**2. Material and methods**

148 Neuroendocrinology and Behavior

important hormone to elaborate urines largely hyperosmotic to plasma. A comparative study of water controlling behavior was done between rat laboratory and *Meriones shawi* demonstrated that the level of AVP is 4-fold greater than in dehydrated rats [11]. AVP levels are highly dependent on the state of hydration and correlate with urinary osmolality [12].

AVP or antidiuretic hormone (ADH), is known to be primarily involved in water absorption in the distal nephron of the kidney in mammals. This peptide is synthesized in the soma of hypothalamic magnocellular neurosecretory cells (MNCSs) located in supraoptic (SON) and paraventricular (PVN) nuclei. After water deprivation the axons MNCs project to the neurohypophysis, where Ca2+ dependent exocytosis in their nerve terminals causes the release of AVP in blood circulation. The small peptide is secreted by the neurohypophysis in response to increases in plasma osmolality. AVP effects on the renal tubule are mediated by hormone binding to V2 type basolateral receptors coupled trough Gs to adenylyl cyclase and activation of the cyclic adenosine monophosphate - Protein kinase A (cAMP-PKA ) cascade [13]. The hydroosmotic action causes a dramatic increase in the osmotic water permeability of connecting cells, principal cells and inner medullary collecting duct cells.

The result is highly concentrated urines produced in response to water restriction.

The success of rodent to survive harsh environment condition goes back to several years ago. However, these animals are faced to substantial anthropogenic threats due to the introduction of heavy metals in environment in the last decades. Cadmium (Cd), a nonessential heavy metal, is widely distributed in the environment due to its use in primary metal industries and phosphate fertilizers [15, 16]. Food and cigarette smoke are the biggest sources of Cd exposure for the general population [17]. In humans, Cd exposure leads to a variety of adverse effects and contributes to the development of serious pathological conditions [18-19] linked to enhanced aging process as well as cancer [20-21]. Cd produces also neurotoxicity with a complex pathology [22-23]. In animals, Cd was shown to be toxic to all tissues such as liver [24], reproductive organs including the placenta, testis and ovaries [17, 25]. Several studies in some industrial sites in Tunisia showed that some habitats of *Meriones shawi* became contaminated by Cd [26-27] *Meriones shawi* have accumulated cadmium on different organs particularly on kidney and liver. It has been reported that kidneys, which play a major role in hydro-mineral maintenance, are considered to be the organ that is most sensitive to Cd, depending on exposure dose, time and administration route [28]. Several studies indicated that the main critical effect of cadmium exposure is kidney dysfunction. Excretion of low molecular weight proteins is characteristic of damage to the proximal tubules of the kidney. The increased excretion of low-molecular weight proteins in the urine is a result of proximal tubular cell damage [29]. This raises the possibility that body fluid homeostasis and vasopressinergic system could be subtly disrupted by Cd exposure. In this study, we were interested to determine whether Cd naturally incorporated in food would alter the water balance in *Meriones shawi* who appears to show a remarkable physiology flexibility of water regulation in both time and space. Effects of Cd exposure upon the water-conserving abilities of this specie were assessed through measurements of water metabolism (total body water (TBW), water influx (Fin), water efflux (Fout) and water turnover rates (WTR) under differing water availabilities. Water fluxes were determined by direct analysis following the principles
