Rapid decrease in rat brain beta adrenergic receptor binding during combined antidepressant alpha-2 antagonist treatment

The delay in onset of the therapeutic effect of antidepressants is believed to be due to a progressive decrease in the density of central beta adrenergic receptors. The changes in beta adrenergic receptor density could result from an increase in the synaptic concentration of norepinephrine, which is secondary to a decrease in the sensitivity of the alpha-2 adrenergic receptor which normally inhibits release and the firing of the locus ceruleus. We have observed an acceleration of beta receptor desensitization with combined administration of antidepressants and alpha-2 adrenergic antagonists. After one day of administration of desipramine (DMI) with phenoxybenzamine, there was a marked decrease in beta adrenergic receptor density. One day of treatment with DMI alone had no significant effect on beta receptor density. Rapid desensitization occurs not only in rat limbic cortex, but also in hippocampus and mesencephalon. Furthermore, combination therapy of DMI with yohimbine or dihydroergotamine, both alpha-2 adrenergic blockers, is similar to DMI-phenoxybenzamine treatment. Combined administration with prazosin, an alpha-1 antagonist, had no effect on antidepressant-induced desensitization. Combined administration of alpha-2 antagonists accelerated beta receptor desensitization by amitriptyline, mianserin, iprindole, tranylcypromine and pargyline. These observations suggest that pharmacological blockade of alpha-2 adrenergic receptors enhances antidepressant-induced decreases in central nervous system beta-adrenergic receptor density.     

Adrenergic and serotonergic receptor binding in rat brain after chronic desmethylimipramine treatment.

The effects of chronic administration of the tricyclic antidepressant agent desmethylimipramine (DMI) on brain adrenergic and serotonergic receptor binding processes were studied. We examined the kinetic properties of alpha adrenergic, beta adrenergic and serotonergic receptor binding sites in cortical and subcortical brain regions of rats treated chronically for various time periods with DMI(6 mg/kg i.p. daily). After 1 week of daily injections, beta receptor binding density in the cortex was significantly decreased. The reduced density of the cortical beta receptors was evident throughout a 12-week administration period. It was not until after 6 weeks of DMI administration that a significant reduction in the subcortical beta receptors was evident. Compared to saline-injected controls, chronic DMI administration lowered [3H]dihydroalprenolol binding in the hippocampus but not in the striatum. After 12 weeks of DMI we detected no differences in alpha adrenergic binding characteristics in the cortex or subcortical forebrain using [3H]dihydroergocryptine as the binding ligand. There was no consistent alteration in the cortical serotonin receptor densities throughout the 12 weeks of DMI administration, and DMI had no effect on the serotonergic binding characteristics in the subcortical forebrain region. We conclude that chronic DMI administration selectively decreases the density of beta adrenergic receptors in rat brain.     

Noradrenergic denervation alters serotonin2-mediated behavior but not serotonin2 receptor number in rats: modulatory role of beta adrenergic receptors

Recent behavioral evidence suggests that enhancement of noradrenergic neurotransmission may alter the functional sensitivity of serotonin2 (5-HT2) receptors in the central nervous system. The present studies have examined the effects of two types of noradrenergic denervation [neurotoxic: via N-(2-chloroethyl)-N-ethyl-2-bromobenzylamine (DSP4) treatment; and pharmacologic: via chronic beta adrenergic receptor blockade] on the 5-HT2-mediated head shake response and cortical beta adrenergic and 5-HT2 receptor number in the rat. No changes in quipazine-induced head shakes were observed 3 days after DSP4 lesion. However, the frequency of head shakes was significantly enhanced 10 days after DSP4 treatment in the presence of a 39% up-regulation of beta adrenergic receptors. Pretreatment with propranolol 10 days after DSP4 lesion selectively antagonized the enhancement of the behavioral response to quipazine without altering base-line response rate, whereas pretreatment with the 5-HT2 antagonist ketanserin totally blocked head shakes in both control and DSP4-treated rats. Pharmacologic denervation achieved by continuous (14 day) administration of the beta adrenergic antagonist propranolol also resulted in a potentiation of the head shake response (274% of control) and an upregulation of beta adrenergic receptors (44%). Conversely, continuous treatment with the beta adrenergic agonist clenbuterol resulted in a marked reduction in head shakes (36% of control) with a concomitant 29% down-regulation of beta adrenergic receptors. 5-HT2 receptor binding was not modified by either DSP4 lesion or continuous administration of beta adrenergic agonists or antagonists. These studies demonstrate that changes in cortical beta adrenergic receptor density may modify 5-HT2-mediated behavior in a manner that is independent of changes in 5-HT2 receptor number.     

Effects of pindolol and propranolol on beta adrenergic receptors on human lymphocytes

The cardiovascular supersensitivity observed after discontinuation of administration of beta adrenergic receptor antagonists may be explained by an increase in the density of beta adrenergic receptors. Thus, a change in the density of receptors has been observed in human lymphocytes after administration of propranolol (Aarons et al., 1980). The effects of pindolol, a beta adrenergic receptor antagonist with intrinsic sympathomimetic activity, were compared with those of propranolol or placebo. Pindolol (10 mg q.i.d.), propranolol (40 mg q.i.d.) or placebo were administered to 12 subjects for 8 days. The density of beta adrenergic receptors was determined by Scatchard analysis of the specific binding of [125I]iodopindolol on membranes prepared from human lymphocytes. Administration of pindolol resulted in a 30 to 50% decrease in the density of beta adrenergic receptors. This decrease was apparent within 1 day of beginning pindolol administration and it persisted for at least 8 days after discontinuation of drug administration. The reversibility of the decrease in receptors observed after pindolol administration was studied in 27 subjects given propranolol, pindolol or placebo for 4 days in a double-blind cross-over trial. Propranolol consistently induced a small increase in the density of beta adrenergic receptors. The density of receptors returned to predrug values within 2 days after discontinuation of propranolol administration. Pindolol induced a 30 to 50% decrease in the density of receptors which, as observed previously, persisted for at least 10 days after discontinuation of treatment.(ABSTRACT TRUNCATED AT 250 WORDS)     

Ability of aged rats to alter beta adrenergic receptors of brain in response to repeated administration of reserpine and desmethylimipramine.

Repeated administration of reserpine to 3-month-old rats produced dose-related increases in [3H]dihydroalprenolol (DHA) binding in pineal gland, cerebral cortex and cerebellum. Reserpine increased DHA binding by increasing the density of beta adrenergic receptors. Brain tissue from 24-month-old rats, however, had an impaired ability to increase receptor density in response to reserpine treatment, even in the pineal gland where the concentration of reserpine was nearly 7 times that found in the glands of young rats given the same dose on the basis of body weight. Repeated administration of desmethylimipramine decreased DHA binding in pineal glands by about 50% and in cerebral cortices by about 25%, but did not alter DHA binding in the cerebellum. The magnitude of these changes was similar in the 3- and 24-month-old rats, although the concentration of desmethylimipramine in the pineal glands and cerebral cortices of the aged rats was significantly higher than that of the young animals. The results indicate that the reserpine-induced decrease in noradrenergic input causes a compensatory increase in beta adrenergic receptor density in rat brain. They suggest further that although aged rats can decrease receptor density in response to increased adrenergic input, they have an impaired ability to increase beta adrenergic receptor density in response to decreased adrenergic input. This finding may explain the decreased density of beta adrenergic receptor found in aged rat brain.     

Characterization of a bromoacetylated derivative of pindolol as a high affinity, irreversible beta adrenergic antagonist in cultured cells

We have utilized cultured cell lines to test the utility of N8-(bromoacetyl)-N1-[3-(4-indolyoxy)-2-hydroxypropyl]-(Z)-1,8-diam ino- p-menthane, BIM, a recently synthesized, irreversible beta adrenergic antagonist. Previously available irreversible antagonists of beta adrenergic receptors have generally exhibited low affinity (typical IC50 values greater than or equal to 1 microM). By contrast, S49 lymphoma cells incubated with 10 nM BIM for 120 min and then washed extensively showed a 70% loss in beta adrenergic receptors, as measured by [125I]iodocyanopindolol binding. This loss, which could be prevented by propranolol, represented a decrease in receptor number without a change in affinity of the remaining receptors for [125I]iodocyanopindolol. The BIM-induced decrease in binding sites was persistent in membranes incubated for several hours after BIM treatment. BIM did not inactivate alpha-1 adrenergic receptors on Madin Darby canine kidney cells, alpha-2 adrenergic receptors on human erythroleukemia cells, nor did BIM treatment alter guanyl-5'-yl-imidodiphosphate-mediated regulation of agonist binding to the beta adrenergic receptors in S49 cell membranes. BIM treatment decreased cyclic AMP (cAMP) accumulation in S49 cells in response to the beta adrenergic agonist isoproterenol, but increased prostaglandin E1-stimulated cAMP accumulation (P = .09) without altering cAMP production in response to forskolin. The inactivation of beta receptors in S49 cells by BIM (IC50 = 0.30 nM) correlated closely with the loss in beta adrenergic receptor-mediated cAMP accumulation in these cells (IC50 = 0.59 nM), implying the absence of substantial receptor reserve for this response. We conclude that BIM is a potent, irreversible, selective beta adrenergic antagonist for the study of beta adrenergic receptors in cultured cells.     

Denervation-induced changes in alpha and beta adrenergic receptors of the rat submandibular gland.

Denervation resulted in a marked increase in the beta adrenergic response and isoproterenol-stimulated cyclic adenosine 3':5'-monophosphate accumulation of dispersed cells prepared from a rat submandibular gland. This increase in beta adrenergic response was paralleled by an increase in the density of beta adrenergic receptors in membranes prepared from these glands. Denervation also produced a moderate increase in alpha adrenergic receptor density in membranes prepared from whole glands. However, the alpha adrenergic response in cells, epinephrine-induced release of potassium, appeared unaltered by denervation. Furthermore, membranes prepared from denervated dispersed cells did not show the increase in alpha adrenergic receptor density seen in membranes from an intact denervated gland. These data demonstrate that removal of noradrenergic nerve terminals affects alpha and beta adrenergic receptors differently. While it is clear that beta adrenergic membrane receptors participate in denervation-induced supersensitivity, the changes in alpha adrenergic membrane receptors are more complex and may not contribute to the supersensitivity seen after denervation. On the basis of competitive binding studies, the adrenergic receptors in membranes from intact submandibular glands were subclassified as beta-1 and alpha-2. Denervation did not alter the binding characteristics of the alpha-2 receptors in this gland, demonstrating that alpha-2 adrenergic membrane receptors can be postsynaptic in this adrenergically innervated tissue.     

Psychopharmacological consequences of activation of beta adrenergic receptors by SOM-1122

SOM-1122 was found to be a high-affinity, partial agonist for beta adrenergic receptors. SOM-1122 inhibited the binding of [125I]iodopindolol to membranes prepared from rat cerebral cortex and cerebellum. GTP regulated the binding of SOM-1122 by increasing the Hill coefficient in both tissues and reducing the affinity of the receptor for SOM-1122 in the cerebellum. SOM-1122 increased the concentration of cyclic AMP in slices of rat cerebral cortex in a dose-dependent manner; this effect was antagonized by propranolol. Two lines of evidence suggested that SOM-1122 was centrally active after peripheral administration. First, SOM-1122 inhibited the binding of [125I]iodopindolol in vivo in a dose-dependent manner. Second, after chronic infusion with SOM-1122 for 7 days, the density of beta adrenergic receptors in the cerebellum was reduced; receptor density also was reduced 18 hr after acute administration of SOM-1122, although to a lesser extent. SOM-1122 was found to be behaviorally active. It reduced locomotor activity and reduced response rate under a multiple fixed-interval, fixed-ratio schedule in a dose-dependent manner. SOM-1122 also reduced response rate and increased reinforcement rate under a differential-reinforcement-of-low-rate schedule. These behavioral actions of SOM-1122 appeared to be due to an interaction of the agonist with beta adrenergic receptors, as they were antagonized by propranolol. The behavioral changes produced by stimulation of beta adrenergic receptors with SOM-1122 were generally similar to those caused by other centrally acting beta adrenergic agonists and by antidepressant drugs.     

Genetic influence on the regulation of beta adrenergic receptors in mice

The regulation of beta adrenergic receptors was investigated in inbred mouse strains in which previous studies revealed differences in the regulation of dopamine receptors. The density of beta adrenergic receptors in the cerebral cortex of BALB/J mice was about one-third of that in CBA/J and C57BL/6J mice. Strain differences in the binding of [125I]iodohydroxypindolol to beta adrenergic receptors were due to changes in the density of beta-1 adrenergic receptors. Chronic administration of propranolol did not result in an increase in the density of beta adrenergic receptors receptors in cortices of C57BL/6J and BALB/cJ mice were observed. In contrast, pretreatment with 6-hydroxydopamine resulted in increases in the density of beta adrenergic receptors in the cerebral cortex of all three strains. Analysis of the effects of these treatments on the subtypes of beta adrenergic receptors revealed that the changes were restricted to changes in the density of beta-1 receptors. The failure to observe a response to propranolol in CBA/J mice expands the extent of deficits reported previously in this strain for striatal dopamine receptor supersensitivity after chronic treatment with haloperidol (Severson et al., Brain Res. 210: 201-215, 1981). CBA/J mice may be a useful model for genetic analysis of mechanisms for the control of receptor sensitivity and to investigate the impairments of the regulation of catecholaminergic receptors observed in aged rodents.     

Selective regulation of beta-2 adrenergic receptors by the chronic administration of the lipophilic beta adrenergic receptor agonist clenbuterol: an autoradiographic study

Many antidepressant drugs, when administered chronically to rats, have been shown to produce decreases in the density of beta adrenergic receptors in the central nervous system. The centrally active beta adrenergic receptor agonist clenbuterol is currently being evaluated clinically as an antidepressant. The chronic administration of this drug to rats resulted in a large decrease in the density of beta adrenergic receptors in some areas of the rat brain but not in others. Thus, autoradiographic studies revealed that the total density of beta adrenergic receptors in the molecular layer of the cerebellum, but not in layers 1 to 3 or layer 4 of the cerebral cortex, was decreased. To examine whether this regional selectivity occurred because of differences in plasticity of cerebellum and cortex or because cerebellum contains mainly beta-2 adrenergic receptors and cortex contains mainly beta-1 adrenergic receptors, separate analyses of the subtypes of beta adrenergic receptors were performed in each area. These experiments indicated that the decrease in receptor density was entirely specific for beta-2 adrenergic receptors, whereas the density of beta-1 receptors was unchanged. Thus, even in layers 1 to 3 and layer 4 of the cerebral cortex, beta-2 receptor density was decreased, with no change in beta-1 receptor density. Using the autoradiographic assay for ligand binding, it was shown that clenbuterol has equal affinity for beta-1 and beta-2 adrenergic receptors, indicating that the selective effect of this drug was not due to a selective affinity for beta-2 receptors.(ABSTRACT TRUNCATED AT 250 WORDS)     

Effects of ethanol administration and withdrawal on neurotransmitter receptor systems in C57 mice

C57BL/6 mice were treated with 7% (v/v) ethanol in a Bio-Serve liquid diet for 7 days. Some animals were then allowed to withdraw from ethanol for a period of 24 hr. The severity of the ethanol withdrawal was assessed by monitoring behavioral changes and by quantitating the decrease in body temperature that occurred during the first 16 hr of withdrawal. Animals withdrawn from ethanol for 24 hr showed a decreased hypothermic response to apomorphine suggesting that changes in dopaminergic systems had occurred. This possibility was further examined in homogenates of striatum by measuring dopamine-stimulated adenylate cyclase activity and the binding of [3H]spiroperidol. However, there were no changes observed in either basal- or dopamine-stimulated adenylate cyclase activity or in the density or affinity or receptors for [3H]spiroperidol. The affinity of apomorphine for the dopamine receptor was also unchanged. In other experiments, alpha and beta adrenergic receptor-mediated increases in cyclic AMP accumulation were assessed in slices of cerebral cortex. There was no change in cyclic AMP accumulation due to either alpha or beta adrenergic receptors. There was, however, a significant decrease in the density of beta adrenergic receptors in both the ethanol-treated mice and in the withdrawn mice. This decrease was restricted to the beta-2 receptor subtype with no change being observed in the density of beta-1 adrenergic receptors. Ethanol administration was also associated with a significant increase in the density of muscarinic cholinergic receptors in the hippocampus and cerebral cortex. The effect was not observed in animals allowed to withdraw for 24 hr.     

Subtype-selective regulation of beta adrenergic receptor-adenylyl cyclase coupling by phorbol esters in 3T3-L1 fibroblasts

To study the epigenetic regulation of beta adrenergic receptor subtypes, we examined the effects of phorbol esters on beta adrenergic receptor coupling to adenylyl cyclase in 3T3-L1 fibroblasts, which express both beta-1 and beta-2 adrenergic receptor subtypes. Pretreatment of intact 3T3-L1 cells with the protein kinase C activator phorbol dibutyrate caused a dose- and time-dependent decrease in subsequent cyclic AMP (cAMP) accumulation mediated by the beta adrenergic agonist isoproterenol. This effect was selective for beta-adrenergic receptor-mediated responses because there was a potentiation of cAMP accumulation caused by other activators such as prostaglandin E1, forskolin or cholera toxin. The inactive phorbol, alpha-phorbol dibutyrate was ineffective at 1 microM in attenuating isoproterenol stimulation, and 25 nM of the protein kinase C inhibitor staurosporine blocked the effects of phorbol ester on beta adrenergic agonist responses. Stimulation of cAMP accumulation by isoproterenol occurred through a greater proportion of beta-2 adrenergic receptors in phorbol dibutyrate-treated cells than in control cells. This was demonstrated using the beta-1 adrenergic selective antagonist ICI 89.406 and the beta-2 adrenergic selective antagonist ICI 118.551 to inhibit competitively isoproterenol-stimulated cAMP accumulation. Beta-2 adrenergic receptor number and subtype in these cells are regulated by glucocorticoids and butyrate. Decreasing the proportion of beta-1 adrenergic receptors and concomitantly increasing beta-2 adrenergic receptors with either glucocorticoids or butyrate decreased the ability of phorbol ester pretreatment to attenuate cAMP accumulation by isoproterenol.(ABSTRACT TRUNCATED AT 250 WORDS)     

Agonist-induced changes in beta adrenergic receptor density and receptor-mediated responsiveness in slices of rat cerebral cortex.

Incubation of slices of rat cerebral cortex with the beta adrenergic receptor agonist (-)-isoproterenol led to a 30 to 50% decrease in the number of binding sites for [125I]iodohydroxybenzylpindolol and to a 60 to 80% decrease in isoproterenol-stimulated cyclic AMP accumulation. The density of beta adrenergic receptors was also decreased following incubation with (-)-norepinephrine but not with (+)-isoproterenol or dopamine and the decrease in receptor density was blocked by co-incubation with the beta adrenergic receptor antagonist sotalol. The half-time for loss of receptors was approximately 3 min and recovery was observed during a 1 hr reincubation of tissue slices or following exposure to guanine nucleotides. A decrease in beta adrenergic receptor density was also observed following chronic treatment with desmethylimipramine which blocks norepinephrine reuptake and thus potentiates the effects of neurally released norepinephrine at adrenergic receptors. The loss of receptors induced in vitro could be reversed by reincubation or by exposure to guanine nucleotides. In contrast, the loss of receptors induced in vivo was not affected by these procedures.     

Regionally specific neural adaptation of beta adrenergic and 5-hydroxytryptamine2 receptors after antidepressant administration in the forced swim test and after chronic antidepressant drug treatment

In the present investigation, experiments were performed in order to determine whether antidepressants are capable of inducing regionally specific adaptation of beta adrenergic and 5-hydroxytryptamine2 (5-HT2) receptors after chronic administration or when combined with the forced swim test. The drugs tested were imipramine, amitriptyline, pargyline and nomifensine. The regional pattern of beta adrenergic or 5-HT2 receptor binding changes induced after chronic treatment with these antidepressants was not uniform. All of the drugs reduced [3H]dihydroalprenolol binding to cortical membranes after chronic treatment but only two, imipramine and pargyline, did so in hippocampus. All of the antidepressants reduced cortical, but not hippocampal, beta adrenergic receptor binding after 2 days of treatment, indicating that the rate of antidepressant-induced neural adaptation is regionally specific. All of the drugs, except nomifensine, induced down regulation of both cortical and hippocampal 5-HT2 receptors after chronic treatment, as measured by [3H]ketanserin binding. The forced swim test accelerated the reduction of [3H] dihydroalprenolol binding in hippocampus induced by imipramine and pargyline while producing no further effect on cortical beta adrenergic receptors. The down-regulation of hippocampal, but not cortical 5-HT2 receptors by imipramine and pargyline was also facilitated in rats processed in the forced swim test. These results provide further support for the view that the forced swim antidepressant drug screen may be of heuristic value as a model of the adaptive neural mechanisms that accompany chronic antidepressant drug treatment. Furthermore, these data provide evidence that multiple neural mechanisms may be involved in the adaptive changes after antidepressant drug treatment.     

Changes in the density of beta adrenergic receptors in rat lymphocytes, heart and lung after chronic treatment with propranolol

Abrupt withdrawal after the chronic administration of propranolol results in clinical syndromes that suggest adrenergic hypersensitivity. Furthermore, propranolol administration has been shown to lead to an increase in the density of beta adrenergic receptors on human lymphocytes. The present studies were designed to assess the relevance of changes measured in lymphocytes to changes that may occur in solid tissues. Direct measurement of the density and properties of beta adrenergic receptors in membrane fragments was performed in vitro using the radioligand [125I]iodohydroxybenzylpindolol. Chronic infusion of propranolol by s.c. implanted osmotic minipumps generated sustained plasma concentrations of propranolol sufficient to cause chronic blockade of beta adrenergic receptors. Infusion of propranolol for 7 days resulted in significant increases in the density of beta adrenergic receptors in rat ventricles, lungs and lymphocytes. A computer-assisted graphic analysis of results obtained in studies with drugs selective for beta-1 or beta-2 receptors revealed increases in the densities of both beta-1 an beta-2 adrenergic receptors. These results are consistent with the hypothesis that change in beta adrenergic receptors on lymphocytes are qualitatively similar to alterations in beta adrenergic receptors in solid tissues not routinely accessible in humans. Increases in the densities of beta-1 and/or beta-2 adrenergic receptors in solid tissues may be related to some of the untoward effects observed in humans after abrupt discontinuation of propranolol administration.     

Regulation of alpha-1 adrenergic receptor density and functional responsiveness in rat brain

The effects of changes in norepinephrine availability on alpha-1 adrenergic receptor density and responsiveness were determined in several regions of rat brain. Receptor density was determined from specific 125I-BE 2254 binding, and responsiveness was determined by norepinephrine-stimulated increases in inositol phosphate accumulation and in cyclic AMP accumulation in the absence or presence of adenosine in slices where beta adrenergic receptors had been inactivated. Adrenergic input was reduced by destroying noradrenergic neurons with DSP-4, depleting amines with chronic reserpine treatment or blocking alpha-1 adrenergic receptors with chronic prazosin treatment. All three treatments caused similar increases in alpha-1 adrenergic receptor density in cerebral cortex but not in striatum or hippocampus. DSP-4 treatment increased the maximal cyclic AMP response to both norepinephrine and phenylephrine but did not alter the maximal inositol phosphate response or the maximal potentiation of the cyclic AMP response to adenosine. DSP-4 treatment also increased the potency of norepinephrine in activating all three responses in cerebral cortex. Adrenergic input was increased by chronic treatment with desmethylimipramine to block norepinephrine reuptake, chronic i.c.v. infusion of norepinephrine or chronic yohimbine treatment to block presynaptic autoreceptors. None of these treatments caused significant changes in alpha-1 adrenergic receptor density or functional responsiveness in any region studied. The results suggest that alpha-1 adrenergic receptor density and responsiveness in rat brain can be increased but not easily decreased. They also suggest that different brain regions are affected differently by alterations of adrenergic input and that the different responses are not coregulated.     

Differential effects of the tricyclic antidepressant desipramine on the density of adrenergic receptors in juvenile and adult rats

Although the tricyclic antidepressants, such as desipramine (DMI), are among the most efficacious treatments for adult depression, they are not effective in treating childhood and adolescent depression. Because the adrenergic nervous system is not fully developed until late adolescence, we hypothesized that the mechanisms regulating receptor density may not yet be mature in young mammals. To test this hypothesis, the effects of DMI treatment on cortical alpha-1-, alpha-2-, and beta-adrenergic receptors were compared in juvenile and adult rats. DMI was delivered either by 4 days of twice daily injections to postnatal day 9 to 13 (4 and 7 mg/kg/day) and adult (20 mg/kg/day) rats, or by 2 weeks of continual drug infusion (osmotic minipumps) to postnatal day 21-35 (15 mg/kg/day) and adult (10 mg/kg/day) rats. These delivery paradigms gave juvenile brain concentrations of DMI similar to those in adult rats. The beta-adrenergic receptor was down-regulated with both treatment paradigms in both juvenile and adult rats. By contrast, in the postnatal day 9 to 13 rats, there was a dose-dependent up-regulation of the alpha-1 in the cortex and alpha-2-adrenergic receptor in the prefrontal cortex, whereas there was no change in density in adult rats. These differences in the alpha-adrenergic receptor regulation after DMI treatment suggest that the lack of efficacy of tricyclic antidepressants in treating childhood depression may be related to immature regulatory mechanisms for these receptors.     

Effects of propranolol on catecholamine synthesis and uptake in the central nervous system of the rat

These studies were undertaken to determine the effects of propranolol on catecholamine synthesis and uptake in the rat central nervous system. The effects of propranolol on catecholamine synthesis were studied in vitro in striatal and hypothalamic synaptosomes, and also in vivo. In addition, the effects of propranolol on catecholamine uptake in striatal and hypothalamic synaptosomes were evaluated. Propranolol inhibited synaptosomal catecholamine synthesis and uptake in both tissues. Norepinephrine uptake in the hypothalamus was most sensitive to propranolol inhibition (IC50 = 5 microM). Dopamine synthesis in striatal synaptosomes was also inhibited markedly, with an IC50 = 8 microM. After in vivo administration, propranolol decreased the accumulation of dopa in the striatum, confirming propranolol's synthesis inhibiting effect in dopaminergic terminals. Studies of soluble striatal tyrosine hydroxylase revealed that propranolol has a direct inhibitory effect on the enzyme. These results indicate that propranolol administration may cause a potentiation of norepinephrine activity specifically at alpha receptors, due to concurrent beta receptor blockade and inhibition of norepinephrine reuptake and a decrease in dopamine activity at dopaminergic receptor sites due to an inhibition of dopamine formation.     

Effect of repeated restraint stress, desmethylimipramine or adrenocorticotropin on the alpha and beta adrenergic components of the cyclic AMP response to norepinephrine in rat brain slices

The cyclic AMP response to catecholamines in rat cortical slices is mediated by a beta adrenergic receptor which is coupled to adenylate cyclase and an alpha adrenergic receptor which potentiates the response to beta receptor stimulation. The present studies examined the effects of repeated restraint stress, adrenocorticotropin or desmethylimipramine administration on the beta and alpha adrenergic components of this response. Restraint was found to produce a small nonsignificant decrease of the beta receptor response accompanied by a significant reduction of the alpha receptor-induced potentiation of the beta response. Desmethylimipramine was found to lower the cyclic AMP response to beta receptor stimulation but not to alter the alpha-induced potentiation of the beta response. Adrenocorticotropin, like restraint stress, was found to reduce only the alpha-induced potentiation of the beta response. Experiments with adenosine and histamine showed that restraint stress lowered the alpha-induced potentiation of cyclic AMP responses to these neurohormones also. It is concluded that restraint stress acts primarily to reduce the response to stimulation of central alpha adrenergic receptors whereas desmethylimipramine acts primarily to reduce the response to stimulation of beta adrenergic receptors. Adrenocorticotropin has the same effect as restraint stress suggesting that pituitary adrenal hormones mediate the stress effect.     

Regulation of serotonin2 (5-HT2) receptors labeled with [3H]spiroperidol by chronic treatment with the antidepressant amitriptyline

Recently, we reported that chronic administration of several antidepressants of different classes produced larger reductions in numbers of serotonin2 (5-HT2) receptors in rat brain labeled by [3H[spiroperidol than in beta adrenergic receptors. In the present study, we examine detailed properties of 5-HT2 receptor regulation by chronic treatment with amitriptyline. Chronic but not acute treatment with the tricyclic antidepressant amitriptyline reduces binding to 5-HT2 receptors by [3H]spiroperidol and beta adrenergic receptor binding of [3H]dihydroalprenolol in brain membranes. The decrease is time-dependent, gradually reversible and represents a change in the number of binding sites with no alteration in drug affinities for 5-HT2 receptors. The effect can be observed at daily doses of 2.5 mg/kg, similar to clinically effective doses in humans. At all doses and time intervals, the decrease in 5-HT2 receptors is more marked than the concurrent change in total beta adrenergic receptor binding. The properties of 5-HT2 receptor reduction after chronic antidepressant treatment indicate that this alteration could be associated with therapeutic response.