Figure 2.

STDP is biased toward t-LTP at sensory synapses onto adult projection neurons. A, B, Representative plots of STDP (expressed as a percentage of baseline EPSC amplitude before pairing, where 100% indicates no change in synaptic efficacy) versus time, showing the effects of Pre → Post (A) and Post → Pre (B) pairing protocols (bar) on the efficacy of monosynaptic primary afferent input. C, D, Neither presynaptic (C) nor postsynaptic (D) stimulation alone (30 stimuli at 0.2 Hz) significantly modulated EPSC amplitude, demonstrating that correlated primary afferent and projection neuron activity was required to modify synaptic strength. E, Across the population of naive lamina I spino-parabrachial neurons sampled, highly correlated Pre → Post pairings (Δt = −10 ms) produced significant t-LTP at primary afferent synapses (n = 5–7 in each group, ***p < 0.001 compared with all other groups; one-way ANOVA with Tukey's multiple-comparisons test). In contrast, Post → Pre pairings (Δt = +10 ms) failed to significantly influence EPSC amplitude at afferent synapses onto projection neurons (p > 0.05 vs control groups).