Heterogeneity in the basic membrane properties of postnatal gonadotropin-releasing hormone neurons in the mouse

The electrophysiological characteristics of unmodified, postnatal gonadotro pin-releasing hormone (GnRH) neurons in the female mouse were studied using whole-cell recordings and single-cell RT-PCR methodology. The GnRH neurons of adult animals fired action potentials and exhibited distinguishable vol tage-current relationships in response to hyperpolarizing and depolarizing current injections. On the basis of their patterns of inward rectification, rebound depolarization, and ability to fire repetitively, GnRH neurons in intact adult females were categorized into four cell types (type I, 48% ;ty pe II, 36%; type III, 11%; type IV, 5%). The GnRH neurons of juvenile anima ls (15-22 d) exhibited passive membrane properties similar to those of adul t GnRH neurons, although only type I (61%) and type II (7%) cells were enco untered, in addition to a group of "silent-type" GnRH neurons (32%) that we re unable to fire action potentials. A massive, action potential-independen t tonic GABA input, signaling through the GABAA receptor, was present at al l ages. Afterdepolarization and afterhyperpolarization potentials (AHPs) we re observed after single action potentials in subpopulations of each GnRH n euron type. Tetrodotoxin (TTX)-independent calcium spikes, as well as AHPs, were encountered more frequently in juvenile GnRH neurons compared with ad ults. These observations demonstrate the existence of multiple layers of fu nctional heterogeneity in the firing properties of GnRH neurons. Together w ith pharmacological experiments, these findings suggest that potassium and calcium channels are expressed in a differential manner within the GnRH phe notype. This heterogeneity occurs in a development-specific manner and may underlie the functional maturation and diversity of this unique neuronal ph enotype.