Learning-related Fos-like immunoreactivity in the chick brain: Time-courseand co-localization with GABA and parvalbumin

Previous work has shown that, after domestic chicks have learned the charac teristics of an object (visual imprinting), there is a learning-related inc rease in the numerical density of Fos-immunopositive neurons in the interme diate and medial part of the hyperstriatum ventrale, a forebrain region tha t is a site of recognition memory for the imprinted object. The present stu dy describes the time-course of this effect and has used double-labelling i mmunocytochemistry to identify neuronal types in which the effect occurs. C hicks were trained by exposure for 1 h to an imprinting (training) stimulus and then given a preference test to determine the strength of imprinting ( i.e. of learning). Strongly imprinted chicks were killed 2, 2.5, 3, 3.5 or 4 h (12 chicks in each group) after the start of training and a further gro up of 12 chicks remained untrained. Sections from the chicks' brains were s tained for Fos-like immunoreactivity, and the numerical density of Fos-posi tive nuclei in the intermediate and medial part of the hyperstriatum ventra le was counted. Relative to untrained chicks, there was a 60% increase in t he number of Fos-positive nuclei in the intermediate and medial part of the hyperstriatum ventrale 2 h after the start of training (P = 0.02), but not at any other time. Sections from 10 trained chicks, two killed at each of the above times after training, and from two untrained chicks were stained with anti-Fos antibody as before and also with an antibody against GABA. Ap proximately 95% of the Fos-positive neurons in the intermediate and medial part of the hyperstriatum ventrale were also immunopositive for GABA. In ne urons immunopositive for GABA, there were significantly (P = 0.02) more Pos -positive nuclei in the intermediate and medial part of the hyperstriatum v entrale 2 h after the start of training than in untrained chicks. Five chic ks killed 2 h after training and five untrained chicks yielded sections for the next experiment; sections were double labelled for (i) Fos and (ii) ei ther Calbindin-D-28k or parvalbumin. Training gave rise to a significant (P = 0.017) increase in numerical density of Fos-positive nuclei of neurons t hat were immunonegative for Calbindin-D-28k. This increase occurred in neur ons that were immunopositive for parvalbumin. The use of alternative antibo dies for GABA, Calbindin-D-28k, and parvalbumin in trained and untrained ch icks confirmed the double-staining pattern observed in the quantitative exp eriments. The results demonstrate that the learning-related increase in Fos-like immu noreactivity following training is transitory and have localized the increa se to a population of neurons immunopositive for GABA and parvalbumin, but not Calbindin-D-28k. (C) 1999 IBRO. Published by Elsevier Science Ltd.