Alpha-adrenergic stimulation of ERK phosphorylation in astrocytes is alpha(2)-specific and may be mediated by transactivation

The highly specific alpha(2)-adrenergic agonist, dexmedetomidine, has hypnotic-sedative, anesthetic-sparing and analgesic effects, and it protects neurons against ischemia. The alpha(1)-adrenergic agonist, phenylephrine, does not share dexmedetomidine's pharmacological properties, although both dexmedetomidine and phenylephrine increase free cytosolic Ca(2+) ([Ca(2+)](i)) in astrocytes, and most of dexmedetomidine's actions in the brain are exerted on postjunctional receptors. alpha(2)-Adrenergic receptors are abundant on astrocytes. Dexmedetomidine-mediated 'down-streamn' signal transduction was therefore investigated in primary cultures of mouse astrocytes and contrasted with that of phenylephrine. The cultures were incubated with dexmedetomidine concentrations known to be pharmacologically active and to act specifically on alpha(2)-adrenergic receptors (25-100 nM). ERK(1/2) phosphorylation was measured using specific antibodies. Peak increases of ERK(1/2) phosphorylation occurred at 50 nM dexmedetomidine, with less effect at higher concentrations. Phenylephrine caused ERK phosphorylation only at a concentration high enough to exert non subtype-specific effects (10 microM), and this effect was counteracted by the alpha(2)-adrenergic antagonist atipamezole. The phosphorylation of ERK was reduced by tyrphostin AG1478, an inhibitor of phosphorylation of the epidermal growth factor receptor (EGFR), and by heparin, which neutralizes heparin-binding epithelial growth factor (HB-EGF), suggesting the involvement of a transactivation process, in which alpha(2)-adrenergic stimulation leads to proteolytic shedding of HB-EGF (and perhaps other EGFR agonists) from transmembrane-spanning precursors.     

Effects of subtypes alpha- and beta-adrenoceptors of the lateral hypothalamus on the water and sodium intake induced by angiotensin II injected into the subfornical organ

The present experiments were conducted to investigate the role of the alpha(1A)-, alpha(1B), beta(1)- and beta(2)-adrenoceptors of the lateral hypothalamus (LH) on the water and salt intake responses elicited by subfornical organ (SFO) injection of angiotensin II (ANG II) in rats. 5-methylurapidil (an alpha(1A)-adrenergic antagonist), cyclazosin (an alpha(1B)-adrenergic antagonist) and ICI-118,551 (a beta(2)-adrenergic antagonist) injected into the LH produced a dose-dependent reduction, whereas efaroxan (an alpha(2)-antagonist) increased the water intake induced by administration of ANG II into the SFO. These data show that injection of 5-methylurapidil into the LH prior to ANG II into the SFO increased the water and sodium intake induced by the injection of ANG II. The present data also show that atenolol (a beta(1)-adrenergic antagonist), ICI-118,551, cyclazosin, or efaroxan injected into the LH reduced in a dose-dependent manner the water and sodium intake to angiotensinergic activation of SFO. Thus, the alpha(1)- and beta-adrenoceptors of the LH are possibly involved with central mechanisms dependent on ANG II and SFO that control water and sodium intake.     

Central serotonergic and adrenergic/imidazoline inhibitory mechanisms on sodium and water intake

Recent studies have shown the existence of two important inhibitory mechanisms for the control of NACL and water intake: one mechanism involves serotonin in the lateral parabrachial nucleus (LPBN) and the other depends on alpha(2)-adrenergic/imidazoline receptors probably in the forebrain areas. In the present study we investigated if alpha(2)-adrenergic/imidazoline and serotonergic inhibitory mechanisms interact to control NaCl and water intake. Male Holtzman rats with cannulas implanted simultaneously into the lateral ventricle (LV) and bilaterally into the LPBN were used. The ingestion of 0.3 M NaCl and water was induced by treatment with the diuretic furosemide (10 mg/kg of body weight)+the angiotensin converting enzyme inhibitor captopril (5 mg/kg) injected subcutaneously 1 h before the access of rats to water and 0.3 M NaCl. Intracerebroventricular (i.c.v.) injection of the alpha(2)-adrenergic/imidazoline agonist clonidine (20 nmol/1 microl) almost abolished water (1.6+/-1.2, vs. vehicle: 7.5+/-2.2 ml/2 h) and 0.3 M NaCl intake (0.5+/-0.3, vs. vehicle: 2.2+/-0.8 ml/2 h). Similar effects were produced by bilateral injections of the 5HT(2a/2c) serotonergic agonist 2,5-dimetoxy-4-iodoamphetamine (DOI, 5 microg/0.2 microl each site) into the LPBN on water (3.6+/-0.9 ml/2 h) and 0.3 M NaCl intake (0.4+/-0.2 ml/2 h). Injection of the alpha(2)-adrenergic/imidazoline antagonist idazoxan (320 nmol) i.c.v. completely blocked the effects of clonidine on water (8.4+/-1.5 ml/2 h) and NaCl intake (4.0+/-1.2 ml/2 h), but did not change the effects of LPBN injections of DOI on water (4.2+/-1.0 ml/2 h) and NaCl intake (0.7+/-0.2 ml/2 h). Bilateral injections of methysergide (4 microg/0.2 microl each site) into the LPBN increased 0.3 M NaCl intake (6.4+/-1.9 ml/2 h), not water intake. The inhibitory effect of i.c.v. clonidine on water and 0.3 M NaCl was still present after injections of methysergide into the LPBN (1.5+/-0.8 and 1.7+/-1.4 ml/2 h, respectively). The results show that the inhibitory effects of the activation of alpha(2)-adrenergic/imidazoline receptors in the forebrain are still present after blockade of the LPBN serotonergic mechanisms and vice versa for the activation of serotonergic mechanisms of the LPBN. Therefore, each system may act independently to inhibit NaCl and water intake.     

Response of supraoptic magnocellular neurons to stimulation of forebrain α-adrenoceptors

Effects of alpha-adrenoceptor agents on electrophysiologically and immunohistochemically identified supraoptic nucleus (SON) vasopressin (VP) units were investigated by intracarotid infusion. Clonidine, an alpha 2-adrenoceptor agonist always excited SON units and alpha 2-adrenoceptor antagonists consistently inhibited them. alpha 1-Adrenoceptor agents produced inconsistent responses. The results implicate forebrain alpha 2-adrenergic receptors in the excitation of SON VP neurons.     

Activation of lateral geniculate neurons by norepinephrine: Mediation by an α-adrenergic receptor

Adrenergic receptors in the vicinity of neurons in the lateral geniculate nucleus (LGN) of the rat were pharmacologically characterized using extracellular single-cell recording and microiontophoretic techniques. Application of norepinephrine (NE) at low iontophoretic currents (1–15 nA) produced a delayed activation of most LGN neurons. This activation was mimicked by various sympathomimetic amines. The relative potency series of agonists was typical of postsynaptic α-adrenergic receptors:epinephrine>NE>phenylephrine α-methylnorepinephrine>dopamine>isoproterenol. The α-antagonists phentolamine, piperoxane and WB-4101, when applied at low iontophoretic currents (<10nA), produced a selective, dose-dependent and reversible blockade of the response to NE. The β-antagonist sotalol had weak and variable effects at these currents. At low currents, the presynaptic α-agonist clonidine was also able to block the response to NE but, at higher currents, produced a partial activation of some units, suggesting that it is a weak agonist. The ability of sympathomimetic amines to activate LGN neurons correlates well with their reported affinities for brainα₁-adrenoceptors labeled with [³H]WB-4101. It is concluded that NE activates neurons in the LGN via a postsynaptic orα₁-adrenergic receptor.     

Alpha 2- and beta-adrenoceptors differentially modulate GABAA- and GABAB-mediated inhibition of red nucleus neuronal firing

In mesencephalic red nucleus (RN), GABA-induced inhibition of neuronal firing is modulated by noradrenaline acting on alpha2- and beta-adrenoceptors. Since both GABAA and GABAB receptors are present in the rat RN, we have recorded the firing activity of RN neurons in vivo from anaesthetized rats to study how GABAA- and GABAB-mediated effects are modulated by either alpha2- or beta-adrenoceptor activation. Both the GABAA agonist isoguvacine and the GABAB agonist baclofen depressed the firing of RN neurons. During simultaneous application of clonidine, an alpha2-adrenoceptor agonist, half of the isoguvacine- and baclofen-mediated responses were modified: isoguvacine-mediated inhibition was enhanced by 97% without any change in effect duration, whereas baclofen responses were either increased or slightly reduced in the same number of cases. Application of isoprenaline, a beta-adrenoceptor agonist, increased isoguvacine effect in 66% of neurons without modifying effect duration; the amount of increase (43%) was significantly lower than that induced by clonidine. On the other hand, in the presence of isoprenaline, baclofen response was reduced in 72% of neurons with respect to both the amount (52%) and the duration (34%) of effect. Taken together, these results indicate that alpha2-adrenoceptors mainly enhance GABAA-induced inhibition and induce mixed effects on GABAB response; on the other side, beta-adrenoceptors exert an opposite modulation on GABA effects, respectively, enhancing and depressing GABAA- and GABAB-mediated responses.     

Microinjection of the α1-agonist methoxamine into the paraventricular hypothalamus induces anorexia in rats

Adrenergic receptors within the paraventricular hypothalamus (PVN) play a prominent role in the control of food intake: stimulation of α2-adrenoceptors induces food intake whereas stimulation of α1-adrenoceptors suppresses food intake. This study further examines the role of PVN α1-adrenoceptors hy examining the effects on food and water intake of the α1-adrenergic agonist methoxamine (100, 200, 400 nMol) microinjected into the rat paraventricular hypothalamus. Methoxamine suppressed food intake but not water intake. Doses of 100, 200, and 400 nMol methoxamine suppressed food intake by 47%. 64%, and 96%, respectively. These results further confirm the hypothesis that administration of α1-agonists into the PVN acts to significantly suppress food intake; an action that is in opposition to the facilitory effects of α2-adrenergic agonists on food intake.     

The resolution of dopamine and β1- and β2-adrenergic-sensitive adenylate cyclase activities in homogenates of cat cerebellum, hippocampus and cerebral cortex

The stimulation of adenylate cyclase by dopamine and various β-adrenergic agonists has been investigated in homogenates from 3 areas of cat brain: the cerebral cortex, cerebellum and hippocampus. The purpose of the study was to determine whether the β-adrenergic receptors coupled to adenylate cyclase could be classified as either β₁ and β₂ subtypes in the different regions studied.The stimulation of adenylate cyclase by the β-adrenergic agonist, (−)isoproterenol (5 × 10⁻⁶M), was completely blocked by the specific β-adrenergic antagonist, (−)alprenolol (10⁻⁵ M), but not by the dopaminergic antagonist, fluphenazine (10⁻⁵ M), whereas the stimulation of adenylate cyclase by (−)epinephrine (10⁻⁴ M) was blocked to varying extents by these two drugs in each of the 3 regions studied. The (−)epinephrine effect was always blocked in the combined presence of (−)alprenolol and fluphenazine. The adenylate cyclase stimulation by (−)epinephrine which is not blocked by (−)alprenolol was due to interaction of (−)epinephrine with a dopaminergic-sensitive adenylate cyclase which has been characterized in cerebral cortex, hippocampus and cerebellum.Regional differences in the affinity of β-adrenergic-sensitive adenylate cyclase for various agonists were investigated in the presence of fluphenazine (10⁻⁵ M). In the cerebellum the potency order was (±)protokylol> (±)hydroxybenzylisoproterenol> (±)isoproterenol> (−)epinephrine> (±)salbutamol> (−)norepinephrine, indicating the presence of a β₂-adrenergic receptor. In the cerebral cortex the potency order was (−)isoproterenol> (±)protokylol> (±)hydroxybenzylisoproterenol> (−)epinephrine= (−)norepinephrine((±)salbutamol being inactive). A similar pattern was found in the hippocampus indicating the presence of a β₁-adrenergic receptor in these two regions. (±)Salbutamol was a partial agonist in the cerebellum and a competitive antagonist in the cerebral cortex.The ratio of the antagonist potencies of (±)practolol and (±)butoxamine preferential β₁- and β₂-adrenergic antagonists respectively, to block the stimulation of adenylate cyclase was 25 in the cerebellum, compared to 0.5 in the cerebral cortex and 1.6 in the hippocampus. These results confirm the presence of a β₂ subtype of receptor coupled to adenylate cyclase in the former and β₁ subtypes in the latter two regions. The comparison between the affinities of a series of β-adrenergic agonists and antagonists for the β-adrenergic receptors coupled with an adenylate cyclase in cerebral cortex and cerebellum with their affinities for well characterized β₂-adrenergic receptors in lung and β₁-adrenergic receptor in heart substantiated this conclusion.     

Receptor-mediated inositide hydrolysis is a neuronal response: comparison of primary neuronal and glial cultures

Cholinergic and adrenergic receptor-stimulated inositide hydrolysis was studied in neuronal and glial cells cultured from brains of 1-day-old Wistar-Kyoto rats. Incubation of the cells with [3H]inositol led to the incorporation of radioactivity specifically into inositol phospholipids. Labeling of the membrane lipids reached a maximum in 2-3 days. Receptor-stimulated breakdown of inositides was determined by following the accumulation of inositol phosphates after incubation of the labeled cells for 60 min with carbachol or norepinephrine in the presence of 10 mM lithium. Carbachol (1 mM) stimulated inositol phosphate production in neurons 30 times higher than that seen in glia. The response stimulated by norepinephrine (75 microM) was 6 times higher in neurons than glia. The response to carbachol was blocked by atropine, and the norepinephrine-induced response was inhibited by prazosin suggesting that the receptors mediating the responses were muscarinic and alpha 1-adrenergic, respectively. These results suggest that muscarinic cholinergic and alpha 1-adrenergic stimulated inositide hydrolysis is primarily a neuronal response and that this biochemical event may be important for transmembrane signaling which occurs during neurotransmission.     

Adrenergic modulation of astroglial phospholipase D activity and cell proliferation

As phospholipase D (PLD) activation has been associated with mitogenic signalling in several cell types, we tested an association between adrenergic activation of PLD and cellular proliferation in primary cultures of rat cortical astrocytes. In 2-week old cultures, PLD activation by noradrenaline (EC50: 0.49 microM) was inhibited by prazosin, a specific antagonist at alpha1-adrenergic receptors (IC50: 0.23 microM). Adrenergic PLD activation was not affected by genistein, an inhibitor of tyrosine kinases, or by Ro 31-8220, an inhibitor of protein kinase C (PKC), but was dose-dependently depressed in the presence of brefeldin A (1-100 microg/ml), an inhibitor of ARF activation. In experiments measuring cell proliferation, noradrenaline potently (EC50: 20 nM) reduced [3H]thymidine incorporation to 20-30% of basal values. This action was mimicked by the beta-specific agonist isoprenaline and was inhibited by the beta-antagonist propranolol in a concentration-dependent manner. The alpha1-adrenergic agonists, phenylephrine and methoxamine, also reduced DNA synthesis. The adrenergic inhibition of astroglial DNA synthesis was not reduced, but further potentiated in the presence of brefeldin A, ethanol, and 1- and 2-butanol; 1-butanol, a substrate of PLD, was equally effective as 2-butanol, a non-substrate. We conclude that adrenergic PLD activation in astrocytes is not involved in mitogenic signalling. The involvement of ARF in the activation of PLD via alpha1-adrenoceptors indicates a role in protein trafficking.     

Role of the alpha 1- and alpha 2-adrenoceptors of the lateral hypothalamus in the dipsogenic response to central angiotensin II in rats.

The present experiments were conducted to investigate the role of the alpha 1- and alpha 2-adrenergic receptors of the lateral hypothalamus (LH) on the drinking response elicited by intracerebroventricular (i.c.v.) injections of carbachol and angiotensin II (AII) in rats. Clonidine (an alpha 2-adrenergic agonist) injected into the LH produced a dose-dependent reduction of the drinking responses elicited by i.c.v. administration of carbachol and AII. The alpha 1-adrenergic agonist phenylephrine injected into the LH reduced the dipsogenic response to i.c.v. AII, but not to carbachol. Injection of yohimbine (an alpha 2-adrenergic antagonist) and prazosin (an alpha 1-adrenergic antagonist) into the LH also reduced the water intake produced by i.c.v. injection of AII. Previous injection of alpha 1- or alpha 2-adrenergic antagonists into the LH increased the antidipsogenic effect of clonidine or phenylephrine injected into the same area on the water intake induced by i.c.v. AII. These results show that the alpha 1- and alpha 2-adrenergic receptors of the LH are involved in the control of drinking responses elicited by i.c.v. injection of AII in rats. They also show that clonidine, but not phenylephrine, suppresses the drinking induced by i.c.v. carbachol. The data suggest that the discharge of central alpha-adrenergic receptors has a dual (inhibitory and excitatory) effect on water intake induced by central AII.     

Epinephrine induced platelet aggregation in rat platelet-rich plasma

Epinephrine at even high concentrations neither caused shape change nor aggregation of rat platelets. However, epinephrine induced aggregation in the presence of low (near-threshold) concentrations of collagen at which concentration only platelet shape change was induced without aggregation. The platelet aggregation was also induced by some other catecholamines and clonidine but not by beta-agonist isoproterenol. The aggregatory potencies of R-(-)-isomers, (-)-epinephrine and (-)-norepinephrine were higher than the corresponding desoxy derivatives, epinine and dopamine. In addition, the epinephrine-induced rat platelet aggregation was inhibited by alpha2-antagonists, yohimbine and phentolamine, but not by alpha1-antagonist prazosin or beta-antagonist propranolol. These results suggested that the epinephrine-induced rat platelet aggregation is occurring through alpha2-adrenergic receptors as is the case with human platelets.     

Effect of acute administration of isoproterenol and angiotensin II, separately and in combination, on water intake and blood pressure of rats

The effects of administration of isoproterenol, a β-adrenergic agonist, and angiotensin II, a peptide, separately and in combination, on water intake and blood pressure of rats were studied. The results of 6 factorially designed studies in which 4 different doses of each compound were administered revealed that water intakes increased directly with the logarithm of increasing doses of each. The effect of simultaneous administration of the 2 compounds on water intake was additive at submaximal doses of each. No interactive effects on water intake were observed when the 2 compounds were administered simultaneously in any study. Reduction in urine output appears to be a more sensitive response to administration of isoproterenol than increase in water intake since it was virtually abolished at a dose (2.5 μg/kg SC) that had no effect on water intake. The lowest doses of angiotensin II (25 and 50 μg/kg SC) had no significant effect on either water intake or urine output. The effect of simultaneous administration of both compounds on urine output was essentially the same as that accompanying administration of isoproterenol alone. Following administration of angiotensin II (150 μg/kg, SC) mean systemic blood pressure of unanesthetized, chronically cannulated rats reached maximal levels within 5 min and returned to pretreatment control level by 60 min. Following administration of isoproterenol (25 μg/kg, SC), mean systemic blood pressure decreased within 5 min, was maximally depressed by 30 min and had returned halfway to the pretreatment control level by 60 min. Simultaneous administration of isoproterenol and angiotensin II failed to induce a significant change in blood pressure. These results are of particular interest since they show that neither the pressor effect of angiotensin II nor the depressor effect of isoproterenol is essential for the induction of drinking by these 2 compounds.     

The alpha(2C)-adrenergic receptor mediates hyperactivity of coloboma mice, a model of attention deficit hyperactivity disorder

Drugs that modify noradrenergic transmission such as atomoxetine and clonidine are increasingly prescribed for the treatment of attention deficit hyperactivity disorder (ADHD). However, the therapeutic targets of these compounds are unknown. Norepinephrine is also implicated in the hyperactivity exhibited by coloboma mice. To identify the receptor subtypes that regulate the hyperactivity, coloboma mice were systematically challenged with adrenergic drugs. The beta-adrenergic receptor antagonist propranolol and the alpha(1)-adrenergic receptor antagonist prazosin each had little effect on the hyperactivity. Conversely, the alpha(2)-adrenergic receptor antagonist yohimbine reduced the activity of coloboma mice but not control mice. Subtype-selective blockade of alpha(2C)-, but not alpha(2A)- or alpha(2B)-adrenergic receptors, ameliorated hyperactivity of coloboma mice without affecting activity of control mice, suggesting that alpha(2C)-adrenergic receptors mediate the hyperactivity. Localized in the basal ganglia, alpha(2C)-adrenergic receptors are in a prime position to impact locomotor activity and are, therefore, potential targets of pharmacotherapy for ADHD.     

Dexmedetomidine and halothane produce similar alterations in electroencephalographic and electromyographic activity in cats

Dexmedetomidine, an α₂-adrenergic agonist, produces sedation and reduces volatile anesthetic requirements. This investigation compared the actions of dexmedetomidine and halothane on the processed EEG and on the electromyogram (EMG) which has not been previously described. Chronically instrumented cats were prepared with arterial and venous cannulae, quadriceps EMG electrodes and EEG electrodes in the lateral geniculate nucleus and over the frontal and occipital cortices. Hemodynamics, EEG and EMG were recorded in the conscious state and after randomly administered halothane or intravenous dexmedetomidine (on separate days). Blink and tail-clamp responses also assessed level of consciousness. Halothane resulted in unconsciousness and a lack of response to tail clamping, while dexmedetomidine produced profound sedation, with preservation of tail-clamp responses. Both agents similarly decreased (P<0.05) the median power frequency from 9.5±0.9 to 5.7±0.4 Hz (2% halothane) and from 9.6±0.7 to 5.9±0.8 Hz (20 μg/kg dexmedetomidine), and 95% power frequency from 23.0±0.2 to 18.2±0.6 Hz (2% halothane) and from 23.0±0.2 to 19.1±0.8 Hz (20 μg/kg dexmedetomidine). Both agents increased the total spectral power and delta band power of the EEG and reduced integrated EMG activity. Halothane and dexmedetomidine produced differing effects on level of consciousness as assessed by response to tail clamping. The results suggest that conventional processing of EEG and EMG parameters are inadequate to assess anesthetic depth in the presence of α₂-adrenergic agonists.     

Receptor subtype and dose dependence of dexmedetomidine-induced accumulation of [C]glutamine in astrocytes suggests glial involvement in its hypnotic-sedative and anesthetic-sparing effects

Dexmedetomidine, a selective α₂-adrenergic agonist, increases accumulation of [¹⁴C]glutamine and its labeled metabolites in primary cultures of mouse astrocytes. The concentration dependence is biphasic and identical to that previously described for dexmedetomidine’s effect on free cytosolic calcium concentration ([Ca²⁺]_i) in astrocytes, and both effects are exerted on the α_2A subtype of the α₂ receptor, suggesting a Ca²⁺-mediated effect. The concentration corresponding to the most potent effect is similar to that with which dexmedetomidine exerts its anesthetic-sparing activity in vivo, and the second peak corresponds to its hypnotic-sedative effect. It is suggested that both effects may be caused by decreased glutamatergic neurotransmission, secondary to reduced availability of glutamine as a glutamate precursor in glutamatergic neurons.     

Characteristics of the β1- and β2-adrenergic-sensitive adenylate cyclases in glial cell primary cultures and their comparison with β2-adrenergic-sensitive adenylate cyclase of meningeal cells

The agonist specificity pattern of the β-adrenergic adenylate cyclase in glial primary cultures was not typical of either β₁- or β₂-adrenergic receptors. The dose-response curves for adrenaline did not correspond to simple mass action kinetics and their computer analysis suggests the presence of both β₁- and β₂-adrenergic-sensitive adenylate cyclase (58 ± 17% and 42 ± 17% respectively).Similar properties of β₁- and β₂-adrenergic-sensitive adenylate cyclases were found by computer analysis of the dose-response curves for isoprenaline in the presence of a constant concentration of practolol (a selective β₁ antagonist) ( 55 ± 10% and 45 ± 10% of β₁- and β₂-sensitive adenylate cyclase respectively).The curves for displacement of [³H]dihydroalprenolol by practolol confirm these results.For purpose of comparison, the β-adrenergic receptors of meningeal cells in cultures were subjected to similar analysis. The results clearly showed that these cells exclusively contained β₂-adrenergic receptors.     

Clonidine antagonism of angiotensin-related drinking: a central site of action

Administration of either isoproterenol (25 μg/kg, s.c.) or angiotensin II (200 μg/kg, s.c.) induces drinking in rats within 0.5–1 h. This drinking was inhibited by prior administration of the presynaptic α-adrenergic agonist clonidine (12 μg/kg, i.p.). Urine output was enhanced by clonidine in the angiotensin II-, but not the isoproterenol-treated group. Drinking in response to peripheral administration of either angiotensin II or isoproterenol was also inhibited by intracerebroventricular (i.v.t.) administration of clonidine (8 μ/kg). This dose of clonidine also enhanced the urine output after angiotensin II. Further, the drinking induced by i.v.t. administration of angiotensin II, at 4 but not 20 ng/kg was inhibited by peripheral administration of clonidine (12 μg/kg, i.p.). When clonidine was administered i.v.t. prior to i.v.t. injection of either angiotensin II (20 ng/kg) or carbachol (1.2 μg/kg), the drinking response to these dipsogens was attenuated. These results suggest that clonidine may act centrally to attenuate drinking at a site, possibly in the nucleus tractus solitarius, that may be considered a final common pathway for this response.     

Moxonidine and central alpha2 adrenergic receptors in sodium intake

Central injections of the alpha(2) adrenergic/imidazoline receptor agonist moxonidine inhibit water and NaCl intake in rats. In the present study, we investigated the possible involvement of central alpha(2) adrenergic receptors on the inhibitory effect of moxonidine in 0.3 M NaCl intake induced by 24 h sodium depletion. Male Holtzman rats with stainless-steel cannulas implanted into the lateral ventricle (LV) were used. Sodium depletion was produced by the treatment with the diuretic furosemide (20 mg/kg of body weight) injected subcutaneously +24 h of sodium-deficient diet. Intracerebroventricular (icv) injections of moxonidine (20 nmol/1 microl) reduced sodium depletion-induced 0.3 M NaCl intake (6.6+/-1.9 ml/120 min vs. vehicle: 12.7+/-1.7 ml/120 min). Pre-treatment with the alpha(2) adrenoreceptor antagonists RX 821002 (80 nmol/1 microl), SK&F 86466 (640 nmol/1 microl) and yohimbine (320 nmol/3 microl) injected icv abolished the inhibitory effect of icv moxonidine on sodium depletion-induced 0.3 M NaCl intake (13.3+/-1.4, 15.7+/-1.7 and 11.8+/-2.2 ml/120 min, respectively). The results show that the activation of alpha(2) adrenoreceptors is essential for the inhibitory effect of central moxonidine on sodium depletion-induced NaCl intake.     

Characterization of β-adrenergic receptors in rat brain and pituitary using a new high-affinity ligand, [I]iodocyanopindolol

The binding of [¹²⁵I]iodocyanopindolol (ICYP) to membrane preparations from rat cerebral cortex, hypothalamus and anterior pituitary gland was characterized in regard to specificity, density, and the proportion of β-adrenergic receptor subtypes. By employing a mixture of ligands specific for α-adrenergic, serotoninergic and dopaminergic receptors, it was possible to eliminate most of the less-specific contributions to ICYP binding profiles, which resulted in narrowing the range of measured dissociation constants to 35–50 pM for all neural tissues studied. These values corresponded well with constant for the ‘slow’ component discernible in ICYP association with cerebral cortical membranes at 37° C. The maximum binding values were 63, 29 and 5.6 fM/mg membrane protein in cortical, hypothalamic and anterior pituitary membrane fractions, respectively.Evaluation of the β-adrenergic receptor subtypes using 4 selective competitors indicated an average 19% content of the β₂-subtype in cortical membranes, while in hypothalamic membranes 47% of the receptors could be assigned to that subtype. In the anterior pituitary as well as in the cerebellum, the receptors were predominantly ofβ₂-subtype. These findings are discussed in terms of possible physiological functions of β-receptors in these tissues, including the regulation of the release of pituitary hormones.