Abstract
Avian and mammalian brains have evolved independently from each other for about 300 million years. During that time, the hippocampal formation (HF) has diverged in morphology and cytoarchitecture, but seems to have conserved much of its function. It is therefore an open question how seemingly different neural organizations can generate the same function. A prominent feature of the mammalian hippocampus is that it generates different neural oscillations, including the gamma rhythm, which plays an important role in memory processing. In this study, we investigate whether the avian hippocampus also generates gamma oscillations, and whether similar pharmacological mechanisms are involved in this function. We investigated the existence of gamma oscillations in avian HF using in vitro electrophysiology in P0–P12 domestic chick (Gallus gallus domesticus) HF brain slices. Persistent gamma frequency oscillations were induced by the bath application of the cholinergic agonist carbachol, but not by kainate, a glutamate receptor agonist. Similar to other species, carbachol‐evoked gamma oscillations were sensitive to GABAA, AMPA/kainate and muscarinic (M1) receptor antagonism. Therefore, similar to mammalian species, muscarinic receptor‐activated avian HF gamma oscillations may arise via a pyramidal‐interneuron gamma (PING)‐based mechanism. Gamma oscillations are most prominent in the ventromedial area of the hippocampal slices, and gamma power is reduced more laterally and dorsally in the HF. We conclude that similar micro‐circuitry may exist in the avian and mammalian hippocampal formation, and this is likely to relate to the shared function of the two structures.
Highlights
Avian and mammalian brains have diverged for over 300 million years since their last common ancestor
Unlike for cortex, there is a general consensus that the hippocampal formations (HF) of both lineages are homologous to each other, as they both derive from the medial pallium (Striedter, 2016)
The questions we aim to address through in vitro electrophysiology are whether (i) the avian HF contains local circuits that generate and sustain gamma frequency oscillations, (ii) these HF micro-circuits exhibit pharmacological similarities to the gamma-generating circuits in the mammalian hippocampus, and (iii) there is a subregional differentiation in the generation of gamma rhythms in the avian HF
Summary
Avian and mammalian brains have diverged for over 300 million years since their last common ancestor The telencephalon of these two lineages is very differently organized (Jarvis et al, 2005), and there is an ongoing debate about whether the avian dorsoventricular ridge (DVR) is homologous to the mammalian neocortex, or whether they derive from different developmental regions (Striedter et al, 2014; Karten, 2015). They seem to exhibit convergent abilities (Emery, 2004, 2006; Clayton & Emery, 2015). The existence of such functionally similar, yet anatomically dissimilar structures between birds and mammals leads us to the question: How do such divergent structures perform such similar functions?
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