Rostral Ventromedial Medulla Neurons Research Articles

The density and distribution of serotonergic appositions onto identified neurons in the rat rostral ventromedial medulla

Neurons in the rostral ventromedial medulla (RVM) contribute to the modulation of nociceptive transmission and to the analgesic effects of opioids. The RVM contains serotonergic terminal arbors, serotonergic neurons and several types of serotonin (5-HT) receptors. Limited evidence suggests that 5-HT acting within RVM decreases nociceptive responsiveness and contributes to opioid analgesia. The present study examines the density and distribution of serotonergic afferents onto physiologically identified neurons in the RVM. In anesthetized rats, RVM neurons were characterized by their response to noxious stimulation as either on (excited), off (inhibited) or neutral (unaffected) cells. Tissue containing intracellularly labeled RVM neurons was processed for 5-HT immunocytochemistry. Five off, five on, and three serotonergic neutral cells were examined with the confocal microscope for appositions between 5-HT immunoreactive (5-HT-IR) processes and intracellularly labeled processes. Serotonergic neutral cells had the highest density of 5-HT-IR appositions. The density of 5-HT-IR appositions onto off cells was slightly lower. On cells demonstrated the lowest density of 5-HT-IR appositions. These results indicate that 5-HT contributes to nociceptive modulation by direct actions on the activity of RVM cells. Because the RVM has several sources of serotonergic input and a number of different 5-HT receptor subtypes, further understanding of the role of RVM 5-HT afferents will require pharmacological studies to determine the action of 5-HT on each cell class and anatomical studies to determine the brainstem origin of serotonergic input to each cell class.

A correlogram analysis of the activity in the rostral ventromedial medulla of awake rats and in rats anesthetized with ketamine or pentobarbital following the administration of morphine

The spiking activity of single units and their corresponding mass activity in the rostral ventromedial medulla (RVM) was recorded simultaneously in rats that were either awake or anesthetized with pentobarbital or ketamine. Autocorrelograms and cross-correlograms were calculated to illustrate the distribution of interspike intervals under each of these conditions. The spontaneous activity in both anesthetized groups, but not the awake group, displayed a low incidence for short intervals (about 1-36 ms) between spikes. However, this low incidence of short interspike intervals was found in some awake rats following the administration of subanesthetic dosages of ketamine, pentobarbital, or morphine. The occurrences, or lack of occurrences of these short interspike intervals may be a marker for the anesthetized state of the rat. A basal spiking pattern, as illustrated by three different types of correlograms, was observed, and these basal patterns did not usually change after the administration of morphine to awake or anesthetized rats, even though the cells became excited or inhibited. Cross-correlograms revealed that the synapses between "on" and "off" cells might sometimes be inhibitory, while at other times they might be excitatory; however, cells of the same class mainly shared excitatory connections. In summary, ketamine and pentobarbital do alter the firing patterns of individual neurons in the RVM, but, no matter what state the rat is in, the activity of individual RVM neurons is strongly related to the activity of other neurons in the local network.

Pronounced changes in the activity of nociceptive modulatory neurons in the rostral ventromedial medulla in response to prolonged thermal noxious stimuli.

1. Brain regions that inhibit nociception can be activated by various environmental stimuli, including prolonged noxious stimuli. The present study tested the effect of such a prolonged noxious stimulus on the activity of nociceptive modulatory neurons in the rostral ventromedial medulla (RVM). These neurons, called ON- and OFF-cells because of their respective burst and pause in activity associated with nocifensor reflexes, have been shown to facilitate and inhibit nociception, respectively. 2. Single-unit activity of ON- and OFF-cells was assessed in lightly halothane- or barbiturate-anesthetized rats exposed to prolonged noxious heat (50 degrees C water). This prolonged noxious stimulus caused an increase in ON-cell and a decrease in OFF-cell activity regardless of anesthetic (halothane or barbiturate) or stimulus location (hindpaw or tail). 3. Surprisingly, and despite the consistent changes in RVM cell activity, the prolonged noxious stimulus caused different effects depending on the reflex used to assess nociception. The hindpaw withdrawal reflex was facilitated when the tail was immersed in hot water, whereas the tail flick reflex was inhibited when the hindpaw was immersed in hot water (see preceding manuscript). Lidocaine inactivation of the RVM shortened the latency for both reflexes but had no effect on tail flick inhibition produced by the noxious conditioning stimulus. In contrast, lidocaine inactivation of the RVM completely reversed the hindpaw reflex facilitation produced by tail heat, indicating the involvement of RVM ON-cells in facilitation of this reflex. 4. These data demonstrate that RVM neurons respond in a consistent manner to noxious stimuli whether applied for a brief or prolonged time: ON-cell activity increases and OFF-cell activity decreases. Moreover, the activation of RVM ON-cells produced by a noxious stimulus is sufficient to enhance some nocifensor reflexes, whereas neural structures other than the RVM appear to mediate the antinociceptive effects produced by a prolonged noxious stimulus.

Serotonin immunoreactivity is contained in one physiological cell class in the rat rostral ventromedial medulla

Neurons in the rostral ventromedial medulla (RVM) are the major source of serotonergic projections to the dorsal horn. A large body of evidence implicates RVM serotonergic neurons in the modulation of spinal nociceptive transmission. Three physiological classes of RVM neurons, on, off, and neutral cells, are postulated to have different nociceptive modulatory effects on spinal nocifensor reflexes. This study was undertaken to determine which RVM cell class(es) contains 5-HT. In anesthetized rats, RVM neurons were identified by their responses to noxious cutaneous stimuli, intracellularly labeled, and processed for 5-HT immunocytochemistry. Labeled neurons were examined with epifluorescence and imaged using a confocal laser microscope. A total of 25 RVM neurons were intracellularly labeled. No off (n = 9) or on (n = 8) cells were serotonergic. Half of the neutral cells (4 of 8) demonstrated 5-HT immunoreactivity. These results call for a reevaluation of the mechanisms of RVM modulatory influence on spinal cord nociceptive transmission. The finding that some neutral cells are serotonergic strongly suggests that serotonergic neutral cells are involved in the modulation of spinal nociceptive transmission. Additionally, inhibition of spinal nociceptive transmission by off cells is unlikely to involve 5-HT release. Finally, since opioid administration does not alter the firing of RVM neutral cells, the results of the present study indicate that serotonergic RVM neurons do not directly mediate the effects of supraspinal opioids in the rat.

Activity of nociceptive modulatory neurons in the rostral ventromedial medulla associated with volume expansion-induced antinociception

A wide range of environmental stimuli have been shown to induce antinociception. Investigation of the brain regions contributing to environmentally induced antinociception (EIA) has focused primarily on the effect of disrupting neural transmission at various CNS sites. In contrast, the present study analyzed changes in the activity of nociceptive modulatory neurons in the rostral ventromedial medulla (RVM) following induction of antinociception by a physiological stimulus, volume expansion. Previous research indicates that 2 classes of RVM neuron, the on- and off-cells, facilitate and inhibit nociception, respectively. In the present study, volume expansion induced by intravenous administration of Ficoll inhibited the tail-flick reflex in 17 of 21 rats and simultaneously increased the firing rate of off-cells and decreased that of on-cells. Changes in cell activity occurred only in those rats in which antinociception was produced. Lidocaine inactivation of the RVM did not block antinociception produced by Ficoll infusion. These findings demonstrate that, although antinociception produced by physiological activation of vagal afferents is associated with an increase in off-cell and a decrease in on-cell activity, these changes are not required for volume expansion-induced antinociception.

A confocal laser microscopic study of enkephalin-immunoreactive appositions onto physiologically identified neurons in the rostral ventromedial medulla

Neurons in the rostral ventromedial medulla (RVM) are important in the opioid modulation of dorsal horn nociceptive transmission. Systemically administered morphine inhibits one class of RVM cells, the on-cells; excites a second class of RVM cells, the off-cells; and has no effect on a third class, neutral cells. In contrast, iontophoretic application of morphine inhibits on-cells but does not alter the activity of either off- or neutral cells. The present study addresses whether the differential sensitivity to exogenous opioids is correlated with a differential termination pattern onto the three classes of RVM neurons by afferents containing endogenous opioids. Intracellular recordings were made from RVM neurons in rats under light halothane anesthesia. Physiologically characterized neurons were injected with Neurobiotin and then subsequently visualized with a Texas red fluorophore. Thick (50 microns) sections containing labeled RVM cells were processed for enkephalin immunoreactivity (ENK-IR) using an FITC fluorophore and then optically sectioned at 1.5 micron intervals using a dual-channel confocal laser scanning microscope. ENK-IR appositions were found on the somata and dendrites of all on-cells. Although ENK-IR varicosities were also apparently apposed to off- and neutral cells, the density of such appositions was significantly less than the density of ENK-IR appositions onto on-cells. The greater overall density of ENK-IR appositions onto on-cells was apparently due to a concentration of appositions on the soma and proximal dendrites of these neurons. These results support a model of RVM function in which endogenous opioid peptides produce an antinociceptive action by a direct inhibitory action on on-cells that facilitate nociceptive transmission. This on-cell inhibition may produce an additional antinociceptive effect by removing a possible on-cell inhibition of off-cells, which are thought to inhibit nociceptive transmission.

Activity of neurons in the rostral medulla of the halothane-anesthetized rat during withdrawal from noxious heat

The physiological and pharmacological properties of two classes of putative nociceptive modulatory neurons have been extensively characterized in the rostral ventromedial medulla (RVM) of the barbiturate-anesthetized rat. ‘On-cells’ show a burst of activity, and ‘off-cells’ a sudden pause immediately preceding the occurence of nocifensor reflexes. In the present study, we have characterized the reflex-related activity of RVM neurons in halothane-anesthetized rats to determine whether the properties of these neurons are dependent on barbiturate anesthesia. Both on- and off-cells were identified in this preparation. Repeated noxious stimulation was associated with a high level of ongoing activity in on-cells, and a low level in off-cells. These data thus demonstrate that the previously described reflex-related changes in RVM neuron activity are not specific to barbiturate-anesthetized preparations, and that a failure to demonstrate off-cells in some studies may result from these neurons being inactive following repeated testing with noxious stimuli.

Circuitry linking opioid-sensitive nociceptive modulatory systems in periaqueductal gray and spinal cord with rostral ventromedial medulla

The interactions among opioid-sensitive nociceptive modulatory systems, which include the midbrain periaqueductal gray, rostral ventromedial medulla and spinal cord, are likely to play a central role in the potent antinociception that results when morphine is administered systemically. The aim of the present study was to investigate the mechanisms through which local application of morphine, either in the periaqueductal gray or at the lumbar spinal cord in the rat, influences the activity of one population of putative nociceptive modulatory neurons in rostral ventromedial medulla, i.e. “on-cells”. Previous studies have shown that the spontaneous and tail-flick-related firing of on-cells is invariably depressed when morphine is given systemically in doses demonstrated to inhibit the tail-flick reflex, and that a similar depression of this activity is produced when morphine is applied directly in the periaqueductal gray or intrathecal space. In the present experiments, on-cells were activated pharmacologically using iontophoretically applied glutamate to provide an indication of whether morphine-induced suppression of on-cell firing reflected a postsynaptic inhibition or a disfacilitation resulting from blockade of an excitatory input to the on-cell. Microinjection of morphine into the periaqueductal gray blocked glutamate-evoked activity of on-cells in parallel with its suppression of the tail-flick reflex, suggesting activation of an inhibitory input to these cells. No change in glutamate-evoked activity occurred in rats in which morphine did not produce antinociception. Intrathecal administration of morphine did not alter the glutamate-evoked activity of these neurons despite blocking the tail-flick reflex, suggesting that morphine acting in the spinal cord removes an excitatory input to on-cells. Thus, in addition to possible direct effects of morphine on rostral ventromedial medulla on-cells, morphine administered systemically will simultaneously engage inhibitory influences from the periaqueductal gray and remove facilitating inputs from the spinal cord. Each of these mechanisms is likely to contribute to the potent antinociception that results when morphine is administered systemically.

Identification of cardiovascular neurons in the rostral ventromedial medulla in anesthetized rats.

Recent studies have identified a region in the rostral ventromedial medulla (RVMM) of rats that appears to be involved in cardiovascular function. Since these studies used either microinjection of lidocaine or electrical stimulation, the exact contribution of intrinsic neurons as opposed to fibers of passage could not be determined. The present study was performed to map the location of neurons in RVMM from which changes in mean arterial pressure could be elicited by the microinjection of the excitatory amino acid analogue N-methyl-D-aspartic acid (NMDA) (20 ng/50 nl), which selectively activates cell bodies in barbiturate-anesthetized rats. Microinjection of NMDA into RVMM most often (53%) elicited pressor responses (31 +/- 7 mm Hg). On the basis of these responses, RVMM was determined to encompass a large portion of the nucleus gigantocellularis 0.5-1.5 mm lateral to the midline, 0.5-3.5 mm above the ventral surface, and extending from the rostral to the caudal pole of the facial nucleus. Depressor responses (-21 +/- 3 mm Hg) were found at all levels of RVMM but were most concentrated and of the largest magnitude in the rostral and caudal poles of RVMM. Microinjection of the inhibitory neurotransmitter glycine (500 mM) was used to determine whether neurons in RVMM were contributing to the maintenance of arterial pressure. Microinjection of glycine decreased arterial pressure (-15 +/- 2 mm Hg) throughout most of RVMM. Unexpectedly, increases in mean arterial pressure (24 +/- 3 mm Hg) were elicited by microinjection of glycine into the same region in RVMM in which NMDA most frequently elicited pressor responses.(ABSTRACT TRUNCATED AT 250 WORDS)

Hyperalgesia during acute opioid abstinence: evidence for a nociceptive facilitating function of the rostral ventromedial medulla

Naloxone-precipitated opioid abstinence is associated with enhancement of reflex responses to noxious stimulation (hyperalgesia). The present experiments in lightly anesthetized rats were designed to determine (1) whether neurons in the rostral ventromedial medulla (RVM) contribute to this enhancement, and (2) whether this enhancement is due to removal of an inhibitory modulatory influence or to activation of a facilitatory influence. In the first experiment, 10 micrograms of morphine was microinjected into the RVM; subsequent administration of naloxone (1 mg/kg, i.v.) shortened tail-flick latency. This is evidence that a synaptic action of opioids within the RVM can contribute to hyperalgesia. In the second experiment, systemic administration of morphine (2 mg/kg, i.v.) was followed by systemic administration of naloxone (1 mg/kg, i.v.), which produced a significant hyperalgesia that could be markedly attenuated by microinjection of 10 micrograms lidocaine into the RVM. That inactivation of RVM reduces the hyperalgesia indicates that the CNS is capable of generating a facilitating action on nociceptive transmission. Previous studies from this laboratory have indicated that a population of RVM neurons, on-cells, shows increased activity during opioid abstinence. The present experiments support the hypothesis that RVM on-cells exert a facilitating influence on nociceptive transmission.

Evidence for GABA-Mediated Control of Putative Nociceptive Modulating Neurons in the Rostral Ventromedial Medulla: Iontophoresis of Bicuculline Eliminates the Off-Cell Pause

This study was undertaken to test the hypothesis that gamma-aminobutyric acid (GABA) is an endogeneous neurotransmitter regulating the activity of a class of putative nociceptive modulatory neurons (termed "off-cells") in the rostral ventromedial medulla (RVM) of the barbiturate-anesthetized rat. Off-cells, which are believed to correspond to the RVM output neuron that inhibits nociceptive processing at the level of the spinal cord, exhibit an abrupt pause in firing that begins immediately prior to the occurrence of the tail flick response (TF), a nocifensive reflex evoked by application of noxious heat to the tail. Single-unit recording and iontophoretic techniques were used to examine the ability of the GABAA receptor antagonist bicuculline methiodide (BIC) to antagonize selectively the characteristic off-cell pause. Iontophoretic application of BIC (5-30 nA) blocked the TF-related pause in each of the off-cells tested. This effect of BIC was generally slow in onset, and outlasted the period of application by several minutes. BIC iontophoresis also eliminated the cyclic alternation between active and silent periods that is often displayed by off-cells in lightly anesthetized rats. BIC application did not have a consistent effect on the firing of two other classes of RVM neurons ("on-cells" and "neutral cells"). Iontophoretically applied BIC antagonized the inhibitory effect of iontophoretically applied GABA, but not that produced by glycine. The glycine receptor antagonist strychnine did not mimic the action of BIC on off-cell activity. These data demonstrate antagonism of a synaptically evoked response using iontophoretic application of BIC, and provide strong evidence that the inhibitory neurotransmitter GABA mediates the TF-related off-cell pause. Taken together with behavioral experiments demonstrating that a GABA-mediated inhibitory process within RVM is crucial in permitting execution of the TF response, the present observations point to the significant functional relevance of GABA transmission within RVM in modulation of nociception.

Activity of most rostral ventromedial medulla neurons reflect EEG/EMG pattern changes.

Studies in lightly anesthetized animals have demonstrated neurons in the rostral ventromedial medulla (RVM) that respond to specific modalities of peripheral stimulation. We initiated this study to investigate putative thermoafferent processing in such RVM neurons. Our results indicate that most RVM neuronal responses are not specific for the applied stimulus but reflect changes in cortical activation. Electroencephalogram/electromyogram (EEG/EMG) and RVM single unit activity was recorded in lightly urethan-anesthetized rats. Five distinct EEG/EMG patterns were observed. Their expression was a function of skin and core temperatures. Pinches and strong thermal manipulations caused a desynchronization of a synchronized EEG pattern and an increase in EMG activity. The activity of 38 of the 51 RVM neurons sampled correlated with changes in EEG/EMG activity. Thirteen neurons were not affected by changes in EEG/EMG activity. One unit responded to temperature manipulations within an EEG/EMG state, suggesting that a small portion of RVM neurons may transmit thermoafferent information.

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.

Opposing actions of norepinephrine and clonidine on single pain-modulating neurons in rostral ventromedial medulla

Opposing actions of norepinephrine and clonidine on single pain-modulating neurons in rostral ventromedial medulla

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. Systematic 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 cell 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 decreased 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. They add to the existing body of evidence implicating both on-and off-cells in modulation of nociceptive transmission and nocifensive reflexes at spinal levels and indicate that these neurons contribute to opiate-induced analgesia.

Arresting but misleading phrases.

This paper discusses some common misconceptions of what a utilitarian approach to medical ethics is and of the conclusions it forces upon those disposed to accept such an approach. It suggests that broad and unargued characterisations of approaches to moral questions as 'utilitarian', 'Hippocratic' or whatever are likely to be misleading and counterproductive. What matters is not what to call the position that people feel inclined to accept but rather what arguments there are in its favour and what arguments there are against it.