Cannabinoid Signaling in Auditory Function and Development

Abstract

Plants of the genus Cannabis have been used by humans for millennia for a variety of purposes. Perhaps most notable is the use of certain Cannabis strains for their psychoactive effects. More recently, several biologically active molecules within the plants of these Cannabis strains, called phytocannabinoids or simply cannabinoids, have been identified. Furthermore, within human cells, endogenous cannabinoids, or endocannabinoids, as well as the receptors and secondary messengers that give rise to their neuromodulatory effects, have also been characterized. This endocannabinoid system (ECS) is composed of two primary ligands-anandamide and 2-arachidonyl glycerol; two primary receptors-cannabinoid receptors 1 and 2; and several enzymes involved in biosynthesis and degradation of endocannabinoid ligands including diacylglycerol lipase (DAGL) and monoacylglycerol lipase (MAGL). Here we briefly summarize cannabinoid signaling and review what has been discerned to date with regard to cannabinoid signaling in the auditory system and its roles in normal physiological function as well as pathological conditions. While much has been uncovered regarding cannabinoid signaling in the central nervous system, less attention has been paid to the auditory system specifically. Still, evidence is emerging to suggest that cannabinoid signaling is critical for the development, maturation, function, and survival of cochlear hair cells (HCs) and spiral ganglion neurons (SGNs). Furthermore, cannabinoid signaling can have profound effects on synaptic connectivity in CNS structures related to auditory processing. While clinical cases demonstrate that endogenous and exogenous cannabinoids impact auditory function, this review highlights several areas, such as SGN development, where more research is warranted.

Keywords: cannabinoid; cochlea; hair cell; hearing; hearing—drug effects; otoprotection; spiral ganglion.

Figure 1

Biosynthesis and degradation of N-arachidonoyl ethanolamine (AEA) and 2-arachidonoyl glycerol (2AG). (A) In the canonical pathway for AEA synthesis and catabolism, an N-acetyl group is added to phosphatidyl ethanolamine (PE) to produce N-arachidonoyl PE, or NAPE, which is then converted to AEA by the enzyme NAPE-PLD. In a non-canonical pathway, NAPE can be hydrolyzed by PLC to produce pAEA, and subsequently dephosphorylated by PTPN22N to produce AEA. Alternatively, NAPE can be converted to lyso-NAPE by the catalytic activity of either ABHD4 or sPLA2, and subsequently, lyso-NAPE is hydrolyzed by ABHD4 and GDE-1 to produce AEA. AEA is hydrolyzed by fatty acid amide hydrolase (FAAH) to produce AA and NAA or, AEA can be oxidized by COX2 to produce prostamide F(2)α. (B) In the canonical biosynthesis pathway for 2AG, PIP2 is hydrolyzed by PLCβ to DAG which is subsequently hydrolyzed by DAGL to produce AG. In the non-canonical pathway, 2-ALPI is synthesized from either PIP2 or 2-API, which is subsequently hydrolyzed by lyso-PI-PLC to generate 2-AG. Degradation of 2AG occurs by hydrolysis by either MAGL or ABHD6 or ABHD12 to produce arachidonic acid and glycerol. Alternatively, 2AG can be oxidized by COX2 to generate to PGE2-G. Abbreviations: NAT, N-acetyltransferase; NAPE-PLD, N-acyl phosphatidylethanolamine-specific phospholipase D; PLC, Phospholipase C; PTPN22, Protein tyrosine phosphatase, non-receptor type 22 (lymphoid); ABHD4, α/β-Hydrolase domain containing 4; sPLA2, Secretory phospholipases A2; GDE-1, Glycerophosphodiester phosphodiesterase-1; FAAH, Fatty acid amide hydrolase; COX2, Cycloxygenase2; DAGL, Diacylglycerol lipase; Lyso-PI-PLC, Lyso phosphatidyl inositol phospholipase C; MAGL, Monoacylglycerol lipase; ABHD6, α/β-Hydrolase domain containing 6; ABHD12, α/β-Hydrolase domain containing 12.

Figure 2

Canonical endocannabinoid signaling pathways. Upon binding of a CB ligand to a Gi/o-coupled GPCR, the Gi/o-α receptor subunit gets detached from the βγ subunits. The liberated Gi/o-α inhibits adenylate cyclase (AC) causing subsequent decreases in cAMP-mediated PKA and CREB activation, leading to downregulation of CREB-induced gene expression. The βγ subunits activate either MAPK or PI3K to regulate gene expression. Alternatively, they can also regulate Ca²⁺ levels via activation of PLC. Inhibition of PKA can also affect other MAPK pathways (not shown). Abbreviations: GPCR, G protein-coupled receptors; AC, Adenylate Cyclase; cAMP, Cyclic adenosine monophosphate; PKA, Protein Kinase A; CREB, cAMP response element-binding protein; MKK, MAP kinase kinase; MAPK, Mitogen-activated protein kinase; ERK, Extracellular signal-related kinase; JNK, c-Jun N-terminal kinase; Raf1, Rapidly Accelerated Fibrosarcoma1; PI3K, Phosphoinositide 3-kinases; Akt, Protein kinase B; mTORC1, mammalian target of rapamycin complex 1; PLC, Phospholipase C.

Figure 3

Endocannabinoid-mediated synaptic suppression. Depolarization and/or calcium influx into a postsynaptic cell leads to endocannabinoid synthesis. ECS ligands are then released into the extracellular space where they can bind cell autonomously to membrane-bound CB receptors (1), or to receptors on presynaptic cells (2,3), or to receptors on glial processes (not shown). Cell autonomous binding of ECS ligands (1) can modulate the strength of post-synaptic responses by self-inhibition via potassium efflux. In presynaptic terminals (2,3) ECS ligands bind to CB receptors which are Gi/o-coupled GPCRs. The binding of these GPCRs leads to Gi/o-α receptor subunit detachment from βγ subunits. The detached Gi/o-α then inhibits adenylate cyclase (AC), which would otherwise drive cAMP-mediated activation of PKA and CREB. However, with AC inhibited the decreased activation of PKA results in decreased vesicle release of neurotransmitters. The presynaptic release is also inhibited by the βγ subunits which inhibit Ca²⁺ entry that would also otherwise drive neurotransmitter release. When inhibitory neurotransmitters are prevented from being released by ECS it is termed depolarization-induced suppression of inhibition (DSI) and when ECS acts on excitatory presynaptic boutons, the inhibition of vesicle release is termed depolarization-induced suppression of excitation (DSE). In addition to DSI and DSE, the inhibition of AC and PKA can lead to decreased activation of CREB thus altering the expression of CREB dependent genes. Furthermore, the GPCR subunits and their effects on PKA and PKC can also activate either MAPK or PI3K signaling cascades to regulate gene expression (see Figure 2).

Figure 4

Distribution of endocannabinoid components in the adult cochlea. Based on immunohistochemical data and gene expression studies from previously published reports, CB1 and CB2 receptors (proteins) and CB1 transcripts (Cnr1) are distributed in the outer hair cells (OHC), inner hair cells (IHC), lateral wall (LW) cells and spiral ganglion neurons (SGN, cell bodies, and axonal projections). Cnr1 transcripts were also found in the pillar cells (PC) and Dieter’s cells (DC). While CB2 receptors appear fairly widely distributed, immunolabeling suggested possible greater intensities of staining around the base of the IHCs where the SGN nerve fibers and the IHC ribbon synapses are located. Transcripts of Dagla are expressed in the sensory HCs, PCs, Deiters cells (DC), and SGN. TRPV1 is expressed in both inner and outer HCs.

Figure 5

In pseudo hue-visualization of the patterns of expression of ECS components in late embryonic and early postnatal cochlear duct. Using data from Kolla et al. (2020); the distribution of several ECS component transcripts are plotted onto a schematic of the mammalian cochlear duct. Purple color indicates areas where transcripts are likely to be detected based on bioinformatic grouping of scRNA-seq data from mouse cochlear epithelia at embryonic days E14 and E16, and postnatal days P1 and P7.