Neuroanatomical distribution and function of the vasopressin V receptor in the rat brain deciphered using specific fluorescent ligands.

It is now accepted that vasopressin, through V/V receptors, centrally regulates cognitive functions such as memory, affiliation, stress, fear and depression. However, the respective roles of these receptor isoforms and their contribution to stress-related pathologies remain uncertain. The development of new therapeutic treatments requires a precise knowledge of the distribution of these receptors within the brain, which has been so far hampered by the lack of selective V markers. In the present study, we have determined the pharmacological properties of three new potent rat V fluorescent ligands and demonstrated that they constitute valuable tools for simultaneous visualization and activation of native V receptors in living rat brain tissue. Thus, d[Leu,Lys-Alexa 647)]VP (analogue 3), the compound with the best affinity-selectivity/fluorescence ratio for the V receptor emerged as the most promising. The rat brain regions most concerned by stress such as hippocampus, olfactory bulbs, cortex and amygdala display the highest V fluorescent labelling with analogue 3. In the hippocampus CA, V receptors are located on glutamatergic, not GABAergic neurones, and are absent from astrocytes. Using AVP-EGFP rats, we demonstrate the presence of V autoreceptors on AVP-secreting neurones not only in the hypothalamus, but also sparsely in the hippocampus. Finally, using both electrophysiology and visualization of ERK phosphorylation, we show analogue 3-induced activation of the V receptor in situ. This will help to analyse expression and functionality of V receptors in the brain and contribute to further explore the AVPergic circuitry in normal and pathological conditions.