Comparison of the effects of ventral medullary lesions on systemic and microinjection morphine analgesia

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

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

Regular physical activity prevents development of chronic muscle pain through modulation of supraspinal opioid and serotonergic mechanisms

The current study shows that blockade of opioid receptors systemically in the periaqueductal gray and the rostral ventromedial medulla prevents analgesia by 8 weeks of wheel running in a chronic muscle pain model. We further show increases in serotonin transporter expression and reversal of hyperalgesia with a selective reuptake inhibitor in the rostral ventromedial medulla in the chronic muscle pain model, and exercise normalizes serotonin transporter expression.

Introduction:

It is generally believed that exercise produces its effects by activating central opioid receptors; there are little data that support this claim. The periaqueductal gray (PAG) and rostral ventromedial medulla (RVM) are key nuclei in opioid-induced analgesia, and opioids interact with serotonin to produce analgesia.

Objectives:

The purpose was to examine central inhibitory mechanisms involved in analgesia produced by wheel running.

Methods:

C57/Black6 mice were given access to running wheels in their home cages before induction of chronic muscle hyperalgesia and compared with those without running wheels. Systemic, intra-PAG, and intra-RVM naloxone tested the role of central opioid receptors in the antinociceptive effects of wheel running in animals with muscle insult. Immunohistochemistry for the serotonin transporter (SERT) in the spinal cord and RVM, and pharmacological blockade of SERT, tested whether the serotonin system was modulated by muscle insult and wheel running.

Results:

Wheel running prevented the development of muscle hyperalgesia. Systemic naloxone, intra-PAG naloxone, and intra-RVM naloxone reversed the antinociceptive effect of wheel running in animals that had received muscle insult. Induction of chronic muscle hyperalgesia increased SERT in the RVM, and blockade of SERT reversed the hyperalgesia in sedentary animals. Wheel running reduced SERT expression in animals with muscle insult. The serotonin transporter in the superficial dorsal horn of the spinal cord was unchanged after muscle insult, but increased after wheel running.

Conclusion:

These data support the hypothesis that wheel running produced analgesia through central inhibitory mechanisms involving opioidergic and serotonergic systems.

Entanglement between thermoregulation and nociception in the rat: the case of morphine

In thermoneutral conditions, rats display cyclic variations of the vasomotion of the tail and paws, the most widely used target organs in current acute or chronic animal models of pain. Systemic morphine elicits their vasoconstriction followed by hyperthermia in a naloxone-reversible and dose-dependent fashion. The dose-response curves were steep with ED50 in the 0.5-1 mg/kg range. Given the pivotal functional role of the rostral ventromedial medulla (RVM) in nociception and the rostral medullary raphe (rMR) in thermoregulation, two largely overlapping brain regions, the RVM/rMR was blocked by muscimol: it suppressed the effects of morphine. "On-" and "off-" neurons recorded in the RVM/rMR are activated and inhibited by thermal nociceptive stimuli, respectively. They are also implicated in regulating the cyclic variations of the vasomotion of the tail and paws seen in thermoneutral conditions. Morphine elicited abrupt inhibition and activation of the firing of on- and off-cells recorded in the RVM/rMR. By using a model that takes into account the power of the radiant heat source, initial skin temperature, core body temperature, and peripheral nerve conduction distance, one can argue that the morphine-induced increase of reaction time is mainly related to the morphine-induced vasoconstriction. This statement was confirmed by analyzing in psychophysical terms the tail-flick response to random variations of noxious radiant heat. Although the increase of a reaction time to radiant heat is generally interpreted in terms of analgesia, the present data question the validity of using such an approach to build a pain index.

Pain Modulation and the Transition from Acute to Chronic Pain

There is now increasing evidence that pathological pain states are at least in part driven by changes in the brain itself. Descending modulatory pathways are known to mediate top-down regulation of nociceptive processing, transmitting cortical and limbic influences to the dorsal horn. However, these modulatory pathways are also intimately intertwined with ascending transmission pathways through positive and negative feedback loops. Models of persistent pain that fail to include descending modulatory pathways are thus incomplete. Although teasing out individual links in a recurrent network is never straightforward, it is imperative that understanding of pain modulation be fully integrated into how we think about pain.

A critical period in the supraspinal control of pain: opioid-dependent changes in brainstem rostroventral medulla function in preadolescence

We have previously shown that the balance of electrically evoked descending brainstem control of spinal nociceptive reflexes undergoes a switch from excitation to inhibition in preadolescent rats. Here we show that the same developmental switch occurs when μ-opioid receptor agonists are microinjected into the rostroventral medulla (RVM). Microinjections of the μ-opioid receptor agonist [D-Ala(2), N-MePhe(4), Gly-ol]-enkephalin (DAMGO) into the RVM of lightly anaesthetised adult rats produced a dose-dependent decrease in mechanical nociceptive hindlimb reflex electromyographic activity. However, in preadolescent (postnatal day 21 [P21]) rats, the same doses of DAMGO produced reflex facilitation. RVM microinjection of δ-opioid receptor or GABA(A) receptor agonists, on the other hand, caused reflex depression at both ages. The μ-opioid receptor-mediated descending facilitation is tonically active in naive preadolescent rats, as microinjection of the μ-opioid receptor antagonist D-Phe-Cys-Tyr-D-Trp-Orn-Thr-Pen-Thr-NH(2) (CTOP) into the RVM at this age decreases spinal nociceptive reflexes while having no effect in adults. To test whether tonic opioid central activity is required for the preadolescent switch in RVM descending control, naloxone hydrochloride was delivered continuously from subcutaneous osmotic mini-pumps for 7-day periods, at various postnatal stages. Blockade of tonic opioidergic activity from P21 to P28, but not at earlier or later ages, prevented the normal development of descending RVM inhibitory control of spinal nociceptive reflexes. Enhancing opioidergic activity with chronic morphine over P7 to P14 accelerated this development. These results show that descending facilitation of spinal nociception in young animals is mediated by μ-opioid receptor pathways in the RVM. Furthermore, the developmental transition from RVM descending facilitation to inhibition of pain is determined by activity in central opioid networks at a critical period of periadolescence.

Descending facilitatory pathways from the rostroventromedial medulla mediate naloxone-precipitated withdrawal in morphine-dependent rats

Opioids produce analgesic effects, and extended use can produce physical dependence in both humans and animals. Dependence to opiates can be demonstrated by either termination of drug administration or through precipitation of the withdrawal syndrome by opiate antagonists. Key features of the opiate withdrawal syndrome include hyperalgesia, anxiety, and autonomic signs such as diarrhea. The rostral ventromedial medulla (RVM) plays an important role in the modulation of pain and for this reason, may influence withdrawal-induced hyperalgesia. The mechanisms that drive opiate withdrawal-induced hyperalgesia have not been elucidated. Here, rats made dependent upon morphine received naloxone to precipitate withdrawal. RVM microinjection of lidocaine, kynurenic acid (excitatory amino acid antagonist) or YM022 (CCK2 receptor antagonist) blocked withdrawal-induced hyperalgesia. Additionally, these treatments reduced both somatic and autonomic signs of naloxone-induced withdrawal. Spinal application of ondansetron, a 5HT3 receptor antagonist thought to ultimately be engaged by descending pain facilitatory drive, also blocked hyperalgesia and somatic and autonomic features of the withdrawal syndrome. These results indicate that the RVM plays a critical role in mediating components of opioid withdrawal that may contribute to opioid dependence.

PERSPECTIVE

Manipulations targeting these descending pathways from the RVM may diminish the consequences of prolonged opioid administration-induced dependence and be useful adjunct strategies in reducing the risk of opioid addiction.

Neural basis for improgan antinociception

Improgan, the prototype compound of a novel class of non-opioid analgesic drugs derived from histamine antagonists, attenuates thermal and mechanical nociception in rodents following intracerebroventricular (i.c.v.) administration. Improgan does not bind to known opioid, histamine or cannabinoid receptors, and its molecular target has not been identified. It is known however, that improgan acts directly in the periaqueductal gray and the rostral ventromedial medulla to produce its antinociceptive effects, and that inactivation of the rostral ventromedial medulla prevents the antinociceptive effect of improgan given i.c.v. Here we used in vivo single-cell recording in lightly anesthetized rats to show that improgan engages pain-modulating neurons in the medulla to produce antinociception. Following improgan administration, OFF-cells, which inhibit nociception, became continuously active and no longer paused during noxious stimulation. The increase in OFF-cell firing does not represent a non-specific neuroexcitant effect of this drug, since ON-cell discharge, associated with net nociceptive facilitation, was depressed. NEUTRAL-cell firing was unaffected by improgan. The net response of rostral ventromedial medulla (RVM) neurons to improgan is thus comparable to that evoked by mu-opioids and cannabinoids, well known RVM-active analgesic drugs. This common basis for improgan, opioid, and cannabinoid antinociception in the RVM supports the idea that improgan functions as a specific analgesic agent.

Activity of murine raphe magnus cells predicts tachypnea and on-going nociceptive responsiveness

In rats, opioids produce analgesia in large part by their effects on two cell populations in the medullary raphe magnus (RM). To extend our mechanistic understanding of opioid analgesia to the genetically tractable mouse, we characterized behavioral reactions and RM neural responses to opioid administration. d-Ala(2), N-Me-Phe(4)-Gly(5)ol-enkephalin, a mu-opioid receptor agonist, microinjected into the murine RM produced cardiorespiratory depression and reduced slow wave electroencephalographic activity as well as increased the noxious heat-evoked withdrawal latencies. As in rat, RM cell types that were excited and inhibited by noxious stimuli, termed on and off cells, respectively, were observed in mice. However, in contrast to findings in rat, opioid doses that suppressed withdrawals did not alter the background discharge rate of murine on and off cells, suggesting that the cellular mechanisms by which the murine RM generates opioid analgesia are substantially different from those in rats. Murine on cell discharge did not predict the latency or magnitude of an ensuing withdrawal but did correlate to the magnitude and latency of concurrent withdrawals. Although opioids failed to alter the background discharge of on and off cells, they reduced the responses of RM neurons to noxious stimulation, further evidence that RM modulates on-going withdrawals. In characterizing the role of RM in respiratory modulation, we found that on cells burst and off cells paused during tachypneic events. The effects of opioids in the murine RM on homeostasis and the association of on and off cell discharge with tachypnea corroborate roles for opioid signaling in RM beyond analgesia.

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.

Differential susceptibility of the PAG and RVM to tolerance to the antinociceptive effect of morphine in the rat

The periaqueductal gray (PAG) and rostral ventromedial medulla (RVM) are part of a nociceptive modulatory system. Microinjection of morphine into either structure produces antinociception. Tolerance develops to ventrolateral PAG mediated antinociception with repeated microinjection of morphine. In contrast, there are no published reports of tolerance to morphine administration into the RVM. Three experiments were conducted to determine whether tolerance develops to morphine microinjections into the RVM. Experiment 1 compared tolerance to the antinociceptive effect of microinjecting morphine (5 microg/0.5 microl) into the PAG and RVM following daily injections for four consecutive days. Experiment 2 assessed tolerance to a range of morphine doses (2.5-20 microg) after injecting morphine into the RVM twice a day for two consecutive days. Experiment 3 followed a similar procedure except twice as many RVM injections were made (8 microinjections in 4 days). The degree to which tolerance developed to the antinociceptive effect of morphine was much greater with microinjections into the PAG compared to the RVM. There was a 64% drop in hot plate latency from the first to the fifth injection of morphine into the PAG, but only a 36% drop in latency following RVM microinjections. Reducing the interdose interval to two injections a day or increasing the total number of injections from 4 to 8 did not enhance the development of tolerance to RVM morphine administration. These data demonstrate that opioid-sensitive neurons in the RVM are relatively resistant to the development of tolerance compared to PAG neurons.

Evidence for an intrinsic mechanism of antinociceptive tolerance within the ventrolateral periaqueductal gray of rats

Repeated microinjections of morphine into the ventrolateral periaqueductal gray produce antinociceptive tolerance. This tolerance may be a direct effect of morphine on cells within the ventrolateral periaqueductal gray or may require activation of downstream structures such as the rostral ventromedial medulla or spinal cord. Experiment 1 examined whether tolerance develops when opioid receptors in the ventrolateral periaqueductal gray are blocked prior to repeated systemic morphine administration. Microinjections of naltrexone hydrochloride (1microg/0.4microl) into the ventrolateral periaqueductal gray blocked antinociception and significantly attenuated the development of antinociceptive tolerance produced from systemic morphine administration. Experiment 2 examined whether tolerance develops when the effects of morphine are isolated to the ventrolateral periaqueductal gray. This was accomplished by microinjecting morphine (5microg/0.4microl) into the ventrolateral periaqueductal gray while simultaneously blocking the descending output through the rostral ventromedial medulla. Inhibition of neurons within the rostral ventromedial medulla by microinjecting the GABA(A) agonist muscimol (10ng/0.5microl) blocked the antinociception produced by microinjection of morphine into the ventrolateral periaqueductal gray but did not block the development of tolerance. These data demonstrate that the ventrolateral periaqueductal gray is both necessary and sufficient to produce tolerance to the antinociceptive effect of morphine. The ventrolateral periaqueductal gray is necessary in that tolerance does not develop if opiate action within the ventrolateral periaqueductal gray is blocked (experiment 1). The ventrolateral periaqueductal gray is sufficient in that tolerance occurs even when morphine's effects are restricted to the ventrolateral periaqueductal gray (experiment 2).

Prostaglandin E2 in the midbrain periaqueductal gray produces hyperalgesia and activates pain-modulating circuitry in the rostral ventromedial medulla

Recent years have seen significant advances in our understanding of the peripheral and spinal mechanisms through which prostaglandins contribute to nociceptive sensitization. By contrast, the possibility of a supraspinal contribution of these compounds to facilitated pain states has received relatively little attention. One possible mechanism through which prostaglandins could act supraspinally to facilitate nociception would be by recruitment of descending facilitation from brainstem pain-modulating systems. The rostral ventromedial medulla (RVM) is now known to contribute to enhanced responding in a variety of inflammatory and nerve injury models. Its major supraspinal input, the midbrain periaqueductal gray (PAG), expresses prostanoid receptors and synthetic enzymes. The aim of the present study was to determine whether direct application of prostaglandin E(2) (PGE(2)) within the ventrolateral PAG is sufficient to produce hyperalgesia, and whether any hyperalgesia could be mediated by recruiting nociceptive modulating neurons in the RVM. We determined the effects of focal application of PGE(2) in the PAG on paw withdrawal latency and activity of identified nociceptive modulating neurons in the RVM of lightly anesthetized rats. Microinjection of PGE(2) (50 fg in 200 nl) into the PAG produced a significant decrease in paw withdrawal latency. The PGE(2) microinjection activated on-cells, RVM neurons thought to facilitate nociception, and suppressed the firing of off-cells, RVM neurons believed to have an inhibitory effect on nociception. These data demonstrate a prostaglandin-sensitive descending facilitation from the PAG, and suggest that this is mediated by on- and off-cells in the RVM.

Oestradiol dampens reflex-related activity of on- and off-cells in the rostral ventromedial medulla of female rats

The present study was conducted to determine whether the ovarian steroid oestradiol alters the activity of nociceptive modulatory neurons in the rostral ventromedial medulla (RVM). Adult female rats were ovariectomized and implanted s.c. with an oestradiol-filled or placebo capsule. Sixteen to 37 days later, rats were anaesthetised for single unit recording from RVM neurons. On-cells were characterised by a burst of activity, and off-cells by a pause in activity immediately preceding reflexive withdrawal of the tail from 51 and 54 degrees C water. Although on- and off-cells were evident in both oestradiol- and placebo-treated rats, the reflex-related on-cell burst and off-cell pause were dampened in oestradiol-treated rats. On-cells from oestradiol-treated rats had a mean activity burst of 9.1+/-2.2 Hz in the 2 s preceding the tail withdrawal reflex to 51 degrees C water, compared with 17.9+/-4.3 Hz for on-cells in placebo controls. Off-cell activity during the 2 s preceding tail withdrawal was 4.8+/-2.2 vs. 0.1+/-0.1 Hz in oestradiol vs. placebo-treated females, respectively. Similar changes in on- and off-cell activity occurred when the tail was placed in 54 degrees C water. The present data demonstrate that oestradiol constrains the magnitude of the shift in RVM on- and off-cell activity associated with nociceptive reflexes.

Orphanin FQ/nociceptin: from neural circuitry to behavior

Orphanin FQ/nociceptin (OFQ/N), the endogenous ligand for the "orphan" opioid receptor ORL-1 (NOP(1)) was first identified in 1995. In the years since its discovery, a large body of evidence has accumulated showing that OFQ/N and its receptor are widely distributed in the nervous system, and showing that OFQ/N has potent and indiscriminate inhibitory actions on neurons in many regions. However, numerous studies investigating the functional role of OFQ/N in physiology or behavior have failed to provide a coherent view. Pain and analgesia have been the best studied, and administration of OFQ/N is reported to have no effect, to produce hyperalgesia, analgesia or anti-hyperalgesia. Effects of OFQ/N receptor antagonists have proved similarly contentious. In an attempt to resolve this controversy, we investigated the actions of OFQ/N on the activity of physiologically characterized neurons in the rostral ventromedial medulla, a region with a well-documented role in pain modulation(Heinricher et al., 1997). The results of those experiments demonstrate that this peptide is neither "anti-opioid" or "anti-hyperalgesic". It is simply inhibitory. For this reason, the effects seen in functional studies will only be fully understood when examined in the context of identified neural circuits.

GABAergic modulation of descending inhibitory systems from the rostral ventromedial medulla (RVM). Dose-response analysis of nociception and neurological deficits

We have examined the effects of muscimol and bicuculline microinjected in the rostral ventromedial medulla (RVM) on motor function and on nociception in three pain tests. In Exp. 1 microinjection of muscimol (6.25-400 ng in 1 microl) in the RVM dose-dependently decreased pain threshold of rats and the ED(50) for muscimol was the same in both the hot plate and tail immersion pain tests. In the hot plate test, but not in the tail immersion test, paw withdrawal latencies increased again with high doses of muscimol (75-400 ng). High doses also produced catalepsy. Exp. 2 examined the effects of muscimol (50 ng) and bicuculline (50 ng) over a range of formalin concentrations (0.25-4%) in the formalin test. Muscimol increased responsiveness to formalin and reduced the slope of the formalin dose-response relation. Bicuculline decreased responses to formalin and reduced the slope of the formalin dose-response relation. It is suggested that RVM cells with inhibitory projections to the dorsal horn are not subject to strong GABAergic influence under mild noxious stimulation. RVM cells are thus active, and spinal dorsal horn relay neurons are inhibited. On the other hand, intense noxious peripheral stimulation may stimulate the release of GABA onto RVM cells, which in turn shuts off descending inhibitory fibers to allow transmission of nociceptor input through the dorsal horn.

Circuitry underlying antiopioid actions of cholecystokinin within the rostral ventromedial medulla

It is now well established that the analgesic actions of opioids can be modified by "anti-analgesic" or "antiopioid" peptides, among them cholecystokinin (CCK). Although the focus of much recent work concerned with CCK-opioid interactions has been at the level of the spinal cord, CCK also acts within the brain to modify opioid analgesia. The aim of the present study was to characterize the actions of CCK in a brain region in which the circuitry mediating the analgesic actions of opioids is relatively well understood, the rostral ventromedial medulla (RVM). Single-cell recording was combined with local infusion of CCK in the RVM and systemic administration of morphine in lightly anesthetized rats. The tail-flick reflex was used as a behavioral index of nociceptive responsiveness. Two classes of RVM neurons with distinct responses to opioids have been identified. OFF cells are activated, indirectly, by morphine and mu-opioid agonists, and there is strong evidence that this activation is crucial to opioid antinociception. ON cells, thought to facilitate nociception, are directly inhibited by opioids. Cells of a third class, NEUTRAL cells, do not respond to opioids, and whether they have any role in nociceptive modulation is unknown. CCK microinjected into the RVM by itself had no effect on tail flick latency or the firing of any cell class but significantly attenuated opioid activation of OFF cells and inhibition of the tail flick. Opioid suppression of ON-cell firing was not significantly altered by CCK. Thus CCK acting within the RVM attenuates the analgesic effect of systemically administered morphine by preventing activation of the putative pain inhibiting output neurons of the RVM, the OFF cells. CCK thus differs from another antiopioid peptide, orphanin FQ/nociceptin, which interferes with opioid analgesia by potently suppressing all OFF-cell firing.

Activation of kappa opioid receptors in the rostral ventromedial medulla blocks stress-induced antinociception

Prior work has shown that kappa opioids may attenuate the effects of analgesic mu receptor agonists in some neural circuits related to pain modulation. This study examined whether hypoalgesia following exposure to a signal for shock is attenuated by infusions of the kappa agonist U69593 into the rostral ventromedial medulla (RVM). Rats were trained with paired or unpaired presentations of white noise and foot shock. On test days, tail flick latencies were measured before, during, and after exposure to the auditory conditioned stimulus (CS). One of three doses of U69593 (0.0445, 0.178 and 1.00 microg) or an equivalent volume of saline was injected into the RVM. Rats that had received noise-shock pairings displayed conditional hypoalgesia (CHA) compared to those given unpaired presentations. Expression of CHA was completely blocked by the highest dose of U69593 (1.00 microg) injected 20 min before testing, indicating an antagonistic effect of U69593 on expression of CHA. These findings are discussed in terms of the evidence for antagonism of morphine- and DAMGO-induced hypoalgesia by kappa agonists.

Expression of antinociception in response to a signal for shock is blocked after selective downregulation of mu-opioid receptors in the rostral ventromedial medulla

Prior work has shown that release of endogenous ligands for mu-opioid receptors in the rostral ventromedial medulla (RVM) is critical for the modulation of spinal nociceptive reflexes observed during stress. In the present study, we used antisense oligodeoxynucleotides (AS ODN) to suppress synthesis of mu-opioid receptors in the RVM prior to activating descending antinociceptive systems with a signal for foot shock. Five groups of rats with RVM cannulae were trained with paired or unpaired exposures to white noise (WN) and foot shock. Over several days, they received RVM infusions of an AS ODN probe targeting exon 1 of the cloned MOR-1 receptor, an inactive missense (MS) ODN with the same base composition in which the sequence for four bases was changed, an AS ODN probe targeting exon 4, or saline. Tail-flick latencies (TFLs) were measured before, during, and after presentation of the auditory signal for shock. Rats given paired training and saline injections displayed longer TFLs than saline control rats given unpaired exposures to WN and shock, confirming the ability of the conditional stimuli (CS) to elicit antinociception. Expression of this conditional hypoalgesia (CHA) was attenuated by pretreatment with the AS ODN probe targeting exon 1, but was unaffected by pretreatment with AS ODN probe targeting exon 4 or MS ODN sequence for exon 1. However, pretreatment with the AS ODN probe targeting exon 1 did not affect expression of conditional freezing to other shock-associated cues. Testing of the same animals several days after the ODN injections showed that the attenuating effect on expression of CHA were reversible. These results support the idea that mu-opioid receptors in the RVM are critically involved in mediating expression of hypoalgesia following stress. They also provide further evidence for dissociation in the mechanisms mediating expression of aversive conditional responses.

Hypoalgesia elicited by a conditioned stimulus is blocked by a mu, but not a delta or a kappa, opioid antagonist injected into the rostral ventromedial medulla

The present study investigated the role of micro, delta, and kappa receptors within the RVM in mediating expression of conditional hypoalgesia (CHA). Five groups of rats with RVM cannulae were given daily sessions of paired or unpaired presentations of an auditory CS (white noise) and foot shock across three consecutive days. On the test day, rats in the Paired condition were injected with the micro antagonist CTAP, the delta antagonist naltrindole, the kappa antagonist nor-BNI, or saline. Rats in the Unpaired condition were injected with saline. TFLs were measured before and after injections, as well as during and after presentations of the CS. The results showed that none of the drugs affected baseline TFLs. During CS presentation, rats in the Paired condition injected with saline showed longer TFLs than those in the Unpaired condition given saline, confirming the presence of CHA. Expression of this response was blocked by CTAP, but was unaffected by naltrindole or nor-BNI. These results suggest that mu, but not delta or kappa, opioid receptors in the RVM mediate expression of CHA.

The contribution of the rostral ventromedial medulla to the antinociceptive effects of systemic morphine in restrained and unrestrained rats

Although there are numerous opioid-sensitive structures in the central nervous system, the contribution of each to the analgesic effect of systemically administered morphine is controversial. One such structure is the rostral ventromedial medulla. In the present study, we tested the hypothesis that the rostral ventromedial medulla is necessary for the full expression of systemic morphine-induced antinociception. Additionally, we examined whether the modulatory effect of the rostral ventromedial medulla on tail-flick latency is dependent on the behavioral state of the animal. In unrestrained rats, inactivation of the rostral ventromedial medulla with either lidocaine (0.5 microl of 4%) or muscimol (50 ng) had no effect on tail-flick latency. In contrast, in restrained rats, inactivation of the rostral ventromedial medulla with either lidocaine (0.5 microl of 4%) or muscimol (50 ng) significantly decreased tail-flick latency. In both conditions, microinjection of morphine (5 microg) into this region significantly increased tail-flick latency. Additionally, in unrestrained rats, muscimol (50 ng) and cholecystokinin tetrapeptide (0.5 ng) infusion into the rostral ventromedial medulla completely reversed systemic morphine-induced analgesia, while lidocaine (0.5 microl of 4%) and cholecystokinin octapeptide (0.25 ng) infusion partially reversed systemic morphine-induced analgesia. These findings demonstrate that the rostral ventromedial medulla does not tonically modulate tail-flick latency in unrestrained rats, but does modulate tail-flick latency when animals are stressed via restraint. These findings also strongly support the hypothesis that the rostral ventromedial medulla is necessary for the full analgesic effects of systemically administered morphine.

Microinjection of morphine into the rostral ventromedial medulla produces greater antinociception in male compared to female rats

The antinociceptive and locomotor effects of microinjecting morphine into the rostral ventromedial medulla (RVM) of male and female rats was assessed. Male rats showed greater antinociception than female rats at all doses and times following morphine administration. Male, but not female rats, also showed a dose dependent decrease in locomotion. These data demonstrate that sex differences in antinociception are mediated at least in part by the RVM.

The influence of temporal summation on a C-fibre reflex in the rat: effects of lesions in the rostral ventromedial medulla (RVM)

In intact rats, an inhibitory mechanism counteracts the increase in excitability of a flexor reflex seen in spinal animals following high-intensity, repetitive stimulation of C-fibres. We tested the hypothesis that the rostral ventromedial medulla (RVM) is involved in these processes. Electromyographic responses elicited by electrical stimulation of the sural nerve, were recorded from the ipsilateral biceps femoris in halothane-anaesthetised, sham-operated or RVM-lesioned rats. There were no significant differences between the C-fibre reflexes in the two groups in terms of their thresholds, latencies, durations or mean recruitment curves. The excitability of the C-fibre reflex was tested following 20 s of high-intensity homotopic electrical conditioning stimuli at 1 Hz. During the conditioning period, the EMG responses first increased in both groups (the wind-up phenomenon), but then decreased in the sham-operated rats and plateaued in the RVM-lesioned rats. These effects were followed by inhibitions that were very much smaller in the RVM-lesioned rats, both in terms of their magnitudes and their durations. It is concluded that the RVM is involved in inhibitory feedback mechanisms elicited by temporal summation of C-fibre afferents that both counteract the wind-up phenomenon and trigger long periods of inhibition.

Analysis of excitatory amino acid transmission within the rostral ventromedial medulla: implications for circuitry

Two classes of neurons with distinct responses to opioids have been identified in the rostral ventromedial medulla (RVM), a region with a well-documented role in nociceptive modulation. 'On-cells' are directly inhibited by opioids, and opioids can thus gain access to the modulatory circuitry of the RVM by an action on these neurons. 'Off-cells' are likely to exert a net inhibitory effect on nociceptive processing, and are activated by opioids. Because the opioid activation of off-cells is indirect, it has been proposed that on-cells function as inhibitory interneurons, and that opioid-induced suppression of on-cell firing in turn activates off-cells via disinhibition. The aim of the present study was to test this possibility. We had previously shown that excitatory amino acid (EAA) neurotransmission is crucial to the nocifensor reflex-related on-cell burst. We therefore infused the non-selective EAA receptor antagonist kynurenate (0.5-2 nmol, 200-500 nl) into the RVM while recording activity of on-, off- and neutral cells in lightly anesthetized rats. Kynurenate infusions produced a significant decrease in on-cell firing, with suppression of the on-cell burst. Off-cells nonetheless continued to display a tail flick-related pause in firing. Tail flick latency was used as an index of nociceptive responsiveness, and was unaffected by kynurenate infusions. These results demonstrate that a burst of on-cell firing is not required in order for the off-cell to exhibit a reflex-related pause in discharge, and do not support the proposed crucial role for on-cells as inhibitory interneurons within the RVM. In addition, preferential suppression of on-cell tiring was not associated with an increase in tail flick latency. This suggests that, under the conditions of these experiments, on-cell discharge is not a potent regulator of moment-to-moment variations in nociceptive responsiveness.

Opioid supraspinal analgesic synergy between the amygdala and periaqueductal gray in rats

Analgesia can be elicited following microinjections of morphine, mu-selective agonists and beta-endorphin into the amygdala. These analgesic responses are mediated by opioid synapses in the periaqueductal gray (PAG) since general (naltrexone), mu (beta-funaltrexamine) and delta2 (naltrindole isothiocyanate) opioid antagonists administered into the PAG significantly reduce both morphine and beta-endorphin analgesia elicited from the amygdala. Supraspinal multiplicative opiate analgesic interactions have been observed between the PAG and rostroventromedial medulla (RVM), the PAG and locus coeruleus (LC), and the RVM and LC. The present study further examined the relationship between the amygdala and PAG in analgesic responsiveness by determining whether multiplicative analgesic interactions occur following paired administration of subthreshold doses of morphine into both structures, beta-endorphin into both structures, morphine into one structure and beta-endorphin into the other structure, or morphine and beta-endorphin into one structure. Co-administration of subthreshold doses of morphine into both the amygdala and PAG results in a profound synergistic interaction on the jump test, but not the tail-flick test. Co-administration of subthreshold doses of beta-endorphin into both structures also results in a profound test-specific synergistic interaction. In both cases, the magnitude of the interaction was similar regardless of the site receiving the fixed dose of the opioid, and the site receiving the variable dose of the opioid. Co-administration of beta-endorphin (1 microg) into the amygdala and morphine (1 microg) into the PAG produced a potent interaction, but co-administration of morphine (1 microg) into the amygdala and beta-endorphin (1 microg) into the PAG failed to produce interactive effects. Finally, co-administration of morphine (1 microg) and beta-endorphin (1 microg) into either the amygdala alone or the PAG alone failed to produce an interaction, indicating the importance of regional opioid activation. These data are discussed in terms of the test-specificity of nociceptive processing in the amygdala, in terms of the multiple modulatory mechanisms mediating beta-endorphin analgesia in the PAG, and in terms of whether the interactions are either mediated by anatomical connections between the amygdala and PAG or by mechanisms initiated by these two sites converging at another site or sites.

Circuitry underlying antiopioid actions of orphanin FQ in the rostral ventromedial medulla

Several laboratories recently identified a 17 amino-acid peptide, termed "nociceptin" or "orphanin FQ (OFQ)", as the endogenous ligand for the LC132 (or "opioid receptor-like1") receptor. Taken together with the fact that the cellular effects of OFQ are to a large extent opioid-like, the close relationship between the LC132 receptor and known opioid receptors raised expectations that the behavioral effects of this peptide would resemble those of opioids. However studies of the role of OFQ in nociception have not provided a unified view. The aim of the present study was to use a combination of electrophysiological and pharmacological techniques to characterize the actions of OFQ in a brain region in which the circuitry mediating the analgesic actions of opioids has been relatively well characterized, the rostral ventromedial medulla (RVM). Single-cell recording was combined with opioid administration and local infusion of OFQ in the RVM of rats lightlyanesthetized with barbiturates. The tail flick reflex was used as a behavioral index of nociceptive responsiveness. Two classes of physiologically identifiable RVM neurons with distinct responses to opioids have been characterized. -cells are activated, although indirectly, by opioids, and there is strong evidence that this activation is crucial to opioid antinociception. -cells, thought to enable nociception, are directly inhibited by opioids. Cells of a third class, cells, do not respond to opioids and whether or not they have any role in nociceptive modulation remains an open question. OFQ infused within the RVM profoundly suppressed the firing of all classes of RVM neurons, blocking opioid-induced activation of -cells. The antinociceptive effects of a micro-opioid agonist infused at the same site were significantly attenuated in these animals. Those of systemically administered morphine, which can produce its antinociceptive effects by acting at a number of CNS sites, were not blocked by RVM OFQ. Inasmuch as activation of -cells can account for the antinociceptive action of opioids within the RVM, these results demonstrate that, at least within the medulla, OFQ can exert a functional "antiopioid" effect by suppressing firing of this cell class. However to the extent that antinociceptive and pronociceptive outflows from various brain regions involved in both transmission and modulation of nociception are active under different conditions, focal application of OFQ in different regions could potentially produce either hypalgesia or hyperalgesia.

Comparison of the effects of nucleus tractus solitarius and ventral medial medulla lesions on illness-induced and subcutaneous formalin-induced hyperalgesias

We have previously demonstrated that illness-inducing agents (lipopolysaccharide (LPS)) and inflammatory agents (subcutaneous (s.c.) formalin) induce hyperalgesia by similar pathways. The present series of experiments compared the effects of medullary lesions on these phenomena. These experiments demonstrate that s.c. formalin-induced hyperalgesia, like illness-induced hyperalgesia, is dependent on the nucleus raphe magnus (NRM) but independent of the nucleus reticularis paragigantocellularis (NRPgc). However, these two forms of hyperalgesia differ with regards to their dependence on the nucleus tractus solitarius (NTS). Illness-induced hyperalgesia is abolished by unilateral (left) NTS lesions, whereas formalin-induced hyperalgesia remains unaffected by this procedure. These data provide further evidence that hyperalgesias induced by illness agents and by inflammatory agents are mediated by similar but not identical pathways. They also illustrate that neural structures have the capacity for opposed actions, in that both the NTS and NRM are documented to underlie hyperalgesia as well as analgesia. This capacity for opposed action may prove to be characteristic of structures involved in pain modulation.

Disinhibition of off-cells and antinociception produced by an opioid action within the rostral ventromedial medulla

Activation of neurons in the rostral ventral medulla, by electrical stimulation or microinjection of glutamate, produces antinociception. Microinjection of opioid compounds in this region also has an antinociceptive effect, indicating that opioids activate a medullary output neuron that exerts a net inhibitory effect on nociception. When given systemically in doses sufficient to produce antinociception, morphine produces distinct, opposing responses in two physiologically identifiable classes of rostral medullary neurons. "Off-cells" are activated, and have been proposed to inhibit nociceptive transmission. "On-cells" are invariably depressed, and may have a pro-nociceptive role. Although on-cell firing is also depressed by iontophoretically applied morphine, off-cells do not respond to morphine applied in this manner. The present study used local infusion of the mu-selective opioid peptide Tyr-D-Ala-Gly-MePhe-Gly-ol-enkephalin (DAMGO) within the rostral medulla to determine whether off-cells are activated by an opioid action within this region that is sufficient to produce a behaviorally measurable antinociception. Activity of on- and off-cells was recorded before and after local infusion of DAMGO noxious heat-evoked tail flick reflex was inhibited in 17 of 28 cases. On-cell firing was profoundly depressed, and this occurred irrespective of the antinociceptive effectiveness of the injection. Off-cells were activated following DAMGO microinjections, but only in experiments in which the tail flick reflex was inhibited. Both reflex inhibition and neuronal effects were reversed following systemic administration of naloxone. These observations thus confirm the role of the on-cell as the focus of direct opioid action within the rostral medulla, and strongly support the proposal that disinhibition of off-cells is central to the antinociception actions of opioids within this region.

Interference with GABA transmission in the rostral ventromedial medulla: disinhibition of off-cells as a central mechanism in nociceptive modulation

Blockade of GABA-mediated synaptic transmission in the rostral ventromedial medulla by local application of GABAA receptor antagonists produces antinociception, indicating that a GABA-mediated inhibition of some population of neurons in this region is normally required if nociceptive information is to be transmitted. The aim of the present study was to elucidate the medullary circuitry mediating this antinociception by recording the activity of putative nociceptive modulating neurons in the rostral ventromedial medulla before and after local infusion of the GABAA receptor antagonist bicuculline methiodide. It was thus possible to correlate changes in the activity of cells of different classes with the ability of the infusion to produce a behaviorally measurable antinociception. One class of medullary neurons, "off-cells," is identified by a pause in firing associated with the occurrence of nocifensor reflexes such as the tail flick evoked by noxious heat. These neurons are uniformly activated following systemic administration of morphine, and are thought to have a net inhibitory effect on nociception. Following local bicuculline administration, off-cells enter a prolonged period of continuous firing that is temporally linked with the period of tail flick inhibition. A second class of neurons, "on-cells," is identified by a burst of activity beginning just before the tail flick, and is directly inhibited by opioids. Unlike off-cells, cells of this class do not show a consistent change in activity associated with inhibition of the tail flick following bicuculline. These data indicate that alterations in the discharges of on-cells would not be able to explain the antinociceptive effect of bicuculline, and therefore point to disinhibition of off-cells as a sufficient basis for antinociception originating within the rostral ventromedial medulla.

The effect of thalamic nucleus submedius lesions on nociceptive responding in rats

Previous studies have shown that the thalamic nucleus submedius (SM) contains nociceptive neurons and is interconnected with spinal, brain-stem and cortical regions associated with nociception. The present study was performed to examine the role of the SM in nociceptive-related behaviors. The effect of SM lesions on nociceptive responding in rats was assessed using both the radiant-heat tail-flick (TF) and the tail-shock 'pain-induced' vocalization (PIV) tests. The results of Exp. 1 indicated that the intensity of electrical shock required for vocalization responses was significantly decreased following SM lesions. No changes in vocalization responses were present in the sham-lesion group. In contrast, both the sham- and SM-lesion groups exhibited a significant post-lesion increase in TF latencies. A second experiment was performed to determine whether the effects of SM lesion on the tail flick may have been masked by conditioned antinociception associated with noxious electrical stimulation of the tail to produce PIV. The results indicated that there was no post-lesion change in TF latencies in either the SM- or sham-lesion group when the antecedent PIV test was omitted. The results suggest that the SM may play a role in supraspinally mediated inhibition of nociceptive input but not in spinally mediated responses to noxious stimuli.

A 9L gliosarcoma transplantation model for studying adoptive immunotherapy into the brains of conscious rats

A rat model for brain tumor immunotherapy is described that closely mimics the type of treatment that could be administered to humans. It involves surgical implantation of a permanent cannula in the brain, through which tumor cells and various effector cells and/or cytokines can be injected. The advantage of this system over more conventional animal surgical procedures is that conscious animals can be treated multiple times while avoiding morbidity and mortality associated with reoperative procedures. Using this system to study adoptive immunotherapy for brain tumors, we provide evidence that the 9L gliosarcoma tumor from the Fischer rat strain can be reduced or destroyed in situ following adoptive immunotherapy with specifically activated cytotoxic T lymphocytes.

Putative role of medullary off- and on-cells in the antinociception produced by dipyrone (metamizol) administered systemically or microinjected into PAG

Recent investigations have shown that non-steroidal antiinflammatory drugs (NSAIDs) may exert an antinociceptive effect when administered at or within the central nervous system (CNS). This might be due to the engagement of CNS substrates that support the analgesic effects of opiates, including the periaqueductal gray matter (PAG) and the rostral ventromedial medulla (RVM). The off- and on-cells of the RVM have been proposed to inhibit and facilitate, respectively, nociceptive transmission. Accordingly, upon heating of a rat's tail the tail-flick (TF) reflex occurs only after off-cells have decreased, and on-cells have increased, their activity. In the present study, i.v. administration (200 and 400 mg/kg) or PAG microinjection (25, 50, 100 and 250 micrograms) of dipyrone (metamizol) to lightly anesthetized rats caused a dose-related retardation of the heat-elicited off-cell pause, on-cell discharge and corresponding TF. Neuronal response and TF retained their mutual time relationship but shifted pari passu toward longer latencies. This antinociception was apparent already 5 min post-injection and reached a maximum in 50-60 min for i.v. administration and 30-35 min for PAG microinjection. These results confirm other authors' findings of the direct antinociceptive action of NSAIDs upon PAG, and provide the first evidence for a plausible involvement of RVM off- and on-cells in such antinociceptive effect.

Neurocircuitry of illness-induced hyperalgesia

We have previously demonstrated that illness-inducing agents such as lithium chloride (LiCl) and the bacterial cell wall endotoxin lipopolysaccharide (LPS) produce hyperalgesia on diverse pain measures. The present series of studies attempted to identify the neurocircuitry mediating these effects. These studies have demonstrated that illness-inducing agents produce hyperalgesia by activating: (a) peripheral nerves rather than by generating a blood-borne mediator (Expt. 1); (b) vagal afferents, specifically afferents within the hepatic branch of the vagus (Expt. 2); (c) as yet unidentified brain site(s) rostral to the mid-mesencephalon (Expt. 6); (d) a centrifugal pathway that arises from the nucleus raphe magnus, and not from the adjacent nucleus reticularis paragigantocellularis pars alpha (Expts. 4 and 5); (e) a centrifugal pathway in the dorsolateral funiculus of the spinal cord (Expt. 3); and (f) the same centrifugal pathways for diverse illness inducing agents (Expts. 3, 7 and 8). These data call for the re-evaluation of a number of assumptions inherent in previous studies of hyperalgesia.

Morphine and diffuse noxious inhibitory controls in the rat: effects of lesions of the rostral ventromedial medulla

The aim of this study was to investigate whether the rostral ventromedial medulla (RVM) participates in the lifting of diffuse noxious inhibitory controls (DNIC) by systemic morphine. The effects of morphine (1 mg/kg i.v.) on DNIC were compared in sham-operated rats and animals with electrolytic lesions of the RVM performed one or three weeks earlier. The C-fibre-evoked responses of spinal dorsal horn convergent neurones were similar in the sham-operated and lesioned animals. DNIC acting on these responses were also similar in these groups. DNIC were similarly reduced naloxone reversibly following morphine injections in sham-operated animals and animals tested one week after lesioning of the RVM. In contrast, DNIC were not significantly altered by morphine in animals tested three weeks after lesioning. The lesions were similar in both groups of animals. This time-dependent attenuation of the effects of morphine indicates that the RVM is not directly involved in the reduction of DNIC induced by systemic morphine. However, it is suggested that lesions in this region can induce a reorganization of brainstem opioidergic systems.

Direct and indirect actions of morphine on medullary neurons that modulate nociception

The rostral ventromedial medulla is part of a neural network through which systemically administered morphine produces antinociception. Two physiologically characterized classes of presumed nociceptive modulating neurons that respond differentially to systemically administered morphine have been identified in this region: the firing of "on-cells" is depressed, whereas "off-cells" become continuously active. On-cells have been proposed to permit or facilitate, and off-cells to inhibit, nociceptive transmission. Because local application of morphine in the rostral ventromedial medulla itself is sufficient to produce antinociception, it is important to determine whether systemically administered morphine exerts its effects on neurons in this region by a direct action. Thus, activity of physiologically characterized neurons was studied before, during and after ionotophoretic administration of morphine. As with systemic administration, iontophoretic application of morphine depresses the activity of on-cells, an effect that is reversed by iontophoretic as well as by systemic administration of naloxone. In contrast, no reliable changes in the firing of off-cells are produced by iontophoretic administration of morphine. Cells of a third class, "neutral cells", are not affected by systemic morphine administration, nor do they respond to iontophoretic application of the drug. The present experiments demonstrate that direct opioid responsiveness in the rostral ventromedial medulla is limited to a single physiologically characterized class of presumed nociceptive modulatory neuron, the on-cell. This implies that the antinociceptive effect exerted by systemically administered morphine involves at least two components within the rostral ventromedial medulla: a direct inhibition of on-cells, and an indirect activation of off-cells. Each of these actions is likely to have a net hypoalgesic effect.

Electroacupuncture analgesia in naive rats: effects of brainstem and spinal cord lesions, and role of pituitary-adrenal axis

Recent studies have shown that analgesia is potentiated by naltrexone (NTX) and naloxone (NAL) pretreatment in rats exposed for the first time to electroacupuncture (EA). In the present study, we have investigated the role of the pituitary-adrenal axis and of brainstem and spinal cord structures in EA analgesia and its potentiation by NTX. The pituitary and adrenal glands do not participate in the production of EA analgesia, but may produce a non-opioid substance which interferes with the development of EA analgesia. Spinalization or dorsolateral funiculi lesions blocked EA analgesia, and intrathecal NTX had no effect. These results indicate that supraspinal structures are necessary to produce and potentiate EA analgesia. Contrary to their critical role in morphine and other models of environmentally produced analgesia nucleus raphe alatus and raphe structures dorsal to it are not necessary for the development of EA analgesia. These structures, however, may contain opiate synapses on which NTX may act as an agonist to potentiate analgesia. The various components which appear to participate in the production of EA analgesia imply a complex circuit of pain modulation systems and indicate that an organism can adapt to distinct environmental conditions with versatile means to avoid pain.

The antinociceptive action of supraspinal opioids results from an increase in descending inhibitory control: correlation of nociceptive behavior and c-fos expression

In an earlier report, we demonstrated that subcutaneous injection of formalin in the rat hindpaw evokes a characteristic pattern of expression of the fos protein product of the c-fos protooncogene in spinal cord neurons, and that systemic morphine reversed the fos-like immunoreactivity in a dose-dependent, naloxone-reversible manner. The present study compared the effects of intracerebroventricular administration of the mu-selective opioid ligand [D-Ala2, NMe-Phe4, Gly-ol5] enkephalin, on the pain behavior and spinal cord fos-like immunoreactivity produced by subcutaneous formalin. Formalin injection produced a biphasic pain behavioral response which lasted about 1 h. There was a significant correlation between the formalin pain score and overall fos-like immunoreactivity in the lumbar enlargement. The greatest numbers of labeled cells and most intense fos-like immunoreactivity were found in laminae I, IIo and V of the L4-5 segments, ipsilateral to the formalin-injected paw. Considerable staining was also found in the ipsilateral ventral horn laminae VII and VIII. [D-Ala2, NMe-Phe4, Gly-ol5]enkephalin produced a dose-related, naloxone-reversible inhibition of both the formalin-evoked pain behavior and fos expression in the cord. The behavioral response to formalin, however, could be completely blocked without eliminating the expression of fos in spinal neurons. Moreover, subpopulations of neurons were differentially regulated. Thus, 100% inhibition of pain behavior was produced at a dose of [D-Ala2, NMe-Phe4, Gly-ol5]enkephalin which reduced fos-like immunoreactivity in the superficial laminae by only 64% and in the neck and ventral cord by 85%. Furthermore, the dose of [D-Ala2, NMe-Phe4, Gly-ol5]enkephalin which produced approximately 50% inhibition of fos-like immunoreactivity in the neck and ventral regions of the spinal cord was without effect in the superficial dorsal horn. Since the potencies for inhibition of pain behavior and fos-like immunoreactivity in the neck and ventral horn were comparable, these data suggest that the activity of neurons in these regions is directly related to the pain behavior produced by nociceptive inputs. Finally, we found that bilateral, midthoracic lesions of the dorsal part of the lateral funiculus blocked both the antinociception and fos suppression produced by intracerebroventricular [D-Ala2, NMe-Phe4, Gly-ol5]enkephalin. These results are consistent with the hypothesis that the analgesic action of supraspinally administered opiates results from an increase in descending inhibitory controls that regulate the firing of subpopulations of spinal cord nociresponsive neurons.

The role of glutamate in opiate descending inhibition of nociceptive spinal reflexes

The present experiment examined descending inhibition of the nociceptive tail-flick reflex produced by microinjection of morphine and glutamate into the periaqueductal gray (PAG) matter and the neurotransmitters mediating the inhibition at the level of the nucleus raphe magnus (NRM). The longlasting opiate analgesia was significantly reduced by microinjection of excitatory amino acid antagonists 1-(p-chlorobenzoyl)-piperazine-2,3-dicarboxylate (PCB, 3.25 mumol) or DL-2-amino-5-phosphono-valerate (APV, 25.38 mumol) into the NRM, whereas the short-lived glutamate analgesia was not. This indicates that although both opiate and non-opiate analgesia may originate in the PAG, the former is relayed through the NRM, whereas the latter is relayed by additional or different nuclei in the medulla. Two observations shed light on the question which receptors mediate the above effect in the NRM. First, PCB blocked morphine analgesia at doses that were 8 times lower than doses of APV that were effective. Second, analgesia produced by injection of glutamate into the NRM was antagonized by PCB (3.25 mumol), whereas APV (25.38 mumol) failed to do so. Together these results indicate that kainate/quisqualate, but not N-methyl-D-aspartate (NMDA), receptors are implicated in the NRM as a relay station in opiate descending inhibition.

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.

Putative nociceptive modulatory neurons in the rostral ventromedial medulla of the rat display highly correlated firing patterns

Recent work in this laboratory has identified two classes of putative nociceptive modulating neurons in the rostral ventromedial medulla (RVM) of the rat: "off-cells," which pause beginning just prior to the tail flick response (TF) evoked by noxious heat, and "on-cells," which accelerate shortly before the occurrence of the TF. In the unstimulated, lightly anesthetized rat, the spontaneous firing pattern of individual on- and off-cells consists of alternating periods of silence and activity lasting from several seconds to a few minutes. In the present study, simultaneous recordings were made from pairs of TF-related neurons, and the relationships among the firing patterns of cells within a class and between cells of different classes were determined. All cells of a given class showed fluctuations in spontaneous discharge that were in phase. On the other hand, there was a striking reciprocity of firing between the two cell classes, such that a decrease in activity of cells of one class was accompanied by an increase in activity of cells of the other class. These observations point to the existence of integrating mechanisms that coordinate the activity of all members of each class of TF-related neurons. Thus, the pattern of activity of any single on- or off-cell provides a useful index of the excitability of all cells of that class. Moreover, because of the highly reciprocal nature of the firing of the two classes, it is possible to infer the current state of both cell populations from the pattern of activity of any single TF-related neuron.

Putative nociceptive modulating neurons in the rostral ventromedial medulla of the rat: firing of on- and off-cells is related to nociceptive responsiveness

In the unstimulated, lightly anesthetized rat, both on- and off-cells exhibit alternating periods of silence and activity lasting from several seconds to a few minutes. In the preceding paper, we showed that the active periods of all cells of the same class are always in phase, whereas the firing of cells of different classes is invariably out of phase. Thus, the pattern of firing of any single on- or off-cell provides a useful indication of the excitability of all on- and off-cells in the rostral ventromedial medulla (RVM). In this study, we measured the latency of the tail flick response (TF) at set intervals while recording from TF-related neurons in RVM, and were able to demonstrate a significant relationship between the spontaneous firing of both on- and off-cells and the latency of the TF response. If noxious heat is applied at a time when an off-cell is spontaneously active (or an on-cell is silent), the TF latency is longer than if the TF trial falls during a period in which the off-cell is silent (or the on-cell is active). This correlation between on- and off-cell firing and changes in TF latency is consistent with a nociceptive modulatory role for either or both cell classes. These findings support the hypothesis that off-cells inhibit and on-cells facilitate spinal nociceptive transmission and reflexes.

Dorsal raphe stimulation modulates nociceptive responses in thalamic parafascicular neurons via an ascending pathway: further studies on ascending pain modulation pathways

A study on the nociceptive responses of single cells within the nucleus parafascicularis (PF) thalami of the rat was undertaken to clarify the reported observations of a pain suppression pathway to this nucleus from the dorsal raphe (DR) nucleus. Two types of nociceptive neuron were identified in the PF which were classified as 'nociceptive-on' and 'nociceptive-off' neurons, respectively. DR stimulation exhibits a simple monophasic 'dose-dependent' relationship between the degree of the inhibition elicited and the stimulation intensity used on the 'nociceptive-off' cells. In contrast, biphasic effects following DR stimulation on the 'nociceptive-on' cells was obtained, with low intensities eliciting suppression while high intensities excited the cells. These effects of low intensity DR stimulation upon the responses of the 'nociceptive-on' cells were diminished but not prevented by transection of the well-known bulbospinal inhibitory fibers descending in the dorsal half of the spinal cord, while the effects of DR stimulation upon the 'nociceptive-off' cells remain unchanged following spinal transection. Thus, our results show that DR stimulation modulates the nociceptive responsiveness of the PF by way of supraspinal pathways in addition to the previously described descending paths.

Immunocytochemical localization of pro-opiomelanocortin neurons in human brain areas subserving stimulation analgesia

The distribution of pro-opiomelanocortin (beta-endorphin, adrenocorticotropic hormone, and 16-K) neurons and fiber projections was evaluated immunocytochemically in 50-mu thick cryostat sections of human diencephalon and midbrain. Specific attention was focused upon regions in which deep brain stimulation has been most effective in the relief of selected chronic pain syndromes. This study revealed a remarkable, nearly point-to-point correlation between clinically effective stimulation sites and the distribution of pro-opiomelanocortin fibers in the human brain. Of particular interest was the dense innervation of the periventricular stratum along the third ventricle, the parafascicular centromedian region of the thalamus, and the periaqueductal gray matter of the midbrain. This study provides anatomical support for the hypothesis that beta-endorphin-containing neuronal systems may contribute to stimulation analgesia in the human.

The failure of morphine to interact with serotonin on nucleus raphe magnus descending inhibition or morphine-induced suppression of cat spinal cord multireceptive neurones

Morphine sulphate, administered in three cumulative doses (0.5, 1.0, and 2.0 mg/kg, i.v.) to alpha-chloralose anaesthetized cats, reduced the nociceptive activity of deep dorsal horn multireceptive neurones but failed to alter the descending nucleus raphe magnus (NRM) phasic inhibition of these neurones. Morphine was also administered to fluoxetine (6.0 mg/kg, i.v.) pretreated animals. Fluoxetine is a selective 5-hydroxytryptamine (5-HT) uptake blocker, which should enhance 5-HT synaptic transmission. In these animals, morphine suppressed the neuronal nociceptive activity to the same extent as seen with morphine alone and did not affect the NRM inhibition. These results do not support the notion that morphine activates a descending serotonergic inhibition from the NRM or that serotonin mediates morphine inhibition of spinal nociceptive transmission in cats.

Morphine microinjected into the periaqueductal gray has differential effects on 3 classes of medullary neurons

The effects of microinjection of 5-10 micrograms of morphine into the midbrain periaqueductal gray (PAG) on the activity of neurons in the rostral ventral medulla (RVM) were studied in lightly anesthetized rats. Based on the relationship between changes in neuronal activity and the occurrence of the tail-flick reflex (TF), RVM neurons were divided into 3 groups: off-cells, on-cells and neutral cells. The off-cells exhibited an abrupt pause and the on-cells an acceleration beginning just prior to the occurrence of the TF. Neutral cell firing did not change at the time of the TF. Microinjections of morphine into the PAG which inhibited the TF had differential effects on the spontaneous activity of the 3 groups of neurons in RVM. Off-cells showed an increase and on-cells a decrease in spontaneous activity which preceded the inhibition of the TF. These microinjections also reduced the TF-related responses of off- and on-cells. The effects on cell activity were reversed by systemically administered naloxone and were not seen following microinjections which failed to block the TF. Neutral cell activity was unchanged following microinjection of morphine into the PAG. These results support the hypothesis that off- and on-cells in the RVM mediate the effects of microinjection of morphine into the PAG on spinal nociceptive reflexes.

The midbrain periaqueductal gray in the rat. I. Nuclear volume, cell number, density, orientation, and regional subdivisions

The midbrain periaqueductal gray is a functionally heterogeneous region which plays an important role in pain modulation. Despite the heterogeneity considerable controversy exists regarding the presence or absence of morphological subdivisions within the region. The present study was designed to evaluate the possibility of morphological subdivisions within the rat periaqueductal gray by using a statistical cluster analysis system. In addition both qualitative and quantitative data concerning neuronal size, shape, and density were obtained. On the basis of measurements of over 12,000 neurons in two planes of section, the mean neuronal length of cell bodies in this region was 14.82 microns and the mean neuronal area was 95.59 microns squared . The mean neuronal density was found to be 16,284 cells per mm3. Neuronal density decreased from rostral to caudal in the periaqueductal gray. The data obtained from cluster maps suggest the presence of four subdivisions within this midbrain region. The medial subdivision contains the smallest neurons and exhibits the lowest cell density. The dorsolateral and ventrolateral divisions contain the largest neurons while the dorsal division displays the highest packing density. These results are discussed in light of recent receptor binding and immunohistochemical studies of this region.

The midbrain periaqueductal gray in the rat. II. A Golgi analysis

This study consists of a detailed analysis of neurons in the midbrain periaqueductal gray of the rat utilizing four variants of the Golgi technique. Neurons were classified into three major categories based on soma shape, number of primary dendrites, number of dendritic bifurcations, interspinous distance, axonal origin, and axon trajectory. Neurons in each category were further subdivided into large and small varieties based predominantly on soma size and dendritic patterns. Both quantitative and qualitative data concerning each neuronal type is provided as well as data relating to its relative distribution among the four periaqueductal gray subdivisions. The small bipolar neuron, characterized by its small size and spindle-shaped soma, was the most prominent cell type observed, composing 37% of the impregnated neurons in our material. This cell type was most numerous in the medial subdivision and least prominent in the dorsolateral subdivision. The small triangular neuron composed 23% of the neuronal population and was relatively evenly distributed through the periaqueductal gray. The remaining four cell types include the large and small multipolar neurons, the large fusiform neurons, and the large triangular neurons. Axons originated from either the perikaryon or a proximal dendrite, with a dendritic origin being most common for large and small triangular neurons and large fusiform neurons. The trajectory of axons in single thick coronal sections originating from periaqueductal gray neurons is typically away from the mesencephalic aqueduct. The exact trajectory is dependent on the location of the neuron. Axons arising from cells in the dorsal subdivision usually project in a dorsal or dorsolateral direction while axons of ventrolateral neurons may project dorsally, laterally, or ventrally. In sum, these data indicate a complex level of internal organization of the periaqueductal gray. The results are discussed in terms of previous immunohistochemical studies of neurons in this region.

Response of serotonin-containing neurons in nucleus raphe magnus to morphine, noxious stimuli, and periaqueductal gray stimulation in freely moving cats

Extracellular single-unit recordings were made in nucleus raphe magnus in unanesthetized, unrestrained cats. Discharge of serotonergic neurons in this region was increased when animals were aroused by noxious stimuli such as pinch and radiant heating of the tail, but these cells were not specifically nociceptive. Peristimulus time histograms indicated that stimulation in the periaqueductal gray was excitatory but alveolar nerve stimulation at a noxious current intensity was no more effective than nonnoxious nerve stimulation in activating serotonergic unit discharge: Similarly, stressful treatments such as physical restraint increased the discharge of some serotonergic neurons, but these cells were activated during any period of behavioral arousal whether or not arousal was the result of aversive treatment. Injection of Formalin into the paw produced pain lasting about 30 min without increasing serotonergic unit discharge above rates observed during undisturbed active waking behavior. The activity of serotonergic neurons was not increased by an analgesic dose of morphine (2 mg/kg, i.p.). These results then are not consistent with the hypothesis that morphine analgesia depends on activation of serotonergic neurons in nucleus raphe magnus or that these cells are specifically involved in modulation of nociception. These neurons may, however, be involved in nociceptive control within the context of a general modulation of sensorimotor processes by serotonin in the central nervous system. We did observe neurochemically unidentified neurons in the medulla whose discharge was more specifically activated by aversive stimuli and also by morphine. It is possible that these neurons are more directly involved in the mediation of opiate and/or stress-induced analgesia.