Correlation between electrophysiological and morphological characteristicsduring maturation of rat supraoptic neurons

The neurohypophysial peptides oxytocin (OT) and vasopressin (AVP) are well known for their role in reproductive functions and fluid balance regulation , respectively. During development, these peptides are thought to act as tr ophic factors on both peripheral and central structures. However, despite t his early developmental function, the maturation of their secreting neurons remains poorly investigated. In this study, we have characterized the elec trical and morphological characteristics displayed by OT and AVP supraoptic (SO) neurons between embryonic day 21 and postnatal day 20. Transient chan ges in passive membrane properties, correlated with a transient increase in the dendritic arborization, were observed at the beginning of the second p ostnatal week (PW2). The action potential matured mostly during PW1 and its threshold progressively hyperpolarized in parallel with the resting membra ne potential. During PW1, SO neurons displayed unique characteristics with a low-threshold Ca2+-dependent depolarizing potential and a prominent hyper polarization-activated current (I-h ). This latter is involved in a depolar izing sag during hyperpolarization and an after hyperpolarizing potential f ollowing a depolarization. During this period, maintaining E-Cl unchanged b y the use of gramicidin-perforated patch recordings revealed excitatory GAB Aergic potentials, that became inhibitory during PW2, whilst glutamatergic potential appeared. The electrical activity was very erratic in young neuro ns and progressively differentiated in the typical firing observed in matur e neurons (tonic and phasic for OT and AVP neurons, respectively) during PW 2-3. These results show that the development of electrical properties of SO neurons is correlated with the maturation of their dendritic arborization.