Systematic examination in the rat of brain sites sensitive to the direct application of morphine: observation of differential effects within the periaqueductal gray

Brain mechanisms for offense, defense, and submission

A preliminary attempt is made to analyze the intraspecific aggressive behavior of mammals in terms of specific neural circuitry. The results of stimulation, lesion, and recording studies of aggressive behavior in cats and rats are reviewed and analyzed in terms of three hypothetical motivational systems: offense, defense, and submission. A critical distinction, derived from ethological theory, is made between motivating stimuli that simultaneously activate functional groupings of motor patterning mechanisms, and releasing and directing stimuli that are necessary for the activation of discrete motor patterning mechanisms. It is suggested that motivating stimuli activate pathways that converge upon sets of homogeneous neurons, called motivational mechanisms, whose activity determines the motivational state of the animal.

A defense motivational mechanism is hypothesized to be located in the midbrain central gray. In addition to tactile, auditory, and visual inputs from the paleospinothalamic tract, lateral lemniscus, and (perhaps) from the pretectum, it may receive inputs from a major forebrain pathway whose functional significance is not yet understood.

A submission motivational mechanism is also thought to be located in the central gray. In addition to inputs for defense, it is thought to receive a necessary input from a “consociate (social familiarity cue) modulator” located in the ventromedial hypothalamus, which can switch behavior from defense to submission. The location of the hypothetical offense motivational mechanism is not known, although the pathways by which it is activated are traced in some detail.

Brain mechanisms of aggression in primitive mammals and in primates are apparently similar to those in rats and cats.

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.

BiP, an endoplasmic reticulum chaperone, modulates the development of morphine antinociceptive tolerance

Morphine is a potent analgesic, but the molecular mechanism for tolerance formation after repeated use is not fully understood. Binding immunoglobulin protein (BiP) is an endoplasmic reticulum (ER) chaperone that is central to ER function. We examined knock-in mice expressing a mutant BiP with the retrieval sequence deleted in order to elucidate physiological processes that are sensitive to BiP functions. We tested the thermal antinociceptive effect of morphine in heterozygous mutant BiP mice in a hot plate test. Paw withdrawal latencies before and after a single administration of morphine were not significantly different between the wild-type and mutant BiP mice. Repeated morphine administration caused the development of morphine tolerance in the wild-type mice. The activation of glycogen synthase kinase 3b (GSK-3b) was associated with morphine tolerance, because an inhibitor of GSK-3β prevented it. On the other hand, the mutant BiP mice showed less morphine tolerance, and the activation of GSK-3b was suppressed in their brain. These results suggest that BiP may play an important role in the development of morphine tolerance. Furthermore, we found that a chemical chaperone which improves ER protein folding capacity also attenuated the development of morphine tolerance in wild-type mice, suggesting a possible clinical application of chemical chaperones in preventing morphine tolerance.

Functional interaction between medial thalamus and rostral anterior cingulate cortex in the suppression of pain affect

The medial thalamic parafascicular nucleus (PF) and the rostral anterior cingulate cortex (rACC) are implicated in the processing and suppression of the affective dimension of pain. The present study evaluated the functional interaction between PF and rACC in mediating the suppression of pain affect in rats following administration of morphine or carbachol (acetylcholine agonist) into PF. Vocalizations that occur following a brief noxious tailshock (vocalization afterdischarges) are a validated rodent model of pain affect, and were preferentially suppressed by injection of morphine or carbachol into PF. Vocalizations that occur during tailshock were suppressed to a lesser degree, whereas, spinal motor reflexes (tail flick and hindlimb movements) were only slightly suppressed by injection of carbachol into PF and unaffected by injection of morphine into PF. Blocking glutamate receptors in rACC (NMDA and non-NMDA) by injecting D-2-amino-5-phosphonovalerate (AP-5) or 6-cyano-7-nitroquinoxaline-2,3-dione disodium (CNQX) produced dose-dependent antagonism of morphine-induced increases in vocalization thresholds. Carbachol-induced increases in vocalization thresholds were not affected by injection of either glutamate receptor antagonist into rACC. The results demonstrate that glutamate receptors in the rACC contribute to the suppression of pain affect produced by injection of morphine into PF, but not to the suppression of pain affect generated by intra-PF injection of carbachol.

T-type channels control the opioidergic descending analgesia at the low threshold-spiking GABAergic neurons in the periaqueductal gray

Endogenous opioids generate analgesic signals in the periaqueductal gray (PAG). However, because cell types in the PAG are difficult to identify, its neuronal mechanism has remained poorly understood. To address this issue, we characterized PAG neurons by their electrical properties using differentially labeled GABAergic and output neurons in the PAG. We found that GABAergic neurons were mostly fast-spiking cells and could be further divided into two distinct classes: with or without low-threshold spikes (LTS) driven by T-type channels. In contrast, the PAG output neurons lacked LTS and showed heterogeneous firing patterns. To reveal the function of the LTS, we examined the mutant mice lacking the alpha1G T-type channels (alpha1G(-/-)). The mutant mice lacked LTS in the fast-spiking GABAergic neurons of the PAG and unexpectedly showed impaired opioid-dependent analgesia; a similar phenotype was reproduced in PAG-specific alpha1G-knockdown mice. Electrophysiological analyses revealed functional expression of mu-opioid receptors in the low threshold-spiking GABAergic neurons. These neurons in the mutant lacking LTS showed markedly enhanced discharge activities, which led to an augmented inhibition of output neurons. Furthermore, the impaired analgesia observed in alpha1G(-/-) mice was reversed by blocking local GABA(A) receptors. These results indicate that alpha1G T-type channels are critical for the opioidergic descending analgesia system in the PAG.

micro-Opioid receptor stimulation in the medial subnucleus of the tractus solitarius inhibits gastric tone and motility by reducing local GABA activity

We examined the effects of altering mu-opioid receptor (MOR) activity in the medial subnucleus of the tractus solitarius (mNTS) on several gastric end points including intragastric pressure (IGP), fundus tone, and the receptive relaxation reflex (RRR). Microinjection of the MOR agonist [d-Ala(2),MePhe(4),Gly(ol)(5)]enkephalin (DAMGO; 1-10 fmol) into the mNTS produced dose-dependent decreases in IGP. Microinjection of the endogenous MOR agonists endomorphin-1 and endomorphin-2 (20 fmol) into the mNTS mimicked the effects of 10 fmol DAMGO. Microinjection of 1 and 100 pmol DAMGO into the mNTS produced a triphasic response consisting of an initial decrease, a transient increase, and a persistent decrease in IGP. The increase in IGP appeared to be due to diffusion to the dorsal motor nucleus of the vagus. The effects of 10 fmol DAMGO in the mNTS were blocked by vagotomy and by blockade of MORs, GABA(A) receptors, and ionotropic glutamate receptors in the mNTS. The RRR response was abolished by bilateral microinjection of the opioid receptor antagonist naltrexone into the mNTS and reduced by intravenous administration of naltrexone. Our data demonstrate that 1) activation of MORs in the mNTS with femtomole doses of agonist inhibits gastric motility, 2) the mechanism of MOR effects in the mNTS is through suppression of local GABA activity, and 3) blockade of MORs in the mNTS prevents the RRR response. These data suggest that opioids play an important role in mediating a vagovagal reflex through release of an endogenous opioid in the mNTS, which, in turn, inhibits ongoing local GABA activity and allows vagal sensory input to excite second-order mNTS neurons.

Mu-opioid receptors transiently activate the Akt-nNOS pathway to produce sustained potentiation of PKC-mediated NMDAR-CaMKII signaling

BACKGROUND

In periaqueductal grey (PAG) matter, cross-talk between the Mu-opioid receptor (MOR) and the glutamate N-methyl-D-Aspartate receptor (NMDAR)-CaMKII pathway supports the development of analgesic tolerance to morphine. In neurons, histidine triad nucleotide binding protein 1 (HINT1) connects the regulators of G protein signaling RGSZ1 and RGSZ2 to the C terminus of the MOR. In response to morphine, this HINT1-RGSZ complex binds PKCgamma, and afterwards, the interplay between PKCgamma, Src and Gz/Gi proteins leads to sustained potentiation of NMDAR-mediated glutamate responses.

METHODOLOGY/PRINCIPAL FINDINGS

Following an intracerebroventricular (icv) injection of 10 nmol morphine, Akt was recruited to the synaptosomal membrane and activated by Thr308 and Ser473 phosphorylation. The Akt activation was immediately transferred to neural Nitric Oxide Synthase (nNOS) Ser1417. Afterwards, nitric oxide (NO)-released zinc ions recruited PKCgamma to the MOR to promote the Src-mediated phosphorylation of the Tyr1325 NMDAR2A subunit. This action increased NMDAR calcium flux and CaMKII was activated in a calcium-calmodulin dependent manner. CaMKII then acted on nNOS Ser847 to produce a sustained reduction in NO levels. The activation of the Akt-nNOS pathway was also reduced by the binding of these proteins to the MOR-HINT1 complex where they remained inactive. Tolerance to acute morphine developed as a result of phosphorylation of MOR cytosolic residues, uncoupling from the regulated G proteins which are transferred to RGSZ2 proteins. The diminished effect of morphine was prevented by LNNA, an inhibitor of nNOS function, and naltrindole, a delta-opioid receptor antagonist that also inhibits Akt.

CONCLUSIONS/SIGNIFICANCE

Analysis of the regulatory phosphorylation of the proteins included in the study indicated that morphine produces a transient activation of the Akt/PKB-nNOS pathway. This activation occurs upstream of PKCgamma and Src mediated potentiation of NMDAR activity, ultimately leading to morphine tolerance. In summary, the Akt-nNOS pathway acts as a primer for morphine-triggered events which leads to the sustained potentiation of the NMDAR-CaMKII pathway and MOR inhibition.

NSAIDs, Opioids, Cannabinoids and the Control of Pain by the Central Nervous System

Nonsteroidal anti-inflammatory drugs (NSAIDs) act upon peripheral tissues and upon the central nervous system to produce analgesia. A major central target of NSAIDs is the descending pain control system. The rostral structures of the descending pain control system send impulses towards the spinal cord and regulate the transmission of pain messages. Key structures of the descending pain control system are the periaqueductal gray matter (PAG) and the rostral ventromedial region of the medulla (RVM), both of which are critical targets for endogenous opioids and opiate pharmaceuticals. NSAIDs also act upon PAG and RVM to produce analgesia and, if repeatedly administered, induce tolerance to themselves and cross-tolerance to opioids. Experimental evidence shows that this is due to an interaction of NSAIDs with endogenous opioids along the descending pain control system. Analgesia by NSAIDs along the descending pain control system also requires an activation of the CB1 endocannabinoid receptor. Several experimental approaches suggest that opioids, NSAIDs and cannabinoids in PAG and RVM cooperate to decrease GABAergic inhibition and thus enhance the descending flow of impulses that inhibit pain.

Opiate effects on aversive midbrain stimulation in rats

Morphine blocked decremental bar pressing of rats to escape electrical stimulation of aversive midbrain sites. These sites included the dorsal central gray area (DCG), the ventral reticular formation (VRF) and the lateral tegmentum (LT). Naloxone, which had no effect alone, blocked the morphine effects of all three aversive sites.

Angiotensin III modulates the nociceptive control mediated by the periaqueductal gray matter

Endogenous angiotensin (Ang) II and/or an Ang II-derived peptide, acting on Ang type 1 (AT(1)) and Ang type 2 (AT(2)) receptors, can carry out part of the nociceptive control modulated by periaqueductal gray matter (PAG). However, neither the identity of this putative Ang-peptide, nor its relationship to Ang II antinociceptive activity was clarified. Therefore, we have used tail-flick and incision allodynia models combined with an HPLC time course of Ang metabolism, to study the Ang III antinociceptive effect in the rat ventrolateral (vl) PAG using peptidase inhibitors and receptor antagonists. Ang III injection into the vlPAG increased tail-flick latency, which was fully blocked by Losartan and CGP 42,112A, but not by divalinal-Ang IV, indicating that Ang III effect was mediated by AT(1) and AT(2) receptors, but not by the AT(4) receptor. Ang III injected into the vlPAG reduced incision allodynia. Incubation of Ang II with punches of vlPAG homogenate formed Ang III, Ang (1-7) and Ang IV. Amastatin (AM) inhibited the formation of Ang III from Ang II by homogenate, and blocked the antinociceptive activity of Ang II injection into vlPAG, suggesting that aminopeptidase A (APA) formed Ang III from Ang II. Ang III can also be formed from Ang I by a vlPAG alternative pathway. Therefore, the present work shows, for the first time, that: (i) Ang III, acting on AT(1) and AT(2) receptors, can elicit vlPAG-mediated antinociception, (ii) the conversion of Ang II to Ang III in the vlPAG is required to elicit antinociception, and (iii) the antinociceptive activity of endogenous Ang II in vlPAG can be ascribed preponderantly to Ang III.

Supraspinal Gbetagamma-dependent stimulation of PLCbeta originating from G inhibitory protein-mu opioid receptor-coupling is necessary for morphine induced acute hyperalgesia

Although alterations in micro-opioid receptor (microOR) signaling mediate excitatory effects of opiates in opioid tolerance, the molecular mechanism for the excitatory effect of acute low dose morphine, as it relates to microOR coupling, is presently unknown. A pronounced coupling of microOR to the alpha subunit of G inhibitory protein emerged in periaqueductal gray (PAG) from mice systemically administered with morphine at a dose producing acute thermal hyperalgesia. This coupling was abolished in presence of the selective microOR antagonist d-Phe-Cys-Tyr-d-Trp-Orn-Thr-Pen-Thr-NH(2) administered at the PAG site, showing that the low dose morphine effect is triggered by microOR activated G inhibitory protein at supraspinal level. When Gbetagamma downstream signalling was blocked by intra-PAG co-administration of 2-(3,4,5-trihydroxy-6-oxoxanthen-9-yl)cyclohexane-1-carboxylic acid, a compound that inhibits Gbetagamma dimer-dependent signaling, a complete prevention of low dose morphine induced acute thermal hyperalgesia was obtained. Phospholipase C beta3, an enzyme necessary to morphine hyperalgesia, was revealed to be associated with Gbetagamma in PAG. Although opioid administration induces a shift in microOR-G protein coupling from Gi to Gs after chronic administration, our data support that this condition is not realized in acute treatment providing evidence that a separate molecular mechanism underlies morphine induced acute excitatory effect.

Transcutaneous electrical nerve stimulation at both high and low frequencies activates ventrolateral periaqueductal grey to decrease mechanical hyperalgesia in arthritic rats

Transcutaneous electric nerve stimulation (TENS) is widely used for the treatment of pain. TENS produces an opioid-mediated antinociception that utilizes the rostroventromedial medulla (RVM). Similarly, antinociception evoked from the periaqueductal grey (PAG) is opioid-mediated and includes a relay in the RVM. Therefore, we investigated whether the ventrolateral or dorsolateral PAG mediates antinociception produced by TENS in rats. Paw and knee joint mechanical withdrawal thresholds were assessed before and after knee joint inflammation (3% kaolin/carrageenan), and after TENS stimulation (active or sham). Cobalt chloride (CoCl(2); 5 mM) or vehicle was microinjected into the ventrolateral periaqueductal grey (vlPAG) or dorsolateral periaqueductal grey (dlPAG) prior to treatment with TENS. Either high (100 Hz) or low (4 Hz) frequency TENS was then applied to the inflamed knee for 20 min. Active TENS significantly increased withdrawal thresholds of the paw and knee joint in the group microinjected with vehicle when compared to thresholds prior to TENS (P<0.001) or to sham TENS (P<0.001). The increases in withdrawal thresholds normally observed after TENS were prevented by microinjection of CoCl(2) into the vlPAG, but not the dlPAG prior to TENS and were significantly lower than controls treated with TENS (P<0.001). In a separate group of animals, microinjection of CoCl(2) into the vlPAG temporarily reversed the decreased mechanical withdrawal threshold suggesting a role for the vlPAG in the facilitation of joint pain. No significant difference was observed for dlPAG. We hypothesize that the effects of TENS are mediated through the vlPAG that sends projections through the RVM to the spinal cord to produce an opioid-mediated analgesia.

Contribution of dopamine receptors to periaqueductal gray-mediated antinociception

RATIONALE

Morphine relieves pain, in part, by acting on neurons within the periaqueductal gray (PAG). Given that the PAG contains a subpopulation of dopamine neurons, dopamine may contribute to the antinociceptive effects mediated by the PAG.

METHODS

This hypothesis was tested by measuring the behavioral and electrophysiological effects of administering dopamine agonists and antagonists into the ventrolateral PAG (vPAG). An initial histological experiment verified the existence of dopamine neurons within the vPAG using dopamine transporter and tyrosine hydroxylase antibodies visualized with confocal microscopy.

RESULTS

Microinjection of cumulative doses of morphine into the vPAG caused antinociception that was dose-dependently inhibited by the dopamine receptor antagonist alpha-flupenthixol. alpha-Flupenthixol had no effect on nociception when administered alone. Injection of the dopamine receptor agonist (-) apomorphine into the vPAG caused a robust antinociception that was inhibited by the D2 antagonist eticlopride but not the D1 antagonist SCH-23390. The effects of dopamine on GABA(A)-mediated evoked inhibitory post-synaptic potentials (eIPSCs) were measured in PAG slices. Administration of met-enkephalin inhibited peak eIPSCs by 20-50%. Dopamine inhibited eIPSCs by approximately 20-25%. Administration of alpha-flupenthixol (20 muM) attenuated eIPSC inhibition by dopamine but had no effect on met-enkephalin-induced inhibition.

CONCLUSIONS

These data indicate that PAG dopamine has a direct antinociceptive effect in addition to modulating the antinociceptive effect of morphine. The lack of an effect of alpha-flupenthixol on opioid-inhibition of eIPSCs indicates that this modulation occurs in parallel or subsequent to inhibition of GABA release.

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.

Descending control of nociception: Specificity, recruitment and plasticity

The dorsal horn of the spinal cord is the location of the first synapse in pain pathways, and as such, offers a very powerful target for regulation of nociceptive transmission by both local segmental and supraspinal mechanisms. Descending control of spinal nociception originates from many brain regions and plays a critical role in determining the experience of both acute and chronic pain. The earlier concept of descending control as an "analgesia system" is now being replaced with a more nuanced model in which pain input is prioritized relative to other competing behavioral needs and homeostatic demands. Descending control arises from a number of supraspinal sites, including the midline periaqueductal gray-rostral ventromedial medulla (PAG-RVM) system, and the more lateral and caudal dorsal reticular nucleus (DRt) and ventrolateral medulla (VLM). Inhibitory control from the PAG-RVM system preferentially suppresses nociceptive inputs mediated by C-fibers, preserving sensory-discriminative information conveyed by more rapidly conducting A-fibers. Analysis of the circuitry within the RVM reveals that the neural basis for bidirectional control from the midline system is two populations of neurons, ON-cells and OFF-cells, that are differentially recruited by higher structures important in fear, illness and psychological stress to enhance or inhibit pain. Dynamic shifts in the balance between pain inhibiting and facilitating outflows from the brainstem play a role in setting the gain of nociceptive processing as dictated by behavioral priorities, but are also likely to contribute to pathological pain states.

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.

Gz mediates the long-lasting desensitization of brain CB1 receptors and is essential for cross-tolerance with morphine

Background

Although the systemic administration of cannabinoids produces antinociception, their chronic use leads to analgesic tolerance as well as cross-tolerance to morphine. These effects are mediated by cannabinoids binding to peripheral, spinal and supraspinal CB1 and CB2 receptors, making it difficult to determine the relevance of each receptor type to these phenomena. However, in the brain, the CB1 receptors (CB1Rs) are expressed at high levels in neurons, whereas the expression of CB2Rs is marginal. Thus, CB1Rs mediate the effects of smoked cannabis and are also implicated in emotional behaviors. We have analyzed the production of supraspinal analgesia and the development of tolerance at CB1Rs by the direct injection of a series of cannabinoids into the brain. The influence of the activation of CB1Rs on supraspinal analgesia evoked by morphine was also evaluated.

Results

Intracerebroventricular (icv) administration of cannabinoid receptor agonists, WIN55,212-2, ACEA or methanandamide, generated a dose-dependent analgesia. Notably, a single administration of these compounds brought about profound analgesic tolerance that lasted for more than 14 days. This decrease in the effect of cannabinoid receptor agonists was not mediated by depletion of CB1Rs or the loss of regulated G proteins, but, nevertheless, it was accompanied by reduced morphine analgesia. On the other hand, acute morphine administration produced tolerance that lasted only 3 days and did not affect the CB1R. We found that both neural mu-opioid receptors (MORs) and CB1Rs interact with the HINT1-RGSZ module, thereby regulating pertussis toxin-insensitive Gz proteins. In mice with reduced levels of these Gz proteins, the CB1R agonists produced no such desensitization or morphine cross-tolerance. On the other hand, experimental enhancement of Gz signaling enabled an acute icv administration of morphine to produce a long-lasting tolerance at MORs that persisted for more than 2 weeks, and it also impaired the analgesic effects of cannabinoids.

Conclusion

In the brain, cannabinoids can produce analgesic tolerance that is not associated with the loss of surface CB1Rs or their uncoupling from regulated transduction. Neural specific Gz proteins are essential mediators of the analgesic effects of supraspinal CB1R agonists and morphine. These Gz proteins are also responsible for the long-term analgesic tolerance produced by single doses of these agonists, as well as for the cross-tolerance between CB1Rs and MORs.

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.

Purification and characterization of a novel antinociceptive toxin from Cobra venom (Naja naja atra)

Snake venoms have demonstrated antinociceptive activity, and certain isolated neurotoxins have demonstrated significant analgesia in animal models. Here we report a novel analgesic toxin which was isolated from Naja naja atra and was given the name 'najanalgesin'. The LD(50) of the crude venom and najanalgesin were 0.89mg/kg and 2.69mg/kg, respectively. We used the writhing test and hot plate test to evaluate the antinociceptive properties of the crude venom and najanalgesin after intraperitoneal (ip) administration. The analgesic mechanism of najanalgesin was also studied. The response latency time was significantly prolonged in the hot plate test after ip administration of the crude venom of Naja naja atra (0.111-0.445mg/kg) in a dose-dependent manner. Najanalgesin (1mg/kg) elicited almost the same antinociceptive effect as that of the crude venom of Naja naja atra at the dose of 0.445mg/kg and remained for 6h after intraperitoneal injection, shown by hot plate test. The percentage of increase in the latency time for the venom and the najanalgesin 3h after drug administration was 96.2% and 112%, respectively. The number of writhes decreased to almost 1/3, 1/6, and 1/12 of the NS (physiological saline) group after intraperitoneal administration of najanalgesin at 0.25, 0.5, and 1.0mg/kg, respectively. Pretreatment with atropine (1mg/kg) or naloxone (3mg/kg) blocked the antinociception of najanalgesin in the hot plate test. Based on the sequence information, najanalgesin is found to be highly homologous with the conventional CTXs (cardiotoxins). To our knowledge, no study had previously reported that a toxin which was homologous with CTXs possessed the antinociceptive activity. Thus, this is the first report that the antinociceptive effect induced by najanalgesin is mediated by cholinergic and opioidergic mechanisms.

Differential responses to morphine-induced analgesia in the tail-flick test

We compared acute and chronic antinociceptive effects of morphine in animals with high reactivity (HR) vs. low reactivity (LR) to novelty. Antinociception was assessed by tail-flick test. Rats were i.p. injected with either saline or morphine (1.5 or 3mg/kg) every 12h for 7 days according to the treatment group. On day 1 of the experiment, LR animals in the 1.5mg/kg morphine group showed significantly higher tail-flick latency than HR. Moreover, significant tolerance to the antinociceptive effects of morphine at the used doses was observed in LR but not HR animals. However, effects of chronic morphine treatment on tail-flick latency in rat groups with similar morphine-induced acute antinociception were undistinguishable. The difference in tail-flick latency between HR and LR rats observed after acute 1.5mg/kg morphine injection was eliminated if beta-funaltrexamine (3mg/kg, i.p.) was administered 24h before the test, an indication that mu opioid receptors are responsible for the difference observed. Studies to anatomically characterize the difference in the acute analgesic effect of morphine in HR vs. LR animals did not however yield any significant difference in mu opioid receptor mRNA levels in locus coeruleus (LC), ventral periaqueductal gray (vPAG), nucleus raphe magnus (NRM) and nucleus reticularis paragigantocellularis (NRPG) between these two groups of animals. In conclusion, our results show that differences in novelty-seeking behavior can predict inter-individual variability in morphine-induced antinociception in rats. Such variability is dependent upon activation of mu opioid receptors, but does not correlate with mu opioid receptor expression in LC, vPAG or ventral medulla.

Nociceptive behavior in animal models for peripheral neuropathy: spinal and supraspinal mechanisms

Since the initial description by Wall [Wall, P.D., 1967. The laminar organization of dorsal horn and effects of descending impulses. J. Neurophysiol. 188, 403-423] of tonic descending inhibitory control of dorsal horn neurons, several studies have aimed to characterize the role of various brain centers in the control of nociceptive input to the spinal cord. The role of brainstem centers in pain inhibition has been well documented over the past four decades. Lesion to peripheral nerves results in hypersensitivity to mild tactile or cold stimuli (allodynia) and exaggerated response to nociceptive stimuli (hyperalgesia), both considered as cardinal signs of neuropathic pain. The increased interest in animal models for peripheral neuropathy has raised several questions concerning the rostral conduction of the neuropathic manifestations and the role of supraspinal centers, especially brainstem, in the inhibitory control or in the abnormal contribution to the maintenance and facilitation of neuropathic-like behavior. This review aims to summarize the data on the ascending and descending modulation of neuropathic manifestations and discusses the recent experimental data on the role of supraspinal centers in the control of neuropathic pain. In particular, the review emphasizes the importance of the reciprocal interconnections between the analgesic areas of the brainstem and the pain-related areas of the forebrain. The latter includes the cerebral limbic areas, the prefrontal cortex, the intralaminar thalamus and the hypothalamus and play a critical role in the control of pain considered as part of an integrated behavior related to emotions and various homeostatic regulations. We finally speculate that neuropathic pain, like extrapyramidal motor syndromes, reflects a disorder in the processing of somatosensory information.

Involvement of post-translational modification of neuronal plasticity-related proteins in hyperalgesia revealed by a proteomic analysis

To clarify roles of an endogenous pain modulatory system of the central nervous system (CNS) in hyperalgesia, we tried to identify qualitative and quantitative protein changes by a proteomic analysis using an animal model of hyperalgesia. Specifically, we first induced functional hyperalgesia on male Wistar rats by repeated cold stress (specific alternation of rhythm in temperature, SART). We then compared proteomes of multiple regions of CNS and the dorsal root ganglion between the hyperalgetic rats and non-treated ones by 2-D PAGE in the pI range of 4.0-7.0. We found that SART changed the proteomes prominently in the mesencephalon and cerebellum. We thus analyzed the two brain regions in more detail using gels with narrower pI ranges. As a result, 29 and 23 protein spots were significantly changed in the mesencephalon and the cerebellum, respectively. We successfully identified 12 protein spots by a MALDI-TOF/TOF MS and subsequent protein database searching. They included unc-18 protein homolog 67K, collapsin response mediator protein (CRMP)-2 and CRMP-4, which were reported to be involved in neurotransmitter release or axon elongation. Interestingly, mRNA expression levels of these three proteins were not changed significantly by the induction of hyperalgesia. Instead, we found that the detected changes in the protein spots are caused by the post-translational modification (PTM) of proteolysis or phosphorylation. Taken together, development of the hyperalgesia would be linked to PTM of these three CNS proteins. PTM regulation may be one of the useful ways to treat hyperalgesia.

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.

Cyclooxygenase-1-derived prostaglandins in the periaqueductal gray differentially control C- versus A-fiber-evoked spinal nociception

Nonsteroidal anti-inflammatory drugs (NSAIDs) exert analgesic effects by inhibiting peripheral cyclooxygenases (COXs). It is now clear that these drugs also have central actions that include the modulation of descending control of spinal nociception from the midbrain periaqueductal gray (PAG). Descending control is a powerful determinant of the pain experience and is thus a potential target for analgesic drugs, including COX inhibitors. Noxious information from the periphery is conveyed to the spinal cord in A- and C-fiber nociceptors, which convey different qualities of the pain signal and have different roles in chronic pain. This in vivo study used different rates of skin heating to preferentially activate A- or C-heat nociceptors to further investigate the actions of COX inhibitors and prostaglandins in the PAG on spinal nociceptive processing. The results significantly advance our understanding of the central mechanisms underlying the actions of NSAIDs and prostaglandins by demonstrating that (1) in the PAG, it is COX-1 and not COX-2 that is responsible for acute antinociceptive effects of NSAIDs in vivo; (2) these effects are only evoked from the opioid-sensitive ventrolateral PAG; and (3) prostaglandins in the PAG exert tonic facilitatory control that targets C- rather than A-fiber-mediated spinal nociception. This selectivity of control is of particular significance given the distinct roles of A- and C-nociceptors in acute and chronic pain. Thus, effects of centrally acting prostaglandins are pivotal, we suggest, to both the understanding of nociceptive processing and the development of new analgesic drugs.

Calcium/calmodulin-dependent protein kinase II supports morphine antinociceptive tolerance by phosphorylation of glycosylated phosducin-like protein

The long isoform of the phosducin-like protein (PhLPl) is widely expressed in the brain and it is thought to influence G-protein signalling by regulating the activity of Gbetagamma dimers. We show that in the mature nervous system, PhLPl exists as both a 38kDa non-glycosylated isoform and as glycosylated isoforms of about 45, 100 and 150kDa. Additionally, neural PhLPl is subject to serine phosphorylation, which augments upon the activation of Mu-opioid receptors (MORs), as does its association with Gbetagamma subunits and 14-3-3 proteins. While the intracerebroventricular (icv) administration of morphine to mice rapidly reduced the association of MORs with G proteins, it increased the serine phosphorylation of these receptors. Moreover, activated Ca2+/calmodulin-dependent protein kinase II (CaMKII) accumulated in the MOR environment and phosphorylated PhLPl was seen to co-precipitate with these opioid receptors. Opioid-induced phosphorylation of PhLPl was impaired by inhibiting the activity of CaMKII and, in these circumstances, the association of PhLPl with Gbetagamma dimers and 14-3-3 proteins was diminished. Furthermore, these events were coupled with the recovery of G protein regulation by the MORs, while there was a decrease in serine phosphorylation of these receptors and morphine antinociceptive tolerance diminished. It seems that CaMKII phosphorylation of PhLPl stabilizes the PhLPl.Gbetagamma complex by promoting its binding to 14-3-3 proteins. When this complex fails to bind to 14-3-3 proteins, the association of PhLPl with Gbetagamma is probably disrupted by GalphaGDP subunits and the MORs recover control on G proteins.

RGS14 prevents morphine from internalizing Mu-opioid receptors in periaqueductal gray neurons

Opioid agonists display different capacities to stimulate mu-opioid receptor (MOR) endocytosis, which is related to their ability to provoke the phosphorylation of specific cytosolic residues in the MORs. Generally, opioids that efficiently promote MOR endocytosis and recycling produce little tolerance, as is the case for [D-Ala(2), N-MePhe(4),Gly-ol(5)] encephalin (DAMGO). However, morphine produces rapid and profound antinociceptive desensitization in the adult mouse brain associated with little MOR internalization. The regulator of G-protein signaling, the RGS14 protein, associates with MORs in periaqueductal gray matter (PAG) neurons, and when RGS14 is silenced morphine increased the serine 375 phosphorylation in the C terminus of the MOR, a GRK substrate. Subsequently, these receptors were internalized and recycled back to the membrane where they accumulated on cessation of antinociception. These mice now exhibited a resensitized response to morphine and little tolerance developed. Thus, in morphine-activated MORs the RGS14 prevents GRKs from phosphorylating those residues required for beta-arresting-mediated endocytosis. Moreover morphine but not DAMGO triggered a process involving calcium/calmodulin-dependent kinase II (CaMKII) in naïve mice, which contributes to MOR desensitization in the plasma membrane. In RGS14 knockdown mice morphine failed to activate this kinase. It therefore appears that phosphorylation and internalization of MORs disrupts the CaMKII-mediated negative regulation of these opioid receptors.

CB1 RECEPTOR ACTIVATION IN THE BASOLATERAL AMYGDALA PRODUCES ANTINOCICEPTION IN ANIMAL MODELS OF ACUTE AND TONIC NOCICEPTION

1. Recent studies have suggested that the basolateral nucleus of the amygdala (BLA) participates in the processing of pain information, especially noxious somatic information. Cannabinoid receptors or CB1 mRNA are expressed more in the BLA than in other nuclei of the amygdala. Thus, the aim of the present study was to examine whether CB1 receptors in the BLA may be involved in modulating acute and/or tonic nociceptive processing. 2. Adult rats were exposed to intra-BLA microinjection of the cannabinoid receptor agonist (R)-(+)-[2,3-dihydro-5-methyl-3-(4-morpholinylmethyl) pyrrolo [1,2,3,-de]-1,4-benzoxazin-6-yl]-1-naphthalenylmethanone mesylate [WIN 55,212-2 (1, 2.5, 5 or 10 microg/side)] and subjected to the tail flick and formalin tests. 3. The rats demonstrated a dose-dependent increase in latency to withdraw from a thermal noxious stimulus in the tail flick test and a decrease in formalin-induced pain behaviours. The antinociceptive effects of the CB1 receptor agonist WIN 55,212-2 (10 microg/side) in both tests were attenuated in the presence of the selective CB1 receptor antagonist, N-(piperidin-1-yl)-5-(4-iodophenyl)-1-(2,4-dichlorophenyl)-4-methyl-1H-pyrazole-3- carboxamide (AM251; 0.55 ng/side). Administration of the CB1 receptor antagonist AM251 (0.55, 5.5, or 55.5 ng/side) alone did not alter the nociceptive thresholds in either test. Bilateral microinjection of the selective CB2 receptor antagonist N-[(1S)-endo-1,3,3-trimethyl bicyclo [2.2.1] heptan-2-yl]-5-(4-chloro-3-methylphenyl)-1-(4-methylbenzyl)-pyrazole-3-carboxamide (SR144528; 1 microg/side) had no effect on the antinociception produced by WIN 55,212-2, suggesting that the antinociceptive actions of WIN 55,212-2 are mediated by CB1 receptors. 4. The findings suggest the existence of a CB1-mediated inhibitory system in the BLA that, when activated, can diminish responsivity to acute and tonic noxious stimuli, but that normally has no tonic effect on the response threshold of these stimuli.

Sumoylated RGS-Rz proteins act as scaffolds for Mu-opioid receptors and G-protein complexes in mouse brain

The RGSZ1 and RGSZ2 proteins, members of the RGS-Rz subfamily of GTPase-activating proteins (GAP), are involved in Mu-opioid receptor desensitization. The expression of these proteins, as well as of their main target the Gz protein, is virtually restricted to the nervous tissue. In synaptosomal membranes, these Rz proteins undergo post-translational modifications such as glycosylation and phosphorylation, and they may covalently attach to small ubiquitin-like modifier (SUMO) proteins. While RGSZ1 exists in conjugated and non-conjugated forms, RGSZ2 is mostly conjugated to SUMO-1, SUMO-2 and SUMO-3 proteins. These sumoylated forms of the GAPs readily associated with Mu-opioid receptors but they associated only poorly with Delta receptors. Furthermore, G alpha i2 and G alpha z subunits co-precipitated with the sumoylated forms of RGSZ1/Z2 proteins, but to a lesser extent with the Ser phosphorylated SUMO-free form of RGSZ1. Upon Mu-opioid receptor activation, there is a strong increase in the association of G alpha proteins with RGSZ2 proteins that persists for intervals longer than 24 h. This effect probably accounts for their role in Mu-opioid receptor desensitization. Only a moderate increase was observed with RGSZ1, the non-sumoylated form of which probably acts as an efficient GAP for these G alpha subunits. Therefore, sumoylation regulates the biological activity of RGS-Rz proteins and it is likely that it serves to switch their behavior, from that of a GAP for activated G alpha subunits to that of a scaffold protein for specific signaling proteins.

Behavioral and electrophysiological evidence for opioid tolerance in adolescent rats

Morphine and other opiates are successful treatments for pain, but their usefulness is limited by the development of tolerance. Given that recent studies have observed differential sensitivity to drugs of abuse in adolescents, the aim of this study was to assess antinociceptive tolerance to morphine in adolescent rats using both behavioral and cellular measures. Early (28-35 days postnatal) and late (50-59 days) adolescent and adult (73-75 days) male rats were injected with morphine (5 mg/kg, s.c.) or saline twice a day for two consecutive days. On Day 3, tolerance to morphine was evident in morphine-pretreated rats when tested on the hot plate test. Although baseline latencies for the early compared to late adolescent rats were faster, the magnitude of the shift in ED(50) for morphine was similar for the two adolescent groups. However, the shift in ED(50) tended to be greater in adolescent compared to adult rats. Subsequent to behavioral testing, whole cell patch-clamp recordings were made from ventrolateral PAG neurons. The opioid agonist, met-enkephalin (ME), activated similar outward currents in PAG neurons of early and late adolescent rats. However, reversal potentials of ME-induced currents were shifted to more hyperpolarized potentials in cells from morphine-pretreated rats. In addition, ME induced larger currents in morphine-pretreated rats with faster hot plate latencies compared to the mean (more tolerant) than in rats with slower latencies. These results indicate that repeated intermittent administration of morphine produces tolerance in adolescent rats that is associated with novel changes in opioid-sensitive ventrolateral PAG neurons.

PAG mu opioid receptor activation underlies sex differences in morphine antinociception

Given the findings that (1) systemic opioid antinociception varies by estrous stage in females and (2) the magnitude of sex differences in opioid antinociception is negatively correlated with opioid agonist efficacy, we hypothesized that sex differences in the function of the descending pain modulatory system are likely influenced by estrous stage in females and by the number of available opioid receptors therein. The present study tested these hypotheses by (1) comparing antinociception produced by morphine microinjection to the ventral periaqueductal gray (vPAG) in females at different stages of the estrous cycle and (2) examining systemic morphine antinociception in males versus females under conditions of reduced vPAG mu opioid receptor availability. When estrous stage of females was not controlled for (Experiment 1), there was no significant sex difference in tail withdrawal antinociception following morphine microinjection (0.3-10microg), although morphine was more potent in males than females in producing immobility. Experiment 2 showed that intra-vPAG morphine produced less antinociception and immobility in estrus than in diestrus females; that is, only estrus females' response to morphine was lower than that of males. Experiment 3 showed that microinjection of the irreversible mu opioid antagonist beta-funaltrexamine (beta-FNA) into the vPAG shifted the systemic morphine dose-effect curve farther to the right in females than in males. That is, a reduction in available vPAG mu opioid receptors had a greater impact on opioid antinociception in females than in males, suggesting that females have fewer vPAG mu opioid receptors than males. Overall, these data suggest that ovarian hormones and PAG mu opioid receptor density contribute to sex differences in antinociception produced by morphine.

Projections from the mesopontine tegmental anesthesia area to regions involved in pain modulation

Pentobarbital microinjected into a restricted locus in the upper brainstem induces a general anesthesia-like state characterized by atonia, loss of consciousness, and pain suppression as assessed by loss of nocifensive response to noxious stimuli. This locus is the mesopontine tegmental anesthesia area (MPTA). Although anesthetic agents directly influence spinal cord nociceptive processing, antinociception during intracerebral microinjection indicates that they can also act supraspinally. Using neuroanatomical tracing methods we show that the MPTA has multiple descending projections to brainstem and spinal areas associated with pain modulation. Most prominent is a massive projection to the rostromedial medulla, a nodal region for descending pain modulation. Together with the periaqueductal gray (PAG), the MPTA is the major mesopontine input to this region. Less dense projections target the PAG, the locus coeruleus and pericoerulear areas, and dorsal and ventral reticular nuclei of the caudal medulla. The MPTA also has modest direct projections to the trigeminal nuclear complex and to superficial layers of the dorsal horn. Double anterograde and retrograde labeling at the light and electron microscopic levels shows that MPTA neurons with descending projections synapse directly on spinally projecting cells of rostromedial medulla. The prominence of the MPTA's projection to the rostromedial medulla suggests that, like the PAG, it may exert antinociceptive actions via this bulbospinal relay.

Neurotensin and pain modulation

Neurotensin (NT) can produce a profound analgesia or enhance pain responses, depending on the circumstances. Recent evidence suggests that this may be due to a dose-dependent recruitment of distinct populations of pain modulatory neurons. NT knockout mice display defects in both basal nociceptive responses and stress-induced analgesia. Stress-induced antinociception is absent in these mice and instead stress induces a hyperalgesic response, suggesting that NT plays a key role in the stress-induced suppression of pain. Cold water swim stress results in increased NT mRNA expression in hypothalamic regions known to project to periaqueductal gray, a key region involved in pain modulation. Thus, stress-induced increases in NT signaling in pain modulatory regions may be responsible for the transition from pain facilitation to analgesia. This review focuses on recent advances that have provided insights into the role of NT in pain modulation.

The cannabinoid receptor agonist, WIN 55,212-2, inhibits cool-specific lamina I medullary dorsal horn neurons

Cannabinoid receptor agonists have been demonstrated to inhibit medullary and spinal cord dorsal horn nociceptive neurons. The effect of cannabinoids on thermoreceptive specific neurons in the spinal or medullary dorsal horn remains unknown. In the present study, single-unit recordings from the rat medullary dorsal horn were performed to examine the effect of a cannabinoid receptor agonists on cold-specific lamina I spinothalamic tract neurons. The cannabinoid CB1/CB2 receptor agonist, WIN 55,212-2 (WIN-2), was locally applied to the medullary dorsal horn and the neuronal activity evoked by cooling the receptive field was recorded. WIN-2 (1 microg/microl and 2 microg/microl) significantly attenuated cold-evoked activity. Co-administration of the CB1 receptor antagonist SR 141716 with WIN-2 did not affect cold-evoked activity. These results demonstrate a potential mechanism by which cannabinoids produce hypothermia, and also suggest that cannabinoids may affect non-noxious thermal discrimination.

Opiate and acetylcholine-independent analgesic actions of crotoxin isolated from crotalus durissus terrificus venom

The venom of Crotalus durissus terrificus is reported to have analgesic activity and the administration of Crotoxin (Cro) to cancer patients is reported to reduce the consumption of analgesics. This study investigated the analgesia induced by Cro and the effects of atropine and naloxone on the antinociceptive activity of Cro in mice and rats. The results showed that Cro at 66.5, 44.3 and 29.5microg/kg (ip) exhibited a dose-dependent analgesic action in mice using the hotplate and acetic acid writhing tests. Cro at 44.3microg/kg (ip) had significant analgesic action in the rat tail-flick test. In the mouse acetic acid-writhing test, intracerebral ventricular administration of Cro 0.3microg/kg produced marked analgesic effects. Microinjection of Cro (0.15microg/kg) into the periaqueductal gray area also elicited a robust analgesic action in rat hotplate test. Atropine at 0.5mg/kg (im) or 10mg/kg (ip) or naloxone at 3mg/kg (ip) failed to block the analgesic effects of Cro. These results suggest that Cro has analgesic effects mediated by an action on the central nervous system. The muscarinic and opioid receptors are not involved in the antinociceptive effects of Cro.

Role of ventral tegmental area, periaqueductal gray and parabrachial nucleus in the discriminative stimulus effects of morphine in the rat

Previous studies have produced mixed results about the role of the ventral tegmental area, periaqueductal gray and parabrachial nucleus in morphine discriminations, perhaps owing to the considerably different methodologies used. The purpose of the present study was to compare the roles of these three brain areas using the same food-reinforced discrimination protocol, to determine whether the schedule of reinforcement influenced maximal substitution produced by site-specific morphine administration and to determine whether the time course of substitution differed by site of morphine administration. Rats were trained to discriminate 3.0 mg/kg subcutaneous morphine from saline under variable interval 15-s or fixed ratio 10 schedules of food reinforcement. Rats were then implanted with one cannula aimed at the lateral ventricle (intracerebroventricular) and one aimed at the ventral tegmental area, periaqueductal gray or parabrachial nucleus. Morphine discrimination curves were obtained by subcutaneous, intracerebroventricular and intrasite routes. When administered subcutaneously, morphine was equipotent in variable interval-trained and fixed ratio-trained rats, although it was more potent in fixed ratio-trained females than fixed ratio-trained males. When administered intracerebroventricularly, morphine (0.3-10 microg) engendered a maximum average of 63% drug-appropriate responding in both variable interval-trained and fixed ratio-trained rats; females showed significantly greater drug-appropriate responding than males, again under the fixed ratio but not under the variable interval schedule. In variable interval-trained rats, intrasite infusions of morphine (0.3-10 microg) produced maximal drug-appropriate responding of approximately 57% (ventral tegmental area), 56% (periaqueductal gray) and 41% (parabrachial nucleus); mean maximal substitution was slightly (< or = 17%) greater in fixed ratio-trained rats. When injected into the ventral tegmental area or periaqueductal gray, but not the parabrachial nucleus, naloxone methiodide (2 microg) significantly decreased drug-appropriate responding following 3.0 mg/kg subcutaneous morphine, in both variable interval-trained and fixed ratio-trained rats. The time course of the discriminative stimulus effects of morphine differed among the three brain sites: intraventral tegmental area morphine produced peak drug-appropriate responding by 15 min after injection, whereas the discriminative stimulus effects of intraperiaqueductal gray and intraparabrachial nucleus morphine peaked at approximately 60 min after injection. Taken together, these results indicate that ventral tegmental area, periaqueductal gray and parabrachial nucleus each play a role in the ability of morphine to function as a discriminative stimulus, regardless of the sex of the subject or the schedule under which the subjects are responding. Ventral tegmental area and periaqueductal gray, however, appear to be more critical than parabrachial nucleus in mediating the discriminative effects of systemic morphine in rats responding under a food reinforcement procedure. The pretreatment time and, to a lesser extent, the schedule of reinforcement are additional variables that should be considered when comparing the relative roles of different brain areas in drug discrimination.