Characterization of alpha1-adrenoceptor subtypes in the pig

Chlorpheniramine exerts anxiolytic-like effects and activates prefrontal 5-HT systems in mice

RATIONALE

The traditional antihistamine chlorpheniramine ameliorates panic attacks, phobias, and lowered mood, and this therapeutic effect is independent of the blockade of histamine H(1) receptors. Since chlorpheniramine inhibits the reuptake of serotonin (5-HT), the anxiolytic-like effect of chlorpheniramine may be produced by an increase in serotonergic function.

OBJECTIVE

To elucidate the mechanisms underlying the anxiolytic-like effects of chlorpheniramine in mice, we examined the involvement of 5-HT systems in the prefrontal cortex that is a crucial region in the regulation of emotional function.

RESULTS

Chlorpheniramine (0.05-5 mg/kg, i.p.) dose-dependently and significantly decreased the duration of freezing behavior in both the elevated open-platform and conditioned fear tests. The anti-freezing effects of chlorpheniramine (5 mg/kg, i.p.) in these tests were inhibited by pretreatment with the non-selective antagonist at 5-HT receptors, methiothepin (0.01 mg/kg, s.c.). In addition, the local injection of chlorpheniramine (10-100 ng/mouse) and 5-HT (1-10 μg/mouse) into the medial part of the prefrontal cortex (mPFC) dose-dependently and significantly decreased the duration of freezing behavior in the elevated open-platform test. In a microdialysis study, chlorpheniramine (0.5 and 5 mg/kg, i.p.) dose-dependently and significantly increased the extracellular 5-HT level in the mPFC. In addition, the local perfusion of chlorpheniramine (10 and 30 μM), but not of the selective H1 receptor antagonist, cetirizine, into the mPFC markedly increased the extracellular 5-HT level in the mPFC.

CONCLUSION

The anxiolytic-like effect of chlorpheniramine is produced, at least in part, by the facilitation of serotonergic neurotransmission in the PFC.

Quantification of [11C]GB67 binding to cardiac alpha1-adrenoceptors with positron emission tomography: validation in pigs

INTRODUCTION

An increase in human cardiac alpha(1)-adrenoceptor (alpha(1)-AR) density is associated with various diseases such as myocardial ischemia, congestive heart failure, hypertrophic cardiomyopathy and hypertension. Positron emission tomography (PET) with an appropriate radioligand offers the possibility of imaging receptor function in the normal and diseased heart. [(11)C]GB67, an analogue of prazosin, has been shown in rats to have potential as a PET ligand with high selectivity to alpha(1)-AR. However, alpha(1)-AR density is up to ten times higher in rat heart compared to that in man. The aim of the present preclinical study was to extend the previous evaluation to a large mammal heart, where the alpha(1)-AR density is comparable to man, and to validate a method for quantification before PET studies in man.

METHODS

Seven [(11)C]GB67 PET studies, with weight-adjusted target dose of either 5.29 MBq kg(-1) (pilot, test-retest and baseline-predose studies) or 8.22 MBq kg(-1) (baseline-displacement studies), were performed in four anaesthetised pigs (39.5 +/- 3.9 kg). Total myocardial volume of distribution (V (T)) was estimated under different pharmacological conditions using compartmental analysis with a radiolabelled metabolite-corrected arterial plasma input function. A maximum possible blocking dose of 0.12 mumol kg(-1) of unlabeled GB67 was given 20 min before [(11)C]GB67 administration in the predose study and 45 min after administration of [(11)C]GB67 in the displacement study. In addition, [(15)O]CO (3,000 MBq) and [(15)O]H(2)O, with weight adjusted target dose of 10.57 MBq kg(-1), were also administered for estimation of blood volume recovery (RC) of the left ventricular cavity and myocardial perfusion (MBF), respectively.

RESULTS

[(11)C]GB67 V (T) values (in ml cm(-3)) were estimated to be 24.2 +/- 5.5 (range, 17.3-31.3), 10.1 (predose) and 11.6 (displacement). MBF did not differ within each pig, including between baseline and predose conditions. Predose and displacement studies showed that specific binding of [(11)C]GB67 to myocardial alpha(1)-ARs accounts for approximately 50% of V (T).

CONCLUSION

The present study offers a methodology for using [(11)C]GB67 as a radioligand to quantify human myocardial alpha(1)-ARs in clinical PET studies.

Characterization of alpha(1)-adrenoceptor subtypes in the eye

The presence of the alpha(1)-adrenoceptor subtypes in various parts of the pig and rabbit eyes was investigated using [(3)H]-prazosin radioligand binding. The characterization of the subtypes was achieved by performing competition experiments with various subtype selective drugs. In the pig retina, both alpha(1A)- and alpha(1B)-adrenoceptors were detected and the proportion of sites was 70% alpha(1A)- and 30% alpha(1B)-adrenoceptors, respectively. In the pig iris, ciliary body and choroid, which are melanin-rich tissues, the non-specific binding of [(3)H]-prazosin was too high to detect any of the alpha(1)-adrenoceptor subtypes. However, in the albino rabbit iris, ciliary body and retina both alpha(1A)- and alpha(1B)-adrenoceptors were detected. The proportion of sites in the iris was 60 % alpha(1A)- and 40% alpha(1B)-adrenoceptors, respectively. In the ciliary body and rabbit retina the proportion of sites were 70% alpha(1A)- and 30% alpha(1B)-adrenoceptors. Only the alpha(1A)-adrenoceptor subtype was detected in the rabbit choroid.