Neurotrophin-3 potentiates excitatory GABAergic synaptic transmission in cultured developing hypothalamic neurones of the rat

1. Neurotrophin-3 (NT-3) supports the survival and differentiation of neuro nes in the central and peripheral nervous systems through a number of mecha nisms that occur in a matter of hours or days. NT-3 may also have a more ra pid mode of action that influences synaptic activity in mature neurones. In the present study the effect of NT-3 on developing GABAergic synapses was investigated in 3- to 7-day-old cultures of rat hypothalamic neurones with whole-cell patch-clamp recording. 2. NT-3 induced a substantial dose-dependent potentiation of the frequency of spontaneous postsynaptic currents (sPSCs; 160 %) in developing neurones during a period when GABA evoked inward (depolarizing) current, as determin ed with gramicidin-perforated patch recordings. The NT-3 effect was long la sting; continued enhancement was found > 30 min after NT-3 wash-out. NT-3 e voked a substantial 202 % increase in total GABA-mediated inward current, m easured as the time-current integral. Action potential frequency was also i ncreased by NT-3 (to 220 %). 3. The frequency of GABA-mediated miniature post-synaptic currents in devel oping neurones in the presence of tetrodotoxin was potentiated (to 140 %) b y NT-3 with no change in the mean amplitude, suggesting a presynaptic locus of the effect. 4. In striking contrast to immature neurones, when more mature neurones wer e studied, NT-3 did not enhance the frequency of GABA-mediated spontaneous postsynaptic currents (sPSCs), but instead evoked a slight (16%) decrease. The frequency of miniature post synaptic currents was also slightly decreas ed (16%) by the NT-3, with no change in amplitude. These results were recor ded during a later period of neuronal maturity when GABA mould evoke outwar d (hyperpolarizing) currents. NT-3 had no effect on the mean amplitude of G ABA-evoked postsynaptic currents in either developing or mature neurones. 5. Intracellular application of K252a, a non-selective tyrosine kinase inhi bitor, did not block the NT-3 effect postsynaptically. In contrast, bath ap plication of K252a prevented the enhancement of sPSCs by NT-3, consistent w ith NT-3 acting through presynaptic induction of tyrosine kinase. Decreasin g extracellular calcium with BAPTA or inhibiting calcium channels with Cd2 blocked the augmentation of sPSC frequency by NT-3, suggesting that an inc rease of calcium entry may be required for the facilitation of NT-3. 6. Together, our results suggest NT-3 enhances GABA release during the deve lopmental period when GABA is depolarizing and calcium elevating, but not l ater when GABA is inhibitory, suggesting that one mechanism through which N T-3 may influence neuronal development is via presynaptic potentiation of G ABA excitation.