Flerobuterol, a beta-adrenoceptor agonist, enhances serotonergic neurotransmission: an electrophysiological study in the rat brain

Effects of salbutamol on the contractile properties of human skeletal muscle before and after fatigue

AIM

The study examined the effects of an oral acute administration of the β2-agonist salbutamol (Sal) (6 mg) vs. placebo on muscle strength and fatigability in 12 non-asthmatic recreational male athletes in a randomized double-blind protocol.

METHODS

Contractile properties of the right quadriceps muscle were measured during electrical stimulations, i.e. twitch, 1-s pulse trains at 20 (P(20) ) and 80 Hz (P(80) ) and during maximal voluntary isometric contraction (MVIC) before (PRE) and after (POST) a fatigue-producing protocol set by an electromyostimulation (30 contractions, frequency: 75 Hz, on-off ratio: 6.25-20s). In addition, the level of muscle voluntary activation was measured.

RESULTS

In PRE and POST conditions, the peak torque (PT) of twitch, P(80) and MVIC were not modified by the treatment. The PT in POST P(20) was slightly, although not significantly, less affected by fatigue in Sal compared with placebo condition. Moreover, twitch half-relaxation time at PRE was smaller under Sal than under placebo (P < 0.05). No significant changes in the degree of voluntary activation were observed with Sal treatment in PRE or POST condition.

CONCLUSION

  Although these findings did not exclude completely an effect of Sal on peripheral factors of human skeletal muscle, oral acute administration of the β2-agonist Sal seems to be without any relevant ergogenic effect on muscle contractility and fatigability in non-asthmatic recreational male athletes.

The neurobiology and control of anxious states

Fear is an adaptive component of the acute "stress" response to potentially-dangerous (external and internal) stimuli which threaten to perturb homeostasis. However, when disproportional in intensity, chronic and/or irreversible, or not associated with any genuine risk, it may be symptomatic of a debilitating anxious state: for example, social phobia, panic attacks or generalized anxiety disorder. In view of the importance of guaranteeing an appropriate emotional response to aversive events, it is not surprising that a diversity of mechanisms are involved in the induction and inhibition of anxious states. Apart from conventional neurotransmitters, such as monoamines, gamma-amino-butyric acid (GABA) and glutamate, many other modulators have been implicated, including: adenosine, cannabinoids, numerous neuropeptides, hormones, neurotrophins, cytokines and several cellular mediators. Accordingly, though benzodiazepines (which reinforce transmission at GABA(A) receptors), serotonin (5-HT)(1A) receptor agonists and 5-HT reuptake inhibitors are currently the principle drugs employed in the management of anxiety disorders, there is considerable scope for the development of alternative therapies. In addition to cellular, anatomical and neurochemical strategies, behavioral models are indispensable for the characterization of anxious states and their modulation. Amongst diverse paradigms, conflict procedures--in which subjects experience opposing impulses of desire and fear--are of especial conceptual and therapeutic pertinence. For example, in the Vogel Conflict Test (VCT), the ability of drugs to release punishment-suppressed drinking behavior is evaluated. In reviewing the neurobiology of anxious states, the present article focuses in particular upon: the multifarious and complex roles of individual modulators, often as a function of the specific receptor type and neuronal substrate involved in their actions; novel targets for the management of anxiety disorders; the influence of neurotransmitters and other agents upon performance in the VCT; data acquired from complementary pharmacological and genetic strategies and, finally, several open questions likely to orientate future experimental- and clinical-research. In view of the recent proliferation of mechanisms implicated in the pathogenesis, modulation and, potentially, treatment of anxiety disorders, this is an opportune moment to survey their functional and pathophysiological significance, and to assess their influence upon performance in the VCT and other models of potential anxiolytic properties.

Effect of the beta-adrenoceptor agonist flerobuterol on serotonin synthesis in the rat brain

The influence of 2- and 14-day treatments with flerobuterol, a preferential beta(2)-adrenoceptor agonist, on regional serotonin (5-HT) synthesis in the rat brain was studied by autoradiography using alpha-[(14)C]methyl-L-tryptophan. Flerobuterol was delivered at a rate of 0.5 mg/kg/day using osmotic pumps implanted s.c. The 2-day flerobuterol treatment significantly increased plasma Trp, both free and total, and decreased plasma Leu and Ile. This resulted in a significant increase in the facilitated transport of Trp. There was a significant increase in the synthesis of 5-HT in the 2-day treatment group in the dorsal and median raphe as well as in all postsynaptic structures, with the exception of the hypothalamus. In contrast, after a 14-day treatment, the enhanced facilitated transport of Trp was no longer present, and the increase in the rate of 5-HT synthesis persisted only in the parietal and occipital cortex and the superior colliculus. These data suggest that flerobuterol, similar to other beta-adrenergic agonists, acutely increases 5-HT synthesis, in part, through an elevation of brain Trp availability.

The serotonergic and noradrenergic systems of the hippocampus: their interactions and the effects of antidepressant treatments

Previous reviews have well illustrated how antidepressant treatments can differentially alter several neurotransmitter systems in various brain areas. This review focuses on the effects of distinct classes of antidepressant treatments on the serotonergic and the noradrenergic systems of the hippocampus, which is one of the brain limbic areas thought to be relevant in depression: it illustrates the complexity of action of these treatments in a single brain area. First, the basic elements (receptors, second messengers, ion channels, ...) of the serotonergic and noradrenergic systems of the hippocampus are revisited and compared. Second, the extensive interactions occurring between the serotonergic and the noradrenergic systems of the brain are described. Finally, issues concerning the short- and long-term effects of antidepressant treatments on these systems are broadly discussed. Although there are some contradictions, the bulk of data suggests that antidepressant treatments work in the hippocampus by increasing and decreasing, respectively, serotonergic and noradrenergic neurotransmission. This hypothesis is discussed in the context of the purported function of the hippocampus in the formation of memory traces and emotion-related behaviors.

Reduced survival after isoprenaline/dopamine in d,l-propranolol intoxicated rats

1. Respiratory and cardiovascular failure are principle toxic effects of beta-blocker overdose. Respiratory arrest is the primary cause of death in beta-blocker intoxicated rats. 2. The effect of beta-adrenoceptor agonists on respiratory and cardiovascular failure in beta-blocker overdose was investigated in a model of acute d,l-propranolol (30 mg kg-1 h-1) intoxication in spontaneously breathing rats. 3. Neither the aselective, hydrophilic beta-agonist isoprenaline (10, 25, 50 micrograms kg-1 min-1), nor the beta 1-selective, lipophilic beta-agonist flerobuterol (1, 3, 10 microgram kg-1 min-1) and the beta 2-selective, lipophilic beta-agonist clenbuterol (10, 25, 50 micrograms kg-1 min-1) had any beneficial effect on cardiovascular and respiratory variables or survival time in d,l-propranolol intoxicated spontaneously breathing rats. 4. Isoprenaline (10 micrograms kg-1 min-1) had no favourable effect on haemodynamic and respiratory variables in artificially ventilated d,l-propranolol intoxicated rats either. 5. Addition of dopamine to isoprenaline resulted in a significant reduction of survival time, primarily caused by a decreased in mean arterial blood pressure, in artificially ventilated d,l-propranolol intoxicated rats. Addition of glucagon to isoprenaline did not affect survival time. 6. Artificial ventilation is the most important supportive measure in d,l-propranolol intoxication in the rat.

Alpha 1-adrenoceptor antagonists differentially control serotonin release in the hippocampus and striatum: a microdialysis study

Using the in vivo microdialysis technique, we have studied the effect of the systemic administration of several alpha 1-adrenoceptor antagonists on the extracellular levels of serotonin (5-HT) in the rat hippocampus. Prazosin, and to a lesser extent, terazosin, decreased these levels by 50-65% for 0.03-0.4 mg/kg, i.v. and by 30-40% for 0.1-0.4 mg/kg, i.v., respectively. In contrast, alfuzosin, an alpha 1-adrenoceptor antagonist with poor brain penetration, did not significantly affect these levels even at the high dose of 0.4 mg/kg, i.v. When perfused into the hippocampus through the dialysis probe, prazosin (1-10 microM) induced a more limited (20-30%) and delayed decrease in 5-HT outflow. These results support the existence of a central noradrenergic facilitatory influence, mediated by alpha 1-adrenoceptors, on serotonergic neurons projecting to the hippocampus. In the striatum prazosin (0.4 mg/kg, i.v.) decreased 5-HT levels to a smaller extent (-35%) than in the hippocampus (-65%), suggesting the existence of differences in the degree of noradrenergic influence on median and dorsal raphé nuclei, which preferentially project to the hippocampus and striatum, respectively.

Actions of lithium on the cyclic AMP signalling system in various regions of the brain--possible relations to its psychotropic actions. A study on the adenylate cyclase in rat cerebral cortex, corpus striatum and hippocampus

It has been estimated that in most industrialized countries 1 person out of every 1000 in the population is undergoing lithium treatment to stabilize their episodic mood disturbances due to manic-depressive illness. Lithium may stabilize mood swings by altering the action of certain neurotransmitters at the synaptic level in the brain. Recent research suggests that lithium alters neurotransmission by affecting neurotransmitter-coupled second messenger systems. A major second messenger system is the adenylate cyclase, which generates intracellular cAMP from ATP. The adenylate cyclases (type I-IV) are regulated by stimulatory and inhibitory receptors, which either stimulate or inhibit the adenylate cyclase activity. The stimulatory and inhibitory neurotransmitter-receptor signals are transferred to the catalytic unit of the adenylate cyclase by Gs and Gi, respectively. The activated receptor induces GTP stimulation of the heterotrimeric G protein, leading to a dissociation of the protein into the active alpha*GTP and the beta gamma complex. The former stimulates the catalytic unit of adenylate cyclase. The stimulation is terminated by a GTPase located on the alpha subunit that converts GTP to inactive GDP. At present, G proteins are known to play a central role in coupling receptors to effector proteins. In addition to extracellular regulation due to neurotransmitters, some adenylate cyclases (type I, III) are regulated by CaM as a consequence of enhanced intracellular concentrations of free Ca2+. The Ca(2+)-dependent stimulation of adenylate cyclase by CaM is assumed to occur by a direct effect on the catalytic unit. The catalytic units sensitive to Ca(2+)-CaM are also subjected to regulation by stimulatory and inhibitory neurotransmitter stimuli. Magnesium is essential for adenylate cyclase activity, since MgATP2- is the enzyme substrate. Furthermore, one Mg2+ site located on the G protein regulates both the receptor agonist affinity and the dissociation of the G protein during the activation cycle. A second Mg2+ site on the catalytic unit is responsible for Mg2+ regulation of the catalytic activity. The present work aimed at investigating the mechanisms by which lithium in vitro and after chronic treatment (ex vivo) affects adenylate cyclase activities in various regions of the rat brain. Lithium in vitro and ex vivo inhibited the selective stimulation of adenylate cyclase by Ca(2+)-CaM in the cerebral cortex. Furthermore, lithium in vitro interacted directly with the catalytic unit of adenylate cyclase.(ABSTRACT TRUNCATED AT 400 WORDS)

Effect of the putative 5-HT1A receptor antagonist NAN-190 on rat brain serotonergic transmission

Based on the fact that 1-(2-methoxyphenyl)-4(4-(2-phtalimido)butyl)piperazine (NAN-190), a high-affinity ligand for 5-HT1A and alpha 1-adrenoceptors, antagonizes the behavioural effects of the 5-HT1A receptor agonist 8-OH-DPAT (8-hydroxy-2-(di-n-propylamino)tetralin), it has been suggested that this drug behaves as a 5-HT1A receptor antagonist. In the present study we examined the effects of this putative 5-HT1A receptor antagonist on rat brain serotonergic neurotransmission. In hippocampal slices of immature rats, NAN-190 but not prazosin potently antagonized (IC50 = 29 nM) the inhibitory effect of 8-OH-DPAT (1 microM) on carbachol-stimulated phosphoinositide turnover but (up to 1 microM) failed to alter the carbachol response. Similarly, NAN-190 (0.1 microM) almost totally prevented the inhibition by 8-OH-DPAT (1 microM) of forskolin-stimulated adenylate cyclase activity in adult rat hippocampal slices but, per se, was without effect on the forskolin response. These results indicate that NAN-190 is a potent antagonist at postsynaptic 5-HT1A receptors in vitro. However, NAN-190 also potently antagonized (IC50 = 0.16 nM) the stimulation by norepinephrine of phosphoinositide turnover in rat cortical slices. In this respect NAN-190 was a 250-fold more potent antagonist than prazosin (IC50 = 49 nM). Thus, in addition to its 5-HT1A receptor antagonist properties, NAN-190 has potent alpha 1-adrenoceptor blocking properties.(ABSTRACT TRUNCATED AT 250 WORDS)