Electroconvulsive Shock Increases α1b-but Not α1a-Adrenoceptor Binding Sites in Rat Cerebral Cortex

Repeated administration of electroconvulsive shock (ECS) increases [3H]prazosin binding to alpha 1-adrenoceptors in rat cerebral cortex. In contrast, [3H]WB4101 binding in cortex has been reported to be unchanged after ECS. [3H]Prazosin labels two alpha 1-adrenoceptor subtypes, termed alpha 1a and alpha 1b, whereas [3H]WB4101 labels the alpha 1a subtype preferentially. The purpose of this study was to determine whether ECS increases one or both alpha 1-adrenoceptor subtypes in rat cerebral cortex. We found that treatment of rats with ECS once daily for 10-12 days increased [3H]prazosin binding in cortex by about 25% but did not significantly alter [3H]WB4101 binding to alpha 1-adrenoceptors. Measurement of alpha 1a and alpha 1b receptors by competition analysis of the selective alpha 1a antagonist 5-methylurapidil against [3H]prazosin and measurement of [3H]prazosin binding in homogenates preincubated with chlorethylclonidine, which alkylates alpha 1b binding sites, also indicated that the ECS-induced increase in alpha 1-adrenoceptors is confined to the alpha 1b subtype. In contrast to its effect on [3H]prazosin binding, ECS did not increase phosphoinositide hydrolysis as measured by [3H]inositol 1-phosphate accumulation in slices of rat cerebral cortex stimulated by either norepinephrine or phenylephrine. The failure of ECS to increase [3H]inositol 1-phosphate accumulation stimulated by phenylephrine, which is a partial agonist for this response, suggests that spare receptors do not account for the apparent absence of effect of ECS on alpha 1-adrenoceptor-mediated phosphoinositide hydrolysis.     

Spontaneous Withdrawal from Long-Term Treatment with Morphine Accelerates the Turnover of α2-Adrenoceptors in the Rat Brain: Up-Regulation of Receptors Associated with Increased Receptor Appearance

Abstract: The aim of this study was to quantify and compare the turnover of brain α₂-adrenoceptors during chronic morphine treatment and after spontaneous morphine withdrawal in rats. The oral administration of increasing doses of morphine (10–90 mg/kg) for 20 days did not alter the specific binding of the agonist [³H]clonidine in the cerebral cortex. However, spontaneous opiate withdrawal (24 h) significantly increased the density of cortical α₂-adrenoceptors (B_max for [³H]clonidine was 21% greater). The recovery of [³H]clonidine binding after irreversible inactivation by N-ethoxycarbonyl-2-ethoxy-1,2-dihydroquinoline (1.6 mg/kg) was assessed in naive, morphine-dependent, and morphine-withdrawn rats to study the process of α₂-adrenoceptor repopulation and to calculate receptor turnover parameters. The simultaneous analysis of receptor recovery curves revealed that the turnover of brain α₂-adrenoceptors in morphine-withdrawn rats was accelerated [appearance rate constant (r) = 21 fmol/mg of protein/day; disappearance rate constant (k) = 0.25 day^?1] compared with those in morphine-dependent (r = 13 fmol/mg of protein/day; k = 0.14 day^?1) and naive (r = 15 fmol/mg of protein/day; k = 0.16 day^?1) rats. Moreover, this analysis also indicated that the increased density of cortical α₂-adrenoceptors observed during morphine withdrawal was due to a significantly higher receptor appearance (Δr = 37–57%) and not to a decreased receptor disappearance, which in fact showed also an increase (Δk = 56–79%). It is proposed that the increased rate of α₂-adrenoceptor production in the brain of morphine-dependent rats during spontaneous withdrawal is most probably mediated by the overactivity of the adenylyl cyclase/cyclic AMP system induced by opiate addiction.     

Changes in extracellular space volume and geometry induced by cortical spreading depression in immature and adult rats

Changes in extracellular space (ECS) diffusion parameters, DC potentials and extracellular potassium concentration were studied during single and repeated cortical spreading depressions (SD) in 13-15 (P13-15), 21 (P21) and 90-day-old (adult) Wistar rats. The real-time iontophoretic method using tetramethylammonium (TMA+)-selective microelectrodes was employed to measure three ECS parameters in the somatosensory cortex: the ECS volume fraction alpha (alpha = ECS volume/total tissue volume), ECS tortuosity lambda (increase in diffusion path length) and the nonspecific TMA+ uptake k'. SD was elicited by needle prick. SD was significantly longer at P13-15 than at P21 and in adults. During SD, alpha in all age groups decreased from 0.21-0.23 to 0.05-0.09; lambda increased from 1.55-1.65 to 1.95-2.07. Ten minutes after SD, alpha (in adults) and lambda (all age groups) increased compared to controls. This increase persisted even 1 hour after SD. When SD was repeated at 1 hour intervals, both alpha and lambda showed a gradual cumulative increase with SD repetition. Our study also shows that cortical SD is, as early as P13, accompanied by severe ECS shrinkage and increased diffusion path length (tortuosity) with values similar to adults, followed by a long-lasting increase in ECS volume and tortuosity when compared to pre-SD values.     

Role of Dopaminergic Neurons in Denervation-Induced α1-Adrenergic Up-Regulation in the Rat Cerebral Cortex

The density of [3H]prazosin binding to alpha 1-adrenoceptors in the rat cortex was measured after selective and mixed noradrenergic or dopaminergic lesions. DSP-4 produced a selective noradrenergic lesion and increased the density of alpha 1-adrenoceptors. 6-Hydroxydopamine produced a selective dopaminergic lesion (after desipramine protection of noradrenergic neurons) and a mixed noradrenergic and dopaminergic lesion that did not change the cortical alpha 1-adrenoceptor binding. On the basis of the results obtained, a hypothesis is put forward that the central dopaminergic system controls the denervation-induced cortical alpha 1-adrenoceptor up-regulation.     

Electroconvulsive shock and brain muscarinic receptors: relationship to anterograde amnesia

Rats were administered one electroconvulsive shock daily for 7 days (ECS X 7) and were killed 24 hours after the last treatment. Muscarinic cholinergic receptor number, as determined by [3H] quinuclidinyl benzilate [( 3H]QNB) binding, was significantly reduced in the cerebral cortex. A parallel group of rats was trained on a passive avoidance task 24 hours following the last ECS and tested for retention of the original avoidance response 24 hours later; these animals exhibited a profound amnesia. Animals tested 1 hour following training were not amnestic, indicating that learning was unimpaired. Animals trained 7 days following ECS X 7 were not amnestic and [3H] QNB binding changes were not demonstrable at this time. A single ECS which does not significantly affect cortical [3H] QNB binding, did not induce amnesia in rats trained 24 hours after the treatment and tested 24 hours later. The parallel, cumulative nature of ECS-induced muscarinic receptor down-regulation and ECS-induced anterograde amnesia suggests a possible causative relationship.     

Effects of Single and Repeated Electroconvulsive Shock Administration on Inositol Phosphate Accumulation in Rat Brain Slices

Accumulation of inositol-1-phosphate after labeling with [3H]inositol and stimulation with noradrenaline, carbachol, and serotonin was measured in rat cortical, caudate nucleus, and hippocampal slices. The response to noradrenaline was significantly increased in cortical slices from animals that had received either a single electroconvulsive shock (ECS) or a series of 10 daily ECS but was unchanged in caudate nucleus or hippocampal slices. The response to carbachol, a muscarinic cholinergic agonist, was unchanged in cortical or caudate nucleus slices but was significantly reduced in hippocampal slices from animals that had received chronic ECS. The response to serotonin in cortical slices was not affected by the treatment. The results are correlated with changes in receptor number, which have been demonstrated to occur after administration of ECS.     

Antidepressant Treatments, Including Sibutramine Hydrochloride and Electroconvulsive Shock, Decrease β1 but Not β2-Adrenoceptors in Rat Cortex

The beta 1- and beta 2-adrenoceptor populations in rat cortex were individually quantified by labelling all of the receptors with [3H]dihydroalprenolol and displacing with isoprenaline (200 microM) or CGP 20712A (1-(2-[(3-carbamoyl-4-hydroxy)phenoxy]ethylamino)-3-[4-(1-methyl-4- trifluoromethyl-2-imidazolyl)phenoxy]-2-propanol methanesulphonate; 100 nM) to define total beta-adrenoceptors and beta 1-adrenoceptors, respectively. Binding parameters for beta 2-adrenoceptors were calculated by the difference. Oral administration of the monoamine reuptake inhibitors sibutramine HCl (3 mg/kg), amitriptyline (10 mg/kg), desipramine (10 mg/kg), or zimeldine (10 mg/kg) for 10 days decreased the total number of beta-adrenoceptors present in rat cortex. This effect was entirely due to a reduction in the number of beta 1-adrenoceptors. Similarly, 10 days of treatment with the monoamine oxidase inhibitor tranylcypromine (10 mg/kg p.o.) or five electroconvulsive shocks (ECSs; 200 V, 2 s) spread over this period also down-regulated beta-adrenoceptors by reducing the content of the beta 1-subtype. By contrast, treatment with clenbuterol (5 mg/kg p.o.) for 10 days reduced the number of cortical beta-adrenoceptors by an effect on the beta 2-adrenoceptor population. The effects of short-term treatment with these drugs were also investigated, and, using the doses shown above, the results of 3 days of administration or a single ECS were determined. Sibutramine HCl and desipramine were alone in producing a reduction in number of beta-adrenoceptors after 3 days. Once again, this was exclusively due to a loss of beta 1-adrenoceptors.(ABSTRACT TRUNCATED AT 250 WORDS)     

Involvement of norepinephrine activity in the regulation of alpha 1 adrenergic receptors in the medial preoptic nucleus of estradiol-treated rats

To establish whether the diurnal decrease in the density of alpha 1 receptors observed in the medial preoptic nucleus (MPN) of estrogen (E2)-treated rats is related to the concomitant diurnal increase in norepinephrine (NE) turnover rates, we quantitated the density of [3H]-Prazosin binding to alpha 1 receptors after blockade of NE turnover with alpha-methyl-paratyrosine (alpha MPT). A series of preliminary studies was performed to rule out an interference of this drug with [3H]-Prazosin binding to alpha 1 adrenergic receptors in vitro and in vivo. Incubation of brain slices with alpha MPT produced a dose-dependent inhibition of [3H]-Prazosin binding to alpha 1 adrenergic receptors with an IC50 of approximately 6 mM. Scatchard analysis demonstrated that alpha MPT exhibited a simple competitive interaction with [3H]-Prazosin binding sites as shown by an increase in the apparent dissociation constant (Kd) of the ligand and no change in the number of alpha 1 receptors (Bmax). In contrast, preincubation of brain slices with alpha MPT and prior in vivo administration of alpha MPT did not affect [3H]-Prazosin binding to alpha 1 adrenergic receptors. Once we established that alpha MPT could be used to suppress NE turnover without interfering with the measurement of alpha 1 receptor densities, we repeatedly injected this drug to ovariectomized (OVX) and E2-implanted rats. The density of alpha 1 adrenergic receptors in MPN was quantitated autoradiographically. Blockade of NE turnover with alpha MPT only partially prevented the reduction in alpha 1 receptor density observed in the E2-treated rats, suggesting that the decrease in the level of [3H]-Prazosin binding sites cannot be completely ascribed to increased NE turnover rates.     

Chronic electroconvulsive shock decreases (+/-) 1-(4-iodo-2,5-dimethoxyphenyl)-2-aminopropane hydrochloride (DOI)-induced wet-dog shake behaviors of dexamethasone-treated rats

Electroconvulsive shock (ECS) therapy is considered to be an effective treatment for depression, but its mechanism of action is still unknown. We investigated the effect of chronic ECS in rats treated for 14 days with dexamethasone (Dex), a glucocorticoid receptor agonist. Chronic injection of sesame oil decreased body weight change and increased serotonin (5-HT)-2A receptor number and DOI (5-HT-2A, 2C receptor agonist)-induced wet-dog shake (WDS) behaviors. Dex treatment for 14 days decreased body weight of rats, but repeated ECS did not reverse this decrease. Dex also abolished plasma corticosterone levels, and ECS failed to restore these levels. These results indicate that chronic ECS does not antagonize the effect of Dex. The treatment with Dex increased 5-HT-2A receptor binding density of rat frontal cortex and the number of DOI-induced WDS behaviors. Chronic ECS reduced the enhanced WDS behaviors by Dex but had little effect on receptor density. These results suggest that chronic ECS might suppress 5-HT-2A receptor function at the postreceptor signaling level rather than at the receptor itself, without changing HPA axis function in Dex-treated rats.     

Glial and neuronal cells express functional chemokine receptor CXCR4 and its natural ligand stromal cell-derived factor 1

Chemokines are a family of proteins that chemoattract and activate cells by interacting with specific receptors on the surface of their targets. The chemokine stromal cell-derived factor 1, (SDF1), binds to the seven-transmembrane G protein-coupled CXCR4 receptor and acts to modulate cell migration, differentiation, and proliferation. CXCR4 and SDF1 are reported to be expressed in various tissues including brain. Here we show that SDF1 and CXCR4 are expressed in cultured cortical type I rat astrocytes, cortical neurons, and cerebellar granule cells. In cortical astrocytes, prolonged treatment with lipopolysaccharide induced an increase of SDF1 expression and a down-regulation of CXCR4, whereas treatment with phorbol esters did not affect SDF1 expression and down-modulated CXCR4 receptor expression. We also demonstrated the ability of human SDF1alpha (hSDF1alpha) to increase the intracellular calcium level in cultured astrocytes and cortical neurons, whereas in the same conditions, cerebellar granule cells did not modify their intracellular calcium concentration. Furthermore, in cortical astrocytes, the simultaneous treatment of hSDF1alpha with the HIV-1 capside glycoprotein gp120 inhibits the cyclic AMP formation induced by forskolin treatment.     

The effect of acute and chronic electroconvulsive shock on [3H]phorbol-dibutyrate binding to rat brain membranes

The present study investigated the effect of single and repeated electroconvulsive shock (ECS) on proteinkinase C in rat cerebral cortex, cerebellum, hippocampus and striatum using [3H]Phorbol-12, 13-butyrate binding. In the postictal period and 24 hr after a single ECS there was no alteration in any brain region. Twenty four hr after 10 once-daily ECS there was a significant decrease the number of binding sites in cerebral cortex (30%) and in cerebellum (20%) without a change in the affinity constant. These findings are discussed with regard to earlier reports on phosphoinositide turnover following chemically and electrically induced seizures.     

Haloperidol-mediated phosphoinositide hydrolysis via direct activation of alpha1-adrenoceptors in frontal cerebral rat cortex.

In addition to its effect on D2 dopamine receptor blockades, haloperidol is able to interact with multiple neurotransmitters (NTs). Its action on phosphoinositide (PI) turnover was studied on cerebral cortex preparations. It induces an increase in inositol phosphate (IP) accumulation, which was only blunted by the alpha1-adrenoceptor blocker prazosin. Haloperidol maximal effect (Emax) was less than the effect of the full agonist norepinephrine (NE), and dose-response curves for both NE in the presence of submaximal doses of haloperidol and haloperidol in the presence of Emax doses of NE showed that haloperidol behaves as a partial agonist of cerebral alpha1-adrenoceptors. Its effect on PI hydrolysis is mediated through phospholipase C activation, as 2-nitro-4-carboxyphenyl-N,N-diphenylcarbamate (NCDC) and 1-[6-([(17beta)-3-methoxyestra- 1,3,5(10)-trien-17-yl]amino)hexyl]-1H-pyrrole-2,5-dione) (U-73122) were able to abrogate both haloperidol and NE actions. Further, the typical neuroleptic exerts a direct activation of alpha1-adrenoceptors as its actions were not modified by cocaine and persisted in spite of chemical rat cerebral denervation with 6-hydroxydopamine (6-OHDA). The possibility that this agonistic action on alpha1-adrenoceptors would be involved in haloperidol side effects is also discussed.     

The effect of repeated electroconvulsive shock treatment and chronic lithium feeding on the release of norepinephrine from rat cortical vesicular preparations

The effect of repeated electroconvulsive shock (ECS) treatment and chronic LiCl feeding on calcium-dependent, K+-evoked release of [3H] norepinephrine from rat cortical vesicular preparation was studied. There was no significant effect of either acute or repeated ECS treatment on [3H]norepinephrine release in cortical vesicles obtained from animals treated for either 1 or 10 days. Release of norepinephrine was examined over a range of CaCl2 concentrations. Clonidine effectively inhibited release of [3H]norepinephrine in cortical vesicles obtained from control and ECS-treated animals. K+-evoked release of [3H]norepinephrine at low (0.2 mM) and high (1.0 mM) CaCl2 concentrations was significantly increased in cortical vesicles obtained from LiCl-treated animals. Clonidine effectively inhibited release of [3H]norepinephrine in cortical vesicles obtained from both control and LiCl-fed animals. These results suggest a possible common mechanism of action of antidepressant drug therapy on presynaptic release of norepinephrine from nerve terminals.     

Differential effects of chronic clorgyline and amfonelic acid on desensitization of striatal dopamine receptors

Chronic treatment of rats with the MAOI clorgyline significantly reduced the density (Bmax) of cortical beta-adrenergic receptors but did not alter either the Bmax or dissociation constant (Kd) of 3H-spiperone binding to striatal DA receptors. Clorgyline co-treatment also did not significantly affect either behavioral supersensitivity to apomorphine or the increase in 3H-spiperone binding induced by chronic haloperidol. In contrast, repeated treatment with the DA uptake inhibitor amfonelic acid elicited behavioral subsensitivity and reduced striatal 3H-spiperone binding. Furthermore, amfonelic acid co-treatment prevented haloperidol-induced behavioral and receptor binding changes. The possible relevance of these findings in relation to drug choice in clinical trials of receptor sensitivity modification are discussed.     

Electroconvulsive shock treatment differentially modulates cortical and subcortical endocannabinoid activity

Previous studies indicate that the endocannabinoid system is a potential target for the treatment of depression. To further examine this question we assessed the effects of electroconvulsive shock (ECS) treatment, both a single session and 10 daily sessions, on endocannabinoid content, CB(1) receptor binding parameters and CB(1) receptor-mediated [(35)S]GTPgammaS binding in the prefrontal cortex, hippocampus, hypothalamus and amygdala. A single ECS session resulted in a general reduction in the binding affinity of the CB(1) receptor in all brain regions examined, as well as reductions in N-arachidonylethanolamine (anandamide) content in the prefrontal cortex and the hippocampus, reduced hydrolysis of anandamide by fatty acid amide hydrolase (FAAH) in the prefrontal cortex and an increase in the binding site density of the CB(1) receptor in the amygdala. Following 10 ECS sessions, all these effects subsided except for the reductions in anandamide content in the prefrontal cortex, which increased in magnitude, as well as the reductions in FAAH activity in the prefrontal cortex. Additionally, repeated ECS treatment resulted in a significant reduction in the binding site density of the CB(1) receptor in the prefrontal cortex, but did not alter CB(1) receptor-mediated [(35)S]GTPgammaS binding. Repeated ECS treatment also significantly enhanced the sensitivity of CB(1) receptor-mediated [(35)S]GTPgammaS binding in the amygdala. Collectively, these data demonstrate that ECS treatment results in a down-regulation of cortical and an up-regulation of subcortical endocannabinoid activity, illustrating the possibility that the role of the endocannabinoid system in affective illness may be both complex and regionally specific.     

Agonist-Induced Regulation of Adrenoceptor Subtypes in Cerebral Cortical Slices

Cerebral cortical slices from rat brain were incubated at 37 degrees C for 2 h in the presence of isoproterenol, noradrenaline, or adrenaline, and binding affinities and densities of adrenoceptor subtypes were subsequently examined in homogenized tissue. The density of alpha 2- and total beta-adrenoceptors was estimated using the radioligands [3H]rauwolscine and [3H]dihydroalprenolol (DHA), respectively. The percentages of beta 1- and beta 2-adrenoceptors were defined by inhibiting the binding of [3H]DHA with the beta 1-selective antagonist metoprolol. Exposure of slices to noradrenaline and adrenaline significantly decreased the maximal number of binding sites (Bmax) of alpha 2-adrenoceptors (48 and 37% respectively) without significantly affecting affinity; isoproterenol had no effect. Exposure to isoproterenol, noradrenaline, and adrenaline significantly decreased the Bmax of beta-adrenoceptors (by 60, 34, and 24%, respectively) but did not affect the affinity. Isoproterenol and adrenaline significantly decreased the density of beta 1-adrenoceptors by 75 and 24% and beta 2-adrenoceptors by 23 and 28%, respectively. Noradrenaline significantly decreased the density of beta 1-adrenoceptors by 42% without affecting the number of beta 2-adrenoceptors. These findings indicate that subtypes of adrenoceptors in rat cerebral cortex are differentially regulated by adrenergic agonists.     

Quantitative Autoradiographic Analysis of the Effects of Electroconvulsive Shock on Serotonin-2 Receptors in Male and Female Rats

Chronic electroconvulsive shock (ECS) is known to increase the level of serotonin-2 (S2) receptors in male rat brain. Using quantitative autoradiography, we have studied the distribution pattern of these receptors in female as well as male rats and the effect of repeated ECS on the receptor level in both sexes. We find that although the distribution of S2 receptors is generally similar in males and females, they respond differently to repeated ECS. In males we found the expected increase in S2 binding, which was localized to specific cortical, hippocampal, and septal regions. In females, no increase was found in the cortex or septum and relatively small increases were found in the hippocampus. It appears that the regulation of S2 receptors by ECS is sex-dependent.     

Effect of prolonged 5-hydroxytryptamine uptake inhibition by paroxetine on cortical beta 1 and beta 2-adrenoceptors in rat brain

The effects of prolonged (21 day) oral administration of the antidepressants paroxetine (0.9 to 8.9 mg/kg/day) and amitriptyline (2.7 to 27 mg/kg/day), on rat brain cortical beta 1- and beta 2-adrenoceptor numbers and affinities were investigated using [3H]-CGP 12177. Although amitriptyline, 27 mg/kg, caused a significant (p less than 0.05) 20% reduction in the number of beta 1-adrenoceptors, paroxetine, at doses up to 8.9 mg/kg p.o., did not influence binding of [3H]-CGP 12177 to cortical beta 1- or beta 2-adrenoceptors. This study with paroxetine provides further evidence that the down-regulation of central beta 1-adrenoceptors in rat brain after repeated administration is not a property of all antidepressant drugs.     

Electroconvulsive Shock and Reserpine: Effects on β-Adrenergic Receptors in Rat Brain

Abstract Electroconvulsive shock (ECS) administered once daily for up to 14 days decreases β-adrenergic receptor binding in the cortex and hippocampus in a time-dependent manner. The decrease in binding in the cortex lasts at least 1 week after the last shock. In the striatum, hypothalamus, or cerebellum, 14 days of ECS did not produce significant changes in β-adrenergic receptor binding. The brain regional pattern of β-adrenergic receptor changes suggests that repeated ECS affects β-adrenergic receptors in brain regions that receive a noradrenergic innervation activated by ECS. The effects of ECS on neurotransmitter receptor binding appear to be highly selective. Of five receptors in the cortex and three receptors in the hippocampus measured, only β-adrenergic receptor binding is decreased. Chronic footshock stress does not alter β-adrenergic receptor binding sites in the cortex, indicating that the effects of ECS are not due to stress alone. The effects of ECS on reserpine-induced alterations in β-adrenergic receptor binding sites were also examined. Ten days of ECS following chronic reserpine injections reverses the increased binding of β-adrenergic receptors     

Interaction of σ-Compounds with Receptor-Stimulated Phosphoinositide Metabolism in the Rat Brain

Sigma-receptors are nonopioid, nondopaminergic receptors that bind with high affinity several antipsychotic drugs and appear to be involved in regulation of posture and movement. The second messenger system coupled to these receptors is still unknown. Recently, an inhibitory effect of various sigma-compounds on carbachol-stimulated phosphoinositide metabolism has been reported. We have investigated the effect of six sigma-compounds on carbachol- and norepinephrine-stimulated 3H-inositol phosphate accumulation in rat cerebral cortex slices. All compounds tested had a dose-dependent inhibitory effect on both systems, although their order of potency differed between neurotransmitters. Pentazocine and 1,3-di-o-tolylguanidine were the most potent inhibitors of carbachol-stimulated phosphoinositide turnover (IC50 = 31.5 and 45.7 microM, respectively), while haloperidol showed the greatest potency on the norepinephrine-coupled system (IC50 = 3.5 microM). In the presence of IC50 concentrations of these inhibitors, the dose-response curves for the agonists were shifted to the right and the EC50 values were significantly increased. Sigma-compounds also inhibited the binding of [3H]quinuclidinyl benzilate to muscarinic receptors and of [3H]prazosin to alpha 1-adrenoceptors in cortical membranes. In the presence of IC50 concentration (11 microM) of 1,3-di-o-tolylguanidine, no differences were found in the maximal number of muscarinic binding sites, whereas the dissociation constant increased approximately fivefold, indicating a decrease of the radioligand's affinity for the receptor. These results indicate that sigma-compounds, at micromolar concentrations, inhibit muscarinic and alpha 1-adrenergic receptor-coupled phosphoinositide metabolism, probably through an interaction with the neurotransmitter recognition sites.