The mechanism of action of the cyclopyrrolone hypnotic drug zopiclone involves allosteric modulation of the GABAA receptor. Zopiclone displaces the binding of [3H]-flunitrazepam with an affinity of 28 nM, and enhances the binding of the channel blocker [35S]-TBPS. The binding of zopiclone, unlike that of hypnotic benzodiazepines, is not facilitated by GABA. Zopiclone does not distinguish between GABAA receptors containing different α-subunits (BZ1 and BZ2 phenotype). Studies with protein-modifying agents (eg diethylpyrocarbonate) and photoaffinity labelling suggest that cyclopyrrolones bind to a domain on the GABAA receptor different from the benzodiazepine binding domain. The consequence of this interaction with the GABAA receptor is to potentiate responses to GABA, as can be demonstrated by electrophysiological methods. Subchronic treatment of mice with high doses of zopiclone does not produce the changes in sensitivity of the GABAA receptor that are observed with hypnotic benzodiazepines.

Discriminant benzamide derivatives (DBD), the prototype of which is DO 710 i.e. N- [(1-propyl 2-pyrrolidinyl) methyl] 5-methyl sulfamoyl 2-methoxy benzamide, were compared to classical neuroleptics such as haloperidol in various behavioral and biochemical tests. Whereas the ID50 (or ED50) of haloperidol for antagonising various apomorphine-induced behavioral responses and producing catalepsy in rats were all around 0.1 mg/kg. DO 710 clearly distinguished catalepsy, HVA increase and apomorphine-induced licking and sniffing in rats (for which the ED50 (or ID50) were 13-54 mg/kg) from apomorphine-induced climbing, yawning and hypothermia (for which the ID50 were 1-2 mg/kg) (fig. 1, 2 and 4). Moreover, DO 710 and other DBD potentiated stereotypies in mice, whereas haloperidol and other classical neuroleptics did not (fig. 3). These features do not seem to be attributable to a heterogenous distribution of DBD in brain, since their ED50 for HVA increase and for inhibiting 3H-N-propylnorapomorphine binding in vivo did not differ in striatum and in limbic regions.

In in vitro binding experiments, DO 710 and other DBD discriminated two classes of 3H-domperidone binding sites in striatum, whereas only one component with a relatively low affinity for DBD could be detected in pituitary (fig.5-6, Table 3). 3H-DO 710 allowed charaterization of a D-2 site with a low affinity for DBD, which was fully sensitive to GTP regulation and present in pituitary and brain, and of a “D-4” site, preferred by DBD, which was little or not sensitive to GTP, present in brain and particularly enriched in olfactory bulb but absent in pituitary (fig. 7). 3H -azidosulpride (i.e. N-[(1-3H- propyl 2-pyrrolidinyl) methyl] 2-methoxy 4-azido 5-methylsulfamoyl benzamide), a photoactivable analog of 3H-DO 710, was used for irreversible labelling of dopamine recognition sites. When subjected to SDS-PAGE, receptor from striatum, pituitary and olfactory bulb co-migrated as a single band of 85 kDa (fig. 8).

These results may suggest the existence of two subclasses of dopamine D-2 receptor as targets for neuroleptic actions, one being preferred by DBD. The recognition subunits of these receptors have similar apparent molecular size.