Associative synaptic potentials in the piriform cortex of the isolated guinea-pig brain in vitro.

The involvement of local and remote associative fibres in the generation of piriform cortex synaptic potentials was investigated in the isolated guinea-pig brain maintained in vitro by arterial perfusion by implementing current source density analysis (CSD) on cortical field potential profiles. Previous hypotheses were verified using acute surgical isolation of piriform cortical areas to study different synaptic events separately. Stimulation of the lateral olfactory tract activated associative potentials throughout the piriform cortex. In the anterior piriform cortex, the current sinks responsible for the generation of associative potentials were located in the superficial portion of layer Ib and in layer III. In the posterior piriform cortex, two associative events were observed: an early sink located in the superficial part of layer Ib, followed by a sink in the deep part of the same layer. In the anterior piriform cortex, local associative synaptic potentials were separated from the component carried by long projective fibres by surgically isolating a small area of cortex monosynaptically activated by lateral olfactory tract stimulation. In this patch of lateral olfactory tract-connected anterior piriform cortex, local associative sinks were observed in the superficial Ib layer and in layer III. Monosynaptic activation of the isolated patch of anterior piriform cortex induced purely associative potentials throughout the piriform cortex. These potentials were mediated by the synaptic activation of apical dendrites in the superficial Ib layer and selectively abolished by severing the long associative fibres. The anterior piriform cortex layer III sink and the posterior piriform cortex deep Ib associative component were evoked by the activation of large population spikes in the monosynaptic anterior piriform cortex and the disynaptic posterior piriform cortex response respectively. These two sinks are presumably generated locally through a polysynaptic circuit, whose activation depends on the degree of cortical excitation. Olfactory signal processing in the guinea-pig piriform cortex during states of normal excitability is supported by the interactions between associative inputs impinging on the synapses located separately on the dendrites of pyramidal neurons. An increase in the synchronization of piriform cortex neuron discharge activates usually silent local circuit synapses.