Correlated pairings of single pre- and postsynaptic action potentials are sufficient to induce long-term plastic changes in developing hippocampal synapses (Bi & Poo, 2001). In contrast, previous work from our group has suggested that postsynaptic burst-spiking is necessary to induce long-term potentiation (LTP) during a pairing protocol in adult acute hippocampal slices (Pike et al. 1999). The aim of the present experiments was to investigate the developmental profile of the effectiveness of single postsynaptic spikes vs. postsynaptic bursting activity for induction of LTP. Hippocampal slices (300 mm) were made from Wistar rats (P12-P29), decapitated under isoflurane anaesthesia in accordance with UK Animals (Scientific Procedures) Act, 1986. Patch-clamp recordings were made from CA1 pyramidal neurones in submerged slices (25-28 °C). Two afferent Schaffer collateral inputs were alternately stimulated at 0.2 Hz and EPSP amplitude measured throughout the experiment.

Following a stable EPSP baseline of at least 12 min, one afferent input was paired firstly with a postsynaptic single spike, repeated 30 times, then 20 min later with a postsynaptic burst, again repeated 30 times.

Single spike pairing induced a significant potentiation of the test input 20 min after pairing in young animals (P12-15: 186 ± 21 % (Student’s t test, mean ± S.E.M.), n = 11, P < 0.05). The effectiveness of single-spike pairing to potentiate the test input was negatively correlated with developmental age (P12-29: n = 34, r = -0.417, P < 0.05). In contrast, burst-spike pairing significantly increased EPSP amplitude in older animals (P25-29: 168 ± 24 %, n = 8, P < 0.05) but caused no further increase in EPSP amplitude in younger animals (P12-15: 100 ± 5 %, n = 9). No significant correlations between developmental age and synaptic efficacy were observed in the control input.

These data suggest that there is a gradual developmental shift in the induction rules for synaptic plasticity in the hippocampus from young animals to adults.

This work was supported by the European Commission FV Grant QLG3-CT-1999-00192 and The Wellcome Trust.

All procedures accord with current UK legislation.