Effects of remoxipride and some related new substituted salicylamides on rat brain receptors

A number of potential neuroleptic drugs of the substituted benzamide type have been compared with some reference neuroleptic drugs regarding their affinities for rat brain receptors using in vitro receptor binding techniques. The effects on dopamine-stimulated adenylate cyclase were also investigated. All 6-methoxysalicylamides were very potent inhibitors of the dopamine-D2 receptor and possessed much less affinity for other receptors as well as for the dopamine-stimulated adenylate cyclase. In contrast, the classical neuroleptic drugs were not selective dopamine-D2 receptor blockers. Several of these neuroleptics were thus potent inhibitors of alpha 1-receptors, 5-HT2 receptors, muscarinic receptors and histamine-H1 receptors. Sulpiride was the most selective dopamine-D2 receptor blocker of the reference compounds with a selectivity ratio of 65. The 6-methoxysalicylamides had selectivity ratios ranging from 18 to 167.

Regulation of anterior pituitary D2 dopamine receptors by magnesium and sodium ions

At D2 3,4-dihydroxyphenylethylamine (dopamine) receptors in anterior pituitary tissue, magnesium ions shifted receptors to agonist high-affinity states, but decreased the affinity of the antagonist [3H]spiperone. Conversely, sodium ions shifted the receptors to agonist low-affinity states, but increased the affinity of [3H]spiperone. Magnesium is proposed to stabilize the hormone-receptor-guanine nucleotide regulatory protein complex, whereas sodium appears to destabilize this ternary complex. Thus, magnesium and sodium appear to mediate their regulatory effects via a common component at the D2 dopamine-receptor ternary complex.

Antagonism of N-methyl-D-aspartate-induced transmitter release in the rat striatum by phencyclidine-like drugs and its relationship to turning behavior

Representatives from several chemical classes known to share behavioral properties with phencyclidine [(1-phenylcyclohexyl)piperidine] (PCP) have been shown to antagonize the excitatory effects of N-methylaspartate on spinal neurons selectively. This study compared the abilities of several of these PCP-like drugs to inhibit N-methyl-D-aspartate (NMDA)-stimulated efflux of acetylcholine in rat striatum. PCP completely inhibited this parameter with an IC50 value of 68 nM. The IC50 values (nanomolar) found for the other drugs tested were as follows: etoxadrol (98), (-)-cyclazocine (120), N-allylnormetazocine (or SKF 10047) (940), ketamine (1600) and ethylketocyclazocine (8300). Morphine had no effect at concentrations as high as 30,000 nM. In addition, 100 nM dexoxadrol, 1-[1-(napthyl)cyclohexyl]piperidine HCl (m-amino PCP) and (-)-cyclazocine inhibited NMDA-induced acetylcholine release by about 50%, whereas the same concentration of their enantiomers (or a structural analog in the case of m-amino-PCP) produced no significant effect. It was also found that 10 mg/kg of dexoxadrol, m-amino-PCP and (-)-cyclazocine induced significant ipsilateral turning in rats with unilateral destruction of the substantia nigra, whereas 10 mg/kg of levoxadrol, 1-[1-(m-nitrophenyl)cyclohexyl]piperidine HCL (m-nitro-PCP) and (+)-cyclazocine produced no such similar effect. In spite of the ability of several dopaminergic antagonists to block turning produced by (+/-)-cyclazocine, we concluded, based on our previously reported studies of the dopaminergic properties of these drugs, that turning is the result of nondopaminergic properties of the PCP-like drugs.(ABSTRACT TRUNCATED AT 250 WORDS)

Chronic treatment with clozapine, unlike haloperidol, does not induce changes in striatal D-2 receptor function in the rat

Comparison has been made of the effects on brain dopamine function of chronic administration of haloperidol or clozapine to rats for up to 12 months. In rats treated for 1-12 months with haloperidol (1.4-1.6 mg/kg/day), purposeless chewing jaw movements emerged. These movements were only observed after 12 months' treatment with clozapine (24-27 mg/kg/day). Apomorphine-induced (0.125-0.25 mg/kg) stereotyped behaviour was inhibited during 12 months treatment with haloperidol. Clozapine treatment was without effect. After 12 months, stereotypy induced by higher doses of apomorphine (0.5-1.0 mg/kg) was enhanced in haloperidol, but not clozapine, treated rats. Bmax for striatal 3H-spiperone binding was elevated throughout 12 months of haloperidol administration, but was not altered by clozapine treatment. Bmax for striatal 3H-NPA binding was only elevated after 12 months of haloperidol treatment; clozapine treatment was without effect. Bmax for 3H-piflutixol binding was not altered by haloperidol treatment, but was increased after 9 and 12 months of clozapine treatment. Dopamine (50 microM)-stimulated adenylate cyclase activity was inhibited after 1 month's haloperidol treatment but normal thereafter. Adenylate cyclase activity was not altered by chronic clozapine treatment. Striatal acetylcholine content was increased after 3 and 12 months of haloperidol or clozapine intake. These findings indicate that the chronic administration of the atypical neuroleptic clozapine does not produce changes in brain dopamine function which mirror those of the typical neuroleptic haloperidol. In particular, chronic administration of clozapine, unlike haloperidol, does not appear to induce striatal D-2 receptor supersensitivity. Unexpectedly, clozapine treatment, unlike haloperidol, altered D-1 receptor function.

Evidence for cholecystokinin-dopamine receptor interactions in the central nervous system of the adult and old rat. Studies on their functional meaning

Evidence has been presented for the existence of interactions between CCK and DA receptors both in striatal and limbic membranes. A similar type of modulation by CCK-8 of DA receptors also exists after chronic neuroleptic treatment indicating that supersensitive DA receptors are also modulated by this peptide. As seen from simulation curves, CCK-8 increases the binding of [3H]DA agonists and reduces the binding of [3H]DA antagonists in striatal membranes, suggesting that CCK-8 may increase striatal DA transmission. Results of this type may underlie some of the non-neuroleptic effects of CCK-8. In the aged brain, the ability of CCK-8 to modulate DA antagonist binding sites is changed such that the binding of [3H]DA antagonists is increased. Thus, in the aged brain, receptor-receptor interactions may be altered, leading to a derangement of heterostatic mechanisms (mechanisms changing chemical transmission without interfering with synaptic homeostasis). It was also demonstrated that during aging there is a preferential disappearance of CCK-like immunoreactivity versus TH immunoreactivity in the nigral DA neurons, especially in the medially located nigral DA cells; furthermore, co-existence in the TH/CCK co-storing terminals in the nucleus accumbens was reduced during aging. Such alterations should also lead to changes in heterostatic regulation because the CCK co-modulation line controlling the DA receptors may be preferentially affected.

Effects of some antidepressant drugs on apomorphine concentration in the central nervous system of rats and apomorphine-induced stereotypy

Pharmacokinetic aspects of interaction between antidepressant drugs (AD) and apomorphine (APO) have been studied. It was found that neither acute nor chronic AD pretreatment affected the distribution of APO-induced stereotypy was not changed. An exception was citalopram (CIT) which given in a single dose to rats significantly potentiated APO-induced gnawing. Single dose of investigated AD delayed the onset of APO-induced sniffing, licking and gnawing, however the overall effect did not reach the statistically significant level. The presented results suggest that changes of APO behavioral effects induced by AD pretreatment are rather due to pharmacodynamic and not pharmacokinetic interaction.

Heterogeneous distribution of dopamine D2 receptors within the rat striatum as revealed by autoradiography of [3H]N-n-propylnorapomorphine binding sites

The precise topographical distribution of dopamine D2 receptors in the rat striatum has been studied autoradiographically through the use of [3H]N-n-propylnorapomorphine (NPA) as a ligand. The highest densities of [3H]NPA binding sites were found in the ventrolateral aspect and medial margin of the striatum, while sparse grain densities were present in the medial and latero-dorsal striatum. Moreover, a clear gradient of [3H]NPA binding was observed along the rostro-caudal axis of the striatum. These heterogeneities in the distribution of dopamine receptors provide further evidence for a functional compartmentalization of the striatum.

Complete conversion of brain D2 dopamine receptors from the high- to the low-affinity state for dopamine agonists, using sodium ions and guanine nucleotide

Since previous work had shown that brain D2 3,4-dihydroxyphenylethylamine (dopamine) receptors were only partly converted from their high-affinity state to their low-affinity state, we here tested whether it was possible to obtain a complete 100% conversion of these receptors into their low-affinity state. It was first essential to resolve the components of [3H]spiperone binding to dopaminergic sites and nondopaminergic sites in rat striatal homogenates. In the presence of 50 microM S-sulpiride (to occlude the dopaminergic sites), therefore, we first determined that the residual binding of [3H]spiperone (approximately 20%) was inhibited by serotonergic agonists much more effectively than dopamine or noradrenaline, thus identifying the serotonergic component of [3H]spiperone binding. Thus, dopamine (or ADTN) inhibited the binding of [3H]spiperone at a high-affinity site (with dissociation constant of 10 nM dopamine), at a low-affinity site (with dissociation constant of 2,000 nM dopamine), and at the serotonergic site (with dissociation constant of 50,000 nM dopamine). In the absence of sodium ions, the high-affinity site was about 50% occupied by [3H]spiperone, and guanine nucleotide had no effect on this proportion. In the presence of 120 mM NaCl, however, the high-affinity site was reduced to 15% and guanine nucleotide completely eliminated this high-affinity site, 100% of the sites having been completely converted to their low-affinity state. Using [3H]N-propyl-norapomorphine to label the high-affinity state of the dopamine receptor, 50% conversion into the low-affinity state occurred at 45 mM LiCl, 69 mM NaCl, and 202 mM KCl. We conclude that it is possible to convert brain D2 dopamine receptors completely into their low-affinity state, in the presence of NaCl and a guanine nucleotide, providing that appropriate allowance is made for the serotonergic component of [3H]spiperone binding.

Synthesis and pharmacological evaluation of aromatic dihydroxylated spiro[indan-1,3'-pyrrolidine] and spiro[indan-2,2'-pyrrolidine] derivatives

Aromatic hydroxylated derivatives of the spiro[indan-1,3'-pyrrolidine] and spiro[indan-2,2'-pyrrolidine] ring systems have been synthesized and evaluated for dopaminergic agonist and antagonist activities. None of these conformationally restricted catecholamines possessed any dopaminergic activity, but 5,6-dihydroxy spiro[indan-1,3'-pyrrolidine] hydrobromide exhibited weak dopamine antagonist properties.

Interactions of apomorphine with serum and tissue proteins

Physical and covalent interactions of apomorphine with serum and tissue proteins could influence the drug's disposition and pharmacological activities in mammals. Ultrafiltration, equilibrium dialysis, and ultraviolet spectrophotometric methods have been used to study the reversible binding of apomorphine to bovine, human, rat, and swine plasma proteins. The degree of binding was generally greater than 90%, but variations were noted in some instances on the basis of drug concentrations and pH over the range of 6.8-7.8. Incubation of [8,9-3H2]apomorphine with bovine serum albumin led to retention of radioactivity and a stoichiometrically controlled released of tritium which arose from the reaction of an electrophilic drug oxidation product and protein, producing drug-protein conjugates. In vitro experiments with mouse striatal brain preparations indicated parallel covalent binding reactions. In vivo experiments in mice indicated accumulation of radioactivity in brain regions and other tissues following daily injections of [8,9-3H2]apomorphine for 14 days. The physical and covalent interactions of apomorphine with mammalian tissue proteins could be the cause of longer disposition half-lives in mammals than those previously reported. The covalent interactions, in particular, may be important in elucidating the mechanism of apomorphine-induced behavioral effects in mice.

In vivo electrochemical monitoring of apomorphine penetration into the rat brain

Differential normal pulse voltammetry was used together with electrochemically treated carbon-fibre electrodes to monitor apomorphine (APO) penetration into the cortex of anaesthetized rats after a peripheral administration of this drug. In vivo cortical APO extracellular concentration and post-mortem APO levels both measured by means of this technique were estimated to be, respectively, 105 +/- 8 nM and 14.6 +/- 1.1 nmol/g of tissue. The maximal extracellular concentration was reached 16 min after a s.c. injection and reached a plateau beyond a dose of 4 mg/kg (s.c.). This suggests that APO is mainly trapped within brain cells.

Effects of discriminant and non-discriminant dopamine antagonists on in vivo accumulation of 3H-N-propyl-norapomorphine in mouse striatum and tuberculum olfactorium

The in vivo accumulation of 3H-N-propyl norapomorphine in mouse striatum and tuberculum olfactorium and its inhibition by a series of classical neuroleptics and discriminant benzamide derivatives previously identified in behavioural and radioligand experiments has been studied. The ID50 values in the two brain areas did not significantly differ with any studied compound. In addition the regional distribution of a discriminant compound related to sulpiride and administered in tritiated form to rats was rather homogeneous. These data do not indicate a preferential accumulation of these compounds in limbic as opposed to striatal areas.

Dopamine D2 receptors in the anterior pituitary: a single population without reciprocal antagonist/agonist states

Although dopamine agonists can recognize two states of the D2 dopamine receptor in the anterior pituitary (D2high and D2low), we examined whether the dopamine antagonists such as [3H]spiperone could recognize these two sites with different affinities. Using up to 30 concentrations of [3H]spiperone, however, we could only detect a single population of binding sites (porcine anterior pituitary homogenates) with a dissociation constant (KD) of 130 pM. When specific [3H]spiperone binding was defined by a low concentration of (+)-butaclamol (100 nM), the apparent density was low. When defined by a high concentration of (+)-butaclamol (10 microM), nonspecific sites became detectable, thus revealing two apparent populations of sites for [3H]spiperone, only one of which was specific for dopamine. Sodium chloride reduced the KD of the single population of specific D2 sites to 64 pM. Guanine nucleotide by itself had no effect on the KD, but enhanced the density by 25%. Since the density-enhancement could be eliminated by extensive washing of membranes, and could be restored by preincubation with dopamine, the nucleotide-induced elevation of D2 density appeared to be a result of the release of tightly bound endogenous dopamine. Thus, monovalent cations and guanine nucleotides appear to have separate regulatory effects on the anterior pituitary D2 receptor that modulate antagonist-receptor interactions. Several maneuvers were used to test whether [3H]spiperone could differentiate between the two agonist-detected subpopulations of sites. Twentyfold different concentrations of [3H]spiperone (47 pM and 1000 pM) were found to label identical proportions of receptors in the D2high and D2low states as detected by the agonist 6,7-dihydroxyaminotetralin (ADTN), suggesting that spiperone labelled equal proportions of D2high and D2low sites without differential affinity for them. In addition, competition of spiperone for D2high sites selectively labelled by the agonist [3H]n-propylnorapomorphine (NPA) had a virtually identical KD for spiperone as did the total D2 receptor population as determined by direct binding studies (75 pM versus 64 pM). [3H]Spiperone also bound to a uniform population of D2low sites induced by preincubation with guanine nucleotide with identical affinity as to the total D2 population. Thus, these data do not support a "reciprocal model" for the D2 receptor (i.e., antagonist having low affinity for D2high and high affinity for D2low in a manner reciprocal to agonists).(ABSTRACT TRUNCATED AT 400 WORDS)

Stereospecific microbiological 10-O-demethylation of R-(-)-10,11-dimethoxyaporphines

The microbiological O-dealkylation of (+) and (-) 10,11-dimethoxyaporphine and the corresponding N-n-propyl analog 10,11-dimethoxy-N-n-propylnoraporphine utilizing the fungus Cunninghamella elegans (ATCC 9245) was found to proceed with regioselectivity for the 10-position, and with a high degree of substrate stereospecificity for the 6a R(-)enantiomer. Only the (R) 10-demethylated products were isolated i.e. (R) iosapocodeine and (R) N-n-propylnor-isoapocodeine. The products were confirmed by comparison with their GC/MS spectra.

Chronic haloperidol does not alter agonist affinity for dopamine receptors in vitro

Agonist competition for [3H]spiperone binding to striatal dopamine D2 receptors was studied in rats rendered supersensitive by chronic treatment with haloperidol. The classical dopamine agonist (-)-N-n-propylnorapomorphine displaced [3H]spiperone biphasically, with IC50 values of 0.5 and 140 nM for the high and low affinity components, respectively. Neither the relative density nor the affinity of either site for (-)-N-propylnorapomorphine was affected by chronic haloperidol treatment. On the other hand, the novel agonist EMD 23 448 displaced [3H]spiperone monophasically. Although this agent only displays potent dopaminergic agonism in supersensitive animals, chronic treatment with haloperidol likewise did not alter the affinity of this drug for [3H]spiperone binding sites. The results suggest that the enhanced in vivo potency of certain agonists in supersensitive animals is probably not mediated by changes in D2 receptor affinity.

Studies on the stability of 3H-dopamine, 3H-apomorphine and 3H-ADTN: effects of sodium ascorbate and EDTA

The rates of decomposition of 3H-dopamine (3H-DA), 3H-apomorphine and 3H-ADTN were determined in Tris buffer at pH 7.4 and in a Tris buffer containing a neostriatal membrane preparation representative of that used in binding experiments. In both the Tris buffer alone and in the neostriatal membrane preparation, 3H-DA was the most stable, 3H-ADTN was intermediate and 3H-apomorphine was the least stable. In the Tris buffer, the extent of decomposition of all three 3H-catechols was greatly retarded by sodium ascorbate. In contrast, in the neostriatal membrane preparation pronounced inhibitory effects of ascorbate were obtained only with 3H-ADTN. Even in the presence of high concentrations of sodium ascorbate (i.e., 0.5 mM), there was an extensive decomposition of 3H-apomorphine in the neostriatal membrane preparation. The data suggest that one exercise great caution in choosing appropriate conditions for binding experiments with these unstable ligands.

N-methyl,N-propargyl-2-aminotetralins:novel dopamine agonists with monoamine oxidase inhibiting properties

A series of mono- (5 and 7) and dihydroxylated (5,6 and 6,7)N-methyl,N-propargyl-2-aminotetralins were studied with respect to their dopamine agonistic and monoamine oxidase inhibitory activities. MAO inhibition was found to be reduced by hydroxylation of the aromatic ring. Among the hydroxylated compounds the 7-OH analogue was the most potent inhibitor in in vitro and ex vivo experiments. Both catecholic structures were equipotent with apomorphine as displacers of the specific in vitro binding of [3H]NPA to rat striatal homogenates. Moreover, the catecholic analogues had a potency comparable to that of apomorphine in the gamma-butyrolactone model whereas the monohydroxy analogues were less active. On the basis of their effectiveness to induce stereotypy in rats and to reverse reserpine-induced hypomotility in mice (both used as indices of postsynaptic dopamine receptor stimulation) the catecholic compounds were more potent than the monohydroxy analogues but much less active than apomorphine. Dopamine agonistic activity was also reflected in decreased HVA levels in the striatum whilst effects on striatal 3-MT levels reflected the balance between dopamine agonistic (decrease in 3-MT) and MAO inhibitory (increase in 3-MT) activity of the various compounds. It was concluded that both the mono- and dihydroxylated compounds have MAO inhibiting and dopamine agonistic activities. The MAO inhibitory activity predominated within the monohydroxy structures whereas the dopamine agonistic effect was predominant for the catecholic compounds. It would thus appear that, at least for the 2-aminotetralins, it is difficult to prepare an analogue which combines a high degree of both MAO inhibitory and DA agonistic activity.

Antagonism of apomorphine-enhanced startle by alpha 1-adrenergic antagonists

The present study investigated the possible involvement of central noradrenergic neurons in mediating the excitatory effect of the dopamine agonist apomorphine on the acoustic startle response in rats. Experiment 1 assessed the effects of intraperitoneal (i.p.) administration of adrenergic antagonists on apomorphine-enhanced startle. The excitation of startle produced by apomorphine (1.0-3.0 mg/kg i.p.) was blocked by the alpha 1-adrenergic antagonists prazosin (0.03-1.0 mg/kg) and WB-4101 (1.0 mg/kg). Prazosin was very potent in this regard, having an ED50 of 0.03 mg/kg. Blockade of beta-adrenergic receptors with propranolol (20 mg/kg) or blockade of peripheral alpha-adrenergic receptors with phentolamine (10 mg/kg) failed to alter the effect of apomorphine. Prazosin did not block the enhancement of startle produced by other drugs (5-methoxy-N,N-dimethyltryptamine, strychnine), nor did it alter the entry of apomorphine into the brain. The alpha 1-adrenergic antagonists piperoxane (0.03 mg/kg), yohimbine (0.03 mg/kg) or RX781094 (0.07 mg/kg) markedly potentiated apomorphine excitation. These data indicated that specific blockade of central alpha 1-adrenergic receptors prevents apomorphine-enhanced startle. In contrast to the effects of alpha 1-adrenergic antagonists, Experiment 2 found that other drugs that produce an acute (clonidine, 0.040 mg/kg) or chronic (intraventricular 6-hydroxydopamine, 2 X 200 micrograms; DSP4, 50 mg/kg i.p.) disruption of noradrenergic transmission failed to affect apomorphine excitation. Thus, the ability of alpha 1-adrenergic antagonists to block apomorphine's excitation of startle cannot be explained by a simple dopamine-norepinephrine interaction. Alternative hypothesis are discussed.

Interactions of dopaminergic agonists and antagonists with dopaminergic D3 binding sites in rat striatum. Evidence that [3H]dopamine can label a high affinity agonist-binding state of the D1 dopamine receptor

The interactions of dopaminergic agonists and antagonists with 3H-agonist labeled D3 dopaminergic binding sites of rat striatum have been characterized by radioligand-binding techniques. When the binding of [3H]dopamine and [3H]apomorphine to D2 dopamine receptors is blocked by the inclusion of D2 selective concentrations of unlabeled spiroperidol or domperidone, these ligands appear to label selectively the previously termed "D3" binding site. Antagonist/[3H]dopamine competition curves are of uniformly steep slope (nH = 1.0), suggesting the presence of a single D3 binding site. The relative potencies of antagonists to inhibit D3 specific [3H]dopamine binding are significantly correlated with their potencies to block D1 dopamine receptors as measured by the inhibition of both dopamine-stimulated adenylate cyclase and [3H]flupentixol-binding activities. The affinities of agonists to inhibit D3 specific [3H]dopamine binding are also correlated with estimates of these agonists' affinities for the high affinity binding component of agonist/[3H]flupentixol competition curves. Both D3 specific [3H] dopamine binding and the high affinity agonist-binding component of dopamine/[3H]flupentixol competition curves show a similar sensitivity to guanine nucleotides. Taken together, these data strongly suggest that the D3 binding site is related to a high affinity agonist-binding state of the D1 dopamine receptor.

Occupation of dopamine receptors by N-n-propylnorapomorphine or spiperone and acetylcholine levels in the rat striatum

In an attempt to quantify the interactions between dopaminergic and cholinergic processes, the consequences of complete or partial activation (with N-n-propylnorapomorphine) or blockade (with spiperone) of dopamine receptors for the acetylcholine levels in the rat striatum were studied. The number of specific striatal binding sites (receptors) of spiperone was nearly three times that of N-n-propylnorapomorphine (76 and 26 pmol g-1 wet weight, respectively). The agonist produced a significant increase in the striatal levels of acetylcholine, but there was no simple relationship between receptor binding and these levels. A linear negative correlation was found between the striatal levels of acetylcholine and specific spiperone binding, showing that further receptor blockade induces a decrease in acetylcholine levels, which is independent of the receptors already occupied by the antagonist. The results of this study are evidence that one striatal dopamine receptor regulates the metabolism of at least 400 molecules of acetylcholine.

Stereotyped behavior and diabetes mellitus in rats: Reduced behavioral effects of amphetamine and apomorphine and reduced in vivo brain binding of [–3H]spiroperidol

Rats made diabetic with streptozotocin showed decreased stereotyped behaviors following administration of amphetamine or apomorphine. Spontaneous activity in an open field was lower in diabetics than in controls, but a low dose of apomorphine produced equivalent fractional decreases in activity in both groups. In vivo accumulation of amphetamine and apomorphine was generally similar in both groups: Reduced tissue access did not appear to be responsible for the decreased behavioral effects of these agents. The in vivo accumulation of spiroperidol in several brain regions was generally less in diabetics than in controls. These data are discussed in terms of altered catecholamine biochemistry and behavior in diabetics.

Effect of chronic treatment with morphine on the binding of 3H-N-n-propylnorapomorphine to striatal membranes of rats: influence of prolyl-leucyl-glycinamide

The effect of chronic administration of morphine to Sprague-Dawley rats on the binding of dopamine receptor agonist, 3H-N-n-propylnorapomorphine (3H-NPA) to striatal membranes was determined. For chronic administration of morphine, rats were implanted subcutaneously with four morphine pellets (each containing 75 mg of morphine free base) during a 3-day period. Placebo pellet-implanted rats served as controls. Two experimental protocols were followed. In one, the rats were sacrificed 70 h after the implantation of first pellet and in the other, the pellets were removed and the animals were sacrificed 24 h later, and the binding of 3H-NPA was determined. Rats receiving morphine showed no difference in the total number of binding sites (Bmax value) but there was a decrease in the apparent dissociation constant (Kd value) compared with placebo controls. Moreover, such a decrease in Kd in morphine-treated rats persisted even 24 h after removal of morphine pellets. The effect of prolyl-leucyl-glycinamide (MIF; 2 mg/kg), which has been shown to inhibit the development of tolerance to and dependence on morphine, on the changes in the binding of 3H-NPA induced by the implantation of morphine pellets was also determined. MIF antagonized the decreases in Kd values induced by the administration of morphine. However, when administered chronically by itself, MIF had no effect on either the Bmax or Kd values of 3H-NPA. It is concluded that the chronic administration of morphine is associated with enhanced activity of dopamine receptors in the central nervous system as revealed by lowering of the Kd value of the agonist, NPA.(ABSTRACT TRUNCATED AT 250 WORDS)

Synthesis of 4-substituted 2H-naphth[1,2-b]-1,4-oxazines, a new class of dopamine agonists

A series of tricyclic oxazines, namely, the 4-substituted 2H-naphth[1,2-b]-1,4-oxazines, have been synthesized and assayed for dopamine agonist activity. One of the members of this series, compound (+)VII-15, was found to be a remarkably potent agonist in vivo when tested in the standard 6-hydroxydopamine lesioned rat assay. The absolute configuration of the compound corresponds to that found in the active isomer of apomorphine. Its activity at the alpha 2 receptor (vs. [3H]clonidine) is relatively low. It also failed to stimulate the synthesis of cAMP in the carp retina assay, thus giving the compound a highly selective profile in favor of the D2 receptor.