Interaction between central gray and nucleus raphe magnus: role of norepinephrine

Effects of 5-hydroxytryptamine applied into nucleus raphe magnus on nociceptive thresholds and neuronal firing rate

The effect of iontophoretically applied 5-hydroxytryptamine on neurones in nucleus raphe magnus, and the effect of microinjection of 5-hydroxytryptamine into nucleus raphe magnus on nociceptive thresholds were examined in the rat. Iontophoretically applied 5-hydroxytryptamine excited 66% and inhibited 6% of the neurones encountered in the nucleus raphe magnus. The excitatory response to 5-hydroxytryptamine was reduced by the putative serotonergic antagonist cinanserin in 21 of 24 cases. In 12 of these neurones the responses to iontophoretically applied glutamate were also examined. In 11 of the 12 studies the responses to glutamate were reduced by cinanserin. Microinjection of 5 microg of 5-hydroxytryptamine into the nucleus raphe magnus produced analgesia as assessed by the tail-flick response to noxious heat stimulation, but no analgesia as assessed by the paw withdrawal response to pressure. Microinjection of 5 microg of 5-hydroxytryptamine into the adjacent nucleus reticularis paragigantocellularis had no analgesic effect in either test. These results indicate that 5-hydroxytryptamine mainly excites neurones in nucleus raphe magnus and that 5-hydroxytryptamine has an action on neurones in nucleus raphe magnus which modulate the nociceptive threshold.

The role of the periaqueductal gray in the modulation of pain in males and females: are the anatomy and physiology really that different?

Anatomical and physiological studies conducted in the 1960s identified the periaqueductal gray (PAG) and its descending projections to the rostral ventromedial medulla (RVM) and spinal cord dorsal horn, as a primary anatomical pathway mediating opioid-based analgesia. Since these initial studies, the PAG-RVM-spinal cord pathway has been characterized anatomically and physiologically in a wide range of vertebrate species. Remarkably, the majority of these studies were conducted exclusively in males with the implicit assumption that the anatomy and physiology of this circuit were the same in females; however, this is not the case. It is well established that morphine administration produces greater antinociception in males compared to females. Recent studies indicate that the PAG-RVM pathway contributes to the sexually dimorphic actions of morphine. This manuscript will review our anatomical, physiological, and behavioral data identifying sex differences in the PAG-RVM pathway, focusing on its role in pain modulation and morphine analgesia.

Noradrenergic pain modulation

Norepinephrine is involved in intrinsic control of pain. Main sources of norepinephrine are sympathetic nerves peripherally and noradrenergic brainstem nuclei A1-A7 centrally. Peripheral norepinephrine has little influence on pain in healthy tissues, whereas in injured tissues it has variable effects, including aggravation of pain. Its peripheral pronociceptive effect has been associated with injury-induced expression of novel noradrenergic receptors, sprouting of sympathetic nerve fibers, and pronociceptive changes in the ionic channel properties of primary afferent nociceptors, while an interaction with the immune system may contribute in part to peripheral antinociception induced by norepinephrine. In the spinal cord, norepinephrine released from descending pathways suppresses pain by inhibitory action on alpha-2A-adrenoceptors on central terminals of primary afferent nociceptors (presynaptic inhibition), by direct alpha-2-adrenergic action on pain-relay neurons (postsynaptic inhibition), and by alpha-1-adrenoceptor-mediated activation of inhibitory interneurons. Additionally, alpha-2C-adrenoceptors on axon terminals of excitatory interneurons of the spinal dorsal horn possibly contribute to spinal control of pain. At supraspinal levels, the pain modulatory effect by norepinephrine and noradrenergic receptors has varied depending on many factors such as the supraspinal site, the type of the adrenoceptor, the duration of the pain and pathophysiological condition. While in baseline conditions the noradrenergic system may have little effect, sustained pain induces noradrenergic feedback inhibition of pain. Noradrenergic systems may also contribute to top-down control of pain, such as induced by a change in the behavioral state. Following injury or inflammation, the central as well as peripheral noradrenergic system is subject to various plastic changes that influence its antinociceptive efficacy.

Analgesic effect of L-threo-3,4-dihydroxyphenylserine (L-DOPS) in patients with chronic pain

Previous pharmacological studies using animals indicated that a systemic administration of L-threo-3,4-dihydroxy-phenylserine (L-DOPS), a precursor of noradrenaline, induces an antinociceptive effect and an increase in the CNS level of noradrenaline which serves an inhibitory role at the spinal dorsal horn and the supraspinal pain afferent system. The aim of the present study was to investigate the analgesic effect of L-DOPS in patients with chronic pain. We selected 18 patients with various kinds of pain. In nine patients, L-DOPS (tablets) was orally administered and pain was assessed by the visual analogue scale (VAS) before and after the L-DOPS administration. Administration of L-DOPS resulted in dose-dependent analgesia. The maximum analgesia (VAS change: from 10 to 4.1 +/- 0.9) was observed 60 min after an administration of 100 mg and it lasted for 2-5 h depending on the patient. In the other nine patients, oral administration of placebo tablets produced only a slight analgesia (VAS change: from 10 to (9.2 +/- 0.3). The difference between the L-DOPS-induced effect and the placebo-induced one was statistically significant. After repeated administration of L-DOPS for 4-5 weeks, neither tolerance nor side effects were observed.

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.

The noradrenaline precursor L-threo-3,4-dihydroxyphenylserine exhibits antinociceptive activity via central alpha-adrenoceptors in the mouse

1. Systemic (s.c. or p.o.) administration of L-threo-3,4-dihydroxyphenylserine (droxidopa, L-threo-DOPS; L-DOPS), a noradrenaline precursor, at a dose-range of 100-800 mg kg-1, produced naloxone-resistant antinociception in a dose-dependent manner in the mouse, as assessed by the tail flick test, kaolin-induced writhing test and formalin-induced nociception test. 2. Antinociception elicited by L-DOPS (400 mg kg-1, s.c.) was not affected by s.c. injection of benserazide, a peripherally preferential L-aromatic amino acid decarboxylase inhibitor, but was suppressed by its intracerebroventricular (i.c.v.) injection. 3. I.c.v. or intrathecal (i.t.) administration of the non-selective alpha-blocker, phentolamine, significantly reduced L-DOPS-induced antinociception. 4. I.c.v. administration of the alpha 1-blocker, prazosin, but not the alpha 2-blocker, yohimbine, abolished the antinociceptive effects of L-DOPS. In contrast, both blockers, when administered i.t., exhibited significant inhibitory effects. 5. These results suggest that systemic L-DOPS produces opioid-independent antinociception, mediated by supraspinal alpha 1-adrenoceptors and by spinal alpha 1- and alpha 2-adrenoceptors and may predict additional therapeutic applications of L-DOPS as an analgesic.

Direct and indirect actions of morphine on medullary neurons that modulate nociception

The rostral ventromedial medulla is part of a neural network through which systemically administered morphine produces antinociception. Two physiologically characterized classes of presumed nociceptive modulating neurons that respond differentially to systemically administered morphine have been identified in this region: the firing of "on-cells" is depressed, whereas "off-cells" become continuously active. On-cells have been proposed to permit or facilitate, and off-cells to inhibit, nociceptive transmission. Because local application of morphine in the rostral ventromedial medulla itself is sufficient to produce antinociception, it is important to determine whether systemically administered morphine exerts its effects on neurons in this region by a direct action. Thus, activity of physiologically characterized neurons was studied before, during and after ionotophoretic administration of morphine. As with systemic administration, iontophoretic application of morphine depresses the activity of on-cells, an effect that is reversed by iontophoretic as well as by systemic administration of naloxone. In contrast, no reliable changes in the firing of off-cells are produced by iontophoretic administration of morphine. Cells of a third class, "neutral cells", are not affected by systemic morphine administration, nor do they respond to iontophoretic application of the drug. The present experiments demonstrate that direct opioid responsiveness in the rostral ventromedial medulla is limited to a single physiologically characterized class of presumed nociceptive modulatory neuron, the on-cell. This implies that the antinociceptive effect exerted by systemically administered morphine involves at least two components within the rostral ventromedial medulla: a direct inhibition of on-cells, and an indirect activation of off-cells. Each of these actions is likely to have a net hypoalgesic effect.

The effects of intracerebroventricular injection of naloxone, phentolamine and methysergide on the transmission of nociceptive signals in rat dorsal horn neurons with convergent cutaneous-deep input

In anaesthetized rats, recordings were made from nociceptive dorsal horn neurons with convergent input from the skin and deep somatic tissues. The results of a previous study have shown that in these neurons the input from deep nociceptors is subjected to a much stronger tonic descending inhibition than is the input from cutaneous nociceptors. The aim of the present study was to find out whether at supraspinal levels opioidergic, adrenergic, or serotoninergic transmitters are involved in this quite specific inhibition of deep nociception. Injections of naloxone, phentolamine, and methysergide into the third ventricle showed that only naloxone is capable of abolishing the tonic inhibition of the deep nociceptive input to spinal neurons. The input from cutaneous nociceptors to the same cells was largely unaffected by naloxone. Thus the effects of intracerebroventricular injection of naloxone resembled those obtained with a spinal cold block in a previous study; with the exception that the increase in background activity--which is prominent during cold block--was missing after the injection of naloxone. The present results demonstrate that the tonic descending inhibition of the deep nociception operates with opioidergic synapses at the supraspinal level. In contrast, supraspinal adrenergic and serotoninergic mechanisms do not appear to contribute to the tonic inhibition. The data confirm and extend previous results which suggested that a particular portion of the descending antinociceptive system may act mainly on the input from deep nociceptors. Pharmacologically, this particular portion seems to be opioidergic in nature.

Putative nociceptive modulating neurons in the rostral ventromedial medulla of the rat: firing of on- and off-cells is related to nociceptive responsiveness

In the unstimulated, lightly anesthetized rat, both on- and off-cells exhibit alternating periods of silence and activity lasting from several seconds to a few minutes. In the preceding paper, we showed that the active periods of all cells of the same class are always in phase, whereas the firing of cells of different classes is invariably out of phase. Thus, the pattern of firing of any single on- or off-cell provides a useful indication of the excitability of all on- and off-cells in the rostral ventromedial medulla (RVM). In this study, we measured the latency of the tail flick response (TF) at set intervals while recording from TF-related neurons in RVM, and were able to demonstrate a significant relationship between the spontaneous firing of both on- and off-cells and the latency of the TF response. If noxious heat is applied at a time when an off-cell is spontaneously active (or an on-cell is silent), the TF latency is longer than if the TF trial falls during a period in which the off-cell is silent (or the on-cell is active). This correlation between on- and off-cell firing and changes in TF latency is consistent with a nociceptive modulatory role for either or both cell classes. These findings support the hypothesis that off-cells inhibit and on-cells facilitate spinal nociceptive transmission and reflexes.

Evidence for excitatory 5-HT2-receptors on rat brainstem neurones

1. The technique of microiontophoresis was used to investigate the identity of the receptor mediating the excitatory effects of 5-hydroxytryptamine (5-HT) upon neurones in the midline of the medullary brainstem of the rat in vivo. 2. The 5-HT1-like receptor agonists 5-carboxamidotryptamine (5-CT) and 8-hydroxy-2-(di-n-propylamino) tetralin (8-OH-DPAT) failed to excite the majority of neurones excited by 5-HT. The mobilities of 5-CT and 8-OH-DPAT when tested in vitro were found not to differ significantly from that of 5-HT, suggesting that the lack of effect of these agonists was not due to a lower rate of release from the microelectrodes. 3. The excitatory responses to 5-HT were attenuated by the 5-HT 2-receptor antagonists ketanserin and methysergide when applied microiontophoretically or administered intravenously (0.3 and 1 mg kg-1 respectively). Excitatory responses to glutamate and noradrenaline were not reduced. 4. The 5-HT3-receptor antagonist MDL 72222 failed to attenuate selectively the excitatory response to 5-HT when applied either by microiontophoresis or administered intravenously (1 mg kg-1). 5. Microiontophoretic application of the alpha 1-adrenoceptor antagonist prazosin did not attenuate excitatory responses to either 5-HT or noradrenaline. Intravenously administered prazosin (0.8 mg kg-1) also failed to attenuate excitatory responses to 5-HT, but did block excitatory responses to noradrenaline. 6. These results suggest that 5-HT2-receptors, but not 5-HT1-like receptors, 5-HT3-receptors or alpha 1-adrenoceptors, are involved in the excitatory response of midline medullary neurones to 5-HT.

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

In urethane-anesthetized rabbits the effects of periaqueductal gray (PAG) and nucleus raphe magnus (NRM) stimulation on the spontaneous and noxious-evoked activity of the lateral reticular nucleus (LRN) neurons were studied. The PAG and the NRM stimulating electrodes were located in the optimal sites for suppressing the jaw-opening reflex (JOR) evoked by the tooth pulp stimulation. It was found that the 12% of neurons tested were affected by one or both stimuli. A total of 80 responsive neurons (52% antidromically activated by the cerebellum) were analyzed. Out of these neurons, 31 showed a convergence to both stimuli, 43 responded only to PAG and 6 only to NRM. Noxious heat stimulation of the contralateral foot was effective in altering the activity of 60% of these neurons. The PAG and NRM stimuli modified the noxious-evoked responses in most of these units. While the excitation was the predominant effect on the spontaneous activity (52 cells), the inhibition was predominant on the noxious-evoked activity (29 cells). These results indicate the presence of connections from PAG and NRM to LRN, probably devoted to the processing of the nociceptive information.

Alterations in nociception following lesions of the A5 catecholamine nucleus

Neurons located in the nucleus raphe magnus (NRM), a region important in the control of nociception, appear to be tonically inhibited by noradrenergic (NA) neurons. Anatomical studies have suggested that the A5 catecholamine nucleus may be the primary source of noradrenergic neurons whose terminals are located in the NRM. The purpose of the present study was to examine the role of A5 neurons in the modulation of nociception. Bilateral electrolytic lesions of the A5 nuclei produced a marked and long lasting antinociception as assessed by both the tail-flick and hot-plate tests. Unilateral A5 lesions also produced a long-lasting elevation in hot-plate latency, but the elevation of tail-flick latency was smaller in magnitude and was only observed one day following the lesion. This finding is consistent with previous studies which have shown that blockade of the NA input to the NRM by the microinjection of NA antagonists also produces antinociception. These data indicate that neurons located in the A5 nucleus may be the origin of this NA projection to the NRM. The elevation in tail-flick latency observed following A5 lesions was significantly attenuated by the intrathecal injection of either the NA antagonist phentolamine or the serotonergic antagonist methysergide. However, the elevation in hot-plate latency was not significantly altered by these monoaminergic antagonists. Similarly, previous studies have shown that the elevation in tail-flick, but not hot-plate latency, produced by the microinjection of NA antagonists in the NRM is attenuated by the intrathecal injection of either phentolamine or methysergide.(ABSTRACT TRUNCATED AT 250 WORDS)

The site and the mode of analgesic actions exerted by clonidine in monkeys

We used intracerebral administration of clonidine in monkeys to map effective sites for analgesia. The jaw opening reflex elicited by tooth pulp stimulation was used for analgesia testing. We found that the most consistently effective sites for analgesia in monkeys are in at least three brain regions: the diencephalic periventricular gray, the dorsal raphe nuclei, and the periaqueductal gray. In addition, the analgesia induced by intracerebral administration of clonidine was effectively antagonized by pretreatment of animals with either naloxone (a narcotic antagonist) or yohimbine (an alpha-adrenergic antagonist). These results suggest the existence of an opiate and an adrenergic antinociceptive mechanism in the diencephalic periventricular gray, the dorsal raphe nuclei, and the periaqueductal gray activated by clonidine in primates.

Single unit studies of identified bulbospinal serotonergic units

Non-serotonergic bulbospinal neurons were identified by conduction velocities greater than 6 m/s. These units were found to fire at rates from 0 to 22 Hz, to respond to sensory stimuli with either excitation or inhibition, and to have unremarkable spike shapes. In iontophoretic experiments, both excitation and inhibition were observed in response to acetylcholine, norepinephrine and serotonin. Serotonergic bulbospinal neurons were identified by their conduction velocities below 6 m/s. These neurons, (which have been shown to be destroyed by 5,7-dihydroxytryptamine), exist as two groups: a a slower-conducting group with conduction velocities below 1.2 m/s, and a faster-conducting group with conduction velocities between 2 and 6 m/s. The neurons of the faster-conducting group were found to be similar to the non-serotonergic group in their firing, spike shapes and responses to sensory stimuli; while the units of the slower-conducting group were consistently found to fire between 0.03 and 6 Hz, to respond to sensory stimuli only with excitation, and to have distinctive spike shapes. Despite these differences, both groups of serotonergic units were found to be consistently inhibited by ACh, NE and 5-HT. In contrast to reports of serotonergic neurons in the midbrain, these units were not generally found to be inhibited by i.v. LSD. It is concluded that the serotonergic neurons of the medullary raphe are distinct both from the non-serotonergic neurons, and from serotonergic neurons in other parts of the brain.

The periaqueductal gray-raphe magnus projection contains somatostatin, neurotensin and serotonin but not cholecystokinin

The retrograde transport-HRP-immunocytochemical technique was employed to ascertain if the periaqueductal gray-raphe magnus projection arises from neurons containing somatostatin, neurotensin, serotonin or cholecystokinin. Following HRP injections into the raphe magnus (NRM) double-labeled cells containing HRP reaction product and somatostatin-, neurotensin- or serotonin-like immunoreactivity were identified in the midbrain periaqueductal gray (PAG). No cholecystokinin-like immunoreactive double-labeled neurons were found in the PAG. These results indicate that the PAG-NRM pathway contains somatostatin, neurotensin and serotonin but not cholecystokinin.

The role of acetylcholine in the function of the nucleus raphe magnus and in the interaction of this nucleus with the periaqueductal gray

The nucleus raphe magnus (NRM) plays an important role in the inhibition of pain. Although this region receives afferents from several areas of the brain, the afferent input from the periaqueductal gray (PAG) has been shown to have significant physiological importance. Together, these two sites constitute the major component of a descending network involved in pain inhibition. In this study the role of acetylcholine (ACh) in the function of the NRM was investigated and the possibility that ACh may be a transmitter between the PAG and the NRM was tested. ACh was applied iontophoretically. Scopolamine and gallamine were used to test the type of cholinergic receptors that are present in the NRM. The results of this study shows the following. (1) The majority of the cells in the NRM are excited by ACh. (2) This response to ACh is partially or totally blocked by scopolamine whereas gallamine does not block the response. (3) There is no correlation between the excitatory response to stimulation of PAG and to ACh. There are cells that respond to PAG stimulation by inhibition but are excited by ACh and there are a few cells that are inhibited by ACh but are excited by PAG stimulation. (4) Scopolamine, at a dose that blocks the ACh response, does not block the response to PAG stimulation. (5) There is no correlation between the response to ACh and the type of projection (direct or indirect) to the spinal cord, as tested by stimulation of the dorsolateral funiculus. From these results it is concluded that ACh is an excitatory transmitter at the NRM region but this transmitter does not mediate the interaction between the PAG and NRM.