Oxytocin in Brattleboro rats: increased synthesis is contrasted by blunted intrahypothalamic release from supraoptic nucleus neurones

Rescue of Vasopressin Synthesis in Magnocellular Neurons of the Supraoptic Nucleus Normalises Acute Stress-Induced Adrenocorticotropin Secretion and Unmasks an Effect on Social Behaviour in Male Vasopressin-Deficient Brattleboro Rats

The relevance of vasopressin (AVP) of magnocellular origin to the regulation of the endocrine stress axis and related behaviour is still under discussion. We aimed to obtain deeper insight into this process. To rescue magnocellular AVP synthesis, a vasopressin-containing adeno-associated virus vector (AVP-AAV) was injected into the supraoptic nucleus (SON) of AVP-deficient Brattleboro rats (di/di). We compared +/+, di/di, and AVP-AAV treated di/di male rats. The AVP-AAV treatment rescued the AVP synthesis in the SON both morphologically and functionally. It also rescued the peak of adrenocorticotropin release triggered by immune and metabolic challenges without affecting corticosterone levels. The elevated corticotropin-releasing hormone receptor 1 mRNA levels in the anterior pituitary of di/di-rats were diminished by the AVP-AAV-treatment. The altered c-Fos synthesis in di/di-rats in response to a metabolic stressor was normalised by AVP-AAV in both the SON and medial amygdala (MeA), but not in the central and basolateral amygdala or lateral hypothalamus. In vitro electrophysiological recordings showed an AVP-induced inhibition of MeA neurons that was prevented by picrotoxin administration, supporting the possible regulatory role of AVP originating in the SON. A memory deficit in the novel object recognition test seen in di/di animals remained unaffected by AVP-AAV treatment. Interestingly, although di/di rats show intact social investigation and aggression, the SON AVP-AAV treatment resulted in an alteration of these social behaviours. AVP released from the magnocellular SON neurons may stimulate adrenocorticotropin secretion in response to defined stressors and might participate in the fine-tuning of social behaviour with a possible contribution from the MeA.

Oxytocin attenuates schizophrenia-like reduced sensorimotor gating in outbred and inbred rats in line with strain differences in CD38 gene expression

Prepulse inhibition (PPI) of the startle response is a measure of sensorimotor gating that is impaired in many clinical conditions, including schizophrenia. The inbred Roman high-avoidance (RHA) rats, compared to their low-avoidance (RLA) counterparts, show distinct schizophrenia-like phenotypes, such as spontaneous deficits in PPI accompanied by decreased medial prefrontal cortex (mPFC) activity and volume. Schizophrenia-like deficits are usually attenuated by antipsychotic drugs, but these drugs often produce severe side effects. In order to reduce these side effects, the neuropeptide oxytocin has been proposed as an alternative natural antipsychotic for schizophrenia. Here, we examined the effects of peripheral oxytocin administration (saline, 0.04, and 0.2 mg/kg) on PPI in the RHA vs. RLA rats, as well as in the outbred heterogeneous stock (HS) rats. Our results showed that oxytocin increased PPI in the HS rats and attenuated PPI deficits in the RHA rats, but it did not significantly affect PPI in the RLAs. To explore whether these divergent effects were associated with differences in oxytocinergic mechanisms, we analyzed gene expression of the oxytocin receptor (OXTR) and the regulator of oxytocin release (CD38) in the mPFC of the Roman rats. Consistent with the differential oxytocin effects on PPI (RHA > RLA), constitutive CD38 expression was reduced in the RHA rats compared to the RLAs, while oxytocin administration increased OXTR expression in both strains. Overall, the present work reveals that oxytocin administration shows antipsychotic-like effects on PPI in outbred and inbred rats, and it suggests that these effects may be related to basal differences in oxytocin-mediated mechanisms in the mPFC.

Exogenous central angiotensin fails to stimulate a sodium appetite in diabetes insipidus Brattleboro rats

We investigated the effect of central administration of angiotensin II (AngII) on a specific salt appetite (SSA) in homozygous diabetes insipidus Brattleboro (DI) rats because this stimulus induces such a response in all other rat strains. DI rats have a deficiency in the synthesis of arginine vasopressin (AVP) and a reduced content of pituitary oxytocin (OT). They are characterized also by polyuria, polydipsia, and they seldom ingest high concentrations of NaCl solutions. We also tested if the appetite can be influenced by neurohypophyseal hormones especially oxytocin (OT) because it inhibits SSA in other animals. DI rats and Long Evans (LE) controls were fed ad libitum and given a choice between water, and either 0.9% or 1.8% NaCl. The data showed a significant increase of daily spontaneous water intake in DI compared with LE rats. Both DI and LE ingested similar small spontaneous volumes of the isotonic NaCl solution, but DI rats drank significantly less hypertonic NaCl than the LE controls. I.c.v infusion of AngII induced significant sodium intake in LE rats, but only raised water intake in DI rats. When combined with i.c.v. Ang II, OVT enhanced salt intake in LE animals while AVP attenuated water intake in both groups of rats and blocked NaCl intake completely in LE rats. In conclusion, DI rats did not demonstrate a SSA in response to central administration of AngII, although the drinking of water was enhanced. In combination with i.c.v. AngII, AVP inhibits water drinking in both DI and LE rats. In the LE controls OT attenuates AngII-induced SSA but has no effect in DI rats.

Animal models for diabetes insipidus

The hormone arginine vasopressin (AVP) is a nonapeptide synthesized by hypothalamic magnocellular nuclei and secreted from the posterior pituitary into the bloodstream. It binds to AVP receptor 2 in the kidney to promote the insertion of aquaporin channels (AQP2) and antidiuretic responses. AVP secretion deficits produce central diabetes insipidus (CDI), while renal insensitivity to the antidiuretic effect of AVP causes nephrogenic diabetes insipidus (NDI). Hereditary and acquired forms of CDI and NDI generate hypotonic polyuria, polydipsia, hyperosmolality, and hypernatremia. The AVP mutant (Brattleboro) rat is the principal animal model of hereditary CDI, while neurohypophysectomy, pituitary stalk compression, hypophysectomy, and mediobasal hypothalamic lesions produce acquired CDI. In animals, hereditary NDI is mainly caused by mutations in AVP2R or AQP2 genes, while acquired NDI is most frequently induced by lithium. We report here on the determinants of the intake and excretion of water and mineral salts and on the different types of DI in humans. We then describe the hydromineral characteristics of these animal models and the responses observed after administration of hypertonic NaCl or when they are fed with low-sodium diets. Finally, we report on the effects of drugs such as AVP analogues and/or oxytocin, another neuropeptide that increases sodium excretion in animal models and humans with CDI, and sildenafil, a compound that increases the expression and function of AQP2 channels in animal models and humans with NDI.

Vasopressinergic control of stress-related behavior: studies in Brattleboro rats

Vasopressin, a nonapeptide, signaling both as hormone in the blood and neuromodulator/neurotransmitter in the brain is considered to be causally involved in the pathological changes underlying anxiety and depression. In the present review we summarize experimental data obtained with Brattleboro rats as a model of congenital vasopressin-deficiency to test the hypothesis that central vasopressin signaling contributes to anxiety- and depression-like behavior. Male, female and lactating rats were studied. We focused on the paraventricular nucleus of the hypothalamus (PVN) and the septum, two brain areas in which vasopressin is proposed to control the endocrine and behavioral stress response, respectively. The presented data support the hypothesis that the behavioral changes seen in these rats are brought about by an altered vasopressin signaling at the brain level. Whereas vasopressin synthesized and released within the hypothalamus is primarily involved in endocrine regulation, vasopressin signaling in other brain areas may contribute to anxiety- and depression-like behavioral parameters. Further studies in this context might focus particularly on the interplay between extra-hypothalamic brain areas such as the septum and the medial amygdala.

Vasopressin signaling at brain level controls stress hormone release: the vasopressin-deficient Brattleboro rat as a model

The nonapeptide arginine vasopressin (AVP) has long been suggested to play an important role as a secretagogue for triggering the activity of the endocrine stress response. Most recent studies employed mutant mice for analyzing the importance of AVP for endocrine regulation under stress. However, it is difficult to compare and draw overall conclusions from all these studies as mixing the genetic material from different mouse strains has consequences on the individual's stress response. Moreover, mice are not ideal subjects for several experimental procedures. Therefore, to get more insight, we used a rather old mutant rat model: the AVP-deficient Brattleboro rat. The present short review is aimed at providing the most interesting results of these studies within the last 8 years that allowed gaining new insights in the potential signal function of AVP in stress and endocrine regulation.