The value of the pre-operated rat in the bioassay of vasopressin

Structure-based design of a novel third-generation antipsychotic drug lead with potential antidepressant properties

Partial agonist activity at the dopamine D₂ receptor (DRD2) is a key feature of third-generation antipsychotics (TGAs). However, TGAs also act as antagonists or weak partial agonists to the serotonin (5-hydroxytryptamine; 5-HT) 2A receptor (5-HT_2AR). Here we present the crystal structures of aripiprazole- and cariprazine-bound human 5-HT_2AR. Both TGAs adopt an unexpected 'upside-down' pose in the 5-HT_2AR binding pocket, with secondary pharmacophores inserted in a similar way to a 'bolt'. This insight into the binding modes of TGAs offered a structural mechanism underlying their varied partial efficacies at 5-HT_2AR and DRD2. These structures enabled the design of a partial agonist at DRD2/3 and 5-HT_1AR with negligible 5-HT_2AR binding that displayed potent antipsychotic-like activity without motor side effects in mice. This TGA lead also had antidepressant-like effects and improved cognitive performance in mouse models via 5-HT_1AR. This work indicates that 5-HT_2AR affinity is a dispensable contributor to the therapeutic actions of TGAs.

Factors influencing the sensitivity of the rat to vasopressin

1. Indirect evidence suggests that the concentration of arginine vasopressin in the plasma of normally hydrated man is about 1 mu-u./ml., but this is usually considered to be below the limit of sensitivity of the standard assay preparation, the water-loaded Wistar rat under ethanol anaesthesia.2. It was found that there was a surprising variation in sensitivity to vasopressin between batches of Wistar rats, and that other varieties of rat (including those with diabetes insipidus) were no more sensitive.3. Three modifications of the standard assay procedure produced an increase in sensitivity:(a) using Wistar rats weighing 100-150 g, rather than larger animals;(b) commencing the assay shortly after surgery, i.e. as soon as the urine flow reached 25 mul./min;(c) infusing vasopressin intravenously (0.5-3 mu-u./min). By using modification (a) with either (b) or (c) it was possible to detect as little as 0.5 mu-u.4. With these modifications antidiuretic activity equivalent to 0.5-2.0 mu-u./ml. of arginine vasopressin was measured in nine samples of plasma from a normally hydrated subject.5. It is suggested that the frequent reports of enhanced sensitivity may have been due to the fortuitous use of a particularly sensitive batch of rats, or to a high endogenous secretion of vasopressin due to operative trauma.