The effects of periaqueductal gray and nucleus raphe magnus stimulation on the spontaneous and noxious-evoked activity of lateral reticular nucleus neurons in rabbits

Role of the red nucleus in suppressing the jaw-opening reflex following stimulation of the raphe magnus nucleus

In a previous study, we found that electrical and chemical stimulation of the red nucleus (RN) suppressed the high-threshold afferent-evoked jaw-opening reflex (JOR). It has been reported that the RN receives bilaterally projection fibers from the raphe magnus nucleus (RMg), and that stimulation of the RMg inhibits the tooth pulp-evoked nociceptive JOR. These facts imply that RMg-induced inhibition of the JOR could be mediated via the RN. The present study first examines whether stimulation of the RMg suppresses the high-threshold afferent-evoked JOR. The JOR was evoked by electrical stimulation of the inferior alveolar nerve (IAN), and was recorded as the electromyographic response of the anterior belly of the digastric muscle. The stimulus intensity was 4.0 (high-threshold) times the threshold. Conditioning electrical stimulation of the RMg significantly suppressed the JOR. A further study then examined whether electrically induced lesions of the RN or microinjection of muscimol into the RN affects RMg-induced suppression of the JOR. Electrically induced lesions of the bilateral RN and microinjection of muscimol into the bilateral RN both reduced the RMg-induced suppression of the JOR. These results suggest that RMg-induced suppression of the high-threshold afferent-evoked JOR is mediated by a relay in the RN.

Cerebellar Golgi cells in the rat receive multimodal convergent peripheral inputs via the lateral funiculus of the spinal cord

We recently showed that the activity of cerebellar Golgi cells can be powerfully modulated by stimulation of peripheral afferents, in a pattern different to local Purkinje cells. Here we have examined the pathways underlying these responses. Graded electrical stimulation of muscle and cutaneous nerves revealed that long-lasting depressions and short-lasting excitations of Golgi cells were evoked by stimulation of cutaneous nerves at stimulus intensities that activated large mechanoreceptive afferents, and grew as additional afferents were recruited. In contrast, none of the neurones responded to stimulation of muscle nerves at intensities that activated group I afferents, although about half responded with long-lasting depressions, but not excitations, to stimuli that recruited group II and III afferents. Selective lesions of the spinal dorsal columns did not affect either of these types of response. After lesions of one lateral funiculus in the lumbar cord the responses evoked by stimulation of the hindlimb contralateral to the lesion were reduced or abolished, leaving responses evoked by ipsilateral hindlimb afferents unaltered. Since both ipsi- and contralateral afferents generate responses in Golgi cells, the convergence from the two sides must occur supraspinally. It is difficult to reconcile these properties with any of the direct spinocerebellar pathways or spinoreticulocerebellar pathways that have been described. Instead, it is likely that the responses are evoked via the multimodal 'wide dynamic range' neurones of the anterolateral system. Golgi cell activity may thus be powerfully enhanced or depressed during arousal via the anterolateral system.

Unilateral application of an inflammatory irritant to the rat temporomandibular joint region produces bilateral modulation of the jaw-opening reflex

The aim of this study was to determine the effect of unilateral acute inflammation of craniofacial deep tissues on the ipsilateral and contralateral jaw-opening reflex (JOR). The effects of mustard oil (MO), injected into the temporomandibular joint region, were tested on the JOR recorded in the digastric muscle and evoked by low-intensity electrical stimulation of the ipsilateral and contralateral inferior alveolar nerve in anesthetized rats. The MO injection induced a long-lasting suppression of the amplitude of both ipsilaterally and contralaterally evoked JOR, although the latency and duration of the JOR were unaffected. The suppressive effect was more prominent for the contralaterally evoked JOR, and observed even when background activity in the digastric muscle was increased by the MO injection. The results indicate that changes in the JOR amplitude following MO injection do not simply reflect alterations in motoneuronal excitability, and suggest that inflammation of deep craniofacial tissues modulates low-threshold sensory transmission to the motoneurons.

Potential role of medullary raphe-spinal neurons in cutaneous vasoconstriction: an in vivo electrophysiological study

In rabbits, raphe magnus/pallidus neurons form a link in the CNS pathway regulating changes in cutaneous blood flow elicited by nociceptive stimulation and activation of the central nucleus of the amygdala. To characterize relevant raphe-spinal neurons, we performed extracellular recordings from the rostral medullary raphe nuclei in anesthetized, paralyzed, mechanically ventilated rabbits. All studied neurons were antidromically activated from the dorsolateral funiculus of the spinal cord (C(8)-T(2)). Of 129 studied neurons, 40% were silent. The remaining neurons discharged spontaneously at 0.3-29 Hz. Nociceptive stimulation (lip squeeze with pliers) excited 63 (49%), inhibited 9 (7%), and did not affect 57 (44%) neurons. The same stimulation also elicited falls in ear pinna blood flow. In neurons activated by the stimulation, the increase in discharge preceded the fall in flow. Electrical stimulation of the spinal trigeminal tract excited 61/63 nociception-activated neurons [onset latencies range: 6-75 ms, mean: 28 +/- 3 (SE) ms], inhibited 9/9 nociception-inhibited neurons (onset latencies range: 9-85 ms, mean: 32 +/- 10 ms), and failed to affect 55/57 neurons insensitive to nociceptive stimulation. Neurons insensitive to nociceptive/trigeminal stimulation were also insensitive to nonnociceptive tactile stimulation and to electrical stimulation of the amygdala. They were either silent (32/45) or discharged regularly at low frequencies. They possessed long-duration action potentials (1.26 +/- 0.08 ms) and slow-conducting axons (6.0 +/- 0.5 m/s). These neurons may be serotonergic raphe-spinal cells. They do not appear to be involved in nociceptive-related cutaneous vascular control. Of the 63 neurons sensitive to nociceptive and trigeminal tract stimulation, 35 also responded to tactile stimulation (wide receptive field). These neurons possessed short action potentials (0.80 +/- 0.03 ms) and fast-conducting axons (30.3 +/- 3.1 m/s). In this subpopulation, electrical stimulation of the amygdala activated nearly all neurons tested (10/12), with a mean onset latency of 34 +/- 3 ms. The remaining 28 neurons sensitive to nociceptive and trigeminal stimulation did not respond to tactile stimuli and were mainly unaffected by amygdala stimulation. It may be that fast-conducting raphe-spinal neurons, with wide multimodal receptive fields and with input from the central nucleus of the amygdala, constitute the bulbo-spinal link in the CNS pathway regulating cutaneous blood flow in response to nociceptive and alerting stimuli.

Nitric oxide producing neurones in the rat medulla oblongata that project to nucleus tractus solitarii

The production of nitric oxide in neurones of the rat medulla oblongata that project to the nucleus tractus solitarii (NTS) was examined by simultaneous immunohistochemical detection of nitric oxide synthase (NOS) and of cholera toxin B-subunit (CTb), which was injected into the caudal zone of the NTS. Neurones immunoreactive for CTb and neurones immunoreactive for NOS were widely co-distributed and found in almost all the anatomical divisions of the medulla. Dual-labelled cells, containing both CTb and NOS immunoreactivities were more numerous ipsilaterally to the injection sites. They were concentrated principally in the more rostral zone of the NTS, raphé nuclei, dorsal, intermediate and lateral reticular areas, spinal trigeminal and paratrigeminal nuclei and the external cuneate and medial vestibular nuclei. Isolated dual-labelled neurones were also scattered throughout most of the divisions of the reticular formation. These observations indicate that many areas of the medulla that are known to relay somatosensory and viscerosensory inputs contain NOS immunoreactive neurones that project to the NTS, and may, therefore, contribute to the dense NOS-immunoreactive innervation of the NTS. The release of nitric oxide from the axon terminals of these neurones may modulate autonomic responses generated by NTS neurones in relation to peripheral sensory stimuli, and thus ultimately regulate sympathetic and/or parasympathetic outflow.

Characterization of neurons in the area of the medullary lateral reticular nucleus responsive to noxious visceral and cutaneous stimuli

In halothane-anesthetized rats, 283 caudal medullary neurons responsive to colorectal distension (CRD) were characterized using extracellular electrodes. Neurons inhibited by CRD (n = 82) were in the area dorsal to the lateral reticular nucleus (LRN). Most neurons excited by CRD (n = 130) were located within or immediately adjacent to the LRN, were excited by noxious heat and/or noxious pinch of at least half the body surface and were called bilateral nociceptive specific (bNS) neurons. bNS neurons had accelerating responses to graded CRD (threshold: 20 +/- 2 mmHg). Ten of twelve bNS neurons tested could be antidromically activated by electrical stimulation of the midline cerebellum. Other neurons excited by CRD (n = 71) had mixed responses to cutaneous stimuli and were generally located in the area dorsal to the LRN. Increases in blood pressure due to intravenous phenylephrine did not significantly alter the spontaneous activity of neurons excited by CRD, but altered spontaneous activity (12 excited, four inhibited) in all neurons tested which were inhibited by CRD. Decreases in blood pressure produced by intravenous nitroprusside produced a reciprocal response in most neurons inhibited by CRD and had a delayed onset (20-30 s after bolus administration) excitatory effect on 21 of 27 units excited by CRD. Combined with other studies, these data suggest a role for neurons within and adjacent to the LRN in the modulation of visceral nociception. They also implicate a role for the cerebellum in visceral nociceptive processing.

Role of norepinephrine in the interaction between the lateral reticular nucleus and the nucleus raphe magnus: an electrophysiological and behavioral study

We have previously demonstrated that the nucleus raphe magnus (NRM) sends a predominantly inhibitory projection to the lateral reticular nucleus (LRN); however, the pharmacology of this pathway is not known. The purpose of this study was to examine the role of norepinephrine in the NRM-LRN system using both electrophysiological and behavioral techniques. Sixty-nine LRN cells were recorded extracellularly. Cells were tested for their response to noxious and innocuous peripheral stimulation applied to the dorsal body surface. The majority of cells were classified as wide dynamic range, with inhibition being the predominant response; receptive fields were located primarily on the tail and hind limbs. The effect of excitatory amino acid glutamate (GLU) administration into NRM (GLU-NRM) was tested on all 69 cells. GLU-NRM inhibited 55 of 69 LRN cells tested; 7 cells were excited and 7 cells did not respond. Thirty-nine LRN cells were tested for their response to norepinephrine (NE) iontophoretically applied in LRN (NE-LRN). Two distinct types of effects were noted. In 9 cells, both NE-LRN and GLU-NRM produced a strong inhibition, with the magnitude of effect between the 2 drugs significantly correlated. In a second group of cells (n = 12), GLU-NRM produced an inhibitory effect while NE-LRN had no effect on the cells' baseline firing rate. However, when the 2 drugs were applied simultaneously, NE-LRN blocked the inhibitory effects of NRM stimulation. The effect of the alpha 2-receptor antagonist yohimbine (YOH) on NRM-evoked responses was tested in 30 LRN cells. The majority of these cells were inhibited by GLU-NRM. Similar to the dichotomous effect noted by NE-LRN, YOH applied iontophoretically in LRN (YOH-LRN) had two predominant effects on NRM-produced inhibition. In 14 of 27 cells, YOH-LRN significantly potentiated the inhibitory effects of NRM stimulation by increasing the duration of the inhibitory epoch an average of 100 sec. In 7 of 27 cells, YOH directly applied in LRN partially antagonized NRM-evoked inhibition. In a second series of experiments, microinjection cannulas were placed within NRM and LRN in order to determine the effect of blocking alpha 2-receptor activity within LRN on NRM stimulation-produced analgesia in an intact animal. Administration of D,L-homocysteic acid in NRM resulted in a significant increase in baseline tail-flick latency of approximately 140%. Pretreatment with YOH (3 micrograms in 0.5 microliter) in LRN resulted in a significant potentiation of this analgesic effect.(ABSTRACT TRUNCATED AT 400 WORDS)

Electrophysiological characterization of the projection from the nucleus raphe magnus to the lateral reticular nucleus: possible role of an excitatory amino acid in synaptic activation

Numerous studies have shown that the lateral reticular nucleus (LRN), located in the caudal ventrolateral medulla, is an important nuclear region in the descending analgesia system. Activation of this brainstem region, either electrically or chemically, results in a reduction in nociceptive threshold. In addition, destruction of LRN abolishes the tonic descending inhibition present on dorsal horn neurons. Recent neuroanatomical tracing studies have shown that the nucleus raphe magnus (NRM), long implicated in nociception, sends direct projections to LRN; however, no information exists regarding the physiological characteristics of this pathway, nor its role in the endogenous descending analgesia system. The purpose of this study was to physiologically characterize the synaptic influence(s) of projections from the NRM to the LRN using electrophysiological recording, electrical and chemical stimulation, and iontophoretic techniques. Sixty-one percent of LRN neurons responded to single pulse stimulation of NRM; 52% of the responsive cells were excited and 48% were inhibited. The mean latency to onset of excitation was 4.9 +/- 1.2 ms. High frequency (100 Hz) electrical stimulation of NRM influenced 69/102 neurons; 52% (36/69) were excited, while 48% (33/69) were inhibited. Microinjection of glutamate into NRM significantly modified the discharge of 83% (93/112) of LRN cells tested; of these, 71% were inhibited, while 29% were excited. In 35 cells the effects of the excitatory amino acid antagonist kynurenic acid (KYN) were studied. In 75% of the cells excited by glutamate administration into the NRM (18/24), KYN partially antagonized this response. In 11 LRN cells inhibited by NRM chemical stimulation, KYN had no effect on this inhibition. Overall, 95% of the LRN cells responsive to NRM stimulation were also responsive to noxious peripheral stimulation, indicating that these cells are receiving ascending information from the spinal cord regarding somatosensory stimulation as well as receiving descending input from the NRM. It is concluded that LRN neurons are highly responsive to both noxious peripheral stimulation and NRM efferent activation, and that this region plays a significant role as an integrator for both ascending and descending information.

Focal dorsal raphe stimulation and pinnal electrical stimulation modulate spontaneous and noxious evoked responses in thalamic neurons

This study investigated the nocieceptive responses of single neurons within the nucleus parafascicularis (PF) thalami of the rat following two modes of electrical stimulation known to induce analgesia. It was found that both focal electrical dorsal raphe stimulation (DRS) and bilateral pinnal (ear) electrical stimulation (PES) converge on the same PF neurons, affecting both the spontaneous discharges and the noxious evoked responses toward these neurons. The effects of different stimulus current intensity, frequency and pulse duration were also examined. It was found that for both DRS and PES at pulse frequency of 10 Hz and current amplitude of 10 microA are the optimal parameters to modulate both the spontaneous and the noxious evoked responses. These stimuli produced prolonged effects related to the duration of stimulation. The external (PES) low current stimulation which was delivered below the sensory threshold was as effective in modulating noxious responses as the invasive DRS in intact animals and in animals with bilateral dorsolateral-funiculus ablation. It was observed that dorsal lateral funiculus ablation (DLFx) did not modify the DRS and the PES effects. These observations further support the existence of an ascending pain modulation pathway.

Electrophysiological identification of spinally projecting neurons in the lateral reticular nucleus of the rat

Eighty-four neurons in the caudal ventrolateral medullary reticular formation were antidromically activated by the stimulation of the dorsolateral funiculus in 49 urethane-anesthetized rats. Of 76 neurons, 37 had no spontaneous discharge. Of the neurons that had spontaneous discharges, 80% had firing rates between 0.1 and 15 Hz. The average conduction velocity, determined among 70 neurons, was 15.20 +/- 1.23 m/s, and 87% had conduction velocities within the range of 2-30 m/s. This study further confirms the existence of spinally-projecting neurons in the lateral reticular nucleus (LRN) of the caudal medulla, and some of them are probably responsible for the descending controls of nociception from the LRN.