Interrogating the contribution of intermediate conductance calcium-dependent K+ channels to hypothalamic vasopressin neuronal excitability in health and disease states.

Erin Shook, Matthew Kirchner and Javier E Stern
Neuroscience Institute; Center for Neuroinflammation and Cardiometabolic Diseases
Georgia State University
Atlanta GA USA

ABSTRACT

Vasopressin (VP) neurosecretory neurons of the hypothalamus are critical regulators of renal water
retention and vascular tone. VP neurons undergo detrimental plastic changes in cardiovascular diseases
such as heart failure (HF), resulting in hyperexcitability and thus altered fluid/electrolyte balance in this
condition. Hyperexcitability is caused in part by changes in intrinsic mechanisms, including the slow
afterhyperpolarization (sAHP), a phenomenon underlain by a calcium-dependent K⁺ current (/_sAHP). The
sAHP is activated by Ca²⁺ and results in an efflux of K⁺ from the cell, hyperpolarizing it, and slowing firing.
Importantly, we found that the sAHP is greatly inhibited in VP neurons from HF rats, contributing to
increased neuronal excitability in this condition. While the features of the sAHP are well characterized,
the identification of the channel underlying the /_sAHP is unknown. Combining patch clamp
electrophysiology and pharmacology in transgenic eGFP-VP rats, we investigated Intermediate
conductance Ca²⁺-dependent K⁺ (IK) channels as a potential candidate responsible for carrying the /_sAHP.
We generated and measured /sAHPs in voltage clamp via fixed steps of twenty spikes at twenty Hz and
measured the resulting tail current at the end of the pulse, representing the sAHP. To account for
rundown of the current over the recording period and for the pharmacological blocker’s slow
mechanism of action, we measured a /sAHP every 60 seconds for at least 15 minutes. Bath application of
TRAM-34 (2 mM), a specific IK channel blocker, failed to inhibit /sAHP peak amplitude or area (p > 0.05;
n=36 SON neurons). Taken together, our studies indicate that IK channels do not contribute to the /sAHP
in VP neurons, despite robust IK expression in the SON. Future studies will aim at targeting other
potential K+ channels contribution to blunted sAHPs and increased excitability of VP neurons during HF.