Common discriminative stimulus properties in rats of muscarinic antagonists, clozapine and the D3 preferring antagonist PNU-99194a: an analysis of possible mechanisms.

Dopamine D3 receptors have been implicated in the aetiology of schizophrenia and the actions of antipsychotic drugs. The initial studies reported here assessed the involvement of such receptors in the in vivo actions of the atypical antipsychotic clozapine and the putative D3-preferring antagonist PNU-99194A in drug discrimination assays. Rats trained to discriminate clozapine consistently generalized to PNU-99194A in two separate studies. However, four other putative D3-preferring antagonists (PD 152255, (+)-S14297, nafadotride and (+)-AJ 76) did not induce generalization to clozapine. In rats trained to discriminate PNU-99194A, which has been suggested to induce a stimulus mediated specifically by D3 antagonism, the D3-preferring antagonist (+)-UH 232 and clozapine both induced full generalization. However, the PNU-99194A-trained animals also generalized fully to the muscarinic antagonists scopolamine and trihexyphenidyl. A possible explanation for the symmetrical generalization observed between clozapine and PNU-99194A is that these drugs have common muscarinic antagonist actions, since muscarinic antagonists have been reported to substitute for clozapine in numerous prior studies. However, in vitro receptor binding studies with M1-M5 receptors indicated that (with the possible exception of the M4 receptor), no muscarinic receptor subtype had high affinity for both clozapine, PNU-99194A and scopolamine. In addition, other binding studies indicated that whereas clozapine and PNU-99194A had high affinity for the D3 receptor, scopolamine did not. It is therefore concluded that: (1) The generalization seen between clozapine, PNU-99194A and muscarinic antagonists may be mediated by common effects 'downstream' from either muscarinic or D3 receptors; (2) D3 antagonism does not play a critical role in the clozapine stimulus (since D3-preferring antagonists did not consistently induce generalization to clozapine); (3) although D3 antagonism plays a role in the PNU-91994A stimulus (since the D3-preferring antagonist (+)-UH 232 induced full generalization, in accord with results from prior studies with other D3-preferring antagonists, the PNU-99194A stimulus also has commonalities with that induced by muscarinic antagonists and clozapine. The in vivo differences observed between PNU-99194A and other D3-preferring antagonists should be borne in mind when this agent is used as a tool to study D3 receptor functioning in vivo. The similarities between the PNU-99194A and clozapine stimuli suggest tentatively that compounds with a profile like PNU-99194A may have antipsychotic actions similar to clozapine. Some preclinical data are suggestive of such effects of PNU-99194A.

Arylacetamides as peripherally restricted kappa opioid receptor agonists.

Analogues of the kappa (kappa) opioid receptor agonist, ICI 199441, were prepared. Ki values for these analogues at the cloned human kappa opioid receptor ranged from 0.058 to 25 nM. Trifluoromethylaryl derivatives were potent analgesics when administered subcutaneously in the rat and were more peripherally restricted than the parent compound, ICI 199441.

New generation dopaminergic agents. 1. Discovery of a novel scaffold which embraces the D2 agonist pharmacophore. Structure-activity relationships of a series of 2-(aminomethyl)chromans.

A series of 2-(aminomethyl)chromans (2-AMCs) was synthesized and evaluated for their affinity and selectivity for both the high- and low-affinity agonist states (D2High and D2Low, respectively) of the dopamine (DA) D2 receptor. The 7-hydroxy-2-(aminomethyl)chroman moiety was observed to be the primary D2 agonist pharmacophore. The 2-methylchroman moiety was discovered to be an entirely novel scaffold which could be used to access the D2 agonist pharmacophore. Attaching various simple alkyl and arylalkyl side chains to the 7-hydroxy 2-AMC nucleus had significant effects on selectivity for the D2High receptor vs the 5HT1A and alpha 1 receptors. A novel DA partial agonist, (R)-(-)-2-(benzylamino)methyl)chroman-7-ol [R-(-)-35c], was identified as having the highest affinity and best selectivity for the D2High receptor vs the alpha 1 and 5HT1A receptors. Several regions of the 2-AMC nucleus were modified and recognized as potential sites to modulate the level of intrinsic activity. The global minimum conformer of the 7-hydroxy-2-AMC moiety was identified as fulfilling the McDermed model D2 agonist pharmacophoric criteria and was proposed as the D2 receptor-bound conformation. Structure-activity relationships gained from these studies have aided in the synthesis of D2 partial agonists of varying intrinsic activity levels. These agents should be of therapeutic value in treating disorders resulting from hypo- and hyperdopaminergic activity, without the side effects associated with complete D2 agonism or antagonism.

Cytotoxic effects of kainate ligands on HEK cell lines expressing recombinant kainate receptors.

Exposure of neurons either for prolonged periods of time or to high concentrations of excitatory amino acids (EAA), such as glutamate, results in neuronal death. Kainate also causes cell toxicity through the glutamate receptors. However, it is unclear whether the kainate receptor itself mediates any of the toxic responses. In the present study, HEK cells expressing the GluR6 +/- KA2 receptor subunit(s) were studied for their susceptibility to toxicity through the kainate receptor by kainate ligands. The natural ligand, glutamate, did not result in toxicity to the recombinant cell lines over that observed with the untransfected HEK cells, whereas kainate produced a 2-3-fold increase in LDH in both the HEK/GluR6 (ANOVA, P = 0.0001) and HEK/GluR6 + KA2 (ANOVA, P = 0.0002) cell lines following treatment with various dosages, but did not affect the HEK cells. Similar 2-3-fold increases in LDH activity were detected in both recombinant cell lines following treatment with 100 nM of SYM2081 ((2S,4R)-4-methylglutamic acid), a dose at which agonistic activity is elicited. The rank order potencies for eliciting toxicity are consistent with the previously reported EC50 values (SYM2081 > kainate > > > glutamate). Surprisingly, the kainate antagonist, NBQX, was the most toxic of the compounds tested although it had an affinity for the kainate receptor similar to glutamate. Treatment with as little as 10 nM elicited a dramatic increase in toxicity (6-10-fold) in the recombinant cell lines. At 1 microM, NBQX was significantly more toxic (Fisher PLSD, P < 0.05) than any of the other compounds tested. Thus, it appears that cell toxicity can be mediated via kainate receptor through two independent mechanisms: activation and blockage of the kainate receptor.