Associative Synaptic Potentials in the Piriform Cortex of the Isolated Guinea‐pig Brain In Vitro

Abstract

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.

Odor-Driven Activity in the Olfactory Cortex of an In Vitro Isolated Guinea Pig Whole Brain With Olfactory Epithelium

“…The present report represents the first attempt to perform a natural stimulation in an in vitro condition. Previous in vitro studies performed on the isolated brain preparation utilized LOT stimulation to mimic olfactory input to the piriform and entorhinal region (Biella and de Curtis 1995;Biella et al 2003;Gnatkowsky et al 2004;Uva et al 2003. Even though electrical tract stimulation is widely accepted to perform system physiology studies, LOT stimulation does not reproduce all the features of natural stimulation and exclude the contribution of the olfactory areas rostral to the site of stimulation.…”

The in vitro isolated whole guinea pig brain as a model to study epileptiform activity patterns

“…The most frequently explored areas in the IWB are the olfactory and limbic regions: activity have been recorded in the olfactory bulb (OB; Uva et al, 2006;Carriero et al, 2009), in the piriform cortex (PC; de Curtis, 1995, 2000;Biella et al, 1996), in the amygdala (Carriero et al, 2009), in the entorhinal and perirhinal cortex (EC and PRC;Biella et al, , 2002Gnatkovsky et al, 2004) and in the hippocampal region such as the CA1 region, the dentate gyrus (DG) and the subiculum (Boido et al, 2014;de Curtis, 2003, 2005). These regions can be easily reached in the IWB when the brain is positioned in the recording chamber with its ventral surface exposed (Fig.…”

“…FPs laminar profiles are recorded with silicon probes and are then analyzed by applying current-source-density (CSD) analysis (de Curtis et al, 1994;Biella and de Curtis, 1995), which is a useful method to distinguish locally generated components from far field components passively propagated in the tissue, and to characterize spatial and temporal arrangement of locally generated responses. Electrolytic lesions performed at the end of the experiments are used to reconstruct the position of the silicon probes tracks on histological sections.…”

Early exposure to urethane anesthesia: Effects on neuronal activity in the piriform cortex of the developing brain

“…To assess intracortical inhibition in the APC [6,7], paired stimulation was delivered with a 30-250-ms inter-pulse interval, and then an average of 10 responses was utilized to build field potential laminar profiles. We used CSD analysis to identify the pattern of current sources and sinks from the field potential profiles.…”

“…Animal brains were dissected out according to the standard procedure [5][6][7][8] /5% CO 2 gas mixture was artery perfused at 6.5 mL/min. Experiments were performed at 32 • C. The ventral view of the isolated guinea pig brain is shown in Fig.…”