Functional Interactions within the Parahippocampal Region Revealed by Voltage 4 Sensitive Dye Imaging in the Isolated Guinea Pig Brain

29 The massive transfer of information from the neocortex to the entorhinal cortex (and 30 viceversa) is hindered by a powerful inhibitory control generated in the perirhinal cortex. In 31 vivo and in vitro experiments performed in rodents and cats support this conclusion, further 32 extended in the present study to the analysis of the interaction between the entorhinal cortex 33 and other parahippocampal areas, such as the postrhinal and the retrosplenial cortices. The 34 experiments were performed in the in vitro isolated guinea pig brain by a combined approach 35 based on electrophysiological recordings and fast imaging of optical signals generated by 36 voltage-sensitive dyes applied to the entire brain by arterial perfusion. 37 Local stimuli delivered in different portions of the perirhinal, postrhinal and 38 retrosplenial cortex evoked local responses that did not propagate to the entorhinal cortex. 39 Neither high frequency and low frequency patterned stimulation nor paired associative stimuli 40 facilitated the propagation of activity to the entorhinal region. Similar stimulations performed 41 during cholinergic neuromodulation with carbachol were also ineffective in overcoming the 42 inhibitory network that controls propagation to the entorhinal cortex. 43 The pharmacological inactivation of GABAergic transmission by local application of 44 bicuculline (1 mM) in area 36 of the perirhinal cortex facilitated the longitudinal (rostro45 caudal) propagation of activity into the perirhinal/postrhinal cortices, but did not cause 46 propagation into the entorhinal cortex. Bicuculline injection in both area 35 and medial 47 entorhinal cortex released the inhibitory control and allowed the propagation of the neural 48 activity to the entorhinal cortex. 49 These results demonstrate that, as for the perirhinal-entorhinal reciprocal interactions, 50 also the connections between the postrhinal/retrosplenial cortices and the entorhinal region 51 are subject to a powerfull inhibitory control. 52