The neurotrophins, nerve growth factor, brain-derived neurotrophic factor, neurotrophin-3 and neurotrophin-4/5, have--in addition to their known effects as neuronal survival factors--recently been found to modulate synaptic transmission in the rat hippocampus and neocortex. Using standard whole-cell patch-clamp recordings, we have now investigated the acute effects of brain-derived neurotrophic factor and neurotrophin-4/5 on unitary (i.e. single cell activated) glutamatergic synaptic connections in microcultures of postnatal rat hippocampal neurons. We show that, in approximately 30% of the cells, glutamatergic synaptic transmission is enhanced to 170 +/- 52% (neurotrophin-4/5, 100 ng/ml) and 143 +/- 35% (brain-derived neurotrophic factor, 100 ng/ml) of control values, respectively. The enhancement is abolished in the presence of the specific Trk tyrosine kinase inhibitor k252a (200 nM). Depending on the particular cell investigated, the enhancement consisted of transient and sustained components in varying quantities. A minority of neurons (10%) showed a depression of glutamatergic synaptic transmission to 64 +/- 14% (brain-derived neurotrophic factor) and 61 +/- 11% of control (neurotrophin-4/5). The enhancement of unitary glutamatergic synaptic transmission is mediated predominantly by presynaptic modifications, as is evident from (i) the concomitant decrease in paired-pulse facilitation, (ii) the concomitant increase in the variance of the evoked unitary synaptic currents and (iii) the enhanced miniature excitatory postsynaptic/autaptic current frequencies that could be observed in the absence of an effect on miniature excitatory postsynaptic/autaptic current amplitudes. Finally, we show that the successful enhancement of synaptic transmission by neurotrophin-4/5 critically depends on the degree of paired-pulse facilitation prior to the start of neurotrophin application, with autapses/synapses initially showing a higher degree of paired-pulse facilitation being enhanced more effectively. Taken together, these results suggest that the brain-derived neurotrophic factor- and neurotrophin-4/5-mediated enhancement of unitary glutamatergic synaptic transmission in hippocampal cultures results predominantly from a presynaptic modulation of transmitter release, and this modulation could participate in the neurotrophin-dependent modification of glutamatergic synaptic transmission in the hippocampus in situ.