Glutamate agonist LY404,039 for treating schizophrenia has affinity for the dopamine D2(High) receptor.

The glutamate agonist LY404,039 has been used to treat schizophrenia. Because all currently used antipsychotics act on dopamine receptors, it was decided to examine whether this glutamate agonist also had an affinity for dopamine D2 receptors in vitro. The present data show that LY404,039 inhibited the binding of [3H]domperidone and [3H]+PHNO by 15.5 +/- 1.5% to the high-affinity state, D2(High), of cloned dopamine D2(Long) receptors and rat striatal tissue with dissociation constants of between 8.2 and 12.6 nM. This high-affinity component of LY404,039 on the binding of [3H]domperidone was inhibited by the presence of guanine nucleotide, indicating an agonist action of the drug at D2(High). LY404,039 also stimulated the incorporation of [35S]GTP-gamma-S into D2(Long) receptors (EC50% = 80 +/- 15 nM) over the same range of concentrations as occurred for the inhibition of [3H]domperidone by LY404,039 at D2(High) (IC50%(High) = 50 +/- 10 nM). A possible clinical antipsychotic action of LY404,039 may depend on the combined stimulation of glutamate receptors and a partial dopamine agonist action that would interfere with neurotransmission at D2(High) receptors.

Glutamate agonists for treating schizophrenia have affinity for dopamine D2High and D3 receptors.

Although the glutamate agonist LY 404,039 has been used to treat schizophrenia, its closest congener LY 379,268 has an affinity for both glutamate and dopamine (DA) D2(High) receptors. Considering that all antipsychotics act on dopamine receptors, and considering that another laboratory reported that LY 379,268 did not have any affinity for the D2(High) receptor, it was necessary to examine whether such glutamate agonists have an affinity for D2 and D3 dopamine receptors in vitro. The present data show that 50-200 nM LY 379,268 inhibited the binding of [(3)H]domperidone and [(3)H](+)PHNO to cloned dopamine D2 receptors consistently and reproducibly by 16% with dissociation constants of 2.1 and 2.5 nM at D2(High), respectively. In addition, LY 379,268 inhibited the binding of [(3)H]domperidone and [(3)H](+)PHNO to cloned dopamine D3 receptors with dissociation constants of 130 and 10 nM, respectively. LY 379,268 also inhibited the binding of [(3)H]domperidone to rat striata with a dissociation constant of 22 nM, predicting a clinical antipsychotic dose of 80-100 mg/day. LY 379,268 appears to act as an agonist at D2(High) and as an antagonist at D3, because guanine nucleotide eliminated the competition at D2(High) but had no effect on the competition at D3. The findings indicate that this type of glutamate agonist, LY 379,268, has a significant affinity for D2(High) and D3 receptors.

Dopamine D2High receptors stimulated by phencyclidines, lysergic acid diethylamide, salvinorin A, and modafinil.

Although it is commonly stated that phencyclidine is an antagonist at ionotropic glutamate receptors, there has been little measure of its potency on other receptors in brain tissue. Although we previously reported that phencyclidine stimulated cloned-dopamine D2Long and D2Short receptors, others reported that phencyclidine did not stimulate D2 receptors in homogenates of rat brain striatum. This study, therefore, examined whether phencyclidine and other hallucinogens and psychostimulants could stimulate the incorporation of [(35)S]GTP-gamma-S into D2 receptors in homogenates of rat brain striatum, using the same conditions as previously used to study the cloned D2 receptors. Using 10 microM dopamine to define 100% stimulation, phencyclidine elicited a maximum incorporation of 46% in rat striata, with a half-maximum concentration of 70 nM for phencyclidine, when compared with 80 nM for dopamine, 89 nM for salvinorin A (48 nM for D2Long), 105 nM for lysergic acid diethylamide (LSD), 120 nM for R-modafinil, 710 nM for dizocilpine, 1030 nM for ketamine, and >10,000 nM for S-modafinil. These compounds also inhibited the binding of the D2-selective ligand [(3)H]domperidone. The incorporation was inhibited by the presence of 200 microM guanylylimidodiphosphate and also by D2 blockade, using 10 microM S-sulpiride, but not by D1 blockade with 10 microM SCH23390. Hypertonic buffer containing 150 mM NaCl inhibited the stimulation by phencyclidine, which may explain negative results by others. It is concluded that phencyclidine and other psychostimulants and hallucinogens can stimulate dopamine D2 receptors at concentrations related to their behavioral actions.

Phencyclidine and glutamate agonist LY379268 stimulate dopamine D2High receptors: D2 basis for schizophrenia.

It has previously been reported that the glutamate ionotropic antagonist phencyclidine directly inhibits the release of prolactin in anterior pituitary cells in culture, suggesting that phencyclidine has a dopamine (DA)-like action on prolactin-releasing cells. It has also been reported that the glutamate metabotropic agonist LY379268 can stimulate the incorporation of [35S]GTP-gamma-S into DA D2Long receptors. The present study was done to examine whether such glutamatergic drugs had similar actions on the DA D2Short receptor. The present results show that phencyclidine, ketamine, and LY379268 also stimulated the incorporation of [35S]GTP-gamma-S into D2Short receptors. The proportion of D2Long and D2Short receptors existing in the high-affinity state were both markedly reduced by NaCl. While phencyclidine and LY379268 each stimulated the incorporation of GTP-gamma-S into D2Long and D2Short receptors, this stimulation was reduced by NaCl, with D2Short being much more sensitive than D2Long to the inhibition by NaCl. The binding of phencyclidine and LY379268 to D2High receptors in vivo was directly confirmed by the i.v. injection of phencyclidine and LY379268 in which 50% inhibited the binding of [3H]PHNO to the striatum ex vivo at 0.25 and 1.5 mg/kg, respectively. The results confirm that glutamate agonists and antagonists have a significant affinity for DA D2High receptors. The psychotogenic action of phencyclidine may stem from a combination or synergistic action of glutamate receptor antagonism and DA D2 agonism. In addition, the antipsychotic clinical action of LY379268 congeners such as LY404039 may be related to a combined or synergistic action of glutamate receptor stimulation together with a partial DA agonist action that reduces endogenous DA neurotransmission.

Dopamine partial agonist action of (-)OSU6162 is consistent with dopamine hyperactivity in psychosis.

Although the Van Rossum hypothesis of dopamine receptor overactivity in schizophrenia is supported by the antagonism of, or reduced dopamine neurotransmission at, dopamine D2 receptors by antipsychotics, it has been claimed that the antipsychotic (-)-OSU6162 has a very low affinity for the dopamine D2 receptor, and has only autoreceptor dopamine D2 receptor-stimulating action, and, therefore, in order to explain its clinical action, the drug must stimulate dopamine D2 receptors that are defective or underactive in schizophrenia. Because the mode of action of (-)-OSU6162 is critical in determining whether psychosis is associated with dopamine D2 receptor overactivity or deficit activity, we measured the potency of (-)-OSU6162 on the binding of [3H]domperidone and on the incorporation of [35S]-GTP-gamma-S into human cloned dopamine D2 receptor-containing cells. We found that (-)-OSU6162 had a dissociation constant of 35 nM at the functional high-affinity site of the dopamine D2 receptor, stimulated the incorporation of [35S]-GTP-gamma-S above 100 nM, and inhibited the incorporating action of 1 microM dopamine with an inhibitory dissociation constant, Ki, of 27 nM, all being properties of a dopamine partial agonist. While not excluding the possibility that dopamine underactivity may exist in the mesocortical system in psychosis, the antipsychotic action of (-)-OSU6162 is consistent with dopamine overactivity in psychosis.