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

Bilateral behavioral and regional cerebral blood flow changes during painful peripheral mononeuropathy in the rat

A unilateral chronic constriction injury (CCI) of the sciatic nerve produced bilateral effects in both pain related behaviors and in the pattern of forebrain activation. All CCI animals exhibited spontaneous pain-related behaviors as well as bilateral hyperalgesia and allodynia after CCI. Further, we identified changes in baseline (unstimulated) forebrain activation patterns 2 weeks following CCI by measuring regional cerebral blood flow (rCBF). Compared to controls, CCI consistently produced detectable, well-localized and typically bilateral increases in rCBF within multiple forebrain structures in unstimulated animals. For example, the hindlimb region of somatosensory cortex was significantly activated (22%) as well as multiple thalamc nuclei, including the ventral medial (8%), ventral posterior lateral (10%) and the posterior (9%) nuclear groups. In addition, several forebrain regions considered to be part of the limbic system showed pain-induced changes in rCBF, including the anterior dorsal nucleus of the thalamus (23%), cingulate cortex (18%), retrosplenial cortex (30%), habenular complex (53%), interpeduncular nucleus (45%) and the paraventricular nucleus of the hypothalamus (30%). Our results suggest that bilateral somatosensory and limbic forebrain structures participate in the neural mechanisms of prolonged persistent pain produced by a unilateral injury.

Does intravenous administration of GABA(A) receptor antagonists induce both descending antinociception and touch-evoked allodynia?

Effects of intravenous administration of picrotoxin (PTX), a GABA(A) receptor antagonist, upon activities of wide dynamic range (WDR) neurons in the lumbar spinal cord were studied in urethane-chloralose anesthetized cats. Intravenous PTX augmented tactile evoked responses of WDR neurons, but reduced nociceptive responses dose-dependently. Spinal transection reversed the suppression of nociceptive responses. In the spinal cat, intravenous PTX enhanced the tactile evoked response. Intravenous PTX enhanced the spontaneous firing of nucleus raphe dorsalis (NRD) and/or ventral periaqueductal gray (PAG) neurons projecting to nucleus raphe magnus. Lidocaine injected into NRD/PAG reversed the antinociceptive action of intravenous PTX. PTX injected into NRD/PAG reduced heat-evoked responses of WDR units. These data suggest that antinociceptive effects of intravenous PTX is primarily due to disinhibitory activation of the descending antinociceptive system originating from NRD and PAG, and that PTX reinforces touch-evoked responses in the spinal cord.

Regional changes in forebrain activation during the early and late phase of formalin nociception: analysis using cerebral blood flow in the rat

This is the first neural imaging study to use regional cerebral blood flow (rCBF) in an animal model to identify the patterns of forebrain nociceptive processing that occur during the early and late phase of the formalin test. We measured normalized rCBF increases by an autoradiographic method using the radiotracer [99mTc]exametazime. Noxious formalin consistently produced detectable, well-localized and typically bilateral increases in rCBF within multiple forebrain structures, as well as the interpeduncular nucleus (Activation Index, AI = 66) and the midbrain periaqueductal gray (AI = 20). Structures showing pain-induced changes in rCBF included several forebrain regions considered part of the limbic system. The hindlimb region of somatosensory cortex was significantly activated (AI = 31), and blood flow increases in VPL (AI = 8.7) and the medial thalamus (AI = 9.0) exhibited a tendency to be greater in the late phase as compared to the early phase of the formalin test. The spatial pattern and intensity of activation varied as a function of the time following the noxious formalin stimulus. The results highlight the important role of the limbic forebrain in the neural mechanisms of prolonged persistent pain and provide evidence for a forebrain network for pain.

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.

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.

Differential distribution of (GABA)A receptor subunits on bulbospinal serotonergic and nonserotonergic neurons of the ventromedial medulla of the rat

Spinally projecting neurons of the ventromedial medulla (VMM) compose an important efferent pathway for the modulation of nociception. These neurons receive a substantial gamma-aminobutyric acid (GABA)-ergic input, but the GABA receptor that mediates this input is unknown. This study examined the distribution of GABA(A) receptor alpha1 and alpha3 subunits in serotonergic and nonserotonergic neurons of the VMM that project to the dorsal horn in the rat. A pledget of Gelfoam soaked in Fluoro-Gold was placed at the thoracolumbar junction of the spinal cord to label spinally projecting neurons. Alternate sections of the medulla were then incubated with a mixture of antisera to either serotonin and the alpha1 subunit, or to serotonin and the alpha3 subunit of the GABA(A) receptor. Nearly 30% of spinally projecting neurons in the VMM were immunoreactive for the alpha1 subunit. A similar percentage of spinally projecting neurons in the VMM were immunoreactive for the alpha3 subunit, although diffuse cellular labeling combined with intense staining of processes in the neuropil precluded a rigorous semi-quantitative estimation of this population. No alpha1-subunit-immunoreactive neurons colocalized serotonin. In contrast, serotonergic neurons were immunoreactive for the alpha3 subunit. However, these double-labeled neurons were a modest percentage of the serotonergic population. A small percentage of spinally projecting serotonergic neurons was immunoreactive for the alpha3 subunit. These results suggest that significant numbers of spinally projecting serotonergic and nonserotonergic neurons of the VMM possess GABA(A) receptors that differ in their respective subunit compositions and that both classes of neurons may mediate the antinociception produced by the microinjection of GABA(A) receptor antagonists in the VMM.

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.

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.