Elsevier

Neuroscience

Volume 119, Issue 2, 27 June 2003, Pages 577-588
Neuroscience

Neuropharmacology
Positional firing properties of postrhinal cortex neurons

https://doi.org/10.1016/S0306-4522(03)00160-XGet rights and content

Abstract

Hippocampal cell firing in awake, behaving rats is often spatially selective, and such cells have been called place cells. Similar spatial correlates have also been described for neurons in the medial entorhinal and perirhinal cortices. All three regions receive sensory associational input from postrhinal cortex, which, in turn, is heavily interconnected with visuospatial neocortical regions. The spatial selectivity of postrhinal cells, however, has never been examined. Here, we report the activity of neurons in postrhinal cortex of freely moving rats performing a spatial task on a four-arm radial maze. Data are also reported for visual association cortex neurons.

The four-arm radial maze was defined by multisensory cues on the surfaces of the maze arms (proximal) and complex visual cues at the surround (distal). On each recording day, rats were run in three conditions: baseline, double cue rotation (proximal +90°; distal −90°), and baseline. In this task, hippocampal place field activity is robust and can be controlled by proximal or distal cues. The majority of postrhinal neurons (64%) exhibited positional correlates during performance on the task; however, characteristics of these postrhinal cells were substantially different from those previously described for hippocampal place cells. Most postrhinal cells with firing fields exhibited split or multiple subfields (93%). Unlike hippocampal place fields, the large majority of postrhinal firing fields (84%) adopted new spatial correlates when experimental cues were rotated, but did so neither predictably nor concordantly.

This is the first report of positional firing correlates in the postrhinal cortex. The data are consistent with the idea that postrhinal cortex participates in visuospatial functions by monitoring changes in environmental stimuli rather than encoding stable spatial cues. Thus, postrhinal neurons appear to participate in higher-level perceptual functions rather than mnemonic functions. We propose that the response properties of postrhinal neurons represent an early step in a spatial pathway that culminates in the specific and stable place fields of the hippocampus.

Section snippets

Subjects

Subjects were four male Long-Evans rats (290–360 g, 2–5 months old) from Charles River Laboratory, Portage, MI, USA. Animals were housed individually and kept on a 12-h light/dark cycle. Animals had ad libitum access to food, but water access other than behavioral rewards during training was limited to 1 h per day for the duration of the experiment. All methods for handling research animals were according to approved IACUC and AAALAC guidelines. Research procedures were designed to minimize

Histology and behavior

Implant placement was evaluated for each subject in Nissl-stained coronal sections. In three out of the four rats, implants were located in anterior POR, which was the intended anatomical location (Fig. 2A, C, left). In the fourth rat, however, the electrode was located in an area slightly anterior to the POR, in lateral visual association cortex (VISL) according to Swanson (Swanson, 1998), (Fig. 2B, C, right). Because of this difference in placement, the recordings from the fourth rat

Discussion

The POR is reciprocally connected with the medial entorhinal cortex (Burwell and Amaral, 1998b), the hippocampus, and the subiculum (Naber et al., 1997). Cells with spatial firing correlates have been identified in each of these regions. This study examined whether cells in the POR also display spatial firing properties. Because the POR is interconnected with the hippocampus, it might be expected that POR cells exhibiting positional firing correlates would show the same properties as place

Acknowledgements

This research was supported by an NSF Career Award (IBN 9875792) to RDB and a Brown University Undergraduate Research and Teaching Fellowship to D.M.H.

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