Pharmacological characterization of the discriminative stimulus effects of clenbuterol in rats

β2-Adrenoreceptor is a regulator of the α-synuclein gene driving risk of Parkinson's disease

Copy number mutations implicate excess production of α-synuclein as a possibly causative factor in Parkinson's disease (PD). Using an unbiased screen targeting endogenous gene expression, we discovered that the β2-adrenoreceptor (β2AR) is a regulator of the α-synuclein gene (SNCA). β2AR ligands modulate SNCA transcription through histone 3 lysine 27 acetylation of its promoter and enhancers. Over 11 years of follow-up in 4 million Norwegians, the β2AR agonist salbutamol, a brain-penetrant asthma medication, was associated with reduced risk of developing PD (rate ratio, 0.66; 95% confidence interval, 0.58 to 0.76). Conversely, a β2AR antagonist correlated with increased risk. β2AR activation protected model mice and patient-derived cells. Thus, β2AR is linked to transcription of α-synuclein and risk of PD in a ligand-specific fashion and constitutes a potential target for therapies.

S(-)Propranolol as a discriminative stimulus and its comparison to the stimulus effects of cocaine in rats

RATIONALE

Racemic propranolol (PRO), a beta-adrenoceptor antagonist, has been evaluated as a test agent but not as a discriminative stimulus. Its S(-) stereoisomer is thought to subserve the effects of (+/-)PRO.

MATERIALS AND METHODS

Rats were trained to discriminate S(-)PRO (5 mg/kg) from saline in a two-lever food-reinforced operant conditioning task.

RESULTS

The S(-)PRO stimulus was shown to be centrally mediated, dose-related, time dependent, and stereoselective: S(-)PRO (ED(50) = 2.2 mg/kg) was twice as potent as (+/-)PRO and approximately four times as potent as R(+)PRO. The S(-)PRO stimulus generalized fully to the beta-adrenoceptor agent pindolol, the alpha(1)-adrenoceptor agonist methoxamine, cocaine, and the serotonergic agents TFMPP and RU 24969; partial generalization occurred to (-)ephedrine and nisoxetine but not to fenfluramine or 5-OMe DMT. The S(-)PRO stimulus was blocked completely (and competitively) when prazosin, an alpha(1)-adrenoceptor antagonist, was given in combination with the training dose of S(-)PRO. Moreover, prazosin exerted antagonism of the S(-)PRO-like effect of (+/-)PRO or R(+)PRO but produced only partial antagonism of the S(-)PRO-like effect of cocaine. In a second study, rats were trained to discriminate 8 mg/kg of cocaine from saline. The cocaine stimulus generalized to S(-)PRO, (+/-)PRO, and R(+)PRO. Prazosin partially attenuated the stimulus effect of cocaine (8 mg/kg) but completely blocked the cocaine-like effects of (+/-), S(-), and R(+)PRO.

CONCLUSIONS

PRO and cocaine exhibited cross-substitution, but their stimulus effects were antagonized differentially by prazosin. PRO (and its optical isomers) can exert a stimulus effect that is based, at least in part, on increased alpha(1)-adrenoceptor activity. PRO might be better characterized as an adrenoceptor partial agonist.

Alpha 1-noradrenergic system role in increased motivation for cocaine intake in rats with prolonged access

In rodents, extended access to cocaine produces an escalation in cocaine self-administration that has face and construct validity for human compulsive drug intake. Here we report that rats with six-hour access (long access, LgA) to cocaine self-administration produced a higher breakpoint for cocaine using a progressive-ratio schedule than rats with one-hour access (short access, ShA), and prazosin (alpha 1 receptor antagonist) reduced the higher breakpoint for cocaine in LgA rats. Additionally, the number of neurons with alpha 1-adrenergic receptor-like immunoreactivity in the bed nucleus of stria terminalis (BNST) was found to be much lower in LgA rats than in ShA and drug-naive rats. In contrast, UK14304 (alpha 2 receptor agonist) and betaxolol (beta 1 receptor antagonist) had no effect on cocaine self-administration in either group. The data suggest that activation of the alpha 1-noradrenergic system, perhaps in the BNST, is associated with increased motivation for cocaine in rats with extended access.

Discriminative stimulus properties of the selective norepinephrine reuptake inhibitor, reboxetine, in rats: a characterization with alpha/beta-adrenoceptor subtype selective ligands, antidepressants, and antagonists at neuropeptide receptors

Although little information is available concerning discriminative stimulus (DS) properties of antidepressants, rats can be trained to recognize the selective norepinephrine (NE) reuptake inhibitor, reboxetine (2.5 mg/kg i.p.). By analogy to reboxetine (effective dose50, 1.1), 'full' (80%) substitution dose50 was obtained with the NE reuptake inhibitors, nisoxetine (4.9), nomifensine (0.5) and BW1555,U88 (1.0). Full substitution was also attained with the NE/serotonin (5-HT) reuptake inhibitors, S33005 (0.3), venlafaxine (4.8) and duloxetine (26.8), and the tricyclics, imipramine (2.5) and clomipramine (2.9). In contrast, the 5-HT reuptake inhibitors, citalopram, sertraline and paroxetine (all >2.5), and the 5-HT reuptake inhibitors/5-HT2 receptor antagonists, nefazodone and trazodone (both >10.0), did not substitute for reboxetine. The 'atypical' antidepressants, mirtazapine (>10.0) and mianserin (>2.5), similarly failed to substitute. DS properties of reboxetine were dose-dependently blocked by the alpha1-adrenoceptor (AR) antagonists, prazosin (inhibitory dose50, 0.3) and WB4101 (0.5), but resistant to the alpha2-AR antagonists, atipamezole (>0.63), idazoxan (>2.5) and RX821,002 (>0.08), and to the beta1-AR and beta2-AR antagonists, betaxolol (>2.5) and ICI118,551 (>10.0). Interestingly, the neurokinin-1 receptor antagonist, GR205,171, stereospecifically substituted for reboxetine (1.1) compared to its less active isomer, GR226,206 (>10.0). The corticotrophin-releasing factor-1 antagonists, DMP695 (>40), CP154,526 (>10.0) and SN003 (>40.0), and the melanin-concentrating hormone-1 antagonist, SNAP-7941 (>40.0), failed to substitute for reboxetine. In conclusion, DS properties of reboxetine are mimicked by antidepressants recognizing NE transporters, and require functionally intact alpha1-ARs for their expression. The neurokinin-1 antagonist, GR205,171, mimics the interoceptive properties of reboxetine, possibly reflecting its elevation of extracellular levels of NE in corticolimbic structures.

The effect of long-term exposure to combinations of growth promoters in Long Evans rats

The aim of the study was to investigate whether the chronic administration (45 days) of clenbuterol (CB) at a growth promoting dose (1 mg kg(-1) bw) and/or dexamethasone (DEX: 0.1 mg kg(-1) bw) may cause the disruption of rat endocrine adrenal function. Blood samples were taken weekly during the whole experiment (S0-S7), and at different days of withdrawal (W0, W5, W10, W15 and W20). Hormone profiles were determined by RIA (ACTH) or EIA (corticosterone and catecholamines). ACTH showed significantly elevated concentrations from S1 until W5 (p<0.05) with CB administration. It began to decrease the day of DEX and CB-DEX administration. DEX showed significantly lowered ACTH concentrations from the day of drug injection (p<0.05). Corticosterone showed significantly elevated levels until W10 (p<0.01) with CB and CB+DEX. DEX showed lowered levels of corticosterone during the whole withdrawal period. Epinephrine presented significantly elevated plasma levels until W5 with CB and CB+DEX. With DEX, epinephrine was also elevated from W5 to W15 (p<0.05). Norepinephrine also presented significantly elevated plasma levels until S7 with CB and CB+DEX (p<0.001). With DEX no differences were found.

CONCLUSION

Long-term administration of CB and/or DEX causes an endocrine adrenal disruption with changes in ACTH, glucocorticoid and catecholamine secretion.

The effect of long-term exposure to combinations of growth promoters in Long Evans rats: part 2. Adrenal morphology (histopathology and immunochemical studies)

The aim of the study was to investigate the effects of long-term exposure (45 days) to growth promoters: clenbuterol (CB: 1 mg kg(-1) bw) and/or dexamethasone (DEX: 0.1 mg kg(-1) bw), in adrenal gland morphology, and the possibility of recovery after the withdrawal of drug treatment. Animals were sacrificed at different days of withdrawal (W0, W5, W10, W15 and W20), and adrenal glands processed for histopathology and immunohistochemistry. Adrenals of CB treatment showed typical features of long-term administration of beta-agonists at W0 such as capillary dilatation in the fasciculata-reticularis zone, and this feature was also presented at W20. Adrenals of CB+DEX treatments showed the same results of CB treatment at days W0 and W20. However, DEX treatment presented the typical results of the exposure to corticoids with the atrophy of adrenal cortex. Immunohistochemistry of adrenal cortex steroidogenic enzymes (P450: scc, 3beta-HSD, aromatase) denoted that neither positive staining nor localization was affected by treatments. Aromatase enzyme was immunolocalized in adrenal medulla cells in controls as well as in treated groups. The immunolocalization of glucocorticoid receptors showed an increase in CB (+++) and CB+DEX (++) treatments compared to the control group (0) and DEX treatment (0). Histopathological and immunohistochemical results are closely related to those found for adrenal endocrine function. We can conclude that chronic administration of growth promoters influence adrenal morphology and glucocorticoid receptor expression.

β 2 Adrenergic antagonist inhibits cerebral cortical oxygen delivery after severe haemodilution in rats

BACKGROUND

Haemodilution has been associated with neurological morbidity in surgical patients. This study tests the hypothesis that inhibition of cerebral vasodilatation by systemic beta2 adrenergic blockade would impair cerebral oxygen delivery leading to tissue hypoxia in severely haemodiluted rats.

METHODS

Under general anaesthesia, cerebral tissue probes were placed to measure temperature, regional cerebral blood flow (rCBF) and tissue oxygen tension (P(Br)O2) in the parietal cerebral cortex or hippocampus. Baseline measurements were established before and after systemic administration of either a beta2 antagonist (10 mg kg(-1) i.v., ICI 118, 551) or saline vehicle. Acute haemodilution was then performed by simultaneously exchanging 50% of the estimated blood volume (30 ml kg(-1)) with pentastarch. Arterial blood gases (ABGs), haemoglobin concentration (co-oximetry), mean arterial blood pressure (MAP) and heart rate (HR) were also measured. Data were analysed using a two-way anova and post hoc Tukey's test [mean (sd)].

RESULTS

Haemodilution reduced the haemoglobin concentration comparably in all groups [71 (9) g litre(-1)]. There were no differences in ABGs, co-oximetry, HR and MAP measurements between control and beta2 blocked rats, either before or 60 min after drug or vehicle administration. In rats treated with the beta2 antagonist there was a significant reduction in parietal cerebral cortical temperature, regional blood flow and tissue oxygen tension, relative to control rats, 60 min after haemodilution (P<0.05 for each). These differences were not observed when probes were placed in the hippocampus.

CONCLUSION

Systemic beta2 adrenergic blockade inhibited the compensatory increase in parietal cerebral cortical oxygen delivery after haemodilution thereby reducing cerebral cortical tissue oxygen tension.

Mercaptoacetate fails to block the feeding-inhibitory effect of the β3-adrenergic receptor agonist CGP 12177A

Peripherally administered beta3-adrenergic receptor (beta3-AR) agonists stimulate lipolysis and inhibit food intake. To test the hypothesis that this inhibition of feeding is due to a substrate-driven increase in hepatic fatty acid oxidation (FAO), we assessed the ability of the FAO inhibitor mercaptoacetate (MA) to reverse the feeding-inhibitory effect of the beta3-AR agonist CGP 12177A (CGP). Adult male Sprague-Dawley rats received intraperitoneal injections of 1 mg/kg CGP, of 45.6 mg/kg MA, or of both drugs, and the effects on food intake, plasma free fatty acids (FFA), and plasma beta-hydroxybutyrate (BHB), an indicator for hepatic FAO, were assessed. Control rats received saline injections. CGP significantly reduced food intake after 0.5 and 6 h and increased plasma FFA and BHB at 0.5 h, suggesting increased lipolysis and hepatic FAO. MA completely reversed the increase in plasma BHB and thus appeared to effectively abolish CGP's effect on hepatic FAO, but MA failed to affect CGP's feeding-inhibitory action. These findings do not support the hypothesis that the beta3-AR agonist CGP inhibits feeding by enhancing hepatic FAO or ketogenesis. Although the beta3-AR agonist CGP reduced saccharin intake in a one-bottle condition taste aversion test, it seems unlikely that the hypophagic effect of CGP is elicited by malaise.

Multi-target strategies for the improved treatment of depressive states: Conceptual foundations and neuronal substrates, drug discovery and therapeutic application

Major depression is a debilitating and recurrent disorder with a substantial lifetime risk and a high social cost. Depressed patients generally display co-morbid symptoms, and depression frequently accompanies other serious disorders. Currently available drugs display limited efficacy and a pronounced delay to onset of action, and all provoke distressing side effects. Cloning of the human genome has fuelled expectations that symptomatic treatment may soon become more rapid and effective, and that depressive states may ultimately be "prevented" or "cured". In pursuing these objectives, in particular for genome-derived, non-monoaminergic targets, "specificity" of drug actions is often emphasized. That is, priority is afforded to agents that interact exclusively with a single site hypothesized as critically involved in the pathogenesis and/or control of depression. Certain highly selective drugs may prove effective, and they remain indispensable in the experimental (and clinical) evaluation of the significance of novel mechanisms. However, by analogy to other multifactorial disorders, "multi-target" agents may be better adapted to the improved treatment of depressive states. Support for this contention is garnered from a broad palette of observations, ranging from mechanisms of action of adjunctive drug combinations and electroconvulsive therapy to "network theory" analysis of the etiology and management of depressive states. The review also outlines opportunities to be exploited, and challenges to be addressed, in the discovery and characterization of drugs recognizing multiple targets. Finally, a diversity of multi-target strategies is proposed for the more efficacious and rapid control of core and co-morbid symptoms of depression, together with improved tolerance relative to currently available agents.

Drug discrimination in neurobiology

Areas of neurobiological interest are identified towards which drug discrimination (DD) studies have made important contributions. DD allows ligand actions to be analyzed at the whole organism level, with a neurobiological specificity that is exquisite and often unrivalled. DD analyses have thus been made of a vast array of CNS agents acting on receptors, enzymes, or ion channels, including most drugs of abuse. DD uniquely offers access to the study of subjective drug effects in animals, using a methodology that also is transposable to humans and has generated unprecedented models of pathology (e.g., chronic pain, opiate addiction). Parametric studies of such independent variables as training dose and reinforcement provide refined insights into the dynamic psychophysiological mechanisms of both drug effects and behavior. Three different mechanisms have been identified by which discriminative, and perhaps other behaviors, can come about. DD also is superbly sensitive to small, partial activation of molecular substrates; this has enabled DD analyses to pioneer the unravelling of molecular mechanisms of drug action (attributing, f.ex., LSD's particular subjective effects to an unusual, partial activation of 5-HT, and perhaps other receptors). DD has both oriented and served as a tool to conduct drug discovery research (e.g., pirenperone-risperidone, loperamide). The DD response arguably constitutes a quantal, rather than graded, variable, and as such allows a comprehension of molecular, pharmacological, and behavioral mechanisms that would have been otherwise inaccessible. Perhaps most important are the following further contributions. One is the notion that particular, different levels of receptor activation are associated with qualities of neurobiological actions that also differ and are unique, this notion arguably constituting the most significant addition to affinity and intrinsic activity since the earliest theoretical conceptions of molecular pharmacology. Another contribution consists of studies that render redundant the notion of tolerance and identify fundamental mechanisms of signal transduction; these mechanisms account for apparent tolerance, dependence, addiction, and sensitization, and appear to operate ubiquitously in a bewildering array of biological systems.

Effects of antidepressants in rats trained to discriminate the beta-2 adrenergic agonist clenbuterol

The ability of indirectly acting agonists such as norepinephrine uptake inhibitors, serotonin reuptake inhibitors, and atypical antidepressants to substitute for clenbuterol, a beta-2 adrenergic agonist, was examined in rats trained to discriminate 0.03 mg/kg clenbuterol and saline using a fixed-ratio 10 (FR 10) schedule with water reinforcement. The beta-2 selective adrenergic agonist clenbuterol produced an orderly dose-response relationship, and its discriminative stimulus effects were antagonized by the beta-adrenergic antagonist propranolol. It was found that the effects of tricyclic antidepressants and selective norepinephrine uptake inhibitors did not generalize to the discriminative stimulus effects of clenbuterol, with the exception of high doses of protriptyline. Moreover, compounds from other drug classes, including fluoxetine and phenelzine, did not substitute for clenbuterol. Atypical antidepressants, such as trazodone, rolipram, and bupropion also did not engender drug-appropriate responding. Prenalterol and dobutamine, both purported to be beta-1 adrenergic receptor agonists, partially substituted for clenbuterol, but at relatively high doses. The present results show that the antidepressants tested do not share discriminative stimulus effects with clenbuterol, a beta-2 adrenergic agonist.