The expression of delta opioid receptor mRNA in adult male zebra finches (Taenopygia guttata)

Abstract

The endogenous opioid system is evolutionarily conserved across reptiles, birds and mammals and is known to modulate varied brain functions such as learning, memory, cognition and reward. To date, most of the behavioral and anatomical studies in songbirds have mainly focused on μ-opioid receptors (ORs). Expression patterns of δ-ORs in zebra finches, a well-studied species of songbird have not yet been reported, possibly due to the high sequence similarity amongst different opioid receptors. In the present study, a specific riboprobe against the δ-OR mRNA was used to perform fluorescence in situ hybridization (FISH) on sections from the male zebra finch brain. We found that δ-OR mRNA was expressed in different parts of the pallium, basal ganglia, cerebellum and the hippocampus. Amongst the song control and auditory nuclei, HVC (abbreviation used as a formal name) and NIf (nucleus interfacialis nidopallii) strongly express δ-OR mRNA and stand out from the surrounding nidopallium. Whereas the expression of δ-OR mRNA is moderate in LMAN (lateral magnocellular nucleus of the anterior nidopallium), it is low in the MSt (medial striatum), Area X, DLM (dorsolateral nucleus of the medial thalamus), RA (robust nucleus of the arcopallium) of the song control circuit and Field L, Ov (nucleus ovoidalis) and MLd (nucleus mesencephalicus lateralis, pars dorsalis) of the auditory pathway. Our results suggest that δ-ORs may be involved in modulating singing, song learning as well as spatial learning in zebra finches.

The Role of the Endogenous Opioid System in the Vocal Behavior of Songbirds and Its Possible Role in Vocal Learning

“…Neuronal fibers immunoreactive for met-enkephalin are distributed across the pallium and are concentrated in song control areas including HVC, RA, LMAN, Area X, and DLM (Ryan et al, 1981;Bottjer and Alexander, 1995). Patterns of δ-OR expression mirrored these results, with δ-OR mRNA being localized to the song control nuclei (LMAN, HVC, RA), all parts of the pallium and hippocampus, and Area X expressing the highest levels of these receptors in adult male zebra finches (Khurshid et al, 2009;Parishar et al, 2021).…”

“…Besides playing a role in modulating the motivation to sing as well as the spectro-temporal features of song, both μ- and δ-ORs may play a role in modulating song learning since they are present in components of the AFP, including Area X, LMAN, and DLM ( Khurshid et al, 2009 ; Parishar et al, 2021 ). These findings are supported by earlier reports demonstrating that these receptors are involved in different kinds of learning.…”

Opioid Modulation of Differential Gene Expression and Neuronal Differentiation in the Ventricular-Subventricular Zone of Adult Male Zebra Finches

“…In these birds, nascent neurons migrate and incorporate into various parts of the telencephalon, including some of the song control nuclei [5], [6], [7]. Earlier studies from our lab [8], [9] demonstrated that opioid receptors (ORs) were expressed by the V-SVZ of adult zebra finches and blocking these receptors with systemic injections of the general opioid agonist naloxone led to an increase in cell proliferation in vivo . Similar results were observed when primary cultures of the adult zebra finch V-SVZ were treated with naloxone [10].…”