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

Evidence that central 5-HT1A-receptors play a role in the von Bezold-Jarisch reflex in the rat

1. The effects of intracisternal (i.c.) application of putative 5-hydroxytryptamine (5-HT)1A antagonists on the reflex bradycardia evoked by injection of phenylbiguanide (i.v.) were investigated in anaesthetized, atenolol-pretreated rats. 2. Intracisternal application of spiperone (100 micrograms kg-1) reversibly attenuated the reflex bradycardia whilst the same dose given i.v. had no effect. The bradycardia was also attenuated by i.c. methiothepin (200 micrograms kg-1), (+/-)-pindolol (100 micrograms kg-1) and buspirone (200 micrograms kg-1) but was not attenuated by antagonists selective for alpha 1-adrenoceptors (alfuzosin; 100 micrograms kg-1), 5-HT2-receptors (BW 501C67; 100 micrograms kg-1) or dopamine D2-receptors ((-)-sulpiride; 100 micrograms kg-1) given i.c. 3. It is concluded that the 5-HT1A-receptor antagonist action of intracisternally applied spiperone, methiothepin, (+/-)-pindolol and buspirone is responsible for the ability of these drugs to attenuate reversibly the excitation of cardiac vagal motoneurones caused by activation of the von Bezold-Jarisch reflex.

Investigation of alpha 2-adrenoceptors in humans

Partial alpha 2-agonists are thought to lower blood pressure principally by stimulation of postsynaptic central nervous system alpha 2-adrenoceptors. It is possible for some to have also a peripheral action, either by acting on the inhibitory presynaptic or as an antagonist at the postsynaptic alpha 2-adrenoceptor. In order to assess these other actions, indices are required for these receptors. Plasma growth hormone is used for the central nervous system and skin blood flow for peripheral postsynaptic alpha 2-adrenoceptors. Plasma insulin is also an index for the latter, but a double-site assay is required to detect decreases within the fasting range. Plasma noradrenaline reflects both central nervous system and peripheral modulation of sympathetic nerve activity, so that "dynamic" tests (described next) are required to dissect these two control mechanisms. Selective peripheral alpha 2-adrenoceptor stimulation or blockade may be attractive for various therapeutic applications such as hypertension, diabetes, and Raynaud's phenomenon. The balance of opposing pre- and postsynaptic alpha 2-adrenoceptor effects will be important and this may require more dynamic tests than already mentioned, such as comparing the effect of alpha-methylnoradrenaline on plasma noradrenaline (presynaptic effect) and skin blood flow (postsynaptic effect) in the presence and absence of the partial alpha 2-agonist or antagonist under investigation. Since alpha-methylnoradrenaline does not penetrate the central nervous system, any blockade of its action by a drug must be peripherally mediated. Examples of all these indices, investigations, and drugs are presented.

Sites of action of alpha 2 agonists and antagonists

The alpha 2 adrenoceptor is found on central and peripheral neurons, on vascular smooth muscle and on endocrine tissue. Although some investigational drugs have widely varying affinities for these sites, clinically available alpha 2 agonists and antagonists act at all alpha 2 receptors. This report illustrates a strategy for evaluating the relative contributions of both central nervous system vs peripheral and of prejunctional vs postjunctional actions of a new drug by measurement of several alpha 2 receptor-mediated hemodynamic and neuroendocrine responses. Evidence is also provided for a temporal dissociation of pre- and postjunctional alpha 2 responses that may permit selective postjunctional alpha 2 blockade to be of clinical value in the treatment of sympathetically induced reductions of skin blood flow.

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)