Shailesh P. Banerjee

Antipsychotic drug effects on glutamatergic activity

Previous work from this laboratory indicated that some antipsychotic drugs possess unique action at N-methyl-D-aspartate (NMDA) receptors. A functional neurochemical assay showed that, at concentrations similar to those found in the cerebrospinal fluid (CSF) of schizophrenics, antipsychotic drugs augment NMDA activity while, at higher concentrations, NMDA activity is suppressed. Using similar analysis, the present paper reports that this pattern of response is also shown by the antipsychotic drugs thioridazine and chlorpromazine. In contrast, promazine, which is structurally similar to chlorpromazine but lacking both D2-effects and antipsychotic potency, had no influence on NMDA receptors. In addition, sulpiride and metoclopramide, drugs with high affinity for D2-dopamine receptors but with weak or no antipsychotic efficacy, also lack effects at the NMDA receptor. Thus, the drugs with clinical efficacy that were tested in the present and previous studies all share unique influence on NMDA receptors. Further work with other antipsychotic agents will be necessary to determine if influence on NMDA receptors contributes to antipsychotic effectiveness.

Modes of direct modulation by taurine of the glutamate NMDA receptor in rat cortex.

Taurine is an endogenous brain substance with robust neuromodulatory and possible neuroprotective properties. Though other mechanisms of action have been reported, its interaction with the NMDA (N-methyl-D-aspartic acid) receptor is undocumented. We investigated taurines interaction with the NMDA receptor using electrophysiological and receptor binding approaches. The effects of taurine on field potential responses in layer-5 of prelimbic cortex in rat brain slices evoked by single-pulse electrical stimulation of ventral medial cortex were determined. Picrotoxin (80 µM) was present in all control and drug solutions to block the Cl(-) channels associated with the GABA-, taurine-, and strychnine sensitive glycine- receptors. A typical response consisted of an NBQX (2,3-dioxo-6-nitro-1,2,3,4-tetrahydrobenzo-[f]-quinoxaline-7-sulfonamide)-sensitive negative wave (N1) followed by a positive wave (P1) and a broad negativity (N2), both sensitive to dl-AP5 (dl-2-amino-5-phosphonopentanoic acid) inhibition. Taurine exerted a 41.5 ± 8.3% (n = 9) voltage reduction within the late phase of N2. This taurine action was prevented by 100 µM AP5, but not by 10 µM nifedipine, supporting a direct modulation of NMDA receptor function by taurine, without requiring the involvement of the L-type Ca(2+) channel. Taurine did not alter specific [(3)H] MK-801 binding to rat cortical membranes in the presence of glycine or glutamate; but inhibited spermine-potentiated specific [(3)H] MK-801 binding to NMDA receptors by 15-20% in the presence of glycine. In addition, taurine reduced the apparent affinity of the NMDA receptor for glycine (in the presence of spermine) by 10-fold. These results show that taurine interacts directly with the NMDA receptor by multiple mechanisms.

Cocaine challenge enhances release of neuroprotective amino acid taurine in the striatum of chronic cocaine treated rats: a microdialysis study.

Drug addiction is a serious public health problem. There is increasing evidence on the involvement of augmented glutamatergic transmission in cocaine-induced addiction and neurotoxicity. We investigated effects of acute or chronic cocaine administration and cocaine challenge following chronic cocaine exposure on the release of excitotoxic glutamate and neuroprotective taurine in the rat striatum by microdialysis. Cocaine challenge, following withdrawal after repeated cocaine exposure markedly increased the release of glutamate, which may cause neurotoxicity. Simultaneously, cocaine challenge after withdrawal also significantly increased the release of taurine, which counteracts glutamate-mediated excitotoxicity and possibly cell death. Thus, the mammalian brain has an endogenous self-protective mechanism against cocaine-mediated neurotoxicity and potentially addiction.

Taurine prevents haloperidol-induced changes in striatal neurochemistry and behavior

Repeated daily administration of haloperidol produces changes in striatal neurochemistry (decreased dopamine synthesis, upregulation of D2 receptors) and behavior (increasing catalepsy). Coadministration of taurine greatly attenuated these neuroleptic-induced changes. Possible mechanisms of taurines mitigating effects are its attenuating influences on glutamatergic transmission and its actions as a GABAA agonist. The possibility was discussed of adding taurine to chronic antipsychotic regimens to block the side-effects typically accompanying such therapy.

Direct Interaction of Taurine with the NMDA Glutamate Receptor Subtype via Multiple Mechanisms

Taurine has neuroprotective capabilities against glutamate-induced excitotoxicity through several identified mechanisms including opening of the Cl(-)channel associated with GABA(A)and glycine receptors, or a distinct Cl(-)channel. No existing work has however shown a direct interaction of taurine with the glutamate NMDA receptor. Here we demonstrate such direct interactions using electrophysiological and receptor binding techniques on rat medial prefrontal cortical (mPFC) slices and well-washed rat cortical membrane. Electrically evoked field potential responses were recorded in layer 4/5 of mPFC in the presence of picrotoxin to prevent opening of Cl(-)channels gated by GABA or taurine. Applied taurine markedly diminished evoked-response amplitude at the peak and latter phases of the response. These phases were predominantly sensitive to the NMDA antagonist, MK-801, but not the AMPA/kainate receptor antagonist CNQX. Furthermore, this taurine effect was blocked by APV pretreatment. Taurine (0.1 mM) decreased spermine-induced enhancement of specific ((3)H) MK-801 binding to rat cortical membrane in the presence of glycine, though it was ineffective in the absence of spermine. Our preliminary work shows that taurine diminished the apparent affinity of NMDA receptor to glycine in the presence of spermine. These results indicate that taurine may directly interact with the NMDA receptor through multiple mechanisms.

Taurine Targets the GluN2b-Containing NMDA Receptor Subtype

As an endogenous neuromodulator in the CNS, Taurine interacts with TAG (6-aminomethyl-3-methyl-4H-1,2,4-benzothiadiazine 1,1-dioxide)-sensitive and TAG-insensitive receptors. Taurine activates the former by opening a Cl− channel and the latter by inhibiting the glutamate NMDA receptor. We sought to resolve the site of taurine’s interaction with the NMDA receptor. We recorded evoked field potential in medial prefrontal cortical slices and compared its dose-dependent inhibition by Ro25-6981, a selective antagonist for the GluN1/GluN2B NMDA receptor sub-type, in the absence or presence of taurine. The result revealed that inhibition of evoked responses mediated by taurine overlapped with that by Ro25-6981, suggesting that taurine modulates NMDA receptor by acting on the NMDA GluN1/GluN2B receptor sub-type. Displacement of specific binding of [3H]spermidine and of [3H]taurine to crude frontal cortical membranes by spermine and spermidine showed that polyamines and taurine may interact at a common binding site, possibly localized at the GluN1 or GluN2B subunit. We also tested for long-term taurine actions on glutamate receptor subunits using western blot determination of NMDA and AMPA receptor subunits expression in synaptosomal membranes prepared from rat frontal cortex following chronic taurine treatment. Thirty daily i.p. injections of taurine (100 mg/kg) significantly increased expression of the NMDA GluN2B, but not GluN1, subunit and decreased expression of the AMPA GluR2 subunit. The up-regulation of the GluN2B subunit suggests its long-term interaction with taurine, and supports its being a major target for taurine action. Down-regulation of the AMPA GluR2 subunit is possibly correlated to an increased recruitment of the GluR2-subunit-lacking, calcium-permeable subtype of AMPA receptor.

Neuropsychopharmacological Actions of Taurine

Taurine, an endogenous amino sulfonic acid, exhibits numerous neuropsychopharmacological activities. Previous studies in our laboratory have shown that it is an effective anti-cataleptic and neuro-protective agent. Current investigations show that acute or chronic administration of psychotropic drug cocaine may increase extracellular release of endogenous taurine which may protect against deleterious effects of the substances of abuse. Taurine administration was found to prevent cocaine-induced addiction by suppressing spontaneous locomotor activity and conditioned place preference. Taurine markedly delayed tail-flick response in rats which was significantly different from that in the group of animal receiving the same volume of saline, thereby indicating that taurine is a potentially valuable analgesic agent. Both taurine and endomorphin-1 were found to suppress the delayed broad negative evoked field potentials in anterior insular cortex (upper layer 5) by partially inhibiting NMDA receptor system. Thus, taurine is a unique psychopharmacological compound with potential for a variety of therapeutic uses including as a neuro-protective, anti-cataleptic, anti-addicting, and analgesic agent.

Mechanisms for metoclopramide-mediated sensitization and haloperidol-induced catalepsy in rats.

Typical antipsychotics such as the dopamine D(2) receptor antagonist, haloperidol are known to cause movement disorders or catalepsy in experimental animals. Catalepsy is believed to result from blockade of dopamine D(2) receptors. In this study two drugs that differ in antipsychotic potency but are similar in blocking dopamine D(2) receptors were used to investigate the mechanism for catalepsy and its sensitization. Metoclopramide is a strong postsynaptic dopamine D(2) receptor blocker with no antipsychotic potency. At low doses of 5 or 10 mg/kg given subcutaneously (s.c.), metoclopramide did not produce catalepsy or movement disturbance for seven days after drug treatment. Also metoclopramide at 10 mg/kg given for five days, failed to induce catalepsy. Haloperidol, another potent dopamine D(2) receptor blocker at 0.5 mg/kg (s.c.) rapidly produced catalepsy and suppressed movement 1 h after a single dose of the drug. Chronic as well as acute treatment with metoclopramide caused sensitization of haloperidol-induced catalepsy. Neurochemical analyses revealed significant dopamine D(2) receptor up-regulation in both frontal cortex and striatum of rats chronically treated with metoclopramide. However, no changes in dopamine D(2) receptor numbers were noted in these areas after chronic treatment with low doses of haloperidol. Significant increases in N-methyl-D-aspartate (NMDA) receptor numbers were observed in both frontal cortex and striatum of metoclopramide treated animals, while haloperidol elicited significant decreases in NMDA receptor numbers in both brain areas. These observations plus previous reports have led us to propose a model for catalepsy and its sensitization. According to this model the increase in NMDA receptors by metoclopramide sensitizes the brain to haloperidol-induced catalepsy. Thus, catalepsy appears to be elicited by simultaneous activation of glutamatergic NMDA and dopamine D(1) receptors as well as a blockade of dopamine D(2) receptors.

Increase of postsynaptic dopaminergic transmission by presynaptic actions of cocaine

Cocaine is believed to modify dopaminergic transmission by blocking reuptake thereby increasing the concentration in the synaptic cleft. The elevated dopamine concentrations at the synapse can affect both post- and presynaptic dopamine receptors. Since the former receptor mediates the functional effects of dopamine while the latter suppresses subsequent release of dopamine, it is unclear whether cocaine would enhance or diminish dopamines influences. Results of the present study indicated that the primary functional result of cocaine is to enhance dopamines postsynaptic effects.