The aim of this research is to characterise the way in which adult, self-renewing, neural progenitor cells (NPCs) acquire their electrical characteristics as they mature into active entities capable of forming neural networks. We have investigated the passive and active electrical properties of isolated, cloned NPCs from the adult rat hippocampus using whole-cell patch-clamp recording techniques. The expression of ionic channels and receptors were studied in both undifferentiated clonal cultures of NPCs and differentiated NPCs that were exposed to combinations of extracellular factors for 7Ð10 days (Chipperfield et al. 2002).

Undifferentiated NPCs had a resting membrane potential (E_m) of -89.3 ± 0.9 mV (mean ± S.E.M.) (n = 24), which changed by 57 mV/10-fold change in external K⁺ concentration similar to that predicted by the Nernst equation for a K⁺-sensitive electrode. Undifferentiated NPCs were electrically inexcitable as depolarizing current pulses failed to elicit action potentials. Under voltage-clamp conditions, voltage ramps from -120 to +20 mV reversed at -90 mV and exhibited marked inward rectification at negative membrane potentials, which was inhibited by bath application of 0.1 mM Ba²⁺. Inward K⁺ currents evoked by hyperpolarizing voltage steps were blocked in a time- and voltage-dependent manner by external Ba²⁺ but were insensitive to 4-aminopyridine and tetraethylammonium ions (TEA).

In contrast, differentiated NPCs were electrically excitable and exhibited phasic action potential firing in response to depolarizing current pulses. A time-dependent voltage sag was observed in response to hyperpolarizing current pulses. The E_m of differentiated NPCs was -57 ± 1.3 mV (n = 26) and the relationship between E_m and external K⁺ concentration was fitted by the Goldman-Hodgkin-Katz voltage equation with P_Na/P_K = 0.08. The currentÐvoltage relationship of differentiated NPCs obtained in response to slow voltage ramps showed marked outward rectification and reversed at ~-60 mV. Voltage-clamped cells held at -100 mV exhibited a transient inward current followed by a sustained outward current in response to depolarizing voltage steps. The transient inward current was abolished in the presence of 300 nM tetrodotoxin (TTX) and the outward current was inhibited by external TEA (5 mM), indicating the presence of functional voltage-dependent TTX-sensitive Na⁺ and delayed rectifier K⁺ channels in differentiated NPCs.

The functional expression of receptors in undifferentiated NPCs was examined in response to brief focal application (²ge³ 100 mM) of various neurotransmitters. In approximately 70 % (32/45) of undifferentiated NPCs, ATP and benzoylbenzoyl-ATP (BzATP) evoked an inward current at negative potentials, whereas acetylcholine, noradrenaline, glutamate and GABA failed to elicit a response (n ²ge³ 12). ATP and BzATP also evoked a response in > 80 % of differentiated NPCs, suggesting that the activation of P2X₇ purinoceptors may play a significant role during differentiation of adult NPCs.