Behavioural Brain Research 123 (2001) 165 – 183 Research report Neurons in rat medial prefrontal cortex show anticipatory rate changes to predictable differential rewards in a spatial memory task Wayne E. Pratt, Sheri J.Y. Mizumori * Uniersity of Utah, Salt Lake City, UT 84112, USA Received 4 December 2000; received in revised form 26 February 2001; accepted 26 February 2001 Abstract The present study electrophysiologically examined the contribution of prelimbic and infralimbic neurons in the medial prefrontal cortex (mPFC) to integration of reward and spatial information while rats performed multiple memory trials on a differentially rewarded eight arm radial maze. Alternate arms consistently held one of two different reward amounts. Similar to previous examinations of the rat mPFC, few cells showed discrete place fields or altered firing during a delay period. The most common behavioral correlate was a change in neuronal firing rate prior to reward acquisition at arm ends. A small number of reward-related cells differentiated between high and low reward arms. The presence of neurons that anticipate expected reward consequences based on information about the spatial environment is consistent with the hypothesis that the mPFC is part of a neural system which merges spatial information with its motivational significance. © 2001 Elsevier Science B.V. All rights reserved. Keywords: Spatial navigation; Neural systems; Reward; Prefrontal cortex; Amygdala; Medial ventral striatum; Electrophysiology www.elsevier.com/locate/bbr 1. Introduction Since the seminal studies of Mogenson and co-work- ers, the medial ventral striatum (mVS) has been viewed as one potential region where traditionally defined memory structures such as the hippocampus and amyg- dala may impact motor structures via pallidal efferents (for review see Ref. [42]). Stimulation of either the hippocampus or amygdala of the rat invokes firing rate changes within the mVS, and chemical excitation of these regions cause behavioral changes that are mVS- dependent [43,47,75 – 77]. This suggests that perhaps one means for the hippocampus to impact behavior is via its projections (from the subiculum) to the mVS [35]. The hippocampus itself has been long implicated as an important structure for rat navigation, as its removal results in drastic impairments in spatial tasks (for review see Ref. [2]) and ‘‘place cells’’ (neurons which fire when a rat occupies a discrete location in its spatial environment) are found in the rat hippocampus [48,49,57]. However, despite the further defining of necessary sensory and mnemonic components that drive hippocampal representations of space, the precise way that these signals affect behavior remains to be determined. To begin to address this issue, Lavoie and Mizumori [29] recorded from mVS neurons in freely moving rats during performance of a win-shift navigation task, to determine what neuronal correlates might be found in a motor structure that receives afferent information from the hippocampus. Recordings revealed that the nucleus accumbens and surrounding striatum encoded not only spatial information, but also motivationally relevant information about the expectation and presence of re- ward within the environment. Furthermore, a subset of * Corresponding author. Address: Department of Psychology, Uni- versity of Washington, Box 351525, Seattle, WA 98195, USA. Tel.: +1-206-543-2699; fax: +1-206-685-3157.. E-mail address: mizumori@u.washington.edu (S.J.Y. Mizumori). 0166-4328/01/$ - see front matter © 2001 Elsevier Science B.V. All rights reserved. PII:S0166-4328(01)00204-2