1. Introduction
The type 1 cannabinoid receptors (CB1Rs) are widely expressed throughout
the central nervous system (CNS) (Domenici et al., 2006; Castillo et
al., 2012; Kendall and Yudowski, 2016a; Barti et al., 2024; B et al.,
2024a), particularly in areas such as the hippocampus, basal ganglia,
and cerebellum. Activations of CB1Rs by endocannabinoids such as
2-arachidonoyl glycerol (2-AG) and anandamide (AEA) can modulate
neurotransmitter release and impact a myriad of physiological functions,
such as stress, learning, memory, cognitive and sensory processing
(Marsicano et al., 2002; Stachowicz, 2023; Barti et al., 2024) (Zou &
Kumar, 2018). CB1Rs are expressed in both excitatory and inhibitory
neurons, and their specific functions can be different depending on the
type of neuron in which they are expressed. (Kendall and Yudowski,
2016b; Busquets-Garcia et al., 2018b, 2018a; Barti et al., 2024). For
instance, Busquets-Garcia et al. have found that CB1Rs are predominantly
expressed on certain subpopulations of GABAergic interneurons in
hippocampus and the deletion of CB1Rs in these neurons impairs
incidental associative learning (Busquets-Garcia et al., 2018a, 2018b).
Recently, Dudok et al. have employed advanced imaging techniques to
observe endocannabinoid (eCB) signaling in the hippocampal CA1 region of
mice during navigation tasks, and demonstrated that CB1Rs are
selectively expressed at synapses of CCK-expressing interneurons and
genetic deletion of CB1Rs in these interneurons results in altered place
cell firing tuning, suggesting that CB1Rs-mediated modulation of
inhibitory synapses is crucial for the proper encoding of spatial
information during behavior (B et al., 2024a). These above researches
highlight CB1Rs importance in modulating synaptic plasticity and
neuronal activity during behavior and underscore the critical role of
eCB signaling in fine-tuning neural circuits involved in spatial
navigation and memory formation. While much has been uncovered regarding
the specific functions of CB1Rs in distinct neuronal subtypes in the
central nervous system, less attention has been paid to central auditory
system specifically.
CB1Rs are highly expressed in the central auditory system across several
species, including rodents, humans, and other mammals (Tzounopoulos et
al., 2007; Zheng et al., 2007; Y et al., 2009; Zhao et al., 2011; X et
al., 2020; S et al., 2022). For instance, the distribution of CB1Rs in
the adult rat brain, including the cochlear nucleus, has been
investigated using receptor binding radioautography and in situ
hybridization histochemistry (Mailleux and Vanderhaeghen, 1992). In the
cochlear nucleus, CB1Rs are predominantly found in regions associated
with synaptic processing, particularly in the dorsal cochlear nucleus
(DCN) (Mailleux and Vanderhaeghen, 1992). Further studies using
immunohistochemistry have confirmed CB1Rs are present in the DCN of mice
(Tzounopoulos et al., 2007; Y et al., 2009; Zhao et al., 2011).
Moreover, Zheng et al. have demonstrated that CB1Rs are localised to
many different cell types in the cochlear nucleus of rat (Zheng et al.,
2007). More recently, Alejando et al. have found that CB1Rs are
expressed in inferior colliculus (IC) and auditory cortex (AC) of
hamster through RT-qPCR and immunostaining (Fuerte-Hortigón et al.,
2021). Consistently, evidence has suggested that CB1Rs are expressed in
AC of human and non-human primate (S et al., 2022). Liu et al. have
investigated the transcriptional abundance of Cnr1 (CB1Rs
encoding gene) mRNA in the brains from adult C57BL/6J mice of two sexes
using RNAscope in situ hybridization (ISH) (X et al., 2020), and CB1Rs
are found in the IC and AC. Although the expression of CB1Rs in the
central auditory system has been extensively studied, the spatial
distribution of CB1Rs across different auditory nuclei is still not
fully understood.
Accumulating evidence indicate that CB1Rs in central auditory system
modulate neurotransmitter release, synaptic plasticity, and may play
roles in normal physiological function (18,19). CB1Rs have been
implicated in the pathology of tinnitus (Zheng et al., 2007, 2015).
Moreover, a previous study has suggested that deleting the CB1Rs gene
does affect basic hearing abilities of mice (Anon, n.d.). However, the
exact roles of CB1Rs in the central auditory system and in different
types of neurons, such as inhibitory interneurons or excitatory
projection neurons, remain largely unexplored.
To address these issues, we first systematically characterized the
neuronal localization and distribution of CB1Rs in several central
auditory nuclei of mice, including the cochlear nucleus (CN), superior
olivary complex (SOC), inferior colliculus (IC), lateral lemniscus (LL),
medial geniculate body (MGB), and auditory cortex (AC), using
fluorescence immunohistochemical techniques. Our findings demonstrated
that CB1R proteins are highly expressed in the CN, SOC and LL. We next
generated conditional knockout mice with Cnr1 specifically deleted from
PV interneurons, a major type of GABAergic interneurons in the central
auditory system, and explored how CB1R deficiency in these neurons
affects hearing function using auditory brainstem response (ABR) tests.
Interestingly, the conditional deletion of Cnr1 in PV interneurons
resulted in hearing loss at 32 KHz and abnormal auditory brainstem
responses. This work may have broader implications for understanding how
cannabinoid signaling influences sensory processing and provide further
insights into CB1Rs as a potential pharmaceutical target for treating
auditory dysfunctions.