Lasting changes in spontaneous multi-unit activity in the chick brain following passive avoidance training

Abstract

Day-old chicks were trained on a one-trial passive avoidance task by pecking at a small, shiny bead coated with either a bitter-tasting substance (methylanthranilate) or water. The undifferentiated spontaneous multi-unit activity recorded bilaterally from anaesthetized chicks 1–13 h after training on the above task exhibited a significant increase in the methylanthranilate-trained over water-control chicks within three structures of the right hemisphere: the hyperstriatum accessorium (47%, P < 0.05), the medial hyperstriatum ventrale (49.1%, P < 0.02) and the medial portion of the paleostriatum augmentatum (47.5%, P <0.02). Within the multi-unit record obtained from both groups of chicks there were periods of short duration (15–20ms) containing high-frequency (400–450 Hz) large-amplitude (⩾200μ V; 450 μV max peak-to-peak) spikes. As a consequence of training, there was a massive increase in the occurrence of these bursting epochs in the medial hyperstriatum ventrale of both the left (320%, P < 0.001) and right (350%, P < 0.001) hemispheres in methylanthranilate-trained compared to water-trained chicks. In addition, the mean number of spikes per burst at this site increased by 66% ( P < 0.001) with no change observed in any other structure sampled. The statistically significant increase in activity within the right hyperstriatum accessorium and medial paleostriatum augmentatum and the non-significant increase in these structures in the left hemisphere was produced almost entirely by tonic spiking. Enhanced spontaneous multi-unit activity recorded under anaesthesia following passive avoidance training in the chick is shown here to be a feature common to several medial forebrain structures. The magnitude of the elevation in bursting frequency and the degree of localization of this effect to the medial hyperstriatum ventrale of methylanthranilate trained chicks would appear to offer strong support to previous biochemical and morphological evidence implicating this structure in the process of memory consolidation for this task. The data reported here represent the first evidence of electrophysiological changes occurring as a consequence of passive avoidance training in the domestic chick.