European Journal zyxwvutsrq of Neuroscience, Vol. 9, pp. 1593-1602, 1997 zyxwvut 0 European Neuroscience Association z Learning-induced Changes in Rat Piriform Cortex Activity Mapped Using Multisite Recording With Voltage Sensitive Dye zyxwv Philippe Litaudon’ , Anne-Marie Mouly2, Regina Sullivan3, Remi Gervais2 and Martine Cattarelli4 ’Neurosciences et Systemes Sensoriels, CNRS, UCB Lyon I, 43 Boulevard du 11 Novembre 1918, 69622 Villeurbanne, France, *I%, CNRS UPR 9075, UCB Lyon I, 8 Avenue Rockfeller, 69373 Lyon, France, 3Department of Zoology, University of Oklahoma, 73019, USA and 41ESGI-CNRS UPR 9054, INRA, 17 rue Sully, BP 1540, 21034, Dijon Cedex, France zyxw Keywords: discriminative learning, electrical stimulation, olfactory cortex, optical recording, plasticity Abstract The piriform cortex (PCx) has a potential role in storage and recall of olfactory information. This study is a first extensive investigation of the spatiotemporal distribution of activity in the PCx induced by learned sensory inputs following conditioning. In a conditioned group, rats chronically implanted with four electrodes in the olfactory bulb were trained to associate the electrical stimulation of a given bulbar electrode with a positive reinforcement, while stimulation of a different electrode predicted a negative reinforcement. In a familiarized group, rats received the same protocol of daily electrical stimulation with no associated reinforcement. At the end of the conditioning or familiarization episode, activity evoked in the PCx was optically mapped using a 144 photodiode array. In the anaesthetized rats, PCx maps were recorded in response to stimulation of each of the four bulbar electrodes using either high (0.5-1 mA) or low (0.1 mA) test current intensities. Low intensity stimulation revealed that conditioning selectively enhanced the probability of occurrence of a signal composed of a single late (5673 ms) component which occurred almost simultaneously on a large PCx area. In the conditioned group, high intensity stimulation through either of the four electrodes revealed a potentiation of the early (17- 30 ms) disynaptic component of the PCx response in the most posterior part of the PCx as well as a homogeneous increase of the late (39-52 ms) component spread over the PCx areas. These data suggest that learning induces synaptic changes at different nodes of the PCx circuitry. Introduction zyxwvutsrqp The piriform cortex (PCx) receives olfactory afferent information from the olfactory bulb (OB) via the lateral olfactory tract (LOT) (Heimer, 1968). The laminar organization of the PCx (see Haberly, 1985, 1990, for review) consists of three main layers: layer I contains the dendrites of pyramidal cells and fibre systems. zyxwvuts This layer is subdivided into layer Ia, that receives afferent fibres from the LOT, and layer zyxwvutsrq Ib, that receives association fibres from other parts of the PCx and from other olfactory cortical areas. Layers I1 and 111 respectively contain superficial and deep pyramidal cell bodies. F’yramidal cells extend long intrinsic axon collaterals reciprocally interconnecting rostra1 and caudal parts of the PCx. This organization, added to the absence of a point-to-point topographical ordering of the OB projections to the PCx, is reflected at the functional level by the fact that in the PCx, an odour is represented by the activation of a very large number of spatially distributed cells. For example, exposure to an odorant induces uniform 2DG uptake across the cortex (Cattarelli et al., 1988). Several lines of evidence suggest that the PCx is also involved in olfactory learning. First, in vitro preparations (Jung et al., 1990; Kanter and Haberly, 1990, 1993; Jung and Larson, 1994), can exhibit long-term potentiation (LTP). Second, LTP was shown to also occur in vivo, in the rat PCx either during olfactory learning (Roman et al., 1987, 1993) or following repeated treatments of high frequency stimulation (Stripling et al. 1988, 1991). Finally, Haberly (1985) suggested that PCx has several features which make it an attractive system for modelling associative content addressable memory. Given these results, there has been increasing interest in the PCx for the developmentof models of learning and memory (Skarda and Freeman, 1987; Wilson and Bower, 1988; Lynch and Granger, 1989; Hasselmo et al., 1992). We decided to investigate further the role of the PCx in olfactory memory and more precisely to look for long-term changes induced by an olfactory learning in the in vivo rat PCx. Taking into account the highly distributed character of odour representation at this level, and the large size of the PCx area, we chose to record PCx activity using multisite optical recording with a voltage sensitive dye, which allows the spatiotemporal mapping of activity of the whole PCx area. Using this technique, we showed in previous experiments that electrical stimulation of the OB produces reliable optically recorded responses in the PCx (Litaudon and Cattarelli, 1995, 1996). The Correspondence to: P. Litaudon, as above Received 4 December 1996, revised 25 February 1997, accepted 3 March I997