Optogenetic Stimulation of Type I GAD65⁺Cells in Taste Buds Activates Gustatory Neurons and Drives Appetitive Licking Behavior in Sodium-Depleted Mice

Abstract

Mammalian taste buds are comprised of specialized neuroepithelial cells that act as sensors for molecules that provide nutrition (e.g., carbohydrates, amino acids, and salts) and those that are potentially harmful (e.g., certain plant compounds and strong acids). Type II and III taste bud cells (TBCs) detect molecules described by humans as "sweet," "bitter," "umami," and "sour." TBCs that detect metallic ions, described by humans as "salty," are undefined. Historically, type I glial-like TBCs have been thought to play a supportive role in the taste bud, but little research has been done to explore their role in taste transduction. Some evidence implies that type I cells may detect sodium (Na 1 ) via an amiloride-sensitive mechanism, suggesting they play a role in Na 1 taste transduction. We used an optogenetic approach to study type I TBCs by driving the expression of the light-sensitive channelrhodopsin-2 (ChR2) in type I GAD65 1 TBCs of male and female mice. Optogenetic stimulation of GAD65 1 TBCs increased chorda tympani nerve activity and activated gustatory neurons in the rostral nucleus tractus solitarius. "N neurons," whose NaCl responses were blocked by the amiloride analog benzamil, responded robustly to light stimulation of GAD65 1 TBCs on the anterior tongue. Two-bottle preference tests were conducted under Na 1 -replete and Na 1 -deplete conditions to assess the behavioral impact of optogenetic stimulation of GAD65 1 TBCs. Under Na 1 -deplete conditions GAD65-ChR2-EYFP mice displayed a robust preference for H 2 O illuminated with 470 nm light versus nonilluminated H 2 O, suggesting that type I glial-like TBCs are sufficient for driving a behavior that resembles Na 1 appetite.

METTL3-mediated m6A RNA methylation regulates dorsal lingual epithelium homeostasis

“…Type I cells resemble glia and are the most abundant cells present in taste buds. However, the specific function of type I cells remains elusive 17 , 18 . Some researchers regard type I cells as salt detector 4 .…”

GAD65Cre drives reporter expression in multiple taste cell types

“…In a recent study, (Baumer-Harrison et al, 2020) used the same constitutive GAD65Cre mice crossed with floxed ChR2 mice to optogenetically stimulate taste buds. To examine reporter expression in taste buds, they used immunocytochemistry to label Type II (PLCb2) and Type III cells (Car4), and counted cells to determine co-localization with the reporter.…”

“…Because they express NTPDase2 for the degradation of ATP released by other cells (Bartel et al, 2006;Vandenbeuch et al, 2013), and because they wrap around Type II and Type III cells (Yang et al, 2020), Type I cells are thought to have a glial-like function, similar to astrocytes in the nervous system. However, several studies have suggested a more complex function of Type I cells including a potential role in amiloride-sensitive salt taste transduction (Vandenbeuch et al, 2008, Baumer-Harrison et al, 2020, release of oxytocin to modulate taste (Sinclair et al, 2010), and release of GABA in response to trigeminal modulation with substance P (Huang and Wu, 2018). In order to examine the potential roles of Type I cells a specific marker is needed to both identify isolated cells for physiology and to manipulate gene expression.…”

Functional expression of TMEM16A in taste bud cells

“…Type II and type III cells express taste receptors and respond to tastants, while type I cells mainly have glial-like functions. Type I cells have also been suggested to be involved in salt transduction because they express amiloride-sensitive Na + channels (Baumer-Harrison et al, 2020;Roper & Chaudhari, 2017;Vandenbeuch et al, 2008;Yang et al, 2020) but a recent study demonstrated that amiloride-sensitive salt taste is transmitted to the nervous system by a unique cell type that expresses the ATP release channel CALHM1/3 and voltage -gated Na + channels, neither of which are found in Type I taste cells (Nomura et al, 2020).…”

“… 2008 ; Roper & Chaudhari, 2017 ; Baumer‐Harrison et al . 2020 ; Yang et al . 2020 ), but a recent study demonstrated that amiloride‐sensitive salt taste is transmitted to the nervous system by a unique cell type that expresses the ATP release channel CALHM1/3 and voltage‐gated Na + channels, neither of which are found in type I taste cells (Nomura et al .…”