Endocannabinoid Receptor Expression in Early Zebrafish Development

Abstract

The endocannabinoid system (ECS) is widely studied due to its interactions with cannabis and its role in modulating physiological responses. While most research has focused on the effects of cannabis on adult ECSs, recent studies have begun to investigate the role of the ECS in developing organisms. However, little is known about the spatial or temporal expression of these receptors during early development. This study combines reverse transcriptase-polymerase chain reaction (RT-PCR) with in situ hybridizations to compile a timeline of the developmental expression of six key cannabinoid receptors; cb1, cb2, trpv1, trpa1a, trpa1b, and gpr55 in zebrafish embryos, starting from as early as 6-h postfertilization (hpf) until 3 days pf (dpf). This time frame is roughly equivalent to 2–10 weeks in human embryonic development. All six genes were confirmed to be expressed within this time range and share similarities with human and rodent expression. Cb1 expression was first detected between 12 and 24 hpf in the retina and CNS, and its expression increased thereafter and was more evident in the olfactory bulb, tegmentum, hypothalamus, and gut. Cb2 expression was relatively high at the 6 and 24 hpf timepoints, as determined by RT-PCR but was undetectable at other times. Trpv1 was first detected at 1 dpf in the trigeminal (Tg) ganglia, Rohon-Beard neurons, and lateral line, and its expression increased in the first 3 dpf. Expression of trpa1a was first detected as late as 3 dpf in vagal (V) neurons, whereas trpa1b was first detected at 1 dpf associated with Tg, glossopharyngeal, and V ganglia. Expression of gpr55 was diffuse and widespread throughout the brain and head region but was undetectable elsewhere in the embryo. Thus, receptor expression was found to be enriched in the central nervous system and within sensory neurons. This work aims to serve as a foundation for further investigation on the role of cannabinoid and cannabinoid-interacting receptors in early embryonic development.