Enhancement of excitability and inhibitory processes in hippocampal dentate gyrus by noradrenaline: a pharmacological study in awake, freely moving rats.

Idazoxan (IDA), an alpha 2 receptor antagonist which increases firing rate of noradrenergic neurons in the locus coeruleus (LC) and release of noradrenaline (NA) in target structures, was used to study the neuromodulatory effects of NA in the hippocampus in awake rats. After IDA the population spike in the dentate gyrus (DG), evoked by a single pulse to the perforant path, was greatly enhanced with no effect of the drug on excitatory postsynaptic potentials (EPSPs). Paired pulses with short interpulse intervals (25-30 ms) produced inhibition of the response to the second pulse which was increased by IDA. This drug effect was independent of its effect on the amplitude of the first spike, since the increase in inhibition was seen at stimulation intensities which did not increase the response amplitude to the initial pulse. Thus both excitability and inhibitory processes can be enhanced in the same population of neurons by an alpha 2 adrenoceptor antagonist.

Preclinical profile of befloxatone, a new reversible MAO-A inhibitor.

Befloxatone, a novel oxazolidinone derivative, is a potent, selective and reversible monoamine oxidase A (MAO-A) inhibitor in vitro (K1A = 1.9-3.6 nM) and ex vivo (ED50 MAO-A = 0.02 mg/kg, p.o.). It does not interact with a large number of receptors, monoamine transporters or other amine oxidases. Binding studies with [3H]-befloxatone in rat brain sections show that it labels with high affinity (Kd = 1.3 nM) a single population of sites with the pharmacological characteristics and regional distribution of MAO-A. In the rat brain, befloxatone (0.75 mg/kg, i.p.) increases tissue levels of monoamines and decreases levels of their deaminated metabolites. Acute administration of befloxatone (0.75 mg/kg, i.p.) induces an increase in extracellular striatal dopamine and cortical norepinephrine but not cortical serotonin levels in the rat. Befloxatone (1 mg/kg, i.p.) potently inhibits the firing rate of serotonergic neurons, partially decreases the firing of noradrenergic neurons and has no effect on the firing of dopaminergic neurons (a mirror image of its effects on monoamine release in terminal regions), suggesting that the relative effects of befloxatone on monoamine release may be governed by autoreceptor-mediated control of monoaminergic neurons at the cell body level. Befloxatone (0.03-0.3 mg/kg, p.o.) exhibits potent activity in behavioural models predictive of antidepressant activity. Befloxatone (up to 1.5 mg/kg, p.o.) does not potentiate the pressor effects of orally administered tyramine at centrally active doses and duodenal [3H]-befloxatone binding is displaced by increasing doses of orally administered tyramine (0.1-40 mg/kg, i.p.). These results suggest that befloxatone is a potent reversible MAO-A inhibitor with antidepressant potential and a wide safety margin with regard to the potentiation of the pressor effect of tyramine.

GABAA and alpha-amino-3-hydroxy-5-methylsoxazole-4-propionate receptors are differentially affected by aging in the rat hippocampus.

We have investigated the age-dependent modifications in the expression of eight different subunits of the gamma-aminobutyric acid, type A (GABA(A)) receptor (alpha1, alpha2, alpha3, alpha5, beta2, beta3, gamma2S, and gamma2L) and all four subunits of the alpha-amino-3-hydroxy-5-methylsoxazole-4-propionate (AMPA) receptor (GluR1-4) in the hippocampus of 24-month-old rats. All aged hippocampi displayed a remarkable increase (aged/adult ratio, 3.53 +/- 0.54) in the mRNA levels of the short version of the gamma2 subunit in parallel with a similar increase in the gamma2 subunit protein (aged/adult ratio, 2.90 +/- 0.62). However, this increase was not observed in the mature receptor. On the other hand, the expression of the different alpha subunit mRNAs increased moderately with aging, displaying a heterogeneous pattern. The most frequent modification consisted in an increase in the expression of the alpha1 subunit mRNA (aged/adult ratio, 1.26 +/- 0.18), in parallel with a similar increase on the alpha1 protein (aged/adult ratio, 1. 27 +/- 0.12) and in the alpha1 incorporated to the assembled GABA(A) receptor (tested by immunoprecipitation; aged/adult ratio, = 1.20 +/- 0.10). However, in the same hippocampal samples, no major modifications were observed on the expression of the AMPA receptor subunits. As a whole, these results indicated the existence of an increased expression of the GABA(A) receptor subunits and a preservation of the AMPA receptor at the hippocampal formation. These modifications could reflect the existence of specific deficiencies (neuronal loss and/or deafferentiation) on the GABAergic system in the aged rats.

Befloxatone, a new reversible and selective monoamine oxidase-A inhibitor. II. Pharmacological profile.

The pharmacological profile of befloxatone, a reversible, selective and competitive inhibitor of monoamine oxidase-A has been investigated in rodents. In mice, befloxatone was more active at potentiating generalized tremors induced by L-5-hydroxytryptophan (ED50, 0.21 mg/kg p.o.) than phenylethylamine-induced stereotypies (ED50, 58 mg/kg p.o.), indicating a very high in vivo selectivity for inhibition of the A form of monoamine oxidase. Befloxatone showed potent activity in behavioral models in rodents predictive of antidepressant activity (forced swimming test, learned helplessness and reserpine reversal) with minimal effective doses of 0.1 to 0.2 mg/kg p.o. In these tests, befloxatone was much more potent (10- to 500-fold) than reference antidepressant compounds (reversible and irreversible monoamine oxidase inhibitors and monoamine reuptake inhibitors). In rats, befloxatone increased rapid eye movement sleep latency and decreased rapid eye movement sleep duration, without rebound effects. Potential anxiolytic activity was observed in the elevated-plus maze test in rats (minimal effective dose, 1-2 mg/kg p.o.). Befloxatone had no effect on motor performance, did not induce sedative or stimulant activity up to doses of 200 mg/kg p.o. and was devoid of anticholinergic activity in mice. Interaction studies with p.o. dietary tyramine (12 mg/kg), carried out in freely moving rats, demonstrated that, in contrast to irreversible monoamine oxidase inhibitors, befloxatone did not potentiate the pressor effect of this amine in the range of doses which showed pharmacological activity in antidepressant behavioral models. Furthermore, of the compounds tested (moclobemide, brofaromine, nialamide and phenelzine), comparison of doses active in antidepressant models and doses potentiating the pressor effects of tyramine demonstrated that befloxatone had the best therapeutic index. The results suggest that befloxatone will show clinical antidepressant activity at low doses and will be devoid of the side effects associated with irreversible monoamine oxidase inhibitors.

SSR591813, a novel selective and partial alpha4beta2 nicotinic receptor agonist with potential as an aid to smoking cessation.

(5aS,8S,10aR)-5a,6,9,10-Tetrahydro,7H,11H-8,10a-methanopyrido[2',3':5,6]pyrano[2,3-d]azepine (SSR591813) is a novel compound that binds with high affinity to the rat and human alpha4beta2 nicotinic acetylcholine receptor (nAChR) subtypes (Ki = 107 and 36 nM, respectively) and displays selectivity for the alpha4beta2 nAChR (Ki, human alpha3beta4 > 1000, alpha3beta2 = 116; alpha1beta1deltagamma > 6000 nM and rat alpha7 > 6000 nM). Electrophysiological experiments indicate that SSR591813 is a partial agonist at the human alpha4beta2 nAChR subtype (EC50 = 1.3 micro M, IA =19% compared with the full agonist 1,1-dimethyl-4-phenyl-piperazinium). In vivo findings from microdialysis and drug discrimination studies confirm the partial intrinsic activity of SSR591813. The drug increases dopamine release in the nucleus accumbens shell (30 mg/kg i.p.) and generalizes to nicotine or amphetamine (10-20 mg/kg i.p.) in rats, with an efficacy approximately 2-fold lower than that of nicotine. Pretreatment with SSR591813 (10 mg/kg i.p.) reduces the dopamine-releasing and discriminative effects of nicotine. SSR591813 shows activity in animal models of nicotine dependence at doses devoid of unwanted side effects typically observed with nicotine (hypothermia and cardiovascular effects). The compound (10 mg/kg i.p.) also prevents withdrawal signs precipitated by mecamylamine in nicotine-dependent rats and partially blocks the discriminative cue of an acute precipitated withdrawal. SSR591813 (20 mg/kg i.p.) reduces i.v. nicotine self-administration and antagonizes nicotine-induced behavioral sensitization in rats. The present results confirm important role for alpha4beta2 nAChRs in mediating nicotine dependence and suggest that SSR591813, a partial agonist at this particular nAChR subtype, may have therapeutic potential in the clinical management of smoking cessation.