GABAergic circuitry in the rostral ventral medulla of the rat and its relationship to descending antinociceptive controls

Cell type-specific dissection of sensory pathways involved in descending modulation

Decades of research have suggested that stimulation of supraspinal structures, such as the periaqueductal gray (PAG) and rostral ventromedial medulla (RVM), inhibits nocifensive responses to noxious stimulation through a process known as descending modulation. Electrical stimulation and pharmacologic manipulations of the PAG and RVM identified transmitters and neuronal firing patterns that represented distinct cell types. Advances in mouse genetics, in vivo imaging, and circuit tracing methods, in addition to chemogenetic and optogenetic approaches, allowed the characterization of the cells and circuits involved in descending modulation in further detail. Recent work has revealed the importance of PAG and RVM neuronal cell types in the descending modulation of pruriceptive as well as nociceptive behaviors, underscoring their roles in coordinating complex behavioral responses to sensory input. This review summarizes how new technical advances that enable cell type-specific manipulation and recording of neuronal activity have supported, as well as expanded, long-standing views on descending modulation.

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.

Long-term, dynamic synaptic reorganization after GABAergic precursor cell transplantation into adult mouse spinal cord

Transplanting embryonic precursors of GABAergic neurons from the medial ganglionic eminence (MGE) into adult mouse spinal cord ameliorates mechanical and thermal hypersensitivity in peripheral nerve injury models of neuropathic pain. Although Fos and transneuronal tracing studies strongly suggest that integration of MGE-derived neurons into host spinal cord circuits underlies recovery of function, the extent to which there is synaptic integration of the transplanted cells has not been established. Here, we used electron microscopic immunocytochemistry to assess directly integration of GFP-expressing MGE-derived neuronal precursors into dorsal horn circuitry in intact, adult mice with short- (5-6 weeks) or long-term (4-6 months) transplants. We detected GFP with pre-embedding avidin-biotin-peroxidase and GABA with post-embedding immunogold labeling. At short and long times post-transplant, we found host-derived synapses on GFP-immunoreactive MGE cells bodies and dendrites. The proportion of dendrites with synaptic input increased from 50% to 80% by 6 months. In all mice, MGE-derived terminals formed synapses with GFP-negative (host) cell bodies and dendrites and, unexpectedly, with some GFP-positive (i.e., MGE-derived) dendrites, possibly reflecting autoapses or cross talk among transplanted neurons. We also observed axoaxonic appositions between MGE and host terminals. Immunogold labeling for GABA confirmed that the transplanted cells were GABAergic and that some transplanted cells received an inhibitory GABAergic input. We conclude that transplanted MGE neurons retain their GABAergic phenotype and integrate dynamically into host-transplant synaptic circuits. Taken together with our previous electrophysiological analyses, we conclude that MGE cells are not GABA pumps, but alleviate pain and itch through synaptic release of GABA.

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.

Selective repression of gene expression in neuropathic pain by the neuron-restrictive silencing factor/repressor element-1 silencing transcription (NRSF/REST)

Neuropathic pain often develops following nerve injury as a result of maladaptive changes that occur in the injured nerve and along the nociceptive pathways of the peripheral and central nervous systems. Multiple cellular and molecular mechanisms likely account for these changes; however, the exact nature of these mechanisms remain largely unknown. A growing number of studies suggest that alteration in gene expression is an important step in the progression from acute to chronic pain states and epigenetic regulation has been proposed to drive this change in gene expression. In this review, we discuss recent evidence that the DNA-binding protein neuron-restrictive silencing factor/repressor element-1 silencing transcription factor (NRSF/REST) is an important component in the development and maintenance of neuropathic pain through its role as a transcriptional regulator for a select subset of genes that it normally represses during development.

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.

Impaired arousal in rat pups with prenatal alcohol exposure is modulated by GABAergic mechanisms

Prenatal alcohol exposure (PAE) increases the risk for The Sudden Infant Death Syndrome (SIDS) in human infants. In rat pups, the arousal response to hypoxia is modulated by medullary raphe GABAergic mechanisms. We hypothesized that arousal to hypoxia is impaired by PAE, and is associated with an increase in medullary GABA and enhanced GABAergic activity. Pregnant dams received an ethanol liquid diet (ETOH), an iso-caloric pair fed diet (PF) or a standard chow diet (CHOW). We first measured the time to arousal (latency), during four episodes of hypoxia in P5, P15, and P21 CHOW, PF, and ETOH pups. We also measured brainstem GABA concentration in the same groups of pups. Finally, we injected artificial cerebrospinal fluid (aCSF), nipecotic acid (NIP) or gabazine into the medullary raphe of P15 and P21 pups receiving the three diets. For statistical analysis, the PF and CHOW groups were combined into a single CONTROL group. Our main finding was that compared to CONTROL, arousal latency to hypoxia is increased in ETOH pups at P15 and P21, and the concentration of brainstem GABA is elevated at P21. NIP administration in CONTROL pups led to arousal latencies similar in magnitude to those in ETOH pups after aCSF injection. NIP injected ETOH pups had no further increases in arousal latency. We conclude that PAE impairs arousal latency and this is mediated or modulated by medullary GABAergic mechanisms.

NAAG peptidase inhibition in the periaqueductal gray and rostral ventromedial medulla reduces flinching in the formalin model of inflammation

Background

Metabotropic glutamate receptors (mGluRs) have been identified as significant analgesic targets. Systemic treatments with inhibitors of the enzymes that inactivate the peptide transmitter N-acetylaspartylglutamate (NAAG), an mGluR3 agonist, have an analgesia-like effect in rat models of inflammatory and neuropathic pain. The goal of this study was to begin defining locations within the central pain pathway at which NAAG activation of its receptor mediates this effect.

Results

NAAG immunoreactivity was found in neurons in two brain regions that mediate nociceptive processing, the periaqueductal gray (PAG) and the rostral ventromedial medulla (RVM). Microinjection of the NAAG peptidase inhibitor ZJ43 into the PAG contralateral, but not ipsilateral, to the formalin injected footpad reduced the rapid and slow phases of the nociceptive response in a dose-dependent manner. ZJ43 injected into the RVM also reduced the rapid and slow phase of the response. The group II mGluR antagonist LY341495 blocked these effects of ZJ43 on the PAG and RVM. NAAG peptidase inhibition in the PAG and RVM did not affect the thermal withdrawal response in the hot plate test. Footpad inflammation also induced a significant increase in glutamate release in the PAG. Systemic injection of ZJ43 increased NAAG levels in the PAG and RVM and blocked the inflammation-induced increase in glutamate release in the PAG.

Conclusion

These data demonstrate a behavioral and neurochemical role for NAAG in the PAG and RVM in regulating the spinal motor response to inflammation and that NAAG peptidase inhibition has potential as an approach to treating inflammatory pain via either the ascending (PAG) and/or the descending pain pathways (PAG and RVM) that warrants further study.

Ultrastructural analysis of rat ventrolateral periaqueductal gray projections to the A5 cell group

Stimulation of neurons in the ventrolateral periaqueductal gray (PAG) produces antinociception as well as cardiovascular depressor responses that are mediated in part by pontine noradrenergic neurons. A previous report using light microscopy has described a pathway from neurons in the ventrolateral PAG to noradrenergic neurons in the A5 cell group that may mediate these effects. The present study used anterograde tracing and electron microscopic analysis to provide more definitive evidence that neurons in the ventrolateral PAG form synapses with noradrenergic and non-catecholaminergic A5 neurons in Sasco Sprague-Dawley rats. Deposits of anterograde tracer, biotinylated dextran amine, into the rat ventrolateral PAG labeled a significant number of axons in the region of the rostral subdivision of the A5 cell group, and a relatively lower number in the caudal A5 cell group. Electron microscopic analysis of anterogradely-labeled terminals in both rostral (n=127) and caudal (n=70) regions of the A5 cell group indicated that approximately 10% of these form synapses with noradrenergic dendrites. In rostral sections, about 31% of these were symmetric synapses, 19% were asymmetric synapses, and 50% were membrane appositions without clear synaptic specializations. In caudal sections, about 22% were symmetric synapses, and the remaining 78% were appositions. In both rostral and caudal subdivisions of the A5, nearly 40% of the anterogradely-labeled terminals formed synapses with non-catecholaminergic dendrites, and about 45% formed axoaxonic synapses. These results provide direct evidence for a monosynaptic pathway from neurons in the ventrolateral PAG to noradrenergic and non-catecholaminergic neurons in the A5 cell group. Further studies should evaluate if this established monosynaptic pathway may contribute to the cardiovascular depressor effects or the analgesia produced by the activation of neurons in the ventrolateral PAG.

Rostral ventromedial medulla μ, but not κ, opioid receptors are involved in electroacupuncture anti-hyperalgesia in an inflammatory pain rat model

It has been reported that intracerebroventricular injection of a μ receptor antagonist blocked 2 but not 100Hz electroacupuncture (EA)-produced analgesia in an uninjured animal model. Because persistent pain changes neural response to external stimulation, we hypothesized that the mechanisms of EA anti-hyperalgesia may be different in persistent pain than in health. Hyperalgesia, decreased paw withdrawal latency (PWL) to a noxious thermal stimulus, was induced by subcutaneously injecting complete Freund's adjuvant (CFA) into the hind paws of rats. Selective antagonists against μ (CTOP: D-Phe-Cys-Tyr-D-Trp-Orn-Thr-Pen-ThrNH2, 6.25 nmol) and κ (Nor-BIN: nor-binaltorphimine, 10 nmol) opioid receptors were infused into the rostral ventromedial medulla (RVM) 10 min before a 30-min EA treatment at acupoint Huantiao (GB30) 1h 30 min post-CFA. PWL was measured before and 2.5 post-CFA. Both 10 Hz and 100 Hz EA-produced anti-hyperalgesia were blocked by intra-RVM μ, but not κ, receptor antagonists. Double immunofluorescence staining demonstrated that μ receptor-containing neurons were GABAnergic and that GABAa receptor-containing neurons were serotonergic in the RVM. The results demonstrated an involvement of RVM μ, but not κ, receptors in EA-produced anti-hyperalgesia. In summary, EA may induce release of endogenous endomorphins that activate μ opioid receptors in GABAnergic neurons to suppress the release of GABA. This removes the tonic inhibition of GABA on serotonergic neurons in the RVM, and activation of these serotonergic neurons inhibits pain. EA may be used as complementary treatment for inflammatory pain.

Novel peptide ligands with dual acting pharmacophores designed for the pathophysiology of neuropathic pain

The conventional design of high affinity drugs targeted to a single molecule has not resulted in clinically useful therapies for pain relief. Recent reviews have suggested that newly designed analgesic drugs should incorporate multiple targets. The distributions of cholecystokinin (CCK) and CCK receptors in the central nervous system (CNS) overlap significantly with endogenous opioid systems and can be dually targeted. CCK has been shown to act as an endogenous "anti-analgesic" peptide and neuropathic pain conditions promote endogenous CCK release in CNS regions of pain modulation. Administration of CCK into nuclei of the rostral ventromedial medulla induces pronociceptive behaviors in rats. RSA 504 and RSA 601 are novel bifunctional compounds developed to target neuropathic pain by simultaneously acting as agonists at two distinct opioid receptors and antagonizing CCK receptors in the CNS. RSA 504 and RSA 601 demonstrate agonist activity in vitro and antihypersensitivity to mechanical and thermal stimuli in vivo using the spinal nerve ligation model of neuropathic pain. Intrathecal administration of RSA 504 and RSA 601 did not demonstrate antinociceptive tolerance over 7 days of administration and did not display motor impairment or sedation using a rotarod. These are the first behavioral studies that demonstrate how multi-targeted molecule design can address the pathology of neuropathic pain. These compounds with δ and μ opioid agonist activity and CCK antagonist activity within one molecule offer a novel approach with efficacy for neuropathic pain while lacking the side effects typically caused by conventional opioid therapies.

Local GABAergic modulation of the activity of serotoninergic neurons in the nucleus raphe magnus

Experiments on rat brainstem sections in membrane potential clamping conditions addressed the effects of serotonin and GABA on serotoninergic neurons in the nucleus raphe magnus. Local application of serotonin stimulated inhibitory postsynaptic currents (IPSC) in 45% of the serotoninergic neurons studied. This response was not seen in the presence of the fast sodium channel blocker tetrodotoxin. The GABAA receptor antagonist gabazine blocked IPSC in both serotonin-sensitive and serotonin-insensitive neurons. Application of GABA evoked generation of a membrane current (IGABA), which was completely blocked by gabazine. These results indicate self-regulation of the activity of serotoninergic neurons in the nucleus raphe magnus via a negative feedback circuit involving local GABAergic interneurons.

Behavioral response and transmitter release during atonia elicited by medial medullary stimulation

Activation of the medial medulla is responsible for rapid eye movement (REM) sleep atonia and cataplexy. Dysfunction can cause REM sleep behavior disorder and other motor pathologies. Here we report the behavioral effects of stimulation of the nucleus gigantocellularis (NGC) and nucleus magnocellularis (NMC) in unrestrained cats. In waking, 62% of the medial medullary stimulation sites suppressed muscle tone. In contrast, stimulation at all sites, including sites where stimulation produced no change or increased muscle tone in waking, produced decreased muscle tone during slow-wave sleep. In the decerebrate cat electrical stimulation of the NGC increased glycine and decreased norepinephrine (NE) release in the lumbar ventral horn, with no change in γ-aminobutyric acid (GABA) or serotonin (5-HT) release. Stimulation of the NMC increased both glycine and GABA release and also decreased both NE and 5-HT release in the ventral horn. Glutamate levels in the ventral horn were not changed by either NGC or NMC stimulation. We conclude that NGC and NMC play neurochemically distinct but synergistic roles in the modulation of motor activity across the sleep-wake cycle via a combination of increased release of glycine and GABA and decreased release of 5-HT and NE. Stimulation of the medial medulla that elicited muscle tone suppression also triggered rapid eye movements, but never produced the phasic twitches that characterize REM sleep, indicating that the twitching and rapid eye movement generators of REM sleep have separate brain stem substrates.

Microinjection of angiotensin II in the caudal ventrolateral medulla induces hyperalgesia

Nociceptive transmission from the spinal cord is controlled by supraspinal pain modulating systems that include the caudal ventrolateral medulla (CVLM). The neuropeptide angiotensin II (Ang II) has multiple effects in the CNS and at the medulla oblongata. Here we evaluated the expression of angiotensin type 1 (AT(1)) receptors in spinally-projecting CVLM neurons, and tested the effect of direct application of exogenous Ang II in the CVLM on nociceptive behaviors. Although AT(1)-immunoreactive neurons occurred in the CVLM, only 3% of AT(1)-positive neurons were found to project to the dorsal horn, using double-immunodetection of the retrograde tracer cholera toxin subunit B. In behavioral studies, administration of Ang II (100 pmol) in the CVLM gave rise to hyperalgesia in both the tail-flick and formalin tests. This hyperalgesia was significantly attenuated by local administration of the AT(1) antagonist losartan. The present study demonstrates that Ang II can act on AT(1) receptors in the CVLM to modulate nociception. The effect on spinal nociceptive processing is likely indirect, since few AT(1)-expressing CVLM neurons were found to project to the spinal cord. The renin-angiotensin system may also play a role in other supraspinal areas implicated in pain modulation.

Periaqueductal gray neurons project to spinally projecting GABAergic neurons in the rostral ventromedial medulla

The analgesic effects of morphine are mediated, in part, by periaqueductal gray (PAG) neurons that project to the rostral ventromedial medulla (RVM). Although much of the neural circuitry within the RVM has been described, the relationship between RVM neurons and PAG input and spinal output is not known. The objective of this study was to determine whether GABAergic output neurons from the PAG target RVM reticulospinal neurons. Immunocytochemistry and confocal microscopy revealed that PAG neurons project extensively to RVM neurons projecting to the spinal cord, and two-thirds of these reticulospinal neurons appear to be GABAergic (contain GAD67 immunoreactivity). The majority (71%) of PAG fibers that contact RVM reticulospinal GAD67-immunoreactive neurons also contained GAD67 immunoreactivity. Thus, there is an inhibitory projection from PAG to inhibitory RVM reticulospinal neurons. However, there were also PAG projections to the RVM that did not contain GAD67 immunoreactivity. Additional experiments were conducted to determine whether the heterogeneity in this projection can be explained by the electrophysiological character of the RVM target neurons. PAG projections to electrophysiologically defined and juxtacellularly filled ON, OFF, and Neutral cells in the RVM were examined. Similar to the pattern reported above, both GAD67- and non-GAD67-immunoreactive PAG neurons project to RVM ON, OFF, and Neutral cells in the RVM. These inputs include a GAD67-immunoreactive projection to a GAD67-immunoreactive ON cell and non-GAD67 projections to GAD67-immunoreactive OFF cells. This pattern is consistent with PAG neurons producing antinociception by direct excitation of RVM OFF cells and inhibition of ON cells.

A comparative study of excitatory and inhibitory amino acids in three different brainstem nuclei

This study was designed to shed more light onto the three different brainstem regions which are implicated in the pain pathway for the level of various excitatory and inhibitory neurotransmitters before and following neuronal stimulation. The in vivo microdialysis technique was used in awake, freely moving adult Sprague-Dawley rats. The neurotransmitters studied included aspartate, glutamate, GABA, glycine, and taurine. The three brainstem regions examined included the mid-brain periaqueductal gray (PAG), the medullary nucleus raphe magnus (NRM), and the spinal trigeminal nucleus (STN). Neuronal stimulation was achieved following the administration of the sodium channel activator veratridine. The highest baseline levels of glutamate (P < 0.0001), aspartate (P < 0.0001), GABA (P < 0.01), taurine (P < 0.0001), and glycine (P < 0.001) were seen in the NRM. On the other hand, the lowest baseline levels of glutamate, GABA, glycine, and taurine were found in the PAG, while that of aspartate was found in the STN. Following the administration of veratridine, the highest release of the above neurotransmitters except for the aspartate and glycine was found in the PAG where the level of glutamate increased by 1,310 +/- 293% (P < 0.001), taurine by 1,008 +/- 143% (P < 0.01), and GABA by 10,358 +/- 1,920% (P < 0.0001) when comparison was performed among the three brainstem regions and in relation to the baseline levels. The highest release of aspartate was seen in the STN (2,357 +/- 1,060%, P < 0.001), while no significant difference was associated with glycine. On the other hand, the lowest release of GABA and taurine was found in the STN (696 +/- 91 and 305 +/- 25%, respectively), and glutamate and aspartate in the NRM (558 +/- 200 and 874 +/- 315%, respectively). Our results indicate, and for the first time, that although some differences are seen in the baseline levels of the above neurotransmitters in the three regions studied, there are quite striking variations in the level of release of these neurotransmitters following neuronal stimulation in these regions. In our opinion this is the first study to describe the pain activation/modulation related changes of the excitatory and inhibitory amino acids profile of the three different brainstem areas.

[Painful legs and moving toes syndrome]

Painful legs and moving toes syndrome is a rare medical picture characterized by involuntary movements of the toes or the whole foot and pain in lower limbs. However, this must be kept in mind due to its association with other diseases and its possibility of being the first symptom. Spinal cord and cauda equina diseases, neuropathies, radiculopathies, drugs and other systemic diseases are the main cause of this syndrome although many cases are still idiopathic. Its diagnosis is essentially clinical and its treatment is complex, including different combinations of drugs and invasive techniques, and generally with a bad response.

Mechanisms for the Anti-nociceptive Actions of the Descending Noradrenergic and Serotonergic Systems in the Spinal Cord

The sensation of pain plays a critical role as an alert and as a protection system against tissue damage from mechanical, chemical, and thermal stimuli. Despite the protective role of pain, the severity of pain sensation is markedly attenuated by the endogenous pain inhibitory systems that predominantly originate at the brain stem. Both behavioral and in vivo extracellular recording studies have sought the loci producing analgesia and clarification of the anti-nociceptive actions. Among those loci, the main descending systems to the spinal dorsal horn are noradrenergic and serotonergic. Although, in vivo studies have provided basic knowledge of these systems, the precise synaptic mechanisms underlying the analgesic actions have not yet been elucidated until recently. The newly developed in vitro slice and in vivo patch-clamp recordings have disclosed the synaptic mechanisms of the noradrenergic and serotonergic effects at the level of spinal dorsal horn. This paper reviews the anti-nociceptive action of these systems, while particularly focusing on the electrophysiological aspects of the systems at the single neuron level in the spinal dorsal horn as well as their origins and responsible receptor subtypes.

Hypothalamic projections to locus coeruleus neurons in rat brain

Locus coeruleus (LC) neurons respond to autonomic and visceral stimuli and discharge in parallel with peripheral sympathetic nerves. The present study characterized the synaptic organization of hypothalamic afferents with catecholaminergic neurons in the LC using electron microscopy. Peroxidase labeling of axon terminals that were anterogradely labeled from the paraventricular nucleus (PVN) was combined with gold-silver labeling of tyrosine hydroxylase in the LC. Approximately 19% of the anterogradely labeled axon terminals formed synaptic specializations with tyrosine hydroxylase-immunoreactive dendrites in the LC. Retrograde transport from the LC combined with immunocytochemical detection of enkephalin and corticotropin-releasing factor (CRF) suggested that most of the LC-projecting PVN neurons (30%) were CRF immunoreactive and few (2%) were enkephalin immunoreactive. Finally, dual retrograde tracing from the LC and median eminence revealed that PVN neurons that project to the LC are a population distinct from that projecting to the median eminence. The present data suggest that a population of hypothalamic neurons is poised to directly modulate the activity of LC neurons and may integrate autonomic responses in brain by influencing LC neurons. Moreover, PVN neurons that use CRF as a neurohormone are distinct from those that use CRF as a neuromodulator to impact on the LC.

The role of descending fibers from the rostral ventromedial medulla in opioid analgesia in rats

There has been controversy as to whether the contribution of descending fibers from the rostral ventromedial medulla to opioid analgesia depends on the nature of the noxious stimulus eliciting pain. In the present study, inactivation of descending fibers by microinjection of muscimol (50 ng) in the rostral ventromedial medulla abolished morphine analgesia in the tail immersion and hot plate tests but decreased morphine analgesia by 60% in the formalin test. Analysis of the dose-response relation for morphine after inactivation of descending fibers revealed that, except for the tail immersion test, high doses of morphine could not overcome the block induced by muscimol. Also, morphine analgesia elicited supraspinally was not detectable when descending fibers were inactivated, suggesting that the analgesic effect of morphine in the brain requires a relay via the rostral ventromedial medulla. The analgesic effect of buprenorphine also depends on the integrity of descending fibers from the rostral ventromedial medulla. The results indicate that descending fibers from the rostral ventromedial medulla are critically important to the analgesic effect of opioids, regardless of the type of noxious stimulation eliciting pain. Residual analgesic effects of opioids after inactivation of descending fibers may be due to peripheral effects in the presence of inflammation.

Interaction between vanilloid and glutamate receptors in the central modulation of nociception

This study investigates the effect of microinjections of capsaicin in the periaqueductal grey matter of rats on nociceptive behaviour and the possible interactions with NMDA and mGlu receptors. Intra-periaqueductal grey microinjection of capsaicin (1-3-6 nmol/rat) increased the latency of the nociceptive reaction in the plantar test. This effect was prevented by pretreatment with capsazepine (6 nmol/rat), which had no effect per se on the latency of the nociceptive reaction. 7-(Hydroxyimino)cyclopropa[b]chromen-1alpha-carboxylate ethyl ester (CPCCOEt, 50 nmol/rat) and 2-Methyl-6-(phenylethynyl)pyridine (MPEP, 50 nmol/rat), antagonists of mGlu(1) and mGlu(5) receptors, respectively, completely blocked the effect of capsaicin. Similarly, pretreatment with DL-2-Amino-5-phosphonovaleric acid (DL-AP5, 5 nmol/rat) and riluzole (4 nmol/rat), an NMDA receptor antagonist and a voltage-dependent Na(+) channels blocker which inhibits glutamate release, respectively, completely antagonized the effect of capsaicin. However, pretreatment with (2S)-alpha-Ethylglutamic acid (30 nmol/rat) and (RS)-alpha-Methylserine-O-phosphate (MSOP, 30 nmol/rat), antagonists of group II and group III mGlu receptors, respectively, had no effects on capsaicin-induced analgesia. Similarly, pretreatment with N-(piperidin-1-yl)-5-(4-chlophenyl)-1-(2,4-dichlorophenyl)-4-methyl-1H-pyrazole-3-carboxamide (SR 141716A, 5 pmol/rat), a selective cannabinoid CB(1) receptor antagonist, did not affect the capsaicin-induced antinociception. In conclusion, this study shows that capsaicin might produce antinociception at the periaqueductal grey level by increasing glutamate release, which activates postsynaptic group I mGlu and NMDA receptors.

Ultrastructural analysis of ventrolateral periaqueductal gray projections to the A7 catecholamine cell group

Stimulation of neurons in the ventrolateral periaqueductal gray produces antinociception that is mediated in part by pontine noradrenergic neurons. Previous light microscopic analysis provided suggestive evidence for a direct projection from neurons in the ventrolateral periaqueductal gray to noradrenergic neurons in the A7 cell group that innervate the spinal cord dorsal horn. Therefore, the present ultrastructural study used anterograde tracing combined with tyrosine hydroxylase immunoreactivity to provide definitive evidence that neurons in the ventrolateral periaqueductal gray form synapses with the somata and dendrites of noradrenergic neurons of the A7 cell group. Injections of the anterograde tracers biotinylated dextran amine or Phaseolus vulgaris leucoagglutinin into the ventrolateral periaqueductal gray of Sasco Sprague-Dawley rats yielded a dense innervation in the region of the lateral pons containing the A7 cell group. Electron microscopic analysis of anterogradely labeled terminals (n=401) in the region of the A7 cell group indicated that approximately 10% of these formed plasmalemmal appositions to tyrosine hydroxylase-immunoreactive dendrites with no intervening astrocytic processes. About 23% of these were asymmetric synapses, 10% were symmetric synapses, and 67% did not exhibit clearly differentiated synaptic specializations. The majority of anterogradely labeled terminals (60%) formed plasmalemmal appositions with dendrites and somata that lacked detectable tyrosine hydroxylase immunoreactivity. About 35% of these were symmetric synapses, 9% were asymmetric synapses and 56% did not form synaptic specializations. Approximately 30% of all anterogradely labeled terminals displayed features characteristic of axo-axonic synapses.The present results provide direct ultrastructural evidence to support the hypothesis that the analgesia produced by stimulation of neurons in the ventrolateral periaqueductal gray is mediated, in part, by activation of spinally projecting noradrenergic neurons in the A7 catecholamine cell group.

Evidence that dopamine-beta-hydroxylase immunoreactive neurons in the lateral reticular nucleus project to the spinal cord in the rat

The existence of noradrenergic projections from the lateral reticular nucleus (LRt) to the dorsal quadrant of cervical, thoracic, or lumbar spinal cord was investigated using a combined method of WGA-apo-HRP-gold retrograde tracing and dopamine-beta-hydroxylase (DBH) immunocytochemistry. Preliminary retrograde tracing studies indicated that LRt neurons projecting to cervical, thoracic, or lumbar spinal cord were characteristically located near the perimeter of the LRt. Double-labeling experiments demonstrated that a portion of these peripherally-located, spinal-projecting neurons were DBH-immunoreactive. Double-labeled neurons were also located at the parvocellular division of the contralateral LRt in the thoracic injection cases. Double-labeled neurons were not observed at the subtrigeminal division in cervical, thoracic, or lumbar injection case. The results suggest the possibility that the noradrenergic LRt-spinal pathway might be involved in a variety of pain processing and cardiovascular regulatory functions in the rat.

Maternal behaviour in lactating rats stimulates c-fos in glutamate decarboxylase-synthesizing neurons of the medial preoptic area, ventral bed nucleus of the stria terminalis, and ventrocaudal periaqueductal gray

Increased activity of the immediate-early gene c-fos can be observed in many areas of the lactating rat brain after dams physically interact with pups and display maternal behaviour. These sites include the medial preoptic area, ventral bed nucleus of the stria terminalis, and the ventrolateral caudal periaqueductal gray, each of which is critical for the normal performance of particular maternal behaviours. The phenotype of cells in these areas that show increased c-fos activity after maternal behaviour, however, is unknown. Via double-label immunocytochemistry, we determined if the population of cells in these sites that express c-fos after maternal behaviour in lactating rats overlaps with the population that expresses the 67,000 mol. wt isoform of glutamate decarboxlyase, the synthesizing enzyme for the inhibitory neurotransmitter GABA. Lactating rats were separated from pups beginning on day 5 postpartum, and 48h later half were allowed to interact with a litter of pups for 60min whereas the other half were not. Dams re-exposed to pups were highly maternal, retrieving and licking them as well as displaying prolonged nursing behaviour that included milk letdown. Both groups of dams had a similar number of 67,000 mol. wt glutamate decarboxylase-immunoreactive cells in each site, although the number of 67,000 mol. wt glutamate decarboxylase-immunoreactive cells per microscopic field was significantly greater in the caudal ventrolateral periaqueductal gray than in the ventral bed nucleus of the stria terminalis, which in turn was greater than the medial preoptic area. In pup-stimulated dams, two to fourfold more Fos-immunoreactive cells were found in these three sites compared with non-stimulated controls. Labeling for Fos immunoreactivity and 67,000 mol. wt glutamate decarboxylase immunoreactivity was heterogeneous within each site. In the medial preoptic area, more Fos-immunoreactive and 67,000 mol. wt glutamate decarboxylase-immunoreactive cells (either single or dual-labeled) were found dorsally than ventrally. In the ventral bed nucleus of the stria terminalis, more Fos-immunoreactive and 67,000 mol. wt glutamate decarboxylase-immunoreactive cells were found medially than laterally. Within the caudal ventrolateral periaqueductal gray, 67,000 mol. wt glutamate decarboxylase-immunoreactive labeling was greatest ventromedially, while high numbers of Fos-immunoreactive nuclei were found both ventromedially and ventrolaterally. In pup-stimulated dams, more than half (53% in the medial preoptic area, 59% in the ventral bed nucleus of the stria terminalis, and 61% in the caudal ventrolateral periaqueductal gray) of the total population of Fos-immunoreactive cells also expressed 67,000 mol. wt glutamate decarboxylase. These results suggest that many of the neurons in these sites that show elevated c-fos activity after maternal behaviour are either local inhibitory interneurons or provide inhibitory input to other neural sites. These inhibitory mechanisms may be critical for the display of postpartum nurturance, possibly facilitating maternal behaviour by removing tonic inhibition on sites necessary for maternal responding or by restricting activity in neural sites that inhibit it.

Periaqueductal gray neurons monosynaptically innervate extranuclear noradrenergic dendrites in the rat pericoerulear region

Previous reports using light microscopy have provided anatomical evidence that neurons in the ventrolateral periaqueductal gray (PAG) innervate the medial pericoerulear dendrites of noradrenergic neurons in the nucleus locus coeruleus (LC). The present study used anterograde tracing and electron microscopic analysis to provide more definitive evidence that neurons in the ventrolateral PAG form synapses with the somata or dendrites of noradrenergic LC neurons. Deposits of either biotinylated dextran amine or Phaseolus vulgaris leucoagglutinin into the rat ventrolateral PAG labeled a moderate to high number of axons in the region of the medial pericoerulear region and Barrington's nucleus, but a relatively low number were labeled in the nuclear core of the LC. Ultrastructural analysis of anterogradely labeled terminals at the levels of the rostral (n = 233) and caudal (n = 272) subdivisions of the LC indicated that approximately 20% of these form synapses with tyrosine hydroxylase-immunoreactive dendrites; most of these were located in the medial pericoerulear region. In rostral sections, about 12% of these were symmetric synapses, 9% were asymmetric synapses, and 79% were membrane appositions without clear synaptic specializations. In caudal sections, about 30% were symmetric synapses, 11% were asymmetric synapses, and 59% were appositions. In both rostral and caudal sections, 60% of the anterogradely labeled terminals formed synapses with noncatecholamine dendrites, and 20% formed axoaxonic synapses. These results provide direct evidence for monosynaptic projections from neurons in the ventrolateral PAG to the extranuclear dendrites of noradrenergic LC neurons. This monosynaptic pathway may mediate in part the analgesia, reduced responsiveness to external stimuli, and decreased excitability of somatic motoneurons produced by stimulation of neurons in the ventrolateral PAG.

Synaptically released 5-HT modulates the activity of tonically discharging neuronal populations in the rostral ventral medulla (RVM)

There is substantial evidence for an important modulating role of monoamines (catecholamines and serotonin, 5-HT) in the rostral ventral medulla (RVM), a region which plays an important role in cardiovascular and nociceptive functions. We investigated in slices the role of endogenous monoamines in the synaptic control of the activity of rat RVM neuronal populations using intracellular recordings in the lateral RVM plus lateral aspect of nucleus paragigantocellularis lateralis. A triple-labelling protocol allowed us to identify the location of impaled neurons and their eventual monoaminergic phenotype within the serotonergic and catecholaminergic populations of the RVM. Focal electrical stimulation revealed the existence of a functional monoaminergic input onto RVM neurons which was mediated by endogenous 5-HT acting at inhibitory 5-HT1A receptors but did not involve noradrenergic neurotransmission. The slow 5-HT-mediated inhibitory postsynaptic potential (IPSP) was only observed in the regularly discharging neurons, which were found to be neither catecholaminergic nor serotonergic. The synaptic release of 5-HT was, itself, under an inhibitory control involving GABAA (gamma-aminobutyric acid) receptors. Moreover, we characterized the effect of the 5-HT-releasing agent fenfluramine on this functional 5-HT-mediated synaptic transmission. Our results show that the effect of fenfluramine is biphasic consisting of an initial prolongation of the serotonergic IPSP followed by a decrease in amplitude. Our data provide a basis for the previously reported inhibitory effects of exogenously applied serotonin agonists/antagonists on the autonomic functions controlled by the RVM. This 5-HT pathway, which functionally links the serotonergic and catecholaminergic regions, might play an important role in cardiovascular and nociceptive functions.

Supraspinal metabolic activity changes in the rat during adjuvant monoarthritis

Pain is a multi-dimensional experience including sensory-discriminative and affective-motivational components. The attribution of such components to a corresponding cerebral neuronal substrate in the brain refers to conclusions drawn from electrical brain stimulation, lesion studies, topographic mappings and metabolic imaging. Increases in neuronal metabolic activity in supraspinal brain regions, suggested to be involved in the central processing of pain, have previously been shown in various animal studies. The present investigation is the first to describe supraspinal structures which show increased metabolic activity during ongoing monoarthritic pain at multiple time-points. Experimental chronic monoarthritis of a hindlimb induced by complete Freund's adjuvant is one of the most used models in studies of neuronal plasticity associated with chronic pain. Such animals show typical symptoms of hyperalgesia and allodynia for a prolonged period. Metabolic activity changes in supraspinal brain regions during monoarthritis were assessed using the quantitative [14C]-2deoxyglucose technique at two, four, 14 days of the disease and, furthermore, in a group of 14-day monoarthritic rats which were mechanically stimulated by repeated extensions of the inflamed joint. Local glucose utilization was determined ipsi- and contralateral to the arthritic hindpaw in more than 50 brain regions at various supraspinal levels, and compared with saline-injected controls. At two and 14 days of monoarthritis significant bilateral increases in glucose utilization were seen in many brain structures, including brainstem, thalamic, limbic and cortical regions. Within the brainstem, animals with 14-day monoarthritis showed a higher number of regions with increased metabolic activity compared with two days. No differences between ipsi- and contralateral sides were detected in any of the experimental groups. Average increases ranged from 20 to 40% compared with controls and maximum values were detected in specific brain regions, such as the anterior pretectal nucleus, the anterior cingulate cortex and the nucleus accumbens. Interestingly, at four days of monoarthritis, the glucose utilization values were in the control range in almost all regions studied. Moreover, in monoarthritic rats receiving an additional noxious mechanical stimulation, the rates of glucose utilization were also comparable to controls in all brain areas investigated. Such patterns of brain metabolic activity agreed with concomitant changes in the lumbar spinal cord, described in the accompanying report. The present data show that a large array of supraspinal structures displays elevated metabolic activity during painful monoarthritis, with a non-linear profile for the time-points investigated. This observation most probably reflects mechanisms of transmission and modulation of nociceptive input arising from the monoarthritis and accompanying its development.

Pain-induced analgesia mediated by mesolimbic reward circuits

We tested the hypothesis that noxious stimuli induce pain modulation by activation of supraspinal structures. We found that intense noxious stimuli (i.e., subdermal injection of capsaicin or paw immersion in hot water) induced profound attenuation of the jaw-opening reflex in the anesthetized rat; forepaw subdermal capsaicin also elevated the mechanical hindpaw-withdrawal threshold in the awake rat. These antinociceptive effects were blocked by previous injection of either a dopamine antagonist (flupentixol) or an opioid antagonist (naloxone) into the nucleus accumbens. Additional experiments in anesthetized animals showed that the antinociceptive effect of noxious stimulation by either capsaicin (>/=100 micrograms) or hindpaw immersion in hot water (>/=45 degrees C for 4 min) correlated with the intensity of the stimulus. The maximal antinociceptive effect of capsaicin was similar in magnitude to that of a high dose of morphine (10 mg/kg) injected subcutaneously. Injection of the GABA(A)-receptor agonist muscimol, but not naloxone, into the rostroventral medulla, a major component of descending pain modulation systems, blocked capsaicin-induced antinociception. Although it is widely thought that painful stimuli may induce analgesia by activating forebrain structures, this is the first demonstration that such a mechanism exists. Furthermore, this mechanism can be engaged by naturalistic stimuli in awake animals. These observations imply that painful stimuli might under certain conditions be rewarding.

Distribution of bulbospinal gamma-aminobutyric acid-synthesizing neurons of the ventral respiratory group of the rat

Spinal respiratory motoneuron activity is controlled primarily by excitatory and inhibitory neurons in the medulla oblongata. To identify bulbospinal inhibitory neurons, immunohistochemistry for glutamic acid decarboxylase (GAD) was combined with retrograde labeling of projections to the C(4) ventral horn with Fluoro-Gold. GAD-immunoreactive bulbospinal neurons were located in the ventrolateral portion of the intermediate reticular nucleus, the ventral portion of the medial reticular nuclei, and the raphe and spinal vestibular nuclei. Small numbers of bulbospinal ventral respiratory group neurons were GAD immunoreactive. These neurons were distributed throughout the rostral ventral respiratory group and the Bötzinger complex. Surprisingly, low numbers of Bötzinger neurons, a population thought to be exclusively inhibitory, were GAD immunoreactive. These results suggest that the rostral ventral respiratory group and the Bötzinger complex both contain heterogeneous bulbospinal neuron populations, only some of which have gamma-aminobutyric acid (GABA)-mediated inhibitory control over phrenic motoneurons. Furthermore, the ventral respiratory group contained many GABAergic neurons that lacked bulbospinal projections.

Ultrastructural evidence that substance P neurons form synapses with noradrenergic neurons in the A7 catecholamine cell group that modulate nociception

Potent antinociception can be produced by electrical stimulation of spinally projecting noradrenergic neurons in the A7 catecholamine cell group and this effect is blocked by intrathecal injection of alpha2-adrenoceptor antagonists. Microinjection of substance P near A7 neurons also produces antinociception that is blocked by intrathecal injection of alpha2-adrenoceptor antagonists. These observations suggest that substance P produces antinociception by activating noradrenergic A7 neurons. However, it is not known whether this effect of substance P is produced by a direct or an indirect action on A7 neurons. Although light microscopic studies have demonstrated the existence of both substance P-containing axon terminals and neurokinin-1 receptors in the region of the A7 cell group, it is not known whether substance P terminals form synapses with noradrenergic A7 neurons. These experiments used double-labeling immunocytochemical methods and electron microscopic analysis to determine whether substance P-containing axons form synapses with noradrenergic neurons in the A7 cell group. Pre-embedding immunocytochemistry, combined with light and electron microscopic analysis, was used to provide ultrastructural evidence for synaptic connections between substance P-immunoreactive terminals labeled with immunoperoxidase and tyrosine hydroxylase-immunoreactive A7 neurons labeled with silver-enhanced immunogold. Tyrosine hydroxylase labeling was found in perikarya and dendrites in the A7 region, and substance P labeling was found in axons and synaptic terminals. Substance P-labeled terminals formed asymmetric synapses with tyrosine hydroxylase-labeled dendrites, but only a few of these were present on tyrosine hydroxylase-labeled somata. Substance P-labeled terminals also formed asymmetric synapses with unlabeled dendrites, and many unlabeled terminals formed both symmetric and asymmetric synapses with tyrosine hydroxylase-labeled dendrites. These results demonstrate that substance P neurons form a significant number of synapses with the dendrites of noradrenergic A7 neurons and support the conclusion that microinjection of substance P in the A7 cell group produces antinociception by direct activation of spinally projecting noradrenergic neurons.

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.

Activity changes in rat raphe magnus neurons at different concentrations of fentanyl in vitro

The nucleus raphe magnus (NRM) is an important descending pain inhibitory system. We postulated that the analgesic action of supraspinally administered opiates results from increased descending inhibitory control of the NRM. We tested whether fentanyl activates NRM neurons in the rat slice preparation using extra-cellular recording. Eighty-seven percent of NRM neurons (total number = 68) tested were spontaneously active with firing frequencies of 0.2-4 spikes/s in artificial cerebrospinal fluid. Application of fentanyl (0.25, 0.5, and 1 mumol/L) increased firing frequencies in 12 of 59 (20%) spontaneously active neurons. In 6 of 9 (67%) silent neurons, fentanyl induced firing activities. Naloxone (1-2 mumol/L) antagonized the increased or induced activities by fentanyl in three neurons. In 13 of 59 (22%) spontaneously active neurons, fentanyl decreased the firing frequencies. Although fentanyl was associated with increased activity in a total of 18 NRM neurons, fentanyl at a higher concentration significantly increased the number of inhibited neurons. The results indicate that fentanyl partly activates the descending inhibitory system originating from the NRM; however, at higher concentrations, it appears also to inhibit this same system.

Differential effects of spinalization on discharge patterns and discharge rates of simultaneously recorded nociceptive and non-nociceptive spinal dorsal horn neurons

Recordings were made simultaneously from 2-5 neurons at the same site in the lumbar spinal dorsal horn of pentobarbital-anesthetized rats. Neurons were classified as low-threshold (LT) or multireceptive (MR) according to their responses to non-noxious mechanical or noxious radiant heat stimuli of the skin. At the same recording sites neurons could be encountered which belong to different classes and/or which had mechanoreceptive fields which did not overlap. Cold blocks of the upper or lower thoracic cord or transsections of the upper cervical cord were made to evaluate the effects of spinalization on both the rate and pattern of background activity and/or noxious heat-evoked responses of different dorsal horn neurons under identical experimental conditions. At 24 of 27 recording sites, spinalization had qualitatively or quantitatively different effects on the rate of background activity of simultaneously recorded neurons. Interspike interval (ISI) means of background activity were significantly reduced in 29 of 65 (44.6%) neurons, prolonged in 23 of 65 (35.4%) neurons, or unchanged in 13 of 65 (20%) neurons. MR neurons displayed a significantly higher incidence of decreased background activity 17 of 45 (37.8%) and a lower incidence of increased background activity (18 of 45, 40%) during spinalization than the LT neurons from which 1 of 12 (8.3%) decreased and 8 of 12 (66.6%) increased background activity. Almost all (95.4%) neurons changed their discharge patterns after spinalization. At 9 of 27 recording sites, the discharge patterns of simultaneously recorded neurons were affected differently by spinalization as revealed by the coefficient of dispersion of the interspike intervals (ISI), indicating changes in the tendency to discharge action potential in clusters (bursts). At the same recording sites the level of noxious heat-evoked responses of simultaneously recorded MR neurons was also differentially affected by spinalization. Nociceptive responses were significantly enhanced in 19 of 37 (51.4%) neurons (137.8 +/- 142.6% of control, mean +/- SD), reduced in 13 of 37 neurons (35.1%) (by 58.9 +/- 20.9%) and/or unchanged in 5 of 37 (13.5%) neurons. It is concluded that no general 'tone' of descending antinociception exists and that tonic descending excitatory and inhibitory systems may be active simultaneously modulating both the level and pattern of neuronal discharges.

Interference with GABA transmission in the rostral ventromedial medulla: disinhibition of off-cells as a central mechanism in nociceptive modulation

Blockade of GABA-mediated synaptic transmission in the rostral ventromedial medulla by local application of GABAA receptor antagonists produces antinociception, indicating that a GABA-mediated inhibition of some population of neurons in this region is normally required if nociceptive information is to be transmitted. The aim of the present study was to elucidate the medullary circuitry mediating this antinociception by recording the activity of putative nociceptive modulating neurons in the rostral ventromedial medulla before and after local infusion of the GABAA receptor antagonist bicuculline methiodide. It was thus possible to correlate changes in the activity of cells of different classes with the ability of the infusion to produce a behaviorally measurable antinociception. One class of medullary neurons, "off-cells," is identified by a pause in firing associated with the occurrence of nocifensor reflexes such as the tail flick evoked by noxious heat. These neurons are uniformly activated following systemic administration of morphine, and are thought to have a net inhibitory effect on nociception. Following local bicuculline administration, off-cells enter a prolonged period of continuous firing that is temporally linked with the period of tail flick inhibition. A second class of neurons, "on-cells," is identified by a burst of activity beginning just before the tail flick, and is directly inhibited by opioids. Unlike off-cells, cells of this class do not show a consistent change in activity associated with inhibition of the tail flick following bicuculline. These data indicate that alterations in the discharges of on-cells would not be able to explain the antinociceptive effect of bicuculline, and therefore point to disinhibition of off-cells as a sufficient basis for antinociception originating within the rostral ventromedial medulla.

Gamma-aminobutyric acid is released in the dorsal horn by electrical spinal cord stimulation: an in vivo microdialysis study in the rat

The mechanism underlying the beneficial effect of electrical stimulation of the posterior surface of the spinal cord in chronic pain states are unknown. The prolonged pain relief following a short stimulation period is believed to imply the activation of long-lasting neurochemical processes, mainly in the spinal cord, but possibly also involving other parts of the central nervous system. Previous studies have demonstrated that substance P and serotonin are released in the cat dorsal horn during spinal cord stimulation (SCS) with electrical parameters similar to those used in the clinic. However, gamma-aminobutyric acid (GABA) has also been hypothesized to play a role in the effect of SCS, but there have been no studies of the possible effects of SCS on GABA release. The authors applied SCS to anesthetized rats and monitored the extracellular concentration of GABA in the lumbar dorsal horns by microdialysis and a sensitive reverse-phase high-performance liquid chromatography technique. After 30 minutes of SCS, the GABA level increased significantly (by almost 270%) in comparison with the basal level recorded before stimulation, from 3.6 +/- 1.0 nmol/L to 13.1 +/- 2.2 nmol/L (mean +/- the standard error of the mean; P < 0.05). The peak release was delayed and appeared in the 30-minute fraction collected after stimulation. Also, perfusion of the dialysis probes with potassium (100 mmol/L) induced an increase of the GABA level. In control experiments without electrical stimulation, slowly decreasing GABA levels were observed throughout the experiments. The present results may suggest an involvement of GABA in the mechanism of SCS-induced pain relief.(ABSTRACT TRUNCATED AT 250 WORDS)

Neuron-specific expression of GABAA-receptor subtypes: differential association of the alpha 1- and alpha 3-subunits with serotonergic and GABAergic neurons

GABAA-receptors in the brain display a striking structural heterogeneity, which is based on a multiplicity of diverse subunits. The allocation of GABAA-receptor subtypes to identified neurons is essential for an analysis of the functional significance of receptor heterogeneity. Among GABA-receptive neurons, well-characterized examples include the serotonergic and GABAergic neurons in the raphe nuclei. The GABAA-receptor subtypes expressed in these two types of neurons were analysed using antisera which recognize selectively the alpha 1- and alpha 3-subunits, and their co-localization with serotonin and glutamate decarboxylase was assessed by confocal laser microscopy in double and triple immunofluorescence staining in the rat. The vast majority of serotonergic neurons express strong alpha 3-subunit-immunoreactivity, but are devoid of alpha 1-subunit staining. In contrast, both the alpha 1- and alpha 3-subunit-immunoreactivities are present in glutamate decarboxylase-positive neurons. Thus, serotonergic and GABAergic neurons selectively express distinct patterns of alpha subunits, suggesting that they possess distinct subtypes of GABAA-receptors. The occurrence of neuron-specific GABAA-receptor subtypes may open new possibilities for the targeting of drugs with selective therapeutic actions.

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.

An animal model for the study of brain transmittor release in response to spinal cord stimulation in the awake, freely moving rat: preliminary results from the periaqueductal grey matter

Electrical spinal cord stimulation (SCS) is an important method in the treatment of certain chronic pain syndromes which are difficult to manage with conventional techniques. The indications for this procedure have gradually narrowed to neuropathic pain states, especially those of peripheral origin, ischaemic pain due to peripheral vascular disease, and treatment-resistant angina pectoris. In spite of the clinical use of this method for more than 20 years, the mechanisms underlying the pain alleviating effect remain largely unknown. For the effect on ischaemic pain, recent animal research indicates a mediation via autonomic pathways. Concerning the effect on neuropathic pain progress in knowledge has been scanty. Data from spinal microdialysis in decerebrated or anaesthetized animals indicate the possible importance of serotonin and substance P in the dorsal horn for pain inhibition by SCS. However, data from experiments on anaesthetized animals are, for several reasons, not likely to truely reflect the mechanisms active in conscious humans under treatment with SCS. To avoid the influence of anaesthesia and to approach the clinical situation, we have developed an animal model enabling simultaneous SCS and supraspinal microdialysis in awake, freely moving rats. The animal model is described and some preliminary data indicating a release of gamma-amino butyric acid (GABA) induced by SCS in the periaqueductal grey matter (PAG), are presented.

GABAergic innervation of serotonergic neurons in the dorsal raphe nucleus of the rat studied by electron microscopy double immunostaining

A double immunocytochemical method combining the preembedding PAP technique and the postembedding immunogold technique was used to examine interactions between GABAergic and serotonergic neurons in the same tissue sections of the dorsal raphe nucleus of the rat. A large number of immunogold stained GABAergic axon terminals were found to be presynaptic to strongly PAP immunostained serotonergic perikarya and dendrites. The types of synapses were mostly symmetrical although a few asymmetrical ones were also found. No axo-axonic synapse between the GABAergic axon terminals and the serotonergic neuronal profiles was found. These results suggest that GABAergic neurons could modulate serotonergic neurons in the dorsal raphe nucleus through synaptic relations.

Interaction between medullary and cervical regulation of renal sympathetic activity

We have reported that electrical or glutamate stimulation of the dorsolateral surface of the cervical spinal cord elicits a 40-60% decrease in renal sympathetic activity (RSA) in anesthetized rats. Because evoked sympatho-inhibition was observed, however, only after transection of the cervical spinal cord at C1, we suggested that unidentified supraspinal neurons affect the regulation of RSA by cervical neurons. In the present experiments, we tested the hypothesis that the modulatory supraspinal neurons are located in the ventrolateral medulla by observing the effects of rostroventral, lateral, medullary (RVLM) injections of the GABAergic agonist, muscimol, on baseline RSA and on our ability to inhibit that activity by cervical stimulation. GABAergic inhibition in the RVLM of chlorolose anesthetized rats elicited changes in RSA that were similar to those observed after transection of the spinal cord, including a 41% decrease in mean arterial pressure and a 44% increase in RSA. Moreover, after muscimol inhibition of RVLM neurons, electrical or glutamate stimulation of the dorsolateral cervical spinal cord elicited a decrease in RSA in otherwise intact rats. These results suggest that neurons in the RVLM interact with neurons in the cervical spinal cord in the regulation of RSA.