Suppression of the jaw-opening reflex by periaqueductal gray stimulation is decreased by paramedian brainstem lesions

The role of dorsomedial and ventrolateral columns of the periaqueductal gray matter and in situ 5-HT₂A and 5-HT₂C serotonergic receptors in post-ictal antinociception

The periaqueductal gray matter (PAG) consists in a brainstem structure rich in 5-hydroxytryptamine (5-HT) inputs related to the modulation of pain. The involvement of each of the serotonergic receptor subtypes found in PAG columns, such as the dorsomedial (dmPAG) and the ventrolateral (vlPAG) columns, regarding post-ictal antinociception have not been elucidated. The present work investigated the participation of the dmPAG and vlPAG columns in seizure-induced antinociception. Specifically, we studied the involvement of serotonergic neurotransmission in these columns on antinociceptive responses that follow tonic-clonic epileptic reactions induced by pentylenetetrazole (PTZ), an ionophore GABA-mediated Cl(-) influx antagonist. Microinjections of cobalt chloride (1.0 mM CoCl2 /0.2 µL) into the dmPAG and vlPAG caused an intermittent local synaptic inhibition and decreased post-ictal antinociception that had been recorded at various time points after seizures. Pretreatments of the dmPAG or the vlPAG columns with the nonselective serotonergic receptors antagonist methysergide (5.0 µg/0.2 µL) or intramesencephalic microinjections of ketanserin (5.0 µg/0.2 µL), a serotonergic antagonist with more affinity to 5-HT2A/2C receptors, decreased tonic-clonic seizure-induced antinociception. Both dmPAG and vlPAG treatment with either the 5-HT2A receptor selective antagonist R-96544 (10 nM/0.2 µL), or the 5-HT2C receptors selective antagonist RS-102221 (0.15 µg/0.2 µL) also decrease post-ictal antinociception. These findings suggest that serotonergic neurotransmission, which recruits both 5-HT2A and 5-HT2C serotonergic receptors in dmPAG and vlPAG columns, plays a critical role in the elaboration of post-ictal antinociception.

Striatal Inhibition of Nociceptive Responses Evoked in Trigeminal Sensory Neurons by Tooth Pulp Stimulation

The noxious evoked response in trigeminal sensory neurons was studied to address the role of striatum in the control of nociceptive inputs. In urethane-anesthetized rats, the jaw opening reflex (JOR) was produced by suprathreshold stimulation of the tooth pulp and measured as electromyographic response in the digastric muscle, with simultaneous recording of noxious responses in single unit neurons of the spinal trigeminal nucleus pars caudalis (Sp5c). The microinjection of glutamate (80 ηmol/0.5 μl) into striatal JOR inhibitory sites significantly decreased the Aδ and C fiber–mediated–evoked response (53 ± 4.2 and 43.6 ± 6.4% of control value, P < 0.0001) in 92% (31/34) of nociceptive Sp5c neurons. The microinjection of the solvent was ineffective, as was microinjection of glutamate in sites out of the JOR inhibitory ones. In another series of experiments, simultaneous single unit recordings were performed in the motor trigeminal nucleus (Mo5) and the Sp5c nucleus. Microinjection of glutamate decreased the noxious-evoked response in Sp5c and Mo5 neurons in parallel with the JOR, without modifying spontaneous neuronal activity of trigeminal motoneurons ( n = 8 pairs). These results indicate that the striatum could be involved in the modulation of nociceptive inputs and confirm the role of the basal ganglia in the processing of nociceptive information.

Opiate anti-nociception is attenuated following lesion of large dopamine neurons of the periaqueductal grey: critical role for D1 (not D2) dopamine receptors

The periaqueductal grey (PAG) area is involved in pain modulation as well as in opiate-induced anti-nociceptive effects. The PAG possess dopamine neurons, and it is likely that this dopaminergic network participates in anti-nociception. The objective was to further study the morphology of the PAG dopaminergic network, along with its role in nociception and opiate-induced analgesia in rats, following either dopamine depletion with the toxin 6-hydroxydopamine or local injection of dopaminergic antagonists. Nociceptive responses were studied through the tail-immersion (spinal reflex) and the hot-plate tests (integrated supraspinal response), establishing a cut-off time to further minimize animal suffering. Heroin and morphine were employed as opiates. Histological data indicated that the dopaminergic network of the PAG is composed of two types of neurons: small rounded cells, and large multipolar neurons. Following dopamine depletion of the PAG, large neurons (not small ones) were selectively affected by the toxin (61.9% dopamine cell loss, 80.7% reduction of in vitro dopaminergic peak), and opiate-induced analgesia in the hot-plate test (not the tail-immersion test) was reliably attenuated in lesioned rats (P < 0.01). After infusions of dopaminergic ligands into the PAG, D(1) (not D(2)) receptor antagonism attenuated opiate-induced analgesia in a dose-dependent manner in the hot-plate test. The present study provides evidence that large neurons of the dopaminergic network of the PAG participate in supraspinal (not spinal) nociceptive responses after opiates through the involvement of D(1) dopamine receptors. This dopaminergic system should be included as another network within the PAG involved in opiate-induced anti-nociception.

Modulation of sympathetic and somatomotor function by the ventromedial medulla

The ventromedial medulla is implicated in a variety of functions including nociceptive and cardiovascular modulation and the control of thermoregulation. To determine whether single microinjections into the ventromedial medulla elicit changes in one or multiple functional systems, the GABA(A) receptor antagonist bicuculline was microinjected (70 nl, 5-50 ng) into the ventromedial medulla of lightly anesthetized rats, and cardiovascular, respiratory, and nociceptive measures were recorded. Bicuculline microinjection into either the midline raphe or the laterally adjacent reticular nucleus simultaneously increased interscapular brown adipose tissue temperature, heart rate, blood pressure, expired [CO(2)], and respiration rate and elicited shivering. Bicuculline microinjection also decreased the noxious stimulus-evoked changes in heart rate and blood pressure, decreased the frequency of heat-evoked sighs, and suppressed the cortical desynchronization evoked by noxious stimulation. Although bicuculline suppressed the motor withdrawal evoked by noxious tail heat, it enhanced the motor withdrawal evoked by noxious paw heat, evidence for specifically patterned nociceptive modulation. Saline microinjections into midline or lateral sites had no effect on any measured variable. All bicuculline microinjections, midline or lateral, evoked the same set of physiological effects, consistent with the lack of a topographical organization within the ventromedial medulla. Furthermore, as predicted by the isodendritic morphology of cells in the ventromedial medulla, midline bicuculline microinjection increased the number of c-fos immunoreactive cells in both midline raphe and lateral reticular nuclei. In summary, 70-nl microinjections into ventromedial medulla activate cells in multiple nuclei and elicit increases in sympathetic and somatomotor tone and a novel pattern of nociceptive modulation.

Raphe magnus neurons respond to noxious colorectal distension

Physiological studies of neurons in raphe magnus (RM) and the adjacent nucleus reticularis magnocellularis (NRMC) have demonstrated that the response to noxious cutaneous stimulation predicts the response to opioid administration and therefore a cell's functional role in nociceptive modulation. Although visceral stimulation, like opioids, elicits antinociception, little is known about how RM and NRMC cells respond to visceral stimulation. Therefore RM and NRMC cells were tested for their responses to both colorectal distension (CRD) and noxious cutaneous heat in halothane-anesthetized rats. Less than a third of serotonergic cells responded to CRD with small increases or decreases in discharge rate. In contrast, almost two-thirds of nonserotonergic cells responded to CRD stimulation with either excitatory (35%) or inhibitory (30%) responses to CRD. The response to heat did not predict the response to CRD with nearly equal proportions of heat-excited, -inhibited, and -unaffected cells being excited, inhibited, or unaffected by CRD. The dissociation between the responses to cutaneous heat and CRD demonstrates that cell classes based on the response to noxious heat are not homogeneous and may play multiple functional roles.

Striatal modulation of the jaw opening reflex

The effect of striatal electrical and chemical conditioning stimulation (L-glutamate 80-160 nmoles/0.5 microl) on the jaw opening reflex (JOR) was studied in Sprague-Dawley male rats anesthetized with urethane. The JOR was evoked by stimulation of the tooth pulp of lower incisors. This response was suppressed by transection of the dental root, which indicates according with the bibliography, a specific activation of the pulp nerves. Three type of responses were obtained on the evoked JOR by conditioning stimulation of the striatum; being the main one the suppression of the reflex elicited by tooth pulp activation. A second type of response was an increase of the tooth-JOR amplitude. This effect was observed more frequently with glutamate stimulation rather than with electrical activation of the striatum. A third response was observed with chemical stimulation but not by electrical stimulation of the striatum. This was a triphasic response which consisted in an increase followed by an inhibition and a late increase of the tooth-JOR amplitude. A biphasic effect, an increase prior to a decrease of the JOR amplitude, was also recorded with a minor frequency. The distribution of effective sites for electrical and chemical stimulation within the striatum are mainly similar located in the rostral aspect of the nucleus, with the inhibitory sites in the middle of the nucleus and intermingled with the excitatory ones. The complex responses (tri/biphasic) were observed ventrally and caudally in the nucleus. On the basis of the results mentioned above, one could assume that the striatum is related to the modulation of the JOR evoked probably by nociceptive stimulation. However, activation of other type of fibers could not be ruled out.

Electrically-evoked inhibitory effects of the nucleus submedius on the jaw-opening reflex are mediated by ventrolateral orbital cortex and periaqueductal gray matter in the rat

In previous studies we have shown that electrical stimulation of the nucleus submedius inhibits the rat radiant heat-induced tail flick reflex, and that this antinociceptive effect is mediated by the ventrolateral orbital cortex and periaqueductal gray. The aim of the present study was to examine whether electrical stimulation of the nucleus submedius could inhibit the rat jaw-opening reflex, and to determine whether electrolytic lesions of the ventrolateral orbital cortex or the periaqueductal gray could attenuate the nucleus submedius-evoked inhibition. Experiments were performed on pentobarbital-anesthetized rats. The jaw-opening reflex elicited by electrical stimulation of the tooth pulp or the facial skin was monitored by recording the evoked digastric electromyogram. Conditioning stimulation was delivered unilaterally to the nucleus submedius 90 ms prior to each test stimulus to the tooth pulp. After that, electrolytic lesions were made in ventrolateral orbital cortex or periaqueductal gray, and the effect of nucleus submedius stimulation on the jaw-opening reflex was re-examined. Unilateral electrical stimulation of nucleus submedius was found to significantly depress the jaw-opening reflex (mean threshold of 28.0+/-1.4 microA, n = 48), and the magnitude of inhibition increased linearly when the stimulus intensity was increased from 20 to 70 microA, resulting in depression of the digastric electromyogram amplitude from 18.4+/-5.4% to 74.0+/-4.9% of the control (P < 0.01, n = 37). The onset of inhibition occured 60 ms after the beginning of nucleus submedius stimulation and lasted about 100 ms, as determined by varying the conditioning-test time interval. Furthermore, ipsilateral lesions of the ventrolateral orbital cortex or bilateral lesions of the lateral or ventrolateral parts of periaqueductal gray eliminated the nucleus submedius-evoked inhibition of the jaw-opening reflex. These data suggest that the nucleus submedius plays an important role in modulation of orofacial nociception, and provide further support for a hypothesis that the antinociceptive effect of nucleus submedius stimulation is mediated by ventrolateral orbital cortex and activation of a descending inhibitory system in the periaqueductal gray.

Physiological properties of raphe magnus neurons during sleep and waking

Neurons in the medullary raphe magnus (RM) that are important in the descending modulation of nociceptive transmission are classified by their response to noxious tail heat as ON, OFF, or NEUTRAL cells. Experiments in anesthetized animals demonstrate that RM ON cells facilitate and OFF cells inhibit nociceptive transmission. Yet little is known of the physiology of these cells in the unanesthetized animal. The first aim of the present experiments was to determine whether cells with ON- and OFF-like responses to noxious heat exist in the unanesthetized rat. Second, to determine if RM cells have state-dependent discharge, the activity of RM neurons was recorded during waking and sleeping states. Noxious heat applied during waking and slow wave sleep excited one group of cells (ON-U) in unanesthetized rats. Other cells were inhibited by noxious heat (OFF-U) applied during waking and slow wave sleep states in unanesthetized rats. NEUTRAL-U cells did not respond to noxious thermal stimulation applied during either slow wave sleep or waking. ON-U and OFF-U cells were more likely to respond to noxious heat during slow wave sleep than during waking and were least likely to respond when the animal was eating or drinking. Although RM cells rarely respond to innocuous stimulation applied during anesthesia, ON-U and OFF-U cells were excited and inhibited, respectively, by innocuous somatosensory stimulation in the unanesthetized rat. The spontaneous activity of >90% of the RM neurons recorded in the unanesthetized rat was influenced by behavioral state. OFF-U cells discharged sporadically during waking but were continuously active during slow wave sleep. By contrast, ON-U and NEUTRAL-U cells discharged in bursts during waking and either ceased to discharge entirely or discharged at a low rate during slow wave sleep. We suggest that OFF cell discharge functions to suppress pain-evoked reactions during sleep, whereas ON cell discharge facilitates pain-evoked responses during waking.

Inhibitory effects of electrical stimulation of ventrolateral orbital cortex on the rat jaw-opening reflex

In previous studies, we have shown that electrically or chemically evoked activation of the ventrolateral orbital cortex (VLO) depresses the rat tail-flick (TF) reflex, and this antinociceptive effect is mediated by the periaqueductal gray (PAG). The aim of the present study was to examine whether electrical stimulation of the VLO could inhibit the rat jaw-opening reflex (JOR), and to determine whether electrolytic lesions of the PAG could attenuate this VLO-evoked inhibition. Unilateral electrical stimulation of the VLO significantly depressed the JOR elicited by tooth pulp or facial skin stimuli, with a mean threshold of 30.5+/-2.3 microA (n=22). Increasing stimulation intensities from 30 to 80 microA resulted in greater reduction of the dEMG amplitude from 22.9+/-5.0% to 69.7+/-3.7% of the baseline value (P<0.01, n=22). The inhibitory effect appeared 50 ms after the beginning of VLO stimulation and lasted about 150 ms, as determined by varying the conditioning-test (C-T) time interval. Unilateral lateral or ventrolateral lesions of the PAG produced only a small attenuation of the VLO-evoked inhibition of the JOR, but bilateral lesions eliminated this inhibition. These findings suggest that the VLO plays an important role in modulation of orofacial nociceptive inputs, and provide further support for the hypothesis that the antinociceptive effect of VLO is mediated by PAG leading to activation of a brainstem descending inhibitory system and depression of nociceptive inputs at the trigeminal level. The role played by VLO in pain modulation is discussed in association with the proposed endogenous analgesic system consisting of medullary cord-Sm-VLO-PAG-medullary cord.

The activity of ON and OFF cells at the rostroventromedial medulla is modulated by vagino-cervical stimulation

In anesthetized rats it was tested whether or not the activity of the ON and OFF cells within the rostral ventromedial medulla (RVM) is modulated by the mechanical stimulation of the uterine cervix (VS). ON cells were identified by an abrupt increase in their firing rate before the tail flick in response to a noxious heat. OFF cells were identified by a sudden decrease in their firing rate before the tail flick. All (27 out of 27) identified ON cells decreased their firing rate immediately after VS was applied. The effect of VS on the activity of the cells persisted for the entire stimulation period. On the other hand, all (19 out of 19) identified OFF cells increased their firing rate immediately after VS. The effect of VS on the activity of these cells also persisted for the entire stimulation period. The activity of the neutral cells showed no change, neither during the application of noxious heat, nor during VS. These results suggest that the analgesic-like effect produced by VS can be mediated by the activity of the antinociceptive circuit at the RVM. Alternatively, it can be suggested that the afferent inflow from the genital tract can induce the activity of the antinociceptive circuit at RVM, either by projections to the periaqueductal gray matter or by direct projections to RVM.

Nuclei within the rostral ventromedial medulla mediating morphine antinociception from the periaqueductal gray

The relative contributions of nuclei within the rostral ventromedial medulla (RVM) involved in mediating morphine induced antinociception from the periaqueductal gray (PAG) were examined. Lidocaine injections (4%) at the time of morphine's maximal response were used to provide a localized neural block and were administered in the nucleus raphe magnus/reticularis gigantocellularis pars alpha (RMg/GiA; commonly referred to as RMg), reticularis gigantocellularis (Gi) and reticularis paragigantocellularis lateralis (LPGi). Microinjection of morphine (6 nmol; 0.5 microliter) into the PAG of awake rats produced an inhibition of the tail-flick reflex that was maximal after 30 min. This response was unaffected by a single medial lidocaine injection (0.5 microliter) into the RMg/GiA or Gi, bilateral injections into the Gi or LPGi or triple injections that included both the RMg/GiA and LPGi. A partial, non-significant block of morphine's response was observed either by bilateral injections (0.5 microliter) into both the Gi and LPGi (% inhibition = 16.4 +/- 24.8) or by bilateral injections in the LPGi and a single medial injection into the Gi (% inhibition = 41.5 +/- 29.8). However, injection of a greater volume of lidocaine (1 microliter) into the RMg/GiA or bilaterally into the LPGi affected adjacent medial and lateral tissue, and completely inhibited morphine's response. Furthermore, triple injections of lidocaine (0.5 microliter) into the Gi or bilateral injections (0.5 microliter) into the Gi and a single medial injection into the RMg/GiA completely blocked morphine's antinociceptive response. These results indicate that morphine antinociception from the PAG is mediated by a large volume of tissue in the RVM containing nuclei located both medially and laterally. Additionally, the principal nuclei involved in this response appear to be the Gi and RMg/GiA.

Periaqueductal gray matter and nucleus raphe magnus involvement in anterior pretectal nucleus-induced inhibition of jaw-opening reflex in rats

In previous studies we have shown that electrical stimulation of the cortex or anterior pretectal nucleus (APT) inhibits the jaw-opening reflex (JOR). In the present study we investigated whether these effects are mediated by a relay in the periaqueductal gray matter (PAG) or rostroventromedial medulla (RVM). Experiments were performed on chloralose-urethane anesthetized rats. The JOR which was elicited by electrical stimulation of the mandibular incisor tooth was monitored by recording the evoked digastric muscle activity. Conditioning stimulation (20 ms train of 0.2 ms pulses at 400 Hz) was delivered to the facial area of the sensorimotor cortex, APT, PAG or nucleus raphe magnus (NRM) 50 ms prior to the test stimulus to the tooth that evoked the JOR. In addition, the effects of microinjections of glutamate into APT, PAG and NRM on the tooth-evoked JOR were also evaluated. The inhibition of the JOR by electrical and glutamate conditioning stimulation was found to be most potent for activation of the NRM and least potent for the APT. Local anesthetic (2% lidocaine, 0.3-0.6 microliters) block of the PAG could partially, significantly (P less than 0.05) and reversibly reduce both the APT and cortical-induced depression of the JOR. Lidocaine block of the ventromedial pons reversibly reduced the PAG, APT and cortical-induced inhibition of the JOR (P less than 0.05). Lidocaine block of the lateral RVM had powerfully (P less than 0.01) and reversibly reduced the PAG-induced inhibition, but had only a small effect (P less than 0.05) on the APT-induced inhibition and no significant effect on the cortical-induced inhibition.(ABSTRACT TRUNCATED AT 250 WORDS)

Somatodendritic morphology of on- and off-cells in the rostral ventromedial medulla

The rostral ventromedial medulla (RVM) contains two classes of physiologically defined neurons, on-cells and off-cells, that are implicated in nociceptive modulation. In a continuing effort to detail the neural circuitry that underlies the activity of these two distinct neuronal types, the somatodendritic morphology of on- and off-cells was studied in the cat, rat, and ferret. In lightly anesthetized animals, on-cells increased and off-cells decreased their discharge rate during a withdrawal reflex evoked by noxious stimuli. Following their physiological characterization by using intracellular recording, on- and off-cells were injected with either horseradish peroxidase or biocytin and their somatodendritic arborizations were examined. Labeled on- and off-cells included fusiform and stellate cells of all sizes as well as large multipolar neurons. Although the somatic shape of both on- and off-cells in RVM was heterogeneous, off-cells tended to be fusiform neurons whose long axis was oriented mediolaterally. The dendritic domains of both on- and off-cells extended bilaterally past the lateral edge of the trapezoid body or pyramid and ventrally to, and sometimes including, the trapezoid body or pyramid. In contrast to their extensive mediolateral spread, the dendritic domains of both cell types were limited to the ventral half of the reticular formation and were compressed along the rostrocaudal axis. The dendritic arbor of individual on- and off-cells extended well beyond the cytoarchitectonic boundaries of any single nuclear region, within the domain delineated as the RVM. The spatial domains of the dendritic arbors of on- and off-cells are further evidence that the on- and off-cells throughout the RVM constitute an integrated unit in the modulation of nociceptive transmission.

Axonal trajectories and terminations of on- and off-cells in the cat lower brainstem

Two physiologically defined classes of pontomedullary raphe neurons were intracellularly labeled in order to determine the target nuclei of their axonal projections. In the lightly anesthetized cat, cells either increased (on-cells) or decreased (off-cells) their discharge rate during the paw withdrawal reflex evoked by noxious pinch or heat. On- and off-cells were injected with horseradish peroxidase and the initial course of labeled axons through the lower brainstem was reconstructed. On-cell projections to the pontomedullary raphe and medial reticular nuclei were sparse. On-cells projected densely to regions of the lateral reticular formation and the ventrolateral medulla at both rostral and caudal medullary levels. In general, on-cells had few collaterals and spare axonal swellings. In contrast to on-cells, most off-cells had axons that collateralized densely within the brainstem raphe and adjacent reticular formation. Such collaterals were either local, within the neuron's dendritic field, or distant, involving a projection of 1-8 mm. One off-cell had a dense terminal field within the sensory trigeminal complex, a projection that may subserve the inhibition of trigeminal sensory neurons produced by raphe magnus stimulation. Well-labeled off-cells had numerous collaterals and dense regions of axonal swellings. In summary, off-cells terminated densely in the raphe magnus and adjacent reticular formation whereas on-cells projected predominantly to the ventrolateral medulla, a region implicated in autonomic control. Local off-cell collaterals provide an anatomical substrate that would enable off-cells to coordinate the activity of on- and off-cells through synaptic contacts.