Different endogenous analgesia systems are activated by noxious stimulation of different body regions

Involvement of opioid and GABA systems in the ventrolateral periaqueductal gray on analgesia associated with tonic immobility

Ventrolateral periaqueductal gray (VL-PAG) contains key neuronal circuits related to the analgesic effect involved in integrated defensive behaviors such as immobility response (IR). The latter is characterized by a reversible state of motor inhibition that can be elicited in rats under several conditions including restriction of movements (tonic immobility: TI). It is known that IR-induced analgesia can be elicited by manipulations or drugs acting on the central nervous system (CNS) at different levels. The aim of this study was to assess the role of the opioid and the GABA systems in TI-elicited analgesia. After inducing TI in naïve rats by neck clamping, the analgesic effect was evaluated by the tail-flick (TF) test. Compared to the control group, rats with TI had increased TF latency evidencing an analgesic effect. An opioid receptor agonist and antagonist were injected systemically, as well as microinjected locally in VL-PAG, as well as GABAA receptor agonist and antagonist were microinjected into VL-PAG. Under both injection schemes, morphine increased TF latency and TI duration, while naloxone blocked TI-induced analgesia. Muscimol reduced TF latency and TI duration while bicuculline increased TF latency but not TI duration. This suggests that TI-elicited analgesia was mediated by opioids at different levels of the CNS especially in the VL-PAG by inhibition of intrinsic tonic GABAergic activity. There were no additive analgesic effects of morphine or bicuculline with tonic immobility, which probably means reach a certain upper limit under such conditions.

Facilitation of a nociceptive flexion reflex in man by nonnoxious radiant heat produced by a laser

Electromyographic recordings were made in healthy volunteers from the knee-flexor biceps femoris muscle of the nociceptive RIII reflex elicited by electrical stimulation of the cutaneous sural nerve. The stimulus intensity was adjusted to produce a moderate pricking-pain sensation. The test responses were conditioned by a nonnoxious thermal (</=40 degrees C) stimulus applied to the receptive field of the sural nerve. This stimulus was delivered by a CO2 laser stimulator and consisted of a 100-ms pulse of heat with a beam diameter of 20 mm. Its power was 22.7 +/- 4.2 W (7.2 mJ/mm2), and it produced a sensation of warmth. The maximum surface temperature reached at the end of the period of stimulation was calculated to be 7 degrees C above the actual reference temperature of the skin (32 degrees C). The interval between the laser (conditioning) and electrical (test) stimuli was varied from 50 to 3, 000 ms in steps of 50 ms. It was found that the nociceptive flexion reflex was facilitated by the thermal stimulus; this modulation occurred with particular conditioning-test intervals, which peaked at 500 and 1,100 ms with an additional late, long-lasting phase between 1,600 and 2,300 ms. It was calculated that the conduction velocities of the cutaneous afferent fibers responsible for facilitating the RIII reflex, fell into three ranges: one corresponding to A delta fibers (3.2 m/s) and two in the C fiber range (1.3 and 0.7 m/s). It is concluded that information emanating from warm receptors and nociceptors converges. In this respect, the present data show, for the first time, that in man, conditioning nonnociceptive warm thermoreceptive A delta and C fibers results in an interaction at the spinal level with a nociceptive reflex. This interaction may constitute a useful means whereby signals add together to trigger flexion reflexes in defensive reactions and other basic motor behaviors. It also may contribute to hyperalgesia in inflammatory processes. The methodology used in this study appears to be a useful noninvasive tool for exploring the thermoalgesic mechanisms in both experimental and clinical situations.

Temporal summation of C-fiber afferent inputs: competition between facilitatory and inhibitory effects on C-fiber reflex in the rat

Long-lasting facilitations of spinal nociceptive reflexes resulting from temporal summation of nociceptive inputs have been described on many occasions in spinal, nonanesthetized rats. Because noxious inputs also trigger powerful descending inhibitory controls, we investigated this phenomenon in intact, halothane-anesthetized rats and compared our results with those obtained in other preparations. The effects of temporal summation of nociceptive inputs were found to be very much dependent on the type of preparation. Electromyographic responses elicited by single square-wave electrical shocks (2 ms, 0.16 Hz) applied within the territory of the sural nerve were recorded in the rat from the ipsilateral biceps femoris. The excitability of the C-fiber reflex recorded at 1.5 times the threshold (T) was tested after 20 s of electrical conditioning stimuli (2 ms, 1 Hz) within the sural nerve territory. During the conditioning procedure, the C-fiber reflex was facilitated (wind-up) in a stimulus-dependent fashion in intact, anesthetized animals during the application of the first seven conditioning stimuli; thereafter, the magnitude of the responses reached a plateau and then decreased. Such a wind-up phenomenon was seen only when the frequency of stimulation was 0.5 Hz or higher. In spinal, unanesthetized rats, the wind-up phenomenon occurred as a monotonic accelerating function that was obvious during the whole conditioning period. An intermediate picture was observed in the nonanesthetized rat whose brain was transected at the level of the obex, but the effects of conditioning were profoundly attenuated when such a preparation was anesthetized. In intact, anesthetized animals the reflex was inhibited in a stimulus-dependent manner during the postconditioning period. These effects were not dependent on the frequency of the conditioning stimulus. Such inhibitions were blocked completely by transection at the level of the obex, and in nonanesthetized rats were then replaced by a facilitation. A similar long-lasting facilitation was seen in nonanesthetized, spinal rats. It is concluded that, in intact rats, an inhibitory mechanism counteracts the long-lasting increase of excitability of the flexor reflex seen in spinal animals after high-intensity, repetitive stimulation of C-fibers. It is suggested that supraspinally mediated inhibitions also participate in long term changes in spinal cord excitability after noxious stimulation.

Cerebrospinal fluid and plasma beta-endorphin in combat veterans with post-traumatic stress disorder

Opioid-mediated analgesia develops in experimental animals following traumatic stress and increased opioid-mediated analgesia has been observed in combat veterans with post-traumatic stress disorder (PTSD). These observations have led to the hypothesis that increased central nervous system (CNS) opioidergic activity exists in patients with PTSD. However, direct CNS data on opioid peptide concentrations and dynamics in patients with PTSD are lacking. We withdrew cerebrospinal fluid (CSF) via a flexible, indwelling subarachnoid catheter over a 6-h period and determined hourly CSF concentrations of immunoreactive beta-endorphin (ir beta END) in 10 well-characterized combat veterans with PTSD and nine matched normal volunteers. Blood was simultaneously withdrawn to obtain plasma for ir beta END. PTSD symptom clusters, as measured by the CAPS, were correlated with neuroendocrine data. Mean CSF ir beta END was significantly greater in patients with PTSD compared with normals and there was a negative correlation between the ir beta END and PTSD intrusive and avoidant symptoms of PTSD. No intergroup difference between plasma ir beta END was found, nor was there a significant correlation between CSF and plasma ir beta END. Immunoreactive beta-lipotropin (ir beta LPH) and pro-opiomelanocortin (irPOMC), both precursors of beta END, were much more plentiful in human CSF than was beta-endorphin itself, as has been previously reported. It remains to be determined whether the increased CNS opioid concentrations predate traumatic stress, thereby conferring a vulnerability to dissociative states and PTSD itself, or result from the trauma. The negative correlation between CSF ir beta END and avoidant and intrusive symptoms suggests that CNS hypersecretion of opioids might constitute an adaptive response to traumatic experience. Poor correlation between CSF and plasma ir beta END limits use of plasma measures to assess CNS opioid activity.

Subnuclear localization of FOS-like immunoreactivity in the rat parabrachial nucleus after nociceptive stimulation

The effect of noxious stimulation on the expression of FOS-like immunoreactivity (FOS-LI) in neurons of the parabrachial nucleus (PB) was studied in awake, freely moving rats. In one series of experiments, the rats were subjected to noxious mechanical stimulation (pinch) of either the nape of the neck or the base of the tail for 20 seconds every 5 minutes for 90 minutes, and then they were killed by transcardial perfusion after 45-210 minutes. Control animals received innocuous mechanical stimulation (brush) of the tail. Noxious stimuli resulted in FOS-LI in neurons in the dorsal part of the lateral PB, with heavy labeling in the superior lateral (PBsl) and the dorsal lateral (PBdl) subnuclei. FOS-LI was also elicited in the central lateral subnucleus (PBcl) and, although much more sparsely, in the external lateral subnucleus and the Kölliker-Fuse nucleus. Tail and neck stimulation resulted in similar labeling patterns, but more neurons, particularly in PBsl, expressed FOS-LI after pinch of the tail than of the neck. In another series of experiments, rats received injection of 5% formalin into one hindpaw. After 75-90 minutes, FOS-LI was seen in the same parts of PB as after noxious mechanical stimulation. The heaviest labeling was seen on the side contralateral to the injection side, with statistically significant (P < 0.05) side differences present in PBsl and PBdl. In a third series of experiments, rats were hemisected at low cervical-upper thoracic segments, allowed 2 weeks to recover, and then given formalin injections in both hindpaws. Significantly more neurons were FOS-labeled in PBdl, PBsl, and PBcl on the side contralateral to the hemisection than on the ipsilateral side. These observations are discussed in relation to the organization of the spinal afferent input and the efferent connections of PB. It is concluded that the FOS-LI expression in PBdl and PBsl and probably also in PBcl, to a large extent, is evoked by the ascending spinal nociceptive input to PB. Because these subnuclei project to several hypothalamic regions, it is suggested that neurons in PB that express FOS after noxious mechanical and chemical stimulation primarily are involved in autonomic and homeostatic responses to behavioral situations that involve tissue-damaging stimuli.

Tail-pinch induced analgesia and immobility: altered responses to noxious tail-pinch by prior pinch of the neck

Noxious pinch to the scruff of the neck using a metal clip produces profound immobility and analgesia. Noxious pinch delivered to the tail fails to induce immobility and results in nociceptive behavior directed at the pinched tail. However, when administered shortly after neck-clip removal, noxious tail-pinch reinstated immobility without any nociceptive response. Prior neck-clip also enhanced the antinociception induced by the tail-pinch as measured by nociceptive response to a leg pinch. Immobility, as well as antinociception, decreased as the time interval between neck-clip removal and the tail-pinch application increased. Pharmacological manipulations which reduce nociception produced a similar alteration in the response to tail-pinch. Thus, following local injections antinociceptive doses of lidocaine to the base of the tail and systemic morphine administration tail-pinch produced marked immobility. Transection of the brain at the intercollicular level provides evidence for supraspinal involvement in post-neck pinch effects. Not only was the ability of prior neck-pinch to confer antinociceptive properties on tail-pinch abolished, but increased responsiveness to noxious tail-pinch was seen. We, therefore, propose that prior neck-pinch confers new stimulus properties on noxious pinch of other body regions resulting in an enhanced antinociceptive effect, which affects both remote regions and the site of stimulation, and the ability to induce immobility.

Clip-induced analgesia: noxious neck pinch suppresses spinal and mesencephalic neural responses to noxious peripheral stimulation

Pinch of the nape of the neck, of mice, with a serrated clip, produces immobility and lack of responsiveness to noxious stimulation. In this study we attempted to determine whether clip application produces true blockade of nociception, independent of its immobilizing effect, and examined the level of the neuroaxis at which such an effect takes place. To this end nociception was measured using indices not requiring a motor response. Neck pinch eliminated the elevation of heart rate induced by noxious pinch of the tail without affecting heart rate by itself providing evidence for its analgesic effect. Direct evidence that neck pinch suppresses the transmission of noxious information is also provided. Neck pinch inhibits neural activity evoked by noxious peripheral stimulation while exerting minimal effects on the effects of nonnoxious stimuli. Thus, sensory evoked activity in the periaqueductal gray area, elicited by noxious electrical stimulation, but not innocuous stimuli, is inhibited by neck pinch. Similarly, neck pinch inhibits the response of spinal cord neurons to noxious but not nonnoxious stimulation. It, therefore, appears that neck pinch produces true analgesia by activating supraspinal systems which in turn acts to inhibit the transmission of nociception both at spinal and supraspinal levels.

Clip induced analgesia and immobility in the mouse: activation by different sensory modalities

In the present study we have attempted to characterize, in mice, a situation which appears to simulate real life predation and elicits simultaneous analgesia and immobility. We utilized pinch produced by clip application to various regions of the body and examined its effect on responsiveness to noxious stimuli and motor behavior. Intense noxious clip was applied to the nape of the neck, back and base of the tail. The area most effective for the elicitation of both clip induced analgesia and immobility was the nape of the neck while tail pinch resulted in analgesia but not immobility. Evidence is provided that different systems are responsible for clip induced immobility and analgesia. Temporal dissociation of clip induced analgesia and immobility could be demonstrated with continuous clip application for 30 min showing a different time course for the analgesic and immobilizing effects. Different stimuli were effective in eliciting clip induced analgesia and immobility with noxious stimuli essential for the induction of clip induced analgesia and innocuous stimuli sufficient for clip induced immobility. Thus, low analgesic doses of local anesthetics injected into the nape of the neck prevented noxious clip from inducing analgesia but immobility was still evident. In contrast, nonnoxious pinch to the nape of the neck elicited immobility but not analgesia and clip induced immobility could still be induced after the administration of high doses of morphine which completely blocked responses to noxious stimuli. These results demonstrate that in a situation resembling natural predation both analgesia and immobility are produced concurrently but that these behavioral phenomena can be elicited differentially and may be mediated by different independent systems.