[3H]p-Aminoclonidine binding to multiple alpha 2-adrenoceptor sites in homogenates of cat frontal cortex and cat spinal cord

Locomotor-activated neurons of the cat. II. Noradrenergic innervation and colocalization with NEα 1a or NEα 2b receptors in the thoraco-lumbar spinal cord

Norepinephrine (NE) is a strong modulator and/or activator of spinal locomotor networks. Thus noradrenergic fibers likely contact neurons involved in generating locomotion. The aim of the present study was to investigate the noradrenergic innervation of functionally related, locomotor-activated neurons within the thoraco-lumbar spinal cord. This was accomplished by immunohistochemical colocalization of noradrenergic fibers using dopamine-β-hydroxylase or NEα(1A) and NEα(2B) receptors with cells expressing the c-fos gene activity-dependent marker Fos. Experiments were performed on paralyzed, precollicular-postmamillary decerebrate cats, in which locomotion was induced by electrical stimulation of the mesencephalic locomotor region. The majority of Fos labeled neurons, especially abundant in laminae VII and VIII throughout the thoraco-lumbar (T13-L7) region of locomotor animals, showed close contacts with multiple noradrenergic boutons. A small percentage (10-40%) of Fos neurons in the T7-L7 segments showed colocalization with NEα(1A) receptors. In contrast, NEα(2B) receptor immunoreactivity was observed in 70-90% of Fos cells, with no obvious rostrocaudal gradient. In comparison with results obtained from our previous study on the same animals, a significantly smaller proportion of Fos labeled neurons were innervated by noradrenergic than serotonergic fibers, with significant differences observed for laminae VII and VIII in some segments. In lamina VII of the lumbar segments, the degree of monoaminergic receptor subtype/Fos colocalization examined statistically generally fell into the following order: NEα(2B) = 5-HT(2A) ≥ 5-HT(7) = 5-HT(1A) > NEα(1A). These results suggest that noradrenergic modulation of locomotion involves NEα(1A)/NEα(2B) receptors on noradrenergic-innervated locomotor-activated neurons within laminae VII and VIII of thoraco-lumbar segments. Further study of the functional role of these receptors in locomotion is warranted.

Distribution of alpha2-adrenoceptor mRNAs in the rat lumbar spinal cord in normal and axotomized rats

Using riboprobe in situ hybridization we have studied the distribution of alpha2A-, alpha2B- and alpha2c-adrenoceptor (AR) mRNAs in the lumbar spinal cord in normal rats and after peripheral axotomy. A strong alpha2A- and alpha2C AR mRNA labelling was found in motoneurons and other cells in the ventral horns. In the dorsal horns strong alpha2A-AR mRNA labelling was found in all layers and in the lateral spinal nucleus, whereas alpha2C-AR mRNA was found in lower numbers of cells in various layers. The alpha2B-AR mRNA signal was only detected in some small cells superficially in the dorsal horn. With regard to axotomy only a marginal effect was observed for alpha2C-AR mRNA in the ventral horn. The results suggest that alpha2-ARs are involved both in sensory and motor processing.

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

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

Pharmacological characterization of [3H]idazoxan, [3H]RX821002 and p-[125I]iodoclonidine binding to alpha 2-adrenoceptors in rat cerebral cortical membranes

Binding characteristics of alpha 2-adrenoceptors in rat cerebral cortical membranes were compared using the antagonist radioligands [3H]idazoxan, [3H]2-(2-methoxy-1,4-benzodioxan-2-yl)-2-imidazoline ([3H]RX821002), and the partial agonist radioligand [125I]2-[2,6-(dichloro-4-iodophenyl)imino]imidazoline ([125I]iodoclonidine). With [3H]RX821002 and alpha 2-adrenoceptor subtype-selective competitors, both alpha 2A/D- and alpha 2C-adrenoceptor subtypes were detected, suggesting rat cortical membranes contain approximately 90% alpha 2A/D-adrenoceptor subtype and 10% alpha 2C-adrenoceptor subtype. Only alpha 2A/D-adrenoceptors were detected with [3H]idazoxan and [125I]iodoclonidine. All three radioligands bound to a single high affinity site (Kd = 0.3-1.6 nM). However, the densities of sites labeled by [3H]idazoxan and [125I]iodoclonidine were 50% greater than the density labeled by [3H]RX821002, likely representing non-adrenoceptor binding sites. The density of [125I]iodoclonidine binding sites in glycylglycine buffer was similar to that labeled by [3H]RX821002. These results suggest that: (1) alpha 2A/D-adrenoceptors are the predominant subtype in rat cerebral cortex, (2) demonstrate that the small number of alpha 2C-adrenoceptors in this tissue can be detected using prazosin to displace [3H]RX821002 binding, and (3) non-adrenoceptor binding with [125I]iodoclonidine can be minimized with the use of glycylglycine buffer.

Supraspinal and spinal monoamine-modified function and the expression of opioid antinociception

Opioids are highly valuable clinical agents for the treatment of pain which are thought to act both at the spinal and supraspinal level. During the course of their actions, they have complex interactions with monoamine systems. These include 5-hydroxytryptamine (5-HT) and noradrenaline (NA), so this topic is discussed using these two transmitter systems, their locations and receptor sub-types, as prime candidates for modulating nociceptive and antinociceptive processes. Several classes of 5-HT receptors, as well as α(2)-adrenoceptors, appear to be clearly involved in antinociception and the functions of systems carrying these receptors may be modified using psychotropic agents. In particular, some antidepressants may acutely augment opioid antinociception and this property may be exploited to delay the onset of opioid tolerance in the sub-acute situation.

Receptor changes in the spinal cord of sheep associated with exposure to chronic pain

There is evidence that post-injury hypersensitivity is partly due to changes in the central nervous system. Sheep with foot rot were used to investigate the effect of chronic pain on some receptors thought to be involved in spinal nociceptive processing systems (alpha 2 adrenoceptor and mu and delta opioid receptors). Saturation binding studies showed a variable distribution of [3H] clonidine (alpha 2 adrenoceptor agonist) in the spinal cord of normal sheep. The number of receptors (Bmax) present in areas thought to be involved in nociceptive processing, laminae I and II and lamina X, increased to 131% and 169% of control sheep values respectively in animals exposed to chronic pain. The affinity of the receptors (KD), however, remained unchanged at approximately 2 nM. There was less [3H]DAGO (mu opioid agonist) and [3H]DPDPE (delta opioid agonist) binding in the sheep spinal cord. Both opioid receptor types being mainly located in the superficial dorsal horn. The [3H]DPDPE binding was unchanged in the sheep with foot rot, whilst the number (Bmax), but not the affinity, of the [3H]DAGO binding sites increased in laminae I and II in lame animals to 130% of the control sheep values. Hence, in animals in chronic pain, the number of alpha 2 adrenoceptors and mu opioid receptors increased mainly in areas of the sheep spinal cord associated with nociception.

Cholecystokinin-8 suppressed 3H-etorphine binding to rat brain opiate receptors

Radio receptor assay (RRA) was adopted to analyse the influence of CCK-8 on 3H-etorphine binding to opiate receptors in rat brain synaptosomal membranes (P2). In the competition experiment CCK-8 (1pM to 1 microM) suppressed the binding of 3H-etorphine. This effect was completely reversed by proglumide at 1 microM. Rosenthal analysis for saturation revealed two populations of 3H-etorphine binding sites. CCK-8 (1pM to 1 microM) inhibited 3H-etorphine binding to the high affinity sites by an increase in Kd (up to +235%) and decrease in Bmax (up to -80%) without significant changes in the Kd and Bmax of the low affinity sites. This effect of CCK-8 (10nM) was also completely reversed by proglumide at 1 microM. Unsulfated CCK-8 (100pM to 1 microM) produced only a slight increase in Kd of the high affinity sites (+64%) without affecting Bmax. The results suggest that CCK-8 might be capable of suppressing the high affinity opioid binding sites via the activation of CCK receptor.

Dipropylacetate-induced shaking behaviour in the rat: a role of spinal alpha 2-adrenoceptors

The role of spinal adrenoceptors in di-n-propylacetate (DPA)-induced shaking behaviour was studied. The alpha 2-agonist p-aminoclonidine in a dose of 5 micrograms was found to suppress the DPA-induced body shakes when injected intrathecally but not when given intraventricularly. There was an enhancement of DPA-induced body shakes after the intrathecal injection of idazoxan while only a slight decrease was found after the intrathecal injection of prazosin. Intrathecal injection of idazoxan but not of prazosin proved to be effective to reverse the DPA-induced body shakes suppressed by p-aminoclonidine. Although the difference in effectiveness after intrathecal and intravenous injection was less than has been described for opiates there are several arguments that indicate an effect confined to the spinal cord. The present results further evidence the notion that the DPA-induced shaking behaviour shares common mechanisms with some of the morphine abstinence symptoms. The data indicate that spinal alpha 2-adrenoceptors are at least partly involved in the suppressive effect of p-aminoclonidine on DPA-induced shaking behaviour.

Binding of [3H]prazosin and [3H]p-aminoclonidine to alpha-adrenoceptors in rat spinal cord

alpha-Adrenoceptors in spinal cord appear to play a role in a number of physiologic processes including the control of blood pressure, pain and motor function. In order to evaluate more clearly these potential roles, the characteristics of binding of [3H]prazosin ([3H]PRZ) to spinal alpha 1 adrenoceptors and [3H]p-aminoclonidine ([3H]PAC) to spinal alpha 2 adrenoceptors were determined. Binding of each ligand to their respective adrenoceptors was saturable and Scatchard analysis revealed binding of each to a single class of adrenoceptors with characteristics of [3H]PRZ binding of Bmax = 78 fmol/mg protein and Kd = 0.75 nM and [3H]PAC binding Bmax = 70 fmol/mg protein and Kd = 1.39 nM. Whereas [3H]PRZ specific binding (Bmax) was unaltered by guanine nucleotides. [3H]PAC binding was increased with addition of 10 microM guanosine triphosphate (GTP) (P less than 0.05) and decreased with either 50 microM GTP or guanyl-5'-yl-imidodiphosphate [Gpp(NH)p] (P less than 0.01). Competition for specific [3H]PRZ and [3H]PAC binding by various alpha 1 and alpha 2 adrenoceptor agonists and antagonists of known pharmacologic activity revealed that [3H]PRZ defines alpha 1 adrenoceptors (Ki = 2.1 nM for prazosin vs 4300 nM for yohimbine) and [3H]PAC defines alpha 2 adrenoceptors (Ki = 1.06 nM for yohimbine vs 15480 nM for prazosin). Regional spinal cord studies demonstrated that dorsal spinal cord in the lumbar region contains the highest density of both [3H]PRZ (Bmax = 93 +/- 14 fmol/mg protein) and [3H]PAC (Bmax = 101 +/- 6 fmol/mg protein) binding. In contrast, lowest binding was evident in thoracic cord with equal levels in both dorsal and ventral regions (Bmax = 44-48 fmol/mg protein). The regional distribution of both alpha 1 and alpha 2 adrenoceptors in spinal cord compares to the localization previously classified functionally utilizing various pharmacological agonists and antagonists at norepinephrine receptors.

Specific [3H]UK 14,304 binding in human cortex occurs at multiple high affinity states with alpha 2-adrenergic selectivity and differing affinities for GTP

[3H]UK 14,034 is a full agonist at alpha 2-adrenergic receptors. Although the characteristics of the binding of the partial alpha 2-adrenergic agonists in postmortem human brain were known, the binding of [3H]UK 14,304 had not been studied in this tissue. Multi-site binding of this radiolabel had been reported in other tissues and guanosine triphosphate (GTP) had been shown to reduce [3H]UK 14,304 binding. We now report that [3H]UK 14,304 labels at least 2 specific binding sites in human brain that both have the characteristics of an alpha 2-adrenergic binding site. GTP decreases agonist binding at both of these sites, but with different potencies at each site.

The effect of unilateral dorsal root ganglionectomies or ventral rhizotomies on alpha 2-adrenoceptor binding to, and the substance P, enkephalin, and neurotensin content of, the cat lumbar spinal cord

The density of alpha 2-adrenoceptor binding sites and the content of substance P, enkephalins, and neurotensin were determined in quadrants of the lumbar spinal enlargement of control cats and of cats upon which either unilateral dorsal root ganglionectomies or unilateral ventral rhizotomies had been performed. The performance of unilateral dorsal root ganglionectomies resulted in a significant decrease (45-55%) of substance P content in the ipsilateral dorsal horn 7 and 21 days postoperatively. The concentration of alpha 2-adrenoceptor binding sites ([3H]rauwolscine Bmax) in the ipsilateral dorsal horn was consistently and significantly decreased at these same postganglionectomy times (20% reduced relative to the contralateral dorsal horn). Enkephalin content 7 and 21 days after ganglionectomies was not significantly different from control, whereas the neurotensin content of the ipsilateral dorsal horn was significantly increased in the 21-day survival cats. The performance of unilateral ventral rhizotomies did not produce any statistically significant changes in the density of alpha 2-adrenoceptor binding sites or in the substance P or enkephalin content of any spinal quadrant. The neurotensin content of both the ipsilateral dorsal and ipsilateral ventral quadrants of the ventral rhizotomized cats was significantly increased. The significant decrease of alpha 2-adrenoceptor binding site concentration in the ipsilateral dorsal horn after unilateral dorsal root ganglionectomies suggests that approximately 20% of the alpha 2-adrenoceptors present within the cat lumbar spinal dorsal gray are located on the axons or terminals of primary sensory afferents. Consistent with this interpretation of the ganglionectomy results, we found significant levels of saturable [3H]rauwolscine binding to homogenates of the cat L4-Sl spinal dorsal root ganglia. Because alpha 2-adrenoceptor binding sites in the ipsilateral ventral lumbar spinal gray were not significantly reduced after unilateral ventral rhizotomies, our results provide no evidence for the location of alpha 2-adrenoceptor on lumbar spinal motoneurons.

Intrathecal 6-hydroxydopamine or cervical spinal hemisection reduces norepinephrine content, but not the density of alpha 2-adrenoceptors, in the cat lumbar spinal enlargement

The effect of the intrathecal administration of the catecholaminergic neurotoxin 6-hydroxydopamine, or of hemisection of the spinal cord at the Cl level, on the density of alpha 2-adrenoceptors and on the norepinephrine, dopamine, and serotonin content in the cat lumbar spinal enlargement was determined 2, 7 or 21 days after performance of each type of lesion. The intrathecal administration of 6-hydroxydopamine produced a time-dependent reduction of norepinephrine content in the cat lumbar spinal enlargement (95% reduction at 21 days) without significantly altering the serotonin content in this same tissue of the same cats. The dopamine content of the dorsal horn was not changed significantly, whereas ventral horn dopamine content was depleted after intrathecal 6-hydroxydopamine. alpha 2-Adrenoceptor binding site density was not significantly different from control either 2 or 21 days after 6-hydroxydopamine, but was increased significantly (50%) over the control density 7 days after 6-hydroxydopamine. Hemisection of the cervical spinal cord produced a bilateral 40-60% reduction of norepinephrine content in both the dorsal and ventral horns of the cat lumbar spinal enlargement 7 and 21 days later. Cervical hemisection did not significantly alter the alpha 2-adrenoceptor binding site density in these same cats either 2, 7, or 21 days after performance of the lesion. It is concluded that alpha 2-adrenoceptors located on the terminals of descending noradrenergic or other spinopetal fibers do not represent a significant fraction of the total population of alpha 2-adrenoceptors present in the dorsal or ventral cat lumbar enlargement.

Clonidine binds to imidazole binding sites as well as alpha 2-adrenoceptors in the ventrolateral medulla

Binding sites labeled by [3H]p-aminoclonidine ([3H]PAC) were characterized in bovine brain membranes prepared from the ventrolateral medulla, the probable site of the antihypertensive action of clonidine and analogs. Comparison was made with [3H]PAC binding to membranes prepared from frontal cortex, which has been studied extensively. Saturation binding isotherms for [3H]PAC were similar in the two regions, although Bmax values were approximately two-fold lower in ventrolateral medulla relative to frontal cortex. Norepinephrine and other phenylethylamines displaced [3H]PAC from a maximum of 70% of the total sites in the ventrolateral medulla. The remaining 30% were norepinephrine-insensitive, non-adrenoceptor sites which displayed high affinity for imidazole compounds. Ligand selectivity differed markedly between ventrolateral medulla and frontal cortex, since some imidazole compounds which potently inhibited [3H]PAC binding in the ventrolateral medulla had no effect in frontal cortex. Imidazole binding sites may mediate, in part, the hypotensive action of clonidine and other imidazole compounds in the ventrolateral medulla. These sites may also participate in the functions of a putative endogenous clonidine-like substance.

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

The lateral reticular nucleus (LRN) and locus coeruleus-subcoeruleus (LC/SC), brainstem structures which overlap the A1 and A6 noradrenergic nuclei respectively, have been implicated in descending modulation of spinal nociceptive transmission. The present studies were designed to examine the role of norepinephrine (NE) in the mediation of inhibition of the nociceptive tail flick reflex produced by focal electrical stimulation in the LRN and LC/SC. Spinal NE was depleted by intrathecal administration of 6-hydroxydopamine (6-OHDA; 20 micrograms) and the threshold electrical stimulation in the LRN and the LC/SC necessary to inhibit the tail flick reflex in lightly pentobarbital-anesthetized rats was determined 9 and 14 days later. Despite a significant depletion (greater than 85%) of lumbar spinal cord NE content, there was no significant change in the tail flick inhibitory stimulation thresholds in the LRN or LC/SC. NE depletion did, however, potentiate the elevation in the inhibitory stimulation threshold in the LRN produced by intrathecal administration of the alpha 2-adrenoceptor antagonist, yohimbine, suggesting that upregulation of spinal adrenoceptors had occurred following 6-OHDA treatment. Adrenoceptor up-regulation was examined quantitatively by characterizing the dose-dependent antinociceptive potency of the selective alpha 2-adrenoceptor agonist clonidine 3, 7, 10, and 14 days following 6-OHDA administration, and analysis of [3H]rauwolscine binding to lumbar spinal cord 9 days following administration of the neurotoxin. The development of supersensitivity, defined as the leftward parallel shift of the dose-response curves for clonidine administered intrathecally, corresponded to the time course of NE depletion following 6-OHDA treatment on the days tested. Binding of [3H]rauwolscine to lumbar spinal cord revealed an elevation in the estimated Bmax without a change in the estimated Kd of the high affinity binding component 9 days following 6-OHDA administration. This study demonstrates that spinal adrenoceptor denervation supersensitivity develops rapidly following intrathecal administration of 6-OHDA and compensates for the selective destruction of spinal noradrenergic nerve terminals. Thus, the absence of effect of NE depletion on the tail flick inhibitory stimulation threshold in the LRN and the LC/SC does not argue against the hypothesis that spinopetal NE-containing neurons in these brainstem loci are involved in modulation of spinal nociceptive transmission.

Characterization of [3H]rauwolscine binding to alpha 2-adrenoceptor sites in the lumbar spinal cord of the cat: comparison to such binding sites in the cat frontal cerebral cortex

The binding of the selective alpha 2-adrenoceptor antagonist radioligand [3H]rauwolscine ([3H]RAUW) to homogenates of cat frontal cerebral cortex and cat lumbar spinal cord was investigated. Experiments were performed at 20 degrees C in 50 mM Tris HCl/l mM Na2EDTA buffer (pH 6.9 at 20 degrees C). At this temperature, specific [3H]RAUW binding, defined as the difference between the amount of [3H]RAUW bound in the absence and presence of 1 microM rauwolscine or 1 microM rauwolscine or 1 microM yohimbine, reaches equilibrium values by approximately 60 min and is reversible with a mean t1/2 of dissociation of 15 min in cortex and 20 min in spinal cord. The kinetically determined Kd of [3H]RAUW (mean K-1/mean K1) was 0.59 nM and 1.68 nM in cortex and spinal cord, respectively. The results of equilibrium saturation experiments, routinely performed at [3H]RAUW concentrations between 0.1 nM and 6.0 nM, indicate that [3H]RAUW binds to saturable sites in both CNS regions of the cat. Scatchard plots of saturation isotherm data were consistently linear and the mean Kd value determined from 10 such experiments was 0.72 nM in frontal cortex and 0.82 nM in lumbar spinal cord. A mean Bmax value of 230 fmol/mg protein was determined for saturable [3H]RAUW binding sites in the cat frontal cortex. In teh cat lumbar spinal gray, a mean Bmax value for saturable [3H]RAUW binding sites of 75 fmol/mg protein was obtained. Saturable [3H]RAUW binding sites in the cat lumbar spinal gray are present at apparently equal density in dorsal and ventral horns. Inhibition experiments, performed at 0.2 nM or 0.4 nM [3H]RAUW, indicate that the binding sites labeled by [3H] RAUW possess a pharmacology characteristic of alpha-adrenoceptors. Thus, rauwolscine, yohimbine, and phentolamine compete for specific [3H]RAUW binding with high affinity and are much more potent inhibitors than corynanthine, prazosin, and propranolol. Mean Hill coefficients, calculated from logit-log plots of competition data, were close to one for all antagonists examined. L-Epinephrine and L-norepinephrine were 15-20 times more potent inhibitors of specific [3H]RAUW binding than were their corresponding D-isomers. The agonist inhibitor potency series: p-aminoclonidine = clonidine = L-epinephrine greater than L-norepinephrine much greater than isoproterenol, is that expected of alpha 2-adrenoceptor sites. Mean Hill coefficients efficients for all agonists were considerably less than one.(ABSTRACT TRUNCATED AT 400 WORDS)

Pharmacology of spinal adrenergic systems which modulate spinal nociceptive processing

Spinopetal pathways may be activated by a variety of brainstem manipulations including microinjections of morphine which are known to modulate spinal nociceptive processing. Based on the ability of these manipulations to release spinal noradrenalin; the ability to reverse the antinociceptive effects by intrathecal adrenergic antagonists and the fact that intrathecal injections of noradrenalin mimic the antinociceptive effect, it appears that the descending modulation may be mediated by descending noradrenergic systems. Examination of the spinal receptor systems with intrathecally administered agents indicates that spinal alpha, but not beta adrenergic receptor agonists produce a powerful analgesia as measured on a variety of reflex and operant measures in mouse, rat, cat, primate and man. On the basis of agonist and antagonist structure-activity relationships it appears that a significant effect can be produced in the absence of any detectable effect on motor function by the occupation of spinal alpha 2 receptors. Distinguishable alpha 1 receptors also appear "analgetically-coupled," but their effects are uniformly contaminated by signs of cutaneous hyperreflexia at doses required to produce analgesia. The ordering of potency with which intrathecal adrenergic antagonists reverse the effects of intrathecal noradrenalin is indistinguishable from that of the reversal by these intrathecal agents of the antinociceptive effects evoked by brainstem morphine. This suggests that the population of spinal receptors acted upon by exogenously administered adrenergic agonists and endogenously released noradrenaline have indistinguishable characteristics.