Hepatic protoporphyria is associated with a decrease in ligand binding for the mitochondrial benzodiazepine receptors in the liver

Protoporphyrin IX (PP) and N-methylprotoporphyrin IX (N-MePP) added in vitro to liver membranes reduced dose-dependently the affinity of [3H]PK 11195 for the mitochondrial benzodiazepine receptors (MBRs), the latter being about 20 times more potent (Ki 4.5 and 0.25 microM). Preincubation of these two porphyrins with liver homogenates for 120 min at 4 degrees resulted in significant inhibition of [3H]PK 11195 binding even after repeated washings of the membranes due to the residual presence in the membranes of about 35 and 5% of PP and N-MePP, respectively. Thus, the hypothesis that an in vivo increase in the hepatic porphyrin content modifies the binding of the isoquinoline PK 11195 to the MBRs was investigated in an experimental model of protoporphyria. PP and N-MePP were allowed to accumulate in vivo through treatment with 3,5-diethoxycarbonyl-1, 4-dihydrocollidine (DDC) (100 mg/kg i.p., once), and rats were killed 5 h after treatment when hepatic porphyrin accumulation was marked (10-fold increase), PP predominating. In the liver, treatment reduced the affinity (Kd) of [3H]PK 11195 for MBRs (from 3.56 to 15.37 nM, P < 0.01) and the maximum number of binding sites (Bmax) (55% decrease, P < 0.05); the affinity (Ki) of RO 5-4864 for [3H]PK 11195 binding sites was also reduced (from 23.9 to 72.99 nM, P < 0.05). No significant differences were found in the brain cortex. Liver and brain diazepam binding inhibitor levels and plasma corticosterone levels were unchanged. The reduction in [3H]PK 11195 binding to MBRs in the liver of DDC-treated rats thus appears to be attributable to a specific effect of the DDC-induced formation of the two protoporphyrins; this conclusion suggests that in hepatic protoporphyria processes modulated by MBRs may be altered.

Anorectic activity of fluoxetine and norfluoxetine in rats: relationship between brain concentrations and in-vitro potencies on monoaminergic mechanisms

The present study was aimed at establishing the importance of brain monoamine uptake and release mechanisms in the anorectic activity of fluoxetine, relating them to the actual brain concentrations of the parent drug and its metabolite norfluoxetine after anorectic doses in rats. Both compounds showed anorectic activity when administered intraperitoneally, norfluoxetine being slightly more active (ED50 = 22.9 mumol kg-1) than fluoxetine (ED50 = 35.0 mumol kg-1) despite the fact that the metabolite is about ten times less potent than the parent drug in inhibiting 5-hydroxytryptamine (5-HT) uptake. Comparing the brain concentrations of norfluoxetine, in terms of maximum concentrations (Cmax) and area under the curve (AUC), after the ED50 of fluoxetine or synthetic norfluoxetine, it also appeared that the metabolite plays a major role in the anorectic effect of the parent drug in rats. Brain Cmax of fluoxetine (48.7 microM) and norfluoxetine (21.7 and 27.3 microM after metabolite and drug, respectively) were several times those blocking 5-HT uptake in-vitro (0.5 microM), making it unlikely that fluoxetine (directly or through its metabolite) reduces food intake by specifically blocking 5-HT neuronal uptake. Brain Cmax of fluoxetine but particularly norfluoxetine were more compatible with those capable in-vitro of affecting catecholaminergic mechanisms, such as inhibition of dopamine and noradrenaline uptake and enhancement of dopamine release. These results together with recent in-vitro findings that the parent compound and its active metabolite induce tritium release from hippocampal synaptosomes previously loaded with [3H]5-HT suggest that mechanisms other than inhibition of 5-HT uptake are involved in the anorectic action of these compounds in rats.

Releasing activities of d-fenfluramine and fluoxetine on rat hippocampal synaptosomes preloaded with [3H]serotonin

Rat hippocampal synaptosomes preloaded with [3H]serotonin and maintained in a superfusion apparatus were exposed for 3 min to d-fenfluramine or fluoxetine. Both drugs evoked a tritium overflow which was reserpine-sensitive requiring the presence of intact synaptic vesicles. However the two drugs displayed different characteristics: 1) the overflow was immediate with d-fenfluramine whereas the releasing activity of fluoxetine showed a delay of about 2 min; 2) d-fenfluramine-induced overflow was already apparent at 0.15 mumol/l whereas the minimal effective concentration of fluoxetine was 2.5 mumol/l. Their concentration-effect curves were differently shaped, the effect of d-fenfluramine being saturable at 5-20 mumol/l (EC50 about 1 mumol/l) while no saturation was observed with fluoxetine up to 10 mumol/l; 3) only 19% of the tritium overflow evoked by fluoxetine (2.5-10 mumol/l) consisted of true [3H]serotonin, compared with 70% when 0.5 mumol/l d-fenfluramine was used; 4) the releasing action of 0.5 mumol/l d-fenfluramine was completely Ca(++)-dependent, while at higher d-fenfluramine concentrations the Ca(++)-independent overflow became more important. The fluoxetine induced overflow was mainly (70%) Ca(++)-independent; 5) the releasing activity of d-fenfluramine was mainly (80%) blocked by the serotonin uptake blockers indalpine, midalcipram and also fluoxetine whereas fluoxetine-induced overflow was insensitive to inhibition of the serotonin carrier. In conclusion, the releasing activity of d-fenfluramine is already present at a very low concentration (0.5 mumol/l) and at this concentration its mechanism of action was Ca(++)-dependent, together with the requirement of a functional serotonin carrier.(ABSTRACT TRUNCATED AT 250 WORDS)

Progress report on the anorexia induced by drugs believed to mimic some of the effects of serotonin on the central nervous system

Some agents that increase serotoninergic transmission in the brain show anorectic activity at doses that do not interfere with the behavior of rats and other animal species. These agents reduce food intake by a mechanism that clearly differs from that involved in the anorectic activity of d-amphetamine. d-Fenfluramine, fluoxetine, and sertraline are three drugs that have already been tested and are used in man. These compounds accumulate in the brain and are metabolized through N-dealkylation. They affect the uptake and release of serotonin at different concentrations, with mechanisms that do not completely overlap. There is pharmacological evidence that d-fenfluramine and sertraline exert their anorectic activity by enhancing the stimulation of 5-HT1nonA receptors whereas fluoxetine seems to affect at anorectic doses both serotoninergic and dopaminergic systems. The role of serotonin in controlling food intake will be discussed, and the effects of agents that reduce serotoninergic transmission will also be considered.

The alpha- and beta-adrenoceptor blocking activities of labetalol and its RR-SR (50:50) stereoisomers

1. We compared the alpha 1-, alpha 2- and beta 1-adrenoceptor blocking potencies of labetalol with those of its two stereoisomers (RR and SR) in pithed rats and in homogenized rat cerebral cortex and heart. 2. In pithed rats, labetalol and the RR-SR combination were given orally either at doses of 25 and 50 mg kg-1 body wt. or intravenously at doses of 1 and 5 mg kg-1 body wt. Prazosin 4 and 20 micrograms kg-1 body wt. and propranolol 1 and 5 mg kg-1 body wt., were given intravenously for comparison studies of potency at alpha 1- and beta 1-adrenoceptors, respectively. Effects were studied before and after i.v. administration of either phenylephrine (at doses which increased the mean arterial pressure by approximately 80 mmHg) or isoprenaline (at doses that increased heart rate by approximately 100 beats min-1). 3. In pithed rats, labetalol and the RR-SR combination antagonized, in a dose-dependent manner, the pressor effect of phenylephrine (P less than 0.05) and the chronotropic effect of isoprenaline (P less than 0.05). Following both oral and intravenous dosing, the RR-SR combination was twice potent as labetalol in terms of alpha 1- and beta 1-adrenoceptor antagonism at equivalent doses. 4. Labetalol and the enantiomers lacked affinity at alpha 2-adrenoceptors while at alpha 1-adrenoceptors the order of potency was prazosin much greater than RR-SR greater than labetalol. At beta 1-adrenoceptors, the affinity of the compound RR-SR was about 3 times that of labetalol.5. As labetalol is a mixture of active (RR and SR) and inactive (SS and SR) enantiomers (in terms of alpha and beta receptor actions), the combination of RR and SR may be a valuable substitute for labetalol in the treatment of systemic hypertension. Although the potential for non-specific side effects (common to all four enantiomers) could be expected to be diminished, recent reports by postmarketing surveillance indicate that the RR isomer (dilevalol) can induce liver toxicity. Interestingly, labetalol is devoid of this effect; whether the combination of RR and SR enantiomers could be of clinical importance warrants further investigation.

Diazepam binding inhibitor (DBI) increases after acute stress in rat

Diazepam binding inhibitor (DBI) acts in brain by binding to GABAA/benzodiazepine receptors (GBR) and to mitochondrial benzodiazepine receptors (MBR). Because DBI acting at MBR, has been shown to be an effector of ACTH-induced steroidogenesis and stress is known to change the level of GBR and MBR, the model of acute noise stress in rats was used to study modifications of DBI and GRB or the content of MBR in various areas of the brain and adrenal gland. It was found that, in the brain of stressed rats, DBI and its processing products (ODN-like immunoreactivity), increased selectively in the hippocampus. This increase in the content of DBI was preceded and followed by a net decrease of GBR and an increase of MBR. Similarly, in adrenal cortex, the content of DBI and MBR increased during the first hour, following acute stress and this increase paralleled the increase in plasma corticosterone. These data suggest that DBI, acting on MBR may regulate steroidogenic function in stress.

[Neurobiology of tianeptine. A new pharmaceutic agent]

In ex vivo experiments, tianeptine increased serotonin uptake in the hippocampus and the cortex acutely and after 72 hours following chronic administration for 15 days. This effect results from an increased maximal rate of uptake without changes in the number or affinity to binding sites for I'3H-imipramine or I'3H-paroxetin. In addition, tianeptine increased extracellular levels of 5-hydroxyindolacetic acid (5-HIAA) in hippocampus and hypothalamus measured with in vivo voltametry. It can thus be concluded that tianeptine also raises 5-hydrotryptamine (5HT) uptake in vivo. The effects of tianeptine on the serotoninergic system, especially the increase in serotonin uptake, are discussed in relation with its effects on behaviour and the dopaminergic and cholinergic systems.

Evidence that imipramine activates 5-HT1C receptor function

The anti-immobility effect of imipramine (15 mg/kg) in the forced swimming test in mice was antagonized by the non-selective 5-hydroxytryptamine (5-HT) antagonist, metitepine (0.5 mg/kg), by the 5-HT1C/5-HT2 antagonist, mesulergine (15 mg/kg), and by the dopamine D2 antagonist, d,l-sulpiride (50 mg/kg). These three antagonists did not alter the behaviour of imipramine-treated mice in an open-field and did not reduce imipramine brain levels. The 5-HT2 antagonist, ritanserin (0.06 mg/kg), the 5-HT1A/5-HTB antagonist, l-propranolol (20 mg/kg), and the 5-HT3 antagonists, endo-2,3-dihydro-N-(8-methyl-8-azabicyclo[3.2.1]oct-3-yl)-2-oxo-1H- benzimidazole-1-carboxamide hydrochloride (DAU 6215; 0.1 mg/kg) and 1,2,3,9-tetrahydro-9-methyl-3[(2-methyl-1H-imidazol-1-yl)methyl]-4H- carbazol-4-one, HCl.2H2O) (GR 38032F; 0.1 mg/kg), failed to reduce imipramine-induced anti-immobility. Subthreshold doses of 8-hydroxy-2-(di-n-propylamino)tetralin hydrochloride (8-OH-DPAT; 0.5 mg/kg) and imipramine (7.5 mg/kg) did not synergize in reducing immobility. d,l-Sulpiride, but not mesulergine, antagonized the effect of desipramine (15 mg/kg) in the forced swimming test. All compounds were administered i.p. 6 min before imipramine or desipramine, given i.p. 30 min before the testing. Imipramine produced 50% inhibition of [3H]mesulergine binding to 5-HT1C receptors at 10 microM, a concentration below that obtained following i.p. imipramine administration. The results suggest a contribution of 5-HT1C receptors in the mechanism of the imipramine effect in the forced swimming test.

Potential antidepressant activity and enhancement of serotonin uptake of a new dibenzothiadiazepine derivative

A molecule, 6-methyl-6,11-dihydro-11-[(N,N-dimethylamino) acetyl]dibenzo[c,fl-[1,2,5]thiadiazepine 5,5-dioxide, (IM/P/3/4, CAS 128377-70-8), was identified in a screening program, which had the scope of finding compounds with antidepressive potential without the common sideeffects of existing antidepressive medication. IM/P/3/4 was found active a) in antagonizing apomorphine (16 mg/kg) and reserpine-induced hypothermia in mice; b) in potentiating yohimbine-induced lethality in mice; c) in reducing immobility of rats forced to swim and of mice suspended by the tail. IM/P/3/4 does not affect a) apomorphine-induced stereotypy; b) amphetamine-induced hypermotility; c) haloperidol-induced catalepsy and water-induced grooming and d) does not induce stereotypy or alter motor activity. The compound also a) reduced the beating of rat right heart atria only at a concentration of 3 x 10-4 mol/l; b) had weak anticholinergic activity; c) antagonized electroshock-induced convulsions and d) prevented indometacin-induced duodenal ulcers. IM/P/3/4 does not have good affinity for noradrenergic, serotonergic, dopaminergic, histaminergic or muscarinic receptors and does not displace imipramine, desipramine and mianserine from their binding sites. IM/P/3/4 increases 5-hydroxyindolacetic acid content and 3H-serotonin uptake in the hypothalamus. The present results suggest that IM/P/3/4 is a potential antidepressant with reduced side effects and with a mechanism of action which is different from that of other antidepressants.

Comparative studies on the anorectic activity of d-fenfluramine in mice, rats, and guinea pigs

The present study compares the anorectic activity of d-fenfluramine and its metabolite d-norfenfluramine in three animal species. d-Fenfluramine and d-norfenfluramine show anorectic activity at increasing doses (ED50) in rats, guinea pigs, and mice, d-norfenfluramine being more active than d-fenfluramine in all three species. Equiactive anorectic activities are reached with different brain levels of d-fenfluramine and d-norfenfluramine, guinea pigs being the most sensitive species, followed by rats then mice. The metabolite most probably plays a major role in the anorectic effect of d-fenfluramine in guinea pigs, contributes to the anorectic activity in rats, but adds little to the action of the parent drug in mice. The different sensitivity to d-fenfluramine and d-norfenfluramine in these three species does not appear to be explained by a number of biochemical parameters, including serotonin uptake or release, receptor subtypes, or 3H-d-fenfluramine binding and uptake.

Effect of acute and chronic administration of buspirone on serotonin and benzodiazepine receptor subtypes in the rat brain: an autoradiographic study

The affinity of buspirone and its main metabolite 1-(2-pyrimidinyl)piperazine (PmP) for serotonin1 (5-HT1) and benzodiazepine receptors was first evaluated by computerized receptor autoradiography. The results confirmed that buspirone is a selective 5-HT1A ligand, since it inhibited the binding of [3H]5-HT with lower IC50 values (about 100 nM) in regions of the brain of the rat where this receptor subtype is predominant (such as hippocampal areas). Larger IC50 values than 3 microM were found in areas of the brain richer in 5-HT1 receptors, other than the 5-HT1A subtype (e.g. striatum, substantia nigra and the ventricles). The PmP was not selective, inhibiting the binding of [3H]5-HT with similar affinity (about 4-10 microM) in all the regions of the brain examined. Neither buspirone nor PmP, up to 100 microM, were active on benzodiazepine receptors. The autoradiographic technique was therefore used to evaluate the effects of acute (10 mg/kg, p.o., 1 hr before killing) and chronic (10 mg/kg, i.p., twice a day for 21 days, 24 hr washout) treatment with buspirone in male rats. Acute treatment reduced the binding of [3H]5-HT in all the regions of the brain studied, including those with low levels of 5-HT1A receptors, indicating the occupancy of 5-HT1 receptors by either buspirone or its metabolite. The binding of [3H]flunitrazepam was decreased (16%) only in the substantia nigra.(ABSTRACT TRUNCATED AT 250 WORDS)

Modulation of [3H]-glutamate binding by serotonin in the rat hippocampus: an autoradiographic study

Serotonin (5-HT) added in vitro (10 microM) increased [3H]-glutamate specific binding in the rat hippocampus, reaching statistical significance in layers rich in N-Methyl-D-Aspartate sensitive glutamate receptors. This effect was explained by a significant increase in the apparent affinity of [3H]-glutamate when 5-HT is added in vitro. Two days after lesion of serotonergic afferents to the hippocampus with 5,7-Dihydroxytryptamine [3H]-glutamate binding was significantly decreased in the CA3 region and stratum lacunosum moleculare of the hippocampus, this reduction being reversed by in vitro addition of 10 microM 5-HT. The decrease observed is due to a significant reduction of quisqualate-insensitive (radiatum CA3) and kainate receptors (strata oriens, radiatum, pyramidal of CA3). Five days after lesion [3H]-glutamate binding increased significantly in the CA3 region of the hippocampus but was not different from sham animals in the other hippocampal layers. Two weeks after lesion [3H]-glutamate binding to quisqualate-insensitive receptors was increased in all the hippocampal layers, while kainate and quisqualate-sensitive receptors were not affected. These data are consistent with the possibility that 5-HT is a direct positive modulator of glutamate receptor subtypes.

Acute noise stress in rats increases the levels of diazepam binding inhibitor (DBI) in hippocampus and adrenal gland

We investigated the effect of acute noise-induced stress on the concentrations of diazepam binding inhibitor (DBI) and its processing products in brain regions and adrenal glands of rats. DBI levels in hippocampus began to increase at 15 and 30 min and became significantly higher (+100%) at 90 and 120 min after stress; they returned to normal values at 360 min. While basal DBI levels were similar in the left and right hippocampus, the stress-induced increase of DBI levels was significantly higher in the left compared to the right side. A significant increase was also detected in the adrenals; here, the time course of DBI increase paralleled that of previously reported plasma corticosterone in stressed rats, being significantly higher 30 min after stress, and recovering to normal values at 60 and 90 min. After acute noise-induced stress, no significant change of DBI levels was detectable in cerebral cortex, striatum, hypothalamus and cerebellum. The present study reports for the first time the occurrence of a modification of DBI and its processing products (ODN-like immunoreactivity) in an experimental model of stress, and suggests a role for these neuropeptides in emotional responses.

Deprivation of growth hormone-releasing hormone early in the rat's neonatal life permanently affects somatotropic function

This work investigated in rats whether passive immunization against the endogenous GHRF in the early postnatal period led to permanent alterations of somatotropic function, similar to those observed in several human growth disorders, e.g. constitutional growth delay (CGD). On postnatal days 1, 2, 4, 6, 8, and 10, rats were given an anti-GHRF-serum (GHRH-Ab, 100 microliters/rat, sc) and were tested 1, 30, and 60 days after this treatment for basal and GHRH-stimulated GH secretion both in vivo and in vitro. GHRH-Ab reduced both basal and GHRF-stimulated GH secretion at all intervals and induced marked and chronic impairment of growth rate. The following differences were observed in the GHRH-Ab treated rats compared to normal rabbit serum-treated controls: 1) GH biosynthesis (incorporation of L-[3H]leucine into the electrophoretic band of GH): reduction of about 70%, 1 day but not 30 days after treatment; 2) Pituitary weight: significant reduction in absolute weight (30-40%) at all posttreatment intervals, and relative weight, 1 and 30 days after treatment. 3) Pituitary GH concentration: significant reduction in GH content (about 40%) but not concentration, at all posttreatment intervals; 4) Percentage of somatotrophs (immunocytochemistry): about 40% reduction 1 day, but not 30 and 60 days after treatment; 5) Hypothalamic somatostatin messenger RNA (mRNA) levels in situ hybridization): selective reduction (40%) in the periventricular nucleus 1 day but not 30 days after treatment; 6) Hypothalamic somatostatin cell number (immunocytochemistry): no significant changes in any hypothalamic area at any interval; 7) Pituitary somatostatin binding (in situ autoradiography): significant reduction, 1 day and 30 days after treatment; 8) Somatostatin inhibition of GH release "in vitro": somatostatin effect on GH release was reduced 30 days after treatment. These and previous data indicate that: 1) Transient deprivation of GHRF in the immediate postnatal period of the rat leads to permanent impairment of growth rate and somatotropic function; 2) GHRF deficiency itself or through reduction of GH secretion impairs somatostatin functions temporarily in the hypothalamus and permanently in the pituitary; 3) This rat model may mimic some forms of growth disorders in humans and holds promise as useful tools for investigating the underlying pathophysiological mechanisms.

Regional distribution of low-affinity GABA receptors coupled to benzodiazepine receptor subtypes in rat brain: an autoradiographic evaluation

Quantitative autoradiography of rat brain coronal sections show that maximum enhancement (more than 80%) of [3H]flunitrazepam binding by GABA occurs in brain regions particularly rich in type I benzodiazepine receptors (inferior colliculus, medium raphe, central gray and substantia nigra); conversely, brain areas where type II predominates show the lowest enhancement by GABA (about 50%). These results, suggesting that the coupling of GABA receptors with type I sites is more efficient than that with type II sites, are at variance with those reported on GABA-benzodiazepine receptors expressed in transfected cells, where the greater GABA potentiation of benzodiazepine binding is due to a subtype of the type II site containing the alpha 3 subunit of the GABAA receptor. One possible explanation of these discrepancies is that the type II receptors found in type I-enriched tissues (inferior colliculus, median raphe, central gray and substantia nigra) are associated with the alpha 3-subunit, while the type II sites present in limbic and cortical regions represent a subpopulation carrying the alpha 2-subunit of the GABAA receptor, with lower GABA potentiation.

Functional implications of decreased renal cortical atrial natriuretic peptide binding in experimental diabetes

Glomerular hyperfiltration in streptozotocin-induced diabetes mellitus in rats may be mediated by atrial natriuretic peptide (ANP). We wanted to evaluate plasma levels of ANP and plasma volume in relation to renal ANP receptor density and affinity in rats 6 weeks after induction of diabetes. Plasma levels of immunoreactive ANP were significantly higher in hyperglycemic diabetic (75.2 +/- 8.3 pg/ml) than in control animals (34.7 +/- 8.1 pg/ml; p less than 0.01). Administration of insulin to keep diabetic rats normoglycemic normalized plasma levels of immunoreactive ANP (30.5 +/- 5.2 pg/ml). In contrast, plasma volume did not show significant differences among the groups (hyperglycemic diabetes, 46.6 +/- 3.8; normoglycemic diabetes, 42.4 +/- 3.2; controls, 43.2 +/- 2.0 ml/kg body wt). No correlation was found between plasma levels of immunoreactive ANP and plasma volume. By autoradiography a significant reduction in the number of renal cortical ANP receptors was observed in hyperglycemic diabetic rats as compared with controls. At variance, ANP receptor affinity did not change either in the cortex or in the medulla in hyperglycemic diabetics in comparison with control animals. The pathophysiological implication of cortical ANP receptor down-regulation was underscored by the blunted response of glomerular filtration rate to ANP infusion in diabetic animals as compared with controls.(ABSTRACT TRUNCATED AT 250 WORDS)

Antagonist properties of 1-(2-pyrimidinyl)piperazine at presynaptic alpha 2-adrenoceptors in the rat brain

The effect of 1-(2-pyrimidinyl)piperazine (PmP), the main metabolite of buspirone, was tested on K(+)-evoked [3H]noradrenaline and [3H]serotonin release from superfused synaptosomes obtained from rat cerebral cortex. PmP had no effects per se, but antagonized the effect of noradrenaline, producing a parallel shift to the right of the noradrenaline inhibition curves. From these data pA2 (apparent affinity) values of 6.8 and 7.3 were calculated for PmP on presynaptic alpha 2-adrenoceptors located on noradrenergic and serotonergic nerve terminals, respectively.

Down-regulation of alpha 2-adrenoceptors involved in growth hormone control in the hypothalamus of infant rats receiving short-term clonidine administration

In infant rats short-term administration of the alpha 2-adrenoceptor agonist, clonidine (CLO), induces refractoriness to the growth hormone (GH)-releasing effect of an acute CLO challenge. CLO reportedly stimulates GH release via increased release of GH-releasing hormone (GHRH) from the hypothalamus. Based on these premises, in this study we investigated the possibility that repeated CLO administration may induce down-regulation of hypothalamic alpha 2-adrenoceptors, involved in GH control, thus prohibiting the GH-releasing effect of the drug. alpha 2-Adrenoceptor binding was determined in different brain regions of 10-day-old rats pretreated for 5 days with CLO (150 micrograms/kg, b.i.d.) and killed 14 h after last CLO administration. [3H]p-Aminoclonidine [( 3H]PAC) was used as the specific ligand of alpha 2-adrenoceptors. Treatment with CLO decreased by about 30% the maximum number of binding sites (Bmax) in areas of the mediobasal hypothalamus (MBH) involved in the stimulatory control of GH secretion, i.e. nucleus periventricularis arcuatus, nucleus ventromedialis hypothalami and nucleus lateralis hypothalami. Reduction of Bmax for [3H]PAC binding was observed also in the nucleus periventricularis hypothalami, an area involved in the inhibitory control of GH secretion and, among extrahypothalamic areas, only in the cortex piriformis. In no brain areas was the affinity constant (Kd) for [3H]PAC binding significantly changed after CLO pretreatment. Binding studies performed with a specific ligand of alpha 1-adrenoceptors, [3H]prazosin, showed that the effect of CLO was specific since no changes in the Bmax or Kd were present in either hypothalamic or extrahypothalamic regions.(ABSTRACT TRUNCATED AT 250 WORDS)

In-vivo radiolabelled oxiracetam binding to rat brain

The in-vivo binding of [3H]oxiracetam has been studied in brain areas of rats examined 30 min after i.c.v. injection. Soluble radioactivity accounted for more than 90% of total radioactivity in all the structures considered and was not affected by co-injection with a 1000-fold excess of unlabelled oxiracetam. Both total and bound radioactivity showed a marked regional distribution, with highest concentrations in the septum, followed by the hippocampus; the cerebral cortex, striatum and cerebellum had the lower concentrations of radioactivity. Computer-assisted quantitative autoradiography with [14C]oxiracetam confirms these findings. Analysis of [3H]oxiracetam bound to membranes indicated that, after co-injection with a 1000-fold excess of unlabelled oxiracetam, there was a significant reduction of binding only in the septum, hippocampus and cerebral cortex. These results suggest that in those cerebral structures oxiracetam binds to saturable sites.

Quantitative autoradiographical analysis of the age-related modulation of central dopamine D1 and D2 receptors

Quantitative autoradiography of [3H]SCH 23390 and [3H](-)-sulpiride binding was performed in the brain of rats of various ages (3, 11 and 24 months) in order to study the changes in D1 and D2 receptor density with age. Binding of [3H]SCH 23390 in the caudate-putamen decreased progressively and markedly at rostral levels in 11- and 24- compared with 3-month-old rats (max. decrease -63%) while at caudal levels significant decrease was observed only in 24-month-old rats. [3H](-)-Sulpiride binding progressively decreased during aging in the caudate-putamen at rostral levels and the decrease was more pronounced laterally (-70% at 24 months), while at caudal levels no significant decrease was observed. D1 and D2 binding sites also decreased in the nucleus accumbens and olfactory tubercle of aged rats, while in the substantia nigra only the D1 receptors appeared to be modified with aging. No change was found in the entopeduncular nucleus, amygdala, frontoparietal, suprarinal-prefrontal and anterior cingulate cortex. The results indicate that the age-associated decrease of D1 and D2 receptors is not widespread, being confined to dopaminergic areas with high density of dopamine receptors.

Reduction of food intake by manipulation of central serotonin. Current experimental results

Serotonergic anorectics are correctly defined only if they enhance 5-HT transmission and have their anorectic effects inhibited by drugs that block 5-HT receptors. Fenfluramine, the prototype indirect 5-HT agonist, and its metabolite, norfenfluramine, act as 5-HT releasers and uptake inhibitors and are both more effective in the dextro form. They lack the stimulant activity and do not cause hyperthermia or stereotypical behaviour as is characteristic of amphetamines. The anorectic effect of those drugs is attenuated by metergoline, a 5-HT receptor antagonist, in animals and man; 5-HT uptake inhibitors such as fluoxetine, zimelidine, SL 810385, and sertraline also cause anorexia, but only sertraline antagonism by metergoline has been reported. The effect of serotonergic anorectics on 5-HT release has been poorly investigated. Quipazine and RU-24969 cause anorexia by acting directly on 5-HT post-synaptic receptors. Serotonergic nerve terminals take up [3H]-D-fenfluramine and bind with high affinity in rat brain; uptake, an active process, appears to occur at a different site than binding, which is not affected by ouabain or low temperature. Anorexia is probably induced by interaction with 5-HT1B receptors in the rat; the human equivalent of this receptor is not known, but the 5-HT1D type is a likely candidate.

Profile of in vitro binding affinities of neuroleptics at different rat brain receptors: cluster analysis comparison with pharmacological and clinical profiles

A series of 21 neuroleptics with different chemical structures (phenothiazines, thioxanthenes, dibenzodiazepines, butyrophenones, benzamides, etc.) was examined for their in vitro interactions with 12 neurotransmitter binding sites in the rat brain (alpha- and beta-noradrenergic, dopaminergic, muscarinic, serotoninergic, histaminic, and opioid receptors, calcium channels, and serotonin uptake binding sites). The biochemical profile obtained from the binding data was compared with reported pharmacological and clinical profiles for this class of compounds by cluster analysis. Cluster analysis on binding data classified the compounds in three main subgroups: benzamides, compounds with an affinity mainly for DA2 and 5-HT2 receptors and inactive at muscarinic receptors, and compounds with a high affinity for alpha 1-adrenergic receptors and muscarinic receptors. The main subgroups resulting from cluster analysis of previously published pharmacological and clinical data for neuroleptics contain compounds common to the present study, with some correlations. The results extend previous observations that a complete binding profile corresponds to the pharmacological and clinical profile of this class of compounds.

Evidence that central 5-HT2 receptors do not play an important role in the anorectic activity of D-fenfluramine in the rat

To gain information on the role of central 5-HT2 receptors in the reduction of food intake caused by D-fenfluramine in rats, different intraperitoneal doses of metergoline, a non-selective 5-HT receptor antagonist and ritanserin, a selective 5-HT2 receptor antagonist, were compared for their ability (a) to antagonize the anorectic effect of D-fenfluramine; (b) to occupy central 5-HT2 receptors in vivo (measured by the binding of [3H]spiperone in the frontal cortex) and (c) to affect the concentrations of D-fenfluramine and its active metabolite, D-norfenfluramine in brain. Metergoline dose-dependently reduced the effect of D-fenfluramine (2.5 mg/kg i.p.) on food intake, with complete antagonism at 1 mg/kg, a dose which occupies about 50% of cortical 5-HT2 receptors. Ritanserin, at a dose (0.5 mg/kg) causing 50% occupation of 5-HT2 receptors, had no effect on anorexia induced by D-fenfluramine and only partially prevented it at doses which caused maximum occupation of 5-HT2 receptors (1-2 mg/kg). Unlike 1 mg/kg metergoline, 1 mg/kg ritanserin significantly reduced the concentrations of D-norfenfluramine in the frontal cortex and hypothalamus of rats 30 min after injection of D-fenfluramine. The results suggest that 5-HT receptors, other than 5-HT2, possibly 5-HT1B, are involved in the anorectic effect of D-fenfluramine in food-deprived rats.

In-vivo (+)-[3H]fenfluramine binding to rat brain: biochemical and autoradiographic studies

The in-vivo binding of (+)-[3H]fenfluramine to rat brain regions is saturable, as shown by the inhibition curves obtained by co-injecting increasing concentrations of unlabelled (+)-fenfluramine: at 2.5 mg kg-1 the inhibition of total bound radioactivity was maximal in all regions. The regional distribution of (+)-[3H]fenfluramine specific binding sites (hypothalamus greater than striatum = cortex greater than brainstem greater than hippocampus greater than cerebellum) closely parallels the regional distribution of 5-hydroxytryptamine uptake. Computer-assisted quantitative autoradiography confirms these findings. The IC50 of (+)-fenfluramine for inhibition of its binding in-vivo is below 0.25 mg kg-1, compatible with the presence of high affinity sites. While the physiological role of (+)-[3H]fenfluramine binding sites in the brain, particularly in hypothalamic nuclei, is being investigated, it has been found that in-vivo labelling could also be obtained in the periphery, in lung and renal cortex. The possibility that this peripheral binding is due to the presence of blood platelets cannot be ruled out.

Effects of aspartame and carbohydrate administration on human and rat plasma large neutral amino acid levels and rat brain amino acid and monoamine levels

Thirty fasted human volunteers were given 0.83 and 8.3 mg aspartame/kg body weight alone, as part of a basal low carbohydrate meal (648 kcal, 10% carbohydrate) or as part of a high energy carbohydrate-rich meal (1290 kcal, 34% carbohydrate). Amino acid concentrations in plasma were determined before and 30, 60 and 180 min after the consumption of aspartame. Under these conditions, which mimic realistic aspartame consumption, aspartame had no significant effect on plasma concentration of any amino acid. In addition, the effect of aspartame alone or with carbohydrates on plasma and brain amino acid levels was studied in rats after acute or subacute (14 d) oral treatment. In subacute dosing experiments aspartame was included in the diet. Brain monoamine concentrations were also measured in the same animals. Plasma concentrations of large neutral amino acids were modified under acute conditions. In contrast, after subacute treatment no significant differences in plasma or brain amino acid concentrations or in brain monoamine concentrations were observed.

Pharmacology of amineptine: synthesis and updating

Amineptine is a tricyclic antidepressant agent with the unique capacity to decrease selectively the uptake of dopamine (DA) without affecting the uptake of noradrenaline (NA) and serotonin (5HT). The effect is obtained both in vitro and in vivo by the use of suitable methodology. Amineptine can be differentiated from amphetamine both on the basis of pharmacological as well as biochemical parameters. In vivo, amineptine increases striatal homovanillic acid without affecting the levels of other metabolites of DA, namely, 3, 4, dihydrozoxyphenylacetic acid (DOPAC) and 3-methoxytyramine (3MT). However, by using relatively high doses of amineptine, the extracellular DOPAC level--assessed by the use of pulse voltammetry--was decreased preferentially in the nucleus accumbens but not in the striatum. Chronic treatment with amineptine, as with other antidepressant agents, induces a down-regulation of beta-adrenergic receptors. Amineptine enters the brain and its pharmacological effects are likely due to the unchanged drug rather than to its two main metabolites.

Catecholamine receptor binding in rat kidney: effect of aging

Binding to several receptors was compared in kidneys from 3- and 24-month-old rats. In crude membrane preparations of aged rat kidneys, the number of beta 2-adrenergic receptors was significantly reduced but the number of total beta-adrenoceptors was unchanged. The high-affinity alpha 1-adrenoceptor component was significantly reduced in old rats, whereas the low-affinity component was unchanged. The number of alpha 2-adrenoceptors showed a non-significant decrease. 3H-spiperone binding sites were similar in young and old rats. For each receptor binding the KD values were the same in young and old animals. The D1 dopamine receptor was significantly reduced in old rats. In our experiments, age-related changes of specific binding sites in the kidney were selective for some receptors studied and did not seem to be due to general aging-induced membrane modifications. Moreover, the renal and central receptors were sensitive to aging differently in the same animal model.

Biochemical characterization of a new highly cardioselective beta-adrenoceptor antagonist

P0160 (1-phenyl-3-(2-(3-(2-cyanophenoxy)-2-hydroxypropyl)amino) ethylhydantoin HCl) is an aryloxypropanolamine which contains a ureido group as part of the hydantoin ring. This molecule was synthesized to obtain a more cardioselective beta-adrenoceptor blocker. Preliminary data have shown that it is as potent as propranolol and four times more cardioselective than atenolol in pharmacological tests in-vitro and in the conscious rat. In the present study we evaluated the interaction of P0160 with beta-adrenoceptors by radioreceptor binding studies and by measuring adenylate cyclase activity coupled to beta-adrenoceptors. The data indicate that P0160 binds with nanomolar affinity to beta-adrenoceptors labelled with [3H]DHA in the rat heart, but with micromolar affinity in the rat lung. Its binding is stereospecific, the S-(-)isomer being 200 times more active than the R-(+) form. P0160's selectivity between cardiac beta 1- and beta 2-receptors was 1388, about 60 times that for metoprolol. Analysis of the thermodynamic characteristics of P0160's interaction with rat heart beta-adrenoceptors indicated antagonist properties of the same order of magnitude as propranolol, as confirmed by adenylate cyclase studies. These data indicate that P0160 is a potent, specific and selective beta 1-adrenoceptor antagonist, and give a molecular explanation for the cardioselective activity found in pharmacological tests.

Different components of 3H-imipramine binding in rat brain membranes: relation to serotonin uptake sites

In the present paper we confirm and extend previous studies showing heterogeneous 3H-imipramine (3H-IMI) binding sites. Inhibition curves of various drugs (serotonin, imipramine, desmethyl-imipramine, d-fenfluramine, d-norfenfluramine and indalpine, a potent serotonin uptake inhibitor) obtained using 2 nM 3H-IMI and in presence of 120 mM NaCl, confirmed the presence of at least three 3H-IMI binding sites: two of these (high and low affinities) were serotonin-insensitive while the third one was selectively inhibited by serotonin and indalpine with nanomolar affinities. Moreover, this last component was found to be selectively modulated by chronic imipramine treatment thus suggesting a closer relation to serotonin uptake mechanism. These data indicate that the use of a more selective inhibitors of the serotonin-sensitive component (like indalpine or serotonin itself) to define non specific 3H-IMI, may be of help in understanding its relation with serotonin uptake system.

Tianeptine, a selective enhancer of serotonin uptake in rat brain

Tianeptine is a tricyclic agent provided with antidepressant activity in experimental models and in clinical trials. In vitro tianeptine and its two principal metabolites have no effects on monoamine uptake, release or neurotransmitter receptor binding. The biochemical effect of tianeptine in vivo after acute or repeated treatment indicates an enhanced serotonin uptake in cortex and hippocampus but not in mesencephalon, with no effect on noradrenaline or dopamine uptake. This enhanced serotonin uptake is not due to decrease in serotonin release, but is related to increase in the Vmax of the uptake carrier for serotonin. The fact that enhancers as well as inhibitors of serotonin uptake are provided with antidepressant activity challenge simple conclusion as to their mechanism of action. The possibility that increased serotonin uptake after repeated treatment may be related to the antidepressant activity exerted by these drugs, and drugs enhancing serotonin uptake might have antidepressant activity, with an earlier onset, is proposed.

Diazepam and desmethyldiazepam differ in their affinities and efficacies at 'central' and 'peripheral' benzodiazepine receptors

The in-vitro binding characteristics of three different ligands ([ 3H]Ro 15-1788, [3H]Ro 5-4864 and [3H]flunitrazepam) and the structural requirements for binding to 'central' and 'peripheral' benzodiazepine receptors have been evaluated in rat cerebral cortex, cerebellum and adrenal glands. [3H]Ro 15-1788 binding was detectable only in the brain. Clonazepam was the most potent inhibitor followed by diazepam and desmethyldiazepam, which showed the same affinity, and by premazepam; Ro 5-4864 did not show appreciable affinity. The same pattern was seen for [3H] flunitrazepam binding in brain areas while in adrenal gland the inhibition pattern was exactly superimposable on that with [3H]Ro 5-4864 in all the areas considered (Ro 5-4864 greater than diazepam greater than desmethyldiazepam greater than clonazepam greater than premazepam). These data confirm and extend previous reports. A methyl group in position 1 enhances the affinity for peripheral benzodiazepine binding sites which are labelled in the adrenal gland by [3H]Ro 5-4864 and [3H]flunitrazepam; in brain areas, [3H]flunitrazepam, like [3H]Ro 15-1788, selectively labels central binding sites. Methylation in position 1 did not change the affinity for these sites. Desmethyldiazepam is less active than diazepam as an anticonvulsant and in other tests. In-vivo experiments were therefore carried out to assess the 'intrinsic activity' of desmethyldiazepam: it appeared that this compound acts as a partial agonist at central benzodiazepine receptors.

Evidence of the involvement of dopamine in the analgesic effect of nefopam

The involvement of brain monoamines in the mechanism of action of nefopam, a new analgesic, was investigated in rats. The study was designed to evaluate the effect of various means of impairing monoaminergic transmission on nefopam analgesia as measured with the hot plate method. Pretreatment with reserpine (2 mg/kg) significantly reduced the antinociceptive action of nefopam (40 mg/kg), indicating that the interaction of this drug with the monoaminergic systems is important for its effects. A role for serotonin (5-HT) or norepinephrine (NE) was ruled out by the fact that selective depletion of 5-HT (using 5,7-dihydroxytryptamine) or NE (using DSP-4 or FLA-63) did not affect nefopam analgesia. A significant reduction of the effect of nefopam was found in rats pretreated with 6-hydroxydopamine (6-OHDA). Also 6-OHDA plus desipramine, which selectively depleted brain DA, markedly reduced the antinociceptive effect of nefopam. The data strongly suggest that a critical dopaminergic synapse is involved in the mechanism by which nefopam inhibits nociceptive responses in rats.

Effect of long term amineptine treatment on pre- and postsynaptic mechanisms in rat brain

The effect of amineptine and its two metabolites on monoamine uptake, release and receptor binding was studied in vitro. Amineptine and its two metabolites did not displace labelled ligands for known neurotransmitters and drug receptor sites. Amineptine and its two metabolites did not influence [3H]-5-hydroxytryptamine ([3H]-5-HT) uptake or release by rat brain synaptosomes. Amineptine inhibited [3H]-dopamine and [3H]-noradrenaline ([3H]-NA) accumulation, with IC50 values of 1.4 and 10 microM, respectively. The effect was retained, though with lower efficacy, by the two metabolites. Amineptine released [3H]-dopamine from preloaded synaptosomes. Metabolite 1 had no effect on catecholamine release, and metabolite 2 was about half as active as the parent compound on [3H]-dopamine release. The releasing effect of amineptine on [3H]-dopamine was potentiated by reserpine pretreatment, suggesting that the drug acts on the cytoplasmic neurotransmitter pool. Chronic treatment with amineptine (20 mg kg-1, twice daily for 15 days followed by a 3 days drug withdrawal period) resulted in a decrease of [3H]-spiperone binding sites in striatum, and of [3H]-dihyroalprenolol and [3H]-clonidine in cortex. Chronic treatment with amineptine reduced basal [3H]-dopamine accumulation in striatal synaptosomes, without affecting [3H]-NA or [3H]-5-HT accumulation. The adaptive changes in the pre- and postsynaptic dopamine mechanisms observed after long term treatment with amineptine are consistent with the drug acting as an indirect dopamine agonist. The down regulation of beta- and alpha 2-noradrenoceptors observed after long term amineptine treatment may play a role in the antidepressant activity of the drug.

Localization of GABAA and GABAB receptor subtypes on serotonergic neurons

The effect of selective destruction of serotonin (5-HT)-containing neurons with 5,7-dihydroxytryptamine (5,7-DHT) on [3H] muscimol and (-)-[3H]baclofen binding was investigated in various rat brain regions. Ten days after intracerebroventricular 5,7-DHT, serotonin levels and [3H]imipramine binding were markedly decreased. 5,7-DHT reduced [3H]muscimol binding only in the mesencephalon, and (-)-[3H]baclofen binding was unmodified in all the areas considered. These results suggest that except in the mesencephalon GABA receptors may not be localized on serotonergic nerve terminals.

Allosteric inhibition as a model to explain flatter displacement curves in binding experiments: application to "heterogeneous" serotonin receptors

Flattened or biphasic inhibition curves are usually interpreted by postulating the presence of two sites which are labelled with the same affinity by the ligand and can be recognized using the appropriate inhibitor with different selective affinities. We found that a priori this type of curve, can be equally fitted by another model, the allosteric model, on account of the mathematical equivalence of the two model functions when the 3H-ligand concentration is kept constant (i.e. inhibition experiments). A new approach consisting of three-dimensional analysis of the experimental data (3H-ligand binding as a function of ligand and inhibitor concentrations, simultaneously) permitted a statistical discrimination between the two models. The examples, used as tool for the present study, are the flattened or biphasic inhibition curves obtained by displacing 3H-serotonin with the neuroleptic spiperone. The results are discussed in relation to the general interpretation of this type of "anomalous" binding data and to the specific field of serotonergic receptor subtypes.

Neurochemical mechanism of action of drugs which modify feeding via the serotoninergic system

The neurochemical mechanisms by which drugs acting on central serotoninergic system modify feeding were reviewed. Fenfluramine, a clinically effective appetite suppressant, releases serotonin from nerve terminals and inhibits its reuptake, and considerable evidence suggests that these effects mediate its anorectic activity. The D isomer of fenfluramine is particularly specific in affecting serotonin mechanisms and causing anorexia. Transmitters other than serotonin such as acetylcholine, catecholamines and GABA are also affected by systemic administration of fenfluramine, but some of these effects are secondary to fenfluramine's action on serotoninergic mechanisms. Moreover, there is no evidence that these brain substances are involved in fenfluramine's ability to cause anorexia. Several studies with drugs affecting different serotonin mechanisms such as release and uptake or mimicking the action of serotonin at post-synaptic receptors suggest that increase serotonin release and direct stimulation of postsynaptic receptors are the most effective mechanisms for causing depression of food intake, although inhibition of serotonin uptake may also contribute in appropriate conditions. Development of serotonin receptor hyposensitivity and, in some instances, decreased serotonin levels may lead to tolerance to the anorectic activity of drugs enhancing serotonin transmission, the degree of this depending critically on the type of effect on serotonin mechanisms and intensity and duration of serotonin receptor activation. Recent evidence suggests that a decrease in serotonin function causes stimulation of feeding. This may lead to development of new strategies for the treatment of clinical anorexias.

Neurochemical effects of buspirone in rat hippocampus: evidence for selective activation of 5HT neurons

The effect of buspirone on neurotransmitter systems in rat hippocampus has been evaluated in vitro and in vivo. In vitro buspirone does not affect the specific binding of 3H-flunitrazepam, 3H-GABA, 3H-dexetimide, but displaces 3H-5HT binding with nanomolar affinity. Oral administration of buspirone does not modify the hippocampal concentrations of GABA, acetylcholine, choline and of 3H-flunitrazepam specifically bound in vivo, but results in a dose-dependent reduction of 5HIAA and noradrenaline concentrations. While the effect on noradrenaline is also obtained in striatum of buspirone-treated animals, the effect on 5HIAA shows a regional specificity. The in vitro and in vivo data suggest that buspirone specifically activates 5HT neurons in hippocampus, and are compared with those obtained with diazepam.

In vivo stereospecific [3H]spiperone binding in rat brain: characteristics, regional distribution, kinetics and pharmacological properties

The time course of [3H]spiperone distribution in the three major pools (specifically and non-specifically membrane-bound and soluble) of different brain areas, was studied in rats given a tracer amount of the drug. In addition, the stereospecificity, dissociation kinetics and pharmacological nature of the in vivo bound [3H]spiperone were investigated. The data show that [3H]spiperone binding sites in the striatum, olfactory tubercles and hypophysis differ clearly from those of the cortical regions. In the prevalently dopaminergic areas the amount of ligand bound to membranes is, up to 24 h post-treatment, proportional to the total 3H present. However a more correct analysis of the data was obtained in all the experiments when membrane-bound was measured instead of total radioactivity. Thus assay of the in vivo specifically bound [3H]spiperone appears essential for a correct evaluation of the density, affinity, regional distribution, pharmacological nature and kinetics of the drug-receptor interaction.

In vivo interaction of premazepam with benzodiazepine receptors: relation to its pharmacological effects

The study of in vivo occupancy of benzodiazepine receptors in rat hippocampus and cerebellum indicates that premazepam reaches only about 70% and 80% occupancy respectively within the dose range used for pharmacological tests in rats. Moreover, at equiactive doses (antileptazol ED50), more brain receptors are occupied by premazepam than by other benzodiazepines, suggesting that premazepam may act as a partial agonist at benzodiazepine receptors, with intrinsic activity lower in cerebellum than in hippocampus. These results may explain the lack of sedative and ataxic properties of premazepam.

Enhancement of diazepam activities induced by denzimol in mice

Denzimol, a new anticonvulsant drug, enhances the depressant and antimetrazol activities of diazepam in mice, in a dose and time-dependent fashion. The depressant and anticonvulsant activities of phenobarbital were not affected by Denzimol. It is suggested that Denzimol induces an increase in the number of the benzodiazepine receptors, and that such increase might be responsible for the enhancement of the diazepam's activities.

Effects of denzimol on benzodiazepine receptors in the CNS: relationship between the enhancement of diazepam activity and benzodiazepine binding sites induced by denzimol in the rat

Denzimol, a new anticonvulsant drug with a pharmacological profile similar to that of phenytoin, enhances the ataxic and antimetrazol activity of diazepam in rats without affecting its activity against picrotoxin-induced seizures. In vivo and ex vivo denzimol enhances the binding of 3H-flunitrazepam in cortex and in hippocampus but not in cerebellum. The possibility of this increase in the number of benzodiazepine binding sites contributing in some way to enhancement of the depressive and anticonvulsant activity of diazepam is discussed.

Increased number of brain benzodiazepine receptors after in-vivo administration of estazolam to rats

Estazolam significantly increased the Kd of [3H]flunitrazepam in-vitro, like other benzodiazepines (BDZs) acting competitively at the receptor site. At variance with other BDZs, estazolam significantly raised the Bmax for [3H]flunitrazepam, at concentrations lower than its Ki for BDZ receptors. This effect may be responsible for the observed increase in [3H]diazepam binding after in-vivo administration of estazolam to rats.