Mary M. Heinricher

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.

Putative pain modulating neurons in the rostral ventral medulla: reflex-related activity predicts effects of morphine.

Three physiologically-defined classes of neurons are found in the rostral ventromedial medulla (RVM), a region which contributes to the antinociceptive action of opiates. The off-cell exhibits an abrupt pause just prior to the occurrence of the tail flick reflex (TF). The on-cell shows a burst of activity beginning just prior to the occurrence of the TF. Neutral cell firing does not change in relation to the TF. Systemic administration of morphine has been shown to produce a consistent increase in the activity of off-cells. In the present studies, the effects of systemically-administered morphine on spontaneous and TF-related activity of on-cells and neutral cells were examined in lightly-anesthetized rats. Measures of spontaneous activity were obtained before and after morphine (1.25-2.5 mg/kg, i.v.). On-cells exhibited an irregular cyclic rate of spontaneous discharge similar to that previously reported for off-cells. In contrast, neutral cells had a nearly constant level of spontaneous activity. After administration of morphine, spontaneous activity ceased for 8 of 8 on-cells, and heat-related activity was eliminated. Administration of naloxone resulted in a return of the periodic firing pattern and the burst associated with the TF. Seven of 8 neutral cells showed no change in firing rate and one showed a decrease rate after morphine administration. These results show that the effect of systemic opiates on an RVM neuron can be predicted based on whether a cell increases or decreases its firing just prior to the occurrence of a nocifensive reflex.(ABSTRACT TRUNCATED AT 250 WORDS)

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.

Medullary pain facilitating neurons mediate allodynia in headache-related pain†

To develop and validate a model of cutaneous allodynia triggered by dural inflammation for pain associated with headaches. To explore neural mechanisms underlying cephalic and extracephalic allodynia.

A possible neural basis for stress-induced hyperalgesia

ABSTRACT Intense stress and fear have long been known to give rise to a suppression of pain termed “stress‐induced analgesia”, mediated by brainstem pain‐modulating circuitry, including pain‐inhibiting neurons of the rostral ventromedial medulla. However, stress does not invariably suppress pain, and indeed, may exacerbate it. Although there is a growing support for the idea of “stress‐induced hyperalgesia”, the neurobiological basis for this effect remains almost entirely unknown. Using simultaneous single‐cell recording and functional analysis, we show here that stimulation of the dorsomedial nucleus of the hypothalamus, known to be a critical component of central mechanisms mediating neuroendocrine, cardiovascular and thermogenic responses to mild or “emotional” stressors such as air puff, also triggers thermal hyperalgesia by recruiting pain‐facilitating neurons, “ON‐cells”, in the rostral ventromedial medulla. Activity of identified RVM ON‐cells, OFF‐cells and NEUTRAL cells, nociceptive withdrawal thresholds, rectal temperature, and heart rate were recorded in lightly anesthetized rats. In addition to the expected increases in body temperature and heart rate, disinhibition of the DMH induced a robust activation of ON‐cells, suppression of OFF‐cell firing and behavioral hyperalgesia. Blocking ON‐cell activation prevented hyperalgesia, but did not interfere with DMH‐induced thermogenesis or tachycardia, pointing to differentiation of neural substrates for autonomic and nociceptive modulation within the RVM. These data demonstrate a top‐down activation of brainstem pain‐facilitating neurons, and suggest a possible neural circuit for stress‐induced hyperalgesia.

Nociceptive facilitating neurons in the rostral ventromedial medulla

&NA; The role of the periaqueductal gray‐rostral ventromedial medulla (RVM) system in descending inhibition of nociception has been studied for over 30 years. The neural basis for this antinociceptive action is reasonably well understood, with strong evidence that activation of a class of RVM neurons termed ‘off‐cells’ exerts a net inhibitory effect on nociception. However, it has recently become clear that this system can facilitate, as well as inhibit pain. Although the mechanisms underlying the facilitation of nociception have not been conclusively identified, indirect evidence points to activation of a class of neurons termed ‘on‐cells’ as mediating descending facilitation. Here we used focal infusion of the tridecapeptide neurotensin within the RVM in lightly anesthetized rats to activate on‐cells selectively. Neurotensin has been shown in awake animals to produce a dose‐related, bi‐directional effect on nociception when applied within the RVM, with hyperalgesia at low doses, and analgesia at higher doses. Using a combination of single cell recording and behavioral testing, we now show that on‐cells are activated selectively by low‐dose neurotensin, and that the activation of on‐cells by neurotensin results in enhanced nociceptive responding, as measured by the paw withdrawal reflex. Furthermore, higher neurotensin doses recruit off‐cells in addition to on‐cells, producing behavioral antinociception. Selective activation of on‐cells is thus sufficient to produce hyperalgesia, confirming the role of these neurons in facilitating nociception. Activation of on‐cells likely contributes to enhanced sensitivity to noxious stimulation or reduced sensitivity to analgesic drugs in a variety of conditions.

Sensitization of Pain-Modulating Neurons in the Rostral Ventromedial Medulla after Peripheral Nerve Injury

Nerve injury can lead to mechanical hypersensitivity in both humans and animal models, such that innocuous touch produces pain. Recent functional studies have demonstrated a critical role for descending pain-facilitating influences from the rostral ventromedial medulla (RVM) in neuropathic pain, but the underlying mechanisms and properties of the relevant neurons within the RVM are essentially unknown. We therefore characterized mechanical responsiveness of physiologically characterized neurons in the RVM after spinal nerve ligation, a model of neuropathic pain that produces robust mechanical hyperalgesia and allodynia. RVM neurons were studied 7–14 d after spinal nerve ligation, and classified as “on-cells,” “off-cells,” or “neutral cells” using standard criteria of changes in firing associated with heat-evoked reflexes. On-cells are known to promote nociception, and off-cells to suppress nociception, whereas the role of neutral cells in pain modulation remains an open question. Neuronal and behavioral responses to innocuous and noxious mechanical stimulation were tested using calibrated von Frey filaments (4–100 g) applied to the hindpaws ipsilateral and contralateral to the injury, and in sham-operated and unoperated control animals. On- and off-cells recorded in nerve-injured animals exhibited novel responses to innocuous mechanical stimulation, and enhanced responses to noxious mechanical stimulation. Neuronal hypersensitivity in the RVM was correlated with behavioral hypersensitivity. Neutral cells remained unresponsive to cutaneous stimulation after nerve injury. These data demonstrate that both on- and off-cells in the RVM are sensitized to innocuous and noxious mechanical stimuli after nerve injury. This sensitization likely contributes to allodynia and hyperalgesia of neuropathic pain states.

GABA-mediated inhibition in rostral ventromedial medulla: role in nociceptive modulation in the lightly anesthetized rat

&NA; Local microinjection of GABAA receptor agonists and antagonists was used to characterize the role of GABA‐mediated inhibitory processes in the nociceptive modulatory functions of the rostral ventromedial medulla (RVM) in the lightly anesthetized rat. Microinjection of selective GABAA receptor antagonists bicuculline methiodide and SR95531 produced a significant increase in tail‐flick (TF) latency. This antinociception was dose related, showed recovery and was attenuated by prior injection of the GABAA receptor agonist THIP at the same site. Microinjection of saline or the glycine receptor antagonist strychnine did not significantly affect TF latency. In contrast, administration of GABAA receptor agonists THIP and muscimol resulted in a significant decrease in TF latency. Microinjections at sites surrounding the RVM did not significantly affect TF latency. These results demonstrate that a GABA‐mediated process within the RVM is crucial in permitting execution of the TF and, presumably, other spinal nociceptive reflexes.

Brain stem neuronal circuitry underlying the antinociceptive action of opiates.

Publisher Summary This chapter discusses the brain stem neuronal circuitry underlying the antinociceptive action of opiates. Opiates are the most potent and reliable analgesic agents. They are of great clinical value because their action on pain is selective: consciousness, motor function, and sensory function other than pain are spared at the usual analgesic doses. Opiates produce analgesia by an action at highly selective receptors located at specific anatomical sites within the central nervous system. The midbrain periaqueductal gray (PAG) and the rostral ventromedial medulla (RVM) are brain stem components of a network that controls nociceptive transmission at the level of the spinal cord. Opiates injected at the brain stem elicit analgesia at doses that are orders of magnitude lower than those required to elicit an equianalgesic effect by systemic administration. Cutting the spinal dorsolateral funiculus, which contains descending projections from the RVM, reduces the effectiveness of a given dose of systemically administered morphine. Injection of the opiate antagonist naloxone into the RVM reverses the analgesic effect of systemically administered opiates. Thus, the RVM and PAG contribute to the analgesic effect of systemically administered opiates.

Microinjection of morphine into various amygdaloid nuclei differentially affects nociceptive responsiveness and RVM neuronal activity

&NA; The goal of the present study was to identify nuclei of the amygdala in which opioid‐sensitive systems can act to recruit nociceptive modulatory circuitry in the rostral ventromedial medulla (RVM) and affect nociceptive responsiveness. In lightly anesthetized rats, 10 &mgr;g of morphine was bilaterally microinjected into basolateral, cortical, medial, central, and lateral nuclei of the amygdala to determine the relative influence on the activity of identified ON, OFF and NEUTRAL cells in the RVM and on the latency of the tail flick reflex evoked by noxious radiant heat. Infusions of morphine into the basolateral nuclei resulted in a substantial, naloxone‐reversible increase in tail flick latency, and significantly increased ongoing firing of OFF cells and depressed that of ON cells. The reflex‐related changes in cell firing were also attenuated. Morphine infusions into the cortical nuclei resulted in a small (approximately 1 s) but significant increase in tail flick latency. As with basolateral microinjections, ongoing activity of the OFF cells was increased, and although the ongoing firing of ON cells was not significantly changed, the reflex‐related burst that characterizes these neurons was reduced. Microinjections in the medial nuclei again altered ongoing activity of both ON cells and OFF cells. However, the duration of the OFF cell pause and tail flick latency were unchanged. NEUTRAL cells were not affected by morphine at any site. Morphine applied within the central, medial lateral and dorsal lateral nuclei had no effect on RVM neurons or on the tail flick. Thus, focal application of morphine within the basolateral nucleus of the amygdala produced hypoalgesia and influenced RVM ON and OFF cells in a manner similar to that seen following systemic or RVM opioid administration. Opioid action within the medial and cortical nuclei also influenced RVM cell activity, but did not prevent the reflex‐related OFF cell pause, and failed to alter the tail flick substantially. These observations, plus the lack of an opioid‐activated influence from the central and lateral nuclei, demonstrate fundamental differences among systems linking the different amygdalar nuclei with the RVM. One way in which the modulatory circuitry of the RVM might be engaged physiologically in behaving animals is via opioid‐mediated activation of the basolateral nucleus.