Enhanced Excitability of Vasopressin Neurons in the Supraoptic Nucleus Following 48 hr. Water Deprivation

Abstract

Arginine vasopressin (AVP) is a neurohypophyseal hormone released from the posterior pituitary by magnocellular neurosecretory cells (MNCs) located within the supraoptic (SON) and paraventricular (PVN) nuclei of the hypothalamus. AVP is involved in the regulation of body fluid homeostasis and influences blood pressure, plasma osmolality, and blood volume. The regulation of AVP release is critical for body fluid homeostasis and is dependent on the activity of MNCs. However the cellular mechanisms leading to changes in AVP release are not fully understood. Water deprivation (WD), a progressive physiological challenge, was used to examine changes in excitatory neurotransmission in MNCs using patch‐clamp electrophysiology. Male Sprague‐Dawley rats weighing 250 – 350 g received bilateral SON infusions of an adeno‐ associated virus (AAV) construct containing AVP gene promoter and GFP reporter using isoflurane anesthesia. Two weeks following AAV infusions, rats were water deprived for 24 hr. or 48 hrs. Controls were always allowed ad libitum access to water. For electrophysiological experiments, rats were anesthetized with isoflurane and coronal brain slices (300 μm) containing the SON were prepared using standard procedures. Whole‐cell patch clamp recordings were obtained from slices superfused with aCSF containing tetrodotoxin (TTX; 0.5 μM) and bicuculine methbromide (Bic; 10 μM). Following baseline mEPSCs recordings, individual NMDAR and AMPA mediated components were pharmacologically isolated with 5 minutes drug application of AMPA antagonist 6‐cyano‐7‐nitroquinoxaline‐2,3‐dione (CNQX; 10uM) or the NMDAR antagonist 2‐amino‐5‐phosphonopentanoic acid (APV; 50 uM). Parameters measured for mEPSCs were amplitude (pA), rise time (ms), decay time (ms), charge transfer, and mEPSC frequency. In GFP labeled SON neurons (i.e. confirmed vasopressinergic neurons), 24 (n = 9; P < 0.05) and 48 hr (n = 9; P <0.05) water deprivation increased the frequency of mEPSCs when compared to controls (n = 13). In cells from the 48 hr WD group, the increase in frequency was accompanied by a reduction in amplitude. The increase in frequency associated with both 24 and 48 hr WD was mediated by an increase of AMPA mediated EPSCs (p < 0.05) whereas the reduction in amplitude specific to 48 hr WD was mediated by a reduction in the amplitude of NMDA mediated EPSCs (p < 0.05). The results suggest that water deprivation may be associated with presynaptic mechanisms that increase AMPA excitatory events that after 48 hr may also be associated with postsynaptic mechanisms that reduce NMDA excitation. Further studies are necessary to understand the individual AMPA and NMDA receptor mechanisms contributing to increases in SON excitability and subsequent vasopressin release in response to water deprivation.Support or Funding InformationSupported by R01 HL119458‐04