Double dissociation of the effects of haloperidol and the dopamine D3 receptor antagonist ABT-127 on acquisition vs. expression of cocaine-conditioned activity in rats.

Dopamine receptors play a critical role in reward-related learning, but receptor subtypes may be differentially involved. D2-preferring receptor antagonists, e.g., haloperidol, attenuate acquisition of cocaine-conditioned motor activity at doses that fail to block expression. We compared haloperidol [4-[4-(4-chlorophenyl)-4-hydroxy-1-piperidyl]-1-(4-fluorophenyl)-butan-1-one] with the D3 receptor-preferring antagonist 2,3-di-tert-butyl-6-{4-[3-(4,5-dimethyl-4H-[1,2,4] triazol-3-yisulfanyl)-propyl]-piperazin-1-y1}-pyrimidine hydrochloride (ABT-127), given at D3 receptor-selective doses [i.e., no displacement of [(3)H]3,5-dichloro-N-[[(2S)-1-ethyl-2-pyrrolidinyl]methyl]-2-hydroxy-6-methoxybenzamide binding, no effects on γ-butyrolactone-induced striatal l-3,4-dihydroxyphenylalanine; haloperidol accumulation; no attenuation of apomorphine-induced stereotypy]. We hypothesized that haloperidol and ABT-127 will produce a doubly dissociable effect on acquisition versus expression of cocaine-conditioned activity. Rats received three 1-h habituation sessions to activity monitors followed by three 1-h cocaine (10 mg/kg) conditioning sessions. The expression phase (no cocaine injections) took place 48 h later. Haloperidol (50 µ/kg) given during the conditioning phase blocked the acquisition of conditioned activity but failed to block the expression of conditioning when given on the test day. In contrast, ABT-127 (1.0 mg/kg), when given during conditioning, failed to block the acquisition of conditioned activity but blocked the expression of conditioning when administered on the test day. Results suggest that D2 receptors are more critically involved in acquisition than initial expression and D3 receptors are more critically involved in expression than acquisition of conditioned activity based on cocaine.

SSR149415, a non-peptide vasopressin V1b receptor antagonist, has long-lasting antidepressant effects in the olfactory bulbectomy-induced hyperactivity depression model.

Olfactory bulbectomy (OBX) in rats causes several behavioral and neurochemical CNS changes, reminiscent of symptoms of human depression. Such depression-like behavior after OBX can be reversed with antidepressants. Recently, a connection between the vasopressin 1b (V1b) receptor and the development of depression has been suggested; therefore, a vasopressin V1b receptor antagonist (SSR149415) was investigated in the OBX model. Male rats received olfactory bulbectomy or sham surgery. After recovery, animals received 14 consecutive daily doses of SSR149415 (10 or 30 mg/kg), imipramine (20 mg/kg), or vehicle (5% hydroxy-propyl methylcellulose). Animals were tested in an open field after acute treatment, on days 7 and 14 of treatment and 1 week after cessation of treatment. Similar to imipramine, repeated, but not acute, administration of SSR149415 completely reversed OBX-induced hyperactivity, leaving activity in shams unaffected. This reversal of OBX-induced hyperactivity in the SSR149415 treated rats was still present 7 days after cessation of treatment. Although the behavioral effects of treatment with SSR149415 were specific for the OBX animals, adrenal gland weights were reduced in both sham and OBX animals treated with 30 mg/kg SSR149415. Chronic but not acute administration of SSR149415 normalizes OBX-induced hyperactivity up to 1 week after cessation of treatment, suggesting that a V1b receptor antagonist may have long-lasting antidepressant activity.

Pyrrolylquinoxalinediones carrying a piperazine residue represent highly potent and selective ligands to the homomeric kainate receptor GluR5.

Pyrrolylquinoxalinediones carrying aminoalkyl residues were evaluated for affinity to the recombinant, homomeric kainate receptors GluR5, GluR6 and GluR7. Most derivatives preferred binding to GluR5. In particular, the piperazine 6e represents a highly potent and selective antagonist to GluR5.

A new pyrrolyl-quinoxalinedione series of non-NMDA glutamate receptor antagonists: pharmacological characterization and comparison with NBQX and valproate in the kindling model of epilepsy.

Antagonists at the ionotropic non-NMDA [AMPA (amino-methyl proprionic acid)/kainate] type of glutamate receptors have been suggested to possess several advantages compared to NMDA (N-methyl-D-aspartate) receptor antagonists, particularly in terms of risk/benefit ratio, but the non-NMDA receptor antagonists available so far have not fulfilled this promise. From a large series of pyrrolyl-quinoxalinedione derivatives, we selected six new competitive non-NMDA receptor antagonists. The basis of selection was high potency and selectivity for AMPA and/or kainate receptors, high in vivo potency after systemic administration, and an acceptable ratio between neuroprotective or anticonvulsant effects and adverse effects. Pharmacological characteristics of these novel compounds are described in this study with special emphasis on their effects in the kindling model of temporal lobe epilepsy, the most common type of epilepsy in humans. In most experiments, NBQX and the major antiepileptic drug valproate were used for comparison with the novel compounds. The novel non-NMDA receptor antagonists markedly differed in their AMPA and kainate receptor affinities from NBQX. Thus, while NBQX essentially did not bind to kainate receptors at relevant concentrations, several of the novel compounds exhibited affinity to rat brain kainate receptors or recombinant kainate receptor subtypes in addition to AMPA receptors. One compound, LU 97175, bound to native high affinity kainate receptors and rat GluR5-GluR7 subunits, i.e. low affinity kainate binding sites, with much higher affinities than to AMPA receptors. All compounds potently blocked AMPA-induced cell death in vitro and, except LU 97175, AMPA-induced convulsions in vivo. In the kindling model, compounds with a high affinity for GluR7 (LU 97175) or compounds (LU 115455, LU 136541) which potently bind to AMPA receptors and low affinity kainate receptor subunits were potent anticonvulsants in the kindling model, whereas the AMPA receptor-selective LU 112313 was the least selective compound in this model, indicating that non-NMDA antagonists acting at both AMPA and kainate receptors are more effective in this model than AMPA receptor-selective drugs. Three of the novel compounds, i.e. LU 97175, LU 115455 and LU 136541, exerted potent anticonvulsant effects without inducing motor impairment in the rotarod test. This combination of actions is thought to be a prerequisite for selective anticonvulsant drug action.

LU 73068, a new non-NMDA and glycine/NMDA receptor antagonist: pharmacological characterization and comparison with NBQX and L-701,324 in the kindling model of epilepsy.

The aim of this study was to assess whether a drug which combines an antagonistic action at both NMDA and non-NMDA receptors offers advantages for treatment of epileptic seizures compared to drugs which antagonize only one of these ionotropic glutamate receptors. The novel glutamate receptor antagonist LU 73068 (4,5-dihydro-1-methyl-4-oxo-7-trifluoromethylimidazo[1,2a]quinoxal ine-2-carbonic acid) binds with high affinity to both the glycine site of the NMDA receptor (Ki 185 nM) and to the AMPA receptor (Ki 158 nM). Furthermore, binding experiments with recombinant kainate receptor subunits showed that LU 73068 binds to several of these subunits, particularly to rGluR7 (Ki 104 nM) and rGluR5 (Ki 271 nM). In comparison, the prototype non-NMDA receptor antagonist NBQX (2,3-dihydroxy-6-nitro-7-sulphamoyl-benzo[f]quinoxaline) binds with high affinity to AMPA receptors only. Both NBQX and LU 73068 were about equieffective after i.p. injection in mice to block lethal convulsions induced by AMPA or NMDA. In the rat amygdala kindling model of temporal lobe epilepsy, LU 73068 dose-dependently increased the focal seizure threshold (afterdischarge threshold, ADT). When rats were stimulated with a current 20% above the individual control ADT, LU 73068 completely blocked seizures with an ED50 of 4.9 mg kg(-1). Up to 20 mg kg(-1), only moderate adverse effects, e.g. slight ataxia, were observed. NBQX, 10 mg kg(-1), and the glycine/NMDA site antagonist L-701,324 (7-chloro-4-hydroxy-3-(3-phenoxy)phenyl-quinoline-2(1H)one), 2.5 or 5 mg kg(-1), exerted no anticonvulsant effects in kindled rats when administered alone, but combined treatment with both drugs resulted in a significant ADT increase. The data indicate that combination of glycine/NMDA and non-NMDA receptor antagonism in a single drug is an effective means of developing a potent and effective anticonvulsant agent.

The G protein coupling T cell antigen receptor/CD3-complex and phospholipase C in the human T cell lymphoma Jurkat is not a target for cholera toxin.

Intact Jurkat cells could be stimulated by monoclonal antibodies against the Tcell antigen receptor complex (OKT3 directed against the CD3 complex, BMA031 directed against constant framework epitopes in the alpha/beta heterodimer). The accumulation of inositol phosphates was inhibited by prior incubation of the cells with cholera holotoxin. The inhibitory effect of cholera toxin (CT) was not cAMP mediated because forskolin (a direct activator of adenylate cyclase) did not mimic the inhibitory effect. When measuring phospholipase C (PLC) in a cell-free assay system by using [3H]inositol-labeled membranes, the enzyme could be stimulated by the poorly hydrolyzable GTP analogue guanosine 5'-O-(thiotriphosphate (GTP gamma S). Both anti-receptor antibodies augmented the GTP gamma S stimulatory effect, while the antibodies alone had no stimulatory capacity. In membranes from CT-pretreated cells, whereas the antibodies lost their stimulatory effect on PLC as in untreated cells, whereas the antibodies lost their stimulatory capacity in the presence of GTP gamma S. These data imply that CT exerts its inhibitory effect on signaling by acting at the receptor level while the PLC regulating G protein is not a target for CT-mediated alterations. This assumption is supported by the finding that in intact Jurkat cells CT, which ADP ribosylated only the alpha-subunit of the stimulatory G protein of the adenylate cyclase, led to a loss of the T cell antigen receptor complex from the cell surface as demonstrated by a decrease of receptor density using flow cytometry analysis. Receptor loss could not be achieved by forskolin treatment or incubation of the cells with the binding subunit of the toxin alone.

Cholera toxin modulates the T cell antigen receptor/CD3 complex but not the CD2 molecule and inhibits signaling via both receptor structures in the human T cell lymphoma Jurkat.

The human T cell lymphoma Jurkat can be activated by stimuli directed either against the T cell antigen receptor-CD3 antigen complex (TcR/CD3) or the CD2 molecule. Stimulation of cells via the TcR/CD3-complex or via the CD2 molecule increases inositol phosphates and cytoplasmic free calcium. Pretreatment of Jurkat cells with cholera toxin leads to a decrease of TcR/CD3 expression on the surface of the cells, while the expression of CD2 is unaffected. In contrast to this distinct effect on the receptor expression, signaling via both pathways is inhibited by cholera toxin. The most convincing explanation for the cholera toxin-mediated inhibition of signaling is that cholera toxin interrupts the signaling pathways at a point where both, stimulation via TcR/CD3 and via CD2, use the same route. The earliest common point of the two signaling pathways, at least in the Jurkat cell line, seems to be the CD3 complex because after its down-regulation (and functional inactivation) both pathways of activation are interrupted.

Effects of nucleotides on the activity of phospholipase C in rabbit thymus lymphocytes. Differences in assays using endogenous [3H]inositol-prelabeled membranes or exogenous [3H]phosphatidylinositol 4,5-bisphosphate as substrate.

The influence of nucleotides and pyrophosphate on phospholipase C from rabbit thymocytes was investigated by using two different methods for the determination of phospholipase C activity. In a first approach the release of radiolabeled inositol phosphates from [3H]inositol-labeled membranes was examined. By a second type of experiment the cleavage of exogenously added radiolabeled phosphatidylinositol 4,5-bisphosphate (PtdIns-4,5-P2) was measured. Using internally labeled membranes only guanosine 5'-O-(thiotriphosphate) exhibited a stimulatory effect on the phospholipase C suggesting the involvement of a G-protein. When exogenous [3H]PtdIns-4,5-P2 was used as substrate, cleavage of PtdIns-4,5-P2 was stimulated by all nucleotides investigated; in addition pyrophosphate showed a stimulatory effect. From these data we conclude that the increased cleavage of exogenous PtdIns-4,5-P2 induced by GTP analogues is not conclusive in terms of the involvement of a G-protein. Rather than induced by a G-protein this activation may be caused by an increased substrate accessibility. Our experiments with endogenous substrate clearly established the regulatory role of G-proteins for membrane-bound phospholipase C.

Phospholipase C in rabbit thymocytes: subcellular distribution and influences of calcium and GTP gamma S on the substrate dependence of cytosolic and plasma membrane-associated phospholipase C.

The subcellular distribution of phospholipase C (PLC) activity in rabbit thymocytes was examined by measuring the enzyme's activity in different subcellular fractions. PLC activity was determined using exogenously added [3H]PIP2 as substrate. Approx. 80% of the activity of the cell homogenate was found in the cytosolic fraction. A minor portion of PLC activity was attached to the particulate fraction. This membrane-associated PLC activity was found to be predominantly bound to the plasma membrane. Both PIP2-cleaving PLCs (the PLC associated with the plasma membrane and the PLC in the cytosol) exhibited maximum activity at pH 5. GTP gamma S stimulated the cytosolic and the membrane-bound PLC. As revealed by computer analysis of the substrate dependence of both basal and GTP gamma S-stimulated PLC activity, GTP gamma S enhanced the Vmax of the enzymes. Calcium, at a concentration of 1 mM, decreased PLC activity, as compared to a calcium concentration of 100 nM. The characteristic increase in Vmax induced by GTP gamma S was observed at a concentration of 1 mM calcium and was similar to that at 100 nM. These data suggest that the stimulatory effect of GTP gamma S is not due to an increased affinity of PLCs to calcium.

Phospholipid-metabolism of a stimulated murine T cell clone.

The release of IP3 and the incorporation of arachidonic acid into phospholipids was measured in a stimulated murine alloantigen-specific, non-cytolytic T cell clone. While Concanavalin A provoked a sharp increase in IP3, Interleukin 2 had no effect on the production of IP3. An increased reacylation of phospholipids with arachidonic acid was seen within the first 4 hours after addition of Concanavalin A, while an effect upon Interleukin 2 was only observed after 8 hours of incubation with Interleukin 2. A similar retarded response to Interleukin 2 was observed in proliferation experiments. These retarded cell responses may be due to changed properties of IL 2 receptors induced by IL 2.