Tolerance to non-opioid analgesics in PAG involves unresponsiveness of medullary pain-modulating neurons in male rats

Opiate analgesia can be hampered by a reduction in pharmacological effectiveness (tolerance), and this crucially depends on the periaqueductal gray matter (PAG). Non-opioids like metamizol (dipyrone) or aspirin also induce PAG-dependent analgesia and tolerance, but the neuronal bases of this tolerance are unknown. Metamizol is a pyrazolon derivative and cyclooxygenase inhibitor with widespread use as an analgesic in Europe and Latin America. Metamizol was microinjected into the PAG of awake male rats, and antinociception was assessed by the tail flick (TF) and hot plate (HP) tests. Microinjection twice daily for 2.5 days caused tolerance to metamizol. The rats were then anesthetized and recordings from pain-facilitating on-cells and pain-inhibiting off-cells of the rostral ventromedial medulla (RVM) were performed. PAG microinjection of morphine or metamizol depresses on-cells, activates off-cells and thus inhibits nociception, including TF and HP. In metamizol-tolerant rats, however, PAG microinjection of metamizol failed to affect on- or off-cells, and this is interpreted as the reason for tolerance. In metamizol-tolerant rats morphine microinjection into PAG also failed to affect RVM neurons or nociception (cross-tolerance). In naïve, non-tolerant rats the antinociceptive effect of PAG-microinjected metamizol or morphine was blocked when CTOP, a μ-opioid antagonist, was previously microinjected into the same PAG site. These results emphasize a close relationship between opioid and non-opioid analgesic mechanisms in the PAG and show that, like morphine, tolerance to metamizol involves a failure of on- and off-cells to, respectively, disfacilitate and inhibit nociception. Cross-tolerance between non-opioid and opioid analgesics should be important in the clinical setting.     

A nonopioid analgesic acts upon the PAG-RVM axis to reverse inflammatory hyperalgesia

Metamizol (dipyrone) and other nonsteroidal anti-inflammatory drugs (NSAIDs) induce antinociception by acting upon peripheral tissues and upon central nervous system structures, notably the periaqueductal grey matter (PAG) and the spinal cord. Inflammation-induced hyperalgesia is prevented by spinal application of NSAIDs before the inflammation, but once central sensitization is established the spinal effect of NSAIDs is uncertain. The present study examines whether the action upon the PAG contributes to the attenuation of inflammation-induced spinal hyperalgesia by NSAIDs. In deeply anaesthetized rats, responses of spinal multireceptive neurons to mechanical stimulation of the ipsilateral paw and leg were recorded. An inflammation in the paw was induced with carrageenan. Fifty minutes later, neuronal responses to innocuous and noxious stimulation had, respectively, increased to 206 and 304% for paw, and 160 and 190% for leg. When metamizol (150 microg in 0.5 microL) was microinjected into PAG before the inflammation, neuronal hyperexcitability was delayed for approximately 60 min and was much reduced by 215 min. More interestingly, microinjection of metamizol into PAG when hyperexcitability was fully developed depressed neuronal responses down to baseline for approximately 1 h. The effect of PAG metamizol was reversed by microinjection of a GABA(A) agonist into the rostral ventromedial medulla (RVM), which indicates that RVM relays the metamizol effect from PAG onto the spinal cord. These results suggest that, upon clinical administration of NSAIDs, a joint action upon PAG and spinal cord contributes to preventing the development of hyperalgesia but it is mainly the action upon PAG which contributes to reducing fully established hyperalgesia.     

Antinociception produced by mu opioid receptor activation in the amygdala is partly dependent on activation of mu opioid and neurotensin receptors in the ventral periaqueductal gray

Exposure to stressful or fear-inducing environmental stimuli activates descending antinociceptive systems resulting in a decreased pain response to peripheral noxious stimuli. Stimulating mu opioid receptors in the basolateral nucleus of the amygdala (BLA) in anesthetized rats produces antinociception that is similar to environmentally induced antinociception in awake rats. Recent evidence suggests that both forms of antinociception are mediated via projections from the amygdala to the ventral periaqueductal gray (PAG). In the present study, we examined the types of neurochemicals released in the ventral PAG that may be important in the expression of antinociception produced by amygdala stimulation in anesthetized rats. Microinjection of a mu opioid receptor agonist into the BLA resulted in a time dependent increase in tail flick latency that was attenuated by preadministration of a mu opioid receptor or a neurotensin receptor antagonist into the ventral PAG. Microinjection of a delta(2) opioid receptor antagonist or an NMDA receptor antagonist into the ventral PAG was ineffective. These findings suggest that amygdala stimulation produces antinociception that is mediated in part by opioid and neurotensin release within the ventral PAG.     

Substance P microinjected into the periaqueductal gray matter induces antinociception and is released following morphine administration

The aims of the present study were to investigate, in rats, the behavioral effects of substance P (SP) microinjected into the ventrolateral periaqueductal gray (PAG) and the effects of the neurokinin 1 (NK-1) receptor antagonist [d-Arg1, d-Trp7, 9, Leu11]-substance P (Spantide). The effect of morphine administration on the release of SP in the ventrolateral PAG was also investigated using microdialysis in awake rats. SP microinjected into the ventrolateral part of the PAG induced significant increases in the hindpaw withdrawal latencies (HWLs) to thermal and mechanical stimulation as an antinociceptive response. The NK-1 receptor antagonist blocked these effects but exhibited no antinociceptive effect alone. Subcutaneous administration of morphine increased basal SP-like immunoreactivity (SP-LI) release in the microdialysate obtained from the ventrolateral PAG of freely moving rats. Our results demonstrate that SP injected into the ventrolateral PAG induces an antinociceptive effect via activation of NK-1 receptors. Morphine administered systemically induces the release of SP in the ventrolateral PAG. We suggest that an increased release of SP in the PAG may contribute to opioid antinociception.     

Antinociception induced by intravenous dipyrone (metamizol) upon dorsal horn neurons: involvement of endogenous opioids at the periaqueductal gray matter, the nucleus raphe magnus, and the spinal cord in rats

Microinjection of dipyrone (metamizol) into the periaqueductal gray matter (PAG) in rats causes antinociception. This is mediated by endogenous opioidergic circuits located in the PAG itself, in the nucleus raphe magnus and adjacent structures, and in the spinal cord. The clinical relevance of these findings, however, is unclear. Therefore, in the present study, dipyrone was administered intravenously, and the involvement of endogenous opioidergic circuits in the so-induced antinociception was investigated. In rats, responses of dorsal spinal wide-dynamic range neurons to mechanical noxious stimulation of a hindpaw were strongly inhibited by intravenous dipyrone (200 mg/kg). This effect was abolished by microinjection of naloxone (0.5 microg/0.5 microl) into the ventrolateral and lateral PAG or into the nucleus raphe magnus or by direct application of naloxone (50 microg/50 microl) onto the spinal cord surface above the recorded neuron. These results show that dipyrone, a non-opioid analgesic with widespread use in Europe and Latin America, when administered in a clinically relevant fashion causes antinociception by activating endogenous opioidergic circuits along the descending pain control system.     

Defensive behaviors evoked from the ventrolateral periaqueductal gray of the rat: comparison of opioid and GABA disinhibition

Microinjection of morphine into the ventrolateral periaqueductal gray (PAG) disinhibits output neurons resulting in immobility and antinociception. Disinhibition also can be produced by microinjection of the GABA antagonist bicuculline. If morphine and bicuculline disinhibit the same class of neurons, then the behavioral effects evoked should be the same. Microinjection of morphine (5 microg/0.4 microl) into the ventrolateral PAG produced antinociception in 46 of 85 rats (54%). Subsets of rats with and without morphine antinociception were subsequently injected with bicuculline (2.5, 5, 10, and 25 ng/0.4 microl) into the same PAG site. Microinjection of bicuculline produced an increase in hot plate latency that was independent of the effect of the prior morphine microinjection. Bicuculline administration also produced an increase in locomotor activity in most rats, not immobility as with morphine microinjections. These differences between morphine and bicuculline microinjections indicate three things: (a) disinhibition of PAG neurons whether by morphine or bicuculline is an effective means of producing antinociception; (b) the circuitry underlying the behavioral effects of morphine and bicuculline differ; (c) the ventrolateral PAG appears capable of supporting a range of defensive behaviors from immobility to flight.     

Opioid peptides (DAGO-enkephalin, dynorphin A(1-13), BAM 22P) microinjected into the rat brainstem: comparison of their antinociceptive effect and their effect on neuronal firing in the rostral ventromedial medulla

The highly mu-selective agonist Tyr-D-Ala-Gly-MePhe-Gly-ol-enkephalin (DAGO) produces potent, dose-dependent naloxone-reversible antinociception when microinjected into the ventrolateral periaqueductal gray (PAG) (ED50 = 0.72 nmol) or rostral ventromedial medulla (RVM) (ED50 = 0.05 nmol) as measured on the rat tail flick (TF) assay. In single-unit recording experiments, DAGO microinjected into the PAG also affected On- and Off-Cell firing in the RVM in the same way as previously demonstrated by our group for morphine. PAG-microinjected DAGO inhibits spontaneous and noxious-evoked On-Cell firing (attenuating the characteristic On-Cell burst) (n = 19), and excites spontaneous Off-Cell firing, preventing the characteristic Off-Cell pause (n = 12) at doses which suppress the TF. These results support a major role for the mu receptor in PAG and RVM mechanisms of opiate antinociception. In our experiments using BAM22P, an endogenous weakly mu-selective opioid peptide, we could not demonstrate a dose-dependent antinociceptive effect, whether the peptide was microinjected supraspinally into the PAG (n = 9) or RVM (n = 11), or intrathecally at the lumbar cord (n = 4). In two animals, a naloxone-reversible antinociceptive effect was observed following the microinjection of 10 nmol BAM 22P into the RVM; however, no effect was seen in 3 animals microinjected with 20 nmol. Dyn A(1-13), a putative endogenous ligand for the kappa receptor, had no antinociceptive effect when microinjected into the ventrolateral PAG, and no effect on the firing (spontaneous or noxious-evoked) of RVM On (n = 3)- or Off (n = 2)-Cells.     

A short-chain α-neurotoxin from Naja naja atra produces potent cholinergic-dependent analgesia

Objective To investigate the analgesia induced by cobrotoxin （CT） from venom of Naja naja atra, and the effects of atropine and naloxone on the antinociceptive activity of CT in rodent pain models. Methods CT was administered intraperitoneally （33.3, 50, 75 μg/kg）, intra-cerebral venticularly （2.4 μg/kg） or microinjected into periaqueductal gray （PAG, 1.2 μg/kg）. The antinociceptive action was tested using the hot-plate test and the acetic acid writhing test in mice and rats. The involvement of cholinergic system and the opioid system in CT-induced analgesia was examined by pretreatment of animals with atropine （0.5 mg/kg, im or 10 mg/kg, ip） or naloxone （3 mg/kg, ip）. The effect of CT on motor activity was tested using the Animex test. Results CT （33.3, 50 and 75 μg/kg, ip） exhibited a dosedependent analgesic action in mice as determined with hot-plate test and acetic acid writhing test. In the mouse acetic acid writhing test, the intra-cerebral ventricle administration of CT 2.4 μg/kg （1/23th of a systemic dose） produced marked analgesic effects. Microinjection of CT 1.2 μg/kg （1/46th of systemic dose） into the PAG also elicited a robust analgesic action in the hot-plate test in rats. Atropine at 0.5 mg/kg （ira） or naloxone at 3 mg/kg （ip） failed to block the analgesic effects of CT, but atropine at 10 mg/kg （ip） did antagonize the analgesia mediated by CT in the mouse acetic acid writhing test. At the highest effective dose of antinociception （75 μg/kg）, CT did not change the spontaneous mobility of mice. Conclusion These results suggest that CT from Naja naja atra venom has analgesic effects. Central nervous system may be involved in CT＇ analgesic effects and the PAG may be the primary central site where CT exerts its effects. The central cholinergic system but not opioid system appears to be involved in the antinociceptive action of CT.     

Highly delta selective antagonists in the RVM attenuate the antinociceptive effect of PAG DAMGO

The present study tested the hypothesis that endogenous opioid peptides acting at the delta-opioid receptor (DOR) in the rostral ventromedial medulla (RVM) contribute to the antinociception elicited by the mu-opioid receptor (MOR) agonist DAMGO in the midbrain periaqueductal gray (PAG). Following microinjection of DAMGO into the PAG, either the highly selective DOR antagonist TIPP[psi] or the DOR2 antagonist naltriben (NTB) was microinjected into the RVM. Both TIPP[psi] (1.0 microg) and NTB (5.0 ng) significantly attenuated the analgesic effect of PAG DAMGO but had no effect when given before PAG saline. These results confirm and extend previous studies suggesting that PAG mu-opioids activate a descending system with a DOR mediated endogenous opioid link in the RVM.     

An analysis of the 'tolerance' which develops to analgetic electrical stimulation of the midbrain periaqueductal grey in freely moving rats

Electrical stimulation of the ventral midbrain periaqueductal grey (PAG) elicits an opioidergic antinociception against noxious heat and pressure in freely moving rats. Recurrent stimulation was associated with a gradual decline and eventual loss of this stimulation-produced antinociception (SPA). This could be reinstated by an increase in current intensity and this reinstatement was preventable by naloxone. The current intensity--antinociception (dose--response) curve was shifted to the right in recurrently stimulated rats and parallel to that in naive animals. The loss of SPA upon repetitive simulation did not represent a conditioning phenomenon. Thus, tolerant rats exposed to all cues which accompanied stimulation revealed no (compensatory) hyperalgesic response--but rather a slight antinociception. Further, SPA recovered spontaneously in tolerant rats. Moreover, 'extinction' by repeated exposure to all cues accompanying stimulation did not restore or accelerate the recovery of SPA in tolerant animals. Tolerant rats showed no depletion in midbrain PAG or other CNS or hypophyseal pools of beta-endorphin, Met-enkephalin or dynorphin indicating that a depletion of endogenous opioid peptides does not underlie the tolerance which develops to stimulation. In fact recurrently stimulated rats did not show any of the pronounced effects upon CNS pools of opioid peptides which are seen with long-term stress. Moreover, repetitively stimulated rats revealed no indications of stress as judged by a diversity of stress-sensitive parameters; basal nociceptive threshold, core temperature, ingestive behaviour, body weight, adrenal weight and hypophyseal secretion of beta-endorphin and prolactin. The data offer two major conclusions. Firstly, the gradual loss of analgesia upon recurrent stimulation of the midbrain PAG does not reflect a generalized debilitation or stress and neither a conditioning phenomenon nor a depletion of pools of endogenous opioid peptides. Rather it closely corresponds to the pharmacological definition of tolerance and may reflect a process occurring at the level of the opioid receptor and coupled processes. This finding explains the cross-tolerance which we observe recurrently stimulated rats to display to morphine. Secondly, this SPA is not a form of stress-induced analgesia and rats undergoing recurrent stimulation reveal no indications of stress as judged by biochemical, physiological and behavioural parameters.     

Comparison of the antinociceptive action of mu and delta opioid receptor ligands in the periaqueductal gray matter, medial and paramedial ventral medulla in the rat as studied by the microinjection technique

In rats stereotaxically implanted with microinjection cannula in either the periaqueductal gray matter (PAG) or the medial/paramedial medullary reticular formation (MRF), microinjection of morphine, sufentanil, D-Ala2-D-Leu5-enkephalin (DADL) or D-Ser2-Thr6-leucine enkephalin (DSTLE) produced dose-dependent elevations in the response latency on tail-flick and hot plate tests. These effects were reversed by naloxone administered by microinjection into the same intracerebral site. Both mu (morphine and sufentanil) and delta (DADL and DSTLE) opioid receptor ligands produced a maximal elevation in the supraspinally mediated hot plate response when administered into either the PAG or the MRF. Similarly, mu and delta receptor ligands produced maximum elevations in the spinally mediated tail-flick response when microinjected into the PAG. In contrast, delta, but not mu, receptor agonists produced a total blockade of the tail-flick response following administration into the MRF. Microinjection of mu (morphine) or delta (DADL) agonists into the PAG or the MRF also resulted in a naloxone-reversible inhibition of the visceral chemical evoked writhing response. These observations suggest that mu and delta opioid receptor linked systems within the MRF but not the PAG produce their antinociceptive effects by discriminable mechanisms with a differential action on spinopetal vs supraspinal modulatory systems.     

Chronic sucrose intake augments antinociception induced by injections of mu but not kappa opioid receptor agonists into the periaqueductal gray matter in male and female rats

Chronic intake of a palatable sucrose solution enhances the antinociceptive potency of systemically administered mu, and kappa opioid receptor agonists. To investigate whether the effects of sucrose on the actions of opioid drugs are mediated within the central nervous system (CNS), antinociception was examined following the administration of mu and kappa opioid receptor agonists into the periaqueductal gray area (PAG). Male and female Long–Evans rats consumed either water and ground chow, or water, chow and a 32% (w/v) sucrose solution. After adaptation to the dietary conditions, a guide cannula was stereotaxically implanted into the PAG. Injections of the mu agonist, morphine (0.0, 2.5, 5.0, 10.0 and 20.0 μg), into the PAG led to dose-related increases in antinociceptive responses on a tail flick test in both male and female rats. Rats which had consumed sucrose displayed significantly greater levels of antinociception than rats not given the sugar. Antinociceptive responses to morphine did not differ as a function of sex. Injections of the kappa agonist, spiradoline (0, 100, 300, 600 μg), into the PAG increased tail flick latencies in male and female rats. However, antinociceptive responses did not vary as a function of diet in rats injected with spiradoline. In both diet conditions, spiradoline led to greater levels of antinociception in female rats than in male rats. These results support the hypothesis that intake of palatable foods and fluids act within the CNS to moderate the behavioral actions of opioid drugs.     

III. Comparison of the antinociceptive action of Mu and delta opioid receptor ligands in the periaqueductal gray matter, medial and paramedial ventral medulla in the rat as studied by the microinjection technique

In rats stereotaxically implanted with microinjection cannula in either the periaqueductal gray matter (PAG) or the medial/paramedial medullary reticular formation (MRF), microinjection of morphine, sufentanil,d-Ala²-d-Leu⁵-enkephalin (DADL) ord-Ser²- Thr⁶-leucine enkephalin (DSTLE) produced dose-dependent elevations in the response latency on tail-flick and hot plate tests. These effects were reversed by naloxone administered by microinjection into the same intracerebral site. Both mu (morphine and sufentanil) and delta (DADL and DSTLE) opioid receptor ligands produced a maximal elevation in the supraspinally mediated hot plate response when administered into either the PAG or the MRF. Similarly, mu and delta receptor ligands produced maximum elevations in the spinally mediated tail-flick response when microinjected into the PAG. In contrast, delta, but not mu, receptor agonists produced a total blockade of the tail-flick response following administraion into the MRF. Microinjection of mu (morphine) or delta (DADL) agonists into the PAG or the MRF also resulted in a naloxone-reversible inhibition of the visceral chemical evoked writhing response. These observations suggest that mu and delta opioid receptor linked systems within the MRF but not the PAG produce their antinociceptive effects by discriminable mechanisms with a differential action on spinopetal vs supraspinal modulatory systems.     

Activation of brain stem nuclei by improgan, a non-opioid analgesic

Improgan is a compound developed from histamine antagonists which shows the pre-clinical profile of a highly effective, non-opioid analgesic when administered into the rodent CNS. Pharmacological studies suggest that improgan activates descending pain-relieving circuits, but the brain and spinal sites of action of this drug have not been previously studied. Presently, the effects of intracerebral and intrathecal microinjections of improgan were evaluated on thermal nociceptive responses in rats. Improgan produced large, dose- and time-related reductions in nociceptive responses following administration into the ventrolateral periaqueductal gray (PAG), the dorsal PAG, and the rostral ventromedial medulla (RVM). The drug had no measurable effects after injections into the caudate nucleus, basolateral amygdala, hippocampus, ventromedial hypothalamus, superior colliculi, ventrolateral medulla, or the spinal subarachnoid space. Inactivation of the RVM by muscimol microinjections completely attenuated antincociceptive responses produced by intraventricular improgan. These findings, taken with earlier results, show that, like opioids and cannabinoids, improgan acts in the PAG and RVM to activate descending analgesic systems. Unlike these other analgesics, improgan does not act in the spinal cord or in CNS areas outside of the brain stem.     

Opioid, cholinergic and α-adrenergic influences on the modulation of nociception from the lateral reticular nucleus of the rat

The lateral reticular nucleus (LRN) has been identified as an area in the caudal medulla involved in the centrifugal modulation of spinal nociceptive transmission and withdrawal reflexes. The data presented in this report further support a role for the LRN in the modulation of nociceptive responses. It was confirmed in the present study that focal electrical stimulation in the LRN inhibits the nociceptive tail-flick (TF) reflex at low intensities of stimulation in lightly pentobarbital-anesthetized rats. Aversive effects, however, were typically produced at similar and higher intensities of stimulation in the LRN in the same rats in the awake state. It was also determined that an inhibitory modulation of nociceptive responses organized both spinally and supraspinally could be activated independently by muscarinic cholinergic or opioid mechanisms in the LRN. Microinjection of morphine into the LRN in conscious rats produced an antinociception in both TF and hot plate (HP) tests which could be attenuated significantly by naloxone, but not atropine, previously microinjected into the same site in the LRN. Carbachol microinjected into the LRN also produced an antinociception which was attenuated significantly by atropine but not naloxone previously microinjected into the same site in the LRN. In contrast, the microinjection of clonidine or norepinephrine into the LRN either did not affect or shortened significantly response latencies in the TF and HP tests. These results further establish that the LRN contributes to the modulation of nociception. Opioid and cholinergic influences in the LRN appear to independently activate inhibition of responding to nociceptive stimuli organized either spinally or supraspinally, although descending inhibition was most clearly activated. An action at alpha 2 adrenoceptors in the LRN, conversely, produces an hyperalgesia as reflected by shortened latencies to respond in TF and HP tests.     

PAG-microinjected dipyrone (metamizol) inhibits responses of spinal dorsal horn neurons to natural noxious stimulation in rats

In addition to their well-known peripheral and spinal effects, non-steroidal antiinflammatory drugs (NSAIDs) are believed to diminish nociceptive responses by acting supraspinally and activating descending modulatory systems. We have herein investigated whether this descending action involves a depression of spinal sensory neurons. In rats under barbiturate anesthesia, responses of lumbar wide-dynamic-range neurons to a noxious clamp in their receptive fields were depressed to 46% of baseline value by the microinjection of 100 μg dipyrone (metamizol) into the periaqueductal gray matter (PAG). These results show that PAG application of NSAIDs activates descending systems which depress the excitation of spinal sensory neurons by natural noxious stimuli.     

5-hydroxytryptamine 1A (5-HT1A) but not 5-HT3 receptor is involved in mediating the nucleus submedius 5-HT-evoked antinociception in the rat

Our previous studies have indicated that the thalamic nucleus submedius (Sm) is involved in modulation of nociception as part of an ascending component of an endogenous analgesic system consisting of spinal cord-Sm-ventrolateral orbital cortex (VLO)-periaqueductal gray (PAG)-spinal cord loop. Microinjection of 5-hydroxytryptamine (5-HT) into Sm produces antinociception and this effect is blocked by 5-HT(2) receptor antagonist. The aim of the present study was to examine whether the 5-HT(1) and 5-HT(3) receptors were also involved in the Sm 5-HT-evoked antinociception. Nociception was assessed in lightly anesthetized rats with radiant-heat-evoked tail flick (TF). 5-HT(1A) and 5-HT(3) receptor antagonists were microinjected into the Sm alone or in combination with a microinjection of 5-HT into the same Sm site. 5-HT(1A) receptor antagonist p-MPPI (0.87 nmol) facilitated the TF reflex; a lower dose (0.43 nmol) of p-MPPI significantly attenuated the Sm 5-HT-evoked inhibition of TF reflex. Microinjection of the 5-HT(3) receptor antagonist LY-278,584 (12 nmol) had no effect either on the TF reflex or on the Sm 5-HT-evoked inhibition. These results suggest that 5-HT(1A) receptor but not 5-HT(3) receptor is involved in mediating the 5-HT-evoked antinociception. Possible mechanisms of Sm 5-HT-induced descending antinociception are discussed.     

Spinal cholinergic and monoamine receptors mediate the antinociceptive effect of morphine microinjected in the periaqueductal gray on the rat tail, but not the feet

The antinociceptive effects of morphine (5 μg) microinjected into the ventrolateral periaqueductal gray were determined using both the tail flick and the foot withdrawal responses to noxious radiant heating in lightly anesthetized rats. Intrathecal injection of appropriate antagonists was used to determine whether the antinociceptive effects of morphine were mediated byα₂-noradrenergic, serotonergic, opioid, or cholinergic muscarinic receptors. The increase in the foot withdrawal response latency produced by microinjection of morphine in the ventrolateral periaqueductal gray was reversed by intrathecal injection of the cholinergic muscarinic receptor antagonist atropine, but was not affected by the a2-adrenoceptor antagonist yohimbine, the serotonergic receptor antagonist methysergide, or the opioid receptor antagonist naloxone. In contrast, the increase in the tail flick response latency produced by morphine was reduced by either yohimbine, methysergide or atropine. These results indicate that microinjection of morphine in the ventrolateral periaqueductal gray inhibits nociceptive responses to noxious heating of the tail by activating descending neuronal systems that are different from those that inhibit the nociceptive responses to noxious heating of the feet. More specifically, serotonergic, muscarinic cholinergic andα₂-noradrenergic receptors appear to mediate the antinociception produced by morphine using the tail flick test. In contrast, muscarinic cholinergic, but not monoamine receptors appear to mediate the antinociceptive effects of morphine using the foot withdrawal response.     

Tolerance effects of non-steroidal anti-inflammatory drugs microinjected into central amygdala, periaqueductal grey, and nucleus raphe Possible cellular mechanism

Pain is a sensation related to potential or actual damage in some tissue of the body.The mainstay of medical pain therapy remains drugs that have been around for decades,like non-steroidal anti-inflammatory drugs (NSAIDs),or opiates.However,adverse effects of opiates,particularly tolerance,limit their clinical use.Several lines of investigations have shown that systemic (intraperitoneal) administration of NSAIDs induces antinociception with some effects of tolerance.In this review,we report that repeated microinjection of NSAIDs analgin,clodifen,ketorolac and xefocam into the central nucleus of amygdala,the midbrain periaqueductal grey matter and nucleus raphe magnus in the following 4 days result in progressively less antinociception compared to the saline control testing in the tail-flick reflex and hot plate latency tests.Hence,tolerance develops to these drugs and cross-tolerance to morphine in male rats.These findings strongly support the suggestion of endogenous opioid involvement in NSAIDs antinociception and tolerance in the descending pain-control system.Moreover,the periaqueductal grey-rostral ventro-medial part of medulla circuit should be viewed as a pain-modulation system.These data are important for human medicine.In particular,cross-tolerance between non-opioid and opioid analgesics should be important in the clinical setting.