Modulation of alpha-1 adrenergic receptors in rat brain following chronic reserpine

Chronic reserpine administration selectively up-regulates beta 1- and alpha 1b-adrenergic receptors in rat brain: an autoradiographic study

Rats were treated for 15 days with reserpine or vehicle. One day after the last treatment, animals were killed and frozen brain sections were prepared for in vitro autoradiography. Binding to beta-adrenergic receptors was measured with [125I]iodocyanopindolol, and binding selective for beta 1 and beta 2 subtypes was assessed by including non-radioactive drugs that selectively mask beta receptor subtypes. Total alpha 1-adrenergic receptor binding was measured with [3H]prazosin, while alpha 1a binding was measured with [3H]WB4101 (in the presence of unlabeled serotonin). Quantitative densitometric analysis revealed that chronic reserpine treatment caused an increase in beta binding throughout the brain, including the cortex, thalamus, amygdala, hippocampus, caudate-putamen and hypothalamus. This effect of reserpine was entirely confined to the beta 1 subtype in all regions examined. [3H]Prazosin binding (alpha 1a plus alpha 1b) was also increased after chronic reserpine in several regions of the cortex and thalamus, as well as the ventral hippocampus and caudal amygdala. No effect of chronic reserpine was seen on [3H]WB4101 binding, indicating that the effect of reserpine on alpha 1 receptors is limited to the alpha 1b subtype. The increase in alpha 1b binding after reserpine administration in rats was generally smaller and less widespread than that seen with beta 1 binding. Thus the effect of reserpine upon noradrenergic neurotransmission demonstrates a high degree of receptor specificity and regional selectivity.

Imipramine and tetrabenazine: effects on monoamine receptor binding sites and phosphoinositide hydrolysis

Treatment of rats for 21 days with tetrabenazine, a drug which depletes monoamines and is used behaviorally to screen for antidepressants, significantly decreased 5-HT2 receptor density, increased alpha 1-adrenoceptor density but did not alter beta-adrenoceptor density in homogenates of frontal cortices labeled with [3H]ketanserin, [3H]prazosin and [3H]dihydroalprenolol, respectively. These effects were not opposite to those of the antidepressant drug imipramine which decreased both 5-HT2 and beta-adrenoceptor density and did not alter alpha 1-adrenoceptor density. Some evidence for antagonistic interactions between the two drugs was found in that imipramine partially prevented the tetrabenazine-induced increase in alpha 1-adrenoceptor density and tetrabenazine partially prevented the imipramine-induced decrease in beta-adrenoceptor density. Neither drug altered phosphoinositide hydrolysis coupled to alpha 1-adrenoceptors. While the effects of tetrabenazine are frequently attributed to its reserpine-like action of depleting monoamines, these results provide the first indication that tetrabenazine alters 5-HT2 and beta-adrenoceptor density in a manner different from that of reserpine.

Modulation of second messenger function in rat brain by in vivo alteration of receptor sensitivity: relevance to the mechanism of action of electroconvulsive therapy and antidepressants

1. The second messengers cyclic AMP and inositol triphosphate are the intracellular mediators for a number of neurotransmitters for which receptors exist on brain neurons. 2. Up- or down-regulation of these receptors in general produce corresponding changes in the associated second messenger systems. 3. Chronic administration of antidepressants including electroconvulsive shock to rats produces a number of changes in cerebral receptors, notably down-regulation of beta-adrenergic and serotonin 5-HT2 receptors and up-regulation of alpha-1 adrenergic receptors. 4. The changes in receptor number induced by such antidepressant treatments are in general accompanied by corresponding changes in the associated second messenger reactions. 5. Antidepressant administration has also been shown to induce increased post-receptor mediated adenylate cyclase activity in cortical membranes, and similar effects have also been reported in striatum after chronic administration of neuroleptics. The relevance of these effects to the mechanism of action of the drugs is discussed.

Electroconvulsive shock and reserpine increase alpha 1-adrenoceptor binding sites but not norepinephrine-stimulated phosphoinositide hydrolysis in rat brain

Repeated treatment of rats with either electroconvulsive shock or reserpine increased the density of alpha 1-adrenoceptor binding sites labeled by [3H]prazosin in rat frontal cerebral cortex. In contrast, norepinephrine-stimulated phosphoinositide hydrolysis, which is mediated by alpha 1-adrenoceptors, was not significantly affected by either treatment. These data suggest that electroconvulsive shock and reserpine might increase only or predominantly a subtype of alpha 1-adrenoceptor that is not coupled to phosphoinositide hydrolysis.

Electroconvulsive shock but not antidepressant drugs increases alpha 1-adrenoceptor binding sites in rat brain

Treatment of rats with electroconvulsive shock once daily for 10-12 days increased the density of alpha 1-adrenoceptors labeled by [3H]prazosin in homogenates of frontal cerebral cortex. A single treatment did not affect [3H]prazosin binding. Repeated treatment with electroconvulsive shock did not appear to affect alpha 1-adrenoceptor binding in the hippocampus or hypothalamus. Treatment of rats with reserpine also increased [3H]prazosin binding in the frontal cortex. In contrast to electroconvulsive shock, three weeks administration of tricyclic antidepressant drugs, monoamine oxidase inhibitors, or atypical antidepressant drugs did not significantly affect [3H]prazosin binding sites in the frontal cortex. The affinities of antidepressant drugs for alpha 1-adrenoceptors ranged from about 50 nM for tricyclic and atypical antidepressants to about 100 microM for monoamine oxidase inhibitors. The high affinities of the tricyclic and atypical antidepressant drugs for alpha 1-adrenoceptors could contribute to clinical differences between these classes of drugs and monoamine oxidase inhibitors. Furthermore, the electroconvulsive shock-induced increase in alpha 1-adrenoceptors could contribute to differences in clinical effects between this treatment and antidepressant drugs.