B-HT 958, a new alpha-adrenoceptor agonist with a high pre/postsynaptic activity ratio

B-HT 920, B-HT 933, and B-HT 958: presynaptic effects on electrically evoked 3H-noradrenaline release from slices of rat brain cortex and hypothalamus

The effects of the three azepine compounds, B-HT 920 (6-allyl-2-amino-5,6,7,8-tetrahydro-4H-thiazolo-[4,5-d]azepine), B-HT 933 [2-amino-6-ethyl-5,6,7,8-tetrahydro-4H-oxazolo[4,5-d]azepine; azepexole] and B-HT 958 (2-amino-6-(p-chloro-benzyl)-4H-5,6,7,8-tetrahydrothiazolo[5,4-d]a zepine) on electrically evoked overflow of 3H-noradrenaline were studied. Slices from parietal cortex (Cx) or nucleus anterior hypothalami (nah) were incubated with 3H-noradrenaline, superfused at 23 degrees C or 37 degrees C in the presence of the noradrenaline uptake inhibitor desipramine (3 mumol/l) and field stimulated at frequencies of 0.3 or 3 Hz. At 37 degrees C/0.3 Hz, B-HT 920 and B-HT 933 concentration-dependently decreased the evoked overflow of tritium in both regions studied, whereas B-HT 958 had no effect. In a second set of experiments each drug was tested under three additional experimental conditions, i.e. 37 degrees C/3 Hz, 23 degrees C/0.3 Hz and 23 degrees C/3 Hz. Increasing the stimulation frequency to 3 Hz or lowering the superfusion temperature to 23 degrees C reduced the effects of B-HT 920 (1 mumol/l) and B-HT 933 (10 mumol/l) as compared to the effects at 37 degrees C/0.3 Hz. When tested at 23 degrees C/3 Hz, both drugs did not significantly affect the evoked overflow of tritium in the Cx or the nah. In contrast, B-HT 958 (10 mumol/l), caused a facilitation of evoked overflow in both regions when the higher stimulation frequency or the lower superfusion temperature was used. Its release-enhancing action was most pronounced at 23 degrees C/3 Hz.(ABSTRACT TRUNCATED AT 250 WORDS)

B-HT 920 and B-HT 958: presynaptic effects on electrically evoked 3H-dopamine release from slices of rat nucleus accumbens

The effects of two thiazoloazepine derivatives, B-HT 920 (6-allyl-2-amino-5,6,7,8-tetrahydro-4H-thiazolo[4,5-d]azepine) and B-HT 958 (2-amino-6-(p-chloro-benzyl)-4H-5,6,7,8-tetrahydrothiazolo[5,4-d]a zepine) on electrically evoked overflow of 3H-dopamine were studied. Slices from nucleus accumbens of the rat were preincubated with 3H-dopamine and superfused at 23 degrees C or 37 degrees C. Electrical field stimulation was applied using frequencies of 0.5 or 5 Hz. At 37 degrees C/5 Hz, B-HT 920 markedly and dose-dependently (0.01-0.1 mumol/l) reduced the stimulation evoked overflow of tritium. Its dose-response curve was shifted to the right at 23 degrees C/0.5 Hz and 23 degrees C/5 Hz, respectively. A similar result was obtained with the dopamine receptor agonist, apomorphine (1 mumol/l). B-HT 958 (0.1-10 mumol/l) also reduced electrically induced overflow of tritium at 37 degrees C/5 Hz, had no effect at 23 degrees C/0.5 Hz, and facilitated tritium overflow at 23 degrees C/5 Hz. Sulpiride (10 mumol/l) completely prevented the effects of B-HT 920 (1 mumol/l) or B-HT 958 (1 mumol/l) at 37 degrees C/5 Hz, whereas phentolamine (1 mumol/l) had no effect on the actions of the two drugs under these experimental conditions. From the patterns of effects obtained under the different experimental conditions it is concluded that B-HT 920 acts as full agonist at presynaptic dopamine autoreceptors whereas B-HT 958 acts as partial agonist.

B-HT 958 lowers blood pressure and heart rate in the rat through stimulation of dopamine receptors

To investigate whether the hypotensive and bradycardiac effects of B-HT 958 (2-amino-6-(p-chlorobenzyl)-4H-5,6,7,8-tetrahydrothiazolo-(5,4-d) azepine) are due to the stimulation of peripheral prejunctional alpha2-adrenoceptors, the selective alpha2-adrenoceptor antagonist idazoxan was given either intravenously (i.v.) or intracerebroventricularly (i.c.v.) to anaesthetized rats before the administration of i.v. B-HT 958. Plasma noradrenaline was used as an approximate index of peripheral sympathetic nervous activity. B-HT 958 350 micrograms kg-1 i.v. caused significant falls in blood pressure and heart rate which were maximal 5 min after dosing (-29.25 +/- 3.2 mmHg and - 52 +/- 6.8 beats min-1 respectively, mean of all control animals). The hypotension and bradycardia were accompanied by significant falls in plasma noradrenaline concentration of 30-40%. Idazoxan 300 micrograms kg-1 i.v. caused a marked, but transient tachycardia and a large sustained rise in plasma noradrenaline concentration. Idazoxan 300 micrograms kg-1 and 1000 micrograms kg-1 i.v. did not prevent B-HT 958-induced falls in mean arterial pressure, heart rate and plasma noradrenaline concentration. Responses to B-HT 958 were unaffected by idazoxan 20 micrograms i.c.v. B-HT 958-induced falls in mean arterial pressure, heart rate and plasma noradrenaline concentration were significantly attenuated by i.v. administration of the dopamine receptor antagonist, sulpiride 300 micrograms kg-1. Sulpiride 10 micrograms and 50 micrograms i.c.v. caused inhibition of B-HT 958 hypotension and bradycardia similar to that of intravenous sulpiride. After i.c.v. sulpiride, B-HT 958 did not cause a significant fall in plasma noradrenaline concentration. A combination of idazoxan 1000 micrograms kg-1 i.v. and sulpiride 300 micrograms kg-1 i.v. did not cause further significant inhibition of B-HT 958 hypotension and bradycardia compared with sulpiride 300 micrograms kg-1 i.v. alone. This combination however had a significantly greater effect in inhibiting B-HT 958 hypotension than had idazoxan 1000 micrograms kg-1 alone, and almost completely blocked the B-HT 958-induced fall in plasma noradrenaline concentration. These results suggest that in the anaesthetized rat the cardiovascular effects of B-HT 958 are due to stimulation of dopamine receptors, probably located within the central nervous system, and not to stimulation of peripheral prejunctional alpha2-adrenoceptors.

B-HT 958--an antagonist at alpha 2-adrenoceptors and an agonist at dopamine autoreceptors in the brain

B-HT 958 (2-amino-6-(p-chlorobenzyl)-4H-5,6,7,8-tetrahydrothiazolo[5,4-d]az epine), a compound chemically related to clonidine-like drugs of the azepine type, was described previously as a mixed agonist-antagonist at peripheral alpha 2-adrenoceptor sites. In the present experiments the actions of B-HT 958 on brain noradrenergic and dopaminergic mechanisms were examined using behavioural, pharmacological and biochemical methods. (i) In the dog, intracisternally injected B-HT 958 (300 micrograms/kg) abolished the reflex bradycardia facilitated by the alpha 2-agonist B-HT 920 (10 micrograms/kg i.ci.). (ii) In the whole mouse brain as well as in the rat hypothalamus and neocortex, but not in the hippocampus, amygdala and nucleus accumbens B-HT 958 (5-20 mg/kg s.c.) accelerated the alpha-methyltyrosine (alpha-MT)-induced disappearance of noradrenaline. (iii) B-HT 958 decreased in a dose-dependent manner the exploratory activity of mice (ED50: 6.3 mg/kg s.c.) and slightly enhanced the motor activity of reserpine-treated mice only in high doses. (iv) B-HT 958 (20 mg/kg s.c.) lowered the level of homovanillic acid in the striatum and nucleus accumbens and dose dependently (1-20 mg/kg) slowed the alpha-MT-induced disappearance of dopamine in these 2 brain regions of the rat as well as in the whole mouse brain. (v) The gamma-butyrolactone-induced increase in DOPA levels was effectively antagonized by B-HT 958 (1 and 5 mg/kg s.c.) in the rat striatum and nucleus accumbens. These behavioural, pharmacological and biochemical observations indicate that B-HT 958 possesses central alpha-adrenoceptor blocking activity and exerts a strong agonistic effect on brain dopamine autoreceptors.

Interactions between the putative calcium entry promotor Bay k 8644 and pressor responses produced by alpha 1- and alpha 2-adrenoceptor agonists in the pithed normotensive rat

Interactions between the putative calcium entry promotor Bay k 8644 and both alpha 1- and alpha 2-adrenoceptor mediated increases in diastolic pressure were studied in the pithed normotensive rat. The alpha 2-adrenoceptor mediated pressor responses elicited by B-HT920, TL-99, DP-6,7-ADTN and B-HT958 were potentiated by Bay k 8644, reflected by a leftward shift and an increase in the maximum of the log dose-pressor response curves. The alpha 1-adrenoceptor-mediated effects elicited by cirazoline, methoxamine, (-)-amidephrine, St 587, (-)-phenylephrine and Sgd 101/75 were less enhanced by Bay k 8644. Only a leftward shift of the dose-response curves was observed, which was most pronounced for (-)-phenylephrine and Sgd 101/75. The alpha 1- and alpha 2-adrenoceptor-mediated pressor components of (-)-noradrenaline were similarly distinguished by Bay k 8644 as observed for the selective alpha 1- or alpha 2-adrenoceptor agonists. Effects of Bay k 8644 on the increase in diastolic pressure mediated by B-HT 920, St 587 and cirazoline were also studied after pretreatment with the calcium entry blocker nifedipine. After additional pretreatment with nifedipine the potentiation by Bay k 8644 observed for B-HT 920 and St 587 was more pronounced. The presence of nifedipine had no effect on the interaction between Bay k 8644 and cirazoline. It is concluded that Bay k 8644 behaves as a mirror image of nifedipine.+

B-HT 958 stimulates dopamine autoreceptors but blocks noradrenaline autoreceptors in the brain

B-HT 958 (2-amino-6-(p-chlorobenzyl)-4H-5,6,7,8-tetrahydrothiazolo 5,4-d azepine) blocked the gamma-butyrolactone-induced increase in the synthesis of dopamine and slowed down the alpha-methyltyrosine-induced disappearance of dopamine in the mouse brain by haloperidol-sensitive mechanisms. In reserpine-treated mice, B-HT 958 produced a most a weak locomotion and no change in the apomorphine-induced increase in motor activity. The motor activity of normal mice was reduced by B-HT 958. At high doses, B-HT 958 accelerated the alpha-methyltyrosine-induced disappearance of noradrenaline and it inhibited the effects of clonidine on the turnover and on the synthesis of noradrenaline in the mouse brain. The findings indicate that the dopamine autoreceptors can be selectively stimulated by B-HT 958 but that the alpha 2-adrenoceptors can be blocked following high doses.

Lack of relationship between intrinsic activity and susceptibility of pressor responses to blockade by nifedipine among the alpha 2-adrenoceptor agonists B-HT 920 and B-HT 958

Following i.v. bolus injections into pithed normotensive rats, the maximal diastolic pressor responses to B-HT 920 and B-HT 958 amounted to 115 and 35 mm Hg, respectively. Prazosin (0.1 mg/kg, i.v., -15 min) was without effect on the log dose-pressor effect curve of B-HT 958, whereas yohimbine (1 mg/kg, i.v., -15 min) shifted this curve about 30-fold to the right, showing the exclusive participation of alpha 2-adrenoceptors in the vasoconstrictor response to B-HT 958. In doses of 10 and 30 mg/kg, B-HT 958 displaced the log dose-vasoconstrictor effect curve of B-HT 920 approximately 6- and 30-fold, respectively, to the right, illustrating the partial agonism of B-HT 958 at postjunctional vascular alpha 2-adrenoceptors. Despite the marked difference in intrinsic activity of B-HT 920 and B-HT 958, the calcium entry blocker nifedipine exhibited a comparable inhibitory action on the vasopressor responses to both agonists. This finding indicates that partial and full agonism at vascular alpha 2-adrenoceptors are not related to the susceptibility of the initiated pressor response to inhibition by calcium entry blockade.

Cardiovascular effects of B-HT 958, an alpha-adrenoceptor agonist with a high pre/postsynaptic activity ratio

B-HT 958 (2-amino-6-(p-chlorobenzyl)-4H-5,6,7,8-tetrahydrothiazolo[5,4-d]az epine), chemically related to clonidine-like drugs of the azepine type, was described previously as a partial alpha 2-adrenoceptor agonist which acted presynaptically mainly as agonist and postsynaptically as antagonist. Following i.v. infusion in anaesthetized cats, 3 mg/kg of B-HT 958 lowered blood pressure, heart rate, cardiac output and total peripheral vascular resistance. A small central nervous component was indicated, since 100 micrograms/kg injected into the vertebral artery was equipotent to 300 micrograms/kg i.v. in lowering blood pressure and heart rate. The drug (1 mg/kg i.v.) decreased the discharge rate of the preganglionic sympathetic splanchnic nerve, but in contrast to the effect of clonidine this could not be demonstrated in decerebrate cats. As blood pressure and heart rate were decreased by B-HT 958 in decerebrate cats, the main site of action was assumed to be peripheral. Also in contrast to clonidine, B-HT 958 did not induce vagal baroreflex bradycardia in anaesthetized dogs with blocked beta-adrenoceptors following intracisternal (30 micrograms/kg as well as 3 mg/kg) injection. In anaesthetized rats the decrease in blood pressure and heart rate caused by 1 mg/kg B-HT 958 i.v. was antagonized by 0.5 mg/kg piperoxan i.v. It is suggested that the cardiovascular effects of B-HT 958 depend on its high selectivity for alpha 2-adrenoceptors and are due to its agonist action presynaptically on peripheral adrenergic nerve terminals.