Cardiovascular effects of B-HT 958, an 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 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.