Effect of spinal norepinephrine depletion on descending inhibition of the tail flick reflex from the locus coeruleus and lateral reticular nucleus in the rat

Locus coeruleus tyrosine hydroxylase positive neurons mediated the peripheral and central therapeutic effects of transcutaneous auricular vagus nerve stimulation (taVNS) in MRL/lpr mice

OBJECTIVE

This study aims to investigate the effects of transcutaneous auricular vagus nerve stimulation (taVNS) on the development of systemic lupus erythematosus (SLE) in MRL/lpr mice.

METHODS

MRL/lpr mice were treated with taVNS for ten weeks. Locus coeruleus (LC) tyrosine hydroxylase positive (TH+) neurons were selectively lesioned by stereotactic injection of 6-hydroxydopamine (6-OHDA) or selectively activated by chemogenetic methods. Sympathetic denervation was conducted by intraperitoneal injection of 6-OHDA.

RESULTS

TaVNS activated the TH + neurons in LC. TaVNS produced central therapeutic effects by reducing the number of hippocampal microglia, and increasing the number of surviving LC TH+ neurons in MRL/lpr mice. TaVNS also retarded the development of lymphadenectasis and splenomegaly, decreased the proportion of double-negative T (DNT) cells, and alleviated nephritis in MRL/lpr mice. The lesion of LC TH+ neurons eliminated both these central and peripheral therapeutic effects of taVNS, while chemogenetic activation of LC TH+ neurons mimicked most central and peripheral protective effects of taVNS in MRL/lpr mice. Furthermore, taVNS regulated the autonomic nervous system in MRL/lpr mice.

CONCLUSION

This study provides direct evidence that taVNS can retard the development of peripheral and central symptoms of SLE, which is mediated by the LC TH+ neurons.

Electrophysiology as a Tool to Decipher the Network Mechanism of Visceral Pain in Functional Gastrointestinal Disorders

Abdominal pain, including visceral pain, is prevalent in functional gastrointestinal (GI) disorders (FGIDs), affecting the overall quality of a patient's life. Neural circuits in the brain encode, store, and transfer pain information across brain regions. Ascending pain signals actively shape brain dynamics; in turn, the descending system responds to the pain through neuronal inhibition. Pain processing mechanisms in patients are currently mainly studied with neuroimaging techniques; however, these techniques have a relatively poor temporal resolution. A high temporal resolution method is warranted to decode the dynamics of the pain processing mechanisms. Here, we reviewed crucial brain regions that exhibited pain-modulatory effects in an ascending and descending manner. Moreover, we discussed a uniquely well-suited method, namely extracellular electrophysiology, that captures natural language from the brain with high spatiotemporal resolution. This approach allows parallel recording of large populations of neurons in interconnected brain areas and permits the monitoring of neuronal firing patterns and comparative characterization of the brain oscillations. In addition, we discussed the contribution of these oscillations to pain states. In summary, using innovative, state-of-the-art methods, the large-scale recordings of multiple neurons will guide us to better understanding of pain mechanisms in FGIDs.

Distinct pathways for norepinephrine- and opioid-triggered antinociception from the amygdala

BACKGROUND

The amygdala has an important role in pain and pain modulation. We showed previously in animal studies that α2 -adrenoreceptor activation in the central nucleus of the amygdala (CeA) mediates hypoalgesia produced by restraint stress, and that direct application of an α2 -agonist in this region produces analgesia.

AIMS

In the present animal experiments, we investigated the pathways through which α2 -sensitive systems in the CeA produce behavioural analgesia. The CeA has dense connections to a descending pain modulatory network, centred in the midbrain periaqueductal grey (PAG) and the rostral ventromedial medulla (RVM), which is implicated in various forms of stress-related hypoalgesia and which mediates the antinociceptive effect of morphine applied in the basolateral amygdala. We investigated whether this circuit mediates the hypoalgesic effects of α2 -adrenergic agonist administration into the CeA as well as the contribution of endogenous opioids and cannabinoids. We also tested the possibility that activation of α2 -receptors in the CeA produces antinociception by recruitment of noradrenergic pathways projecting to the spinal cord.

RESULTS

Hypoalgesia resulting from bilateral application of the α2 -adrenergic agonist clonidine in the CeA was not reversed by chemical inactivation of the RVM or by systemic injections of naloxone (μ-opioid antagonist) or rimonabant (CB1 antagonist). By contrast, spinal α2 -receptor blockade (intrathecal idazoxan) completely prevented the hypoalgesic effect of clonidine in the CeA, and unmasked a small but significant hyperalgesia.

CONCLUSION

In rats, adrenergic actions in the CeA mediating hypoalgesia require spinal adrenergic neurotransmission but not the PAG-RVM pain modulatory network, or opiate or cannabinoid systems.

Depletion of endogenous noradrenaline does not prevent spinal cord plasticity following peripheral nerve injury

The present study examined the role of endogenous noradrenaline on glial and neuronal plasticity in the spinal cord in rats after peripheral nerve injury. An intrathecal injection of dopamine-β-hydroxylase antibody conjugated to saporin (DβH-saporin) completely depleted noradrenergic axons in the spinal cord and also reduced noradrenergic neurons in the locus coeruleus (A6) and A5 noradrenergic nucleus in the brainstem and noradrenergic axons in the paraventricular nucleus of the hypothalamus. DβH-saporin treatment itself did not alter mechanical withdrawal threshold, but enhanced mechanical hypersensitivity and intrathecal clonidine analgesia after L5-L6 spinal nerve ligation. In the spinal dorsal horn of spinal nerve ligation rats, DβH-saporin treatment increased choline acetyltransferase immunoreactivity as well as immunoreactivity in microglia of ionized calcium binding adaptor molecule 1[IBA1] and in astrocytes of glial fibrillary acidic protein, and brain-derived nerve growth factor content. DβH-saporin treatment did not, however, alter the fractional release of acetylcholine from terminals by dexmedetomidine after nerve injury. These results suggest that endogenous tone of noradrenergic fibers is not necessary for the plasticity of α2-adrenoceptor analgesia and glial activation after nerve injury, but might play an inhibitory role on glial activation.

PERSPECTIVE

This study demonstrates that endogenous noradrenaline modulates plasticity of glia and cholinergic neurons in the spinal cord after peripheral nerve injury and hence influences the pathophysiology of spinal cord changes associated with neuropathic pain.

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

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

Noradrenergic pain modulation

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

Activation of μ-opioid receptors excites a population of locus coeruleus-spinal neurons through presynaptic disinhibition

The nucleus locus coeruleus (LC) plays an important role in analgesia produced by opioids and by modulation of the descending noradrenergic pathway. The functional role of micro-opioid receptors (muOR) in regulation of the excitability of spinally projecting LC neurons has not been investigated. In the present study, we tested the hypothesis that activation of presynaptic mu-opioid receptors excites a population of spinally projecting LC neurons through attenuation of gamma-aminobutyric acid (GABA)-ergic synaptic inputs. Spinally projecting LC neurons were retrogradely labeled by a fluorescent dye injected into the spinal dorsal horn of rats. Whole-cell current- and voltage-clamp recordings were performed on labeled LC neurons in brain slices. All labeled LC noradrenergic neurons were demonstrated by dopamine-beta-hydroxylase (DbetaH) immunofluorescence. In 37 labeled LC neurons, (D-Ala(2),N-Me-Phe(4),Gly-ol(5))-enkephalin (DAMGO) significantly increased the discharge activity of 17 (45.9%) neurons, but significantly inhibited the firing activity of another 15 (40.5%) cells. The excitatory effect of DAMGO on seven labeled LC neurons was diminished in the presence of bicuculline. DAMGO significantly decreased the frequency of GABA-mediated miniature inhibitory postsynaptic currents (mIPSCs) in all nine labeled LC neurons. However, DAMGO had no effect on glutamate-mediated miniature excitatory postsynaptic currents (mEPSCs) in 12 of 15 neurons. Furthermore, DAMGO significantly inhibited the peak amplitude of evoked inhibitory postsynaptic currents (eIPSCs) in all 11 labeled neurons, but had no significant effect on the evoked excitatory postsynaptic currents (eEPSCs) in 10 of these 11 neurons. Thus, data from this study suggest that activation of micro-opioid receptors excites a population of spinally projecting LC neurons by preferential inhibition of GABAergic synaptic inputs. These findings provide important new information about the descending noradrenergic modulation and analgesic mechanisms of opioids.

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.

Regional distribution of alpha(2C)-adrenoceptors in brain and spinal cord of control mice and transgenic mice overexpressing the alpha(2C)-subtype: an autoradiographic study with [(3)H]RX821002 and [(3)H]rauwolscine

Behavioral studies on gene-manipulated mice have started to elucidate the neurobiological functions of the alpha(2C)-adrenoceptor (AR) subtype. In this study, we applied quantitative receptor autoradiography to investigate the potential anatomical correlates of the observed functional effects of altered alpha(2C)-AR expression. Labeling of brain and spinal cord sections with the subtype non-selective alpha(2)-AR radioligand [(3)H]RX821002 and the alpha(2C)-AR-preferring ligand [(3)H]rauwolscine revealed distinct binding-site distribution patterns. In control mice, [(3)H]rauwolscine binding was most abundant in the olfactory tubercle, accumbens and caudate putamen nuclei, and in the CA1 field of the hippocampus. A mouse strain with overexpression of alpha(2C)-AR regulated by a gene-specific promoter showed approximately two- to four-fold increased levels of [(3)H]rauwolscine binding in these regions. In addition, dramatic increases in [(3)H]rauwolscine binding were seen in the nerve layer of the olfactory bulb, the molecular layer of the cerebellum, and the ventricular system of alpha(2C)-AR-overexpressing mice, representing "ectopic" alpha(2C)-AR expression. Competition-binding experiments with several alpha(2)-AR ligands confirmed the alpha(2C)-AR identity of these sites. Our results provide quantitative evidence of the predominance of the alpha(2A)-AR subtype in most regions of the mouse CNS, but also disclose the wide distribution of alpha(2C)-AR in the normal mouse brain, although at relatively low density, except in the ventral and dorsal striatum and the hippocampal CA1 area. alpha(2C)-AR are thus present in brain regions involved in the processing of sensory information and in the control of motor and emotion-related activities such as the accumbens and caudate putamen nuclei, the olfactory tubercle, the lateral septum, the hippocampus, the amygdala, and the frontal and somatosensory cortices. The current results may help in specifying an anatomical framework for the functional roles of the alpha(2A)- and alpha(2C)-AR subtypes in the mouse CNS.

Immunocytochemical localization of the alpha3, alpha4, alpha5, alpha7, beta2, beta3 and beta4 nicotinic acetylcholine receptor subunits in the locus coeruleus of the rat

The presence of nicotinic acetylcholine receptors (nAChRs) within the locus coeruleus (LC) has been examined using a wide range of techniques. However, the expression pattern of individual nicotinic receptor subunits has not been described. Using immunocytochemistry, we demonstrate the distribution of the alpha3, alpha4, alpha5, alpha7, beta2, beta3 and beta4 nAChR subunits within the LC. Most nAChR subunits were expressed on neuronal perikarya within the LC nucleus. The alpha3, alpha4, alpha7 and beta3 immunoreactive neurons were evenly distributed in the dorsal and ventral LC whereas the alpha5, beta2 and beta4 nAChR subunits were preferentially confined to the upper dorsal section. In addition to neuronal perikarya, alpha4, alpha5 and beta2 immunoreactive fibers were observed. With the exception of the alpha3 subunit, punctate labeling was observed within and immediately surrounding the LC. These data are consistent with the presence of multiple nAChRs within the LC and extend these findings to show the distribution pattern of each nAChR subunit throughout the LC nucleus.

Trigeminal nociception-induced cerebral Fos expression in the conscious rat

Little is known about trigeminal nociception-induced cerebral activity and involvement of cerebral structures in pathogenesis of trigeminovascular headaches such as migraine. Neuroimaging has demonstrated cortical, hypothalamic and brainstem activation during the attack and after abolition with sumatriptan. This has led to the conclusion that the dorsal raphe and locus coeruleus may initiate events that generate migraneous headache. Using a conscious rat model of trigeminal nociception and cerebral Fos expression as histochemical markers of neuronal activity, we characterized the pattern of brain activity after noxious trigeminal stimulation with capsaicin (250 and 1000 nm). A significantly increased Fos immunoreactivity was found in the trigeminal nucleus caudalis (layers I and II), the area postrema, the nucleus of the solitary tract, the parvicellular reticular nucleus, the locus coeruleus, the parabrachial nucleus and the raphe nuclei. In addition, the ventrolateral periaqueductal grey, the intralaminar thalamic and various hypothalamic areas, showed an enhanced Fos expression after the intracisternal administration of capsaicin. Other responding areas were the amygdala, the upper lip and forelimb regions of the primary somatosensory cortex, and the insula. Many of these areas participate in (anti)-nociception, although we cannot exclude the possibility that in conscious animals the pain-associated physiological and behavioural responses that are an intrinsic and necessary part of coping with pain have generated the increased Fos expression. Trigeminal stimulation-induced locus coeruleus, dorsal raphe and hypothalamic activation are opposed to a suggested pathogenic role of these nuclei in migraine and cluster headache, respectively.

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.

Fluorescent double-label study of lateral reticular nucleus projections to the spinal cord and periaqueductal gray in the rat

Following injections of WGA-HRP into either the spinal cord or periaqueductal gray, labeled neurons were observed bilaterally along the periphery of the lateral reticular nucleus (LRN) magnocellular division. The possibility that some of these neurons in the LRN provide collateral axonal branches to both the periaqueductal gray and the spinal cord was investigated in rats using a retrograde double-labeling method employing two different fluorescent tracers, True Blue and Nuclear Yellow. Following sequential injection of the two fluorescent axonal tracers into the spinal cord and periaqueductal gray in the same animal, a modest number of double-labeled neurons were observed bilaterally near the medial and dorsal perimeter of the magnocellular division of the LRN. The labeled neurons were distinctly multipolar in shape and measured approximately 15-18 mu in their greatest transverse diameter. No double-labeled neurons were observed in the parvocellular or subtrigeminal divisions of the LRN. Based upon these observations, it is suggested that collaterals of the LRN-spinal pathway provide feedback information to the periaqueductal gray that might then be used to modulate the participation of the latter cell group in a variety of pain processing and cardiovascular regulatory functions.

The organization and function of endogenous antinociceptive systems

Much progress has been made the understanding of endogenous pain-controlling systems. Recently, new concepts and ideas which are derived from neurobiology, chaos research and from research on learning and memory have been introduced into pain research and shed further light on the organization and function of endogenous antinociception. These most recent developments will be reviewed here. Three principles of endogenous antinociception have been identified, as follows. (1) Supraspinal descending inhibition: the patterns of neuronal activity in diencephalon, brainstem and spinal cord during antinociceptive stimulation in midbrain periaqueductal gray (PAG) or medullary nucleus raphe magnus have now been mapped on the cellular level, using the c-Fos technique. Results demonstrate that characteristic activity patterns result within and outside the PAG when stimulating at its various subdivisions. The descending systems may not only depress mean discharge rates of nociceptive spinal dorsal horn neurons, but also may modify harmonic oscillations and nonlinear dynamics (dimensionality) of discharges. (2) Propriospinal, heterosegmental inhibition: antinociceptive, heterosegmental interneurons exist which may be activated by noxious stimulation or by supraspinal descending pathways. (3) Segmental spinal inhibition: a robust long-term depression of primary afferent neurotransmission in A delta fibers has been identified in superficial spinal dorsal horn which may underlie long-lasting antinociception by afferent stimulation, e.g. by physical therapy or acupuncture.

The role of alpha 2-adrenoceptors of the medullary lateral reticular nucleus in spinal antinociception in rats

We attempted to find out the role of alpha 2-adrenoceptors of the medullary lateral reticular nucleus (LRN) in antinociception in rats. Spinal antinociception was evaluated using the tail-flick test, and supraspinal antinociception using the hotplate test. Antinociceptive effects were determined following local electric stimulation of the LRN, and following microinjections of medetomidine (an alpha 2-adrenoceptor agonist; 1-10 micrograms), atipamezole (an alpha 2-adrenoceptor antagonist; 20 micrograms) or lidocaine (4%) into the LRN. The experiments were performed using intact and spinalized Hannover-Wistar rats with a unilateral chronic guide cannula. Electric stimulation of the LRN as well as of the periaqueductal gray produced a significant spinal antinociceptive effect in intact rats. Medetomidine (1-10 micrograms), when microinjected into the LRN, produced no significant antinociceptive effect in the tail-flick test in intact rats. However, following spinalization, medetomidine in the LRN (10 micrograms) produced a significant atipamezole-reversible antinociceptive effect in the tail-flick test. In the hot-plate test, medetomidine (10 micrograms) in the LRN produced a significant atipamezole-reversible increase of the paw-lick latency in intact rats. Microinjection of atipamezole (20 micrograms) or lidocaine alone into the LRN produced no significant effects in the tail-flick test. The results are in line with the previous evidence indicating that the LRN and the adjacent ventrolateral medulla is involved in descending inhibition of spinal nocifensive responses. However, alpha 2-adrenoceptors in the LRN do not mediate spinal antinociception but, on the contrary, their activation counteracts antinociception at the spinal cord level.(ABSTRACT TRUNCATED AT 250 WORDS)

Spinal antinociception mediated by a cocaine-sensitive dopaminergic supraspinal mechanism

The role of dopaminergic descending supraspinal processes in mediating the antinociceptive action of cocaine was studied in the rat using a combination of extracellular neuronal recording and behavioral techniques. Neurons in the superficial laminae (I-II) of the spinal dorsal horn with receptive fields on the tail were recorded in anesthetized rats using insulated metal microelectrodes. Stimulation of the receptive field with either high intensity transcutaneous electrical pulses or with an infrared CO2 laser beam produced a biphasic increase in dorsal horn unit discharge. Conduction velocity estimates indicated that the early discharge corresponded to activity in A delta whereas the late response corresponded to activity in C afferent fibers. Cumulative doses of cocaine (0.1-3.1 mg/kg i.v.) inhibited the late response to either electrical or laser stimulation in a dose-related manner. The early response to laser, but not electrical, stimulation was also suppressed by cocaine. Neurons in the spinal dorsal horn with receptive fields on the ipsilateral hindpaw were activated by natural noxious (pinch) or innocuous (tap) somatic stimulation. Cocaine selectively suppressed nociceptively evoked dorsal horn unit discharge. This antinociceptive effect was dose-related (0.3-3.1 mg/kg, i.v.) and antagonized by eticlopride (0.05-0.1 mg/kg, i.v.), a selective D2 dopamine receptor blocker. The same doses of cocaine failed to inhibit the responses of dorsal horn neurons to low threshold innocuous stimulation. Complete thoracic spinal cord transection eliminated the antinociceptive effect of cocaine on dorsal horn neurons and also eliminated the cocaine-induced attenuation of the tail-flick reflex.(ABSTRACT TRUNCATED AT 250 WORDS)

Intrathecal 5-methoxy-N,N-dimethyltryptamine in mice modulates 5-HT1 and 5-HT3 receptors

The antinociceptive effects of intrathecally administered 5-methoxy-N,N-dimethyltryptamine (5-MeO-DMT), a potent 5-HT receptor agonist, were studied in three behavioral tests in mice: the tail-flick test and the intrathecal substance P and N-methyl-D-aspartic acid (NMDA) assays. Intrathecal administration of 5-MeO-DMT (4.6-92 nmol/mouse) produced a significant prolongation of the tail-flick latency. This action was blocked by 5-HT3 and gamma-aminobutyric acidA (GABAA) receptor antagonists but not by 5-HT2, 5-HT1A, 5-HT1B or 5-HT1S receptor antagonists. Binding studies indicated that 5-MeO-DMT had very low affinity for 5-HT3 receptors. 5-MeO-DMT inhibited biting behavior while increasing scratching behavior induced by intrathecally administered substance P. The inhibition of biting behavior was antagonized by intrathecal co-administration of 5-HT1B and GABAA receptor antagonists while 5-HT1A, 5-HT1S, 5-HT2 and 5-HT3 receptor antagonists had no effect. 5-MeO-DMT-enhanced scratching behavior was inhibited by all the antagonists used except ketanserin and bicuculline, suggesting the involvement of 5-HT1A, 5-HT1B, 5-HT1S, 5-HT3 and GABAA receptors. NMDA-induced biting behavior was inhibited by 5-MeO-DMT pretreatment; this action was antagonized by 5-HT1B, 5-HT3 and GABAA receptor antagonists. The involvement of these receptors in 5-MeO-DMT action suggests that it may promote release of 5-HT (5-hydroxytryptamine, serotonin).

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)

The function of noradrenergic neurons in mediating antinociception induced by electrical stimulation of the locus coeruleus in two different sources of Sprague-Dawley rats

Although noradrenergic neurons in the nucleus locus coeruleus are known to project to the spinal cord, these neurons appear to innervate different regions of the spinal cord in Sprague-Dawley rats obtained from two different vendors. Recent anatomical studies demonstrated that the noradrenergic neurons in the locus coeruleus in Sasco Sprague-Dawley rats primarily innervate the ventral horn, whereas Harlan Sprague-Dawley rats have coeruleospinal projections that terminate in the dorsal horn of the spinal cord. This report describes the results of behavioral experiments that were designed to determine the functional significance of these anatomical differences. Electrical stimulation of neurons in the locus coeruleus produced antinociception in both Harlan and Sasco rats. The antinociception in Harlan rats was readily reversed by intrathecal injection of yohimbine, a selective alpha 2-adrenoceptor antagonist, or by phentolamine, a non-selective alpha 2-adrenoceptor antagonist. In contrast, these antagonists did not alter the antinociception produced by locus coeruleus stimulation in Sasco rats. Finally, the alpha 2-antagonist, idazoxan, did not alter the antinociceptive effect of locus coeruleus stimulation in either group of rats. These observations indicate that coeruleospinal noradrenergic neurons in Harlan and Sasco Sprague-Dawley rats have different physiological functions. Thus, electrical stimulation of noradrenergic neurons in the locus coeruleus that innervate the spinal cord dorsal horn (Harlan rats) produces antinociception, but stimulation of coeruleospinal noradrenergic neurons that project to the ventral horn (Sasco rats) does not produce antinociception. It is likely that genetic differences between these outbred stocks of rats account for the fundamental differences in the projections of coeruleospinal neurons and their function in controlling nociception.

Synergistic brainstem interactions for morphine analgesia

Morphine is a potent analgesic when microinjected into the periaqueductal gray (PAG), the rostral ventral medulla (RVM) which contains the nuclei raphe magnus and reticularis gigantocellularis and the dorsolateral pons (DLP) which includes the locus coeruleus. Coadministration of low morphine doses which are inactive alone into combinations of these three regions elicits dramatic analgesic responses, implying the existence of synergy. The most effective combination is the PAG/RVM, whereas the PAG/DLP and RVM/DLP combinations are much less efficacious. In addition to fixed combinations, inclusion of a low morphine dose in one region shifts the analgesic dose-response curves in the others. The marked synergy between the PAG and the RVM is sensitive to naloxonazine, implying a role for mu 1 receptors. Thus, these studies indicate the presence of intrinsic brainstem mu 1 receptor systems with synergistic interactions which can be pharmacologically distinguished from the brainstem mu 2 receptors mediating supraspinal/spinal synergy.

6-Hydroxydopamine treatment of neonatal rats. I. Effects on the development of the spinal cord noradrenergic system

The spinal cord contains noradrenergic (NA) pathways which descend from cell bodies in the medulla oblongata and pons to terminate at all levels in the spinal gray matter. The present studies sought to determine the patterns of postnatal development of pre- and postsynaptic elements of NA transmission in the spinal cord. Significant presynaptic development is evident at birth as reflected by substantial high-affinity uptake of norepinephrine (NE) into synaptosomes (0.65-0.90 pmol/mg protein). There is a subsequent increase in uptake on postnatal day (PND) 5, followed by a decrease in 5-10 days to essentially adult levels, starting on PND 20 (0.30-0.35 pmol/mg protein). This decrease in NE uptake occurs coincident with increases in the density of postsynaptic alpha 1 and beta adrenergic receptors and also NE-stimulated accumulation of 3',5'-cyclic adenosine monophosphate (cAMP). Peaks in the development of alpha 1 receptors (PND 10) and beta receptors (PND 20) and NE-stimulated cAMP accumulation (PND 15) were also followed by decreases to adult levels. The neurotoxin 6-hydroxydopamine (6-OHDA) was administered at birth to determine the effects of denervation on the development of the spinal NA systems. At each day following 6-OHDA, synaptosomal uptake of [3H]NE was reduced by two-thirds compared with control values. alpha 1 and beta adrenergic receptor binding are uniformly increased along with a parallel increase in NE-stimulated accumulation of cAMP. While uniformly increased over control, the pattern of postnatal increases and decreases in receptors and cAMP accumulation is maintained.(ABSTRACT TRUNCATED AT 250 WORDS)

The effects of iontophoretic clonidine on neurones in the rat superficial dorsal horn

Clonidine and glutamate were applied by iontophoresis to cells in the superficial 3 laminae of the spinal cord in the anaesthetised rat. Only cells that were excited by glutamate (up to 150 nA) were studied. Some spontaneously active cells could be excited by clonidine (up to 100 nA). However, when applied to non-spontaneous cells, clonidine had no effect at any dose level. When ejected in a cyclic pattern alternating with glutamate ejection, clonidine powerfully amplified the response of many cells to the glutamate stimulus. This effect was seen only on cells with small-amplitude spikes and low-threshold (LT) receptive fields. The amplification was often sustained and could outlast the clonidine ejection by several minutes. Clonidine had a long-lasting inhibitory effect on the responses to glutamate of cells with high-threshold (HT) or wide-dynamic-range (WDR) receptive fields. Clonidine appeared to selectively decrease the responsiveness of WDR cells to noxious stimulation. It is suggested that an amplification of the response of LT cells to other excitatory inputs could contribute to the analgesic action of clonidine.

Selective blockade by yohimbine of descending spinal inhibition from lateral reticular nucleus but not from locus coeruleus in rats

The present study was undertaken to compare the effects of the alpha 2-adrenoceptor antagonist yohimbine on inhibition of C-fiber-evoked responses of dorsal horn neurons produced by electrical stimulation of the lateral reticular nucleus (LRN) and the Locus coeruleus (LC) in the rat. In the majority of neurons, C-fiber-evoked responses were significantly inhibited by 53.84 +/- 5.02% and 57.63 +/- 5.79% of control by LRN and LC stimulation, respectively, whereas in less than half of the neurons, A-fiber-evoked responses were reduced by 20.99 +/- 6.06% and 21.78 +/- 4.48% of control, respectively. After systemic or spinal administration of yohimbine, LC-induced inhibition of C-fiber-evoked responses was not affected. In contrast, LRN-induced inhibition was markedly attenuated by yohimbine. The results suggest that alpha 2-adrenoceptors may be involved in mediation of inhibition of spinal nociception induced by stimulation of LRN but not by LC.

Desipramine potentiates spinal antinociception by 5-hydroxytryptamine, morphine and adenosine

The effects of pretreatment with desipramine, a selective noradrenaline (NA) uptake blocker, on spinal antinociception by 5-hydroxytryptamine (5-HT), morphine and an adenosine analog (NECA) in the rat hot-plate test were examined to determine if endogenous NA is involved in the spinal action of these agents. Desipramine, 25 mg/kg, had no significant intrinsic effect in the hot-plate test but potentiated spinal antinociception by NA and 5-HT. Potentiation was more prominent at higher doses of NA and 5-HT. Desipramine also enhanced the action of morphine and NECA, but, in these instances, the greatest enhancement occurred at lower doses. These results, in conjunction with others, suggest that 5-HT releases NA from the spinal cord while morphine and NECA interact synergistically with endogenously released NA.

8-Phenyltheophylline reverses the antinociceptive action of morphine in the periaqueductal gray

Morphine was injected into the periaqueductal gray region of the rat and 8-phenyltheophylline, an adenosine receptor antagonist, was injected intrathecally 15 or 30 min later, to determine whether supraspinally-administered morphine activated descending mechanisms to release adenosine (or a nucleotide which is metabolized to adenosine) from the spinal cord. 8-Phenyltheophylline (10 micrograms) reversed the antinociceptive action of morphine in the hot plate but not the tail-flick test. A combination of methysergide/phentolamine (15 micrograms each) reversed the action of morphine in both tests. 8-Phenyltheophylline retained the ability to reverse the action of morphine in the hot plate test in rats pretreated with 6-hydroxydopamine (to induce degeneration of descending noradrenergic pathways) but reversal was no longer observed in rats pretreated with 5,7-dihydroxytryptamine (after pretreatment with desipramine, to induce degeneration of descending serotonergic pathways). These results indicate that a component of the supraspinal antinociceptive action of morphine is due to release of adenosine or nucleotide, within the spinal cord and this release is dependent on intact serotonergic pathways.

The projection of noradrenergic neurons in the A7 catecholamine cell group to the spinal cord in the rat demonstrated by anterograde tracing combined with immunocytochemistry

Noradrenergic neurons located in the A5, A7 and locus coeruleus/subcoeruleus (LC/SC) catecholamine cell groups innervate all levels of the spinal cord. However, the specific spinal cord terminations of these neurons have not been clearly delineated. This study determined the spinal cord terminations of the A7 catecholamine cell group using the anterograde tracer Phaseolus vulgaris leucoagglutinin (PHA-L) in combination with dopamine-beta-hydroxylase (DBH) immunocytochemistry. In addition, the spinal cord projections of A7 neurons were examined by measuring the reduction in the density of DBH-immunoreactive axons in specific regions of the spinal cord after a unilateral electrolytic lesion of the A7 cell group. The results of these experiments indicate that noradrenergic neurons in the A7 cell group project primarily in the ipsilateral dorsolateral funiculus and terminate most heavily in the dorsal horn (laminae I-IV).

Projections from the periaqueductal gray to the rostromedial pericoerulear region and nucleus locus coeruleus: anatomic and physiologic studies

Previous studies showed that the nucleus locus coeruleus (LC) receives two major afferent inputs from 1) nucleus paragigantocellularis and 2) nucleus prepositus hypoglossi, both in the rostral medulla. Recent reports suggested that the midbrain periaqueductal gray (PAG) projects to the rostromedial pericoerulear area and LC. Since the PAG is a major site for control of central antinociception, and since descending noradrenergic fibers have been implicated in pain modulation, we have investigated in detail the functional anatomy of projections from PAG to the dorsolateral pontine tegmentum. A combined anatomical and electrophysiological approach was used to assess the organization and synaptic influence of PAG on neurons in the rostromedial pericoerulear region and in LC proper. Injections of the tracer wheatgerm agglutinin conjugated to horseradish peroxidase encompassing LC proper and the rostromedial pericoerulear area retrogradely labeled neurons in PAG located lateral and ventrolateral to the cerebral aqueduct; injections restricted to LC proper did not consistently label PAG neurons. Deposits of the anterograde axonal tracer Phaseolus vulgaris leucoagglutinin into this same region of PAG labeled axons that robustly innervated the zone rostral and medial to LC. Only sparse fibers were observed in LC proper. Consistent with these results, focal electrical stimulation of LC antidromically activated only a few PAG neurons (6 of 100); all of these driven cells were located lateral and ventrolateral to the cerebral aqueduct. The majority of neurons in the rostromedial pericoerulear area were robustly activated by single pulse stimulation of PAG. In contrast, single pulse electrical stimulation of lateral PAG produced weak to moderate synaptic activation of some LC neurons; stimulation of ventrolateral PAG produced predominant inhibition of LC discharge, perhaps through recurrent collaterals subsequent to antidromic activation of neighboring LC cells. Taken together, these results indicate that PAG strongly innervates the region rostral and medial to LC, including Barrington's nucleus, but only weakly innervates LC proper. Although recent studies indicate that the dendrites of LC neurons ramify heavily and selectively in the rostromedial pericoerulear region, the results of the present physiological studies suggest that PAG preferentially targets rostromedial pericoerulear neurons rather than LC dendrites.

Projections of neurons in the ventromedial medulla to pontine catecholamine cell groups involved in the modulation of nociception

Stimulation of neurons in the nucleus raphe magnus (RMg) or the adjacent gigantocellular nucleus pars alpha (Gi alpha) and paragigantocellular nucleus (PGi) produces antinociception which is partially mediated by bulbospinal noradrenergic neurons. Since no norepinephrine-containing neurons are located in either the RMg or the Gi alpha/PGi, it is likely that neurons located in these nuclei have axonal connections with the spinally-projecting catecholamine neurons located in the A5, A6 (locus coeruleus), or A7 catecholamine cell groups. To provide evidence for such connections, the anterograde tracer, Phaseolus vulgaris-leucoagglutinin (PHA-L), was injected into the RMg or Gi alpha/PGi and labeled axons were identified near catecholamine-containing neurons labeled with dopamine-beta-hydroxylase-immunoreactivity (D beta H-ir). A dense field of PHA-L-positive terminals was seen within the A7 cell group which was mainly ipsilateral to PHA-L injections made into either the RMg or the Gi alpha/PGi. Many PHA-L-positive terminals were closely apposed to D beta H-ir A7 perikarya or proximal dendrites. A modest number of terminals was seen within the A5 and LC cell groups. In the second experiment, a unilateral injection of the retrograde tracer, Fluoro-Gold, was made into the A7 cell group and brainstem sections were processed for serotonin (5-HT) immunocytochemistry. Many neurons retrogradely labeled with Fluoro-Gold were seen in the RMg, but a much larger number were found in the Gi alpha/PGi. Less than 5% of these Fluoro-Gold-labeled cells contained 5-HT-immunoreactivity. The results of these experiments indicate that the RMg and Gi alpha/PGi have a substantial population of non-serotonergic neurons which project to the A7 noradrenergic cell group.

The projections of locus coeruleus neurons to the spinal cord

Spinally projecting noradrenergic neurons located in the locus coeruleus/subcoeruleus (LC/SC) are a major source of the noradrenergic innervation of the spinal cord. However, the specific terminations of these neurons have not been clearly defined. The purpose of this chapter is to describe the results of experiments that used the anterograde tracer Phaseolus vulgaris leucoagglutinin in combination with dopamine-beta-hydroxylase immunocytochemistry to more precisely determine the spinal cord terminations of neurons located in the LC/SC. The results of these experiments indicate that the axons of LC neurons are located primarily in the ipsilateral ventral funiculus and terminate most heavily in the medial part of laminae VII and VIII, the motoneuron pool of lamina IX, and lamina X. LC neurons provide a moderately dense innervation of the ventral part of the dorsal horn, but only a very sparse innervation of the superficial dorsal horn. The SC projects ipsilaterally in the ventrolateral funiculus and terminates diffusely in the intermediate and ventral laminae of the spinal cord. Finally, the results of preliminary experiments indicate that different rat substrains may have LC neurons that exhibit qualitatively different termination patterns in the spinal cord. More specifically, LC neurons in some rat substrains innervate the dorsal horn, while those in other substrains primarily innervate the ventral horn and intermediate zone.

Imipramine-fentanyl antinociception in a rabbit tooth pulp model

Antinociception of imipramine (I) and its effect in combination with fentanyl (F) was evaluated in rabbits using electrically-induced lick chew responses via tooth pulp stimulation as the model of nociception. Acute i.v. injections of I elicited a graded dose response comparable to i.v. morphine (M) with I ED 50 = 4.35 mg/kg (2.31-8.14, 95% CL) and M ED 50 = 1.81 mg/kg (1.11-3.90), with no differences in the slopes between the two curves. The lethal dose of I was 10 mg/kg. An i.v. dose of I twice the ED 50 elicited an antinociceptive effect of more than 50% maximum possible effect (MPE) for 90 minutes with peak effect of 82% MPE occurring at 15 minutes. These effects of I were not reversed by a morphine-reversal dose of naloxone (0.1 mg/kg i.v.) but were reversed with a ten fold dose of naloxone. F ED 50 values (mcg/kg) were lowered from 11.35 to 2.70, 0.74 and 0.33 with increasing pretreatment doses of I (1.0, 2.1 and 3.2 mg/kg). These magnitudes of potency increases of F were 4.2, 15.3 and 34.4 fold respectively. A single i.v. ED 50 dose of I extended the time to 50% MPE of an ED 90 dose of F from 26 minutes to 77 minutes; of a 2 X ED 50 dose of F from 17 minutes to 28 minutes. Data points for three different combinations of I and F fell significantly within the synergistic field of an ED 50 isobologram and a polynomial equation described the curve best fitting the data points. F alone (i.v. ED 50 dose) increased the PaCO2 values to 74% above controls and three different combinations with I showed no increases in PaCO2 values above controls. I alone did not significantly cause any change in PaCO2 values from controls.

Descending noradrenergic influences on pain

Multiple separate and distinct supraspinally organized descending inhibitory systems have been identified which are capable of powerfully modulating spinal nociceptive transmission. Until recently, brainstem sites known to be involved in the centrifugal modulation of spinal nociceptive transmission were few in number, being limited to midline structures in the midbrain and medulla (e.g., periaqueductal gray and nucleus raphe magnus). However, with continued investigation, that number has increased and brainstem sites previously thought to be primarily involved in cardiovascular function and autonomic regulation (e.g., nucleus tractus solitarius; locus coeruleus/subcoeruleus (LC/SC); A5 cell group; lateral reticular nucleus) also have been demonstrated to play a role in the modulation of spinal nociceptive transmission. Spinal monoamines (norepinephrine (NE) and serotonin) have been shown to mediate stimulation-produced descending inhibition of nociceptive transmission from these brainstem sites. The majority of NE-containing fibers and terminations in the spinal cord arise from supraspinal sources; thus, the LC/SC, the parabrachial nuclei, the Kölliker-Fuse nucleus and the A5 cell group have all been suggested as possible sources of the spinal noradrenergic (NA) innervation involved in the centrifugal modulation of spinal nociceptive transmission. Several lines of evidence suggest that the LC/SC plays a significant role in a functionally important descending inhibitory NA system. Focal electrical stimulation in the LC produces an antinociception and increases significantly the spinal content of NA metabolites. The inhibition of the nociceptive tail-flick withdrawal reflex produced by electrical stimulation in the LC/SC has been demonstrated to be mediated by postsynaptic alpha 2-adrenoceptors in the lumbar spinal cord. Similarly, electrical or chemical stimulation given in the LC/SC inhibits noxious-evoked dorsal horn neuronal activity. Thus, results reported in electrophysiological experiments confirm those reported in functional studies and the NA coeruleospinal system appears to play a significant role in spinal nociceptive processing.

Antagonism of stimulation-produced antinociception from ventrolateral pontine sites by intrathecal administration of alpha-adrenergic antagonists and naloxone

Focal electrical stimulation of the ventrolateral pontine tegmentum in conscious rats induced antinociception in approximately one-half of the animals screened, as indicated by a marked suppression of the thermally evoked tail-flick flexion reflex. The effectiveness of ventrolateral pontine stimulation in elevating tail-flick latency was significantly reduced by intrathecal microinjection of 30 micrograms of the non-selective alpha-adrenergic antagonist phentolamine, and was largely abolished by a 60-micrograms dose of this drug. The blockade of ventrolateral pontine stimulation-produced antinociception by phentolamine was maximal by 15 min postinjection, and was still evident 60 min after drug microinjection. Ventrolateral pontine stimulation-produced antinociception was also attenuated by intrathecal administration of the alpha 2-selective antagonist yohimbine (37 micrograms) and the opioid antagonist naloxone (30 micrograms), but not the alpha 1 antagonist WB-4101 (37 micrograms), the beta-adrenergic antagonist propranolol (111.6 micrograms) nor the serotonergic antagonist methysergide (30 micrograms). However, the antagonism of pontine stimulation-produced antinociception by naloxone was unlike that of phentolamine and yohimbine, in that it developed slowly and was only evident at 60 min postinjection. Hence naloxone's site of action may be distant from the injection site. These data indicate that the thermal antinociception produced by stimulation of the ventrolateral pons is mediated through spinal alpha 2-receptors and opioid receptors of uncertain location. The close proximity of many of the effective electrode placements to the rostral A5 and ventral subcoerulear A7 noradrenergic cell groups suggests that noradrenergic spinopetal projections arising from these groups are involved in mediating the antinociception induced by stimulating these sites.

Direct catecholaminergic innervation of primate spinothalamic tract neurons

Catecholaminergic axonal varicosities identified by immunocytochemical staining for dopamine-beta-hydroxylase were observed at the light microscopic level apposing the somata of retrogradely labeled spinothalamic tract neurons in the monkey spinal cord. Three retrogradely labeled and two intracellularly labeled spinothalamic neurons were serially sectioned and examined at selected intervals at the electron microscopic level. Electron microscopic study revealed that axonal boutons directly contacted the somata and/or dendrites of lamina I, IV, and V spinothalamic tract neurons. All of the profiles apposing one of the retrogradely labeled lamina I spinothalamic tract neurons were categorized from eight planes of section spaced at 1-micron intervals. Of the 305 profiles counted that were adjacent to this soma, 17 (5.6%) stained positively for dopamine-beta-hydroxylase. Of these 17 appositions, three were followed in serial sections to confirm that they had synaptic thickenings and alignment of vesicles along the membrane contacting the spinothalamic tract soma. Catecholaminergic boutons were observed apposing the somata and dendrites of intracellularly filled STT cells characterized as high threshold and wide dynamic range neurons. These observations clearly indicate a direct innervation of spinothalamic tract neurons by catecholaminergic neurons, providing anatomical data to support previous physiological findings demonstrating that catecholamines modulate nociceptive transmission.

Adenosine mediates calcium-induced antinociception and potentiation of noradrenergic antinociception in the spinal cord

Intrathecal (i.t.) coadministration of calcium (Ca2+), 50 micrograms potentiates the spinal antinociceptive action of morphine and noradrenaline (NA) but not cyclohexyladenosine, 5'-N-ethylcarboxamido adenosine or 5-hydroxytryptamine in the rat tail flick test. This dose of Ca2+ has no intrinsic effect in this test. Higher doses of Ca2+ (200-400 micrograms) produce antinociception in the tail flick and hot plate tests, which is completely blocked by pretreatment with the adenosine receptor antagonists theophylline, 50 micrograms and 8-phenyltheophylline, 3 micrograms. 8-Phenyltheophylline also eliminates potentiation of the antinociceptive action of NA by 50 micrograms Ca2+. The intrinsic antinociceptive effect of Ca2+ is blocked by i.t. pretreatment with the neurotoxins capsaicin, 50 micrograms and 6-hydroxydopamine, 50 micrograms but not 5,7-dihydroxytryptamine, 50 micrograms. Antinociception also is blocked by pretreatment with phentolamine but not by methysergide. These results suggest that the antinociceptive action of high doses of Ca2+ is due to release of adenosine (or a nucleotide which is metabolized to adenosine) from the spinal cord. At lower doses, the release of adenosine is insufficient to cause antinociception, but potentiates the action of NA. Adenosine appears to originate from capsaicin-sensitive small diameter primary afferent nerve terminals. A subsequent interaction of adenosine with spinal adrenergic receptors contributes to antinociception.

Acute increases in arterial blood pressure produced by occlusion of the abdominal aorta induces antinociception: peripheral and central substrates

Occlusion of the abdominal aorta proximal to the renal arteries results in an increase in arterial blood pressure, inhibition of forepaw and hindpaw withdrawal to a noxious mechanical stimulus, and inhibition of the tail-flick reflex to noxious heat. Occlusion of the abdominal aorta distal to the renal arteries does not elevate arterial blood pressure and produces no antinociceptive effects. Occlusion of the vena cava lowers arterial blood pressure and produces no antinociception. The inhibitory effects of occlusion of the abdominal aorta depend upon activation of high pressure baroreceptors since bilateral sinoaortic denervation, but not bilateral vagotomy, eliminates the inhibition with respect to all behavioral measures. The inhibitory effects with respect to the tail-flick reflex also depend upon activation of a descending inhibitory system since reversible cold block of the spinal cord at the level of the second thoracic vertebra eliminates the antinociception. This antinociception is also eliminated following intrathecal administration of the noradrenergic receptor antagonist phentolamine, but not by intrathecal administration of either methysergide or naloxone. These data support the view that activation of high pressure baroreceptors by increases in arterial blood pressure produces antinociception via activation of a spinopetal noradrenergic system.

Distribution of phenylethanolamine N-methyltransferase cell bodies, axons, and terminals in monkey brainstem: an immunohistochemical mapping study

Adrenaline (epinephrine) is an important candidate transmitter in descending spinal control systems. To date intrinsic spinal adrenergic neurons have not been reported; thus adrenergic input is presumably derived from brainstem sites. In this regard, the localization of adrenergic neurons in the brainstem is an important consideration. Maps of adrenergic cell bodies and to a lesser extent axons and terminal fields have been made in various species, but not in monkeys. Thus, the present study concerns the organization of adrenergic systems in the brainstem of a monkey (Macaca fascicularis) immunohistochemically mapped by means of an antibody to the enzyme phenylethanolamine N-methyltransferase (PNMT). PNMT-immunostained cell bodies are distributed throughout the medulla in two principal locations. One concentration of labeled cells is in the dorsomedial medulla and includes the nucleus of the solitary tract (NTS), the dorsal motor nucleus of the vagus (X), and an area ventral to X in a region of the reticular formation (RF) known as the central nucleus dorsalis (CnD) of the medulla. A few scattered cells are observed in the periventricular gray just ventral to the IVth ventricle and on midline in the raphe. The second major concentration of PNMT-immunostained cells is located in the ventrolateral RF, lateral and dorsolateral to the inferior olive (IO), including some cells in the rostral part of the lateral reticular nucleus (LRN). Terminal fields are located in the NTS, X, area postrema (AP), and the floor of the IVth ventricle in the medulla and pons. A light terminal field is also observed in the raphe, particularly raphe pallidus (RP). A heavy terminal field is present in locus coeruleus (LC). Fibers labeled for PNMT form two major fiber tracts. One is in the dorsomedial RF extending as a well-organized bundle through the medulla, pons, and midbrain. A second tract is located on the ventrolateral edge of the medulla and caudal pons. Fibers in this tract appear to descend to the spinal cord. A comparison with maps of other catecholamine neurons in primates is discussed, confirming that the distribution of the adrenergic system in monkeys is similar to that described in the human.

Effects of iontophoresis of noradrenaline and stimulation of the periaqueductal gray on single-unit activity in the rat superficial dorsal horn

Recordings were made with a new form of low-noise carbon fibre microelectrode from 75 units in the superficial laminae of the lumbar dorsal horn of the anaesthetized rat. The response of each unit to adequate stimulation of its peripheral receptive field, to noradrenaline (NA) applied iontophoretically, and to electrical stimulation of the periaqueductal gray (PAG) was investigated. Only units that could be excited by iontophoresis of glutamate (10-100 nA) were analyzed. Recording sites in the spinal cord and stimulation sites in the brainstem were identified histologically at the end of each experiment. Forty-six units with low-threshold receptive fields and small spike amplitudes were found, mainly located in laminae II and III. Both stimulation of the PAG and NA iontophoresis excited the majority (32/46) of these units. The rest were unaffected. Eight high-threshold (HT) units were located in the region of lamina I. Twenty-one wide-dynamic-range (WDR) units were found mainly in deeper laminae. Both WDR and HT units were inhibited by NA and PAG stimulation. This inhibition affected both glutamate-evoked activity and responses to nociceptive stimuli. We suggest that the small LT units are inhibitory interneurones of the substantia gelatinosa (SG), which act on the WDR and HT units to produce nociceptive-specific inhibition. The inhibition can be modality-specific without necessarily being presynaptic because of the laminar arrangement of the dorsal horn. The PAG could thus exert its known antinociceptive effects at least partly via descending noradrenergic axons that excite these SG cells.

Adaptive changes in alpha-2 adrenoceptor mediated responses: analgesia, hypothermia and hypoactivity

The acute effects of the alpha-2 adrenoceptor agonists, clonidine and guanfacine, upon antinociception, hypothermia and motor activity were compared under conditions of receptor antagonism, denervation, and chronic administration of a tricyclic antidepressant compound. The analgesic actions of clonidine and guanfacine were antagonised by idazoxan, an alpha-2 receptor antagonist, but potentiated by pretreatment with the noradrenaline neurotoxin DSP4, and attenuated by chronic treatment with desipramine (DMI). Clonidine- and guanfacine-induced hypothermia was antagonised by idazoxan, potentiated by prior treatment with DSP4 and attenuated by chronic administration with DMI. Both clonidine and guanfacine produced decreases in motor activity that were attenuated by idazoxan but unaffected by prior DSP-4 treatment. Chronic DMI administration also attenuated clonidine-induced hypoactivity but potentiated guanfacine-induced hypoactivity. These diverse results describe both similar and differential adaptive mechanisms modulating the functional effect of alpha-2 receptor systems in the central nervous system.

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

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

Lesions to ascending noradrenergic and serotonergic pathways modify antinociception produced by intracerebroventricular administration of morphine

Ascending noradrenergic and serotonergic pathways were lesioned by injection of 6-hydroxydopamine into the dorsal bundle or 5,7-dihydroxytryptamine into the ventromedial tegmentum respectively, and the antinociceptive effect of morphine, administered by intracerebroventricular injection assessed using the tail-flick test 3-14 days later. Lesions of the dorsal bundle selectively depleted levels of noradrenaline (NA) in the forebrain and increased the antinociceptive effect of morphine early, but not later, in the time course of action. Lesions to the locus coeruleus depleted NA in the forebrain and spinal cord but had no effect on the antinociceptive action of morphine. Lesions of the ventromedial tegmentum selectively depleted 5-hydroxytryptamine (5-HT) in the forebrain and transiently reduced the action of morphine. 5,7-Dihydroxytryptamine given intraventricularly depleted 5-HT in the forebrain and spinal cord and also transiently reduced the antinociceptive effect of morphine. These results indicate that aminergic pathways, projecting to the forebrain, are involved in the suppression of the tail-flick reflex produced by injection of morphine into the lateral ventricle.

Evidence for functional contact between cografted locus coeruleus and spinal cord in oculo: electrophysiological studies

The functional consequences of the locus coeruleus innervation of the spinal cord are not yet clearly understood. In a recent histological study it was shown that intraocular spinal cord grafts will become innervated by tyrosine hydroxylase-positive nerve fibers from a cografted locus coeruleus. In the present study we use this intraocular model of the descending coeruleo-spinal pathway to investigate functional contact between locus coeruleus and the spinal cord. We have pharmacologically characterized the receptor mediation of norepinephrine-induced, as well as locus coeruleus-mediated depressions of spinal cord neurons grafted in oculo. We found that electrical stimulation of the locus coeruleus part of the double grafts predominantly caused an inhibition of cografted spinal cord neurons. Norepinephrine-induced inhibition of the firing rate of single grafted spinal cord neurons was antagonized by phentolamine, an alpha-adrenergic antagonist, but was unaffected by timolol, a beta-adrenergic antagonist. Similarly, inhibition of the firing rate of grafted spinal cord neurons by stimulation of cografted locus coeruleus was antagonized by phentolamine but not by timolol. Interestingly, single spinal cord grafts were more sensitive to the depressant effects of perfused norepinephrine than was the spinal cord cografted with locus coeruleus. We conclude that spinal cord grafts can be functionally innervated by cografted locus coeruleus and that the noradrenergic inputs to spinal cord from cografted locus coeruleus are alpha-adrenergically mediated. Furthermore, the postsynaptic receptors in single spinal cord grafts appear to be supersensitive to norepinephrine application.