Organization of tyrosine hydroxylase- and serotonin-immunoreactive brainstem neurons with axon collaterals to the periaqueductal gray and the spinal cord in the rat

Insight gained from using animal models to study pain in Parkinson's disease

Pain is one of the key non-motor symptoms experienced by a large proportion of people living with Parkinson's disease (PD), yet the mechanisms behind this pain remain elusive and as such its treatment remains suboptimal. It is hoped that through the study of animal models of PD, we can start to unravel some of the contributory mechanisms, and perhaps identify models that prove useful as test beds for assessing the efficacy of potential new analgesics. However, just how far along this journey are we right now? Is it even possible to model pain in PD in animal models of the disease? And have we gathered any insight into pain mechanisms from the use of animal models of PD so far? In this chapter we intend to address these questions and in particular highlight the findings generated by others, and our own group, following studies in a range of rodent models of PD.

Effects of Salmon Calcitonin on the Concentrations of Monoamines in Periaqueductal Gray in Formalin Test

Background:

The receptors of salmon calcitonin, located on certain areas of the brain such as the periaqueductal gray matter, are responsible for pain modulation.

Aims:

The effects of intracerebroventricular injection of salmon calcitonin on the behavioral response to pain and on the levels of monoamines in the periaqueductal gray were explored using a biphasic animal model of pain.

Study Design:

Animal experiment.

Methods:

A total of 45 male rats were divided into four groups (n=6). Salmon calcitonin was injected into the lateral ventricle of the brain (1.5 nmol, with a volume of 5 μL). After 20 min, 2.5% formalin was subcutaneously injected into the right leg claw, and pain behavior was recorded on a numerical basis. At the time of the formalin test, the periaqueductal gray area was microdialized. High-performance liquid chromatography method was used to gauge the levels of monoamines and their metabolites.

Results:

Intracerebroventricular injections of salmon calcitonin resulted in pain reduction in the formalin test (p<0.05). The dialysate concentrations of serotonin, dopamine, norepinephrine, 5-hydroxyindoleacetic acid, 3,4-dihydroxyphenylacetic, and 4-hydroxy-3-methoxyphenylglycol increased in the periaqueductal gray area in different phases of the formalin pain test (p<0.05).

Conclusion:

Salmon calcitonin reduced pain by increasing the concentrations of monoamines and the metabolites derived from them in the periaqueductal gray area.

Acute nociceptive stimuli rapidly induce the activity of serotonin and noradrenalin neurons in the brain stem of awake mice

Nociception is an important type of perception that has major influence on daily human life. There are some descending pathways related to pain management and modulation, which are collectively known as the descending antinociceptive system (DAS). Noradrenalin (NA) in the locus coeruleus (LC) and serotonin (5-HT) in the rostral ventromedial medulla (RVM) are components of the DAS. Most 5-HT neurons in the dorsal raphe (DR) have ascending projections rather than descending projections, and they project to the thalamus that modulates nociception. Both the DAS and the DR are believed to be involved in pain-emotion symptoms. In this study, we utilized a fiber photometry system to specifically examine the activity of LC NA neurons and RVM/DR 5-HT neurons using mice carrying tetracycline-controlled transactivator transgene (tTA) under the control of either a dopamine β-hydroxylase promoter or a tryptophan hydroxylase-2 promoter and site-specific infection of an adeno-associated virus carrying a TetO G-CaMP6 gene. After confirmation of specific expression of G-CaMP6 in the target populations, changes in green fluorescent signal intensity were recorded in awake mice upon exposure to acute nociceptive stimulation consisting of a pinch and application of heat (55 °C) to the tail. Both stimuli resulted in rapid and transient (<15 s) increases in the activity of LC NA neurons and RVM/DR 5-HT neurons while the control stimuli did not induce any changes. The present results clearly indicate that acute nociceptive stimuli increase the activity of LC NA neurons and RVM/DR 5 H T neurons and suggest a possible therapeutic target for pain treatment.

Corticotropin-Releasing Factor in the Brain and Blocking Spinal Descending Signals Induce Hyperalgesia in the Latent Sensitization Model of Chronic Pain

Latent sensitization is a model of chronic pain in which an injury triggers a period of hyperalgesia followed by an apparent recovery, but in which pain sensitization persists but is suppressed by opioid and adrenergic receptors. One important characteristic of latent sensitization is that hyperalgesia can be triggered by acute stress. To determine whether the effect of stress is mimicked by the activation of corticotropin-releasing factor (CRF) signaling in the brain, rats with latent sensitization induced by injecting complete Freund's adjuvant (CFA, 50 μl) in one hind paw were given an intracerebroventricular (i.c.v.) injection of CRF. The i.c.v. injection of CRF (0.6 μg, 10 μl), but not saline, induced bilateral mechanical hyperalgesia in rats with latent sensitization. In contrast, CRF i.c.v. did not induce hyperalgesia in rats without latent sensitization (injected with saline in the hind paw). To determine whether descending pain inhibition mediates the suppression of hyperalgesia in latent sensitization, rats with CFA-induced latent sensitization received an intrathecal injection of lidocaine (10%, 1 μl) at the cervical-thoracic spinal cord to produce a spinal block. Lidocaine-injected rats, but not rats injected intrathecally with saline, developed bilateral mechanical hyperalgesia. Intrathecal lidocaine did not induce hyperalgesia in rats without latent sensitization (injected with saline in the hind paw). These results show that i.c.v. CRF mimicked the hyperalgesic response triggered by stress during latent sensitization, possibly by blocking inhibitory spinal descending signals that suppress hyperalgesia.

Brainstem catecholaminergic neurones and breathing control during postnatal development in male and female rats

KEY POINTS

The brainstem catecholaminergic (CA) modulation on ventilation changes with development. We determined the role of the brainstem CA system in ventilatory control under normocapnic and hypercapnic conditions during different phases of development [postnatal day (P)7-8, P14-15 and P20-21] in male and female Wistar rats. Brainstem CA neurones produce a tonic inhibitory drive that affects breathing frequency in P7-8 rats and provide an inhibitory drive during hypercapnic conditions in both males and females at P7-8 and P14-15. In pre-pubertal rats, brainstem CA neurones become excitatory for the CO₂ ventilatory response in males but remain inhibitory in females. Diseases such as sudden infant death syndrome, congenital central hypoventilation syndrome and Rett syndrome have been associated with abnormalities in the functioning of CA neurones; therefore, the results of the present study contribute to a better understanding of this system.

ABSTRACT

The respiratory network undergoes significant development during the postnatal phase, including the maturation of the catecholaminergic (CA) system. However, postnatal development of this network and its effect on the control of pulmonary ventilation ( V̇E ) is not fully understood. We investigated the involvement of brainstem CA neurones in respiratory control during postnatal development [postnatal day (P)7-8, P14-15 and P20-21], in male and female rats, through chemical injury with conjugated saporin anti-dopamine β-hydroxylase (DβH-SAP). Thus, DβH-SAP (420 ng μL^-1 ), saporin (SAP) or phosphate buffered solution (PBS) was injected into the fourth ventricle of neonatal Wistar rats of both sexes. V̇E and oxygen consumption were recorded 1 week after the injections in unanaesthetized neonatal and juvenile rats during room air and hypercapnia. The resting ventilation was higher in both male and female P7-8 lesioned rats by 33%, with a decrease in respiratory variability being observed in males. The hypercapnic ventilatory response (HCVR) was altered in male and female lesioned rats at all postnatal ages. At P7-8, the HCVR for males and females was increased by 37% and 30%, respectively. For both sexes at P14-15 rats, the increase in V̇E during hypercapnia was 37% higher for lesioned rats. A sex-specific difference in HCRV was observed at P20-21, with lesioned males showing a 33% decrease, and lesioned females showing an increase of 33%. We conclude that brainstem CA neurones exert a tonic inhibitory effect on V̇E in the early postnatal days of the life of a rat, increase variability in P7-8 males and modulate HCRV during the postnatal phase.

Monoaminergic descending pathways contribute to modulation of neuropathic pain by increasing-intensity treadmill exercise after peripheral nerve injury

This study characterizes the impact of increasing-intensity treadmill exercise (iTR) on noradrenergic (NE) and serotonergic (5HT) modulation of neuropathic pain. Following sciatic nerve transection and repair (SNTR) rats developed significant mechanical and thermal hyperalgesia that was partially prevented by iTR performed during the first 2weeks after injury. Marked decrease in the expression of 5HT_2A and α_1A and β-, but not α_2A adrenergic receptors in the spinal cord dorsal horn was associated to SNTR and recovered by iTR, particularly in lamina II. iTR significantly increased 5HT_2A in periaqueductal grey (PAG), raphe magnus (RM) and dorsal raphe nucleus (DRN), with a pattern suggesting reorganization of serotonergic excitatory interconnections between PAG and DRN. iTR also increased the expression of α_1A in locus coeruleus (LC) and DRN, and β₂ in LC, indicating that exercise enhanced activity of NE neurons, likely by activating autologous projections from DRN and PAG. iTR hypoalgesia was antagonized by blockade of β₂ and 5HT_2A receptors with administration of butoxamine and ketanserin. The neurotoxin DSP4 was injected to induce depletion of NE projections from LC before starting iTR. DSP4 treatment worsened mechanical hyperalgesia, but iTR hypoalgesia was similarly produced. Moreover, 5HT_2A expression in LC further increased after DSP4 injection, all these results suggesting an intrinsic regulation of 5HT and NE activity between PAG, DRN and LC neurons activated by iTR. Finally, iTR significantly reduced microglial reactivity in LC and increased non-microglial BDNF expression, an effect that was reverted by butoxamine, implicating BDNF regulation in central 5HT/NE actions on neuropathic pain.

Afferent and efferent connections of C1 cells with spinal cord or hypothalamic projections in mice

The axonal projections and synaptic input of the C1 adrenergic neurons of the rostral ventrolateral medulla (VLM) were examined using transgenic dopamine-beta hydroxylase Cre mice and modified rabies virus. Cre-dependent viral vectors expressing TVA (receptor for envelopeA) and rabies glycoprotein were injected into the left VLM. EnvelopeA-pseudotyped rabies-EGFP glycoprotein-deficient virus (rabies-EGFP) was injected 4-6 weeks later in either thoracic spinal cord (SC) or hypothalamus. TVA immunoreactivity was detected almost exclusively (95 %) in VLM C1 neurons. In mice with SC injections of rabies-EGFP, starter cells (expressing TVA + EGFP) were found at the rostral end of the VLM; in mice with hypothalamic injections starter C1 cells were located more caudally. C1 neurons innervating SC or hypothalamus had other terminal fields in common (e.g., dorsal vagal complex, locus coeruleus, raphe pallidus and periaqueductal gray matter). Putative inputs to C1 cells with SC or hypothalamic projections originated from the same brain regions, especially the lower brainstem reticular core from spinomedullary border to rostral pons. Putative input neurons to C1 cells were also observed in the nucleus of the solitary tract, caudal VLM, caudal spinal trigeminal nucleus, cerebellum, periaqueductal gray matter and inferior and superior colliculi. In sum, regardless of whether they innervate SC or hypothalamus, VLM C1 neurons receive input from the same general brain regions. One interpretation is that many types of somatic or internal stimuli recruit these neurons en bloc to produce a stereotyped acute stress response with sympathetic, parasympathetic, vigilance and neuroendocrine components.

Central nervous system control of gastrointestinal motility and secretion and modulation of gastrointestinal functions

Although the gastrointestinal (GI) tract possesses intrinsic neural plexuses that allow a significant degree of autonomy over GI functions, the central nervous system (CNS) provides extrinsic neural inputs that regulate, modulate, and control these functions. While the intestines are capable of functioning in the absence of extrinsic inputs, the stomach and esophagus are much more dependent upon extrinsic neural inputs, particularly from parasympathetic and sympathetic pathways. The sympathetic nervous system exerts a predominantly inhibitory effect upon GI muscle and provides a tonic inhibitory influence over mucosal secretion while, at the same time, regulates GI blood flow via neurally mediated vasoconstriction. The parasympathetic nervous system, in contrast, exerts both excitatory and inhibitory control over gastric and intestinal tone and motility. Although GI functions are controlled by the autonomic nervous system and occur, by and large, independently of conscious perception, it is clear that the higher CNS centers influence homeostatic control as well as cognitive and behavioral functions. This review will describe the basic neural circuitry of extrinsic inputs to the GI tract as well as the major CNS nuclei that innervate and modulate the activity of these pathways. The role of CNS-centered reflexes in the regulation of GI functions will be discussed as will modulation of these reflexes under both physiological and pathophysiological conditions. Finally, future directions within the field will be discussed in terms of important questions that remain to be resolved and advances in technology that may help provide these answers.

Spinal cord stimulation modulates supraspinal centers of the descending antinociceptive system in rats with unilateral spinal nerve injury

BACKGROUND

The descending antinociceptive system (DAS) is thought to play crucial roles in the antinociceptive effect of spinal cord stimulation (SCS), especially through its serotonergic pathway. The nucleus raphe magnus (NRM) in the rostral ventromedial medulla is a major source of serotonin [5-hydroxytryptamine (5-HT)] to the DAS, but the role of the dorsal raphe nucleus (DRN) in the ventral periaqueductal gray matter is still unclear. Moreover, the influence of the noradrenergic pathway is largely unknown. In this study, we evaluated the involvement of these serotonergic and noradrenergic pathways in SCS-induced antinociception by behavioral analysis of spinal nerve-ligated (SNL) rats. We also investigated immunohistochemical changes in the DRN and locus coeruleus (LC), regarded as the adrenergic center of the DAS, and expression changes of synthetic enzymes of 5-HT [tryptophan hydroxylase (TPH)] and norepinephrine [dopamine β-hydroxylase (DβH)] in the spinal dorsal horn.

RESULTS

Intrathecally administered methysergide, a 5-HT1- and 5-HT2-receptor antagonist, and idazoxan, an α2-adrenergic receptor antagonist, equally abolished the antinociceptive effect of SCS. The numbers of TPH-positive serotonergic and phosphorylated cyclic AMP response element binding protein (pCREB)-positive neurons and percentage of pCREB-positive serotonergic neurons in the DRN significantly increased after 3-h SCS. Further, the ipsilateral-to-contralateral immunoreactivity ratio of DβH increased in the LC of SNL rats and reached the level seen in naïve rats, even though the number of pCREB-positive neurons in the LC was unchanged by SNL and SCS. Moreover, 3-h SCS did not increase the expression levels of TPH and DβH in the spinal dorsal horn.

CONCLUSIONS

The serotonergic and noradrenergic pathways of the DAS are involved in the antinociceptive effect of SCS, but activation of the DRN might primarily be responsible for this effect, and the LC may have a smaller contribution. SCS does not potentiate the synthetic enzymes of 5HT and norepinephrine in the neuropathic spinal cord.

Columnar distribution of catecholaminergic neurons in the ventrolateral periaqueductal gray and their relationship to efferent pathways

The periaqueductal gray (PAG) is a critical brain region involved in opioid analgesia and provides efferents to descending pathways that modulate nociception. In addition, the PAG contains ascending pathways to regions involved in the regulation of reward, including the substantia nigra (SN) and the ventral tegmental area (VTA). SN and VTA contain dopaminergic neurons that are critical for the maintenance of positive reinforcement. Interestingly, the PAG is also reported to contain a population of dopaminergic neurons. In this study, the distribution of catecholaminergic neurons within the ventrolateral (vl) PAG was examined using immunocytochemical methods. In addition, the catecholaminergic PAG neurons were examined to determine whether these neurons are integrated into ascending (VTA, SN) and descending rostral ventral medulla (RVM) efferent pathways from this region. The immunocytochemical analysis determined that catecholaminergic neurons in the PAG are both dopaminergic and noradrenergic and these neurons have a distinct rostrocaudal distribution within the ventrolateral column of PAG. Dopaminergic neurons were concentrated rostrally and were significantly smaller than noradrenergic neurons. Combined immunocytochemistry and tract tracing methods revealed that catecholaminergic neurons are distinct from, but closely associated with, both ascending and descending efferent projection neurons. Finally, by electron microscopy, catecholaminergic neurons showed close dendritic appositions with other neurons in PAG, suggesting a possible nonsynaptic mechanism for regulation of PAG output by these neurons. In conclusion, our data indicate that there are two populations of catecholaminergic neurons in the vlPAG that form dendritic associations with both ascending and descending efferents suggesting a possible nonsynaptic modulation of vlPAG neurons.

Inhibition of A5 Neurons Facilitates the Occurrence of REM Sleep-Like Episodes in Urethane-Anesthetized Rats: A New Role for Noradrenergic A5 Neurons?

When rapid eye movement (REM) sleep occurs, noradrenergic cells become silent, with the abolition of activity in locus coeruleus (LC) neurons seen as a key event permissive for the occurrence of REM sleep. However, it is not known whether silencing of other than LC noradrenergic neurons contributes to the generation of REM sleep. In urethane-anesthetized rats, stereotyped REM sleep-like episodes can be repeatedly elicited by injections of the cholinergic agonist, carbachol, into a discrete region of the dorsomedial pons. We used this preparation to test whether inhibition of ventrolateral pontine noradrenergic A5 neurons only, or together with LC neurons, also can elicit REM sleep-like effects. To silence noradrenergic cells, we sequentially injected the α₂-adrenergic agonist clonidine (20–40 nl, 0.75 mM) into both A5 regions and then the LC. In two rats, successful bilateral clonidine injections into the A5 region elicited the characteristic REM sleep-like episodes (hippocampal theta rhythm, suppression of hypoglossal nerve activity, reduced respiratory rate). In five rats, bilateral clonidine injections into the A5 region and then into one LC triggered REM sleep-like episodes, and in two rats injections into both A5 and then both LC were needed to elicit the effect. In contrast, in three rats, uni- or bilateral clonidine injections only into the LC had no effect, and clonidine injections placed in another six rats outside of the A5 and/or LC regions were without effect. The REM sleep-like episodes elicited by clonidine had similar magnitude of suppression of hypoglossal nerve activity (by 75%), similar pattern of hippocampal changes, and similar durations (2.5–5.3 min) to the episodes triggered in the same preparation by carbachol injections into the dorsomedial pontine reticular formation. Thus, silencing of A5 cells may importantly enable the occurrence of REM sleep-like episodes, at least under anesthesia. This is a new role for noradrenergic A5 neurons.

Neurochemistry of bulbospinal presympathetic neurons of the medulla oblongata

This review focuses on presympathetic neurons in the medulla oblongata including the adrenergic cell groups C1-C3 in the rostral ventrolateral medulla and the serotonergic, GABAergic and glycinergic neurons in the ventromedial medulla. The phenotypes of these neurons including colocalized neuropeptides (e.g., neuropeptide Y, enkephalin, thyrotropin-releasing hormone, substance P) as well as their relative anatomical location are considered in relation to predicting their function in control of sympathetic outflow, in particular the sympathetic outflows controlling blood pressure and thermoregulation. Several explanations are considered for how the neuroeffectors coexisting in these neurons might be functioning, although their exact purpose remains unknown. Although there is abundant data on potential neurotransmitters and neuropeptides contained in the presympathetic neurons, we are still unable to predict function and physiology based solely on the phenotype of these neurons.

The Origin of Brainstem Noradrenergic and Serotonergic Projections to the Spinal Cord Dorsal Horn in the Rat

Although it has been proposed that the locus coeruleus is the predominant, if not exclusive, brainstem origin of the noradrenergic innervation of the spinal dorsal horn, pharmacological studies argue otherwise. In this study we made localized injections of the retrograde tracer wheatgerm agglutinin conjugated to apo-horseradish peroxidase gold (WGA:apoHRP-Au), in conjunction with immunocytochemical labeling for tyrosine hydroxylase (TH) or serotonin (5-HT), to identify the brainstem source of the noradrenaline (NA) and 5-HT innervation of the dorsal horn of the rat. Our studies were concentrated in the C5 spinal segment. The pattern of labeling was only studied in animals in which the tracer injection was restricted to the dorsal horn. In these rats, TH-immunoreactive neurons in widespread regions of the brainstem, including the locus coeruleus, subcoeruleus, A5, and A7 cell groups, were found to project to the dorsal horn. In terms of absolute numbers of double-labeled cells, no one noradrenergic cell group predominated. As expected, dorsal-horn-projecting 5-HT-immunoreactive neurons were found within the 5-HT populations of the rostroventromedial medulla and caudal pons, including the nucleus raphe magnus, nucleus paragigantocellularis (PGi), and ventral portions of the nucleus gigantocellularis (Gi). The majority of retrogradely labeled 5-HT-immunoreactive cells were, however, located off the midline, in the ipsilateral PGi and ventral Gi. Finally, a large number of retrogradely labeled, non-5-HT cells were found intermingled among the 5-HT cells of this region. Our results provide evidence that the noradrenergic regulation of nociceptive transmission at the spinal cord level arises from direct spinal projections of several brainstem noradrenergic cell groups.

Catecholaminergic systems in stress: structural and molecular genetic approaches

Stressful stimuli evoke complex endocrine, autonomic, and behavioral responses that are extremely variable and specific depending on the type and nature of the stressors. We first provide a short overview of physiology, biochemistry, and molecular genetics of sympatho-adrenomedullary, sympatho-neural, and brain catecholaminergic systems. Important processes of catecholamine biosynthesis, storage, release, secretion, uptake, reuptake, degradation, and transporters in acutely or chronically stressed organisms are described. We emphasize the structural variability of catecholamine systems and the molecular genetics of enzymes involved in biosynthesis and degradation of catecholamines and transporters. Characterization of enzyme gene promoters, transcriptional and posttranscriptional mechanisms, transcription factors, gene expression and protein translation, as well as different phases of stress-activated transcription and quantitative determination of mRNA levels in stressed organisms are discussed. Data from catecholamine enzyme gene knockout mice are shown. Interaction of catecholaminergic systems with other neurotransmitter and hormonal systems are discussed. We describe the effects of homotypic and heterotypic stressors, adaptation and maladaptation of the organism, and the specificity of stressors (physical, emotional, metabolic, etc.) on activation of catecholaminergic systems at all levels from plasma catecholamines to gene expression of catecholamine enzymes. We also discuss cross-adaptation and the effect of novel heterotypic stressors on organisms adapted to long-term monotypic stressors. The extra-adrenal nonneuronal adrenergic system is described. Stress-related central neuronal regulatory circuits and central organization of responses to various stressors are presented with selected examples of regulatory molecular mechanisms. Data summarized here indicate that catecholaminergic systems are activated in different ways following exposure to distinct stressful stimuli.

Physiological and morphological properties of, and effect of substance P on, neurons in the A7 catecholamine cell group in rats

The A7 catecholamine cell group consists of noradrenergic (NAergic) neurons that project to the dorsal horn of the spinal cord. Here, we characterized their morphology and physiology properties and tested the effect of substance P (Sub-P) on them, since the results of many morphological studies suggest that A7 neurons are densely innervated by Sub-P-releasing terminals from nuclei involved in the descending inhibitory system, such as the lateral hypothalamus and periaqueductal gray area. Whole cell recordings were made from neurons located approximately 200 microm rostral to the trigeminal motor nucleus (the presumed A7 area) in sagittal brainstem slices from rats aged 7-10 days. After recording, the neurons were injected with biocytin and immunostained with antibody against dopamine-beta-hydroxylase (DBH). DBH-immunoreactive (ir) cells were presumed to be NAergic neurons. They had a large somata diameter ( approximately 20 microm) and relatively simple dendritic branching patterns. They fired action potentials (AP) spontaneously with or without blockade of synaptic inputs, and had similar properties to those of NAergic neurons in other areas, including the existence of calcium channel-mediated APs and a voltage-dependent delay in initiation of the AP (an indicator of the existence of A-type potassium currents) and an ability to be hyperpolarized by norepinephrine. Furthermore, in all DBH-ir neurons tested, Sub-P caused depolarization of the membrane potential and an increase in neuronal firing rate by acting on neurokinin-1 receptors. Non-DBH-ir neurons with a smaller somata size were also found in the A7 area. These showed great diversity in firing patterns and about half were depolarized by Sub-P. Morphological examination suggested that the non-DBH-ir neurons form contacts with DBH-ir neurons. These results provide the first description of the intrinsic regulation of membrane properties of, and the excitatory effect of Sub-P on, A7 area neurons, which play an important role in pain regulation.

Spinal and pontine α2-adrenoceptors have opposite effects on pain-related behavior in the neuropathic rat

Descending noradrenergic pathways contribute to feedback inhibition of pain by releasing norepinephrine in the spinal cord. Noradrenergic nuclei in the pons contain abundant alpha(2)-adrenoceptors. We assessed the contribution of pontine alpha(2)-adrenoceptors to endogenous regulation of pain in nerve-injured rats. Tactile allodynia and mechanical hyperalgesia were assessed in the injured dermatome and heat nociception in an uninjured dermatome. Atipamezole, an alpha(2)-adrenoceptor antagonist, or saline was administered systemically or microinjected into the locus coeruleus, the lateral parabrachial nucleus, the central nucleus of the amygdala, the midbrain periaqueductal gray, and/or through an intrathecal (i.t.) catheter to the spinal cord. Atipamezole administered systemically, into the amygdala or the periaqueductal gray had no significant effects on pain behavior. Atipamezole (0.3-5 microg) microinjected into the pons, the locus coeruleus or the lateral parabrachial nucleus, produced a selective and dose-related antiallodynia, which was reversed by i.t. administration of atipamezole (5 microg). I.t. administration of atipamezole alone (5 microg) produced thermal hypersensitivity in the non-neuropathic segment (tail) of nerve-injured animals. In sham-operated controls, i.t. administration of atipamezole had no effect. Suppression of heat nociception in uninjured dermatomes of nerve-injured but not the control animals following i.t. administration of atipamezole indicates that nerve injury produced a tonic activation of noradrenergic feedback inhibition acting on spinal alpha(2)-adrenoceptors. In parallel, antiallodynia induced by pontine administration of atipamezole indicates that nerve injury induces a tonic activation of pontine alpha(2)-adrenoceptors that promotes neuropathic hypersensitivity by attenuating descending inhibition. Thus, spinal and pontine alpha(2)-adrenoceptors have opposite effects on pain-related behavior in neuropathic animals.

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.

The organization of the brainstem and spinal cord of the mouse: relationships between monoaminergic, cholinergic, and spinal projection systems

Information regarding the organization of the CNS in terms of neurotransmitter systems and spinal connections in the mouse is sparse, especially at the level of the brainstem. An overview is presented of monoaminergic and cholinergic systems in the brainstem and spinal cord that were visualized immunohistochemically in inbred C57BL/6 and outbred CD-1 mice. This information is complemented with data on spinal cord-projecting systems that were characterized using retrograde tracing, spinal hemisections, and double labeling techniques. Attention is given to differences in these systems related to spinal levels. The data are discussed with reference to studies in the rat, and to standardized information as provided in the atlas of the mouse brain. Although the overall organization of these systems in the mouse is largely similar to those in the rat, species differences are present in relative location, size and/or connectivity of cell groups. For example, catecholaminergic neurons in the (ventro)lateral pons (A5 and A7 cell groups) in the mouse project to the spinal cord mainly via contralateral, and not ipsilateral, pathways. The data further supplement information as provided in standardized brainstem sections of the C57BL/6 mouse [Paxinos, G., Franklin, K.B.J., 2001. The mouse brain in stereotaxic coordinates. Academic Press, San Diego], especially with respect to the size and/or location of the catecholaminergic retrorubral field (A8 group), A5, A1, and C1 cell groups, and the serotonergic B4 group, reticulotegmental nucleus (B9 group), lateral paragigantocellular nucleus and raphe magnus nucleus (B3 group). Altogether this study provides a comprehensive overview of the spatial relationships of neurochemically and anatomically defined neuronal systems in the mouse brainstem and spinal cord.

Antinociception induced by stimulation of ventrolateral periaqueductal gray at the freezing threshold is regulated by opioid and 5-HT2A receptors as assessed by the tail-flick and formalin tests

It has been suggested that antinociception is part of the animal's defensive reaction to threatening situations. Chemical or electrical stimulation of the ventrolateral portion of the periaqueductal gray (vlPAG) produces both defensive freezing behavior and antinociception, supporting the view that the vlPAG is a critical structure in the coordination of the defensive reaction. The present study indicated that electrical stimulation of the vlPAG, at a current intensity sufficient to induce defensive freezing, caused a decrease in reactivity to a phasic escapable noxious stimulus (as measured in the tail-flick test) and to a tonic, inescapable noxious stimulus (as measured in the formalin test). These antinociceptive effects were reversed by microinjections of the opioid antagonist naltrexone or the specific 5-HT2A receptor antagonist ketanserin into the stimulation sites. These results suggest that (a) activation of neural circuits of the vlPAG, responsible for the production of freezing behavior, reduces the reactivity to nociceptive stimuli (as evaluated by the tail-flick and formalin tests) and that (b) opioid- and 5-HT2A-mediated mechanisms are called into action for regulating the antinociceptive response that accompanies the freezing behavior induced by vlPAG stimulation.

Noxious activation of spinal or vagal afferents evokes distinct patterns of fos-like immunoreactivity in the ventrolateral periaqueductal gray of unanaesthetised rats

The consequences of a severe traumatic injury--deep pain and haemorrhage--usually evoke a passive emotional coping reaction characterised by: quiescence and immobility, decreased vigilance, hypotension and bradycardia. Results of studies utilising microinjections of excitatory amino acids suggest that passive coping reactions are mediated, at least in part, by activation of the midbrain, ventrolateral periaqueductal gray (vlPAG) region. Further, experiments in anaesthetised rats, using the expression of the immediate-early gene, c-fos, as a marker of neuronal activation, report that pain arising from muscles, joints or viscera selectively activates the vlPAG. Anaesthesia alone, however, evokes substantial Fos-like immunoreactivity (IR) within the vlPAG and this may have obscured any differences in patterns of Fos expression following noxious deep somatic versus noxious visceral activation. In these experiments, in unanaesthetised rats, the effects of noxious spinal versus noxious vagal primary afferent activation were re-examined and distinct rostrocaudal patterns of Fos-expression were observed. Specifically: (i) injection of algesic substances into muscle, which preferentially activates spinal afferents, evoked Fos expression predominantly within the caudal vlPAG; whereas, (ii) noxious manipulations whose effects are mediated by (cardiopulmonary) vagal activation evoked preferential Fos-expression within the rostral vlPAG. On the other hand, hypotensive haemorrhage evoked substantial Fos expression along the entire rostrocaudal extent of the vlPAG, a finding which fits with suggestions that haemorrhagic shock is triggered by a combination of: (i) spinally-relayed nociceptive signals originating from ischaemic tissue, and (ii) vagally-relayed signals reflecting poor cardiac filling.

The neuropharmacology of centrally-acting analgesic medications in fibromyalgia

As demonstrated above, the anatomy and neuropharmacology of the pain pathways within the CNS, even to the level of the midbrain, are extraordinarily complex. Indeed, discussions of the effects of these agents on the neuropharmacology of the thalamus, hypothalamus, and cortex were excluded from this review owing to their adding further to this complexity. Also, the dearth of data regarding FMS pain pathophysiology necessitated a relatively generic analysis of the pain pathways. As mentioned in the introduction, the current thought is that central sensitization plays an important role in FMS. However, we see in this chapter that the behavioral state of central sensitization may be a result of alterations in either the ascending systems or in one or more descending systems. Studies to assess the presence or relative importance of such changes in FMS are difficult to perform in humans, and to date there are no animal models of FMS. Accepting these limitations, it is apparent that many drugs considered to date for the treatment of FMS do target a number of appropriate sites within both the ascending and descending pain pathways. The data regarding clinical efficacy on some good candidate agents, however, is extremely preliminary. For example, it is evident from the present analysis that SNRIs, alpha 2 agonists, and NK1 antagonists may be particularly well suited to FMS, although current data supporting their use is either anecdotal or from open-label trials [114,149]. Other sites within the pain pathways have not yet been targeted. Examples of these include the use of CCKB antagonists to block on-cell activation or of nitric oxide synthetase antagonists to block the downstream mediators of NMDA activation. Efficacy of such agents may give considerable insight into the pathophysiology of FMS. Finally, as indicated previously, FMS consists of more than just chronic pain, and the question of how sleep abnormalities, depression, fatigues, and so forth tie into disordered pain processing is being researched actively. Future research focusing on how the various manifestations of FMS relate to one another undoubtedly will lead to a more rational targeting of drugs in this complex disorder.

Catecholaminergic innervation of the sympathetic preganglionic cell column of the filefish Stephanolepis cirrhifer

Nerve fibers immunoreactive for enzymes synthesizing catecholamines were examined in the central autonomic nucleus, a column of sympathetic preganglionic neurons, in the filefish Stephanolepis cirrhifer. Varicose nerve fibers immunoreactive for tyrosine hydroxylase were densely distributed in the rostral part, sometimes in contact with perikarya but were sparse in the caudal part of this nucleus. Fluorescent double labeling distinguished noradrenergic nerve fibers immunoreactive for both tyrosine hydroxylase and dopamine beta hydroxylase, and dopaminergic fibers immunoreactive only for tyrosine hydroxylase. In the brainstem, catecholaminergic neurons were observed in the locus coeruleus, the caudal dorsomedial reticular zone of the medulla, and the area postrema. Double labeling of tyrosine hydroxylase and dopamine beta hydroxylase showed that the neurons in the locus coeruleus were all noradrenergic, and those in the caudal dorsomedial medulla were mostly noradrenergic, whereas the area postrema contained both noradrenergic and dopaminergic neurons. No catecholaminergic neurons were found in the ventral region of the brainstem. After application of DiI to the central autonomic nucleus, retrogradely labeled neurons were seen in the caudal dorsomedial medulla but not in the locus coeruleus or the area postrema. These findings suggest that the sympathetic preganglionic neurons of the filefish may receive noradrenergic axonal projections from neurons in the caudal dorsomedial medulla. In the light of previous studies, inputs of these catecholaminergic fibers to the central autonomic nucleus may be involved in regulation of sympathetic activity of peripheral organs, together with serotoninergic and peptidergic inputs to this nucleus.

Differential expression of catecholamine synthetic enzymes in the caudal ventral pons

The analysis of colocalization of multiple catecholamine biosynthetic enzymes within the ventrolateral part of the medulla oblongata of the rat revealed distinct subpopulations of neurons within the C1 region (Phillips et al., J Comp Neurol 2001, 432:20-34). In extending this study to include the caudal pons, it was shown for the first time that the A5 cell group could be distinguished by the presence of immunoreactivity to tyrosine hydroxylase (TH), aromatic l-amino acid decarboxylase (AADC), and dopamine beta hydroxylase (DBH). A novel cell group was also identified. The cells within this new group were immunoreactive to DBH but not TH, AADC, or phenylethanolamine N-methyltransferase (PNMT) and will be referred to as the TH-, DBH+ cell group. The TH-, DBH+ neurons were not immunoreactive for either the dopamine or noradrenaline transporters, suggesting that these neurons do not take up these transmitters. A5 neurons were immunoreactive for the noradrenaline transporter but not the dopamine transporter (as previously shown). Retrograde tracing with cholera toxin B revealed that the TH-, DBH+ neurons do not project to the thoracic spinal cord or to the rostral ventrolateral medulla, but A5 neurons do. A calbindin immunoreactive cell group is located in a region overlapping TH-, DBH+ cell group. However, only a few neurons were immunoreactive for both markers. The physiological role of the TH-, DBH+ cell group remains to be determined.

Catecholamine systems in the brain of vertebrates: new perspectives through a comparative approach

A comparative analysis of catecholaminergic systems in the brain and spinal cord of vertebrates forces to reconsider several aspects of the organization of catecholamine systems. Evidence has been provided for the existence of extensive, putatively catecholaminergic cell groups in the spinal cord, the pretectum, the habenular region, and cortical and subcortical telencephalic areas. Moreover, putatively dopamine- and noradrenaline-accumulating cells have been demonstrated in the hypothalamic periventricular organ of almost every non-mammalian vertebrate studied. In contrast with the classical idea that the evolution of catecholamine systems is marked by an increase in complexity going from anamniotes to amniotes, it is now evident that the brains of anamniotes contain catecholaminergic cell groups, of which the counterparts in amniotes have lost the capacity to produce catecholamines. Moreover, a segmental approach in studying the organization of catecholaminergic systems is advocated. Such an approach has recently led to the conclusion that the chemoarchitecture and connections of the basal ganglia of anamniote and amniote tetrapods are largely comparable. This review has also brought together data about the distribution of receptors and catecholaminergic fibers as well as data about developmental aspects. From these data it has become clear that there is a good match between catecholaminergic fibers and receptors, but, at many places, volume transmission seems to play an important role. Finally, although the available data are still limited, striking differences are observed in the spatiotemporal sequence of appearance of catecholaminergic cell groups, in particular those in the retina and olfactory bulb.

The organization of preoptic-medullary circuits in the male rat: evidence for interconnectivity of neural structures involved in reproductive behavior, antinociception and cardiovascular regulation

The present studies used anatomical tract-tracing techniques to delineate the organization of pathways linking the medial preoptic area and the ventral medulla, two key regions involved in neuroendocrine, autonomic and sensory regulation. Wheatgerm agglutinin-horseradish peroxidase injections into the ventromedial medulla retrogradely labeled a large number of neurons in the medial preoptic area, including both the median and medial preoptic nuclei. The termination pattern of preoptic projections to the medulla was mapped using the anterograde tracers Phaseolus vulgaris leucoagglutinin and biotinylated dextran amine. Tracer injections into the preoptic area produced a dense plexus of labeled fibers and terminals in the ventromedial and ventrolateral pons and medulla. Within the caudal pons/rostral medulla, medial preoptic projections terminated heavily in the nucleus raphe magnus; strong anterograde labeling was also present in the pontine reticular field. At mid-medullary levels, labeled fibers focally targeted the nucleus paragigantocellularis, in addition to the heavy fiber labeling present in the midline raphe nuclei. By contrast, very little labeling was observed in the caudal third of the medulla. Experiments were also conducted to map the distribution of ventral pontine and medullary neurons that project to the medial preoptic area. Wheatgerm agglutinin-horseradish peroxidase injections in the preoptic area retrogradely labeled a significant population of neurons in the ventromedial and ventrolateral medulla. Ascending projections from the medulla to the preoptic area were organized along rostral-caudal, medial-lateral gradients. In the caudal pons/rostral medulla, retrogradely labeled cells were aggregated along the midline raphe nuclei; no retrograde labeling was present laterally at this level. By contrast, in the caudal half of the medulla, cells retrogradely labeled from the medial preoptic area were concentrated as a discrete zone dorsal to the lateral reticular nucleus; labeled cells were not present in the ventromedial medulla at this level. The present findings suggest that the medial preoptic area and ventral midline raphe nuclei share reciprocal connections that are organized in a highly symmetrical fashion. By contrast, preoptic-lateral medullary pathways are not reciprocal. These preoptic-brainstem circuits may participate in antinociceptive, autonomic and reproductive behaviors.

Mu- and delta-opioid receptor mRNAs are expressed in spinally projecting serotonergic and nonserotonergic neurons of the rostral ventromedial medulla

The rostral ventromedial medulla (RVM) is an important mediator of the supraspinal component of opioid antinociception. Previous studies have suggested that activation of the cloned mu- and delta-opioid receptors (MOR1 and DOR1 respectively) in the RVM produces the antinociception mediated by spinally projecting neurons. In the present study, we investigated the expression of mRNA encoding either MOR1 or DOR1 in the RVM of rats. In addition, we examined quantitatively the expression of MOR1 and DOR1 mRNAs in spinally projecting RVM neurons including serotonergic (5HT) cells by using in situ hybridization, immunocytochemistry, retrograde tract-tracing, and the physical disector. Brainstem neurons were labeled in 14 male Sprague-Dawley rats by applying Fluoro-Gold (FG) topically to the dorsal surface of the lumbosacral spinal cord. Five-micrometer-thick cryostat sections were cut and in situ hybridization was performed by using full-length cRNA probes labeled with 35S-UTP. We found that 43% of RVM projection neurons expressed MOR1 mRNA and 83% of RVM projection neurons expressed DOR1 mRNA. Of 192 retrogradely labeled cells in the RVM, 51 cells (27%) were immunoreactive for 5HT. Of this population, half appeared to be labeled for the mRNA encoding MOR1 and over three-fourths appeared to be labeled for the mRNA encoding DOR1. Thus, we conclude that bulbospinal neurons express MOR1 and DOR1; moreover, MOR1 and DOR1 are expressed by significant proportions of 5HT neurons projecting to or through the dorsal spinal cord.

Midbrain periaqueductal gray (PAG) inhibits nociceptive inputs to sacral dorsal horn nociceptive neurons through alpha2-adrenergic receptors

Modulation of sacral spinal dorsal horn neurons by the ventrolateral PAG was studied by extracellular recording combined with microiontophoretic applications of alpha-adrenergic agonists or antagonists. Bicuculline (BIC, 15 ng) microinjected into the ventrolateral PAG produced a consistent inhibition of the responses of nociceptive dorsal horn neurons. After PAG-BIC applications, the total number of spikes per heat stimulation period was significantly decreased to a mean of 37 +/- 19% (n = 8) of the pre-BIC control. Local iontophoresis of the selective alpha2-adrenoceptor antagonists idazoxan or yohimbine but not the selective alpha1 antagonist benoxathian significantly reversed PAG-BIC-evoked inhibition. At low ejection currents, clonidine, an alpha2-adrenoceptor agonist, markedly reduced noxious heat-evoked responses but had no consistent action on the responses to iontophoresed excitatory amino acids [EAA; N-methyl--aspartate (NMDA) or kainic acid]. At ejection currents higher than required to block descending inhibition, idazoxan potentiated responses to both heat and EAA iontophoresis. At higher ejection currents, EAA responses were inhibited by clonidine. This indicates that both presynaptic and postsynaptic alpha2 receptors are capable of inhibiting the recorded neurons. Activation of the alpha1 adrenoceptors by iontophoresis of methoxamine often led to a marked increase in the responses to kainic acid and, to a lesser extent, to NMDA iontophoresis or noxious heat. Together with previously reported work, the current experiments demonstrate that PAG neurons inhibit nociceptive dorsal horn neurons primarily through an indirect alpha2 adrenoceptor mechanism. In this same population of dorsal horn neurons, norepinephrine has a direct alpha1-mediated excitatory effect.

Substance P receptor (NK1)-immunoreactive neurons projecting to the periaqueductal gray: distribution in the spinal trigeminal nucleus and the spinal cord of the rat

Substance P receptor (SPR)-immunoreactive neurons projecting to the periaqueductal gray (PAG) were examined in the rat spinal trigeminal nucleus and spinal cord by a retrograde tracing method combined with immunofluorescence histochemistry. After injection of Fluoro-gold (FG) into the PAG, SPR-immunoreactive neurons labeled with FG were observed mainly in the lateral spinal nucleus and lamina I of the medullary and spinal dorsal horns and additionally in laminae V and X of the spinal cord.

Opioid-induced release of neurotensin in the periaqueductal gray matter of freely moving rats

The midbrain periaqueductal gray matter (PAG) is an important region for endogenous pain suppression. Nerve terminals containing opioid peptides and neurotensin (NT), as well as high densities of opioid- and NT-receptors, have been demonstrated in the ventromedial PAG. Local administration of opioids or NT in this region induces antinociception in experimental animals. In the present microdialysis study, the effect of opioids on the release of NT in the ventromedial PAG was investigated. Perfusion of the microdialysis probe with 10 microM morphine induced a significant increase (P < 0.05; n = 5) of the extracellular level of NT-like immunoreactivity (NT-LI), while perfusion with a 10-fold higher concentration of morphine had no significant effect on the NT-LI release in the PAG. Also perfusion of the dialysis probe with the mu-opioid receptor-specific agonist [D-Ala2-N-Me-Phe4-Gly5-ol]-enkephaline (DAGO) (1 or 100 microM) induced a significant (P < 0.05; n = 7-9) increase of the NT-LI level. The increase in NT-LI release in response to 1 microM DAGO was both calcium-dependent and naloxone-reversible. Since opioid agonists generally inhibit neuronal activity, an indirect mechanism, involving inhibition of tonically active inhibitory neurons, e.g. gamma-aminobutyric acid (GABA) neurons, could be of importance for the opioid induced release of NT. However, local administration in the PAG of the GABA(A) antagonist bicuculline (0.1-10 microM) or the GABA(A) agonist muscimol (1-100 microM) had no significant effect on the extracellular NT-LI level in the PAG, suggesting that GABAergic mechanisms are not involved in the opioid-induced release of NT-LI. In conclusion, the present data provide in vivo evidence that mu-opioid receptors mediate stimulation of neurotensin release in the PAG.

Neuroanatomy of the pain system and of the pathways that modulate pain

We review many of the recent findings concerning mechanisms and pathways for pain and its modulation, emphasizing sensitization and the modulation of nociceptors and of dorsal horn nociceptive neurons. We describe the organization of several ascending nociceptive pathways, including the spinothalamic, spinomesencephalic, spinoreticular, spinolimbic, spinocervical, and postsynaptic dorsal column pathways in some detail and discuss nociceptive processing in the thalamus and cerebral cortex. Structures involved in the descending analgesia systems, including the periaqueductal gray, locus ceruleus, and parabrachial area, nucleus raphe magnus, reticular formation, anterior pretectal nucleus, thalamus and cerebral cortex, and several components of the limbic system are described and the pathways and neurotransmitters utilized are mentioned. Finally, we speculate on possible fruitful lines of research that might lead to improvements in therapy for pain.

Lower brainstem catecholamine afferents to the rat dorsal raphe nucleus

A large body of data suggests that the activation of alpha 1 receptors by a tonic noradrenergic input might be responsible for the tonic discharge of the serotonergic neurons of the dorsal raphe nucleus (DRN). To test this hypothesis, it was necessary to determine the origin of the noradrenergic and adrenergic innervation of these neurons. For this purpose, we combined small iontophoretic injections of the sensitive retrograde tracer cholera toxin b subunit (CTb) in the different subdivisions of the DRN with tyrosine hydroxylase immunohistochemistry. After CTb injections in the ventral or dorsal parts of the central DRN, a small number of double-labeled cells was observed in the locus coeruleus (A6 noradrenergic cell group), the A5 noradrenergic group, the dorsomedial medulla (C3 adrenergic cell group), and the lateral paragigantocellular nucleus (C1 adrenergic cell group). After CTb injections in the lateral wings or the dorsal part of the rostral DRN, a similar number of double-labeled cells was seen in C3. Slightly more double-labeled cells were seen in A6 and A5. In addition, a substantial to large number of double-labeled cells appeared in C1, the commissural part of the nucleus of the solitary tract (A2 noradrenergic cell group) and the caudoventrolateral medulla (A1 noradrenergic cell group). These results indicate that the noradrenergic and adrenergic inputs to the DRN arise from all the catecholaminergic cell groups of the lower brainstem except the A7 noradrenergic group. They further reveal the existence of a topographical organization of these afferents to the different subdivisions of the DRN.

Differential responses of lateral and ventrolateral rat periaqueductal grey neurones to noradrenaline in vitro

1. The action of noradrenaline on the membrane properties of rat periaqueductal grey (PAG) neurones was examined using intracellular recordings in brain slices maintained in vitro. Morphological properties and the anatomical location of neurones were characterized by use of intracellular staining within biocytin. 2. Noradrenaline (0.3-100 microM) depolarized 66% (81/123) and hyperpolarized 30% (37/123) of neurones. The alpha 1- and alpha 2-adrenoceptor agonists phenylephrine and UK 14304 produced depolarizations and hyperpolarizations in all PAG neurones tested, respectively. Neurones depolarized by noradrenaline were more responsive to phenylephrine, whereas neurones hyperpolarized by noradrenaline were more responsive to UK 14304. 3. The UK 14304-induced hyperpolarizations reversed polarity at -108 +/- 2 mV (n = 11). The reversal potential increased when the extracellular potassium concentration was raised (slope = 57.8 mV/log[K+]o mM) in a manner similar to that predicted for potassium conductance. 4. The phenylephrine-induced depolarizations did not reverse polarity at negative potentials (n = 25), or did so at potentials (-119 +/- 2 mV, n = 13) more negative than the UK 14304-induced hyperpolarizations. Superfusion with low calcium (0.1 mM), high magnesium (10 mM) and either cobalt (2-4 mM), or cadmium (100 microM) usually reduced the response to phenylephrine and produced reversals near that predicted for potassium conductance. 5. The majority of the ventrolateral PAG neurones were depolarized by noradrenaline (85%, 62/73). In contrast, almost equal proportions of the lateral PAG neurones were hyperpolarized (54%, 20/37) and depolarized (46%, n = 17/37) by noradrenaline. PAG neurones depolarized or hyperpolarized by noradrenaline could not be differentiated on morphological grounds. 6. These results suggest that the net effect of noradrenaline on lateral and ventrolateral PAG neurones is to bias activity in favour of a ventrolateral PAG-mediated response pattern, which includes quiescence, hyporeactivity, hypotension and bradycardia.

Descending antinociceptive mechanisms in the brainstem: their role in the animal's defensive system

The identification of specialized mechanisms in the mammalian brainstem that function to inhibit the rostral transmission of nociceptive (pain-related) information in the spinal cord led to an explosion of research into the neuroanatomical and neurochemical substrates of these antinociceptive systems. As outlined in the present paper, most attention was directed at those mechanisms in the periaqueductal grey (PAG) and rostral ventromedial medulla (RVM). However, comparatively little attention has been paid to the functional role of these mechanisms in animal behaviour. The purpose of the present paper is to review research into the behavioural significance of those antinociceptive mechanisms in the PAG and RVM. It is concluded that these mechanisms function as part of the animal's fear or defensive system, serving to make a threatened animal insensitive to noxious stimulation and thereby allowing that animal to engage in defensive responses instead of recuperative activities. Further, it is argued that the organization of these antinociceptive circuits reflects the animal's increasing capacity for early detection of danger. Specifically, nociception itself is held to signify the presence of immediate threat, and consequently, nociceptive input directly activates antinociceptive circuits at either the spinal level (during intense noxious stimulation) or RVM (following exposure to moderate noxious stimuli). In contrast, events that are themselves innocuous but which signal threat (either learned or innate danger signals) activate fear and defensive systems in the amygdala and PAG which engage the descending antinociceptive projections in the RVM.

Repeated spinal cord stimulation decreases the extracellular level of gamma-aminobutyric acid in the periaqueductal gray matter of freely moving rats

Most of the previous experimental studies on the antinociceptive effects of electrical spinal cord stimulation (SCS) have focused on short-lasting effects mainly depending on spinal mechanisms. However, patients treated with SCS for chronic pain often report pain relief exceeding the period of stimulation for several hours. The long lasting effect of SCS might not only involve spinal, but also supraspinal mechanisms. A supraspinal region of major importance for the coordination of descending pain inhibition is the periaqueductal grey matter (PAG). The aim of the present microdialysis study, performed in awake freely moving rats, was to investigate if repeated SCS (two 30 min periods separated by a 90 min resting period) alters the extracellular neurotransmitter concentrations in the ventrolateral PAG. In a first series of experiments significantly decreased (-30%; P < 0.05; n = 7) gamma-aminobutyric acid (GABA) levels were detected immediately after the second SCS session. Neither the concentration of serotonin nor that of substance P-like immunoreactivity (SP-LI) was affected by SCS. The decrease of GABA after two SCS sessions was confirmed in a second series of experiments (-30%; P < 0.05; n = 7). No spontaneous decline of GABA was observed in sham-stimulated animals (n = 6). The glutamate concentration was also determined in this latter series of experiments and a significant decrease (-23%; P < 0.05; n = 5) was observed after the second SCS session. As GABA-neurons in the PAG exert a tonic depressive effect on the activity in descending pain inhibitory pathways, a decreased extracellular GABA level in this region, as detected following repeated SCS, might indicate an increased pain inhibition.

Direct projections from the anterior pretectal nucleus to the ventral medulla oblongata in rats

The anterior pretectal nucleus has recently been implicated in the descending modulation of nociception. Electrical stimulation of the nucleus was found to reduce the nociceptive responses of deep dorsal horn neurons and to inhibit spinally integrated withdrawal reflexes. It is believed that at least part of the descending inhibitory effects of the anterior pretectal nucleus are mediated by reticulospinal cells of the ventrolateral medulla. The purpose of the present study was to trace the direct medullary projections of the anterior pretectal nucleus, to describe their topographical organization and to reveal the chemical nature of some of their putative target cells. The connections were studied using anterograde tract-tracing with Phaseolus vulgaris leucoagglutinin. Direct projections from the anterior pretectal nucleus to the ipsilateral rostral ventral medulla were found in all cases. A dense innervation of the dorsal inferior olive, the gigantocellular reticular nucleus pars ventralis and pars alpha and the ventral pontine reticular nucleus was found from all aspects of the anterior pretectal nucleus. Descending labelled terminals were also observed in the gigantocellular reticular nucleus proper and, laterally, in the lateral paragigantocellular nucleus and in the region of the A5 noradrenergic cell group. A relatively lower density of labelled terminals was noted in the medullary raphe nuclei and in the rostroventrolateral reticular nucleus. Following tract-tracer injections into five distinct subregions of the anterior pretectal nucleus, the topographical organization of the projection was examined and the relatively highest density and most widespread projection was found to originate from the caudoventral part of the anterior pretectal nucleus. A combined tract-tracing and immunolabelling study revealed that some of the descending, labelled terminals were in close proximity of tyrosine hydroxylase-immunoreactive dendrites in the C1 and A5 cell groups. Some labelled fibres were also noted among the serotonin-immunoreactive cells in the lateral extension of the B3 cell population. The existence of direct projections to the ventral medulla and pons correlates well with physiological data which showed that the descending, antinociceptive effects of the anterior pretectal nucleus are relayed via the rostral ventrolateral medulla. The data are also in keeping with pharmacological studies that suggested the role of catecholaminergic cells in the mediation of these descending effects. It is proposed that the rostral ventral medullary projections provide a path through which antinociceptive effects of the anterior pretectal nucleus are mediated to the spinal cord.

Neuropathology and immunohistochemistry of the brain-stem in neonates with congenital hydrocephalus: comparative studies between aqueductal stenosis and Arnold-Chiari malformation

Neuropathological and immunohistochemical studies were done on the brain-stem of neonates who had congenital hydrocephalus with aqueductal stenosis or Arnold-Chiari malformation (ACM). The infants with aqueductal stenosis showed heterogeneity in their clinicopathological findings while the infants with ACM were relatively similar in neuropathological findings. There were prominent astrogliosis, decreased immunoreactivity with antisera to tyrosine hydroxylase and myelin basic protein in the periaqueductal area, and an increased reactivity with antiserum to substance P in the tegmentum of most patients with aqueductal stenosis and other malformations. In ACM, there was little gliosis in the tegmentum and periaqueductal area and minimal immunoreactivity of tyrosine hydroxylase, myelin basic protein and substance P. In both groups of cases, the cells in the periaqueductal region differ in neurotransmitter/neuromodulator immunoreactivity and degree of myelination reflecting a difference possibly in their maldevelopment.

The dorsal raphe: an important nucleus in pain modulation

The dorsal raphe nucleus (DRN) is an important nucleus in pain modulation. It has abundant 5-HT neurons and many other neurotransmitter and/or neuromodulator containing neurons. Its vast fiber connections to other parts of the central nervous system provide a morphological basis for its pain modulating function. Its descending projections, via the nucleus raphe magnus or directly, modulate the responses caused by noxious stimulation of the spinal dorsal horn neurons. In ascending projections, it directly modulates the responses of pain sensitive neurons in the thalamus. It can also be involved in analgesia effects induced by the arcuate nucleus of the hypothalamus. Neurophysiologic and neuropharmacologic results suggest that 5-HT neurons and ENKergic neurons in the DRN are pain inhibitory, and GABA neurons are the opposite. The studies of the intrinsic synapses between ENKergic neurons, GABAergic neurons, and 5-HT neurons within the DRN throw light on their relations in pain modulation functions, and further explain their functions in pain mediation.

Origin of serotoninergic afferents to the hypoglossal nucleus in the rat

The hypoglossal nucleus contains serotonin and several different serotonin receptors, and serotonin is present in fibers and terminals contacting hypoglossal motoneurons. Serotonin alters the excitability of hypoglossal motoneurons, and may influence hypoglossal motoneuron activity in a variety of physiological processes. Since the hypoglossal nucleus contains no serotoninergic somata, the present study sought to identify the sources of serotoninergic afferents to the hypoglossal nucleus. Fluorogold was injected into the hypoglossal nucleus and serotoninergic immunofluorescence was utilized in a dual-fluorescence technique to identify the sources of serotoninergic afferents to the hypoglossal nucleus. The results demonstrate that most serotoninergic afferents to the hypoglossal nucleus originate from the nuclei raphe pallidus and obscurus, while fewer originate from the nucleus raphe magnus and the parapyramidal region. Other regions of the medial tegmental field and the pons that contain both serotoninergic neurons and neuronal afferents to the hypoglossal nucleus contain no double-labeled neurons.

Innervation of serotonergic medullary raphe neurons from cells of the rostral ventrolateral medulla in rats

The rostral ventral medulla has been shown to consist of three distinct subregions: the midline or raphé region, the lateral paragigantocellular-gigantocellular region and the rostro-ventrolateral reticular nucleus. All three regions have been shown to contribute to central vaso-regulation and to project towards sympathetic preganglionic neurons of the thoracic spinal cord. Therefore it is of particular interest to describe the interconnections between the three regions and to see if local afferents reach cells which have been implicated in the regulation of descending inputs. Following injections of the anterograde tract tracer Phaseolus vulgaris leucoagglutinin into the lateral paragigantocellular nucleus or the rostroventrolateral reticular nucleus, labelled axons were traced into the medullary raphé nuclei and the contralateral rostral ventrolateral medulla. Efferents originating from both regions innervated the raphé pallidus, raphé obscurus and raphé magnus. However the distribution of terminals originating from the two regions was different in the contralateral ventrolateral medulla oblongata. The data indicate that the connection between the ipsi- and contralateral equivalents of both the lateral paragigantocellular-gigantocellular region and the rostroventrolateral reticular nucleus are stronger than the cross-connection between the ipsi- and contralateral parts of the two different regions. In the second part of the study, the existence of direct projections from the rostroventrolateral reticular nucleus and the lateral paragigantocellular-gigantocellular region onto serotonin-immunogold-labelled cells of the ventromedial medulla were investigated. The correlated light and electron microscopic analysis revealed direct synaptic contacts between axons originating from both the lateral paragigantocellular-gigantocellular region and the rostroventrolateral reticular nucleus, and serotonin-immunoreactive cells of the raphé obscurus and raphé pallidus. The results of the present light microscopic tract-tracing study revealed a different pattern of the intramedullary projection of the lateral paragigantocellular-gigantocellular region and the rostroventrolateral reticular nucleus. These data are in support of the proposed parcellation of the two cytoarchitectonically different areas of the rostral ventrolateral medulla into two functionally distinct subdivisions. Furthermore, the direct anatomical connection revealed in the present study between cells of the rostral ventrolateral and ventromedial medulla oblongata indicates the possibility that vasoregulatory effects of some cells of the rostral ventrolateral medulla oblongata might be executed via direct projections onto serotonin-immunoreactive cells of the medullary raphé nuclei.

GABAergic regulation of noradrenergic spinal projection neurons of the A5 cell group in the rat: an electron microscopic analysis

Recent studies have demonstrated an important contribution of the A5 noradrenergic cell group of the rostral medulla in the regulation of nociceptive messages at the level of the spinal cord. These noradrenergic controls parallel those arising from the serotonin-containing neurons of the nucleus raphe magnus. In the present study, we used postembedding immunogold staining to identify GABA-immunoreactive terminals that synapse upon identified spinally projecting noradrenergic neurons of the A5 cell group in the rat. A5 projection neurons were identified by Fluoro-Gold transport from the spinal cord; sections containing retrogradely labelled cells were then immunoreacted for tyrosine hydroxylase (TH) to identify the catecholamine-containing, presumed noradrenergic, neurons. Double-labelled A5 cells were intracellularly filled with Lucifer Yellow (LY) and then the LY was photo-oxidized to an electron-dense product. Seven intracellularly filled TH-immunoreactive projection neurons were studied with postembedding immunocytochemistry. Each A5 neuron received a significant GABA-immunoreactive terminal input. Out of a pooled total of 151 terminal profiles found in apposition to intracellularly labelled somatic and dendritic profiles, 31 (20.5%) were GABA-immunoreactive. The proportion of GABA-immunoreactive terminals that contacted somatic profiles (12/72; 17%) was similar to the proportion that contacted TH-labelled dendritic profiles (19/79; 24%). There was a discernible synaptic specialization in about 50% of the labelled terminals that contacted the TH projection neuron. Both symmetric and asymmetric synaptic specializations were found. Labelled terminals contained round or pleiomorphic vesicles, but not flat vesicles; many also contained dense-core vesicles. Our results indicate that noradrenergic neurons of the A5 cell group, which contribute to both antinociceptive and cardiovascular controls through their projection to the spinal cord, are regulated by local GABAergic, presumably inhibitory, mechanisms. Whether the initiation of A5 neuron activity results from a lifting of tonic GABAergic inhibitory control, as has been proposed for the neurons of the nucleus raphe magnus, remains to be determined.

Dual projections of single cholinergic and aminergic brainstem neurons to the thalamus and basal forebrain in the rat

Compelling evidence indicates that cholinergic basal forebrain neurons are strongly activated during waking, and concurrently thalamic spindle activity is suppressed and thalamocortical sensory transmission is facilitated. Both thalamus and basal forebrain are known to receive projections from brainstem cholinergic and aminergic neuronal pools that are involved in wake/sleep regulation. The present study addressed the question of whether single cholinergic and aminergic neurons contributed to both of these ascending projections, by using two fluorescent retrograde tracers combined with immunofluorescence. Cholinergic neurons projecting to both the basal forebrain and thalamus were found in the pedunculopontine and laterodorsal tegmental nuclei, representing an average of 8.0% of the total cholinergic cell population in these nuclei. Serotonergic neurons with dual projections were observed in the dorsal, median and caudal linear raphe nuclei, accounting for a mean of 4.7% of total serotonergic neurons in these nuclei. Relatively few noradrenergic neurons (2.0%) in the locus ceruleus projected to both target structures, and a very small subpopulation of histaminergic neurons (1.5%) in the tuberomammillary hypothalamic nucleus had dual projections. Of all brainstem neurons with dual projections, cholinergic and serotonergic neurons accounted for an overwhelming majority, with noradrenergic followed by histaminergic neurons representing the remaining minority. These data suggest that through dual projections, cholinergic and aminergic brainstem neurons can concurrently modulate the activity of neurons in the thalamus and basal forebrain during cortical arousal.

Evidence for projections from medullary nuclei onto serotonergic and dopaminergic neurons in the midbrain dorsal raphe nucleus of the rat

The anterograde tracer Phaseolus vulgaris-leucoagglutinin was injected into the medial nucleus of the solitary tract and into the rostral dorsomedial medulla. A sequential two-color immunoperoxidase staining was accomplished in order to demonstrate the co-distribution of presumed terminal axons with chemically distinct neurons in the dorsal raphe nucleus of the midbrain central gray, i.e., B7 serotonergic and A10dc dopaminergic neurons. Black-stained efferent fibers from the medial nucleus of the solitary tract and the rostral dorsomedial medulla intermingled with brown-stained serotonergic (5-hydroxytryptamine-immunoreactive) or dopaminergic (tyrosine hydroxylase-immunoreactive) neurons. Light microscopy revealed that the black-stained efferent axons exhibited numerous en passant and terminal varicosities that were often found in close apposition to brown-stained serotonergic and dopaminergic somata, and to proximal and distal dendrites and dendritic processes. The close association of immunoreactive elements suggests the presence of axo-somatic and axo-dendritic synaptic contacts of medullary fibers with serotonergic and dopaminergic neurons in the dorsal raphe nucleus. These projections could be involved in the modulation of dorsal raphe neurons, depending on the autonomic status of an animal.

Raphespinal and reticulospinal axon collaterals to the hypoglossal nucleus in the rat

Neurons in the medial tegmental field project directly to spinal somatic motoneurons and to cranial motoneuron pools such as the hypoglossal nucleus. The axons of these neurons may be highly collateralized, projecting to multiple levels of the spinal cord and to many diverse regions at different levels of the neuraxis. We employed a double fluorescent retrograde tracer technique to examine whether medial tegmental neurons that project to the spinal cord also project to the hypoglossal nucleus. Injections of Diamidino Yellow into the hypoglossal nucleus and Fast Blue into the spinal cord produced large numbers of double labeled neurons in the medial tegmental field, particularly in the caudal raphe nuclei and adjacent ventromedial reticular formation. In these structures the number of neurons projecting to both the hypoglossal nucleus and the spinal cord was equivalent to the number of neurons projecting to multiple levels of the spinal cord observed in control animals. Fewer neurons projecting to both the hypoglossal nucleus and the spinal cord were observed in several other nuclei and subregions of the medial tegmental field, while almost no such neurons were observed in the lateral tegmental field or other pontomedullary structures. These results demonstrate that neurons of the caudal raphe nuclei and adjacent ventromedial reticular formation project to both the spinal cord and the hypoglossal nucleus, and support the concept that the diffuse projections to motoneuron pools from the medial tegmental field globally modulate both spinal and cranial somatic motoneuron excitability.

Organization of medullary adrenergic and noradrenergic projections to the periaqueductal gray matter in the rat

The periaqueductal or midbrain central gray matter (CG) in the rat contains a dense network of adrenergic and noradrenergic fibers. We examined the origin of this innervation by using retrograde and anterograde axonal tracers combined with immunohistochemistry for the catecholamine biosynthetic enzymes tyrosine hydroxylase (TH), dopamine beta-hydroxylase (DBH), and phenylethanolamine N-methyltransferase (PNMT). Following injections of the fluorescent tracers Fast Blue or Fluorogold into the CG, double-labeled neurons in the medulla were identified mainly in the noradrenergic A1 group in the caudal ventrolateral medulla (VLM) and A2 group in the medial part of the nucleus of the solitary tract (NTS); and in the adrenergic C1 group in the rostral ventrolateral medulla and C3 group in the rostral dorsomedial medulla. Injections of Phaseolus vulgaris-leucoagglutinin (PHA-L) into these cell groups resulted in a distinct pattern of axonal labeling in various subdivisions of the CG. Anterogradely labeled fibers originating in the medial NTS were predominantly found in the lateral portion of the dorsal raphe nucleus and in the adjacent part of the lateroventral CG (CGlv). Following PHA-L injections into the C3 region the anterogradely labeled fibers were diffusely distributed in the CGlv and the dorsal raphe nucleus at caudal levels, but rostrally tended to be located laterally in the CGlv. In contrast, ascending fibers from the caudal and rostral VLM terminated in the rostral dorsal part of the CGlv and in the dorsal nucleus of the CG, whereas ventral parts of the CG, including the dorsal raphe nucleus, contained few afferent fibers. Double-label studies with antisera against DBH and PNMT confirmed that noradrenergic neurons in the A1 and A2 groups and adrenergic neurons in the C1 and C3 groups contributed to these innervation patterns in the CGlv. Noradrenergic and adrenergic projections from the medulla to the CG may play an important role in a variety of autonomic, sensory and behavioral processes.