Gonadotropin Inhibitory Hormone (GnIH) expressing neurons, through projections and interactions with gonadotropin releasing hormone (GnRH)-expressing and POMC-expressing neurons in the arcuate nucleus of the hypothalamus are suggested to serve as a fulcrum for neuroendocrine regulation of reproduction and appetite. Relatively little is known of the intrinsic membrane and extrinsic synaptic mechanisms regulating activity of GnIH neurons. Here, using the whole-cell patch clamp technique, the electrophysiological and pharmacological profile of GFP-labelled GnIH neurons has been investigated in rats in vitro.
Whole-cell recordings were obtained from 35 GnIH-GFP expressing neurons of the rat dorsomedial hypothalamus (DMH). Passive membrane properties included a mean resting membrane potential, firing rate and input resistance of 39.3 ± 0.7 mV, 0.94 ± 0.18 Hz and 1489 ± 98 mΩ, respectively. DMH GnIH neurons expressed: a 4-AP-sensitive transient outward rectification in 91% neurons; a Cs+-sensitive hyperpolarisation activated conductance in 57% of neurons; a Ni2+ sensitive T-type calcium conductance found in 74% of GnIH neurons. The AMPA receptor antagonist NBQX (10µM; n=5) and the GABA receptor antagonist bicuculine (20µM; n=5) revealed both GABAergic and glutamatergic synaptic connections to GnIH neurons.
Thyrotropin-releasing hormone (TRH; 400nM) induced depolarisation in all neurons tested (n=7) associated with a decrease in input resistance and reversal potential around 0mV, indicating activation of a non-selective cation conductance. Conversely, 5-HT (50µM) induced membrane hyperpolarisation 9 of 14 neurons, associated with a decrease in input resistance and reversal potential around -90mV consistent with activation of one or more K+ conductances. 5-HT-induced depolarisation in 5 neurons was associated with an increase in input resistance and reversal potential around –90 mV, suggesting block of one or more K+ conductances.
These data provide the first description of the intrinsic membrane properties and extrinsic synaptic/neuromodulatory mechanisms regulating excitability of GnIH neurons.