Learning-induced modulation of the GABAB-mediated inhibitory synaptic transmission: mechanisms and functional significance

Complex olfactory-discrimination (OD) learning results in a series of intrinsic and excitatory synaptic modifications in piriform cortex pyramidal neurons that enhance the circuit excitability. Such overexcitation must be balanced to prevent runway activity while maintaining the efficient ability to store memories. We showed previously that OD learning is accompanied by enhancement of the GABA(A)-mediated inhibition. Here we show that GABA(B)-mediated inhibition is also enhanced after learning and study the mechanism underlying such enhancement and explore its functional role. We show that presynaptic, GABA(B)-mediated synaptic inhibition is enhanced after learning. In contrast, the population-average postsynaptic GABA(B)-mediated synaptic inhibition is unchanged, but its standard deviation is enhanced. Learning-induced reduction in paired pulse facilitation in the glutamatergic synapses interconnecting pyramidal neurons was abolished by application of the GABA(B) antagonist CGP55845 but not by blocking G proteingated inwardly rectifying potassium channels only, indicating enhanced suppression of excitatory synaptic release via presynaptic GABA(B)-receptor activation. In addition, the correlation between the strengths of the early (GABA(A)-mediated) and late (GABA(B)-mediated) synaptic inhibition was much stronger for each particular neuron after learning. Consequently, GABA(B) -mediated inhibition was also more efficient in controlling epileptic-like activity induced by blocking GABA(A) receptors. We suggest that complex OD learning is accompanied by enhancement of the GABA(B) -mediated inhibition that enables the cortical network to store memories, while preventing uncontrolled activity.