Initial observations on the localization of mRNA for α and β adrenergic receptors in brain and peripheral tissues of rat using in situ hybridization

Therapeutic Potential of Centhaquine Citrate, a Selective Alpha-2B Adrenoceptor Agonist, in the Management of Circulatory Shock

Shock is a life-threatening condition marked by inadequate tissue perfusion and organ dysfunction with high morbidity and mortality. Activation of the sympatho-adrenergic system is pivotal in response to all four major categories (i.e., hypovolemic, distributive, cardiogenic, and obstructive). In addition, exogenous vasopressors are often used to maintain organ perfusion pressure and decrease the size of the intravascular compartment. These agents preferentially constrict the arterial system but may lead to microcirculatory failure, especially at higher doses. This review outlines the sympatho-adrenergic system response after shock, discusses various vasopressors currently used as resuscitative agents, and reports the rationale for using a predominant venous vasopressor in shock. We also discuss the preliminary evidence for and ongoing research into a novel venous vasopressor, centhaquine citrate.

Sex-dependent noradrenergic modulation of premotor cortex during decision-making

Rodent premotor cortex (M2) integrates information from sensory and cognitive networks for action planning during goal-directed decision-making. M2 function is regulated by cortical inputs and ascending neuromodulators, including norepinephrine (NE) released from the locus coeruleus (LC). LC-NE has been shown to modulate the signal-to-noise ratio of neural representations in target cortical regions, increasing the salience of relevant stimuli. Using rats performing a two-alternative forced choice task after administration of a β-noradrenergic antagonist (propranolol), we show that β-noradrenergic signaling is necessary for effective action plan signals in anterior M2. Loss of β-noradrenergic signaling results in failure to suppress irrelevant action plans in anterior M2 disrupting decoding of cue-related information, delaying decision times, and increasing trial omissions, particularly in females. Furthermore, we identify a potential mechanism for the sex bias in behavioral and neural changes after propranolol administration via differential expression of β2 noradrenergic receptor RNA across sexes in anterior M2, particularly on local inhibitory neurons. Overall, we show a critical role for β-noradrenergic signaling in anterior M2 during decision-making by suppressing irrelevant information to enable efficient action planning and decision-making.

Systemic administration of a β2-adrenergic receptor agonist reduces mechanical allodynia and suppresses the immune response to surgery in a rat model of persistent post-incisional hypersensitivity

Beta 2 adrenergic receptor (β2 AR) activation in the central and peripheral nervous system has been implicated in nociceptive processing in acute and chronic pain settings with anti-inflammatory and anti-allodynic effects of β2-AR mimetics reported in several pain states. In the current study, we examined the therapeutic efficacy of the β2-AR agonist clenbuterol in a rat model of persistent postsurgical hypersensitivity induced by disruption of descending noradrenergic signaling in rats with plantar incision. We used growth curve modeling of ipsilateral mechanical paw withdrawal thresholds following incision to examine effects of treatment on postoperative trajectories. Depletion of spinal noradrenergic neurons delayed recovery of hypersensitivity following incision evident as a flattened slope compared to non-depleted rats (-1.8 g/day with 95% CI -2.4 to -1.085, p < 0.0001). Chronic administration of clenbuterol reduced mechanical hypersensitivity evident as a greater initial intercept in noradrenergic depleted (6.2 g with 95% CI 1.6 to 10.8, p = 0.013) and non-depleted rats (5.4 g with 95% CI 1.2 to 9.6, p = 0.018) with plantar incision compared to vehicle treated rats. Despite a persistent reduction in mechanical hypersensitivity, clenbuterol did not alter the slope of recovery when modeled over several days (p = 0.053) or five weeks in depleted rats (p = 0.64). Systemic clenbuterol suppressed the enhanced microglial activation in depleted rats and reduced the density of macrophage at the site of incision. Direct spinal infusion of clenbuterol failed to reduce mechanical hypersensitivity in depleted rats with incision suggesting that beneficial effects of β2-AR stimulation in this model are largely peripherally mediated. Lastly, we examined β2-AR distribution in the spinal cord and skin using in-situ hybridization and IHC. These data add to our understanding of the role of β2-ARs in the nervous system on hypersensitivity after surgical incision and extend previously observed anti-inflammatory actions of β2-AR agonists to models of surgical injury.

Cellular and subcellular localization of alpha-1 adrenoceptors in the rat visual cortex

Noradrenaline is thought to play modulatory roles in a number of physiological, behavioral, and cellular processes. Although many of these modulatory effects are mediated through alpha-1 adrenoceptors, basic knowledge of the cellular and subcellular distributions of these receptors is limited. We investigated the laminar distribution pattern of alpha-1 adrenoceptors in rat visual cortex, using immunohistochemistry at both light and electron microscopic levels. Affinity-purified anti-alpha-1 antibody was confirmed to react only with a single band of about 70-80 kDa in total proteins prepared from rat visual cortex. Alpha-1 adrenoceptors were widely distributed though all cortical layers, but relatively high in density in layers I, II/III, and V. Immunoreactivity was observed in both neuronal perikarya and processes including apical dendrites. In double-labeling experiments with anti-microtubule-associated protein 2, anti-neurofilament, anti-glial fibrillary acidic protein, anti-glutamic acid decarboxylase 65/67, anti-2-3-cyclic nucleotide 3-phosphodiesterase, and anti-tyrosine hydroxylase antibodies, alpha-1 adrenoceptors were found mainly in dendrites and somata of microtubule-associated protein 2-immunopositive neurons. About 20% of alpha-1 adrenoceptors were in GABAergic neurons. A small number of alpha-1 adrenoceptors were also distributed in axons of excitatory neurons, astrocytes, oligodendrocytes and noradrenergic fibers. Using an immunoelectron microscopic technique, numerous regions of alpha-1 adrenoceptor immunoreactivity were found in cell somata, on membranes of dendrites, and in postsynaptic regions. Moreover, a small number of immunoreaction products were also detected in axons and presynaptic sites. These findings provide the first quantitative evidence regarding the cellular and subcellular localization of alpha-1 adrenoceptor immunoreactivity in visual cortex. Moreover, the ultrastructural distribution of alpha-1 adrenoceptor immunoreactivity suggests that alpha-1 adrenoceptors are transported mainly into dendrites and that they exert effects at postsynaptic sites of neurons.

alpha(2a) and alpha(2c) adrenoceptors on spinal neurons controlling penile erection

The thoracolumbar and lumbosacral spinal cord contain respectively sympathetic and parasympathetic preganglionic neurons that supply the organs of the pelvis including the penis. These neurons are influenced by supraspinal information and receive aminergic projections from the brainstem. The presence of the alpha(1)- and alpha(2)-adrenoceptor subtypes has been demonstrated in the rat spinal cord. In this species, we looked for the presence of alpha(2a)- and alpha(2c)-adrenoceptor subtypes in the sympathetic and parasympathetic preganglionic neurons controlling erection. In adult male rats, transsynaptic axonal transport of pseudorabies virus injected into the penis was combined with immunohistochemistry against alpha(2a)- and alpha(2c)-adrenoceptor subtypes. At 4 days survival time, neurons infected with the pseudorabies virus were solely found in the intermediolateral cell column and dorsal gray commissure of segment T12-L2 and in the intermediolateral cell column of segment L6-S1. Neurons and fibers immunoreactive for alpha(2a)- and alpha(2c)-adrenoceptor subtypes were mainly present in the intermediolateral cell column, the dorsal gray commissure and the ventral horn of the T12-L2 and L5-S1 spinal cord, the dorsal horn displayed only immunoreactive fibers. Pseudorabies virus-infected neurons in the autonomic nuclei were both immunoreactive for alpha(2a)- and alpha(2c)-adrenoceptor subtypes and closely apposed by alpha(2a)- and alpha(2c)-immunoreactive fibers. The results suggest an intraspinal modulation of the noradrenergic and adrenergic control of the autonomic outflow to the penis by pre- and postsynaptic alpha(2) adrenoceptors.

Effect of antisense knock-down of alpha(2a)- and alpha(2c)-adrenoceptors on the antinociceptive action of clonidine on trigeminal nociception in the rat

Although activation of alpha(2)-adrenoceptors is known to play an important role in mediating antinociception, the contribution of various alpha(2)-adrenoceptor subtypes in modulating trigeminal nociception remains unknown since subtype specific agonists and antagonists are not available. The present study investigated the functional role of alpha(2)-adrenoceptor subtypes in modulating the N-methyl-D-aspartate-induced nociceptive behavior in the medullary dorsal horn by using antisense oligodeoxynucleotides to selectively knock-down the receptor subtypes. Microinjection of N-methyl-D-aspartate (2 nmol in 10 microl) through a cannula implanted dorsal to the medullary dorsal horn produced a total of 164.9+/-8.8 scratches in the facial region (n=14), and the scratching behavior lasted for 77.8+/-5.2s (n=14). Microinjection of clonidine, an alpha(2)-agonist (7 microg in 5 microl), 15 min prior to administration of N-methyl-D-aspartate, produced a reduction of 71.6% (n=12) in the number of scratches and a reduction of 57.5% (n=12) in the duration. The inhibitory effect of clonidine was blocked by idazoxan (n=4) and yohimbine (n=4), alpha(2) antagonists. In rats pretreated with the antisense probe to the alpha(2A) adrenoceptor, clonidine only produced a reduction of 7.3% in the number of scratches (n=12) and a reduction of 9% in the duration (n=12). The antinociceptive effect of clonidine recovered completely 4 days after termination of the alpha(2A) antisense oligodeoxynucleotide treatment. In contrast to the alpha(2A) antisense-treated animals, clonidine reduced the number of scratches and the duration by 85.5% (n=9) and 82.1% (n=9), respectively, in rats pretreated with the sense probe to the alpha(2A) adrenoceptor. The effect of clonidine was not altered in rats pretreated with the antisense or the sense probes to the alpha(2C) adrenoceptor. In the alpha(2C) antisense pretreated rats, clonidine reduced the number of scratches and the duration by 60.8% (n=11) and 44.5 % (n=11), respectively. In the sense-pretreated rats, clonidine produced a reduction of 69.1% in the number of scratches (n=9) and a reduction of 55.1% in the duration (n=9). In order to assess the effectiveness of the antisense treatment, the receptor expression was examined by immunohistochemistry. Antisense treatment reduced alpha(2A) and alpha(2C) receptor immunoreactivity in the medullary dorsal horn compared to the sense and the vehicle-treated animals. Quantitative image analysis revealed a significant decrease in pixel intensity following the antisense treatment. These results indicate that activation of alpha(2A) adrenoceptor plays an important role in mediating the antinociceptive effect of clonidine in the medullary dorsal horn in the rat.

Descending control of pain

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

Alpha1-adrenergic receptors in human spinal cord: specific localized expression of mRNA encoding alpha1-adrenergic receptor subtypes at four distinct levels

alpha1-Adrenergic receptors (alpha1ARs) are important in lower urinary tract syndromes such as benign prostatic hypertrophy and bladder irritability. Spinal cord alpha1ARs have been postulated to play a role in modulating these diseases, yet alpha1AR subtype (alpha1a, alpha1b, alpha1d) neuronal localization in human spinal cord has not been described. We therefore tested the hypothesis that alpha1AR subtype distribution varies according to specific spinal cord tract and level. In situ hybridization was performed to identify cell bodies containing alpha1AR subtype mRNA at four levels of human spinal cord (cervical enlargement, thoracic, lumbar, sacral). alpha1AR mRNA is present in ventral gray matter only (ventral>dorsal; sacral>lumbar=thoracic>cervical). Signaling cell bodies were detected in anterior horn motor neurons at all levels; dorsal nucleus of Clarke and intermediolateral columns in cervical enlargement, thoracic and lumbar spinal cord regions; and parasympathetic nucleus in sacral spinal cord. Although all three alpha1AR subtypes are present throughout human spinal cord, alpha1d mRNA predominates overall. If confirmed at a protein level, these findings may contribute to the development of new therapeutic strategies in the treatment of several human diseases.

Alpha2B adrenoceptor mRNA expression during rat brain development

The expression of alpha2B adrenoceptor mRNA in developing and adult rat brain was examined, using in situ hybridization with S-labeled riboprobes. In the adult we have detected more widespread expression than previously reported, with moderate to strong hybridization signals in the fundus striati, olfactory tubercles, septum, thalamus and Purkinje cells of the cerebellum. In addition, there was low expression in the endopiriform nucleus, claustrum, cortex, caudate-putamen and spinal trigeminal nucleus of the brain stem. During embryonic development, there was intense, transient mRNA expression in the developing vascular plexus and vasculature which disappeared by birth. In most brain areas which exhibit mRNA expression in the adult, expression started during late embryonic development; with the exception of the thalamus, where expression was differentially regulated in sensory and non-sensory thalamic nuclei starting at the end of the first postnatal week. In addition, there was transiently upregulated expression in the caudate-putamen and cerebellar Purkinje cells during late embryonic and early postnatal development, respectively. This transient expression correlates with the time of neuronal migration and differentiation in these structures and complements the developmental expression of alpha2A and alpha2C adrenoceptors. These results suggest that alpha2B adrenoceptors may play a role in angiogenesis and in mediating neurotrophic functions of norepinephrine in some brain areas.

Regulation of pineal alpha1B-adrenergic receptor mRNA: day/night rhythm and beta-adrenergic receptor/cyclic AMP control

Mammalian pineal function is regulated by norepinephrine acting through alpha1beta- and beta1-adrenergic receptors (ARs). Noradrenergic stimulation of alpha1beta-ARs potentiates the beta1-AR-driven increase in cAMP, serotonin N-acetyltransferase, and melatonin production. In the present study, we describe a 3-fold daily rhythm in mRNA-encoding alpha1beta-ARs in the pineal gland, with a peak at midnight. Pharmacological studies indicate that this increase in alpha1beta-AR mRNA is due to activation of beta-ARs. Second messenger studies indicate that alpha1beta-AR mRNA is increased by agents that increase cAMP, including dibutyryl cAMP, cholera toxin, forskolin, or vasoactive intestinal peptide. These observations indicate that alpha1beta-AR mRNA can be physiologically regulated by a beta-AR-dependent enhancement of cAMP. It also was observed that in vivo and in vitro changes in alpha1beta-AR mRNA are not accompanied by similar changes in alpha1beta-AR binding, indicating that turnover of alpha1beta-AR protein is significantly slower than that of alpha1beta-AR mRNA and that post-transcriptional mechanisms play an important role in regulating alpha1beta-AR binding.

Localization of alpha 2A- and alpha 2B-adrenergic receptor subtypes in brain

Recent studies have shown that all three subtypes of alpha2-adrenergic receptor (alpha2-AR) are found in brain. The purpose of this study was to map the subtype localization of the alpha2A- and alpha2B-ARs in brain structures. RNase protection shows that both the alpha2A- and alpha2B-ARs are detectable in cortex, cerebellum, pons-medulla, and hypothalamus. We tested probes derived from the alpha2A- and alpha2B-AR cDNAs on cell lines that express each of the alpha2-AR subtypes to establish the subtype specificity of these probes for in situ hybridization. Then we used the alpha2A- and alpha2B-AR probes for in situ hybridization on sagittal and coronal sections of rat brain. Both alpha2A and alpha2B mRNA were detected throughout the brain. Overall, there appears to be a greater expression of message for alpha2A- than alpha2B-AR in most brain areas, with the exception of the thalamus. Developing these probes for in situ hybridization is an important step for further studies on the exact role of the alpha2-AR subtypes in neurons that modulate cardiovascular function.

Ontogenic expression of two alpha-1 adrenergic receptor subtypes in the rat brain

alpha-1A/D and alpha-1B adrenergic receptor subtype mRNA expression was studied during pre- and postnatal rat brain development. Oligonucleotide probes were generated to distinguish these two homologous subtypes by in situ histochemical analysis in E14, E16, E19, P0, P8, P14, P21, and adult animals. alpha-1B adrenergic receptor mRNA expression was noted as early as the E14 animal and demonstrated specific regional and temporal expression. alpha-1A/D adrenergic receptor mRNA expression was limited in the E19 and P0 animal but increased in intensity with aging. Specific regional and temporal expression differed between the two subtypes. The regional localization for both subtypes appeared to be stable after P21 but the intensity of expression for both subtypes decreased between P21 and adulthood, which is a finding that correlates with previous ligand-binding data. Different subtypes of homologous receptors have very different ontogenic patterns of distribution and may account for previous discrepancies in ligand-binding data and differential tissue responses to various ligands during development.

Noradrenergic regulation of immediate early gene expression in rat forebrain: differential effects of alpha 1- and alpha 2-adrenoceptor drugs

Noradrenergic (NAergic) transmission in the rat cerebral cortex has recently been shown to be involved in the regulation of the basal expression of NGFI-A, an immediate early gene (IEG) which encodes a zinc-finger transcription factor. The present study further investigated the role of the NAergic system in mediating cortical IEG expression and possible topographical changes in expression of NGFI-A mRNA in rat forebrain after alpha 1- and alpha 2-adrenoceptor (AR) agonist and antagonist treatment. Expression of c-fos and c-jun, which encode leucine-zipper class transcription factors, was also studied. Male Sprague-Dawley rats were injected intraperitoneally with either an alpha 1-AR agonist (methoxamine, 5 or 10 mg/kg); an alpha 1-AR antagonist (prazosin, 5 mg/kg); an alpha 2-AR agonist (clonidine, 0.5 mg/kg); or an alpha 2-AR antagonist (methoxyidazoxan, 5 mg/kg) and killed after 1 h. IEG mRNA levels were detected by quantitative in situ hybridization histochemistry using 35S-labelled oligonucleotides. High basal levels of NGFI-A mRNA were present in cortical layers IV and VI, hippocampal CA1, piriform cortex, amygdala and caudate putamen. alpha 1-AR agonist and antagonist treatment had essentially no effect on IEG mRNA, despite producing characteristic behavioral and peripheral effects at the doses used. Methoxyidazoxan significantly increased (mean%) NGFI-A mRNA in: cerebral cortex (44); caudate putamen (82); amygdala (92); and CA1 of hippocampus (48), while clonidine significantly decreased NGFI-A mRNA in the various cortical layers to a similar extent (27-37%). Basal c-fos mRNA expression was lower than that for NGFI-A in forebrain areas including cortex, caudate putamen and hippocampus.(ABSTRACT TRUNCATED AT 250 WORDS)

Increased alpha 1B-adrenoreceptor mRNA levels in the rat kidney after thyroidectomy

Oligonucleotide probes were designed to sequences of the rat alpha 1B- and alpha 2B-adrenergic receptor mRNA and used for in situ hybridization histochemistry on tissue sections of kidneys from control and thyroidectomized rats. Both alpha 1B- and alpha 2B-receptor mRNA labelling was demonstrated in proximal tubule cells in the outer stripe of the outer medulla, with tubular rays radiating into the cortex. Thyroidectomy induced a more than 4-fold increase in mRNA for the alpha 1B-receptor in the kidney, whereas no change in alpha 2B-receptor mRNA levels could be demonstrated in thyroidectomized rats as compared to control animals. The results suggest that thyroid hormone plays an important role in regulating expression of alpha 1B-receptors in renal tubule cells.

Expression of multiple alpha adrenergic receptor subtype messenger RNAs in the adult rat brain

Multiple subtypes of alpha adrenergic receptors with CNS expression (alpha 1A, alpha 1B, alpha 2A and alpha 2C) have been identified through pharmacological and molecular biological means. To characterize the localization of these subtypes and attempt to correlate subtype expression with physiological significance, the expression of the mRNAs encoding the alpha 1A, alpha 1B, alpha 2A and alpha 2C adrenergic receptor subtypes was examined in the adult rat brain by in situ hybridization histochemistry. Each subtype demonstrated a unique pattern of distribution, with the alpha 1 adrenergic receptors more restricted in their distribution and the alpha 2 receptors more widespread. The alpha 1A was primarily localized in the olfactory bulb, intermediate layers of the cortex, the hippocampus and the reticular nucleus of the thalamus. The alpha 1B was expressed in intermediate and deep layers of the cortex, thalamus, hippocampus, dorsal raphe and cerebellum. Although the alpha 2A message was relatively low in abundance, it was identified in the olfactory bulb, cortex, hippocampus, locus coeruleus, pons and cerebellum. The alpha 2C messenger RNA was localized in the cortex (particularly cingulate), hippocampus, caudoputamen, pons and cerebellum. Multiple alpha adrenergic receptor subtypes have significant sequence homology and similar pharmacologic properties; however, they each possess a unique pattern of messenger RNA distribution throughout the brain. The multiplicity of subtypes of alpha adrenergic receptors in specific brain regions may dictate the physiological and pharmacological responses to catecholamines.

Cellular localization of messenger RNA for beta-1 and beta-2 adrenergic receptors in rat brain: an in situ hybridization study

Selective, 35S-labeled, oligonucleotide probes were designed from sequences of the rat beta-1 and beta-2 adrenoceptor messenger RNAs for use in situ hybridization experiments on sections of unfixed rat brain and spinal cord. After hybridized sections were exposed to film or dipped in autoradiographic emulsion, specific and selective labeling patterns characteristic for each receptor messenger RNA and region of the central nervous system were observed. For example, labeling for beta-1 messenger RNA was found in the anterior olfactory nucleus, cerebral cortex, lateral intermediate septal nucleus, reticular thalamic nucleus, oculomotor complex, vestibular nuclei, deep cerebellar nuclei, trapezoid nucleus, abducens nucleus, ventrolateral pontine and medullary reticular formations, the intermediate gray matter of the spinal cord and in the pineal gland, while beta-2 messenger RNA labeling was strongest in the olfactory bulb, piriform cortex, hippocampal formation, thalamic intralaminar nuclei and cerebellar cortex. In some of these regions the beta-1 labeling seemed mainly confined to the cell nucleus. Whether or not this apparently nuclear labeling is specific, i.e. indicates synthesis of beta-1 receptor, remains to be established. However, all labeling patterns described disappeared when excess unlabeled probes were added to their respective radiolabeled probes or when sense probes were employed. Since the in situ method labels only cell bodies that produce the messenger RNA for these two beta receptor subtypes, a comparison between these maps and those of past autoradiographic studies mapping the location of central beta receptors using drugs as radioligands may produce further insights regarding the pre- and postsynaptic localization of these receptors in the various parts of the central nervous system circuitry.

Diverse tissue expression of rat alpha 2-adrenergic receptor genes

Previously, we have reported two major alpha 2-adrenergic receptor transcripts in rat brain of 3.8 and 3.0 kb and the cloning and characterization of the rat brain complementary DNA (cDNA) (RB alpha 2C) specific for the 3.0-kb messenger RNA. In this report, we used rat brain cDNAs specific for the 3.0 and 3.8 kb transcripts, which encode the alpha 2C- and alpha 2A-adrenergic receptors, respectively, and the RNG alpha 2 cDNA, which encodes for the nonglycosylated alpha 2B-adrenergic receptor in rat, to study tissue-specific expression of the three alpha 2-adrenergic receptor genes in rat. To eliminate cross-hybridization of probes with transcripts from other alpha 2 genes, we subcloned fragments that encode for the highly divergent third cytoplasmic loop of each rat alpha 2-adrenergic receptor cDNA and used RNase protection analysis to detect specific transcripts. We show that the three rat alpha 2-adrenergic receptor genes have diverse patterns of tissue expression, and although transcripts specific for each alpha 2-adrenergic receptor gene are found in brain and kidney, the levels of expression of each subtype differ in these tissues. We speculate on the significance of tissue-specific expression of the alpha 2-adrenergic receptor genes.

Distributions of mRNAs for alpha-2 adrenergic receptor subtypes in rat brain: an in situ hybridization study

Selective 35S-labeled oligonucleotide probes were designed to sequences of the rat alpha-2A (RG20), alpha-2B (RNG), and alpha-2C (RG10) adrenoreceptor mRNAs for use in in situ hybridization experiments on sections of unfixed rat brain, spinal cord and kidney. After hybridized sections were exposed to film or dipped in autoradiographic emulsion, specific and selective labeling patterns characteristic for each probe and region of the central nervous system were observed. Alpha-2A mRNA labeling was most pronounced in neurons in layer six of the cerebral cortex, hypothalamic paraventricular nucleus, reticular thalamic nucleus, pontine nuclei, locus coeruleus, vestibular nuclei, trapezoid nuclei, deep cerebellar nuclei, nucleus tractus solitarii, ventrolateral medullary reticular formation, and the intermediolateral cell column of the thoracic spinal cord. In some of these locations, the receptor mRNA, in all probability, is present in noradrenaline and perhaps adrenaline neurons. The alpha-2B probe, which primarily labels the kidney, gave only a very light signal in the thalamus in the central nervous system after extended exposure times. Alpha-2C mRNA labeling was primarily observed in the olfactory bulb, cerebral cortex, islands of Calleja, striatum, hippocampal formation, cerebellar cortex, and dorsal root ganglia. Labeling patterns disappeared when excess unlabeled probes were added to their respective radiolabeled probes, or when sense probes were employed. When a hybrid antisense probe homologous to all three alpha-2 probes was used, labeling patterns also disappeared. The present study therefore justifies the pharmacological subclassification of alpha-2 receptors by providing anatomical evidence for specific and selective cell groups in the rat central nervous system containing mRNA for three alpha-2 receptor subtypes.

The depressive effect of intrathecal clonidine on the spinal flexor reflex is enhanced after sciatic nerve section in rats

The effect of intrathecal (i.t.) alpha 2-adrenoceptor agonist, clonidine, on the spinal nociceptive flexor reflex was studied in decerebrate, spinalized, unanesthetized rats with intact sciatic nerves or in rats in which the sciatic nerve had been ipsilaterally sectioned. In rats with intact nerves i.t. clonidine caused a dose-dependent biphasic effect on flexor reflex excitability. At low dose (10 ng) the effect of clonidine was purely facilitatory, whereas with 50-100 ng clonidine the initial facilitation was often followed by reflex depression. Long-lasting, strong reflex depression was observed after i.t. injection of high doses of clonidine (1 and 10 micrograms). Four to 18 days after sciatic nerve section, the depressive effect of clonidine on the flexor reflex was dramatically enhanced. Depression was frequently observed already with doses of 5 and 10 ng, and maximal depression was reached at 100 ng and 1 micrograms in axotomized rats. The facilitatory effect of low doses of clonidine on the reflex was also observed, although somewhat less frequently than in normals. The depressive effect of clonidine on the flexor reflex was reversed by the selective alpha 2-receptor antagonist, atipamezole (20 micrograms, i.t.), in rats with both intact and sectioned sciatic nerves. The present results revealed an increased sensitivity and effectiveness of the depression of spinal reflex mechanisms by i.t. clonidine after sciatic nerve section, which is opposite to the decreased sensitivity to i.t. morphine after axotomy that we observed previously.(ABSTRACT TRUNCATED AT 250 WORDS)

Further evidence for the existence of two forms of alpha 2B-adrenoceptors in rat

Analysis of saturation isotherms of the novel alpha 2-adrenoceptor antagonist radioligand [3H]-MK 912 revealed that the ligand labelled a homogenous population of alpha 2B-adrenoceptors in the neonatal rat lung with a Kd of 0.77 nM and a Bmax of 231 fmol/mg protein. In rat kidney, combined saturation and competition experiments, using [3H]-MK 912 and the alpha 2A-adrenoceptor selective drug guanfacine, revealed that approximately 81% of the sites labelled by [3H]-MK 912 were alpha 2B-adrenoceptors and approximately 19% alpha 2A-adrenoceptors; the Kds of [3H]-MK 912 being 1.1 and 2.0 nM and the Bmax 134 and 33 fmol/mg protein, respectively. The kidney alpha 2B-adrenoceptors were studied separately by using approximately 1.5 nM [3H]-MK 912 in the presence of 0.32 microM guanfacine, the latter which blocked ligand binding to alpha 2A-adrenoceptors completely. Analysis of drug competition curves obtained during these conditions revealed that 18 out of 20 different agonists and antagonists yielded steep and uniphasic competition curves which modelled best into one site fits. However, both guanoxabenz and LT 11 appeared to inhibit [3H]-MK 912 binding at two sites; the Kds of guanoxabenz differing approximately 120-fold and that of LT 11 differing approximately 35-fold for the two sites. Moreover, the addition of mutual fixed concentrations of either 20 microM guanoxabenz or 20 microM LT 11 completely prevented the binding of the other compound to its high affinity site, indicating that both compounds labelled the same site with the high affinity. The analysis indicated that 29% of the sites were of high and 71% of low affinity. However, in the neonatal rat lung guanoxabenz and LT 11 (as well as 15 other compounds) yielded competition curves which modelled only into one site fits. The Kds obtained in the lung correlated well with the Kds obtained in the kidney for alpha 2B-adrenoceptors; for guanoxabenz and LT 11 the values from the lung were close to those determined in the kidney for the low affinity site for guanoxabenz and LT 11. Moreover, when the rat RNG alpha 2B-adrenoceptor was expressed in COS-7 cells and its binding properties tested using [3H]-MK 912 binding, guanoxabenz, LT 11 as well as a number of other drugs inhibited the ligand binding at a single alpha 2-adrenoceptor site; the drug Kds being practically the same as those found for the neonatal rat lung. It is suggested that rat alpha 2B-adrenoceptors may exist in two forms: alpha 2B1 and alpha 2B2.(ABSTRACT TRUNCATED AT 400 WORDS)