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

Lateral habenular norepinephrine contributes to states of arousal and anxiety in male rats

Recent research has identified the lateral habenula (LHb) as a brain region playing an important role in the production of stressful and anxiogenic states. Additionally, norepinephrine (NE) has long been known to be involved in arousal, stress and anxiety, and NE projections to the LHb have been identified emanating from the locus coeruleus (LC). The current research was devised to test the hypothesis that NE release within the LHb contributes to the occurrence of anxiogenic behaviors. Male rats were implanted with bilateral guide cannula aimed at the LHb and subsequently treated with intracranial (IC) infusions of the selective α2 adrenergic autoreceptor agonist, dexmedetomidine (DEX) (0, 0.5, 1.0 μg/side), prior to assessment of ambulatory and anxiogenic behavior in tests of spontaneous locomotion, open field behavior, and acoustic startle-response. Results demonstrated that DEX administration significantly reduced the overall locomotor behavior of subjects at both doses indicating that infusion of even small doses of this α2 agonist into the LHb can have profound effects on the subjects' general levels of alertness and activity. DEX was also found to attenuate anxiety as evidenced by a reduction in the magnitude of a startle-response to an acoustic 110 dB stimulus. Taken together, these results identify a role for NE release within the LHb in both arousal and anxiety.

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.

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.

Regulation of alpha(2A)-adrenoceptor expression by chronic stress in neurons of the brain stem

Alpha(2)-Adrenoceptors are supposed to be important regulatory elements in responses to stress. Previous receptor binding studies in male tree shrews have shown that chronic psychosocial stress down-regulates binding sites for alpha(2)-adrenergic ligands in several brain stem nuclei. The aim of the present study was to quantify effects of chronic subordination stress on expression of the alpha(2)-adrenoceptor subtype A gene in identified neurons of the brain stem. We partially cloned the alpha(2A)-adrenoceptor cDNA of the tree shrew (1.22 kb) and localized receptor RNA expression in brain stem neurons by in situ hybridization using a 35S-labeled cRNA probe (1.06 kb). To identify neurons expressing receptor mRNA, brain sections were first immunocytochemically stained with antibodies against tyrosine hydroxylase, phenylethanolamine-N-methyltransferase, or glutamate, and then processed for in situ hybridization. Furthermore, expression of receptor-specific RNA was quantified in single neurons of animals which had been psychosocially stressed during 4 weeks and in unstressed controls. We found strong in situ hybridization in the noradrenergic neurons of the locus coeruleus, but only weak labeling of A2 neurons in the solitary tract nucleus and no labeling of A1 neurons in the caudal ventrolateral medulla. Adrenergic neurons in the solitary tract nucleus (group C2) did not express the alpha(2A)-adrenoceptor, and C1 neurons in the rostral ventrolateral medulla showed only a minor labeling by the in situ probe. In contrast, large glutamatergic neurons in the lateral reticular nucleus were strongly labeled by the probe. Chronic psychosocial stress reduced alpha(2A)-adrenoceptor RNA expression in locus coeruleus neurons (-24.0%), in solitary tract neurons (-31.0%), and in neurons of the lateral reticular nucleus (-18.8%). These findings show that stress not only decreases the expression of the alpha(2A)-adrenergic autoreceptor in the locus coeruleus but also of alpha(2A)-heteroreceptors in glutamatergic neurons.

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.

Alpha 2-adrenoceptor agonists and stress-induced analgesia in rats: influence of stressors and methods of analysis

The present experiments were designed to investigate the role of housing and handling conditions during testing, as well as data analysis, on the outcome of antinociceptive testing of alpha 2-adrenoceptor agonists, fentanyl, and a high dose of chlordiazepoxide in the tail withdrawal reaction test (TWR test) in rats. Dose-response curve data were obtained with fentanyl, clonidine, xylazine, dexmedetomidine, and 40.00 mg/kg chlordiazepoxide and were compared under normal TWR test conditions and during immobilization or immobilization with continuous painful stimulation. Data were analyzed in terms of all-or-none criteria as well as percentage maximum possible effect (%MPE) analysis over the total measurement period or at any specific time point during testing. The results indicate that stress, induced by immobilization and immobilization with long-term-applied paw pressure, unmasked possible antinociceptive properties of the various alpha 2-adrenoceptor agonists and potentiated the effects of fentanyl. Stress also unmasked the positive effects of benzodiazepines. The manner of data analysis was shown to significantly affect the outcome measured in stress and nonstress conditions. The MPE analysis, particularly at one time point, appeared much more sensitive than the all-or-none criteria. The data indicate that the housing and handling conditions of animals during testing, together with data analysis, may affect the outcome of different classes of compounds in the TWR test, and this knowledge may help control for false positive results.

alpha 2A subtype of presynaptic alpha 2-adrenoceptors modulates the release of [3H]-noradrenaline from rat spinal cord

The modulation of noradrenaline (NA) release from rat spinal cord slices was investigated and the subtype of presynaptic alpha 2-adrenoceptors involved in the negative feedback modulation was characterized using in vitro perfusion experiments. Rat spinal cord slices were loaded with [3H]NA and the release of radioactivity at rest and in response to field stimulation was determined. The alpha 2-adrenoceptor agonists, clonidine and dexmedetomidine inhibited the stimulation-evoked release of NA from spinal cord slices, whereas alpha 2-adrenoceptor antagonists yohimbine and CH-38083 (7,8-(methylenedioxy)-14-alpha-hydroxyalloberbane HCI), enhanced it. ARC 239, a selective alpha 2B-antagonist, had no effect on the release. In contrast, BRL 44408 a selective alpha 2A-antagonist increased the release of NA. Our results indicate that the negative feedback modulation of NA release from noradrenergic fibres in the spinal cord is mediated via alpha 2A subtype of alpha 2-adrenoceptors.

Supraspinal influence on hindlimb withdrawal thresholds and mustard oil-induced secondary allodynia in rats

We examined the role of supraspinal structures in secondary allodynia induced by mustard oil in awake rats. To produce allodynia (=unpleasent sensation evoked by innocuous stimuli), mustard oil (50%) was applied for 2 min to the skin of the ankle of one hindlimb. Mechanical hypersensitivity of the skin was tested by determining the hindlimb withdrawal threshold to a series of monofilaments applied to the glabrous foot pad (=distal to the mustard oil-treated ankle). In intact rats, mustard oil produced a secondary allodynia in the mustard oil-treated hindlimb as indicated by a decreased withdrawal threshold to mechanical test stimuli applied to the glabrous skin (=outside the mustard oil-treated ankle), whereas the withdrawal threshold in the contralateral (=control) hindlimb was not changed. Following spinalization, mustard oil treatment produced no secondary allodynia, but the interpretation of this finding was complicated by a concomitant bilateral elevation of hindlimb withdrawal thresholds to mechanical skin stimulation. However, the spinalized rats had shorter tail-flick latencies to radiant heat than intact rats. Administration of an opioid antagonist, naloxone (1 mg/kg, SC), had no effect on withdrawal thresholds in spinalized animals. Importantly, microinjection of lidocaine (4%) into the nucleus raphe magnus in rats with an intact spinal cord had a selective antiallodynic effect when the injection volume was 1.0 microl but not when it was 0.5 microl. Lidocaine (4%, 0.5 microl) in the lateral reticular nucleus of the medulla also attenuated the spinal hypersensitivity, however, concomitantly with motor side effects, due to which this finding maybe artificial. It is concluded that brain stem spinal pathways, originating adjacent to but not within the raphe magnus, contribute to the behavioral expression of secondary allodynia induced by neurogenic inflammation of the skin. Furthermore, there is a differential tonic control of various spinal reflexes by the brain stem as indicated by the dissociative effects of spinalization on mechanically induced hindlimb withdrawal vs. heat-induced tail-flick reflex.

Different roles of alpha 2-adrenoceptors of the medulla versus the spinal cord in modulation of mustard oil-induced central hyperalgesia in rats

We attempted to determine the roles of spinal versus medullary alpha 2-adrenoceptors in modulation of central hyperalgesia in rats. Central hyperalgesia was produced by applying mustard oil (50%) to the skin of the ankle of one hindpaw. The threshold for eliciting a hindlimb flexion reflex was determined by applying a series of calibrated monofilaments to the glabrous skin of the hindpaw contralaterally (= control) or ipsilaterally to the mustard oil-treated ankle (= outside the area of primary hyperalgesia). Medetomidine (an alpha 2-adrenoceptor agonist; 1 micrograms), atipamezole (an alpha 2-adrenoceptor antagonist; 2.5 micrograms) or saline was microinjected into the lateral reticular nucleus of the medulla, the nucleus raphe magnus, or intrathecally to the lumbar spinal cord 12 min before the mustard oil treatment. Following saline injections, mustard oil produced a significant decrease of the hindlimb withdrawal threshold in the mustard oil-treated limb but not in the contralateral limb. Atipamezole in the lateral reticular nucleus produced a complete reversal of the hyperalgesia but no effect on the threshold of the intact limb. However, atipamezole in the raphe magnus nucleus or in the lumbar spinal cord did not produce a significant attenuation of the hyperalgesia. Medetomidine in the spinal cord, but not in the lateral reticular nucleus, reversed the hyperalgesia. At this dose range (up to 3 micrograms), medetomidine in the spinal cord of nonhyperalgesic control rats did not produce any significant change in the withdrawal response of hindlimbs or in the tail-flick latency. The results indicate that neurogenic inflammation induces significant plastic changes in the function of alpha 2-adrenergic pain regulatory mechanisms. In rats with mustard oil-induced central hyperalgesia, an alpha 2-adrenoceptor antagonist produces an antihyperalgesic effect due to an action on the caudal ventrolateral medulla, whereas an alpha 2-adrenoceptor agonist produces an enhanced antinociceptive effect due to a direct action on the spinal cord.