Abstract
The opioid system in the brain is responsible for processing affective states such as pain, pleasure, and reward. It consists of three main receptors, mu- (μ-ORs), delta- (δ-ORs), and kappa- (κ-ORs), and their ligands – the endogenous opioid peptides. Despite their involvement in the reward pathway, and a signaling mechanism operating in synergy with the dopaminergic system, fewer reports focus on the role of these receptors in higher cognitive processes. Whereas research on opioids is predominated by studies on their addictive properties and role in pain pathways, recent studies suggest that these receptors may be involved in learning. Rodents deficient in δ-ORs were poor at recognizing the location of novel objects in their surroundings. Furthermore, in chicken, learning to avoid beads coated with a bitter chemical from those without the coating was modulated by δ-ORs. Similarly, μ-ORs facilitate long term potentiation in hippocampal CA3 neurons in mammals, thereby having a positive impact on spatial learning. Whereas these studies have explored the role of opioid receptors on learning using reward/punishment-based paradigms, the role of these receptors in natural learning processes, such as vocal learning, are yet unexplored. In this review, we explore studies that have established the expression pattern of these receptors in different brain regions of birds, with an emphasis on songbirds which are model systems for vocal learning. We also review the role of opioid receptors in modulating the cognitive processes associated with vocalizations in birds. Finally, we discuss the role of these receptors in regulating the motivation to vocalize, and a possible role in modulating vocal learning.
Highlights
Acoustic communication is important for the survival of animals living in large social groups
In a letter to the Royal Society of London, he wrote about the stages of song learning in young birds, differentiating begging calls for food, distance calls for communication and learnt vocalizations
These findings suggested that vocal learners chose to learn from a live heterospecific tutor rather than from the taped vocalizations of conspecifics, showing that social interactions were important for vocal learning
Summary
Acoustic communication is important for the survival of animals living in large social groups. 1997; Hardy and Parker, 1997; Dowsett-Lemaire, 2008; Gammon and Altizer, 2011; Balsby et al, 2012; Janik, 2014; Mello, 2014; Reichmuth and Casey, 2014; Stoeger and Manger, 2014; Favaro et al, 2016; Mori et al, 2018; Johnson and Clark, 2020; Vernes and Wilkinson, 2020; Dalziell et al, 2021) Neuroethologists study these vocal learners to understand the intricacies and origins of human speech acquisition. One such avian species which learns to produce complex vocal signals are songbirds Like humans, these birds live in intricate social groups wherein vocal communication is essential (Zann, 1996; Fernandez et al, 2017; Tchernichovski et al, 2017). In a letter to the Royal Society of London, he wrote about the stages of song learning in young birds, differentiating begging calls for food, distance calls for communication and learnt vocalizations He stated that the initial phase of song production in young birds was very similar to babbling in human babies, which was strongly influenced by the social environment (Barrington, 1773). We will discuss the anatomical distribution of opioid ligands and receptors in the brain of songbirds and how these receptors may modulate vocalization and vocal learning
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