductal gray slices

A novel member of the opioid related receptor family, the nociceptin/orphanin FQ (N/OFQ) peptide (NOP) receptor was identified and demonstrated to be involved in many physiological functions including pain regulation. [Nphe(1)]N/OFQ-(1-13)-NH(2) (Nphe) is a novel peptide antagonist of NOP receptors, developed using peripheral preparations. We have quantitatively investigated the interaction of Nphe with N/OFQ, the endogenous ligand of NOP receptors, in the midbrain ventrolateral periaqueductal gray (PAG), a crucial brain region for N/OFQ-induced reversal of opioid analgesia, using the patch-clamp recording technique in brain slices. N/OFQ concentration-dependently activated an inwardly rectifying K(+) current in response to hyperpolarization ramps from -60 to -140 mV. Nphe concentration-dependently attenuated the K(+) current activated by N/OFQ without changing its reversal potential. The presence of Nphe right-shifted the concentration-response curve to N/OFQ in a parallel manner. The Schild plot analysis yielded a slope of 1.16 and a pA(2) value of 6.64 that is similar to those obtained in peripheral preparations. At concentrations up to 3 microM, Nphe affected neither the membrane current per se, nor the inwardly rectifying K(+) current activated by [D-Ala(2), N-Me-Phe(4),Gly-ol(5)]-enkephalin or baclofen, a mu-opioid and GABA(B) receptor agonist, respectively. It is concluded that Nphe acts as a pure, selective and competitive antagonist at native NOP receptors of ventrolateral PAG neurons.

Does ethanol activate G-protein coupled inwardly rectifying K+ channels?

G-protein coupled inwardly rectifying K+ (GIRK) channels have been reported to be targets of ethanol actions. We investigated if ethanol affects native GIRK channels in rat brain tissues at clinically relevant concentrations using brain slices containing the ventrolateral periaqueductal gray (PAG), an area related to pain regulation. Ethanol did not affect the membrane current elicited by hyperpolarization ramps at concentrations up to 150 mM. However, at 200-300 mM, which is above the lethal level, it activated a barium-sensitive GIRK current in 30-57% of neurons. In neurons unresponsive to ethanol, baclofen, the mu-opioid or nociceptin successfully activated GIRK channels. It is suggested that GIRK channels of the ventrolateral PAG are unlikely to be targets of the analgesic action of ethanol.

Synergistic mu-opioid and 5-HT1A presynaptic inhibition of GABA release in rat periaqueductal gray neurons

The periaqueductal gray (PAG) plays a critical role in descending antinociception. In mechanically dissociated rat PAG neurons, pharmacologically separated spontaneous GABAergic miniature inhibitory postsynaptic currents (mIPSCs) were recorded using the nystatin-perforated patch technique. Both DAMGO, a specific mu-opioid receptor agonist, and serotonin inhibited mIPSC frequency in a dose-dependent manner without affecting mIPSC amplitude, respectively, in the same PAG neurons. The presynaptic opioid effect was blocked by a specific mu-opioid receptor antagonist, CTOP. The presynaptic serotonergic effect was mimicked by a specific 5-HT(1A) receptor agonist, 8-OH-DPAT, and blocked by the specific antagonist, NAN-190. These opioidergic and serotonergic inhibitions of GABA release employed the similar intracellular mechanism of opening 4-AP-sensitive K(+) channels via GTP-binding proteins (G-proteins). Subthreshold concentrations of DAMGO (3 nM) significantly decreased mIPSC frequency with subthreshold concentrations of serotonin (3 nM) and this effect was completely blocked by pretreatment with N-ethylmaleimide (NEM), a PTX-sensitive G-protein inhibitor. In contrast, maximum doses of DAMGO (10 microM) did not further inhibit mIPSC frequency with maximum doses of serotonin (10 microM). In conclusion, activation of presynaptic mu-opioid and 5-HT(1A) receptors synergistically inhibited GABA release. These results suggest a cellular mechanism within PAG for the analgesic effectiveness of combined therapies using opioids in conjunction with classes of anti-depressants which increase synaptic serotonin levels.

Enkephalin release and opioid receptor activation does not mediate the antinociceptive or sedative/hypnotic effects of nitrous oxide

In previous studies using Fos expression as a marker of neuronal activation, we showed that nitrous oxide (N(2)O) activates bulbospinal noradrenergic neurons in rats and that destruction of these neuronal pathways leads to loss of N(2)O antinociceptive action. Based on previous rat studies it has been proposed that these noradrenergic neurons are activated through opioid receptors through the release of endogenous opioid ligands in the periaqueductal gray. Using mice with a disrupted preproenkephalin gene (Penk2 -/-) and the opioid receptor antagonist naltrexone, we investigated the role of enkephalinergic mechanisms and opioid receptor activation in the behavioral and bulbospinal neuron responses to N(2)O in mice. The antinociceptive response to N(2)O was investigated using the tail-flick, hot-plate, and von Frey assays, the sedative/hypnotic response was measured using rotarod and loss of righting reflex, and bulbospinal neuronal activation was assessed with pontine Fos immunostaining. No differences were observed between wild-type and Penk2 -/- mice for the antinociceptive, sedative/hypnotic, and pontine neuronal activation effects of N(2)O. Similarly, naltrexone did not block N(2)O-induced antinociception, sedation, or hypnosis. We conclude that neither enkephalin nor opioid receptors participate in either the antinociceptive or the sedative/hypnotic actions of N(2)O in mice.

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.

Cellular and synaptic adaptations mediating opioid dependence

Although opioids are highly effective for the treatment of pain, they are also known to be intensely addictive. There has been a massive research investment in the development of opioid analgesics, resulting in a plethora of compounds with varying affinity and efficacy at all the known opioid receptor subtypes. Although compounds of extremely high potency have been produced, the problem of tolerance to and dependence on these agonists persists. This review centers on the adaptive changes in cellular and synaptic function induced by chronic morphine treatment. The initial steps of opioid action are mediated through the activation of G protein-linked receptors. As is true for all G protein-linked receptors, opioid receptors activate and regulate multiple second messenger pathways associated with effector coupling, receptor trafficking, and nuclear signaling. These events are critical for understanding the early events leading to nonassociative tolerance and dependence. Equally important are associative and network changes that affect neurons that do not have opioid receptors but that are indirectly altered by opioid-sensitive cells. Finally, opioids and other drugs of abuse have some common cellular and anatomical pathways. The characterization of common pathways affected by different drugs, particularly after repeated treatment, is important in the understanding of drug abuse.

Endogenous opiates: 1999

This paper is the twenty-second installment of the annual review of research concerning the opiate system. It summarizes papers published during 1999 that studied the behavioral effects of the opiate peptides and antagonists, excluding the purely analgesic effects, although stress-induced analgesia is included. The specific topics covered this year include stress; tolerance and dependence; learning, memory, and reward; eating and drinking; alcohol and other drugs of abuse; sexual activity, pregnancy, and development; mental illness and mood; seizures and other neurologic disorders; electrical-related activity; general activity and locomotion; gastrointestinal, renal, and hepatic function; cardiovascular responses; respiration and thermoregulation; and immunologic responses.

[Phe1psi(CH2-NH)Gly2]nociceptin-(1 - 13)-NH2 activation of an inward rectifier as a partial agonist of ORL1 receptors in rat periaqueductal gray

1. [Phe1psi(CH2-NH)Gly2]nociceptin-(1 - 13)-NH2 (Phepsi), a tridecapeptide analogue of orphanin FQ/nociceptin (OFQ/N), was introduced as a competitive antagonist of opioid receptor-like orphan receptor (ORL1) in guinea-pig ileum and mouse vas deferens preparations in vitro but was recently found to act as an agonist in vivo. 2. In the periaqueductal gray, a site enriched with both OFQ/N and ORL1 and involved in OFQ/N-induced hyperalgesia and anti-analgesia, the effects of Phepsi and OFQ/N on the membrane current were studied using whole cell patch clamp recording technique in rat brain slices. 3. OFQ/N (0.01 - 1 microM) activated an inwardly rectifying type of K+ channels in ventrolateral neurons of PAG. Phepsi (0.03 - 1 microM), like OFQ/N, also activated this inward rectifier but had only 30% efficacy of OFQ/N. 4 At maximal effective concentration (1 microM), Phepsi reversed the increment of K+ conductance induced by OFQ/N (300 nM) by 46%. On the other hand, Phepsi also prevented the effect of OFQ/N if pretreated before OFQ/N. 5 It is suggested that Phepsi acts as a partial agonist of ORL1 that mediates the activation of inwardly rectifying K+ channels in ventrolateral neurons of rat periaqueductal gray.