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

The physiology and processing of pain: a review

Despite the many advances in our understanding of the mechanisms underlying pain processing, pain continues to be a major healthcare problem in the United States. Each day, millions of Americans are affected by both acute and chronic pain conditions, costing in excess of $100 billion for treatment-related costs and lost work productivity. Thus, it is imperative that better treatment strategies be developed. One step toward improving pain management is through increased knowledge of pain physiology. Within the nervous system, there are several pathways that transmit information about pain from the periphery to the brain. There is also a network of pathways that carry modulatory signals from the brain and brainstem that alter the incoming flow of pain information. This article provides a review to the physiology and processing of pain.

The restless legs syndrome

The restless legs syndrome (RLS) is one of the commonest neurological sensorimotor disorders at least in the Western countries and is often associated with periodic limb movements (PLM) during sleep leading to severe insomnia. However, it remains largely underdiagnosed and its underlying pathogenesis is presently unknown. Women are more affected than men and early-onset disease is associated with familial cases. A genetic origin has been suggested but the mode of inheritance is unknown. Secondary causes of RLS may share a common underlying pathophysiology implicating iron deficiency or misuse. The excellent response to dopaminegic drugs points to a central role of dopamine in the pathophysiology of RLS. Iron may also represent a primary factor in the development of RLS, as suggested by recent pathological and brain imaging studies. However, the way dopamine and iron, and probably other compounds, interact to generate the circadian pattern in the occurrence of RLS and PLM symptoms remains unknown. The same is also the case for the level of interaction of the two compounds within the central nervous system (CNS). Recent electrophysiological and animals studies suggest that complex spinal mechanisms are involved in the generation of RLS and PLM symptomatology. Dopamine modulation of spinal reflexes through dopamine D3 receptors was recently highlighted in animal models. The present review suggests that RLS is a complex disorder that may result from a complex dysfunction of interacting neuronal networks at one or several levels of the CNS and involving numerous neurotransmitter systems.

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.

Estrogen receptor-alpha and -beta immunoreactive neurons in the brainstem and spinal cord of male and female mice: relationships to monoaminergic, cholinergic, and spinal projection systems

For many populations of estrogen-sensitive neurons it remains unknown how they are associated with central nervous system circuitries that mediate estrogen-induced modulation of behavioral components. With the use of double-labeling immunohistochemistry and tracing techniques, the relationships of estrogen receptor (ER)-alpha- and ER-beta-immunoreactive (IR) neurons in the mouse brainstem and spinal cord to monoaminergic, cholinergic, and spinal projection systems are explored. Similar distributions of ER-IR neurons were present in females and males, with differences in labeling intensity of ER-alpha immunoreactivity among males and estrogen-, and oil-treated females. Barrington's nucleus, the ventrolateral medulla, and the nucleus of the solitary tract contained spinal-projecting ER-alpha-IR neurons, whereas ER-alpha-IR neurons in the periaqueductal gray, parabrachial nucleus, and catecholaminergic A1 cell group received spinal input. Numerous tyrosine hydroxylase (TH)-IR ER-alpha-IR neurons were present in the ventral periaqueductal gray, nucleus of the solitary tract, A1 cell group, and lumbosacral cord. The dorsal raphe nucleus contained ER-alpha-IR and ER-beta-IR neurons that colocalized with serotonin (5HT), and the reticulotegmental nucleus contained 5HT-IR ER-alpha-IR neurons. Fibers IR for vesicular acetylcholine transporter (VAChT), TH, and 5HT were located among ER-alpha-IR neurons in the dorsal horn and spinal autonomic regions. Robust staining for TH and VAChT, but not 5HT, was present among ER-alpha-IR neurons in the lumbosacral lateral collateral pathway. Possible modulatory actions of estrogen on each of these ER-IR populations are discussed in the context of their specific function, including micturition, sexual behavior, ejaculation, cardiovascular and respiratory control, tactile and nociceptive sensory processing, anti-nociception, endocrine regulation, and feeding.

Nucleus retroambiguus-spinal pathway in the mouse: Localization, gender differences, and effects of estrogen treatment

Nucleus retroambiguus (NRA)-motoneuronal projections are species-specific and serve expiration, Valsalva maneuvers, vocalization, and sexual behavior. In cat and monkey, estrogen induces sprouting of NRA-spinal axons. This pathway may thus serve as a model to study mechanisms through which estrogen induces neuronal plasticity. In this study, NRA-spinal projections are described in adult mice by using anterograde and retrograde tracing techniques, with attention to gender, strain (CD-1 and C57BL/6), and estrogen-induced changes (in ovariectomized females). Labeled NRA-spinal neurons at the level of the decussation of the corticospinal tract were most numerous after tracer injections into the thoracic and upper lumbar cord. They were medium-sized and had axons that descended through the contralateral cord. A group of small neurons was labeled in the NRA immediately rostral to the decussation of the corticospinal tract after cervical and thoracic, but not after lumbar injections. This group projected mainly via an ipsilateral pathway. The main projections from the caudal NRA involved motoneurons in the thoracic and upper-lumbar cord that supply abdominal wall and cremaster muscles. Pelvic floor motoneurons did not receive substantial input. NRA-spinal projections, especially those involving the upper lumbar cord, were sexually dimorphic, being more extensive in males than in females. Moreover, they were more distinct in estrogen-treated females than in control females. Strain differences were not observed. The unique features of the caudal NRA-spinal pathway in the mouse are discussed in the framework of possible functions of this system, such as mating behavior and related social behaviors, parturition, thermoregulation, and control of balance.

Effects of neurotoxic destruction of descending noradrenergic pathways on cannabinoid antinociception in models of acute and tonic nociception

The effects of neurotoxic destruction of catecholaminergic projections to the spinal cord on cannabinoid antinociception were examined in models of acute and tonic nociception. High performance liquid chromatography was used to quantify monoamine levels in sham-operated and lesioned rats. Intrathecal administration of the catecholamine neurotoxin 6-hydroxydopamine (6-OHDA) induced a selective depletion of norepinephrine (by approximately 85% of control) in rat lumbar spinal cord without altering levels of dopamine or serotonin. By contrast, brain levels of monoamines did not differ in sham-operated and lesioned rats. Pain behavior was similar in sham-operated and lesioned rats receiving vehicle in models of both acute and tonic nociception. The cannabinoid agonist WIN55,212-2 (5 or 10 mg/kg, i.p.) produced antinociception in the tail-flick test in sham-operated rats. The antinociceptive effect of WIN55,212-2 was attenuated relative to control conditions in rats depleted of spinal norepinephrine. WIN55,212-2 suppressed tonic pain behavior in the formalin test in sham-operated rats during phase 2 (15-60 min post formalin) of nociceptive responding. By contrast, in lesioned rats, WIN55,212-2 suppressed pain behavior during phase 1 (0-9.9 min) and phase 2A (10-39.9 min), but not during phase 2B (40-60 min). The cannabinoid agonist suppressed formalin-evoked Fos protein expression, a marker of neuronal activity, in the lumbar dorsal horn of sham-operated rats, but no suppression was observed in lesioned rats. The number of formalin-evoked Fos-like immunoreactive (FLI) cells was greater in lamina I and II of lesioned rats relative to sham-operated rats. These data indicate that the suppressive effect of the cannabinoid on formalin-evoked Fos protein expression in the superficial dorsal horn was attenuated following destruction of descending noradrenergic pathways. Our data are consistent with the hypothesis that cannabinoids produce antinociception, in part, by modulating descending noradrenergic systems and support a differential involvement of noradrenergic projections to the spinal cord in cannabinoid modulation of acute versus tonic nociception.

Effects of c-fos antisense oligodeoxynucleotide on 5-HT-induced upregulation of preprodynorphin, preproenkephalin, and glutamic acid decarboxylase mRNA expression in cultured rat spinal dorsal horn neurons

Effects of c-fos antisense oligodeoxynucleotide (ASO) on serotonin (5-HT)-induced upregulation of preprodynorphin (ppDyn), preproenkephalin (ppEnk), and glutamic acid decarboxylase (GAD), a special chemical marker for gamma-aminobutyric acid (GABA) neurons, mRNAs in cultured spinal dorsal horn neurons were investigated in order to extend our understanding of expressions of opioid peptides and GABA in spinal cord regulated by the descending serotonergic efferents. Reverse transcription-polymerase chain reaction analysis revealed a time-course increase in the expression of mRNAs encoding c-fos, ppDyn, ppEnk, and GAD after administration of 5-HT (100 nM). Administration of c-fos ASO (0.02 nM) 30 min prior to 5-HT application markedly blocked the expression of c-fos gene. Moreover, c-fos ASO pretreatment significantly decreased the 5-HT-induced upregulation of ppDyn and ppEnk mRNAs, but failed to affect the expression level of GAD mRNA. These results suggest that the serotoningic raphe-spinal efferents might play an important role in regulating the synthesis of enkephalin, dynorphin, and GABA in the spinal dorsal horn neurons. The immediate early oncogene c-fos might be involved in the 5-HT-induced increase in ppDyn and ppEnk expression. However, under the present experimental conditions, c-fos does not seem to be associated with the upregulation of GAD mRNA induced by 5-HT.

The activation of 5-HT(3) receptors evokes GABA release in the spinal cord

We carried out experiments using a microdialysis method to determine whether activation of 5-HT(3) receptors increases the concentration of GABA in the dorsal horn of the spinal cord. Intrathecal perfusion of a selective 5-HT(3) receptor agonist, 1-phenylbiguanide (0.3, 1.0, and 3.0 mM) dose-dependently, increased GABA concentration, while concentrations of glutamate and glycine were not changed. The concentration of aspartate was increased only by 3.0 mM of 1-phenylbiguanide. Our results have provided direct evidence that activation of 5-HT(3) receptors evokes GABA release in the spinal dorsal horn, possibly producing analgesia.

The locus coeruleus-noradrenergic system: modulation of behavioral state and state-dependent cognitive processes

Through a widespread efferent projection system, the locus coeruleus-noradrenergic system supplies norepinephrine throughout the central nervous system. Initial studies provided critical insight into the basic organization and properties of this system. More recent work identifies a complicated array of behavioral and electrophysiological actions that have in common the facilitation of processing of relevant, or salient, information. This involves two basic levels of action. First, the system contributes to the initiation and maintenance of behavioral and forebrain neuronal activity states appropriate for the collection of sensory information (e.g. waking). Second, within the waking state, this system modulates the collection and processing of salient sensory information through a diversity of concentration-dependent actions within cortical and subcortical sensory, attention, and memory circuits. Norepinephrine-dependent modulation of long-term alterations in synaptic strength, gene transcription and other processes suggest a potentially critical role of this neurotransmitter system in experience-dependent alterations in neural function and behavior. The ability of a given stimulus to increase locus coeruleus discharge activity appears independent of affective valence (appetitive vs. aversive). Combined, these observations suggest that the locus coeruleus-noradrenergic system is a critical component of the neural architecture supporting interaction with, and navigation through, a complex world. These observations further suggest that dysregulation of locus coeruleus-noradrenergic neurotransmission may contribute to cognitive and/or arousal dysfunction associated with a variety of psychiatric disorders, including attention-deficit hyperactivity disorder, sleep and arousal disorders, as well as certain affective disorders, including post-traumatic stress disorder. Independent of an etiological role in these disorders, the locus coeruleus-noradrenergic system represents an appropriate target for pharmacological treatment of specific attention, memory and/or arousal dysfunction associated with a variety of behavioral/cognitive disorders.

Projections from the nucleus reticularis magnocellularis to the rat cervical cord using electrical stimulation and iontophoretic injection methods

The aim of this study is to clarify the fiber distribution of the nucleus reticularis magnocellularis (NRMC) and adjacent areas in the rat spinal cord. Biotinylated dextran amine was injected iontophoretically through a glass capillary into the areas, in which a single cell responded to noxious electrical stimulation of the sciatic nerve and to a pinch of the thigh skin with multiple spikes. Labeled fibers descended bilaterally through the ventral funiculi of the medulla oblongata and then through the ventral and lateral funiculi of the cervical cord with an ipsilateral predominance, and terminated in the spinal gray (laminae I-X). A single fiber sometimes ran through several laminae while bifurcating many short branches with axon varicosities and terminal buttons in one transverse section, that is, through laminae V, VII and X, through laminae V, IIl-IV and I-II, and through laminae VII to I-II. The present study showed that the wide distribution of a single fiber and a mass of fibers descending from the NRMC and adjacent areas might modulate not only somatic sensory and motor functions but also autonomic functions in the spinal cord.

Serotonin potentiation of glycine-activated whole-cell currents in the superficial laminae neurons of the rat spinal dorsal horn is mediated by protein kinase C

The modulatory effects of serotonin (5-HT) on glycine (Gly)-activated whole-cell currents were investigated in neurons acutely dissociated from the superficial laminae (I and II) of the rat spinal dorsal horn using the nystatin-perforated patch recording configuration under voltage-clamp conditions. Our results demonstrate that (1). Gly acted on strychnine (STR)-sensitive Gly receptors and elicited inward Cl(-) currents (I(Gly)) at a holding potential of -40 mV; (2). 5-HT potentiated I(Gly) without affecting the reversal potential of I(Gly); (3). the agonist (alpha-methyl-5-HT) and antagonist (ketanserine) of 5-HT(2) receptor mimicked and blocked the potentiating effect of 5-HT on I(Gly), respectively; (4). bisindolylmaleimide I (BIM), a selective inhibitor of protein kinase C (PKC), reduced the potentiating effect of 5-HT on I(Gly); and (5). 5-HT-induced enhancement of I(Gly) was not affected by pretreatment with 1,2-bis-(2-aminophenoxy) ethane-N,N,N',N'-tetraacetic acid tetrakis (acetoxy-methyl) ester (BAPTA AM), a Ca(2+) chelator. These results indicate that (1). the potentiation of 5-HT on I(Gly) is mediated by 5-HT(2) receptor and through Ca(2+)-independent PKC intracellular signal transduction pathway; and (2). the interactions between 5-HT and Gly might modulate the transmission of nociceptive information through the spinal cord.

The antinociceptive effect of tramadol in the formalin test is mediated by the serotonergic component

The aim of this study was to investigate the neurotransmissions involved in the antinociceptive effect of tramadol in the formalin test, which is an animal model of acute and tonic pain. A subcutaneous injection of formalin produces a biphasic nociceptive response: phase 1 (0-10 min-acute pain) and phase 2 (21-60 min-tonic pain). Nociceptive activity is reduced greatly during the 10 min between these two phases. We measured in mice the effects of (+/-)-tramadol, and of (+)- and (-)-tramadol administered before the induction of pain by formalin, in the presence and absence of drugs that act on the opioidergic, serotonergic and noradrenergic systems (naloxone, ketanserin, fluoxetine, maprotiline). With respect to animals treated with formalin alone, (+/-)-tramadol and its enantiomers significantly reduced the duration of nociceptive behaviours (lifting, licking, favouring, shaking, and flinching of the formalin-treated paw) during phase 2. This effect was prevented by the 5-HT(2) receptor antagonist ketanserin, but not by naloxone which, on the contrary, was able to prevent the antinociceptive effect of morphine. Naloxone and ketanserin did not affect the duration of nociceptive behaviour in animals not treated with tramadol. Fluoxetine (a selective 5-hydroxytryptamine (5-HT) reuptake inhibitor), but not maprotiline (a selective norepinephrine reuptake inhibitor), potentiated the antinociceptive effect of (+/-)-tramadol. In conclusion, we demonstrate that the serotonergic pathway is responsible for the antinociceptive effect of tramadol in phase 2 of the formalin test, and that this effect is mediated by 5-HT(2) receptors.

Ondansetron inhibits the analgesic effects of tramadol: a possible 5-HT(3) spinal receptor involvement in acute pain in humans

To investigate a possible antinociceptive role of serotonin receptor subtype 3 (5-HT(3)), we evaluated the effects of a coadministration of ondansetron, a 5-HT(3) selective antagonist, and tramadol, a central analgesic dependent on enhanced serotonergic transmission. Fifty-nine patients undergoing ear, throat, and nose surgery, using tramadol for 24-h postoperative patient-controlled analgesia (bolus = 30 mg; lockout interval = 10 min) were randomly allocated either to a group receiving ondansetron continuous infusion (1 mg. mL(-1). h(-1)) for postoperative nausea and vomiting (Group O) or to a control group receiving saline (Group T). Pain and vomiting scores and tramadol consumption were evaluated at 4, 8, 12, and 24 h. Pain scores were never >4, according to a 0-10 numerical rating scale, in both groups. Group O required significantly larger doses of tramadol at 4 h (213 versus 71 mg, P < 0.001), 8 h (285 versus 128 mg, P < 0.002), and 12 h (406 versus 190 mg, P < 0.002). Vomiting scores were higher in Group O at 4 h (P < 0.05) and 8 h (P = 0.05). We conclude that ondansetron reduced the overall analgesic effect of tramadol, probably blocking spinal 5-HT(3) receptors.

IMPLICATIONS

Serotonin is an important neurotransmitter of the descending pathways that down-modulate spinal nociception. In postoperative pain, ondansetron, a selective 5-HT(3) receptor antagonist, increased the analgesic dose of tramadol. We suggest that, when antagonized for antiemetic purpose, 5-HT(3) receptors foster nociception, because of their site-dependent action.

Electrophysiologic characterization of the antinociceptive actions of S18616, a novel and potent alpha2-adrenoceptor agonist, after acute and persistent pain states

alpha (2)-Adrenoceptor (AR) agonists are active in behavioral models of persistent pain involving tissue and nerve damage. We evaluated the spinal effect of a novel, potent, and selective alpha (2)-AR agonist, [7,8](2-chlorobenzo)-2-amino-1-aza-3-oxa[4,5]spirodeca-1,7-diene (S18616), on the responses of dorsal horn neurons in halothane-anesthetized rats. Intrathecal administration of S18616 (0.1 to 3.0 microg) dose-dependently suppressed C- and A delta-fiber evoked responses but not the A beta-fiber evoked response. Drug effects were reversed by the alpha (2)-AR antagonists, atipamezole and idazoxan (100 microg). In rats with unilateral spinal nerve (L5-L6) ligation performed 2 weeks before study, S18616 (0.1 to 3.0 microg) dose-dependently suppressed the C- and A delta-fiber evoked responses and blocked "wind-up" in these neurons. The potency was comparable between nerve-injured and sham-operated rats, and S18616 was equally effective against responses to thermal and high-intensity mechanical stimuli. Interestingly, the effectiveness of S18616 on the low-intensity mechanical evoked response was significantly enhanced after nerve injury. Finally, S18616 (0.3 and 3.0 microg) reduced the neuronal responses produced by intraplantar injection of formalin. In conclusion, S18616 dose-dependently and potently inhibits the responses of dorsal horn neurons to peripheral stimulation in normal, inflamed, and neuropathic rats. These data support the use of spinal S18616 and other alpha (2)-AR agonists in the management of clinical pain.

Descending control of pain

Upon receipt in the dorsal horn (DH) of the spinal cord, nociceptive (pain-signalling) information from the viscera, skin and other organs is subject to extensive processing by a diversity of mechanisms, certain of which enhance, and certain of which inhibit, its transfer to higher centres. In this regard, a network of descending pathways projecting from cerebral structures to the DH plays a complex and crucial role. Specific centrifugal pathways either suppress (descending inhibition) or potentiate (descending facilitation) passage of nociceptive messages to the brain. Engagement of descending inhibition by the opioid analgesic, morphine, fulfils an important role in its pain-relieving properties, while induction of analgesia by the adrenergic agonist, clonidine, reflects actions at alpha(2)-adrenoceptors (alpha(2)-ARs) in the DH normally recruited by descending pathways. However, opioids and adrenergic agents exploit but a tiny fraction of the vast panoply of mechanisms now known to be involved in the induction and/or expression of descending controls. For example, no drug interfering with descending facilitation is currently available for clinical use. The present review focuses on: (1) the organisation of descending pathways and their pathophysiological significance; (2) the role of individual transmitters and specific receptor types in the modulation and expression of mechanisms of descending inhibition and facilitation and (3) the advantages and limitations of established and innovative analgesic strategies which act by manipulation of descending controls. Knowledge of descending pathways has increased exponentially in recent years, so this is an opportune moment to survey their operation and therapeutic relevance to the improved management of pain.

Contributions of the medullary raphe and ventromedial reticular region to pain modulation and other homeostatic functions

The raphe magnus is part of an interrelated region of medullary raphe and ventromedial reticular nuclei that project to all areas of the spinal gray. Activation of raphe and reticular neurons evokes modulatory effects in sensory, autonomic, and motor spinal processes. Two physiological types of nonserotonergic cells are observed in the medullary raphe and are thought to modulate spinal pain processing in opposing directions. Recent evidence suggests that these cells may modulate stimulus-evoked arousal or alerting rather than pain-evoked withdrawals. Nonserotonergic cells are also likely to modulate spinal autonomic and motor circuits involved in thermoregulation and sexual function. Medullary serotonergic cells have state-dependent discharge and are likely to contribute to the modulation of pain processing, thermoregulation, and sexual function in the spinal cord. The medullary raphe and ventromedial reticular region may set sensory, autonomic, and motor spinal circuits into configurations that are appropriate to the current behavioral state.

The NK1 receptor is essential for the full expression of noxious inhibitory controls in the mouse

Behavioral analysis of the NK1 receptor gene knock-out (NK1-/-) mouse indicated that substance P was closely involved in orchestrating the physiological and behavioral response of the animal to major environmental stressors. In particular, endogenous pain control mechanisms, such as stress-induced analgesia were substantially impaired in mutant mice, suggesting a reduction in descending inhibitory controls to the spinal cord from the brainstem. To directly test the integrity of descending controls in NK1-/- mice, we have analyzed c-Fos expression in laminae I-II of the lumbar and cervical cord and in the rostral ventromedial medulla in an experimental paradigm known to require recruitment of descending inhibitory controls. Anesthetized mice were stimulated with water at 50 degrees C either on their forepaw, hindpaw, or on both the hindpaw plus forepaw concurrently. Wild-type mice, naive or treated with an NK1 antagonist (RP67580) or its inactive isomer (RP68651), were compared with NK1-/- mice. C-Fos expression at the lumbar laminae I-II level was significantly reduced, whereas it was significantly greater in the raphe magnus and pallidus nuclei in the double stimulation situation in wild-type compared with NK1-/- mice. Blocking the NK1 receptor pharmacologically reproduced, in an enantiomere-selective manner, the data from NK1-/- mice, with no evidence for recruitment of descending inhibition at the lumbar cord level after forepaw stimulation. The present study demonstrates that the NK1 receptor is essential for the full development of noxiously evoked descending inhibition.

Antinociceptive effect of clomipramine in monoarthritic rats as revealed by the paw pressure test and the C-fiber-evoked reflex

The antinociceptive effect of clomipramine was studied in monoarthritic rats by using the paw pressure test and the C-fiber-evoked reflex. Monoarthritis was produced by intra-articular injection of complete Freund's adjuvant into the tibio-tarsal joint. Joint circumference as well as vocalization threshold to graded paw pressure were evaluated weekly during a 14-week period after the intra-articular injection. At week 8, monoarthritic and vehicle-injected control rats were given either clomipramine or saline and both the paw pressure threshold and inhibition of the C-fiber-evoked reflex response were evaluated. Results showed that (i) 1.5, 3.0, and 6.0 mg/kg, i.v. of clomipramine induced significantly greater dose-dependent antinociception to paw pressure testing in the monoarthritic group, as compared to the control one; and (ii) 0.75, 1.5, 3.0, and 6.0 mg/kg, i.v. of clomipramine exerted significantly higher dose-dependent inhibition of the C-reflex activity in monoarthritic rats than in controls. Results suggest that the higher sensitivity to clomipramine in monoarthritic rats could be related to adaptive changes occurring in monoamine metabolism or in other neurotransmitter systems during chronic pain.

An excitatory role for 5-HT in spinal inflammatory nociceptive transmission; state-dependent actions via dorsal horn 5-HT(3) receptors in the anaesthetized rat

The involvement of 5-HT(3) receptor mediated modulation of formalin and carrageenan induced inflammatory transmission was investigated. The effects of the selective 5-HT(3) receptor antagonist ondansetron on the electrically evoked responses of dorsal horn neurones in normal animals were compared to those following carrageenan. The effect of pre-treatment on the formalin response was also studied. Ondansetron had no significant effect on the electrically evoked responses of dorsal horn neurones in normal animals or following carrageenan induced inflammation, but significantly inhibited both phases of the formalin response. Our results suggest that 5-HT(3) receptors in the spinal cord have no significant role under normal conditions. However, during formalin (but not carrageenan) induced inflammation this system is activated, maintaining the response of nociceptive spinal neurones to peripheral formalin.

Serotonin potentiates the response of neurons of the superficial laminae of the rat spinal dorsal horn to gamma-aminobutyric acid

Employing the Nystatin-perforated whole-cell patch-clamp recording technique, the modulatory effects of serotonin (5-HT) on gamma-aminobutyric acid (GABA)-activated whole-cell currents were investigated in neurons acutely dissociated from the superficial laminae (laminae I and II) of the rat spinal dorsal horn. The results showed: (1) GABA acted on GABA(A) receptors and elicited inward Cl(-) currents (I(GABA)) at a holding potential (V(H)) of -40 mV; (2) 5-HT potentiated GABA-induced Cl(-) current without affecting the reversal potential of I(GABA) and the apparent affinity of GABA to its receptor; (3) alpha-methyl-5-HT, a selective agonist of 5-HT(2) receptor, mimicked the potentiation effect of 5-HT on I(GABA), whereas ketanserine, an antagonist of 5-HT(2) receptor, blocked the potentiation effect of 5-HT; (4) Chelerythrine, an inhibitor of protein kinase C, reduced the potentiation effect of 5-HT on I(GABA). The present results indicate: (1) The potentiation of 5-HT on I(GABA) is mediated by 5-HT(2) receptor and through a protein kinase-dependent transduction pathway; (2) The interactions between 5-HT and GABA might play an important role in the modulation of nociceptive information transmission at spinal cord level.

The role of vagal visceral afferents in the control of nociception

We have shown that activity in subdiaphragmatic vagal afferents modulates mechanical hyperalgesic behavior in the rat. Subdiaphragmatic vagotomy decreases paw-withdrawal threshold to mechanical stimulation (baseline and after intradermal injection of bradykinin), thus enhancing mechanical hyperalgesic behavior. Most of this decrease is generated by an endocrine signal released by the adrenal medullae because denervation or removal of the adrenal medullae prevents or reverses these changes. This novel mechanism may imply that: (a) the brain is able to regulate sensitivity of nociceptors all over the body by a neuroendocrine mechanisms, (b) sensitivity of nociceptors can be influenced by changes in parts of the body which are remote from the location of the sensitized nociceptors and (c) circulating catecholamines can influence nociceptors in a way which is different from those reported so far (see Jänig and McLachlan, 1994; Jänig, 1996a; Jänig et al., 1996).

Localization of calcitonin receptor mRNA in the mouse brain: coexistence with serotonin transporter mRNA

To elucidate the sites of and mechanisms of analgesic effect of centrally injected calcitonin, we examined expression of calcitonin receptor mRNA in the mouse brain by in situ hybridization techniques. Calcitonin receptor mRNA was expressed in various brain regions, including the preoptic area, dorsomedial hypothalamic nucleus, lateral hypothalamic area, periaqueductal gray, dorsal raphe nucleus, locus coeruleus, lateral parabrachial nucleus, gigantocellular reticular nucleus alpha part, lateral paragigantocellular nucleus, raphe magnus nucleus and solitary tract nucleus, which are known to play important roles in pain modulation. In addition, a double in situ hybridization technique demonstrated the intense expression of calcitonin receptor mRNA on serotonergic neurons in some raphe nuclei and the lateral paragigantocellular nucleus, suggesting the involvement of central serotonergic pathways in analgesic effect of calcitonin.

Periaqueductal gray matter input to cardiac-related sympathetic premotor neurons

The periaqueductal gray matter (PAG) serves as the midbrain link between forebrain emotional processing systems and motor pathways used in the defense reaction. Part of this response depends upon PAG efferent pathways that modulate cardiovascular-related sympathetic outflow systems, including those that regulate the heart. While it is known that the PAG projects to vagal preganglionic neurons, including possibly cardiovagal motoneurons, no information exists on the PAG circuits that may affect sympathetically mediated cardiac functions and, thus, the purpose of this study was to use neuroanatomical methods to identify these pathways. First, viral transneuronal retrograde tracing experiments were performed in which pseudorabies virus (PRV) was injected into the stellate ganglion of rats. After 4 days survival, five PAG regions contained transynaptically infected neurons; these included the dorsomedial, lateral and ventrolateral PAG columns as well as the Edinger-Westphal and precommissural nuclei. Second, the descending efferent PAG projections were studied with the anterograde axonal marker Phaseolus vulgaris leuco-agglutinin (PHA-L) with a particular focus on determining whether the PAG projects to the intermediolateral cell column (IML). Almost no axonal labeling was found throughout the thoracic IML suggesting that the PAG modulates sympathetic functions by indirect pathways involving synaptic relays through sympathetic premotor cell groups, especially those found in the medulla oblongata. This possibility was examined by a double tracing study. PHA-L was first injected into either the lateral or ventrolateral PAG and after 6 days, PRV was injected into the ipsilateral stellate ganglion. After an additional 4 days survival, a double immunohistochemical procedure for co-visualization of PRV and PHA-L was used to identify the sympathetic premotor regions that receive an input from the PAG. The PAG innervated specific groups of sympathetic premotor neurons in the hypothalamus, pons, and medulla as well as providing reciprocal intercolumnar connections within the PAG itself (Jansen et al., Brain Res. 784 (1998) 329-336). The major route terminates in the ventral medulla, especially within the medial region which contains sympathetic premotor neurons lying within the raphe magnus and gigantocellular reticular nucleus, pars alpha. Both serotonergic and non-serotonergic sympathetic premotor neurons in these two regions receive inputs from the PAG. Weak PAG projections to sympathetic premotor neurons were found in the rostral ventrolateral medulla (including to C1 adrenergic neurons), locus coeruleus, A5 cell group, paraventricular and lateral hypothalamic nuclei. In summary, both the lateral and ventrolateral PAG columns appear to be capable of modulating cardiac sympathetic functions via a series of indirect pathways involving sympathetic premotor neurons found in selected sites in the hypothalamus, midbrain, pons, and medulla oblongata, with the major outflow terminating in bulbospinal regions of the rostral ventromedial medulla.

Differential distribution of (GABA)A receptor subunits on bulbospinal serotonergic and nonserotonergic neurons of the ventromedial medulla of the rat

Spinally projecting neurons of the ventromedial medulla (VMM) compose an important efferent pathway for the modulation of nociception. These neurons receive a substantial gamma-aminobutyric acid (GABA)-ergic input, but the GABA receptor that mediates this input is unknown. This study examined the distribution of GABA(A) receptor alpha1 and alpha3 subunits in serotonergic and nonserotonergic neurons of the VMM that project to the dorsal horn in the rat. A pledget of Gelfoam soaked in Fluoro-Gold was placed at the thoracolumbar junction of the spinal cord to label spinally projecting neurons. Alternate sections of the medulla were then incubated with a mixture of antisera to either serotonin and the alpha1 subunit, or to serotonin and the alpha3 subunit of the GABA(A) receptor. Nearly 30% of spinally projecting neurons in the VMM were immunoreactive for the alpha1 subunit. A similar percentage of spinally projecting neurons in the VMM were immunoreactive for the alpha3 subunit, although diffuse cellular labeling combined with intense staining of processes in the neuropil precluded a rigorous semi-quantitative estimation of this population. No alpha1-subunit-immunoreactive neurons colocalized serotonin. In contrast, serotonergic neurons were immunoreactive for the alpha3 subunit. However, these double-labeled neurons were a modest percentage of the serotonergic population. A small percentage of spinally projecting serotonergic neurons was immunoreactive for the alpha3 subunit. These results suggest that significant numbers of spinally projecting serotonergic and nonserotonergic neurons of the VMM possess GABA(A) receptors that differ in their respective subunit compositions and that both classes of neurons may mediate the antinociception produced by the microinjection of GABA(A) receptor antagonists in the VMM.

Physiological identification of pontomedullary serotonergic neurons in the rat

Spinal serotonin is derived entirely from bulbar sources and plays an important role in spinal modulatory processes, including pain modulation. Establishing the electrophysiological properties of SEROTONERGIC bulbospinal neurons in the pontomedullary raphe and reticular formation is critical to understanding the physiological role of serotonin in the spinal cord. Neurons were characterized by their responses to noxious stimulation and their background discharge pattern in the lightly anesthetized rat. Characterized cells were intracellularly labeled with Neurobiotin, which was visualized with a Texas Red fluorophore. Sections containing the labeled cells were processed for serotonin immunocytochemistry with the use of a Bodipy fluorophore. Forty-seven intracellularly labeled cells were tested for serotonin immunoreactivity. The labeled neurons were located in raphe magnus, the nucleus reticularis magnocellularis, and the adjacent reticular and raphe nuclei at levels from the inferior olivary complex to the superior olivary complex. SEROTONERGIC cells were located in the raphe nuclei, in nucleus reticularis magnocellularis pars alpha, and in nucleus reticularis magnocellularis pars beta or nucleus reticularis gigantocellularis. Thirteen intracellularly labeled cells contained serotonin immunoreactivity. The background discharge rate of SEROTONERGIC cells average 1.8 Hz (range: 0.5-3.1 Hz). Discharge was steady and without sustained pauses or bursts in firing. Most serotonin-immunoreactive cells were unaffected or slightly excited by pinch and were unaffected by noxious heat. Three SEROTONERGIC cells were weakly excited by both noxious pinch and heat, whereas two SEROTONERGIC cells were briefly inhibited by these stimuli. Cells that lacked serotonin immunoreactivity were heterogeneous and included ON, OFF, and NEUTRAL cells. Nonserotonergic cells differed from SEROTONERGIC cells in having an irregular discharge pattern and/or a high mean discharge rate. A linear discriminant function, employing background discharge characteristics as independent variables, was calculated that successfully classified 13 of 13 SEROTONERGIC and 32 of 33 nonserotonergic neurons. The probability of misclassification with the use of this discriminant function was estimated to be < 10%. Employing the discriminant function on a test group of cells whose immunochemical content was unknown revealed a population of SEROTONERGIC-LIKE cells that resembled the labeled SEROTONERGIC cells in background discharge pattern, response to noxious stimulation, and nuclear location. The discharge of pontomedullary SEROTONERGIC neurons is slow and steady, suggesting that these neurons may have a role in the tonic, rather than phasic, modulation of spinal processes.

Evidence for rhythmic firing being caused by feedback inhibition in pinch-inhibited raphe magnus neurons

Raphe magnus cells that are inhibited by skin pinching fire spontaneously with strongly preferred interspike intervals (mean cycle 85 ms, n = 33). In pentobarbital-anesthetized rats, mid-cycle cathodal activation (0.3 ms) or end-cycle anodal black (30-60 ms) at approximately 1 Hz through the extracellular recording microelectrode delayed expected spikes; respective post-stimulus latencies peaked on average at 1.17 (n = 14) and 0.40 (n = 6) cycles. Feedback inhibition following random excitation, but not free-running intrinsic or afferent oscillations, may therefore cause the rhythm.

[Substance P, somatostatin and monoaminergic transmitters in the cerebrospinal fluid of patients with chronic idiopathic trigeminal neuralgia]

The etiology of trigeminal neuralgia is unknown, but both peripheral and central causes have been suggested. To investigate the role of central neurochemical mechanisms we measured epinephrine, norepinephrine and their breakdown product, vanilly mandelic acid (VMA), in the cerebrospinal fluid (CSF) of 16 patients (53.3 +/- 8.3 years) suffering from trigeminal neuralgia. As markers for the dopaminergic system, we determined CSF levels of dopamine and its metabolite homovanillic acid (HVA). As a marker for the serotonergic system, we measured CSF levels of serotonin metabolite 5-hydroxyindoleacetic acid (5-HIAA). In addition, levels of the neuropeptides substance P and somatostatin were determined. The concentration of norepinephrine (P < 0.01), VMA (P < 0.05) and HVA (P < 0.05) were significantly decreased in patients with trigeminal neuralgia and correlated with the duration of the disease and depression scores. 5-HIAA was also significantly decreased (P < 0.05) compared to control patients. Whereas substance P was significantly elevated (P < 0.05), somatostatin was significantly decreased (P < 0.05). Various correlations between the classical neurotransmitters and the neuropeptides could be established. We hypothesize than the sum of complex neurochemical changes plays a role in the etiology of trigeminal neuralgia, which can be separated in local and more central proceedings. The increase in substance P, a major nociceptive neuromodulator, supports the concept of a local neurogenic inflammation, possibly located in the trigeminovascular system. Depending on the duration of the disease and depression, the loss of serotonergic, dopaminergic and noradrenergic innervation seems to reflect more central changes, possibly due to alterations in their antinociceptive descending pathways.

The ventrolateral medulla of the rat is connected with the spinal cord dorsal horn by an indirect descending pathway relayed in the A5 noradrenergic cell group

The pathway conveying the descending inhibitory noradrenergic input elicited from the caudal ventrolateral medulla (VLM) onto the spinal cord dorsal horn was studied in the rat. Retrograde labeling with cholera toxin subunit B (CTb) injected into the dorsal horn was combined with immunostaining for dopamine-beta-hydroxylase (DBH) in the VLM and other brainstem nuclei containing noradrenergic cells. CTb-labeled neurons occurred in the lateral part of the VLM (VLMlat), located ventrolaterally to the DBH-immunoreactive cells of the A1 noradrenergic cell group. Neuronal profiles stained for CTb and DBH (double labeled) occurred in the A5 (31%), A6 (57%), and A7 (12%) noradrenergic cell groups. To ascertain whether noradrenergic cells targeting the spinal cord in those groups received projections from the VLMlat, this area was injected with the anterograde tracer biotinylated dextran amine (BDA). Labeled terminal fibers with boutons en passant were apposed to numerous double-stained neurons in the A5 cell group. Similar appositions occurred in small amounts in the ventral subcoerulear component of the A6. Correlated light and electron microscopic analyses of the labeled appositions revealed that the BDA-labeled axonal boutons contained spherical vesicles and were presynaptic at asymmetrical contacts to somata and dendritic profiles of the double-stained A5 neurons. These data indicate the occurrence of an indirect dysynaptic pathway connecting the VLM to the spinal cord, with a relay in the A5 cells. This pathway may convey the antinociceptive effects mediated by alpha 2-adrenoreceptors, which have been previously observed in the spinal cord following VLM stimulation.

Pharmacology of N-methyl-D-aspartate-evoked [3H]noradrenaline release in adult rat spinal cord

N-Methyl-D-aspartate (NMDA) produced a concentration-related increase in [3H]noradrenaline release from adult rat cervical spinal cord slices. Its potency was relatively low and the response concentrated in dorsal spinal regions although also observed in ventral slices. NMDA did not increase the release of radiolabelled glutamate, aspartate, gamma-aminobutyric acid (GABA), acetylcholine or serotonin. In comparison with previously characterised NMDA responses in the striatum, (release of dopamine, GABA, acetylcholine or spermidine) the spinal response was particularly sensitive to MK-801 and magnesium and to L-689,560 but not to other glycine receptor antagonists (7-chlorokynurenate, CNQX (6-cyano-7-nitroquinoxaline-2,3-dione), DNQX (6,7-dichloroquinoxaline-2,3-dione), (+)-HA966). Dextrorphan and dextromethorphan produced partial or biphasic inhibition curves suggesting a subdivision of NMDA receptors. NMDA-evoked [3H]noradrenaline release was moderately sensitive to CPP and CGP37849 but insensitive to arcaine. These characteristics distinguish the native spinal NMDA receptor subtype(s) from those so far characterised in the striatum suggesting a unique spinal NMDA receptor subtype.

Contacts between serotoninergic fibres and dorsal horn spinocerebellar tract neurons in the cat and rat: a confocal microscopic study

Contacts between serotoninergic nerve fibres and dorsal horn dorsal spinocerebellar tract neurons were analysed in order to investigate the morphological basis of actions of serotonin upon dorsal spinocerebellar tract neurons. In a series of experiments dorsal spinocerebellar tract neurons were labelled with intracellularly injected rhodamine-dextran in the cat. The neurons were monosynaptically excited by group II muscle afferents and cutaneous afferents and were identified by antidromic activation following stimuli applied in the cerebellum. In the second series of experiments dorsal spinocerebellar tract neurons were labelled by retrograde transport of Fluorogold injected into the cerebellum in the rat. In both series, serotoninergic fibres were labelled by using a specific anti-serotonin antiserum and were revealed by immunofluorescence. Appositions between the serotoninergic fibres and the cells were inspected with a dual channel confocal microscope. The merged images obtained with the two channels of the microscope were viewed in single optical planes 2 microns apart and in rotated three-dimensional reconstructions. Serotoninergic nerve fibres were found in apposition to cell bodies of all feline dorsal spinocerebellar tract neurons (n = 7) and of 75% of rat dorsal spinocerebellar tract neurons (n = 90). The numbers of putative contacts on cell bodies varied between less than 100 and nearly 300 (mean 160) in the cat and between about five and 30 in the rat. Contacts with dendrites of feline neurons were seen on 96% of 72 dendrites within 300 microns from soma and on 91% of 23 dendrites at distances of 300-500 microns. The number of such contacts varied from less than five to 150 on a single dendrite within these ranges of distances. Their total number within 100 microns from the soma was comparable or exceeded the number of contacts on the soma.

Transneuronal labeling of CNS neuropeptide and monoamine neurons after pseudorabies virus injections into the stellate ganglion

The viral transneuronal labeling method was used in combination with immunohistochemical procedures to identify CNS neuropeptide and monoamine neurons that innervate the sympathetic preganglionic neurons (SPNs) which project to the stellate ganglion--the principal source of the sympathetic supply to the heart. Transneuronal labeling was found at three CNS levels: spinal cord, brainstem, and hypothalamus. In the thoracic spinal cord, apart from the pseudorabies virus (PRV)-labeled stellate SPNs, PRV-labeled neurons were localized in laminae I/II, IV, V, VII, and X as well as in the lateral spinal nucleus and lateral funiculus. In the C1-C4 spinal segments, labeled neurons were found in the lateral funiculus as well as laminae V and VII of the spinal gray matter. PRV-labeled cells were identified in lamina V and the dorsolateral funiculus of the lumbar spinal cord. Three medullary areas were consistently labeled: rostral ventromedial medulla (RVMM), rostral ventrolateral medulla (RVLM), and caudal raphe nuclei. The greatest concentration of labeling was found in the RVMM. This projection arose from adrenergic, serotonergic (5-HT), thyrotropin releasing hormone (TRH), substance P, somatostatin, enkephalin, and vasoactive intestinal peptide (VIP) immunoreactive neurons. The RVLM projection originated mainly from C1 adrenergic neurons, some of which contained immunoreactive neuropeptide Y (NPY). C3 adrenergic-NPY neurons lying near the floor of the 4th ventricle were also labeled. Enkephalin-, somatostatin- and VIP-immunoreactive RVLM neurons also contributed to this projection. 5-HT neurons of the caudal raphe nuclei (raphe pallidus, raphe obscurus, and raphe magnus) were labeled; some of these contained substance P or TRH-immunoreactivity with an occasional neuron staining for all three putative neurotransmitters. In the pons, catecholamine neurons in the A5 cell group, subcoeruleus and Kolliker-Fuse nuclei were labeled. The midbrain contained relatively few infected cells, but some were present in the Edinger-Westphal and precommissural nuclei. Forebrain labeling was concentrated in the paraventricular hypothalamic nucleus (PVN) with lesser amounts in the lateral hypothalamic area (LHA) and the perifornical region. In the PVN, oxytocin-immunoreactive neurons accounted for the greatest chemically-defined projection while corticotrophin releasing factor (CRF), vasopressin-, and angiotensin II-immunoreactive neurons provided successively lesser inputs. In the LHA, angiotensin II-immunoreactive neurons were labeled. In summary, this study provides the first detailed map of the chemically-coded CNS neurons involved in the control of the cardiosympathetic outflow.

Distribution of spinal cord projections from the medullary subnucleus reticularis dorsalis and the adjacent cuneate nucleus: a Phaseolus vulgaris-leucoagglutinin study in the rat

The distribution and organization of descending spinal projections from the dorsal part of the caudal medulla were studied in the rat following injections of Phaseolus vulgaris-leucoagglutinin into small areas of the subnucleus reticularis dorsalis (SRD) and the adjacent cuneate nucleus (Cu). The caudal aspect of the Cu projected only to the dorsal horn of the ipsilateral cervical cord via the dorsal funiculus. These projections were mainly to laminae I, IV, and V. More ventrally located reticular structures projected to the full length of the cord. Fibers originating from the SRD travelled through the ipsilateral dorsolateral funiculus and terminated within the deep dorsal horn and upper layers of the ventral horn, mainly in laminae V-VII. Fibers originating from subnucleus reticularis ventralis (SRV) travelled ipsilaterally through the lateral and ventrolateral funiculi and bilaterally through the ventromedial funiculus. These fibers terminated within the ventral horn. The density of labeling within the gray matter varied at different levels of the cord was as follows: cervical > sacral > thoracic > lumbar. The reciprocal connections between the caudal medulla and the spinal cord suggest that the former is an important link in feedback loops that regulate spinal outflow.

Normal distribution of alpha 2-adrenoceptors in the rat spinal cord and its modification after noradrenergic denervation: a quantitative autoradiographic study

The distribution of alpha 2 (alpha 2)-adrenoceptors along cervical, thoracic, lumbar, and sacral segments of the spinal cord of normal rats has been studied by quantitative autoradiography using the specific alpha 2-antagonist [3H]rauwolscine as a ligand. In addition, the influence of noradrenergic (NA) denervation [obtained either by complete transection of the spinal cord at vertebrae level T8-T9 or by selective lesion of NA spinal cord system carried out by intracisternal injection of 6-hydroxydopamine (6-OHDA)] on eventual variations of alpha 2-adrenoceptor density at spinal cord target cells was studied in parallel. In control rats, the quantitative analysis of alpha 2-adrenoceptor densities revealed the presence of these receptors throughout the whole gray matter with a preferential location in the superficial dorsal horn. This pattern was the same at all rostro-caudal levels of the cord and appeared very well correlated with the distribution of NA terminals revealed by immunohistochemistry, particularly in the superficial layers of the dorsal horn. After total transection of the spinal cord (caudally to the section) and 6-OHDA-induced lesion, an increase of alpha 2-adrenoceptor density was mainly observed within the distal dorsal horn thus evidencing supersensitivity in this area, while modifications were not detectable in other regions of the spinal gray matter, except at the lumbar level where other dorsal, central, and intermediate zones were significantly enriched.

Effects of neonatal capsaicin treatment on descending modulation of spinal nociception from the rostral, medial medulla in adult rat

Stimulation-produced modulation from the rostral, medial medulla (RMM) on the spinal nociceptive tail-flick (TF) reflex and on lumbar spinal dorsal horn neuron responses to noxious cutaneous stimuli was studied in adult rats treated as neonates with capsaicin or vehicle. In vehicle-treated rats (n = 7), both descending facilitatory and inhibitory influences on the TF reflex were produced from the RMM. At 11/23 sites in the RMM, electrical stimulation produced biphasic modulatory effects. Electrical stimulation facilitated the spinal nociceptive TF reflex at low intensities (5-25 microA) and inhibited the TF reflex at greater intensities (50-200 microA). The mean threshold intensity of stimulation to inhibit the TF reflex (cut-off time = 7.0 s) was 66 microA (n = 11). At 11 of 23 sites, electrical stimulation only inhibited the TF reflex; the mean threshold intensity of stimulation to inhibit the TF reflex was 50 microA (n = 11). At one stimulation site, electrical stimulation only facilitated the TF reflex at the intensities tested (5-100 microA). In capsaicin-treated rats (n = 6), the proportion of sites from which electrical stimulation only inhibited the TF reflex was significantly less (3/27 sites = 11%) than in vehicle-treated rats (11/23 = 48%). The threshold intensity of stimulation to inhibit the TF reflex from these three sites was 50 microA. The number of sites in RMM from which electrical stimulation only facilitated the TF reflex was significantly greater in capsaicin-treated rats (15/27 = 56%) than in vehicle-treated rats (1/23 = 4%). Neither the number of sites in RMM from which electrical stimulation produced biphasic modulatory effects on the TF reflex (48% and 33%, respectively) nor the intensities of stimulation or magnitudes of facilitation or inhibition of the TF reflex significantly differed between vehicle- and capsaicin-treated rats. In electrophysiological experiments, all units studied responded to non-noxious and noxious intensities of mechanical stimulation applied to the glabrous skin of the plantar surface of the ipsilateral hind foot and also to noxious heating of the skin (50 degrees C). The number of sites where electrical stimulation produced only facilitatory effects on responses of spinal dorsal horn neurons to noxious stimulation (thermal or mechanical) of the skin was significantly increased from 13% of the total sites in vehicle-treated rats to 40% in capsaicin-treated rats.(ABSTRACT TRUNCATED AT 400 WORDS)

Whole-cell patch-clamp recordings from visualized bulbospinal neurons in the brainstem slices

The purpose of this study was to develop a method for electrophysiological characterization of retrogradely labeled bulbospinal neurons in the specific cytoarchitectonic regions in the brainstem slices. Several days after the spinal cord was injected with the carbocyanine dye, DiI, retrogradely labeled bulbospinal neurons were visualized by epifluorescence optics in the brainstem slices with the aid of a silicon intensifier tube (SIT) camera. Labeled somata were routinely seen in the caudal raphe nuclei, rostroventral medial and lateral portions of the medulla, the A5 group and in other medullary sites known to project to the spinal cord. Electrophysiological properties of the DiI-labeled neurons were assessed by whole-cell recordings using micropipettes filled with biocytin. The slices were subsequently processed for dual visualization of biocytin and serotonin or a marker for noradrenergic neurons, tyrosine hydroxylase (TH). The electrophysiological properties of bulbospinal neurons were correlated with their morphology and neurochemical content. This technique may be useful in other areas of CNS for studying morphology, neurochemical content and physiology of retrogradely labeled neurons.

Neurotransmitters in subcortical somatosensory pathways

Investigations during recent years indicate that many different neuroactive substances are involved in the transmission and modulation of somesthetic information in the central nervous system. This review surveys recent developments within the field of somatosensory neurotransmission, emphasizing immunocytochemical findings. Increasing evidence indicates a widespread role for glutamate as a fast-acting excitatory neurotransmitter at different levels in somatosensory pathways. Several studies have substantiated a role for glutamate as a neurotransmitter in primary afferent neurons and in corticofugal projections, and also indicate a neurotransmitter role for glutamate in ascending somatosensory pathways. Other substances likely to be involved in somatosensory neurotransmission include the neuropeptides. Many different peptides have been detected in primary afferent neurons with unmyelinated or thinly myelinated axons, and are thus likely to be directly involved in primary afferent neurotransmission. Some neurons giving rise to ascending somatosensory pathways, primarily those with cell bodies in the dorsal horn, are also immunoreactive for peptides. Recent investigations have shown that the expression of neuropeptides, both in primary afferent and ascending tract neurons, may change as a result of various kinds of peripheral manipulation. The occurrence of neurotransmitters in intrinsic neurons and neurons providing modulating inputs to somatosensory relay nuclei (the dorsal horn, the lateral cervical nucleus, the dorsal column nuclei and the ventrobasal thalamus) is also reviewed. Neurotransmitters and modulators in such neurons include acetylcholine, monoamines, GABA, glycine, glutamate, and various neuropeptides.

Alpha 2A-adrenergic receptors are present in lower brainstem catecholaminergic and serotonergic neurons innervating spinal cord

A subtype-specific polyclonal antibody was used for the immunohistochemical detection of alpha 2A-adrenergic receptors (alpha 2A-ARs) in the rat lower brainstem (medulla and pons). Using dual-label fluorescence histochemistry, punctate alpha 2A-AR-like immunoreactivity (alpha 2A-AR-LIR) was identified in noradrenergic, adrenergic, and serotonergic neurons of the pontomedullary region. Confocal microscopic examination of material simultaneously labeled for TH-LIR and alpha 2A-LIR revealed that the clusters of alpha 2A-LIR were located intracellularly. Lower medullary neurons with spinal projections to segment T3 were retrogradely labeled using FITC-conjugated microbeads and the material was processed for simultaneous detection of alpha 2A-LIR and either TH-LIR or 5-HT-LIR. Using this triple-label approach, we found that virtually all medullary serotonergic cells (raphe pallidus, raphe obscurus and parapyramidal area) including those with identified spinal projections contain punctate alpha 2A-AR-LIR. In contrast, fewer than 10% of dorsal raphe serotonergic cells examined for comparison were immunoreactive. The triple labeling approach also indicated that more than 95% of the TH-immunoreactive cells of the dorsal and ventrolateral medulla, including those with demonstrable spinal projections (A5 noradrenergic and C1/C3 adrenergic) had detectable amounts of alpha 2A-AR-LIR. The presence of alpha 2A-ARs in a large fraction of bulbospinal pre-sympathetic neurons (noradrenergic A5, adrenergic C1 and C3 and serotonergic raphe cells) could explain the powerful and relatively selective effect of clonidine and other centrally acting alpha 2A-AR agonists on sympathetic efferent activity and hypertension.

Real-time monitoring of endogenous noradrenaline release in rat brain slices using fast cyclic voltammetry: 3. Selective detection of noradrenaline efflux in the locus coeruleus

Fast cyclic voltammetry (FCV) at carbon fibre microelectrodes was used to monitor 'real time' endogenous noradrenaline (NA) efflux in superfused slices of rat locus coeruleus (LC) following local electrical stimulation. When stimulated with a standard train (30 pulses, 100 Hz, 0.2 ms, 10 mA, every 5 min), efflux of monoamine was constant over the experimental period (2.5 h): Amine efflux declined by only 16 +/- 5% while uptake half-life lengthened by only 9 +/- 8%. When calibrated in solutions of NA, peak amine efflux corresponded to 0.31 +/- 0.04 microM (mean +/- S.E.M., n = 28) and was removed by uptake with a half-life of 2.93 +/- 0.28 s (n = 16). The released compound was confirmed as NA on the basis of pharmacological and electrochemical criteria. Stimulated monoamine efflux was reversibly reduced by 78% by omission of Ca2+ from the superfusate for 30 min (P < 0.05). Ro 4-1284 (1 microM), a fast-acting reserpine-like drug, decreased amine efflux by 86% (P < 0.05). The monoamine oxidase inhibitor pargyline (2 microM) increased efflux by 30% (P < 0.05). Desipramine (0.05 microM), a selective NA uptake blocker, significantly increased amine efflux (by 96%, P < 0.05) and uptake half-life (by 314%, P < 0.05). Fluvoxamine (0.5 microM), the selective serotonin (5HT) uptake blocker, increased efflux by 59% (P < 0.05) and the uptake half-life by 122% (P < 0.05). Vanoxerine (GBR 12909: 0.3 microM), the dopamine (DA) uptake blocker, had no effect on amine efflux or uptake half-life. The voltammogram of the released amine had single oxidation and reduction peaks.(ABSTRACT TRUNCATED AT 250 WORDS)

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

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

Identification of periaqueductal gray and dorsal raphe nucleus neurons projecting to both the trigeminal sensory complex and forebrain structures: a fluorescent retrograde double-labeling study in the rat

The midbrain periaqueductal gray (PAG) including the dorsal raphe nucleus (DR) has been known to contain serotoninergic neurons projecting to many brain regions. Employing fluorescent retrograde double labeling combined with immunofluorescence histochemistry for serotonin (5-HT), we examined in the rat whether or not single PAG/DR neurons with 5-HT send their axons to both the trigeminal sensory complex and forebrain regions. Stereotaxic injections of Diamidino Yellow (DY) and Fast Blue (FB) were performed unilaterally; DY was injected into the caudal spinal trigeminal nucleus or principal sensory trigeminal nucleus, and FB into the ventrolateral orbital cortex, nucleus accumbens or amygdala. A small percentage of PAG/DR neurons were doubly labeled with DY and FB, and the majority of them showed 5-HT-like immunoreactivity (5-HT-LI). Most of these 5-HT-LI PAG/DR neurons that were indicated to send their axons simultaneously to both the trigeminal sensory complex and forebrain regions were distributed in the ventrolateral PAG subdivision and ventral aspects of the medial PAG subdivision at the middle and caudal PAG levels, bilaterally with a predominant distribution on the side ipsilateral to the injections. This indicates a possible role of these PAG/DR neurons in the limbic or affective-motivational aspect of the pain-related neural system.

Descending projections from the medullary dorsal reticular nucleus make synaptic contacts with spinal cord lamina I cells projecting to that nucleus: an electron microscopic tracer study in the rat

An ultrastructural study is made of the synaptic contacts occurring between structures labelled anterogradely and retrogradely in the superficial dorsal horn following injections of cholera toxin subunit B or horseradish peroxidase in the dorsal reticular nucleus of the medulla oblongata of the rat. Both tracers revealed labelled axonal boutons in lamina I with round synaptic vesicles and a few large granular vesicles making asymmetrical synaptic contacts upon labelled somata and dendrites. After injections of Phaseolus vulgaris leucoagglutinin in the dorsal reticular nucleus, labelled boutons identical to those revealed by the two other tracers were presynaptic to unlabelled somata and dendrites. In addition, dorsoreticular neurons were labelled retrogradely following injections of cholera toxin subunit B into the superficial dorsal horn of the cervical enlargement. These observations show the occurrence of a reciprocal connection between dorsal reticular and lamina I neurons. Considering the putative excitatory nature of the axodendritic contacts in lamina I, a positive feedback circuit is suggested, whereby the nociceptive signals transmitted to the dorsal medullary reticular formation by marginal neurons are intensified.