Why study bird brain?
A comparative approach gives us insights into how brains evolve. Birds and mammals share a common ancestor more than 300 million years in the past. Their forebrains are organized very differently, and although the hippocampus is agreed by all to be homologous in both lineages, its internal organization is again very different. Nevertheless, it performs very similar functions. Therefore, by investigating the micro-circuitry of the avian hippocampus, we can start to understand whether ancestral circuits may have been conserved, even as the larger-scale organization of the cells changed over evolutionary time. In addition, comparative approaches to brain structure-function relationships, by highlighting the similarities and differences, can give us deeper insights into the range of possible architectures that can deliver similar functionality in the brain. The commonalities in the micro-circuitry between classes despite the huge phylogenetic distance could imply that these are essential for gamma rhythmogenesis and hence evolutionarily conserved. The differences in the circuit could imply evolutionary divergence and show that several different microcircuits could generate similar functionality. Thus, we can expect to learn more about the mammalian Hippocampal Formation (HF) by studying the avian HF.
How does this work help chicken welfare?
Pre-slaughter electrical stunning is used in farm animals to ensure the animals are unconscious and insensible to pain prior to slaughter. However, earlier investigations in chickens support that at times stunning may induce a state similar to 'petit mal' ('absence seizure' in humans) which is not associated with unconsciousness. To address this welfare concern, it is pertinent to understand the neural circuitry in the chicken hippocampus since mammalian hippocampus is known to play a role in ictogenesis (seizure generation). A better understanding of the avian hippocampal micro-circuitry might lead us to formulate a testable hypothesis on the reasons for the electrically induced seizures observed in chickens. This in turn could be used to improve stunning techniques and reduce animal suffering during the slaughter procedures for billions of broiler chickens annually.
Receptor pharmacology of gamma oscillations induced in the Avian Hippocampus in vitro
Gamma rhythms are a physiological feature of the mammalian hippocampus and play an important role in memory processing. However, such oscillations have not been explored in the avian hippocampal formation (HF) whose neuroanatomy is unlike its mammalian counterpart. We therefore investigate how divergent structures perform convergent functions. If similar micro-circuitry underlies the avian HF, then we would predict that similar network properties should be detectable.
We investigate the existence of gamma oscillations in avian HF, the underlying mechanisms of rhythmogenesis and the role of different receptors in this activity.
We euthanized newly hatched chicks by cervical dislocation. We employed in vitro electrophysiology to record local field potentials in chick brain slices (400 µm). Bath application of various agonists and antagonists allowed us to elucidate the receptor pharmacology of avian hippocampal gamma oscillations in vitro.
In P0 - P4 chick HF brain slices, persistent gamma frequency oscillations (peak power: 64 ± 24.8 µV2/Hz; peak frequency: 36 ± 1.4 Hz; n = 27 slices) were induced by the bath application of the cholinergic agonist, carbachol (10 µM). However, the bath application of kainate (50 - 800 nM), a glutamate receptor agonist, did not elicit gamma. Similar to other species, carbachol-evoked gamma oscillations were sensitive to GABA-A, AMPA/kainate and muscarinic (M1) receptor antagonism.
We conclude that in juvenile chick HF, gamma rhythmogenesis is cholinergic in nature. This is unlike in adult mammals where both cholinergic and glutamatergic mechanisms are known to exist. However, similar to mammalian species, muscarinic acetylcholine receptor (mAChR) activated avian HF gamma oscillations are likely to arise via a pyramidal-interneuron gamma (PING) based mechanism.
Here is a poster about this work
This work was published after peer-review:
Pradeep D, Lynch, N.M., Crutwell, J., Cunningham, M.O. and Smulders, T.V., 2018. In vitro characterization of gamma oscillations in the hippocampal formation of the domestic chick. European Journal of Neuroscience, 48(8), pp.2807-2815.