beta-Adrenergic receptor involvement in 6-hydroxydopamine-induced supersensitivity in rat cerebral cortex

Activating cAMP/PKA signaling in skeletal muscle suppresses the ubiquitin-proteasome-dependent proteolysis: implications for sympathetic regulation

Although we have recently demonstrated that plasma catecholamines induce antiproteolytic effects on skeletal muscle (Graça FA, Gonçalves DAP, Silveira WA, Lira EC, Chaves VE, Zanon NM, Garófalo MAR, Kettelhut IC, Navegantes LCC. Am J Physiol Endocrinol Metab. 305: E1483-E1494, 2013), the role of the muscle sympathetic innervation and, more specifically, norepinephrine (NE) in regulating the ubiquitin (Ub)-proteasome system (UPS) remains unknown. Based on previous findings that chemical sympathectomy acutely reduces UPS activity, we hypothesized that muscle NE depletion induces adrenergic supersensitivity in rat skeletal muscles. We report that surgical sympathetic denervation (SDEN), a condition in which only muscle NE from both hindlimbs is depleted, transiently reduced the overall proteolysis and the UPS activity (∼25%) in both soleus and extensor digitorum longus muscles. This antiproteolytic response was accompanied by increased activity of adenylyl cyclase (112%), levels of cyclic adenosine monophosphate (cAMP; 191%), and the serine phosphorylation of cAMP response element-binding protein (32%). In extensor digitorum longus from normal rats, NE (10(-4) M) in vitro increased the levels of cAMP (115%) and the serine phosphorylation of both cAMP response element-binding protein (2.7-fold) and forkhead box class O1 transcription factor. Similar effects were observed in C2C12 cells incubated with forskolin (10 μM). In parallel, NE significantly reduced the basal UPS (21%) activity and the mRNA levels of atrophy-related Ub-ligases. Similar responses were observed in isolated muscles exposed to 6-BNZ-cAMP (500 μM), a specific PKA activator. The phosphorylation levels of Akt were not altered by SDEN, NE, forskolin or 6-BNZ-cAMP. Our results demonstrate that SDEN induces muscle adrenergic supersensitivity for cAMP leading to the suppression of UPS, and that the suppressive effects of NE on UPS activity and expression of Ub-ligases can be mediated by the activation of cAMP/PKA signaling, with the inhibition of forkhead box class O1 transcription factor.

Sensation seeking: A comparative approach to a human trait

A comparative method of studying the biological bases of personality compares human trait dimensions with likely animal models in terms of genetic determination and common biological correlates. The approach is applied to the trait of sensation seeking, which is defined on the human level by a questionnaire, reports of experience, and observations of behavior, and on the animal level by general activity, behavior in novel situations, and certain types of naturalistic behavior in animal colonies. Moderately high genetic determination has been found for human sensation seeking, and marked strain differences in rodents have been found in open-field behavior that may be related to basic differences in brain neurochemistry. Agonistic and sociable behaviors in both animals and humans and the trait measure of sensation seeking in humans have been related to certain common biological correlates such as gonadal hormones, monoamine oxidase (MAO), and augmenting of the cortical evoked potential.

The monoamine systems in the rodent brain are involved in general activity, exploratory behavior, emotionality, socialization, dominance, sexual and consummately behaviors, and intracranial self-stimulation. Preliminary studies have related norepinephrine and enzymes involved in its production and degradation to human sensation seeking. A model is suggested that relates mood, behavioral activity, sociability, and clinical states to activity of the central catecholamine neurotransmitters and to neuroregulators and other transmitters that act in opposite ways on behavior or stabilize activity in the arousal systems. Stimulation and behavioral activity act on the catecholamine systems in a brain–behavior feedback loop. At optimal levels of catecholamine systems activity (CSA) mood is positive and activity and sociability are adaptive. At very low or very high levels of CSA mood is dysphoric, activity is restricted or stereotyped, and the organism is unsocial or aggressively antisocial. Novelty, in the absence of threat, may be rewarding through activation of noradrenergic neurons.

Neuromodulatory transmitter systems in the cortex and their role in cortical plasticity

Cortical neuromodulatory transmitter systems refer to those classical neurotransmitters such as acetylcholine and monoamines, which share a number of common features. For instance, their centers are located in subcortical regions and send long projection axons to innervate the cortex. The same transmitter can either excite or inhibit cortical neurons depending on the composition of postsynaptic transmitter receptor subtypes. The overall functions of these transmitters are believed to serve as chemical bases of arousal, attention and motivation. The anatomy and physiology of neuromodulatory transmitter systems and their innervations in the cerebral cortex have been well characterized. In addition, ample evidence is available indicating that neuromodulatory transmitters also play roles in development and plasticity of the cortex. In this article, the anatomical organization and physiological function of each of the following neuromodulatory transmitters, acetylcholine, noradrenaline, serotonin, dopamine, and histamine, in the cortex will be described. The involvement of these transmitters in cortical plasticity will then be discussed. Available data suggest that neuromodulatory transmitters can modulate the excitability of cortical neurons, enhance the signal-to-noise ratio of cortical responses, and modify the threshold for activity-dependent synaptic modifications. Synaptic transmissions of these neuromodulatory transmitters are mediated via numerous subtype receptors, which are linked to multiple signal transduction mechanisms. Among the neuromodulatory transmitter receptor subtypes, cholinergic M(1), noradrenergic beta(1) and serotonergic 5-HT(2C) receptors appear to be more important than other receptor subtypes for cortical plasticity. In general, the contribution of neuromodulatory transmitter systems to cortical plasticity may be made through a facilitation of NMDA receptor-gated processes.

Long-term stress degenerates, but imipramine regenerates, noradrenergic axons in the rat cerebral cortex

Exposed to a forced walking stress for 2 weeks, some rats became persistently inactive (depression-model rats), whereas others gradually recovered from exhaustion (spontaneous recovery rats). We also studied rats exposed to short-term stress, rats without stress, and the model rats treated with imipramine or saline. We examined the density of noradrenergic axons in the frontal cortex using retrograde labeling of the locus coeruleus with horseradish peroxidase injected into the cortex and immunohistochemical staining of cortical axons with dopamine beta-hydroxylase antiserum. The density was significantly lower in the depression-model rats, but tended to be higher in the recovery rats and short-term stressed rats. Chronic treatment with imipramine significantly increased the density in the model rats. There was also a correlation between the density of noradrenergic axons and the recovery rate of activity. Our results suggest that cortical noradrenergic degeneration is involved in the pathogenesis of depression.

Involvement of nerve growth factor in visual cortex plasticity

The physiological role of nerve growth factor (NGF), the prototype member of the neurotrophin family, has been widely studied. NGF has been shown to promote survival, sprouting and differentiation of sympathetic ganglion cells and sensory neurons in the peripheral nervous system; it has also been shown to support survival and regeneration of cholinergic neurons in the central nervous system. Recent evidence indicates that NGF is also involved in the neuronal plasticity of the visual cortex. Exogenous supplies of NGF have been shown to interfere with normal processes underlying activity- and age-dependent synaptic modifications in both developing and adult visual cortex. In parallel to these physiological effects, numerous neuronal markers in the visual cortex have been found to be influenced by NGF. Several proposals have been introduced to explain the physiological role of NGF in visual cortex plasticity. Although the mechanisms underlying NGF effects in the visual cortex are still under active investigation, current evidence implies that NGF, and perhaps other neurotrophins as well, may be useful for preventing or correcting inappropriate or anomalous connections in the visual cortex, and thus for treating visual dysfunctions such as amblyopia and strabismus.

Effect of chemical sympathectomy on (-)-isoprenaline-induced changes in cardiac beta-adrenoceptor subtypes in the guinea-pig and rat

1. Quantitative autoradiography was used to determine beta-adrenoceptor densities in cardiac regions of guinea-pigs and rats after chemical sympathectomy with 6-hydroxydopamine, and to examine how chemical sympathectomy affected beta-adrenoceptor changes following infusion of (-)-isoprenaline (400 micrograms kg-1 hr-1, 7 days). 2. Seven days after 6-hydroxydopamine (100 mg kg-1, i.v.), cardiac tissue levels of noradrenaline were reduced by 94.0 +/- 3.5% (guinea-pig) and 86.0 +/- 7.0% (rat). The blood pressure increase in rats to tyramine (0.5 mg, i.v.) was reduced from 118 mmHg in controls to 4.4 mmHg in 6-hydroxydopamine-treated animals. 3. There were no changes 7 and 14 days after 6-hydroxydopamine treatment in total, beta 1-and beta 2-adrenoceptor density in the atrioventricular conducting system and atrial and ventricular myocardium in both species. 4. In control animals, (-)-isoprenaline infusion produced selective reductions in beta 2-adrenoceptor density, whilst beta 1-adrenoceptor density remained unchanged. 5. In 6-hydroxydopamine treated guinea-pigs or rats, (-)-isoprenaline infusion caused no change in beta 1-adrenoceptors except in the right bundle branch whilst beta 2-adrenoceptors were reduced in the atrioventricular conducting system (atrioventricular node, bundle of His, right and left bundle branches) and myocardium (interventricular septum and atria). 6. The differential effect of (-)-isoprenaline on beta 1- and beta 2-adrenoceptors is not therefore due to the occupation of beta 1-adrenoceptors by noradrenaline or to prior down-regulation of beta 1-adrenoceptors by noradrenaline, since it persists after depletion of noradrenaline.

Effects of phospholipase A2 inhibitors on the antidepressant-induced axonal regeneration of noradrenergic locus coeruleus neurons

In previous experiments, we have shown that antidepressants such as desipramine (DMI) can induce axonal regeneration of noradrenergic locus coeruleus neurons. In this article, we suggest that phospholipase A2 is involved in the molecular mechanism of the antidepressant-induced regeneration of brain noradrenergic axons. The effects of the PLA2 inhibitors, mepacrine (MEP) or 4-bromphenacyl bromide (BPB), upon the DMI-induced regeneration of noradrenergic axons in the rat cerebral cortex were assessed by either histofluorescence or immunohistochemistry using an antibody to dopamine-beta-hydroxylase. Symmetrical sites of the frontal cortex in both hemispheres were pretreated with 6-hydroxydopamine (6-OHDA). Then, the same cortical site of one hemisphere was infused with DMI by means of osmotic minipumps for more than 2 weeks, while the corresponding cortical site of the other hemisphere was infused with DMI plus MEP or BPB. It was found that the PLA2 inhibitors could attenuate the DMI-induced regeneration of noradrenergic axons. Thus, if axonal retraction or degeneration of brain noradrenergic neurons is involved in the pathogenesis of clinical depression, elucidating the malfunction of the PLA2 systems may provide substantial evidence to aid in our understanding of the cause of depression at the molecular level.

Degeneration of locus coeruleus axons in stress-induced depression model

Antidepressants such as desipramine induce axonal regeneration of brain noradrenergic neurons. This novel action of antidepressants suggests the involvement of degeneration or retraction of brain noradrenergic axons in the pathophysiology of clinical depression. The present study was designed to further confirm this view in an animal model of stress-induced depression. The depression model was produced by exposing rats to prolonged forced walking stress. To see if axonal degeneration of noradrenergic neurons occurred in the depression model, the density of noradrenergic axons in the cerebral cortex was assessed by three different methods, antidromic stimulation technique, retrograde tracing with horseradish peroxidase and immunohistochemical staining with dopamine-beta-hydroxylase antiserum. These methods all assured of degenerative changes of noradrenergic axon terminals in the depression model. Furthermore, it was found that repeated treatments of the depression-model rats with imipramine could cause regeneration of cortical noradrenergic axons. These findings support the view that degeneration or retraction of noradrenergic axons is involved in the pathophysiology of depression.

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.

Effects of intracortical infusion of anticholinergic drugs on neuronal plasticity in kitten striate cortex

During a critical period of postnatal development the mammalian visual cortex is highly susceptible to experience-dependent alterations of neuronal response properties. These modifications are facilitated by the neuromodulators noradrenaline and acetylcholine. To identify the cholinergic mechanisms responsible for this facilitation, muscarinic and nicotinic antagonists were infused into the visual cortex of kittens while the animals were subject to monocular deprivation. Subsequently the ocular dominance of cortical cells was assessed by single-unit recording. Ocular dominance changes were suppressed by scopolamine and pirenzepine but not by gallamine, hexamethonium and mecamylamine. This blocking effect was concentration-dependent, and control experiments revealed that it was not due to suppression of neuronal responses to light. It is concluded from these results that acetylcholine facilitates neuronal plasticity in the visual cortex through mechanisms activated by muscarinic M1 receptors.

Regulation of beta 1-adrenergic receptor mRNA and ligand binding by antidepressant treatments and norepinephrine depletion in rat frontal cortex

The number of beta 1-adrenergic receptor (beta 1AR) binding sites is decreased by chronic antidepressant treatments, including electroconvulsive seizure (ECS) and imipramine, whereas administration of agents that deplete norepinephrine (NE) increases the number of beta 1AR binding sites in cerebral cortex. The present study was carried out to examine the influence of these treatments on levels of beta 1AR mRNA in frontal cortex to study the molecular mechanisms that underlie the regulation of beta 1ARs in brain. Levels of beta 1AR mRNA were measured by RNase protection analysis using a riboprobe derived from rat beta 1AR cDNA, and the levels of beta AR binding were measured using the nonselective ligand [3H]CGP-12177. Studies to verify the specificity of the RNase protection assay revealed that the distribution of beta 1AR mRNA was in agreement with the reported distribution of beta 1AR ligand binding: Levels of beta 1AR mRNA were highest in cerebral cortex or frontal cortex, intermediate in neostriatum, hippocampus, lung, and heart, and lowest in cerebellum, kidney, and liver. Chronic ECS treatment (once daily for 10 days) significantly decreased levels of beta AR ligand binding and resulted in a corresponding, time-dependent down-regulation of beta 1AR mRNA levels in frontal cortex. However, imipramine administration regulated levels of beta 1AR mRNA in a biphasic manner, with treatments for 7-14 days increasing and treatments for 18-21 days decreasing levels of beta 1AR mRNA in frontal cortex. In contrast, levels of [3H]CGP-12177 ligand binding were decreased at all time points examined (3-21 days).(ABSTRACT TRUNCATED AT 250 WORDS)

NE/DA interactions in prefrontal cortex and their possible roles as neuromodulators in schizophrenia

The monoaminergic innervation of the rat prefrontal cortex arises from well-defined mesencephalic nuclei, with noradrenergic (NE) neurons located in the locus coeruleus, dopaminergic (DA) neurons located in the ventral tegmental area, and serotonergic (5-HT) neurons originating in the raphe nuclei. Specific destruction of the NE bundle was found to induce morphological (i.e., sprouting) as well as metabolic (i.e., changes in rate of DA utilization) modifications of mesocortical DA neurons, suggesting that these two catecholaminergic systems have functional interactions within the prefrontal cortex. This was substantiated by experiments showing that DA afferents modulate the sensitivity of cortical post-synaptic beta-adrenergic receptors and that, reciprocally, NE neurons control the sensitivity of cortical D1 receptors. Behavioural and pharmacological data have further indicated that the stimulation of cortical alpha-1 adrenergic receptors inhibits cortical DA transmission at D1 receptors. Secondly, we have attempted to analyze how such interactions between neuromodulatory systems may be related to the development of mental diseases such as schizophrenia. On the basis of studies in the literature describing the effects produced by the ingestion of hallucinogenic drugs or data collected regarding REM sleep, it is postulated that two modes of brain functioning exist: analogical and cognitive. Each mode is characterized by differences in the relative activities of NE, DA and 5-HT neurons. At birth, during REM sleep, and following the ingestion of hallucinogens, the mode of brain functioning is essentially analogical; in contrast, both analogic and cognitive modes are postulated to coexist in the awake state. Oscillations between these two modes are under the control of monoaminergic systems on which an increase in cortical DA release favours the cognitive processing mode, whereas intermittent activations of NE neurons would switch the brain into the analogical mode of processing. It is proposed that schizophrenic patients with "positive" symptoms suffer from an abnormal preponderance of the analogical mode while awake, whereas "negative" symptoms are due to the excessive presence of the cognitive mode. Although pure biological deficits cannot be excluded, these dysfunctions could be related to the absence of particular environmental variables early in the development of these patients. This condition is probably required to establish normal regulatory control of monoaminergic neuronal activity.

Autoradiographic identification of D1 dopamine receptors labelled with [3H]dopamine: distribution, regulation and relationship to coupling

On the basis of experiments made on striatal membranes, Leff and Creese [Molec. Pharmac. (1985) 27, 184-192] have proposed that tritiated dopamine binds to a high-affinity agonist state of D1 dopamine receptors (D1h) which adopt this conformation when they are associated with the GTP-binding protein involved in the transduction process. Quantitative autoradiography was thus used to look for the distribution of these D1h sites in the rat brain and to compare it with that of D1 receptors labelled with [3H]7-chloro-8-hydroxy-3-methyl-1-phenyl-2,3,4,5-tetrahydro-1H-3-benz aze pine [( 3H]SCH23390), a D1 antagonist. The effects of unilateral 6-hydroxydopamine lesion of the ascending dopamine pathways on the density of [3H]dopamine D1h and [3H]SCH23390 binding sites in the striatum and the nucleus accumbens were also analysed. In the striatum, when D2 receptors were blocked by spiroperidol (20 nM), [3H]dopamine was found to bind specifically to dopamine receptors of the D1 type. Complementary experiments made with dopamine uptake blockers indicated that high-affinity dopamine uptake sites were not labelled by [3H]dopamine under our experimental conditions. The anatomical distribution of [3H]dopamine D1h binding sites was found to be markedly different from that of [3H]SCH23390 binding sites. This was particularly the case in the substantia nigra, some amygdaloid nuclei and the prefrontal cortex--structures in which the ratios between [3H]SCH23390 and [3H]dopamine binding sites were more than seven-fold higher than that observed in the striatum. [3H]SCH23390 binding was not significantly affected in either the striatum or the nucleus accumbens six weeks after a complete unilateral destruction of ascending dopamine pathways. In contrast, a marked decrease in [3H]dopamine D1h binding sites was found in both structures, but this effect was lower in the medioventral (-60%) than in the laterodorsal (-81%) part of the striatum, even though dopamine denervation was uniform throughout the structure. Preincubation of the sections with dopamine (0.5 microM) led to a partial recovery (+126%) in the lesioned striatum and an increase of [3H]dopamine labelling in the control striatum (+68%). This suggest that the presence of dopamine stabilizes the D1h state of D1 receptors. The absence or low amount of dopamine, either due to dopamine denervation or naturally occurring (prefrontal cortex), would then impair the [3H]dopamine D1h binding. In addition, a lower coupling of D1 receptors with adenylate cyclase was observed in the substantia nigra when compared to that in the striatum: this may explain the relatively weak [3H]dopamine binding in the substantia nigra.(ABSTRACT TRUNCATED AT 400 WORDS)

Dopamine receptor 'supersensitivity' occurring without receptor up-regulation

Unilateral 6-hydroxydopamine (6-OHDA)-induced lesions of the substantia nigra have been widely used to study various aspects of dopamine neurobiology, and to screen for antiparkinsonian drugs. This study examined the role of receptor alterations in the pharmacological supersensitivity seen in response to lesioning of central dopamine pathways in rats by intracisternal (IC) administration of 6-OHDA (200 micrograms), as well as by bilateral (BIL) or unilateral (UNI) infusion of 6-OHDA into the substantia nigra (8 micrograms/side). Both IC and BIL lesions resulted in permanent decreases in dopamine concentration in the striatum, the major terminal projection from the substantia nigra. When challenged with apomorphine (0.3 mg/kg), IC-lesioned rats exhibited bursts of rapid locomotion interspersed by rearing, whereas BIL-lesioned rats displayed intense grooming or gnawing and nose poking of the cage floor; these behaviors were not seen in respective sham (i.e. vehicle)-lesioned rats injected with apomorphine. Scatchard analysis of saturation isotherms of both D1 [( 3H]SCH23390 binding sites) and D2 [( 3H]spiperone binding sites) dopamine receptors in the striatum revealed no difference in either the maximum number of binding sites (Bmax), or the dissociation constant (Kd) of either receptor type when BIL and IC lesioned rats were compared to appropriate controls. Conversely, the UNI lesioned rats had, under identical conditions of analysis, the expected increase in the density of D2 receptors on the lesioned side. There was no change in dopamine-sensitive adenylate cyclase activity in the striata of supersensitive IC-lesioned rats, but there was a shift to the left in the dose-response curve in striata from rats bilaterally-lesioned in the substantia nigra, similar to what occurs in UNI lesioned rats. Together, these data clearly demonstrate that although increases in receptor density and changes in cAMP systems are seen in the UNI model, neither mechanism is a requirement for functional supersensitivity in response to 6-OHDA lesions. These data suggest that other cellular events (e.g. alterations in receptor interactions) may play a role in the response to insult, and raise questions about the utility of the unilateral model as a screen for antiparkinsonian drugs.

Stimulatory and inhibitory cyclic AMP responses in rabbit ciliary processes after cervical ganglionectomy

Cyclic AMP production in response to agonists which act at a variety of receptors to either stimulate or inhibit cyclic AMP production has been studied in intact, dissected ciliary processes from rabbit eyes after unilateral surgical removal of the cervical ganglion. Cyclic AMP responses to stimulatory ligands vasoactive intestinal peptide (VIP), isoproterenol, and forskolin and inhibitory agonists neuropeptide Y (NPY), the synthetic somatostatin analogue SMS 201-995, and alpha-adrenergic agents were investigated in tissues from normal eyes and compared to the same responses in tissues from sympathetically denervated eyes. Neither stimulated cyclic AMP production nor inhibition of stimulated cyclic AMP production was significantly different in tissues from denervated vs. normal eyes. Inhibition of VIP-stimulated cyclic AMP production by epinephrine and paraaminoclonidine in tissues from both normal and denervated eyes was blocked by the alpha 2-adrenergic antagonist yohimbine but not by the alpha 1-adrenergic antagonist prazosin. These data indicate that the VIP, NPY, somatostatin, and alpha 2- and beta 2-adrenergic receptors which regulate cyclic AMP production in rabbit ciliary processes are postjunctional and suggest that ligands known to modulate cyclic AMP levels in this tissue may exert effects on aqueous humor formation independently of adrenergic innervation.

Regulation of the development of locus coeruleus neurons in vitro

Primary dissociated cultures of embryonic locus coeruleus (LC) neurons were established. Norepinephrine (NE) uptake was used as an index of maturation of the noradrenergic (NA) neurons from the LC. When LC cells were cocultured with a low density of hippocampal target cells, NE uptake was stimulated. However, increasing the concentration of hippocampal cells resulted in a significant decrease in NE uptake. The target stimulatory effect was mediated by both neurons and glia, whereas the inhibitory effect was mediated by direct contact between target glia and LC neurons and detected only in the presence of serum. In addition to the effect of target, we also tested the effect of elevated intracellular cAMP levels on NE uptake versus GABA uptake. GABA uptake was an index of development of non-NA cells. Increasing intracellular cAMP resulted in selective stimulation of NE uptake. These studies illustrate the potential of dissociated LC cultures in studying the regulation of NA axonal outgrowth.

Axonal sprouting of noradrenergic locus coeruleus neurons following repeated stress and antidepressant treatment

Plastic changes in axon terminals of NA LC neurons following repeated stress and antidepressant treatments were examined using electrophysiological or morphological methods. For stress treatment, rats restrained in a small cage were immersed up to the neck in warm water for 10 min daily. Electrophysiological experiments were performed under urethane anesthesia on the day following the termination of stress treatment. To quantify the density of cortical axon terminals arising in the LC, the percentage of LC neurons activated antidromically from the cerebral cortex was assessed. The percentage of LC neurons showing antidromic response to cortical stimulation was increased in the animals stressed for two weeks but not for one week. Since threshold currents for antidromic activation were not changed by the stress treatment, the observed changes were interpreted as morphological (axonal sprouting) rather than physiological consequences in NA axon terminals of LC neurons. To test the ability of antidepressants to induce the regeneration of central NA axons, local injections of 6-OHDA were made bilaterally into the symmetrical sites of the FC. Two weeks after the 6-OHDA injections, the same cortical site of one hemisphere was infused with the antidepressant MPL, DMI, or MIA, and the corresponding site of the other hemisphere with SAL. The density of glyoxylic acid-induced catecholamine fibers was greater in the cortical hemisphere infused with the antidepressants than that infused with SAL. These findings indicate that repeated mild stress and antidepressant treatments induce sprouting of NA LC axons in the cerebral cortex. Axonal sprouting of LC neurons can explain both the delayed onset of the clinical response to antidepressants and subsensitivity of beta-adrenoceptors following repeated stress and antidepressant treatments, and may be a common mechanism for the clinical efficacy of antidepressant drugs and electroconvulsive shock. Furthermore, the findings suggest the possibility that axonal retraction or degeneration of central NA neurons may be involved, at least in part, in the pathology of clinical depression.

Involvement of dopamine neurons in the regulation of beta-adrenergic receptor sensitivity in rat prefrontal cortex

The contribution of dopamine (DA) afferents to the regulation of beta-adrenergic receptor sensitivity (isoproterenol-stimulated adenylate cyclase activity) in the rat prefrontal cortex was investigated by comparing the effects of lesions affecting either both DA and noradrenaline (NA) or NA fibers alone. Bilateral 6-hydroxydopamine (6-OHDA) lesions made in the ventral tegmental area destroyed ascending DA and to a variable extent ascending NA fibers innervating the prefrontal cortex. Two opposite effects were observed depending on the extent of cortical NA denervation: (a) When NA denervation was complete (less than 4% of controls), a marked increase in the isoproterenol-sensitive adenylate cyclase activity (+78%) was found. The amplitude of this denervation supersensitivity was similar to that occurring following complete and selective destruction of NA innervation induced by bilateral 6-OHDA injections made into the pedunculus cerebellaris superior. (b) When 6-OHDA injections into the ventral tegmental area led to a partial destruction of cortical NA afferents (10-40% of control values), a hyposensitivity of the isoproterenol-induced adenylate cyclase activity (-30%) was observed. This effect contrasted with the moderate supersensitivity seen in rats with partial, but selective, destruction of NA innervation (pedunculus cerebellaris superior lesions). The hyposensitivity of beta-adrenergic receptors obtained in rats with partial lesions of cortical NA fibers, but devoid of cortical DA innervation, suggests that DA neurons may regulate, under certain conditions, the denervation supersensitivity of beta-adrenergic receptors.

Effects of acute and chronic cocaine administration on somatostatin level and binding in the rat brain

The effects of acute and chronic cocaine (40 mg/Kg i.p.) in vivo administration, on 125I-Tyr11-somatostatin binding and somatostatin-like immunoreactivity (SLI) in the rat frontoparietal cortex, hippocampus and olfactory bulb were explored. Acute and chronic cocaine administration did not affect the levels of SLI in the three brain areas studied. Acute cocaine administration resulted in an 55% and 32% decrease in the total number of specific somatostatin receptors in the hippocampus and olfactory bulb respectively, but not in the frontoparietal cortex. Somatostatin receptor affinity increased in the hippocampus and was unaltered in frontoparietal cortex and olfactory bulb. After two weeks of daily cocaine injections the somatostatin binding in the hippocampus and olfactory bulb returned to control values. The in vitro addition of cocaine to a brain membrane preparation obtained from untreated rats did not markedly affect somatostatin binding characteristics. These results are suggestive of a possible role for somatostatin in the limbic structures as a response to cocaine administration.

Regulation of adrenergic receptors in intraocular hippocampal transplants: role of noradrenergic innervation

Hippocampal tissue transplanted into the anterior chamber of the eye offers a unique system in which development can be studied in the absence of the noradrenergic innervation. This system was used to determine the extent to which noradrenergic innervation regulates the development of adrenergic receptors. In addition to examining single denervated transplants, transplants grown with innervation from the superior cervical ganglia of the host rat or from locus coeruleus cotransplants were also examined to determine whether the source of norepinephrine and extent of innervation in oculo regulate the development and density of adrenergic receptors. In vitro autoradiographic analysis of ligand binding to both alpha 1- and beta-adrenergic receptors with 125I-BE 2254 and 125I-pindolol, respectively, was used to characterize adrenergic receptors in the intraocular transplants. Quantitative analysis of the receptors showed an up-regulation of both alpha 1- and beta-adrenergic receptors in tissue grown in the absence of norepinephrine, but in general there was not a high degree of correlation between norepinephrine content and receptor density. Although high-performance liquid chromatography (HPLC) analysis of catecholamines revealed higher than normal amounts of norepinephrine in hippocampal transplants innervated by the superior cervical ganglia or a locus coeruleus cotransplant, the density of alpha 1 and beta receptors was quite comparable with values found in the literature for normal adult hippocampus. These results suggest that the relationship between receptor number and density of innervation may differ significantly from what is observed in response to pharmacological manipulation of norepinephrine systems in the adult brain.

Characterization of beta-adrenoceptors in the Syrian hamster Harderian gland: sexual differences and effects of either castration or superior cervical ganglionectomy

Scatchard analysis of saturation isotherms of [125I]-iodopindolol was used to characterize beta-adrenoceptor density (Bmax) and affinity constant (Kd) in female and male hamster Harderian glands. Single-point experiments were also completed in intact females, intact males, and castrated or superior cervical ganglionectomized males. Scatchard analysis described a single population of binding sites with a Bmax of 292.2 +/- 45.1 fmol/mg protein (X +/- SEM, n = 6) in females and 18.2 +/- 3.0 fmol/mg protein (n = 6, P less than .001) in males. The affinity also varied significantly (P less than .05) with a Kd of 1.08 +/- 0.18 versus 0.26 + 0.05 nM (n = 6) in the Harderian gland of females and males, respectively. Single-point [125I]-IPIN (400 pM) binding values in females were 67.3 +/- 4.0, in intact males were 12.8 +/- 3.2, and in castrated males were 31.2 +/- 4.2 fmol/mg protein (n = 7-9). Superior cervical ganglionectomy induced no significant changes in receptor binding. The results indicate pronounced sexual differences in the density and affinity of beta-adrenoceptors in the hamster Harderian gland, which may be sex hormone dependent.

The role of muscarinic acetylcholine receptors in ocular dominance plasticity

During a critical period of postnatal development neuronal connections in the visual cortex are susceptible to experience-dependent modifications. In normally reared kittens the majority of neurons respond to visual stimulation of either eye. A few days of monocular deprivation, however, are sufficient to render most cortical neurons unresponsive to visual stimuli presented to the deprived eye. Among other factors the cholinergic projection to striate cortex has been identified as having a permissive role in this use-dependent modification of synaptic transmission. In order to analyze further the influence of acetylcholine in cortical plasticity, we tested whether the blockade of muscarinic or nicotinic receptors interfered with ocular dominance plasticity. At four weeks of age kittens had one eyelid sutured closed and osmotic minipumps implanted, which delivered scopolamine (1 nmol/h) or hexamethonium (1 or 10 nmol/h) into the striate cortex of one hemisphere and vehicle solution (saline) into the other. After one week, ocular dominance distributions were determined in area 17 with single unit recording. In the control hemispheres, most neurons became unresponsive to the deprived eye, while in the scopolamine-treated hemispheres most neurons remained binocular. In contrast to the effects of scopolamine, the intracortical infusion of hexamethonium had no effect on ocular dominance plasticity. These results demonstrate that blockade of muscarinic, but not nicotinic receptors renders kitten striate cortex resistant to the effects of monocular deprivation.

Idazoxan and brain alpha 2-adrenoceptors in the rabbit

The effect of intravenous infusion of idazoxan on the depressor response to intracisternal clonidine 1 microgram/kg and on [3H]yohimbine binding in the fore- and hindbrain of the rabbit was examined. Idazoxan was infused either acutely (30 min) or chronically (5 days) at doses of 0.56 or 1.1 mg/h. Idazoxan 1.1 mg/h reduced the fall in blood pressure after clonidine. This attenuation of the depressor response was observed in the groups that were given the higher dose of idazoxan both acutely and chronically. The extent of attenuation was not modified by the duration of treatment. The low dose of idazoxan given acutely had no significant effect on the response to clonidine but the chronically infused group showed an enhanced response. A significant increase in the number of [3H]yohimbine binding sites (83%) was observed in the forebrain after 5 days infusion of 1.1 mg/h idazoxan with no change in the hindbrain. The lower dose of infusion did not cause any significant change in [3H]yohimbine binding in either brain region. Thus it appears that the susceptibility of the alpha 2-adrenoceptor binding sites to up-regulation by idazoxan may depend on the brain region observed.

Effects of alpha-methyl-p-tyrosine on monoamines and catecholamine receptors in rat cerebral cortex and neostriatum

The effects of inhibiting the synthesis of catecholamines using alpha-methyl-p-tyrosine (alpha-MPT) were investigated in four cortical regions (cingulate, somatosensory, visual and entorhinal-piriform) as well as in the neostriatum (caudate-putamen). After acute (48 hours) treatments with alpha-MPT the endogenous NA levels were significantly reduced in all regions examined. The DA contents were also decreased in regions known to possess a dense dopaminergic innervation (neostriatum, cingulate and entorhinal-piriform cortices) but not in the somatosensory and visual areas, where DA is normally present in small amounts. Serotonin and 5-HIAA levels were either unaffected or increased. After such catecholamine synthesis inhibitions, there were no changes in the binding parameters (Bmax and Kd) of [3H]prazosin (alpha 1-receptors), [3H]idazoxan (alpha 2-receptors), [3H]dihydroalprenolol (total beta receptors) in the cerebral cortex nor in [3H]SCH23390 sites (dopamine D1 receptors) in both cerebral cortex and neostriatum. The results indicate that acute catecholamine depletions but with conservation of the fibers do not produce receptor modifications.

Development of beta 1 and beta 2 adrenergic receptors in baboon brain: an autoradiographic study using [125I]iodocyanopindolol

[125I]iodocyanopindolol (ICYP) autoradiography was used to investigate the temporal development and distribution of beta 1 and beta 2 receptors in brains of baboons at ages embryonic day 100 (E100), full-term gestation (El80), and 3 years. In all brain regions examined, with the exception of the hippocampus, binding to beta 1 receptors exceeded that to beta 2 receptors. The highest densities of beta 1 receptors were found in the caudate nucleus, putamen, globus pallidus, substantia nigra, and cerebral cortex; intermediate receptor densities were observed in most nuclei of thalamus, and the lowest concentrations were in the hippocampus. At E100, beta receptors were identified in the striatum, globus pallidus, and thalamus. During maturation, the number of beta 1 receptors declined in cortical areas but increased in the head of the caudate and putamen. Significant differences in the developmental distribution of beta receptors during development were also detected: at E100 and E180 beta 1 receptors appeared as patches in the caudate and putamen, but by 3 years of age they were more homogeneously distributed in both regions; changes also occurred in the distribution of binding within cortical layers. Autoradiograms of [125I]ICYP and [3H]mazindol binding show overlapping patches of labeling in the E180 striatum, suggesting a possible developmental association between beta receptors and dopamine high-affinity uptake carrier sites. This study demonstrates that noradrenergic receptors in the primate forebrain undergo significant developmental reorganization with regional variations.

Evidence that noradrenergic activity does not mediate the inhibition of luteinizing hormone secretion caused by vasoactive intestinal peptide

We have recently shown that intracerebroventricular (i.c.v.) infusion of vasoactive intestinal peptide (VIP) in ovariectomized (OVX) rats causes a pronounced inhibition of pulsatile luteinizing hormone (LH) secretion. Since there is increasing evidence for an interaction between VIP- and norepinephrine-secreting systems in the brain, we tested the hypothesis that disruption of central noradrenergic activity would attenuate VIP-induced suppression of pulsatile LH release. Seven days prior to blood sampling, we administered either 6-hydroxydopamine (30 micrograms) or vehicle into the third cerebral ventricle of long-term OVX rats. This treatment reduced hypothalamic norepinephrine concentrations to 36% of control values. On the day of blood sampling, serial 300-microliter blood samples were taken from conscious, unrestrained animals at 5-min intervals for a total of 4 h. Each sample was replaced with an equal volume of a blood replacement mixture. Immediately following a control bleeding period of 2 h, VIP was administered i.c.v. to each animal for 2 h (at a dose of 1.8 nmol/h). The suppression of pulsatile LH secretion caused by VIP infusion was virtually identical in the 6-hydroxydopamine- and vehicle-pretreated groups. These results indicate that a substantial depletion of hypothalamic norepinephrine does not attenuate VIP-induced suppression of pulsatile LH secretion. This provides evidence that centrally administered VIP inhibits pulsatile LH secretion by a mechanism independent of noradrenergic transmission.

Alpha 2-adrenergic receptors in brown adipose tissue of infant rats--II. Studies on function and regulation

1. Clonidine inhibited the forskolin- and MIX-induced rate of lipolysis in brown fat adipocytes isolated from interscapular brown fat of 7-day-old rats. Its effect could be prevented by the alpha 2-antagonist yohimbine. 2. Pertussis toxin prevented the above effect of clonidine, thus indicating that alpha 2-adrenoceptors are linked with adenylate cyclase via the Ni regulatory subunit. 3. Chemical sympathectomy of 5-day-old rats by 6-hydroxydopamine increased the number of low-affinity alpha 2 sites in brown fat. 4. Chronic administration of yohimbine to 2-3-week-old rats also increased the density of alpha 2-adrenoceptors in brown fat. 5. It is suggested that brown fat of infant rats possesses functional alpha 2-adrenoceptors.

Temporal effects of intrahypothalamic 5,7-dihydroxytryptamine: relationship between serotonin levels and [3H]serotonin binding

The relationship between serotonin (5-HT) levels and [3H]5-HT binding in discrete hypothalamic areas was examined in separate groups of animals at various times, following unilateral intrahypothalamic injection of 5,7-dihydroxytryptamine (5,7-DHT). Seven days post-5,7-DHT lesion, 5-HT levels were significantly decreased in both the ipsilateral and contralateral ventromedial and dorsomedial hypothalamic nuclei (VMN, DMN). In the lateral hypothalamic area (LHA), 5-HT levels were significantly decreased only ipsilaterally. Fifty days postlesion, 5-HT levels in the ipsilateral VMN remained significantly below sham, while the DMN and LHA returned to sham values. Seven days after 5,7-DHT there was a significant increase in [3H]5-HT labeling densities in the ipsilateral and contralateral ventromedial hypothalamic area as well as in the ipsilateral LHA. In contrast, in the dorsomedial hypothalamic area there was no increase in [3H]5-HT binding. Fifty days postlesion, no significant differences in [3H]5-HT binding between 5,7-DHT and sham were observed in any areas examined. This data provides further evidence for the regeneration of 5-HT fibers in the hypothalamus and demonstrates that the relationship between [3H]5-HT binding and 5-HT levels varies from one hypothalamic area to another.

Timing of 6-hydroxydopamine administration influences its effects on visual cortical plasticity

We recorded from the visual cortex of 4 groups of monocularly deprived kittens. Three groups were treated with intraventricular 6-hydroxydopamine (6-OHDA) at different times relative to monocular deprivation (MD). One group received only vehicle solution and MD. 6-OHDA caused the greatest decrease in plasticity in the kittens receiving 6-OHDA throughout the deprivation period; that is, these kittens were the least affected by MD. 6-OHDA caused a smaller decrease in plasticity in kittens receiving 6-OHDA just prior to eyelid suture and a still smaller decrease in kittens waiting a week between 6-OHDA treatment and eyelid suture. The kittens in all groups receiving 6-OHDA were equally depleted of norepinephrine (NE). We conclude that 6-OHDA decreases plasticity in the visual cortex; however, the time course of this decrease is better related to the time course of the 6-OHDA treatment than to the time course of NE depletion.

Noradrenergic sub-sensitivity in the cerebral cortex of the tottering mouse, a spontaneously epileptic mutant

The glycogenolytic action of norepinephrine (NE) was examined in the tottering mouse, a spontaneously epileptic mutant which presents a noradrenergic hyperinnervation of various CNS areas, including the cerebral cortex. The potency and efficacy of NE in promoting glycogenolysis were markedly decreased in cerebral cortical slices prepared from homozygous tottering (tg/tg) when compared to control C57BL/6j (+/+) mice, indicating a sub-sensitive response to a cellular action of NE. The metabolic nature of this adaptive change suggests that an impaired capacity of NE in mobilizing energy substrates may be related to the expression of the epileptic symptomatology in this mutant.