Vanessa A. Mitchell

Actions of the FAAH inhibitor URB597 in neuropathic and inflammatory chronic pain models

While cannabinoid receptor agonists have analgesic activity in chronic pain states, they produce a spectrum of central CB1 receptor‐mediated motor and psychotropic side effects. The actions of endocannabinoids, such as anandamide are terminated by removal from the extracellular space, then subsequent enzymatic degradation by fatty‐acid amide hydrolase (FAAH). In the present study, we compared the effect of a selective FAAH inhibitor, URB597, to that of a pan‐cannabinoid receptor agonist HU210 in rat models of chronic inflammatory and neuropathic pain. Systemic administration of URB597 (0.3 mg kg−1) and HU210 (0.03 mg kg−1) both reduced the mechanical allodynia and thermal hyperalgesia in the CFA model of inflammatory pain. In contrast, HU210, but not URB597, reduced mechanical allodynia in the partial sciatic nerve‐ligation model of neuropathic pain. HU210, but not URB597, produced a reduction in motor performance in unoperated rats. The effects of URB597 in the CFA model were dose dependent and were reduced by coadministration with the cannabinoid CB1 antagonist AM251 (1 mg kg−1), or the CB2 and SR144528 (1 mg kg−1). Coadministration with AM251 plus SR144528 completely reversed the effects of URB597. These findings suggest that the FAAH inhibitor URB597 produces cannabinoid CB1 and CB2 receptor‐mediated analgesia in inflammatory pain states, without causing the undesirable side effects associated with cannabinoid receptor activation.

Actions of N-arachidonyl-glycine in a rat neuropathic pain model.

While cannabinoid receptor agonists reduce the abnormal pain sensations associated with animal models of neuropathic pain states they also produce CB(1) receptor mediated side effects. Recently, a number of arachidonic acid-amino acid conjugates, including N-arachidonyl-glycine (NAGly), have been identified which are structurally related to the endocannabinoid arachidonyl ethanolamide (anandamide). In the present study we examined the effect of NAGly in a rat model of neuropathic pain. Intrathecal administration of NAGly (700 nmol) and the pan-cannabinoid receptor agonist HU-210 (30 nmol) reduced the mechanical allodynia induced by partial ligation of the sciatic nerve. The NAGly induced anti-allodynia was dose dependent and, unlike HU-210, was unaffected by the cannabinoid CB(1) and CB(2) receptor antagonists, AM251 and SR144528 (30 nmol). The NAGly degradation products, arachidonic acid and glycine (700 nmol), did not reduce allodynia. HU-210, but not NAGly produced a reduction in rotarod latency. These findings suggest that NAGly may provide a novel analgesic approach to alleviate neuropathic pain.

Glutamate Spillover Modulates GABAergic Synaptic Transmission in the Rat Midbrain Periaqueductal Grey via Metabotropic Glutamate Receptors and Endocannabinoid Signaling

Glutamate spillover regulates GABAergic synaptic transmission at several CNS synapses via presynaptic ionotropic and metabotropic glutamate receptors (mGluRs). We have previously demonstrated that activation of group I–III mGluRs inhibits GABAergic transmission in the midbrain periaqueductal gray (PAG), a region involved in organizing behavioral responses to threat, stress, and pain. Here, we examined the role of glutamate spillover in the modulation of GABAergic transmission in the PAG. Using whole-cell recordings from rat PAG slices, we found that evoked IPSCs were reduced by the nonspecific glutamate transport blockers dl-threo-β-benzyloxyaspartic acid (TBOA) and l-trans-pyrrolidine-2,4-dicarboxylic acid, but not by the glial GLT1-specific blocker dihydrokainate. In contrast, TBOA had no effect on evoked IPSCs when glutamate uptake into the postsynaptic neuron was selectively impaired. TBOA increased the paired-pulse ratio of evoked IPSCs and reduced the rate but not the amplitude of spontaneous miniature IPSCs. The effect of TBOA on evoked IPSCs was abolished by the broad-spectrum mGluR antagonist (2S)-2-amino-2-[(1S,2S)-2-carboxycycloprop-1-yl]-3-(xanth-9-yl) propanoic acid (100 μm), reduced by the mGluR5-specific antagonist 2-methyl-6-(phenylethynyl)pyridine hydrochloride (MPEP) and mimicked by the mGluR1/5 agonist (RS)-3,5-dihydroxyphenylglycine (DHPG). Furthermore, the effects of both TBOA and DHPG were reduced by the cannabinoid CB1 receptor antagonist 1-(2,4-dichlorophenyl)-5-(4-iodophenyl)-4-methyl-N-1-piperidinyl-1H-pyrazole-3-carboxamide (AM251). Finally, although MPEP and AM251 had no effect on single evoked IPSCs, they increased evoked IPSCs during repetitive stimulation. These results indicate that neuronal glutamate transporters limit mGluR5 activation and endocannabinoid signaling, but may be overwhelmed during conditions of elevated glutamate release. Thus, neuronal glutamate transporters play a key role in regulating endocannabinoid-mediated cross talk between glutamatergic and GABAergic synapses within the PAG.

Actions of N-arachidonyl-glycine in a rat inflammatory pain model.

BackgroundWhile cannabinoid receptor agonists have analgesic activity in inflammatory pain states they produce a range of side effects. Recently, it has been demonstrated that the arachidonic acid-amino acid conjugate, N-arachidonyl-glycine (NA-glycine) is effective in acute pain models.ResultsIn the present study we examined the effect of NA-glycine in a rat model of inflammatory pain. Intrathecal administration of NA-glycine (70 – 700 nmol) and the pan-cannabinoid receptor agonist HU-210 (10 nmol) reduced the mechanical allodynia and thermal hyperalgesia induced by intraplantar injection of Freunds complete adjuvant (FCA). The actions of HU-210, but not NA-glycine were reduced by the cannabinoid CB1 receptor antagonist AM251. The cannabinoid CB2 receptor antagonist SR144528 also had no effect on the actions of NA-glycine. In contrast, N-arachidonyl-GABA (NA-GABA, 700 nmol) and N-arachidonyl-alanine (NA-alanine, 700 nmol) had no effect on allodynia and hyperalgesia. HU-210, but not NA-glycine produced a reduction in rotarod latency.ConclusionThese findings suggest that NA-glycine may provide a novel non-cannabinoid receptor mediated approach to alleviate inflammatory pain.

Role of 5‐HT1 receptor subtypes in the modulation of pain and synaptic transmission in rat spinal superficial dorsal horn

BACKGROUND AND PURPOSE 5‐HT receptor agonists have variable nociceptive effects within the spinal cord. While there is some evidence for 5‐HT1A spinally‐mediated analgesia, the role of other 5‐HT1 receptor subtypes remains unclear. In the present study, we examined the spinal actions of a range of 5‐HT1 agonists, including sumatriptan, on acute pain, plus their effect on afferent‐evoked synaptic transmission onto superficial dorsal horn neurons.

ACTIONS OF THE ENDOCANNABINOID TRANSPORT INHIBITOR AM404 IN NEUROPATHIC AND INFLAMMATORY PAIN MODELS

1 Although cannabinoid receptor agonists have analgesic activity in chronic pain states, they produce a spectrum of central cannabinoid CB1 receptor‐mediated motor and psychotropic side‐effects. The actions of endocannabinoids, such as anandamide, are terminated by uptake and subsequent intracellular enzymatic degradation. In the present study, we examined the effect of acute administration of the anandamide transport inhibitor AM404 in rat models of chronic neuropathic and inflammatory pain. 2 Systemic administration of AM404 (10 mg/kg) reduced mechanical allodynia in the partial sciatic nerve ligation (PNL) model of neuropathic pain, but not in the complete Freunds adjuvant (CFA) model of inflammatory pain. 3 The effect of AM404 in the PNL model was abolished by coapplication with the selective cannabinoid CB1 receptor antagonist AM251 (1 mg/kg). AM404 did not produce a reduction in motor performance in either the PNL or CFA models. 4 These findings suggest that acute administration of AM404 reduces allodynia in a neuropathic pain model via cannabinoid CB1 receptor activation, without causing the undesirable motor disruption associated with cannabinoid receptor agonists.

Actions of the dual FAAH/MAGL inhibitor JZL195 in a murine inflammatory pain model.

The analgesic efficacy of cannabinoids in chronic pain models is limited by side-effects. It has been proposed that this might be overcome by using agents which indirectly activate the endocannabinoid system. We examined the analgesic and side-effect profile of the dual FAAH/MAGL inhibitor JZL195 in an inflammatory pain model. The effect of systemic injections of a range of doses of JZL195 and the pan-cannabinoid receptor agonist WIN55212 were performed 1 day following intraplantar injection of CFA in C57BL/6 mice. JZL195 and WIN55212 both reduced mechanical allodynia and thermal hyperalgesia, and produced catalepsy and sedation in a dose dependent manner. Unlike WIN55212, JZL195 reduced allodynia at doses below those at which side-effects were observed. The effects of JZL195 and WIN55212 were abolished by co-application with the CB1 antagonist AM251. The CB2 antagonist also reduced the JZL195 anti-allodynia, and reversed the WIN55212 anti-allodynia. The reduction in allodynia produced by JZL195 was greater than that produced individually by the FAAH and MAGL inhibitors, URB597 and JZL184. These findings suggest that JZL195 reduces inflammation induced allodynia at doses below those which produce side-effects, and displays greater efficacy that FAAH or MAGL inhibitors. Thus, dual FAAH/MAGL inhibition has the potential to alleviate inflammatory pain with reduced cannabinoid-like side-effects.

Neurotensin inhibition of GABAergic transmission via mGluR-induced endocannabinoid signalling in rat periaqueductal grey.

Neurotensin modulates pain via its actions within descending analgesic pathways which include brain regions such as the midbrain periaqueductal grey (PAG). The aim of this study was to examine the cellular actions of neurotensin on PAG neurons. Whole cell patch clamp recordings were made from rat midbrain PAG slices in vitro to examine the postsynaptic effects of neurotensin and its effects on GABAA mediated inhibitory postsynaptic currents (IPSCs). Neurotensin (100–300 nm) produced an inward current in subpopulations of opioid sensitive and insensitive PAG neurons which did not reverse over membrane potentials between –50 and –130 mV. The neurotensin induced current was abolished by the NTS1 and NTS1/2 antagonists SR48692 (300 nm) and SR142948A (300 nm). Neurotensin also produced a reduction in the amplitude of evoked IPSCs, but had no effect on the rate and amplitude of TTX‐resistant miniature IPSCs. The neurotensin induced inhibition of evoked IPSCs was reduced by the mGluR5 antagonist MPEP (5μm) and abolished by the cannabinoid CB1 receptor antagonist AM251 (3μm). These results suggest that neurotensin produces direct neuronal depolarisation via NTS1 receptors and inhibits GABAergic synaptic transmission within the PAG. The inhibition of synaptic transmission is mediated by neuronal excitation and action potential dependent release of glutamate, leading to mGluR5 mediated production of endocannabinoids which activate presynaptic CB1 receptors. Thus, neurotensin has cellular actions within the PAG which are consistent with both algesic and analgesic activity, some of which are mediated via the endocannabinoid system.

Postsynaptic actions of substance P on rat periaqueductal grey neurons in vitro.

The postsynaptic actions of substance P on rat midbrain periaqueductal grey (PAG) neurons were examined using whole-cell patch-clamp recordings in brain slices. Substance P produced an inward current in a subpopulation (60%) of PAG neurons. The substance P induced current was concentration dependent (EC50=27 nM) and was reduced by the NK1, NK2 and NK3 antagonists L-732,138 (20 microM), GR 159897 (3 microM) and SB 218795 (3 microM). The selective NK1, NK2 and NK3 agonists [Sar9,Met(O2)11]-Substance P (100 nM), GR 64349 (300-500 nM) and senktide (300 nM) also produced inward currents in subpopulations of neurons. A greater proportion of substance P-sensitive neurons (70%) than substance P-insensitive neurons (31%) responded to the mu/delta opioid agonist met-enkephalin (10 microM). Substance P reduced the outward current produced by met-enkephalin. The reversal potential of the substance P induced current varied from -5 mV to below -140 mV in the absence of met-enkephalin, and was -105 mV in the presence of met-enkephalin. These results indicate that substance P acts via NK1, NK2 and NK3 receptors to excite subpopulations of opioid-sensitive and insensitive PAG neurons by increasing a non-selective cation conductance and by reducing a K+ current. In addition, substance P has anti-opioid actions that are largely mediated by a reduction in the opioid induced K+ current.

Release of immunoreactive brain-derived neurotrophic factor in the spinal cord of the rat following sciatic nerve transection

Using the antibody microprobe method, the sites of spinal release of immunoreactive brain-derived neurotrophic factor (BDNF) was studied in normal rats, and rats with prior sciatic nerve transection. In normal rats, a significant basal release of immunoreactive BDNF was found in the superficial dorsal horn. Following sciatic nerve transection (performed 14 days previously), release of BDNF was found throughout the whole of the dorsal horn, extending into deeper laminae. Electrical stimulation of the ipsilateral sciatic nerve at a strength adequate to excite either A fibres (20 Hz at 2x threshold voltage) or A and C fibres (2 Hz at 20x threshold voltage) did not alter the basal release of immunoreactive BDNF in normal or in nerve-injured rats. The results suggest that BDNF is released from the central terminals of primary afferent fibres, but such release is not solely dependent upon action potential invasion of these terminals. The increased extent of release following nerve transection is consistent with the hypothesis that BDNF plays a role in the central response to peripheral nerve injury.