Alpha 2-adrenoceptor blocking properties of idazoxan stereoisomers: stereoselectivity for presynaptic alpha 2-adrenoceptors

Modulation of theta rhythmicity in the medial septal neurons and the hippocampal electroencephalogram in the awake rabbit via actions at noradrenergic alpha2-receptors

The modulation of the firing discharge of medial septal neurons and of the hippocampal electroencephalogram (EEG) mediated by actions on alpha2-adrenoreceptors (ARs) was investigated in awake rabbits. Bilateral i.c.v. infusion of a relatively low dose (0.5 microg) of the alpha2-AR agonist clonidine produced a reduction in the theta rhythmicity of both medial septal neurons and the hippocampal EEG. In contrast, a high dose of clonidine (5 microg) increased the percentage and degree of rhythmicity of theta bursting medial septal neurons as well as the theta power of the hippocampal EEG. On the other hand, administration of alpha2-AR antagonist idazoxan produced the opposite dose-dependent effect. While a low dose of the antagonist (20 microg) produced an increase in both the theta rhythmicity of medial septal neurons and the theta power of the hippocampal EEG, a high dose (100 microg) caused a reduction of theta rhythmicity in both the medial septum and hippocampus. These results suggest that low doses of alpha2-ARs agents may act at autoreceptors regulating the synaptic release of noradrenaline, while high doses of alpha2-ARs drugs may have a predominant postsynaptic action. Similar results were observed after local injection of the alpha2-AR drugs into the medial septum suggesting that the effects induced by the i.c.v. infusion were primarily mediated at the medial septal level. We suggest that noradrenergic transmission via the postsynaptic alpha2-ARs produces fast and strong activation of the septohippocampal system in situations that require urgent selective attention to functionally significant information (alert, aware), whereas the action via the presynaptic alpha2-ARs allows a quick return of the activity to the initial level.

Responses to alpha 2-adrenoceptor blockade by idazoxan in healthy male and female volunteers

Seven male and five female volunteers underwent double-blind infusions of the alpha 2-adrenoceptor antagonist idazoxan (100 and 200 micrograms/mg) and placebo in random order. Blood pressure, plasma norepinephrine, growth hormone and subjective responses were measured. The higher dose of idazoxan produced increases in blood pressure, norepinephrine and growth hormone and slight increases in anxiety. Both subject age and sex appeared to influence the magnitude of responses.

The effects of (±)-idazoxan and its stereoisomers on mouse vas deferens motilityin vitro.A comparison with yohimbine

The effects of idazoxan (IDZ) and its stereoisomers were compared to that of a classical alpha 2-antagonist, yohimbine (YOH), on para-aminoclonidine (PAC)- and norepinephrine (NE)-mediated inhibition of the twitch response evoked in the mouse vas deferens by low-frequency (0.1 Hz) field stimulation. (+/-)-IDZ and (+)-IDZ antagonized the inhibitory effects of PAC, (+)-IDZ being twice as potent as (+/-)-IDZ; in contrast, antagonism by (-)-IDZ failed to meet Schild criteria for a competitive mechanism. YOH completely reversed the inhibition of twitch response induced by NE, but not that induced by PAC; in the latter case, residual inhibition was almost fully reversed by (+)-IDZ and to a lesser extent by (+/-)-IDZ, while (-)-IDZ proved ineffective. These results provide pharmacological evidence of alpha 2-receptor heterogeneity at the vas deferens level. A possible additional mechanism involving imidazoline binding sites is discussed.