AGE-DEPENDENT LONG-TERM POTENTIATION OF INHIBITORY SYNAPTIC TRANSMISSION IN RAT VISUAL-CORTEX

Long-term potentiation (LTP) of inhibitory synaptic transmission in ra t visual cortex was investigated using intracellular recording in slic e preparations. The inhibitory postsynaptic potential (IPSP) of layer V cells evoked monosynaptically by layer IV stimulation was recorded w hile excitatory postsynaptic potentials (EPSPs) were blocked by adding NMDA and non-NMDA receptor antagonists to the perfusate. Separate pre synaptic fiber groups were activated by two stimulating electrodes pla ced in layer IV. One electrode was used to test the effect of conditio ning stimulation; the other served as a control. Trains of high-freque ncy stimulation (50 Hz, 1 sec) were applied at an interval of 10 sec a s a conditioning stimulation. The LTP of IPSP induced by the condition ing stimulation had properties similar to those known for LTP of EPSP in hippocampal CA1: (1) it was specific to the conditioned pathway; (2 ) repetitive application of the conditioning stimulation saturated the LTP; (3) LTP was induced by conditioning stimulation at intensities h igher than a threshold; (4) weak conditioning stimulation to one pathw ay, which alone failed to induce LTP, induced associative LTP when str ong conditioning stimulation was simultaneously applied to the other p athway; (5) weak conditioning stimulation sometimes induced short-term potentiation. In contrast to these similarities, LTP of IPSP was indu ced by a wider range of stimulus frequencies than that capable of gene rating LTP of EPSPs in CA1. It was induced not only by a brief high-fr equency stimulation but also by an intermediate-frequency (2 Hz) stimu lation continued for 5 min. In addition, the LTP induction was not dep endent on postsynaptic membrane potential. Finally, the LTP of IPSP wa s more easily induced in developing than in mature rats. This age depe ndence suggests that LTP of IPSPs may contribute to the improvement of response selectivity of visual cortical cells through visual experien ce during postnatal development.