Three vasopressin receptors, all belonging to the class I G-protein coupled receptors, are known. The V1a receptor is expressed in the brain, liver, vascular smooth muscle, lung, uterus and testis, the V1b or V3 receptor is expressed in the brain and pituitary gland, the V2 receptor is expressed in the kidney where it regulates water reabsorption and mediates the antidiuretic effects of vasopressin (Robben, et al.)1. Compounds with activity at the V2 receptor may therefore cause side-effects on blood homeostasis.
The oxytocin receptor is related to the Vasopressin receptor family and mediates the effects of the neurohormone oxytocin in the brain and the periphery. Oxytocin is believed to have central anxiolytic effects (Neumann)2. Central oxytocin receptor antagonism might therefore lead to anxiogenic effects, which are regarded as undesired side-effects.
In the brain vasopressin acts as a neuromodulator and is elevated in the amygdala during stress (Ebner, et al.)3. It is known that stressful life events can trigger major depression and anxiety (Kendler, et al.)4 and that both have very high comorbidity, with anxiety often preceding major depression (Regier, et al.)5. The V1a receptor is extensively expressed in the brain and particularly in limbic areas like the amygdala, lateral septum and hippocampus which are playing an important role in the regulation of anxiety. Indeed V1a knock-out mice show a reduction in anxious behavior in the plus-maze, open field and light-dark box (Bielsky, et al.)6. The down-regulation of the V1a receptor using antisense oligonucleotide injection in the septum also causes a reduction in anxious behavior (Landgraf, et al.)7. Vasopressin or the V1a receptor are also implicated in other neuropsychological disorders: genetic studies linked sequence polymorphism in the promoter of the human V1a receptor to autistic spectrum disorders (Yirmiya, et al.)8, intranasal administration of vasopressin was shown to influence aggression in human males (Thompson, et al.)9 and vasopressin levels were found to be elevated in schizophrenic patients (Raskind, et al.)10 and patients with obsessive-compulsive disorder (Altemus, et al.)11.
Autistic Spectrum Disorders (ASD) are a clinically heterogeneous condition characterized by defects in socialization and language. ASD include a wide range of abnormalities including a genuine incapacity to organize affective relations, behavioral anomalies in reciprocal social interactions, verbal and non-verbal communication, limited interest in the surrounding environment associated with stereotyped movements and repetitive plays (Bourreau et al, 2009)12. Research to date indicates that a genetic predisposition may be involved, but also environmental factors have to be taken into consideration (Bourgeron, 2009)13. There is at present no efficient biological/pharmaceutical treatment to ASD.
The suprachiasmatic nucleus (SCN) is the endogenous clock of the body regulating circadian rhythmicity and is known to be rich in vasopressin neurons (Kalsbeek et al. 2010)14, producing and releasing vasopressin with a 24 h circadian rhythm (Schwartz et al. 1983)15. A major regulatory effect of vasopressin on circadian rhythm could not be demonstrated by the prior art. The Brattleboro rat, a rat strain naturally lacking vasopressin due to a point mutation, has no obvious defect in its circadian rhythm (Groblewski et al. 1981)16. Injection of vasopressin directly in the hamster SCN had no effect on circadian phase shift (Albers et al. 1984)17. In contrast, the vasopressin receptors were shown to modulate the circadian clock in a more subtle way. Yamaguchi et al (2013)18 demonstrated that V1a knock-out and V1a/V1b double knock-out mice show faster reentrainment to the new light/dark cycle after a circadian phase advance or a phase delay, an experiment mimicking jet-lag in humans. The same result was obtained after chronic administration of a mixture of V1a and V1b small molecule antagonists through a minipump directly on the SCN.