Suppression of a hind limb flexion withdrawal reflex by microinjection of glutamate or morphine into the periaqueductal gray in the rat

A cholinergic circuit that relieves pain despite opioid tolerance

Chronic pain is a tremendous burden for afflicted individuals and society. Although opioids effectively relieve pain, significant adverse outcomes limit their utility and efficacy. To investigate alternate pain control mechanisms, we explored cholinergic signaling in the ventrolateral periaqueductal gray (vlPAG), a critical nexus for descending pain modulation. Biosensor assays revealed that pain states decreased acetylcholine release in vlPAG. Activation of cholinergic projections from the pedunculopontine tegmentum to vlPAG relieved pain, even in opioid-tolerant conditions, through ⍺7 nicotinic acetylcholine receptors (nAChRs). Activating ⍺7 nAChRs with agonists or stimulating endogenous acetylcholine inhibited vlPAG neuronal activity through Ca2+ and peroxisome proliferator-activated receptor α (PPAR⍺)-dependent signaling. In vivo 2-photon imaging revealed that chronic pain induces aberrant excitability of vlPAG neuronal ensembles and that ⍺7 nAChR-mediated inhibition of these cells relieves pain, even after opioid tolerance. Finally, pain relief through these cholinergic mechanisms was not associated with tolerance, reward, or withdrawal symptoms, highlighting its potential clinical relevance.

Brainstem Modulation of Nociception by Periaqueductal Gray Neurons Expressing the μ-Opioid Receptor in Mice

BACKGROUND

Pharmacologic manipulations directed at the periaqueductal gray have demonstrated the importance of the μ-opioid receptor in modulating reflexive responses to nociception. The authors hypothesized that a supraspinal pathway centered on neurons in the periaqueductal gray containing the μ-opioid receptor could modulate nociceptive and itch behaviors.

METHODS

The study used anatomical, optogenetic, and chemogenetic approaches in male and female mice to manipulate μ-opioid receptor neurons in the periaqueductal gray. Behavioral assays including von Frey, Hargreaves, cold plantar, chloroquine-induced itch, hotplate, formalin-induced injury, capsaicin-induced injury, and open field tests were used. In separate experiments, naloxone was administered in a postsurgical model of latent sensitization.

RESULTS

Activation of μ-opioid receptor neurons in the periaqueductal gray increased jumping (least-squares mean difference of -3.30 s; 95% CI, -6.17 to -0.44; P = 0.023; n = 7 or 8 mice per group), reduced itch responses (least-squares mean difference of 70 scratching bouts; 95% CI, 35 to 105; P < 0.001; n = 8 mice), and elicited modestly antinociceptive effects (least-squares mean difference of -0.7 g on mechanical and -10.24 s on thermal testing; 95% CI, -1.3 to -0.2 and 95% CI, -13.77 to -6.70, and P = 0.005 and P < 0.001, respectively; n = 8 mice). Last, the study uncovered the role of the periaqueductal gray in suppressing hyperalgesia after a postsurgical state of latent sensitization (least-squares mean difference comparing saline and naloxone of -12 jumps; 95% CI, -17 to -7; P < 0.001 for controls; and -2 jumps; 95% CI, -7 to 4; P = 0.706 after optogenetic stimulation; n = 7 to 9 mice per group).

CONCLUSIONS

μ-Opioid receptor neurons in the periaqueductal gray modulate distinct nocifensive behaviors: their activation reduced responses to mechanical and thermal testing, and attenuated scratching behaviors, but facilitated escape responses. The findings emphasize the role of the periaqueductal gray in the behavioral expression of nociception using reflexive and noxious paradigms.

EDITOR’S PERSPECTIVE

Endogenous antinociceptive system and potential ways to influence It

The biological significance of pain is to protect the organism from possible injury. However, there exists a situation, where, in the interest of survival, it is more important not to perceive pain. Spontaneous suppression of pain or weakening of nociception is mediated by an endogenous antinociceptive (analgesic) system. Its anatomical substrate ranges from the periaqueductal gray matter of the midbrain, through the noradrenergic and serotonergic nuclei of the brain stem to the spinal neurons, which receive "pain" information from nociceptors. Moreover, the activity of this system is under significant control of emotional and cognitive circuits. Pain can be moderated primarily through stimulation of positive emotions, while negative emotions increase pain. Paradoxically, one pain can also suppress another pain. Analgesia can be induced by stress, physical exercise, orosensory stimulation via a sweet taste, listening to music, and after placebo, i.e. when relief from pain is expected. Since pain has sensory, affective, and cognitive components, it turns out that activation of these entire systems can, in specific ways, contribute to pain suppression.

Ventrolateral Periaqueductal Gray Matter Neurochemical Lesion Facilitates Epileptogenesis and Enhances Pain Sensitivity in Epileptic Rats

The ventrolateral periaqueductal gray matter (vlPAG) plays a critical role in the pathogenesis of migraine and few studies have shown that vlPAG might be involved in the pathophysiology of epilepsy. But its roles in epileptogenesis and comorbid relationship between migraine and epilepsy have never been reported. In this study, the impairments of vlPAG neuronal network during spontaneous recurrent seizure (SRS) development after status epilepticus (SE) were investigated, and the pain sensitivity as well as the SRS investigated after neurochemical lesion to vlPAG to determine the role of vlPAG in epileptogenesis and in migraine comorbidity with epilepsy. Neuronal loss and alterations of excitatory and inhibitory neural transmission within vlPAG accompanied the development of epileptogenesis induced by SE. On the other hand, neurochemical lesion to vlPAG enhanced frequency and duration of spontaneous seizure event and frequency of epileptiform inter-ictal spike discharges in electroencephalography (EEG), but decreased pain threshold in epileptic rats. This indicates an involvement of the pain regulating structure, vlPAG, in the pathogenesis of epilepsy. This may imply that vlPAG network alterations could be a possible underlying mechanism of the interactive comorbid relationship between epilepsy and migraine.

The Role of Glutamatergic and Dopaminergic Neurons in the Periaqueductal Gray/Dorsal Raphe: Separating Analgesia and Anxiety

The periaqueductal gray (PAG) is a significant modulator of both analgesic and fear behaviors in both humans and rodents, but the underlying circuitry responsible for these two phenotypes is incompletely understood. Importantly, it is not known if there is a way to produce analgesia without anxiety by targeting the PAG, as modulation of glutamate or GABA neurons in this area initiates both antinociceptive and anxiogenic behavior. While dopamine (DA) neurons in the ventrolateral PAG (vlPAG)/dorsal raphe display a supraspinal antinociceptive effect, their influence on anxiety and fear are unknown. Using DAT-cre and Vglut2-cre male mice, we introduced designer receptors exclusively activated by designer drugs (DREADD) to DA and glutamate neurons within the vlPAG using viral-mediated delivery and found that levels of analgesia were significant and quantitatively similar when DA and glutamate neurons were selectively stimulated. Activation of glutamatergic neurons, however, reliably produced higher indices of anxiety, with increased freezing time and more time spent in the safety of a dark enclosure. In contrast, animals in which PAG/dorsal raphe DA neurons were stimulated failed to show fear behaviors. DA-mediated antinociception was inhibitable by haloperidol and was sufficient to prevent persistent inflammatory pain induced by carrageenan. In summary, only activation of DA neurons in the PAG/dorsal raphe produced profound analgesia without signs of anxiety, indicating that PAG/dorsal raphe DA neurons are an important target involved in analgesia that may lead to new treatments for pain.

Periaqueductal gray and emotions: the complexity of the problem and the light at the end of the tunnel, the magnetic resonance imaging

The periaqueductal gray (PAG) is less referred in relationship with emotions than other parts of the brain (e.g. cortex, thalamus, amygdala), most probably because of the difficulty to reach and manipulate this small and deeply lying structure. After defining how to evaluate emotions, we have reviewed the literature and summarized data of the PAG contribution to the feeling of emotions focusing on the behavioral and neurochemical considerations. In humans, emotions can be characterized by three main domains: the physiological changes, the communicative expressions, and the subjective experiences. In animals, the physiological changes can mainly be studied. Indeed, early studies have considered the PAG as an important center of the emotions-related autonomic and motoric processes. However, in vivo imaging have changed our view by highlighting the PAG as a significant player in emotions-related cognitive processes. The PAG lies on the crossroad of networks important in the regulation of emotions and therefore it should not be neglected. In vivo imaging represents a good tool for studying this structure in living organism and may reveal new information about its role beyond its importance in the neurovegetative regulation.

Divergent Modulation of Nociception by Glutamatergic and GABAergic Neuronal Subpopulations in the Periaqueductal Gray

The ventrolateral periaqueductal gray (vlPAG) constitutes a major descending pain modulatory system and is a crucial site for opioid-induced analgesia. A number of previous studies have demonstrated that glutamate and GABA play critical opposing roles in nociceptive processing in the vlPAG. It has been suggested that glutamatergic neurotransmission exerts antinociceptive effects, whereas GABAergic neurotransmission exert pronociceptive effects on pain transmission, through descending pathways. The inability to exclusively manipulate subpopulations of neurons in the PAG has prevented direct testing of this hypothesis. Here, we demonstrate the different contributions of genetically defined glutamatergic and GABAergic vlPAG neurons in nociceptive processing by employing cell type-specific chemogenetic approaches in mice. Global chemogenetic manipulation of vlPAG neuronal activity suggests that vlPAG neural circuits exert tonic suppression of nociception, consistent with previous pharmacological and electrophysiological studies. However, selective modulation of GABAergic or glutamatergic neurons demonstrates an inverse regulation of nociceptive behaviors by these cell populations. Selective chemogenetic activation of glutamatergic neurons, or inhibition of GABAergic neurons, in vlPAG suppresses nociception. In contrast, inhibition of glutamatergic neurons, or activation of GABAergic neurons, in vlPAG facilitates nociception. Our findings provide direct experimental support for a model in which excitatory and inhibitory neurons in the PAG bidirectionally modulate nociception.

Presynaptic glycine receptors increase GABAergic neurotransmission in rat periaqueductal gray neurons

The periaqueductal gray (PAG) is involved in the central regulation of nociceptive transmission by affecting the descending inhibitory pathway. In the present study, we have addressed the functional role of presynaptic glycine receptors in spontaneous glutamatergic transmission. Spontaneous EPSCs (sEPSCs) were recorded in mechanically dissociated rat PAG neurons using a conventional whole-cell patch recording technique under voltage-clamp conditions. The application of glycine (100 µM) significantly increased the frequency of sEPSCs, without affecting the amplitude of sEPSCs. The glycine-induced increase in sEPSC frequency was blocked by 1 µM strychnine, a specific glycine receptor antagonist. The results suggest that glycine acts on presynaptic glycine receptors to increase the probability of glutamate release from excitatory nerve terminals. The glycine-induced increase in sEPSC frequency completely disappeared either in the presence of tetrodotoxin or Cd²⁺, voltage-gated Na⁺, or Ca²⁺ channel blockers, suggesting that the activation of presynaptic glycine receptors might depolarize excitatory nerve terminals. The present results suggest that presynaptic glycine receptors can regulate the excitability of PAG neurons by enhancing glutamatergic transmission and therefore play an important role in the regulation of various physiological functions mediated by the PAG.

Projections from the rat cuneiform nucleus to the A7, A6 (locus coeruleus), and A5 pontine noradrenergic cell groups

Stimulation of neurons in the cuneiform nucleus (CnF) produces antinociception and cardiovascular responses that could be mediated, in part, by noradrenergic neurons that innervate the spinal cord dorsal horn. The present study determined the projections of neurons in the CnF to the pontine noradrenergic neurons in the A5, A6 (locus coeruleus), and A7 cell groups that are known to project to the spinal cord. Injections of the anterograde tracer, biotinylated dextran amine in the CnF of Sasco Sprague-Dawley rats labeled axons located near noradrenergic neurons that were visualized by processing tissue sections for tyrosine hydroxylase-immunoreactivity. Anterogradely labeled axons were more dense on the side ipsilateral to the BDA deposit. Both A7 and A5 cell groups received dense projections from neurons in the CnF, whereas locus coeruleus received only a sparse projection. Highly varicose anterogradely labeled axons from the CnF were found in close apposition to dendrites and somata of tyrosine hydroxylase-immunoreactive neurons in pontine tegmentum. Although definitive evidence for direct pathways from CnF neurons to the pontine noradrenergic cell groups requires ultrastructural analysis, the results of the present studies provide presumptive evidence of direct projections from neurons in the CnF to the pontine noradrenergic neurons of the A7, locus coeruleus, and A5 cell groups. These results support the suggestion that the analgesia and cardiovascular responses produced by stimulation of neurons in the CnF may be mediated, in part, by pontine noradrenergic neurons.

Quantitative assessment of nocifensive behavioral responses and the underlying neuronal circuitry

This paper reviews several recently developed animal models that allow a quantitative assessment of the magnitude of nocifensive behavioral responses across a range of noxious stimulus intensities. Models discussed in detail include: (a) the rodent tail flick reflex, and a modification that allows measurement of tail flick magnitude, (b) rat hindlimb flexion withdrawal reflex elicited by noxious thermal stimulation of the paw, and (c) a learned operant response (nose bar press) evoked by noxious thermal stimulation of the rat's tail. These models are discussed in terms of their advantages over previous methods measuring response threshold, their fulfillment of criteria for ideal pain assessment models, and the neuronal circuitry underlying the behavioral response.

Spinal processing of noxious and innocuous cold information: differential modulation by the periaqueductal gray

In addition to cold being an important behavioral drive, altered cold sensation frequently accompanies pathological pain states. However, in contrast to peripheral mechanisms, central processing of cold sensory input has received relatively little attention. The present study characterized spinal responses to noxious and innocuous intensities of cold stimulation in vivo and established the extent to which they are modulated by descending control originating from the periaqueductal gray (PAG), a major determinant of acute and chronic pain. In lightly anesthetized rats, hindpaw cooling with ethyl chloride, but not acetone, was sufficiently noxious to evoke withdrawal reflexes, which were powerfully inhibited by ventrolateral (VL)-PAG stimulation. In a second series of experiments, subsets of spinal dorsal horn neurons were found to respond to innocuous and/or noxious cold. Descending control from the VL-PAG distinguished between activity in nociceptive versus non-nociceptive spinal circuits in that innocuous cold information transmitted by non-nociceptive class 1 and wide-dynamic-range class 2 neurons remained unaltered. In contrast, noxious cold information transmitted by class 2 neurons and all cold-evoked activity in nociceptive-specific class 3 neurons was significantly depressed. We therefore demonstrate that spinal responses to cold can be powerfully modulated by descending control systems originating in the PAG, and that this control selectively modulates transmission of noxious versus innocuous information. This has important implications for central processing of cold somatosensation and, given that chronic pain states are dependent on dynamic alterations in descending control, will help elucidate mechanisms underlying aberrant cold sensations that accompany pathological pain states.

The lower limb flexion reflex in humans

The flexion or flexor reflex (FR) recorded in the lower limbs in humans (LLFR) is a widely investigated neurophysiological tool. It is a polysynaptic and multisegmental spinal response that produces a withdrawal of the stimulated limb and resembles (having several features in common) the hind-paw FR in animals. The FR, in both animals and humans, is mediated by a complex circuitry modulated at spinal and supraspinal level. At rest, the LLFR (usually obtained by stimulating the sural/tibial nerve and by recording from the biceps femoris/tibial anterior muscle) appears as a double burst composed of an early, inconstantly present component, called the RII reflex, and a late, larger and stable component, called the RIII reflex. Numerous studies have shown that the afferents mediating the RII reflex are conveyed by large-diameter, low-threshold, non-nociceptive A-beta fibers, and those mediating the RIII reflex by small-diameter, high-threshold nociceptive A-delta fibers. However, several afferents, including nociceptive and non-nociceptive fibers from skin and muscles, have been found to contribute to LLFR activation. Since the threshold of the RIII reflex has been shown to correspond to the pain threshold and the size of the reflex to be related to the level of pain perception, it has been suggested that the RIII reflex might constitute a useful tool to investigate pain processing at spinal and supraspinal level, pharmacological modulation and pathological pain conditions. As stated in EFNS guidelines, the RIII reflex is the most widely used of all the nociceptive reflexes, and appears to be the most reliable in the assessment of treatment efficacy. However, the RIII reflex use in the clinical evaluation of neuropathic pain is still limited. In addition to its nocifensive function, the LLFR seems to be linked to posture and locomotion. This may be explained by the fact that its neuronal circuitry, made up of a complex pool of interneurons, is interposed in motor control and, during movements, receives both peripheral afferents (flexion reflex afferents, FRAs) and descending commands, forming a multisensorial feedback mechanism and projecting the output to motoneurons. LLFR excitability, mediated by this complex circuitry, is finely modulated in a state- and phase-dependent manner, rather as we observe in the FR in animal models. Several studies have demonstrated that LLFR excitability may be influenced by numerous physiological conditions (menstrual cycle, stress, attention, sleep and so on) and pathological states (spinal lesions, spasticity, Wallenberg's syndrome, fibromyalgia, headaches and so on). Finally, the LLFR is modulated by several drugs and neurotransmitters. In summary, study of the LLFR in humans has proved to be an interesting functional window onto the spinal and supraspinal mechanisms of pain processing and onto the spinal neural control mechanisms operating during posture and locomotion.

Antinociception following application of DAMGO to the basolateral amygdala results from a direct interaction of DAMGO with Mu opioid receptors in the amygdala

Previous studies from our laboratory have shown that application of the mu opioid agonist DAMGO into the basolateral region of the amygdala (BLA) suppresses the radiant heat tail flick (TF) reflex in anesthetized rats. This antinociceptive effect can be blocked by lesions of brainstem regions such as the periaqueductal gray (PAG) or the rostral ventromedial medulla (RVM) or by functional inactivation of neurons in these regions, suggesting the activation of brainstem-descending antinociceptive systems from the amygdala. However, little is known about the direct interaction of DAMGO with mu receptors in the amygdala. In the present series of experiments, the BLA was pretreated with opioid receptor antagonists and a G protein inhibitor prior to TF testing with application of DAMGO into the same site. Rats pretreated with the non-selective opioid antagonist naltrexone (1.25-3.75 microg/0.25 microl per side) or the G protein inhibitor pertussis toxin (0.25 microg) failed to show inhibition of TF reflexes following infusion of DAMGO (0.168-0.50 microg), indicating that DAMGO works through G-protein-coupled opioid receptors in the BLA. Furthermore, pretreatment with the mu antagonist beta-FNA (1.00-2.00 microg) attenuated antinociception induced by DAMGO injection, suggesting DAMGO's action on mu receptors in the BLA. Accordingly, we confirm a direct interaction of DAMGO with G-protein-coupled mu receptors in the BLA contributing to induction of opioid antinociception in the amygdala.

Modulation of Chelidonii herba on glycine-activated and glutamate-activated ion currents in rat periaqueductal gray neurons

BACKGROUND

Chelidonii herba is classified as Papaver somniferum L. Aqueous extract from C. herba is traditionally used for disorders with symptoms like pain, bloating, abdominal cramp after meals.

METHODS

Modulation of C. herba on glycine-activated and glutamate-activated ion currents in the acutely dissociated periaqueductal gray (PAG) neurons was investigated by the nystatin-perforated patch-clamp technique.

RESULTS

C. herba inhibited glycine-activated ion current and increased glutamate-activated ion current. C. herba-induced inhibition on glycine-activated ion current is implicated in opioid receptors and GTP-binding proteins (G-proteins). Increased glutamate-activated ion current induced by C. herba is linked neither by opioid receptors nor GTP-binding proteins.

CONCLUSIONS

Suppressed glycine-induced response and elevated glutamate-induced response by C. herba may increase neuronal excitability in PAG, results in activation of descending pain control system, and this mechanism can be suggested as one of the analgesic actions of C. herba.

Modulation of cyclooxygenase-2 on glycine- and glutamate-induced ion currents in rat periaqueductal gray neurons

Cyclooxygenase (COX) is a key enzyme in the conversion of arachidonic acid to prostaglandins. Two isoforms of COX are known: COX-1 and COX-2. In the present study, the modulatory effect of COX-2 on glycine- and glutamate-induced ion currents in periaqueductal gray (PAG) neurons was investigated using the nystatin-perforated patch clamp method. Continuous application of lipopolysaccharides on PAG neurons resulted in increased glycine-induced ion current and decreased glutamate-induced ion current. In contrast, continuous application of celecoxib, selective COX-2 inhibitor, resulted in decreased glycine-induced ion current and increased glutamate-induced ion current. These results demonstrate that COX-2 modulates neuronal activity of PAG, and it can be suggested that COX-2 participates in the regulation of the descending pain control system in the level of PAG neurons.

Noxious stimulation increases glutamate and arginine in the periaqueductal gray matter in rats: a microdialysis study

The periaqueductal gray matter (PAG) is an important center in the modulation of behavioral responses during nociception and stress. In the present experiment, extracellular excitatory amino acid overflow in the PAG was measured every 30 s during noxious stimulation. A combination of in vivo brain microdialysis in freely moving rats and capillary zone electrophoresis with laser induced-fluorescence detection allowed us to detect short lasting changes of excitatory amino acid in dialysates. A formalin injection in the hindpaw of the rat increased glutamate, arginine and aspartate concentration in PAG dialysates. This increase was calcium and nerve impulse-dependent, suggesting neuronal and glial origin of glutamate and arginine, respectively. Handling, pinching or saline injection in the hind paw did not increase glutamate showing that this neurochemical phenomenon is related to painful and persistent noxious stimulation. The results suggest that a rapid excitation of the PAG occurs during noxious stimulation. The role of glutamate and arginine in analgesia is discussed.

The effects of an intrathecal NMDA antagonist (AP5) on the behavioral changes induced by colorectal inflammation with turpentine in rats

Visceral pain, especially that associated with inflammation of visceral organs, is poorly understood and difficult to treat clinically. The purpose of this study was to investigate the effects of intrathecal 2-amino-5-phosphonovaleric acid (AP5, a competitive NMDA antagonist) upon a visceromotor response to distension of colonic tissue inflamed by exposure to turpentine. All experiments were conducted under pentobarbital anesthesia. Animals were prepared with a laminectomy from T12 to L1 to facilitate intrathecal drug administration. Colonic distension thresholds for a visceromotor response were determined in the presence and absence of AP5. Animals were divided into two groups. The NS group received 50 microl of saline intrathecally and the AP5 group 10 mM of AP5 in 50 microl saline. After baseline measurements, intrathecal drugs were administered. Five minutes later, the effects of intrathecal drugs were measured, then 1 ml of 25% turpentine was administered anorectally. Subsequent measurements were made every 5 minutes for the next 90 minutes. Visceromotor thresholds to colorectal distension (CRD) were significantly decreased 50 min after turpentine administration in the NS group. There was no threshold change in the AP5 group. This study suggests that the administration of the competitive NMDA receptor antagonist AP5 in this model blocks the effect of turpentine sensitization on visceromotor response to CRD. The absence of AP5 effects in animals not sensitized by turpentine suggests that NMDA systems may be involved in the sensitization.

Functional characteristics of the midbrain periaqueductal gray

The major functions of the midbrain periaqueductal gray (PAG), including pain and analgesia, fear and anxiety, vocalization, lordosis and cardiovascular control are considered in this review article. The PAG is an important site in ascending pain transmission. It receives afferents from nociceptive neurons in the spinal cord and sends projections to thalamic nuclei that process nociception. The PAG is also a major component of a descending pain inhibitory system. Activation of this system inhibits nociceptive neurons in the dorsal horn of the sinal cord. The dorsal PAG is a major site for processing of fear and anxiety. It interacts with the amygdala and its lesion alters fear and anxiety produced by stimulation of amygdala. Stimulation of PAG produces vocalization and its lesion produces mutism. The firing of many cells within the PAG correlates with vocalization. The PAG is a major site for lordosis and this role of PAG is mediated by a pathway connecting the medial preoptic with the PAG. The cardiovascular controlling network within the PAG are organized in columns. The dorsal column is involved in pressor and the ventrolateral column mediates depressor responses. The major intrinsic circuit within the PAG is a tonically-active GABAergic network and inhibition of this network is an important mechanism for activation of outputs of the PAG. The various functions of the PAG are interrelated and there is a significant interaction between different functional components of the PAG. Using the current information about the anatomy, physiology, and pharmacology of the PAG, a model is proposed to account for the interactions between these different functional components.

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.

Responses of motor units during the hind limb flexion withdrawal reflex evoked by noxious skin heating: phasic and prolonged suppression by midbrain stimulation and comparison with simultaneously recorded dorsal horn units

In rats anesthetized with sodium pentobarbital, we quantitatively analyzed descending modulation from the midbrain of a nociceptive flexion withdrawal reflex and responses of associated spinal neurons. We monitored the isometric force of hind limb withdrawal elicited by noxious heat stimuli (42-54 degrees C, 10 sec) on the hind paw. In one series of experiments, single-fiber EMG electrodes recorded responses of single muscle fibers (i.e., motor units) in biceps femoris during the hind limb withdrawal, without and during electrical stimulation in the midbrain periaqueductal gray (PAG) or lateral midbrain reticular formation (LRF). In a second series, responses of single lumbar dorsal horn neurons were also recorded simultaneously. Withdrawal force and associated motor unit responses were suppressed for prolonged periods (4 to greater than 60 min) following the initial episode of PAG or LRF stimulation in 40% of the rats, while they were suppressed phasically (i.e., only during brain stimulation) in the remainder. Motor unit responses increased in a graded fashion with increasing skin stimulus temperature from threshold (45 degrees C) to 54 degrees C. During PAG stimulation, the slope of the rate coding function was reduced with no change in threshold temperature. During LRF stimulation the rate coding function was shifted toward higher temperatures with increased threshold (47 degrees C). In 14 experiments 43 paired recordings were made from a dorsal horn and a motor unit during hind limb withdrawals. Mean latency to onset and peak of the heat-evoked response was shorter for dorsal horn compared to motor units. In 6/14 rats withdrawal force and motor unit responses were significantly suppressed for more than 8 min following mechanical placement of the stimulating electrodes and/or the initial episode of midbrain stimulation, while the simultaneously recorded dorsal horn unit responses remained constant. Following supplemental administration of pentobarbital (10-30 mg/kg i.v.), withdrawals and motor unit responses to heat were suppressed while dorsal horn unit responses were unchanged or enhanced. Also, in 12/42 cases, withdrawals and motor unit responses decremented markedly during the initial 3 trials of heat, while simultaneously recorded dorsal horn unit responses remained stable. These results indicate that the withdrawal reflex and associated motor units can be markedly suppressed in the absence of concomitant changes in responsiveness of dorsal horn neurons, and are discussed in terms of the neurocircuitry of spinal flexor reflexes and their descending modulation.

Reciprocal connections between the periaqueductal gray matter and other somatosensory regions of the cat midbrain: a possible mechanism of pain inhibition

Lectin-conjugated horseradish peroxidase was injected or implanted in crystalline form into various parts of the periaqueductal gray matter (PAG) in the cat. After varying survival periods, the animals were fixed and the mesencephalon was sectioned and incubated for HRP histochemistry. Outside PAG, labelled cells and terminal labelling were observed in the cuneiform, parabrachial and intercollicular nuclei, in the deep and intermediate gray layers of the superior colliculus, in the anterior and posterior pretectal nuclei and in the nucleus of Darkschewitsch. This study has shown that the region of PAG that is known to receive heavy ascending somatosensory input from the spinal cord and to be part of descending pain-inhibiting systems, also has reciprocal connections with other somatosensory areas of the midbrain. The results are discussed in relation to nociception and nociceptive inhibiting mechanisms.