Functional Differentiation along the Rostro-Caudal Axis of the Avian Hippocampal Formation

Introduction: Different functional domains can be identified along the longitudinal axis of the mammalian hippocampus. We have recently hypothesized that a similar functional gradient may exist along the longitudinal axis of the avian hippocampal formation (HF) as well. If the 2 gradients are homologous, we would expect the caudal HF to be more responsive to acute stress than the rostral HF. Methods: We restrained 8 adult Dekalb White hens in a bag for 30 min under red-light conditions and compared FOS-immunoreactive (FOS-ir) cell densities in different hippocampal subdivisions to control hens. Results: Although we could find no evidence of an activated stress response in the hypothalamic-pituitary-adrenal axis of the restrained birds, we did find a significant increase in FOS-ir cell densities in the rostral HF of the restrained birds compared to controls. Conclusion: We speculate that the HF response is not due to an acute stress response, but instead, it is related to the change in spatial context that was part of taking the birds and restraining them in a different room. We see no activation in the caudal HF. This would be consistent with our hypothesis that the longitudinal axis of the avian HF is homologous to the long axis of the mammalian hippocampus.