Effect of repetitive electroconvulsive treatment on sensitivity to pain and on [3H]nitrendipine binding sites in cortical and hippocampal membranes

The effect of electroconvulsive shock (ECS) on the responsiveness to pain (measured by the hot-plate test) and on the characteristics ofl-type calcium channels (measured as [³H]nitrendipine binding sites) in the cortex and hippocampus was tested on the Wistar rat. In animals receiving a single ECS, the calcium channel density and affinity 24 h after treatment did not differ from the controls; the response to pain was also at the control level. Repeated ECS (eight once-daily shocks) resulted in an increased responsiveness to pain (shortening of response latency) and in an increase in the density of cortical, but not hippocampal, calcium channels. The K_D value for [³H]nitrendipine binding sites in either brain region remained unaltered by ECS. The calcium channel antagonist nifedipine, which by itself did not significantly alter the response to pain, prevented the enhancement of pain sensitivity brought about by ECS. The results suggest activation of calcium-dependent mechanisms by repeated ECS and confirm the involvement of calcium channels in pain mechanisms.     

The effect of repeated combined treatment with nifedipine and antidepressant drugs or electroconvulsive shock on the hippocampal corticosteroid receptors in rats

The effect of nifedipine, a calcium channel antagonist, on changes in the density of glucocorticoid (GR) and/or mineralocorticoid receptors (MR), induced by long-term treatment with antidepressant drugs (imipramine and amitriptyline) or electroconvulsive shock (ECS) was investigated in the rat hippocampus. Long-term treatment with imipramine or amitriptyline significantly increased the density of GR, while chronic ECS significantly elevated the density of both GR and MR. Nifedipine administered repeatedly had no effect on the basal GR and MR levels, however when the rats were pretreated with nifedipine, the antidepressant drugs and ECS were unable to increase the density of GR, or GR and MR, respectively. These results indicate that, unlike in other models, nifedipine blocks the effects of antidepressant drugs and ECS on GR and/or MR. They also show, that antidepressant drugs and ECS differ in their effect on the density of GR and MR.     

A study of the sensitivity of rat brain alpha2-adrenoceptors during chronic antidepressant treatments

Summary Two experimental approaches were used to study the effect of chronic antidepressant treatments on the functioning of central alpha₂-adrenoceptors. In one, the effect of antidepressant treatment on the ability of a low dose of clonidine (25 g/kg) to lower rat brain 3-methoxy-4-hydroxyphenylethyleneglycol sulphate (MHPG-SO₄) levels was studied. In the other, potential treatment-induced alterations in rat frontal cortex ³H-clonidine binding were determined. The same cortical tissue was also studied in parallel for ³H-dihydroalprenolol binding.Of the tricyclics imipramine, amitriptyline and nortriptyline, only chronic imipramine (10 mg/kg b.i.d. for 14 days) attenuated the clonidine-induced reduction in brain MHPG-SO₄ levels. Repeated electroconvulsive shock therapy (100 mA for 1 s once daily for 10 days), but not chronically administered pargyline, also antagonized the neurochemical response to clonidine.The following long-term antidepressant treatments were investigated for their effect on rat frontal cortex ³H-clonidine and ³H-dihydroalprenolol binding: desipramine, imipramine, nortriptyline, amitriptyline, mianserin, iprindole, nisoxetine, pargyline and electroconvulsive shock therapy. Only chronic pargyline (25 mg/kg once daily for 14 days) altered cortical ³H-clonidine binding. Scatchard analysis revealed that the drug-induced decrease was due to a reduction in the number of recognition sites with no change in affinity. With the exceptions of mianserin and nisoxetine, all other seven forms of antidepressant treatment decreased cortical ³H-dihydroprenolol binding, Scatchard analysis revealing a diminution in the number of recognition sites.The results of this, and previous studies, indicate that under the dosage regimens employed only some forms of long-term antidepressant treatments induce a down-regulation in the functioning of rat brain presynaptic alpha₂-adrenoceptors, as assessed by the clonidine-MHPG-SO₄ model. No correlation was found between treatment-induced alterations in the characteristics of cortical ³H-clonidine recognition sites and the effect of identical treatment on the clonidine-induced reduction in rat brain MHPG-SO₄ levels.To unequivocally assume that drug-induced changes in central ³H-clonidine recognition sites reflect alterations in the functioning of central presynaptic alpha₂-adrenoceptors may be open to question.     

Role of calcium channels in effects of antidepressant drugs on responsiveness to pain

The effect of acute and chronic treatment with three antidepressant drugs on the cortical L-type calcium channel (measured as [³H]nitrendipine binding sites) and on the responsiveness to pain (assessed in the hot-plate test) was tested on the Wistar rat. Acute administration of antidepressants did not affect the characteristics of calcium channels and did not significantly prolong the hot-plate latency. However, a combination of antidepressants with nifedipine brought about analgesia. Chronic administration of imipramine did not significantly affect the characteristics of calcium channels but produced a moderate analgesic effect. In contrast, chronic administration of citalopram and chlorprothixene increased the density of [³H]nitrendipine binding sites and induced hyperalgesia, which was nullified by acute administration of nifedipine. The results confirm that calcium channels may be involved in analgesia and hyperalgesia and indicate that chronic treatment with some antidepressant may induce an increase in the density of cortical calcium channels.     

Lithium does not prevent ECS-induced decreases in β-adrenergic receptors

Electroconvulsive shock (ECS) reduces the number of rat cortical -adrenergic receptors. Lithium has been reported in several systems to prevent receptor changes induced by other agents. However, the present experiment reports that chronic lithium does not prevent the reduction in dihydroalprenolol binding induced by ten daily ECS treatments.     

Involvement of hippocampal neuropeptide Y in mediating the chronic actions of lithium, electroconvulsive stimulation and citalopram

Neuropeptide Y (NPY) has been considered to be involved in the pathogenesis of affective disorders, and chronic treatment with lithium or electroconvulsive stimuli (ECS) has been shown to increase mRNA and peptide levels of NPY in rat brain tissue. Consequently, parameters reflective of NPYergic neurotransmission were studied in the hippocampus of rats following chronic treatment with lithium, ECS or the selective serotonin re-uptake inhibitor (SSRI), citalopram. Lithium (28 days, diet) and ECS (10 days, once daily) treatments caused a marked increase in levels of preproNPY mRNA in the CA1 area and dentate gyrus (DG). This increase was accompanied by an increase in extracellular levels of NPY in the dorsal hippocampus of freely moving rats as determined by microdialysis, suggesting that lithium and ECS treatments lead to an increased biosynthesis and release of NPY in this area. (125)I-peptide YY (PYY) binding was reduced by 40 and 60% respectively in the DG following the same treatments, showing that the increased release is accompanied by a down-regulation of corresponding binding sites. In contrast, citalopram (10 mg/kg i.p., twice daily for 28 days) caused a 100% increase in (125)I-PYY binding in CA, CA3 and DG while levels of preproNPY mRNA and extracellular NPY in the hippocampus were unaffected. The results indicate that various agents and stimuli exerting antidepressant effects in humans, such as chronic lithium, ECS and citalopram all increase NPYergic neurotransmission in the hippocampus by distinct modes of action. Moreover, NPY (6 microg) given intracerebroventricularly (i.c.v.) induced an antidepressant-like effect in the forced swim test. It is hypothesised that the increase in NPYergic neurotransmission may be associated with the mechanism of action of various antidepressant treatments in the alleviation of depression.     

Calcium antagonist binding in cat brain tolerant to electroconvulsive shock

Cats subjected to daily (25-30 days) electroconvulsive shock (ECS) demonstrated an elevation of their electroconvulsive threshold or tolerance to ECS. [3H] Nitrendipine binding was measured to brain regions from non-tolerant (sham shocked) and ECS tolerant cats 24 hr following the last shock. ECS produced a significant increase (45%) in the density of [3H] nitrendipine binding sites in the cerebral cortex and a significant decrease (33%) in the apparent affinity of [3H] nitrendipine in the cerebellum. No changes in binding were observed in the hippocampus. The effects of ECS were also investigated in the rat, an animal not displaying tolerance to repeated ECS. [3H] Nitrendipine binding to rat brain was measured 10 min and 24 hr following one shock (acute) or ten shocks delivered transauricularly once daily (chronic). Twenty-four hours following chronic ECS, there was a significant increase (19%) and decrease (11%) in the density, but no change in the apparent affinity of [3H] nitrendipine binding sites in the cerebral cortex and hippocampus respectively. No significant change in [3H] nitrendipine binding was observed in rat cerebellum 24 hr following chronic ECS. There were no changes in [3H] nitrendipine binding in the cerebral cortex and hippocampus 10 min and 24 hr following acute ECS. These results indicate that ECS can alter [3H] nitrendipine binding to calcium channel linked dihydropyridine binding sites in the central nervous system. It is suggested that changes in [3H] nitrendipine binding in the cat cerebellum may be involved in the development of tolerance to ECS.     

Concurrent nimodipine attenuates the withdrawal signs and the increase of cerebral dihydropyridine binding after chronic morphine treatment in rats

Summary The effect of chronic administration of dihydropyridine calcium channel antagonist nimodipine (1 mg/kg/day) given concurrently with morphine on the signs of morphine withdrawal and on the [³H]nitren-dipine binding in the rat brain has been investigated. Chronic morphine administration in increasing daily doses from 20 mg/kg to 70 mg/kg for 24 days and consequent withdrawal for 24 h induced loss of body weight, wet dog shakes, episodes of writhing and yawning behaviour. The density of [³H]nitrendipine binding was elevated in the cortex and limbic structures but not in the striatum after chronic morphine treatment. Chronic concurrent administration of nimodipine prevented the loss of body weight and reduced the scores of wet dog shakes and writhing, but did not affect yawning behaviour at 24h after morphine withdrawal. The concurrent nimodipine treatment also prevented the rise in the density of central dihydropyridine binding sites which occurred upon chronic morphine treatment. These results suggest that chronic nimodipine treatment attenuates the development of the withdrawal signs which occur upon the termination of chronic morphine treatment by preventing the up-regulation of the central dihydropyridine-sensitive binding sites. Correspondence to: L. Ahtee at the above address     

Non-selectivity of amitriptyline for subtypes of brain muscarinic receptors demonstrated in binding and functional assays

The characteristics of interaction of amitriptyline, a tricyclic antidepressant, with rat brain muscarinic receptors were assessed using both radioligand binding and functional assays. In competition studies, amitriptyline displaced muscarinic ligand binding from a single high-affinity site in homogenates of various brain regions which have a different distribution of M1 and M2 receptor subtypes. The affinity of amitriptyline for muscarinic receptors was also comparable in all brain regions. Furthermore, amitriptyline identified a single species of muscarinic receptors in intact cells dissociated from the cerebral cortex and in cerebrocortical slices. The non-selectivity of amitriptyline for muscarinic receptor subtypes in these preparations was in contrast to the selectivity exhibited by pirenzepine. This non-selective nature of amitriptyline was also evident in functional assays, since this antidepressant was equipotent at antagonizing M1-mediated increase in phosphoinositide hydrolysis and M2-mediated inhibition of cyclic AMP formation in dissociated cortical cells. Atropine was also equipotent at blocking both responses but was 20- to 30-fold more potent than amitriptyline. These results demonstrate that amitriptyline behaves as a non-selective muscarinic antagonist using both radioligand binding and functional measurements.     

Repeated treatment with d-fenfluramine or metergoline alters cortex binding of 3H-serotonin and serotonergic sensitivity in rats

28-day treatment with d-fenfluramine, a serotonin (5HT) releaser and uptake inhibitor, caused significant reduction (23%) of ³H-5HT binding sites (B_max) in the rat cortex. These sites were significantly increased (31%) in cortical membranes of rats which had received metergoline, a potent serotonin antagonist, for 28 days. Parallel changes were found in the anorectic activity of metachlorophenylpiperazine (m-CPP), a potent central 5HT agonist: chronic treatment with d-fenfluramine or metergoline caused respectively a decrease and in the effect of m-CPP on food intake. The data show that changes in 5HT central receptor number and sensitivity may occur after chronic treatment with drugs acting on brain serotonin.     

Atipamezole,Benzodiazepines, Bicucullin and Tifluadom Antagonize the Effect of TRH on Rat Duodenum and Displace it from Brain and Anterior Pituitary Receptors

Abstract: The mechanism of action of the TRH induced inhibition of contractions of the transmurally stimulated rat duodenum has been studied. The effect of TRH was not antagonized by atropine, pentolinium, phenoxybenzamine, sotalol, methysergide, domperidone, diphenhydramine, cimetidine, aminophylline, antazolin, indomethacin, morphine, naloxone or tetrodotoxin. In contrast, the adrenergic α₂-antagonist atipamezole, the benzodiazepines chlordiaxepoxide and midazolam or GABA-A-antagonist bicucullin but not picrotoxin or SR-95531 attenuated the response to TRH. An opioid-kappa-receptor agonist having benzodiazepine structure, tifluadom, but not MR 2034 also diminished the response to TRH. However, these actions were not modified by the α₂-agonist medetomidine, benzodiazepine antagonist flumazenil, GABA-agonists muscimol or baclofen or naloxone, respectively. While the binding of [³H][3-Me-His²]TRH to the rat anterior pituitary, hypothalamus, cortex and brainstem homogenates was saturable and of high affinity, no saturable binding was observed in the duodenal smooth muscle. Agents that were effective in the duodenal preparation displaced [³H][3-Me-His²]TRH from its binding sites in brain homogenates and the inhibitory constants (K_i) were (in μM): 0.038-0.107 (TRH), 0.19-5.8 (chlordiazepoxide), 0.021-8.9 (midazolam), 1.5-17 (tifluadom), 60-210 (bicucullin) and 150-530 (atipamezole). Atipamezole, bicucullin and chlordiazepoxide caused competitive displacement indicated by the increased K_D of the labelled ligand but no change in the B_max while tifluadom increased K_D and decreased B_max. It is concluded that the inhibitory effect of TRH on the contractions of the duodenal smooth muscle is mediated directly by the smooth muscle and it is apparently specific for TRH. The competitive displacement of [³H][3-Me-His²]TRH binding in brain homogenates by benzodiazepines, bicucullin and atipamezole suggest a similar mechanism for their antagonistic effect of the TRH-induced inhibition of duodenal contractions.     

Interaction of Dihydropyridine Calcium Channel Agonists and Antagonists with Adenosine Receptors

Abstract: We have confirmed our previous ( Fredholm et al. 1986a ) finding that the dihydropyridine calcium channel agonist Bay K 8644 can displace [³H]-R-PIA from its binding site, the adenosine A₁-receptor. Bay K 8644 had an apparent K_i of 5.2 × 10^-6 M. The effect was shared by the two dihydropyridine calcium channel antagonists nifedipine and felodipine (K_i 4.2 and 8.7 × 10^-6 M, respectively). By contrast, two non-dihydropyridine calcium channel antagonists, verapamil and diltiazem, did not affect binding. Bay K 8644 displaced [³H]-R-PIA from its binding sites in a solubilized preparation. [³H]-XAC, a novel, potent A₁-receptor antagonist ligand, was also displaced by the dihydropyridine compounds with a similar or slightly higher potency as the displacement of R-PIA. This suggests a direct interaction with the adenosine receptor rather than an effect on regulatory GTP-binding proteins. However, at 1 μmol/l neither Bay K 8644 nor nifedipine significantly attenuated cyclic AMP accumulation in rat hippocampi or the R-PIA-mediated adenylate cyclase inhibition. The results show that dihydropyridine compounds that act as agonists or antagonists on L-type calcium channels can also affect adenosine receptors. The potency of the compounds for this effect is much lower than their potency as calcium channel agonists or antagonists. The results may therefore be of more experimental than clinical significance.     

Phencyclidine increases the affinity of dihydropyridine calcium channel antagonist binding in rat brain

Summary Phencyclidine (PCP) significantly reduces the apparent dissociation constant (K _D) of the dihydropyridine (DHP) calcium channel antagonist, [³H]nitrendipine, in synaptosomal membranes of rat and mouse brain without significantly effecting the maximum binding capacity (B _max). At an optimum concentration of PCP (10 M) the apparentK _D of [³H]nitrendipine was reduced from 178±9 pM to 112±9 pM in rat forebrain, a 58% increase in affinity. The structural derivatives of PCP, P-Br-PCP {1-[1-(4-bromophenyl-cyclohexyl)piperidine]}, m-NH₂-PCP {1-[1-(3-anilo)-cyclohexyl]piperidine}, (±)-PCMP [1-(1-phenyl)-cyclohexyl-3-methylpiperidine] also increased the apparent affinity of [³H]nitrendipine in the following order, p-Br-PCP PCMp>PCP>m-NH₂-PCP. Local anesthetics either reduced the apparent affinity of [³H]nitrendipine or had no effect. Kinetic analysis revealed that PCP both increased the microassociation rate constant and decreased the microdissociation rate contant of [³H]nitrendipine. The magnitude of this enhanced binding varied with the brain region studied; the greatest increase in apparent affinity of [³H]nitrendipine was observed in striatum, while no significant increase in affinity was observed in brainstem. In some brain areas, PCP was more effective in reducing theK _D in crude homogenates than in washed tissue. PCP (10 M) did not alter theK _D of [³H]nitrendipine to rat cardiac tissue. Both Ca²⁺ and Mg²⁺ inhibited the effect of PCP, while monovalent ions were ineffective in this regard. These data are consistent with an allosteric modulation of DHP calcium channel antagonist binding sites by PCP and structural derivatives that is not mediated through the brain PCP binding site. This modulation of DHP binding sites may account for some of the psychopharmacologic actions PCP and related compounds in vivo.     

Treatment with antidepressants and down regulation of beta-adrenergic receptors

The effect of chronic and acute treatment with several antidepressant drugs and electroconvulsive shock (ECS) on the responsiveness of norepinephrine (NE) receptor coupled adenylate cyclase and beta-adrenergic receptor binding sites in rat brain has been studied by several groups of investigators. It has been consistently reported that chronic administration (2 to 4 weeks of treatment) of antidepressant drugs or ECS causes a decreased accumulation of NE stimulated cyclic AMP accumulation in rat brain. It has also been reported that chronic treatment with antidepressant drugs or ECS causes decreased binding of [³H]dihydroalprenolol (DHA) or [¹²⁵I]hydroxybenzylpindolol (I-HYP) (ligands used for studies of beta-adrenergic receptors) in rat brain. However, such effects are not observed after acute or single administration of these drugs or ECS. Since there appears to be a close correlation between the time course of the effects of these drugs on NE receptor coupled adenylate cyclase and beta-adrenergic receptor binding, and since most of the antidepressant drugs tested thus far produce such effects, it has been suggested that down regulation or decreased responsiveness of beta-adrenergic receptors may be related to the therapeutic effects manifested by antidepressant drugs. These observations offer the possibility that determination of beta-adrenergic receptor responsiveness after chronic treatment with drugs may prove useful in the preclinical screening of potential antidepressant drugs. In this paper, studies related to down regulation of beta-adrenergic receptors, including our own, are reviewed.     

Interactions of DPI 201-106, a novel cardiotonic agent,with cardiac calcium channels

Summary The interaction of DPI 201-106, a novel cardiotonic agent, with the calcium entry blocker receptor complex was studied using porcine cardiac sarcolemmal membranes. DPI 201-106 and the chemically-related calcium antagonist, cinnarizine, produce concentration-dependent inhibition of nitrendipine, gallopamil and diltiazem binding to their respective sites in these vesicles. This effect of DPI 201-106 is not stereoselective since resolved stereoisomers of this compound display equal potency in inhibiting each of the binding reactions. Equilibrium ligand binding studies revealed that DPI 201-106 and cinnarizine cause mixed inhibitory patterns at the aralkylamine and benzothiazepine sites (i. e. both K _d and B _max values were affected) while mainly increasing K _d at the dihydropyridine site. The kinetics of ligand dissociation from the three calcium entry blocker receptors, together with measurements of dihydropyridine association kinetics, further demonstrate that DPI 201-106 interacts at a unique site in the receptor complex and allosterically modulates binding of nitrendipine, gallopamil and diltiazem.The functional consequences of the above interactions with the calcium channel were studied in isolated cardiac preparations. In guinea-pig atria, DPI 201-106 increased force of contraction. This inotropic effect is seen only with the S(–) enantiomer and is unaltered by nitrendipine-, verapamil- or diltiazem-pretreatment, indicating DPI 201-106 does not act as a stimulant of this channel. Furthermore, DPI 201-106 did not alter the inotropic action of Bay K 8644, a calcium channel stimulant. Spontaneous rate of guinea-pig right atria is decreased by both DPI 201-106 and cinnarizine. In addition, potassium-induced contractures in cat papillary muscles are reduced by both agents. These latter two effects differed from the positive inotropic effect in that 1) both enantiomers of DPI 201-106 are equally potent in decreasing heart rate and reducing potassium-induced contractures and 2) these effects of DPI 201-106 require larger concentrations than those needed to elicit a positive inotropic effect (10^–5 mol/l vs. 10^–6 mol/l). Therefore, in addition to its positive inotropic action, DPI 201-106 shares receptor binding and pharmacological attributes of the calcium channel antagonist, cinnarizine. Send offprint requests to P. K. S. Siegl at the above address     

The acute and chronic effects of D-amphetamine, chlorpromazine, amitriptyline and lithium chloride on adenosine 5-triphosphatases in different regions of the rat brain.

The hypothesis that adenosine triphosphatase plays a primary role in the re-uptake process of biogenic amines in the rat brain was tested. Four centrally acting drugs, known to alter the turnover of biogenic amines in different ways, were studied for their in vivo effects on Na⁺ K⁺ ATPase and Mg²⁺ (EC 3.6.1.5) ATPase activities in high-speed supernatant fractions from ten brain regions. The acute and chronic administration of amphetamine, amitriptyline, chlorpromazine and lithium chloride altered the activities of the ATPases in several of the brain regions studied. In most cases, the regions in which the activity of these enzymes was most affected coincided with the areas in which other investigators have shown these drugs to have a pharmacological action. Amphetamine and amitriptyline increased the activity of the ATP'ases in several regions whereas chlorpromazine and lithium chloride had the opposite effects. While it seems possible that different classes of drugs can affect ATP'ases in different ways depending on their neuropharmacological profile, there is little evidence from this study to substantiate the view that changes in ATP'ase activity is directly involved in the activity of the re-uptake transport system for brain biogenic amines.     

Repeated treatment with antidepressant drugs and/or electroconvulsive shock (ECS) does not affect the quinpirole-induced elevation of the serum corticosterone concentration in rats

The selective dopamine D2 receptor agonist quinpirole (0.03-1 mg/kg intra peritoneally) increased dose-dependently the serum corticosterone level in rats. The effect of the maximum dose of quinpirole (0.3 mg/kg intraperitoneally) was antagonized in a dose- dependent manner by the selective D2 receptor antagonist sulpiride (3-30 mg/kg intra peritoneally), but not by the selective D(1) receptor antagonist SCH-23390 (3 mg/kg intra peritoneally). Imipramine and amitriptyline (10 mg/kg per os), administered acutely or repeatedly (twice daily for 14 days), did not affect the corticosterone response to quinpirole (0.1 mg/kg intraperitoneally). The response was modified neither in animals treated repeatedly with electroconvulsive shock (ECS) (seven shocks every 2 days), nor after their combined repeated treatment with imipramine and ECS. The above results indicate that repeated treatment with antidepressant drugs and/or ECS does not affect the sensitivity of dopamine D2 receptors involved in the corticosterone response to quinpirole.     

Pilocarpine Treatments Differentially Affect α2-Adrenoceptors which Modulate the Firing Rate of Locus coeruleus Neurones and the Synthesis and Release of Noradrenaline in Rat Brain

Abstract: We have investigated the effect of treatments with the muscarinic acetylcholine receptor agonist, pilocarpine, on the sensitivity of central α₂-adrenoceptors that regulate the firing activity of rat locus coeruleus, the tyrosine hydroxylase activity in the rat cortex, hippocampus and hypothalamus, and the K⁺-evoked release of [³H]noradrenaline from rat cortical and hippocampal synaptosomes. Short-term (4 days), but not acute, treatment with pilocarpine caused a small but statistically significant increase in the inhibitory effect of the α₂-adrenoceptor agonist clonidine on the firing rate of locus coeruleus neurones, with a decrease in the ED₅₀ of 29% (P<0.001). However, no change in the effect of clonidine on the locus coeruleus was observed after longer pilocarpine (11 days) treatment. In the rat cerebral cortex, but not in hippocampus or hypothalamus, chronic (19 days) treatment with pilocarpine caused a decrease in the inhibitory effect of clonidine on tyrosine hydroxylase activity (55%, P<0.05), but did not change the stimulatory effect of the α₂-adrenoceptor antagonist idazoxan. Moreover, treatments (4, 11 and 19 days) with pilocarpine did not alter the inhibitory effect of clonidine [10^?8-10^?5 M] on the K⁺-evoked release of [³H]noradrenaline from rat cortical and hippocampal synaptosomes. These results indicate that administration of pilocarpine slightly potentiates some but not all the functional responses mediated by brain presynaptic α₂-adrenoceptors. In conclusion, these results do not support the hypothesis that chronic treatments with pilocarpine lead to a suitable model of α₂-adrenoceptor supersensitivity.     

Effects of prenalterol on beta adrenergic responsiveness and receptors in the cerebral cortex of the rat

The activity of the beta-adrenoceptor agonist, prenalterol, at beta adrenoceptors in the cerebral cortex of the rat and the effect of chronic intraperitoneal infusion of prenalterol on the biochemical responsiveness and density of cerebral cortical beta adrenoceptors was studied. Whereas isoproterenol caused a four-fold rise in the content of cyclic AMP in slices of cerebral cortex, prenalterol did not produce a significant increase in cyclic AMP. However, prenalterol inhibited the isoproterenol-stimulated increase in cyclic AMP in cortical slices in a concentration-dependent manner. Using an in vivo binding technique, prenalterol (3.8 mg/kg/hr) infused intraperitoneally through osmotic minipumps, penetrated the brain and significantly inhibited the binding of the beta adrenoceptor antagonist, [125I]iodopindolol (125I-IPIN), to cortical beta adrenoceptors. Infusion of prenalterol (3.8 mg/kg/hr) for 7 days resulted in a small (20%), but significant, reduction in the ability of isoproterenol to stimulate maximally the accumulation of cyclic AMP in slices of cerebral cortex. No alteration in the Bmax or KD of the binding of [125I]iodopindolol was observed in homogenates of cortex obtained from prenalterol-treated rats. Furthermore, no change was observed in the binding of the hydrophilic ligand [3H]CGP-12177 in homogenates of cortex. In this study, then, prenalterol exhibited properties in vitro of a beta adrenoceptor antagonist, but did cause modest desensitization of beta adrenoceptor responsiveness when administered continuously in vivo.     

Biochemical evidence for a new benzodiazepine receptor antagonist

1-(3-Chlorophenyl)-3-diethylcarbamoyl-1 H-1,2,4-triazole (CP-32,961) inhibited [³H]diazepam binding to rat cortical membrances and [³H]flunitrazepam binding to mouse brain in vivo. Its inhibition of the binding of [³H]Ro 15-1788 (benzodiazepine receptor antagonist) to these membranes was not facilitated by added GABA; CP-32,961 exhibited a GABA ratio of 0.84 compared to 2.44 for diazepam. Cerebellar cyclic GMP content in rats was raised by CP-32,961, which also further elevated the increased cyclic GMP levels induced by isoniazid. These neurochemical actions are similar to those shown by ethyl β-carboline-3-carboxylate (β-CCE) and suggest that CP-32,961 is a benzodiazepine receptor antagonist with inverse agonist activity.