Sensitivity Of Α2-adrenoceptors Research Articles

Wistar Kyoto rats exhibit decreased serotonin neuronal firing and increased norepinephrine burst activity but dampened hippocampal α2-adrenoceptor sensitivity.

Wistar Kyoto (WKY) rats manifest abnormalities in the function of monoamine receptors and transporters, as well as levels of these neurotransmitters in the brain. The present study assessed alterations in the firing activity of serotonin (5-hydroxytryptamine [5-HT]), norepinephrine (NE), and dopamine (DA) neurons, as well as the activity of 5-HT and NE receptors and transporters in the hippocampus. In vivo electrophysiological recordings were conducted in male WKY and Wistar rats. Extracellular single-unit recordings of 5-HT, NE, and DA neurons were performed. Recordings of pyramidal neurons were conducted in the medial prefrontal cortex (mPFC) and the hippocampus, where direct application of 5-HT and NE by iontophoresis was also carried out. The mean firing rate of 5-HT neurons was significantly decreased in WKY compared to Wistar rats. The burst activity of NE neurons was significantly increased in WKY, while their mean firing activity was not changed. There was no alteration in the firing, burst, and population activity of DA neurons in WKY animals. In the hippocampus, a decrease in sensitivity of α2-adrenoceptors, but not 5-HT receptors, was observed. There was, however, no change in the activity of 5-HT and NE transporters. The firing activity of mPFC pyramidal neurons was similar in WKY versus Wistar rats. In WKY rats, there was a decrease in the firing activity of 5-HT neurons. There was also an enhanced burst activity of NE neurons, accompanied by a reduction in sensitivity of the α2-adrenoceptor in the hippocampus, inferring a decrease in NE transmission.

Chronic fluoxetine reverses the effects of chronic corticosterone treatment on α2-adrenoceptors in the rat frontal cortex but not locus coeruleus

Disruption of the hypothalamic-pituitary-adrenal axis is an established finding in patients with anxiety and/or depression. Chronic corticosterone administration in animals has been proposed as a model for the study of these stress-related disorders and the antidepressant action. Alterations of the central noradrenergic system and specifically of inhibitory α2-adrenoceptors seem to be part of the pathophysiology of depression and contribute to the antidepressant activity. The present study evaluates in male rats the effect of chronic corticosterone treatment during 35 days (16–20 mg kg−1 day−1) on the sensitivity of α2-adrenoceptors expressed in the somatodendritic and terminal noradrenergic areas locus coeruleus (LC) and prefrontal cortex (PFC), respectively. Further, the effect of chronic fluoxetine treatment (5 mg kg−1, i.p., since the 15th day) on the sensitivity of α2-adrenoceptors was examined under control conditions and in corticosterone-treated rats. The α2-adrenoceptor functionality was analysed in vitro by agonist-mediated [35S]GTPγS binding stimulation and in vivo through the modulation of noradrenaline (NA) release evaluated by dual-probe microdialysis. The concentration-effect curves of the [35S]GTPγS binding stimulation by the agonist UK14304 (5-bromo-N-(4,5-dihydro-1H-imidazol-2-yl)-6-quinoxalinamine) demonstrated a desensitization of cortical α2-adrenoceptors induced by corticosterone (-logEC50 = 6.7 ± 0.2 vs 8.2 ± 0.3 in controls) that was reverted by fluoxetine treatment (-logEC50 = 7.5 ± 0.3). Local administration of the α2-adrenoceptor antagonist RS79948 ((8aR,12aS,13aS)-5,8,8a,9,10,11,12,12a,13,13a-decahydro-3-methoxy-12-(ethylsulfonyl)-6H-isoquino[2,1-g][1,6]naphthyridine) (0.1–100 μmol L−1) into the LC induced a concentration-dependent NA increase in the PFC of the control group (Emax = 191 ± 30%) but non-significant effect was observed in corticosterone-treated rats (Emax = 133 ± 46%), reflecting a desensitization of α2-adrenoceptors that control the firing of noradrenergic neurons. Fluoxetine treatment did not alter the corticosterone-induced desensitization in this area (Emax = 136 ± 19%). No effect of fluoxetine on α2-adrenoceptor functionality was observed in control animals (Emax = 223 ± 30%). In PFC, the local administration of RS79948 increased NA in controls (Emax = 226 ± 27%) without effect in the corticosterone group (Emax = 115 ± 26%), suggesting a corticosterone-induced desensitization of terminal α2-adrenoceptors. Fluoxetine administration prevented the desensitization induced by corticosterone in the PFC (Emax = 233 ± 33%) whereas desensitized α2-adrenoceptors in control animals (Emax = −24 ± 10%). These data indicate that chronic corticosterone increases noradrenergic activity by acting at different α2-adrenoceptor subpopulations. Treatment with the antidepressant fluoxetine seems to counteract these changes by acting mainly on presynaptic α2-adrenoceptors expressed in terminal areas.

Chronic citalopram administration desensitizes prefrontal cortex but not somatodendritic α2-adrenoceptors in rat brain

Selective serotonin reuptake inhibitors (SSRIs) regulate brain noradrenergic neurotransmission both at somatodendritic and nerve terminal areas. Previous studies have demonstrated that noradrenaline (NA) reuptake inhibitors are able to desensitize α2-adrenoceptor-mediated responses. The present study was undertaken to elucidate the effects of repeated treatment with the SSRI citalopram on the α2-adrenoceptor sensitivity in locus coeruleus (LC) and prefrontal cortex (PFC), by using in vivo microdialysis and electrophysiological techniques, and in vitro stimulation of [35S]GTPγS binding autoradiography. Repeated, but not acute, treatment with citalopram (5 mg/kg, i.p., 14 days) increased extracellular NA concentration selectively in PFC. The α2-adrenoceptor agonist clonidine (0.3 mg/kg, i.p.), administered to saline-treated animals (1 ml/kg i.p., 14 days) induced NA decrease in LC (Emax = −44 ± 4%; p < 0.001) and in PFC (Emax = −61 ± 5%, p < 0.001). In citalopram chronically-treated rats, clonidine administration exerted a lower decrease of NA (Emax = −25 ± 7%; p < 0.001) in PFC whereas the effect in LC was not different to controls (Emax = −36 ± 4%). Clonidine administration (0.625–20 μg/kg, i.v.) evoked a dose-dependent decrease of the firing activity of LC noradrenergic neurons in both citalopram- (ED50 = 3.2 ± 0.4 μg/kg) and saline-treated groups (ED50 = 2.6 ± 0.5 μg/kg). No significant differences between groups were found in ED50 values. The α2-adrenoceptor agonist UK14304 stimulated specific [35S]GTPγS binding in brain sections containing LC (144 ± 14%) and PFC (194 ± 32%) of saline-treated animals. In citalopram-treated animals, this increase did not differ from controls in LC (146 ± 22%) but was lower in PFC (141 ± 8%; p < 0.05). Taken together, long-term citalopram treatment induces a desensitization of α2-adrenoceptors acting as axon terminal autoreceptors in PFC without changes in somatodendritic α2-adrenoceptor sensitivity.

Dysfunctional inhibitory mechanisms in locus coeruleus neurons of the wistar kyoto rat.

Background:The noradrenergic nucleus locus coeruleus (LC) has functional relevance in several psychopathologies such as stress, anxiety, and depression. In addition to glutamatergic and GABAergic synaptic inputs, the activation of somatodendritic α2-adrenoceptors is the main responsible for LC activity regulation. The Wistar Kyoto (WKY) rat exhibits depressive- and anxiety-like behaviors and hyperresponse to stressors. Thus, the goal of the present study was to investigate in vitro the sensitivity of α2-adrenoceptors, as well as the glutamatergic and GABAergic synaptic activity on LC neurons of the WKY strain.Methods:For that purpose patch-clamp whole-cell recordings were done in LC slices.Results:The α2-adrenoceptors of LC neurons from WKY rats were less sensitive to the effect induced by the agonist UK 14 304 as compared to that recorded in the Wistar (Wis) control strain. In addition, the GABAergic input to LC neurons of WKY rats was significantly modified compared to that in Wis rats, since the amplitude of spontaneous GABAergic postsynaptic currents was reduced and the half-width increased. On the contrary, no significant alterations were detected regarding glutamatergic input to LC neurons between rat strains.Conclusions:These results point out that in WKY rats the inhibitory control exerted by α2-adrenoceptors and GABAergic input onto LC neurons is dysregulated. Overall, this study supports in this animal model the hypothesis that claims an imbalance between the glutamatergic-GABAergic systems as a key factor in the pathophysiology of depression.

Chronic Pain Leads to Concomitant Noradrenergic Impairment and Mood Disorders

Patients suffering chronic pain are at high risk of suffering long-lasting emotional disturbances characterized by persistent low mood and anxiety. We propose that this might be the result of a functional impairment in noradrenergic circuits associated with locus coeruleus (LC) and prefrontal cortex, where emotional and sensorial pain processes overlap. We used a chronic constriction injury of sciatic nerve as a model of neuropathic pain in male Sprague-Dawley rats to assess the time-dependent changes that might potentially precipitate mood disorders (2, 7, 14, and 28 days after injury). This was measured through a combination of behavioral, electrophysiological, microdialysis, immunohistochemical, and Western blot assays. As expected, nerve injury produced an early and stable decrease in sensorial pain threshold over the testing period. By contrast, long-term neuropathic pain (28 days after injury) resulted in an inability to cope with stressful situations, provoking depressive and anxiogenic-like behaviors, even more intense than the aversiveness associated with pain perception. The onset of these behavioral changes coincided with irruption of noradrenergic dysfunction, evident as: an increase in LC bursting activity; in tyrosine hydroxylase expression and that of the noradrenaline transporter; and enhanced expression and sensitivity of α2-adrenoceptors in the LC. Long-term neuropathic pain leads to anxio-depressive-like behaviors that are more predominant than the aversion of a painful experience. These changes are consistent with the impairment of noradrenergic system described in depressive disorders.

Effects of Acute, Repeated and Chronic Variable Stress on In Vivo Tyrosine Hydroxylase Activity and on α2-adrenoceptor Sensitivity in the Rat Brain

We assessed the effects of a single tail pinch and two chronic stress regimes, repeated and variable, on in vivo tyrosine hydroxylase activity and on α2-adrenoceptor sensitivity in two brain regions. After administering a 3,4-dihydroxyphenylalanine (DOPA) decarboxylase inhibitor, tyrosine hydroxylase activity, measured as the accumulation of DOPA, and noradrenaline (NA) content were determined by using high-performance liquid chromatography. A single tail pinch for 5 min induced an enhancement of DOPA content in hippocampus (28%) and hypothalamus (67%) which was still present 24 h later. This increase could account for the lack of changes in NA content in both regions after the application of this stressor. However, tyrosine hydroxylase activity was unmodified 24 h after exposure to both repeated (5 min of tail pinch, twice daily, for 14 days) and chronic variable stress (one of 5 different stressors, once daily, for 14 days) although there was an enhancement of NA levels in hippocampus (45 and 54%, respectively) and hypothalamus (24.5 and 36%, respectively). The sensitivity of the α2-adrenoceptors which regulate [3H]-NA release in hippocampal and hypothalamic synaptosomes was not modified by the acute or chronic stress protocols assayed. The results show that both paradigms of chronic stress had similar effects on the noradrenergic indices evaluated.

Effects of N-(2-chloroethyl)-N-ethyl-2-bromobenzylamine (DSP4) on alpha2-adrenoceptors which regulate the synthesis and release of noradrenaline in the rat brain.

N-(2-chloroethyl)-N-ethyl-2-bromobenzylamine (DSP4) induces a degeneration of noradrenergic axons originating in the locus coeruleus. The sensitivity of alpha2-adrenoceptors which regulate the synthesis and release of noradrenaline was investigated in three brain regions which receive an unequal innervation from locus coeruleus, 21 days after DSP4 (50 mg/kg) administration. After giving treated rats a dopa decarboxylase inhibitor, the in vivo tyrosine hydroxylase activity and the tissue concentrations of noradrenaline were also evaluated. Relevant reductions of noradrenaline levels were found in hippocampus and parietal cortex (91% and 77.5%, respectively; P<0.001) together with a less pronounced reduction in hypothalamus (32%, P<0.01). The administration of the neurotoxin led to decreases of the basal tyrosine hydroxylase activity, determined as the accumulation of 3,4-dihydroxyphenylalanine, in hippocampus and parietal cortex (75% and 50.5%, respectively; P<0.001), but not in hypothalamus. The inhibitory effect of clonidine on tyrosine hydroxylase activity was markedly reduced in hippocampus of rats treated with DSP4 (10+/-5% vs 57+/-3% in the control group, P<0.001) but was not changed in parietal cortex and hypothalamus. Moreover, in hippocampus, a lack of functionality of the alpha2-adrenoceptors which regulate K(+)-evoked [3H]noradrenaline release was determined. However, in cortical synaptosomes the concentration-effect curve for the oxymetazoline shifted to the right. The administration of the neurotoxin did not modify the inhibitory effects of the agonist in hypothalamus. These results support the previously described selectivity of DSP4 for noradrenergic terminals arising from locus coeruleus and suggest a more severe lesioning of the hippocampus than the parietal cortex.

Sex differences in visceral fat lipolysis and metabolic complications of obesity.

Cardiovascular complications of obesity are more common in men than women. Sex differences in visceral fat lipolysis may be of importance in this respect, since increased release of free fatty acids (FFAs) from visceral fat to the liver by the portal venous system has been thought to cause several metabolic complications due to obesity, such as hypertension, hyperlipidemia, and glucose intolerance. The aim of this study was to investigate sex differences in clinical characteristics and visceral fat mobilization in obesity. Obese subjects (22 male and 23 female) undergoing elective surgery were matched for body mass index and age. The males had both higher waist-to-hip ratio (WHR), sagittal diameter, blood pressure, fat-cell volume, plasma insulin, glucose, and triglyceride and lower HDL cholesterol levels than the females. The rate of norepinephrine-induced FFA and glycerol release was twofold higher in men (P = .02). No significant reutilization of FFA was observed. The difference in maximum norepinephrine-induced rate of lipolysis between men and women was independent of both WHR and sagittal diameter and was an independent regressor for levels of plasma glucose and plasma HDL cholesterol. Fat-cell volume was an independent regressor for plasma triglycerides and blood pressure. No sex differences in the lipolytic sensitivity to beta 1- or beta 2-adrenoceptor-specific agonists or in the antilipolytic effect of insulin were observed. However, the lipolytic beta 3-adrenoceptor sensitivity was 12 times higher (P = .004) and the antilipolytic alpha 2-adrenoceptor sensitivity 17 times lower (P = .003) in men. Furthermore, lipolysis induced by agents acting at the adenylate cyclase and protein kinase A levels were almost twofold enhanced in men. However, no sex difference in maximum hormone-sensitive lipase activity was observed. In conclusion, in obesity, catecholamine-induced rate of FFA mobilization from visceral fat to the portal venous system is higher in men than women. This phenomenon is partly due to a larger fat-cell volume but also to a decrease in the function of alpha 2-adrenoceptors, an increase in the function of beta 3-adrenoceptors, and an increased ability of cyclic AMP to activate hormone-sensitive lipase. These factors may contribute to gender-specific differences in metabolic and cardiovascular disturbances accompanied by obesity.

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In vivo microdialysis was used to measure the effects of long-term treatment of rats with desipramine upon the regulation by α2-adrenoceptors of serotonin (5-HT) release from the serotonergic neurons in the hippocampus. Extracellular levels of 5-HT were estimated by assaying its concentrations in the perfusate using HPLC-ECD. When the α2-adrenoceptor agonist brimonidine was added to the perfusion solution, the K+-evoked 5-HT release was significantly inhibited in a concentration-dependent manner. Treatment with pertussis toxin did not alter the K+-evoked release of 5-HT but abolished the inhibitory effect of brimonidine. Long-term desipramine treatment significantly reduced the inhibitory effect of brimonidine upon the K+-evoked 5-HT release, and both the 5-HT content of the hippocampus and the basal 5-HT concentration in the dialysate were significantly increased. The present study suggests that long-term administration of desipramine causes a functional subsensitivity of the presynaptic α2-adrenoceptors that regulate serotonergic neuronal function in the rat hippocampus. It also supports the concept that changes in the sensitivity of α2-adrenoceptors that regulate neurotransmitter release play an important role in the mechanism of antidepressant drug action.

The effects of phosphatidylserine on the decreased cardiovascular response to clonidine microinfused into the nucleus tractus solitarii of old rats

1. The cardiovascular effects of direct microinfusions of clonidine into the nucleus tractus solitarii (NTS) in rats of different ages were studied. 2. Clonidine microinfused into the NTS produced a dose-dependent hypotensive and bradycardic response, which was reduced by previous microinfusion of yohimbine but not of propranolol or prazosin. 3. Unilateral microinfusion of clonidine into the NTS produced a significantly smaller hypotensive and bradycardic response in 16 month old rats than in 3 month old rats, whereas in older animals (24 month old) the same doses of clonidine were ineffective. In 16 month old rats, yohimbine given into the NTS produced a smaller pressor and tachycardic response than in 3 month old rats whereas in 24 month old animals yohimbine, at the same doses, was unable to change blood pressure and heart rate. 4. Oral administration of phosphatidylserine (50 mg kg-1 day-1 for 30 consecutive days) enhanced the hypotensive and bradycardic response to clonidine microinfused into the NTS in 16 month old rats. The same schedule of treatment with phosphatidylserine did not affect the cardiovascular response to clonidine in young (3 month old) rats and did not restore responsiveness to clonidine in older rats (24 month old). 5. Methoxamine and phentolamine, given intravenously, produced similar pressor and depressor responses, respectively in 3, 16 and 24 month old rats, suggesting that the decreased sensitivity to clonidine in old rats was not due to peripheral cardiovascular changes. 6. In conclusion, the present results indicate that, during aging, there is a decreased sensitivity of alpha 2-adrenoceptors in the brain. Such an alteration can be reversed by chronic treatment with phosphatidylserine during the initial but not the later stages of aging.

The effect of single dose and chronic moclobemide treatment on clonidin induced changes in blood pressure and norepinephrine and MHPG levels in plasma in man

MHPG levels are decreased after single dose and chronic Moclo administration. However, our study does not show convincing evidence for decreased sensitivity of α2 adrenoceptors after chronic treatment with therapeutic doses of Moclo.

Antidepressant administration has a differential effect on rat brain α2-adrenoceptor sensitivity to agonists and antagonists

The aim of this study was to characterize the action of antidepressants on rat brain α 2-adrenoceptor function by examining their effect on the yohimbine-sensitive component of the cAMP response to adrenoceptor agonists. Using a prelabeling technique for measuring cAMP accumulation, it was found that chronic (2 weeks) administration of antidepressants increased the potency of yohimbine, an α 2-adrenoceptor antagonist, to inhibit norepinephrine-stimulated cAMP accumulation in rat brain cerebral cortical slices. In contrast, antidepressant treatment decreased the ability of the α-adrenoceptor agonist 6-fluoronorepinephrine to augment isoproterenol-stimulated cAMP accumulation. The results suggest that antidepressants alter rat brain α 2-adrenergic function by shifting the receptor to antagonist-preferring state. Such an effect may contribute to the change in noradrenoceptor responsiveness that is characteristic of antidepressant administration.

Enhancement of central and peripheral α1-adrenoceptor sensitivity and reduction of α2-adrenoceptor sensitivity following chronic imipramine treatment in rats

After chronic imipramine treatment (20 mg/kg i.p., once daily for 14 days) the dose-response curve of the isolated rat anococcygeal muscle to phenylephrine shifted to the left, and furthermore, the -log K A value (affinity) for phenylephrine was significantly increased without affecting the affinity for guanfacine. On the other hand, such treatment caused a shift to the right of the dose-response curve to guanfacine on aortic strips and the affinity of the α-adrenoceptor for guanfacine was lowered without any accompanying changes in the affinity value for phenylephrine. However the relative efficacies of phenylephrine or guanfacine were not influenced by imipramine in either preparation. The ability of phenylephrine to displace [ 3H]prazosin from its specific binding sites was significantly enhanced after chronic imipramine treatment. These results may indicate that following chronic imipramine treatment the α 1-adrenoceptors of both central and peripheral tissues responded with supersensitivity to an α 1-preferential agonist, and the α 2-adrenoceptors with reduced sensitivity to an α 1-preferential agonist.