Hyperalgesia during naloxone-precipitated withdrawal from morphine is associated with increased on-cell activity in the rostral ventromedial medulla

Reflex-related activation of putative pain facilitating neurons in rostral ventromedial medulla requires excitatory amino acid transmission

Although the importance of the rostral ventromedial medulla in pain modulation is generally accepted, the recognition that it can exert both pain facilitating and pain inhibiting influences, and that its constituent neuronal population is physiologically and pharmacologically heterogeneous, is relatively recent. A class of neuron which may be a source of facilitating influences from the rostral ventromedial medulla has been identified in electrophysiological experiments. These neurons, termed "on-cells," are characterized by a sudden burst of activity beginning just before nocifensive reflexes. This burst of firing is thought to be a significant factor in brainstem control of nociceptive transmission under physiological conditions. The aim of the present study was to determine whether an excitatory amino acid is involved in generation of the reflex-related burst that defines on-cells, and more generally, to examine the role of excitatory amino acid neurotransmitters within the rostral ventromedial medulla of the rat. Iontophoretic application of the broad-spectrum excitatory amino acid receptor antagonist kynurenate significantly reduced the reflex-related on-cell burst, whereas ongoing firing was unaffected. Spontaneous activity of other medullary neurons was unchanged. These data demonstrate that release of an endogenous excitatory amino acid neurotransmitter is necessary for the activation of on-cells that is associated with nocifensive reflexes. In contrast, these receptors evidently play a much less significant role in maintaining the ongoing activity of any cell class in the rostral ventromedial medulla in lightly anaesthetized rats.

Contribution of sacral spinal cord neurons to the autonomic and somatic consequences of withdrawal from morphine in the rat

In this study, we monitored Fos-like immunoreactivity in the sacral spinal cord to identify neurons that are likely to contribute to the autonomic manifestations of opioid antagonist-precipitated withdrawal in morphine-tolerant rats. Injection of systemic antagonist increased the Fos-like immunoreactivity throughout the first sacral segment, particularly in laminae I/II, X, and in the sacral parasympathetic nucleus (SPN). Selective peripheral withdrawal, with a hydrophilic antagonist that does not cross the blood-brain barrier (BBB), induced diarrhea, but no other withdrawal signs were evident. Compared to rats that withdrew systemically, peripherally withdrawal evoked significantly less Fos-like immunoreactivity in laminae V/VI, X and the SPN. By contrast, selective spinal withdrawal, by intrathecal injection of an opioid antagonist that does not cross the BBB, provoked hyperactivity of the hindlimbs and tail, but no diarrhea. These animals demonstrated significantly increased Fos-like immunoreactivity in laminae I/II, V/VI, the SPN, and the ventral horn compared to rats that withdrew systemically. Animals treated neonatally with capsaicin, to eliminate C-fiber input, demonstrated withdrawal behavior similar to intact withdrawing rats, except that the capsaicin-pretreated rats had significantly greater weight loss. However, this group had less Fos-like immunoreactivity in laminae V/VI, X and SPN compared to the intact withdrawing rats. These data suggest that withdrawal from morphine evokes hyperactivity of sacral neurons, particularly those involved in regions that process nociceptive and autonomic information. Peripheral withdrawal is sufficient to induce diarrhea, but it does not fully explain the associated weight loss. Unmyelinated primary afferents may contribute a tonic peripheral inhibition of circuits that regulate gut motility and intestinal fluid transport. Taken together, these data suggest that chronic exposure to opioids induces a latent sensitization in sacral cord neurons that can be manifested as neuronal hyperactivity during withdrawal; this mechanism may underlie withdrawal-induced hyperalgesia and gut hypermotility.

Endogenous opioids acting at a medullary mu-opioid receptor contribute to the behavioral antinociception produced by GABA antagonism in the midbrain periaqueductal gray

This study examined the contribution of endogenous opioids to the antinociception produced by microinjection of the GABAA receptor antagonist, bicuculline, into the rat midbrain ventrolateral periaqueductal gray region. Microinjection of bicuculline (40 ng/0.4 microliter) into the periaqueductal gray produced robust antinociception as measured by the tail-flick latency to noxious heat. This antinociception was partially reversed by intravenous administration of the non-selective opioid antagonist naloxone hydrochloride (1 and 5 mg/kg), indicating that endogenous opioid release is necessary for this effect. To determine whether opioid release in the rostral ventromedial medulla, a major projection target of the periaqueductal gray, contributes to this effect, we microinjected another opioid antagonist, naltrexone, into the rostral ventromedial medulla. Naltrexone in the rostral ventromedial medulla (5 and 10 micrograms/microliter) significantly attenuated bicuculline antinociception elicited from the periaqueductal gray. Cys2, tyr3, orn5, pen7-amide (26.5 nmol), a selective mu-opioid receptor antagonist, also reversed the antinociception when microinjected into the rostral ventromedial medulla. Microinjections of naltrexone (10 micrograms/microliter) or cys2, tyr3, orn5, pen7-amide at sites in the medulla dorsal to the rostral ventromedial medulla were ineffective. None of the antagonists altered baseline tail-flick latencies. These results support the hypothesis that a population of periaqueductal gray neurons produces antinociception through a mu-opioid receptor-mediated action of endogenous opioids in the rostral ventromedial medulla. Thus, two opioid-sensitive pain-modulating brainstem sites are linked by an endogenous opioid synapse in the rostral ventromedial medulla.

Mechanisms of chronic pain

Chronic pain differs from acute pain in that it serves no useful function, causes suffering, limits activities of daily living, and increases costs of healthcare payments, disability, and litigation fees. Pain perception begins with activation of peripheral nociceptors and conduction through myelinated A delta and unmyelinated C fibers to the dorsal root ganglion. From here, signals travel via the spinothalamic tract to the thalamus and the somatosensory cortex. Modulation of sensory input (i.e., pain) occurs at many levels. Nociceptors are also neuroeffectors, and transmission can be modulated by their cell bodies, which secrete inflammatory mediators, neuropeptides, or other pain-producing substances. Descending pathways from the hypothalamus, which has opioid-sensitive receptors and is stimulated by arousal and emotional stress, can transmit signals to the dorsal horn that modulate ascending nociceptive transmissions. Modulation to alter the perception of pain also can occur at higher centers (e.g., frontal cortex, midbrain, medulla) by opioids, anti-inflammatory agents, as well as antagonists and agonists of neurotransmitters. This article will review our current knowledge of the mechanisms involved in (1) the transduction of tissue injury or disease signals (nociception and nociceptive receptors); (2) the transmission of signals rostrally to the thalamus and higher nervous system centers (involving perception of the quality, location, and intensity of noxious signals); and (3) the modulation of ascending sensory messages at all levels (periphery, spinal cord, and higher centers).

Spinal cord mechanisms of opioid tolerance and dependence: Fos-like immunoreactivity increases in subpopulations of spinal cord neurons during withdrawal [corrected]

Tolerance to the analgesic effects of morphine results in part from the development of a compensatory response in neurons that express the opioid receptor or of neural circuits in which those neurons participate. According to this formulation, withdrawal of morphine results in an overshoot of several neuronal properties because of the unopposed action of the compensatory response system. To identify the population of spinal cord neurons that underlies this state, we monitored expression of Fos-like immunoreactivity, after naltrexone-precipitated abstinence in normal and morphine-tolerant rats. After daily (five days) implantation of morphine or placebo pellets, the rats received an injection of saline or naltrexone and behavior was monitored for 1 h. The rats were then killed, their spinal cords removed and 50-microns transverse sections of the lumbar cord were immunostained with a rabbit polyclonal antiserum directed against Fos. Naltrexone injection in the placebo group did not increase spinal cord Fos expression. Naltrexone-precipitated abstinence resulted in an increase in Fos expression at all levels of the spinal cord; the greatest increase and densest staining was in laminae I through VI. Importantly, when withdrawal was precipitated in anesthetized rats, we recorded a significant reduction in Fos expression, particularly in laminae III through VI, but there was persistent expression in the superficial dorsal horn, particularly in lamina I. These results suggest that spinal cord nociresponsive neurons are sensitized during the development of tolerance. This sensitization is unmasked by the administration of naltrexone and is manifested by fos induction in laminae I/II in awake or anesthetized withdrawing animals. The underlying mechanisms of tolerance development may be similar to those that underlie injury-induced central sensitization and hyperalgesia.

Mechanisms of hyperalgesia and morphine tolerance: a current view of their possible interactions

Over the last several years, compelling evidence has accumulated indicating that central hyperactive states resulting from neuronal plastic changes within the spinal cord play a critical role in hyperalgesia associated with nerve injury and inflammation. Such neuronal plastic changes may involve activation of central nervous system excitatory amino acid (EAA) receptors, subsequent intracellular cascades including protein kinase C translocation and activation as well as nitric oxide production, leading to the functional modulation of receptor-ion channel complexes. Similar EAA receptor-mediated cellular and intracellular mechanisms have now been implicated in the development of tolerance to the analgesic effects of morphine, and a site of action involved in both hyperalgesia and morphine tolerance is likely to be in the superficial laminae of the spinal cord dorsal horn. These observations suggest that hyperalgesia and morphine tolerance, two seemingly unrelated phenomena, may be interrelated by common neural substrates that interact at the level of EAA receptor activation and related intracellular events. This view is supported by recent observations showing that thermal hyperalgesia develops when animals are made tolerant to morphine antinociception and that both hyperalgesia and reduction of the antinociceptive effects of morphine occur as a consequence of peripheral nerve injury. The demonstration of interrelationships between neural mechanisms underlying hyperalgesia and morphine tolerance may lead to a better understanding of the neurobiology of these two phenomena in particular and pain in general. This knowledge may also provide a scientific basis for improved pain management with opiate analgesics.

N-methyl-D-aspartate receptors are implicated in hyperresponsiveness following naloxone reversal of alfentanil in isolated rat spinal cord

In isolated neonatal rat spinal cord, naloxone administered after an opioid increases a nociceptive-related slow ventral root potential (sVRP) to levels above pre-drug controls. We studied the role of N-methyl-D-aspartate (NMDA) receptors in this phenomenon, which may be related to acute tolerance and to hyperalgesia on antagonist-precipitated withdrawal. Naloxone (200 nM) alone produced no significant effect on sVRP area, while naloxone (560 nM) increased area to 121 +/- 17.7% of control (mean +/- SD). Following 200 nM alfentanil, naloxone (200 nM) was associated with a significant rebound in sVRP area to 138 +/- 18.0% of pre-drug control. Hyperresponsiveness developed within 7 min of initial alfentanil exposure. The non-competitive NMDA antagonist MK-801 (20 nM) had no effect on sVRP area when applied alone; higher concentrations produced irreversible depression. MK-801 (20 nM) co-applied with 200 nM alfentanil blocked the rebound increase in sVRP area following naloxone 200 nM and also the increase following naloxone alone (560 nM). The results suggest that alfentanil induces a rapid NMDA receptor-dependent change in spinal cord neuronal excitability.

Medullary on-cell activity during tail-flick inhibition produced by heterotopic noxious stimulation

Reflex responses and neuronal excitation elicited by noxious stimuli applied to a given body site can be inhibited by application of noxious stimulation to another, even distant body region. Such heterotopic noxious stimulation (HNS) has been proposed to act via 'diffuse noxious inhibitory controls' (DNIC) which involve supraspinal components. The so-called on-cells of the rostral ventromedial medulla (RVM) in rats are thought to facilitate nociceptive transmission. Experimental manipulations that inhibit on-cells also inhibit withdrawal reflexes and nociceptive thalamic responses. In the present study on-cell activity was recorded in relation to the tail flick (TF) elicited by noxious heat applied to the tail both before and during either immersion of a paw in water above 56 degrees C or application of strong pinch to various body regions. Such HNS elicited strong activation of on-cells, followed by depression even when HNS continued. When this depression was intense, tail-heating failed to elicit vigorous on-cell firing, and TF was retarded or abolished. These results are compatible with the hypothesis that antinociception elicited by HNS involves depression of on-cell firing and hence lack of facilitation of nociceptive transmission.

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.

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.

Neural substrates of opiate withdrawal

Drug withdrawal is an integral part of most types of dependence and, to a large extent, opiate withdrawal has been considered the prototypic, classic measure of opiate dependence. The opiate withdrawal syndrome is characterized by multiple behavioral and physiological signs such as behavioral activation, ptosis, diarrhea, 'wet dog' shakes and motivational dysfunction, which may be represented in the CNS at multiple sites. It seems that the activating effects associated with the opiate withdrawal syndrome may be mediated by the nucleus locus coeruleus. Other signs such as wet dog shakes may involve sites in the hypothalamus important for temperature regulation. Certain other signs such as diarrhea and lacrimation may be dependent on peripheral opiate receptors. The motivational aspects of opiate withdrawal as demonstrated by the aversive stimulus effects or negative reinforcing effects (e.g. disrupted lever-pressing for food and place aversions) may involve those elements of the nucleus accumbens that are known to be important for the acute reinforcing effects of opiates in nondependent rats. Evidence exists at the cellular and molecular level for both 'within-system' and 'between-system' adaptations to dependence. Elucidation of the neural networks, cellular mechanisms and molecular elements involved in opiate withdrawal may provide not only a model for our understanding of the adaptive processes associated with drug dependence but also of those associated with other chronic insults to CNS function.

Endogenous opiates: 1990

This paper, an examination of works published during 1990, is thirteenth in a series of our annual reviews of the research involving the behavioral, nonanalgesic, effects of the endogenous opiate peptides. The specific topics this year include stress; tolerance and dependence, eating; drinking; gastrointestinal, renal, and hepatic functions; mental illness; learning, memory, and reward; cardiovascular responses; respiration and thermoregulation; seizures and other neurological disorders; electrical-related activity; locomotor activity; sex, pregnancy, development, and aging; immunological responses; and other behavior.

Spatial and temporal variation of microstimulation thresholds for inhibiting the tail-flick reflex from the rat's rostral medial medulla

Suppression of the tail-flick reflex by microstimulation of the rostral medial medulla in rats lightly anesthetized with barbiturates was studied with regard to spatial and temporal variations in electrical threshold. Trains of constant-current pulses with linearly descending amplitudes (called 'ramps') were passed through the extracellular brain microelectrode during noxious heating of the tail. The pulse amplitude at the time of the reflex, after allowance for conduction and reaction latencies, was taken as the threshold reading. This new method revealed a range of vertical electrode positions corresponding roughly to the nucleus raphe magnus, where the thresholds tended to be lowest (a mean of 4.1 microA for 0.4-ms pulses delivered at 50 Hz). In confirmation of the technique's validity neither the duration of the ramp nor its starting amplitude, within their useful range, significantly affected the measured threshold. Pronounced temporal fluctuation was seen in thresholds measured every 2 min. Spatial variability within the low-threshold region and differences between preparations were statistically much smaller sources of variation. The temporal fluctuation appeared to have a stationary mean for at least 20 min under constant conditions of anesthesia. In some experiments, action potentials from single neurons were recorded through the stimulating electrode, and classified into those inhibited during the tail-flick (off-cells), those excited (on-cells), and those unaffected (neutral cells). The thresholds where off-cells exhibited their maximum action potential were on average significantly lower than corresponding thresholds for on-cells. Short-range (less than 0.2 mm) spatial variations in the threshold appeared however to be uncorrelated with the distance to an individual recorded off-cell or on-cell.(ABSTRACT TRUNCATED AT 250 WORDS)

Somatodendritic morphology of on- and off-cells in the rostral ventromedial medulla

The rostral ventromedial medulla (RVM) contains two classes of physiologically defined neurons, on-cells and off-cells, that are implicated in nociceptive modulation. In a continuing effort to detail the neural circuitry that underlies the activity of these two distinct neuronal types, the somatodendritic morphology of on- and off-cells was studied in the cat, rat, and ferret. In lightly anesthetized animals, on-cells increased and off-cells decreased their discharge rate during a withdrawal reflex evoked by noxious stimuli. Following their physiological characterization by using intracellular recording, on- and off-cells were injected with either horseradish peroxidase or biocytin and their somatodendritic arborizations were examined. Labeled on- and off-cells included fusiform and stellate cells of all sizes as well as large multipolar neurons. Although the somatic shape of both on- and off-cells in RVM was heterogeneous, off-cells tended to be fusiform neurons whose long axis was oriented mediolaterally. The dendritic domains of both on- and off-cells extended bilaterally past the lateral edge of the trapezoid body or pyramid and ventrally to, and sometimes including, the trapezoid body or pyramid. In contrast to their extensive mediolateral spread, the dendritic domains of both cell types were limited to the ventral half of the reticular formation and were compressed along the rostrocaudal axis. The dendritic arbor of individual on- and off-cells extended well beyond the cytoarchitectonic boundaries of any single nuclear region, within the domain delineated as the RVM. The spatial domains of the dendritic arbors of on- and off-cells are further evidence that the on- and off-cells throughout the RVM constitute an integrated unit in the modulation of nociceptive transmission.