Examination of the D2/5-HT2 affinity ratios of resolved 5,6,7,8,9,10-hexahydro-7,10-iminocyclohept[b]indoles: an enantioselective approach toward the design of potential atypical antipsychotics.

Enantiomers of several N-substituted 5,6,7,8,9,10-hexahydro-7,10-iminocyclohept[b]indoles were obtained by the resolution of 2-fluoro-5,6,7,8,9,10-hexahydro-7,10-iminocyclohept[b]indole and 5,6,7,8,9,10-hexahydro-7,10-iminocyclohept[b]indole followed by N-alkylation. These, as well as the racemates, were evaluated for their affinity for the 5-HT2 and D2 receptors. Those compounds possessing the 7S,10R stereochemistry were consistently recognized by the 5-HT2 and D2 receptors as the eutomer. 2-Fluoro-11-[4-(4-fluorophenyl)-4-oxobutyl]-5,6,7,8,9,10-hexahydro-7S,10 R- iminocyclohept[b]indole [(7S,10R)-8] had the highest affinity for the 5-HT2 receptor (Ki = 0.80 nM), while its distomer (7R,10S)-8 was the most selective member of this class of bridged gamma-carbolines (D2/5-HT2 = 562). Incorporation of a benzoyl or isosteric benzisoxazole moiety tethered by a four-carbon spacer to a bridged gamma-carboline nucleus, possessing the 7S,10R absolute configuration, produced high affinity ligands for the 5-HT2 and D2 receptors.

Haloperidol treatment differentially regulates [3H]DTG and [3H](+)-3-PPP labeled sigma binding sites.

The effect of repeated haloperidol administration on sigma binding sites in brain membranes was assessed using [3H](+)-3-(3-hydroxyphenyl)-N-(1- propyl)piperidine ((+)-3-PPP) and [3H]1,3-di-o-tolylguanidine (DTG). Administration of haloperidol (1 mg/kg, i.p.) to guinea pigs for 14 consecutive days followed by a 4 day drug-free period prior to sacrifice resulted in 75% and 6% decreases in the specific binding of [3H](+)-3-(3-hydroxyphenyl)-N-(1- propyl)piperidine and [3H]1,3-di-o-tolylguanidine, respectively, when measured using a single concentration (2 nM) of radioligand. Scatchard analysis revealed a reduction in both the maximum number of [3H](+)-3-(3-hydroxyphenyl)-N-(1-propyl)piperidine binding sites and the affinity of these sites for the radioligand; the potency of 1,3-di-o-tolylguanidine to inhibit [3H](+)-3-(3-hydroxyphenyl)-N-(1-propyl)piperidine binding was also reduced. In parallel studies, the potency of 1,3-di-o-tolylguanidine to inhibit [3H]1,3-di-o-tolylguanidine binding was unaffected by haloperidol treatment, but the potency of (+)-3-(3-hydroxyphenyl)-N-(1-propyl)piperidine against [3H]1,3-di-o-tolylguanidine was reduced 3-fold. Phenytoin, which increased (10-fold) the potency of dextromethorphan to inhibit [3H]1,3-di-o-tolylguanidine binding in control membranes, had no effect in membranes obtained from haloperidol-treated animals.(ABSTRACT TRUNCATED AT 250 WORDS)

NPC 16377, a potent and selective sigma-ligand. I. Receptor binding, neurochemical and neuroendocrine profile.

6-[6-(4-Hydroxypiperidinyl)hexyloxy]-3-methylflavone HCI (NPC 16377), a structurally novel compound, was found to be a highly potent and selective ligand for sigma-sites. Although 5-fold less potent than haloperidol and 2-fold less potent than ifenprodil to inhibit 1,3-di-o-tolylguanidine binding, NPC 16377 (IC50 = 36 nM) was more potent than alpha-(4-fluorophenyl)-4-(5-fluoro-2-pyrimidinyl)-1-piperazinyl butanol (BMY 14802), rimcazole and the atypical antipsychotic, clozapine. A similar rank order of potency was observed when [3H](+)-3-(3-hydroxyphenyl)-N-(1-propyl)piperdine was used as the radioligand. Like BMY, rimcazole and clozapine, NPC 16377 (IC50 = 2671 nM) had low affinity for dopamine type 2 receptors. Additionally, the compound was only weakly active in 35 additional receptor binding assays including those for serotonin2 and serotonin1C receptors. In vivo, NPC 16377 potently inhibited the binding of [3H]-(+)-N-allylnormetazocine to sigma sites after both intraperitoneal and oral administration. At doses 30-fold in excess of the ID50 to inhibit [3H](+)N-allylnormetazocine, NPC 16377 failed to displace [3H]raclopride from dopamine type 2 binding sites. Unlike haloperidol, BMY 14802, ifenprodil and clozapine, behaviorally effective doses of NPC 16377 did not increase dopamine turnover in the frontal cortex, nucleus accumbens or corpus striatum of rats. In contrast, each of these agents increased circulating levels of both adrenocorticotropin and corticosterone, but only NPC 16377 decreased circulating plasma levels of prolactin. The results of the current study are consistent with the notion that NPC 16377 is a potent, selective and orally active sigma site ligand. At behaviorally relevant doses the compound produces neuroendocrine effects both similar to, and different from, neuroleptics, other sigma-ligands and atypical antipsychotics, while having no effect on dopamine turnover. Given these data, NPC 16377 should prove to be a useful compound to explore further the physiological and functional significance of sigma-sites in brain.

NPC 16377, a potent and selective sigma-ligand. II. Behavioral and neuroprotective profile.

6-[6-(4-Hydroxypiperidinyl)hexyloxy]-3-methylflavone HCI, (NPC 16377), a potent and highly selective sigma-site ligand, was evaluated in tests predictive of antipsychotic and neuroprotective potential and for toxicity. Like haloperidol, clozapine and remoxipride, and the sigma-ligands BMY 14802, ifenprodil and rimcazole, NPC 16377 reversed amphetamine-induced hyperactivity and apomorphine-induced climbing in mice. Additional evidence for antipsychotic activity was obtained in rats with NPC 16377, clozapine, BMY 14802, ifenprodil, haloperidol and rimcazole, all of which reduced conditioned avoidance responses at doses that did not reduce escape behavior. NPC 16377 did not induce catalepsy in mice, suggesting a decreased liability for producing extrapyramidal side effects. NPC 16377 extended survival time for mice exposed to a hypoxic environment. In a model of global ischemia using conscious gerbils, NPC 16377 prevented damage to hippocampal CA1 neurons after either intraperitoneal or oral administration. NPC 16377 did not disrupt prepulse inhibition or block the disruption of prepulse inhibition induced by the phencyclidine site-selective ligand (+)MK-801. In rats trained to discriminate phencyclidine from saline, NPC 16377 did not substitute for the psychotomimetic. These data are consistent with the notion that selective sigma-agents may possess antipsychotic and neuroprotective activities. Moreover, the results from prepulse inhibition and drug discrimination experiments suggest that NPC 16377 is devoid of phencyclidine-like effects.

Bridged gamma-carbolines and derivatives possessing selective and combined affinity for 5-HT2 and D2 receptors.

A series of 5,6,7,8,9,10-hexahydro-7,10-iminocyclohept[b]indoles and 6,7,8,9,10,11-hexahydro-7,11-imino-5H-cyclooct[b]indoles was prepared. Structural modifications of the lead compound, 11-[4-(4-fluorobenzoyl)propyl]-5,6,7,8,9,10-hexahydro-7,10- iminocyclohept[b]indole (5, Ki = 0.82 nM vs [3H]ketanserin) enabled the identification of the functionality necessary for high affinity at serotonin 5-HT2 and dopamine D2 receptors in ligand binding studies. The indole ring, as well as the benzoyl or isosteric benzisoxazole moiety, were essential for high affinity. Variations of the length of the side chains resulted in ligands having either selective affinity for the 5-HT2 receptor or a combination of 5-HT2 and D2 affinity. In vivo binding studies were performed on selected members in this series. The most potent member, 2-fluoro-11-[4-(4-fluorobenzoyl)butyl]-5,6,7,8,9,10-hexahydro-7,10- iminocyclohept[b]indole (36) had an ED50 of < 1 mg/kg at the 5-HT2 and D2 receptors following oral administration.

The envelope glycoprotein of HIV-1 alters NMDA receptor function.

Human immunodeficiency virus (HIV-1) infection often results in central nervous system (CNS) dysfunction, yet the mechanism(s) of action for HIV-1 in the CNS are not fully understood. In the present study gp120, the HIV-1 envelope glycoprotein, was shown to selectively inhibit N-methyl-D-aspartate (NMDA) receptor function. In addition to inhibiting radioligand binding to rat NMDA receptors, gp120 inhibited NMDA-induced currents in Xenopus oocytes, attenuated NMDA-stimulated calcium flux and cytotoxicity in cultured cerebellar granule cells, and provided partial protection against NMDA-induced lethality in vivo. These findings suggest that NMDA receptor complex is a possible site of action of HIV-1 within the CNS.

Synthesis and in vitro evaluation of 5,6,7,8,9,10-hexahydro-7,10-iminocyclohept[b]indoles: high-affinity ligands for the N,N'-di-o-tolylguanidine-labeled sigma binding site.

A series of 5,6,7,8,9,10-hexahydro-7,10-iminocyclo[b]indoles substituted at the 5 and/or 11 positions was synthesized from tropinone. Affinity for sigma binding sites was determined using [3H]-N,N'-di-o-tolylguanidine ([3H]DTG) and [3H]-(+)-3-(3-hydroxyphenyl)-N-1-propylpiperidine ([3H]-(+)-3-PPP) and for the dopamine D2 receptor labeled with [3H]sulpiride. Nearly all compounds studied in this series possessed a higher affinity for [3H]DTG than [3H]-(+)-PPP-labeled sigma sites, suggesting that [3H]DTG and [3H]-(+)-3-PPP radioligands label pharmacologically distinct sigma binding sites, as reported previously. Substitution at the 11 position with side chains containing a four-carbon tether resulted in compounds having the highest affinity for the [3H]DTG-labeled sigma site. The most potent and selective member of this series was 11-[4-(2-furanyl)butyl]-5,6,7,8,9,10-hexahydro-7,10-iminocyclohept [b] indole (40). Enantioselectivity was investigated by preparing the (+)- and (-)-isomers of 40. These studies revealed that (+)-40 was more potent at the [3H]-DTG-labeled sigma site whereas (-)-40 had a higher affinity at sigma sites labeled with [3H]-(+)-PPP. Racemic 40 was observed to possess a higher affinity than either of its respective enantiomers at both the [3H]DTG- and [3H]-(+)-3-PPP-labeled sites, suggesting an allosteric interaction.

Pharmacological profile of NPC 17742 [2R,4R,5S-(2-amino-4,5-(1, 2-cyclohexyl)-7-phosphonoheptanoic acid)], a potent, selective and competitive N-methyl-D-aspartate receptor antagonist.

2R,4R,5S-(2-amino-4,5-(1,2-cyclohexyl)-7-phosphonoheptanoic acid) (NPC 17742), the most potent isomer of the mixture 2-amino-4,5-(1,2-cyclohexyl)-7-phosphonoheptanoic acid (NPC 12626), was evaluated for activity in tests associated with receptors for excitatory amino acids. In receptor binding assays, NPC 17742 was selective for the N-methyl-D-aspartate (NMDA) receptor with a potency comparable to that of D(-, -3-(2-carboxypiperazine-4-yl)propyl-1-phosphonic acid. Like (+/-)cis-4-phosphono-methyl-2-piperidine carboxylic acid (CGS 19755) and (+/-)(E)-2-amino-4-methyl-5-phosphono-3-penteneoic acid (CGP 37849), NPC 17742 competitively inhibited NMDA-induced enhancement of 1-[(2-thienyl)cyclohexyl]piperidine binding to the NMDA receptor ionophore and partially inhibited [3H]glycine binding to strychnine-insensitive sites. In contrast, NPC 17742 and CGP 37849 inhibited Mg(++)-stimulated 1-[(2-thienyl)cyclohexyl]piperidine binding in a noncompetitive fashion. In voltage-clamped Xenopus oocytes expressing excitatory amino acid receptors, NPC 17742 (pKB = 6.91) was equipotent with CGP 37849 (pKB = 7.17) in inhibiting NMDA-induced inward currents. Likewise, NPC 17742 (ED50 = 2.68 mg/kg) was equipotent with CGP 37849 and CGS 19755 in blocking NMDA-induced convulsions, but was less potent than these two compounds in the maximal electroshock test. Unlike CGP 37849 or CGS 19755, NPC 17742 potently antagonized seizures induced by pentylenetetrazol. In a model of global ischemia, low doses of NPC 17742 given either before or after ischemic result were effective in blocking damage to hippocampal CA1 neurons. The pharmacologic responses to NPC 17742 occurred at doses 30- to 300-fold lower than the acute lethal dose.(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of strychnine-insensitive glycine receptor antagonists and sigma agents on working memory performance: comparison with dizocilpine and scopolamine.

The strychnine insensitive glycine receptor antagonists (+/-) HA 966 (2.5, 3.5, 4.25 and 5.0mg/kg) and 7 chlorokynurenic acid (5.0, 10.0, and 15.0mg/kg), the putative sigma agents NPC 16377 (5.0 and 8.0mg/kg), BMY 14802 (5.0, 7.5 and 10.0mg/kg), and ifenprodil (5.0 and 7.0mg/kg) and the reference agents scopolamine and dizocilpine [(+) MK 801] were evaluated in a nonspatial delayed matching to sample working memory task in rats. (+/-) HA 966 impaired accuracy at the longest retention interval and decreased response probability measures. 7-Chlorokynurenic acid was essentially without effect. The noncompetitive NMDA antagonist dizocilpine reduced accuracy at all retention intervals, decreased the probability of a choice response and increased the probability of an intertrial interval response. The anticholinergic agent scopolamine selectively reduced accuracy at the longest retention interval but did not affect other performance measures. Sigma agents decreased response probability measures but did not affect accuracy at any retention interval. The results support the notion that sigma agents, glycine antagonists and NMDA antagonists produce different effects in cognitive tasks including working memory performance.

Biochemical and behavioral studies following subchronic administration of GABA uptake inhibitors in mice.

N-(4,4-Diphenyl-3-butenyl) nipecotic acid (SKF(R)-89976A) and N-(4,4-diphenyl-3-butenyl) guvacine (SKF 100330A) are potent inhibitors of the uptake of GABA and have anticonvulsant properties. In the present study, the effects of these compounds on several behavioral and biochemical measures were determined, following subchronic administration. Administration of SKF(R)-89976A (8.9 mg/kg) for 14 days caused a small but significant reduction in its potency to protect against pentylenetetrazole-induced seizures, whereas treatment with SKF 100330A (13.6 mg/kg) had no significant effect. The percentage of animals rendered cataleptic by administration of either GABA uptake inhibitor was reduced by treatment for as few as 4 days and treatment with SKF(R)-89976A for 14 days resulted in a 4-fold increase in the CD50 for induction of catalepsy. The binding of [3H]GABAA and [3H]GABAB in membranes from the forebrain of the mouse were not influenced by treatment with drug nor was synaptosomal uptake of [3H]GABA. Likewise, the binding of [3H]sulpiride in striatal membranes of the mouse was unaffected by repeated exposures to SKF(R)-89976A. These results demonstrate that prolonged administration of GABA uptake inhibitors produced only a small reduction in anticonvulsant potency, whereas liability to side-effects, as demonstrated by the reduction in catalepsy, was substantially reduced.

Opposite effects of spermine and arcaine on responses of N-methyl-D-aspartate receptors expressed in Xenopus oocytes.

The ability of polyamines to modulate N-methyl-D-aspartate (NMDA) receptor function was investigated in Xenopus oocytes injected with rat brain mRNA. Whereas spermine and spermidine augmented NMDA/glycine-induced inwards currents, arcaine (1,4-diguanidinobutane) and 1,10-diaminodecane inhibited the response. The potency of arcaine to inhibit NMDA/glycine-induced currents was unaffected by spermine; similarly, arcaine did not influence the potency of spermine, but did reduce the maximal response to spermine. These findings demonstrate that polyamines exert both positive and negative modulatory control of the NMDA receptor expressed in Xenopus oocytes, and suggest that spermine and arcaine act at distinct sites in the NMDA receptor complex.

Possible cerebroprotective and in vivo NMDA antagonist activities of sigma agents.

The recent finding that ifenprodil binds with high affinity to sigma sites suggests that other sigma agents may have ifenprodil-like cerebroprotectant and functional N-methyl-D-aspartate (NMDA) antagonist effects. The present study, compared the in vivo effects of ifenprodil and the sigma agents, BMY 14802, caramiphen and haloperidol, in three tests sensitive to NMDA antagonists and purported cerebroprotectant drugs. When administered at or below the rotorod TD50 dose, all four compounds significantly increased survival time in an hypoxic environment (4% O2 in nitrogen). Caramiphen and ifenprodil (ED50 = 52 and 61 mg/kg, respectively) also blocked maximal electroshock-induced seizures, whereas BMY 14802 and haloperidol were ineffective. Finally, caramiphen (ED50 = 95 mg/kg) antagonized seizures and lethality induced by administration of NMDA (250 mg/kg, IP). BMY 14802, haloperidol and ifenprodil only partially antagonized NMDA-induced seizures, but did enhance the anticonvulsant potency of the noncompetitive NMDA antagonist, MK-801. Together, these findings suggest that sigma agents may have cerebroprotective effects.

[3H]DTG and [3H](+)-3-PPP label pharmacologically distinct sigma binding sites in guinea pig brain membranes.

The interaction of various compounds with sigma binding sites was examined in membranes prepared from whole guinea pig brain. Whereas [3H](+)-3-(3-hydroxyphenyl)-N-(1-propyl)piperidine labeled a single population of binding sites exhibiting a Kd of 43 nM, [3H]1,3-di-o-tolylguanidine bound to two sites having Kds of 35 and 212 nM, and to a greater maximum number of sites than [3H](+)-3-(3-hydroxyphenyl)-N-(1-propyl)piperidine. Haloperidol, 1,3-di-o-tolylguanidine, BMY 14802, and (-)-pentazocine each displayed nearly equal affinity for binding sites labeled by [3H](+)-3-(3-hydroxyphenyl)-N-(1-propyl)piperidine and [3H]1,3-di-o-tolylguanidine, whereas (+)-3-(3-hydroxyphenyl)-N-(1-propyl)piperidine was 3 times more potent in inhibiting [3H](+)-3-(3-hydroxyphenyl)-N-(1-propyl)piperidine than [3H]1,3-di-o-tolylguanidine binding. In contrast, (+)-SKF 10,047, (+)-cyclazocine and (+)-pentazocine exhibited more than 9-fold higher affinity for [3H](+)-3-(3-hydroxyphenyl)-N-(1-propyl)piperidine than [3H]1,3-di-o-tolylguanidine binding sites. Dextromethorphan was 15-fold more potent against [3H](+)-3-(3-hydroxyphenyl)-N-(1-propyl)piperidine than [3H]1,3-di-o-tolylguanidine, inhibited [3H](+)-3-(3-hydroxyphenyl)-N-(1-propyl)piperidine binding in a biphasic manner, and inhibited [3H]haloperidol and [3H](+)-SKF 10,047 binding with potencies similar to those obtained against [3H]1,3-di-o-tolylguanidine and [3H](+)-3-(3-hydroxyphenyl)-N-(1-propyl)piperidine, respectively. Phenytoin increased [3H](+)-3-(3-hydroxyphenyl)-N-(1-propyl)piperidine and [3H](+)-SKF 10,047 binding, but did not enhance [3H]1,3-di-o-tolylguanidine or [3H]haloperidol binding. However, the potency of dextromethorphan to inhibit [3H]1,3-di-o-tolylguanidine binding was increased in the presence of phenytoin.(ABSTRACT TRUNCATED AT 250 WORDS)

Pharmacological characterization of sigma binding sites in guinea pig brain membranes.

The discovery of sigma binding sites has prompted investigation into the functional role of these sites. Binding studies have revealed that sigma sites exhibit a unique pharmacological profile, and have provided evidence favoring the existence of a multiplicity of sigma binding sites in central nervous system. However, the findings that chemicals having diverse structures and therapeutic applications are all potent sigma agents, and that sigma binding sites are present in peripheral tissues, have raised concerns about the physiological and pharmacological relevances of this site. Furthermore, an endogenous ligand for the sigma binding site has not yet been identified. Finally, there is a lack of data regarding the functional coupling of sigma binding sites, although recent studies have provided some clues as to a possible signalling mechanism (Karbon et al., 1990). Besides raising questions about the relevance of sigma sites, the paucity of information on their functional properties has made it difficult to distinguish sigma agonists from and antagonists. While haloperidol is assumed to be a sigma antagonist, the finding that prolonged administration of this neuroleptic decreases the number of sigma sites would seem to argue in favor of it being an agonist for this site. Regardless of the precise nature of the sigma binding site, studies have suggested that it may represent the site of action for a number of important drugs. For example, haloperidol, a butyrophenone antipsychotic, exhibits high affinity for sigma binding sites, and several psychotomimetics, including PCP and (+)-benzomorphans, also bind this site.(ABSTRACT TRUNCATED AT 250 WORDS)

gamma-Aminobutyric acidB receptor activation modifies agonist binding to beta-adrenergic receptors in rat brain cerebral cortex.

The interaction of isoproterenol with beta-adrenergic receptor (beta AR) binding sites was measured in membranes prepared from rat brain cerebral cortical slices previously incubated in the presence or absence of gamma-aminobutyric acid (GABA) receptor agonists. Both GABA and baclofen, but not isoguvacine, altered beta AR agonist binding by increasing the affinity of both the low- and high-affinity binding sites and by increasing the proportion of low-affinity receptors. The response to baclofen was stereoselective, and the effect of GABA was not inhibited by bicuculline. The results suggest that GABAB, but not GABAA, receptor activation modifies the coupling between beta AR and stimulatory guanine nucleotide-binding protein, which may in part explain the ability of baclofen to augment isoproterenol-stimulated cyclic AMP accumulation in brain slices.

Augmentation of neurotransmitter receptor-stimulated cyclic AMP accumulation in rat brain: differentiation between the effects of baclofen and phorbol esters.

The augmentation of isoproterenol or vasoactive intestinal peptide (VIP)-stimulated cyclic AMP accumulation in rat brain slices by the GABAB agonist baclofen was compared to that mediated by tumor-promoting phorbol esters. The protein kinase C inhibitor H7 and desensitization of protein kinase C reduced the cyclic AMP augmenting effect of the phorbol ester, but not baclofen. Incubation of brain slices in the presence of both baclofen and a phorbol ester amplified the cyclic AMP response to isoproterenol or VIP to a greater degree than that found with either baclofen or the phorbol ester alone, with the increased augmentation appearing to be additive. These findings indicate that although stimulation of GABAB receptors or protein kinase C activation by phorbol esters have similar effects on transmitter-stimulated cyclic AMP production in brain, these augmenting actions appear to be independently mediated.

Phorbol esters enhance neurotransmitter-stimulated cyclic AMP production in rat brain slices.

The effect of phorbol esters on cyclic AMP production in rat CNS tissue was examined. Using a prelabeling technique for measuring cyclic AMP accumulation in brain slices, it was found that phorbol 12-myristate, 13-acetate (PMA) enhanced the cyclic AMP response to forskolin and a variety of neurotransmitter receptor stimulants while having no effect on second messenger accumulation itself. A short (15-min) preincubation period with PMA was required to obtain maximal enhancement, whereas the augmentation was lessened by prolonged exposure (3 h) to the phorbol. The response to PMA was concentration dependent (EC50 = 1 microM) and regionally selective, being most apparent in forebrain, and was not influenced by removal of extracellular calcium or by inhibition of phosphodiesterase or phospholipase A2. Only those phorbols known to stimulate protein kinase C augmented the accumulation of cyclic AMP. Moreover, the membrane substrates phosphorylated by endogenous C kinase and by a partially purified preparation of this enzyme were similar. The results suggest that phorbol esters, by activating protein kinase C, modify the cyclic AMP response to brain neurotransmitter receptor stimulation in brain by influencing a component of the adenylate cyclase system beyond the transmitter recognition site.

An examination of the involvement of phospholipases A2 and C in the alpha-adrenergic and gamma-aminobutyric acid receptor modulation of cyclic AMP accumulation in rat brain slices.

Experiments were undertaken to define the role of two calcium-associated enzyme systems in modulating transmitter-stimulated production of cyclic nucleotides in rat brain. Cyclic AMP (cAMP) accumulation was examined in cerebral cortical slices using a prelabeling technique. The enhancement of isoproterenol-stimulated cAMP production by alpha-adrenergic and gamma-aminobutyric acid-B (GABAB) agonists was reduced by exposing the tissue to EGTA, a chelator of divalent cations, or quinacrine, a nonselective inhibitor of phospholipase A2. Likewise, chronic (2 weeks) administration of corticosterone decreased the alpha-adrenergic and GABAB receptor modulation of second messenger production. Neither cyclooxygenase nor lipoxygenase inhibitors selectively influenced the facilitating response of alpha-adrenergic and GABAB agonists. Other experiments revealed that although norepinephrine and 6-fluoronorepinephrine stimulated inositol phosphate (IP) production in cerebral cortical slices with potencies equal to those displayed in the cyclic nucleotide assay, selective alpha 1-adrenergic agonists were less efficacious on IP formation and were without effect in the cAMP assay. Conversely, a selective alpha 2-adrenergic receptor agonist facilitated the cAMP response to a beta-adrenergic agonist without affecting IP formation. The rank orders of potency of a series of alpha-adrenergic antagonists suggest that IP accumulation is mediated solely by alpha 1-adrenergic receptors, whereas the augmentation of cAMP accumulation is regulated by a mixed population of alpha-adrenergic sites. The results suggest that the alpha-adrenergic and GABAB receptor-mediated enhancement of isoproterenol-stimulated cAMP formation appears to be more closely associated with phospholipase A2 than phospholipase C and may be mediated by arachidonate or some other fatty acid.

Phorbol esters down-regulate protein kinase C in rat brain cerebral cortical slices.

The effect of phorbol esters on cyclic AMP production in rat cerebral cortical slices was studied using a prelabelling technique to measure cyclic nucleotide accumulation. Cholera toxin-stimulated cyclic AMP accumulation was enhanced approximately 2-fold by phorbol 12-myristate, 13-acetate (PMA) which alone had no effect on cyclic AMP production. The augmentation by PMA was maximal within the first hour of incubation, decreasing progressively thereafter. Protein kinase C activity was decreased 80-90% during a 3 hr exposure to PMA, as was 3H-phorbol 12,13-dibutyrate binding. Both phosphatidyl serine and arachidonic acid were found to enhance protein kinase C activity in a concentration-dependent manner, an effect that was attenuated by prolonged incubation of the brain tissue with PMA. The results indicate that exposure of brain slices to phorbol esters causes a down-regulation of rat brain protein kinase C, and that this modification corresponds with a decrease in the ability of PMA to augment cyclic AMP production, suggesting a functional relationship between the two systems in rat brain.