α3β4∗ Nicotinic Acetylcholine Receptors Strongly Modulate the Excitability of VIP Neurons in the Mouse Inferior Colliculus

Abstract

The inferior colliculus (IC), the midbrain hub of the central auditory system, receives extensive cholinergic input from the pontomesencephalic tegmentum. Activation of nicotinic acetylcholine receptors (nAChRs) in the IC can alter acoustic processing and enhance auditory task performance. However, how nAChRs affect the excitability of specific classes of IC neurons remains unknown. Recently, we identified vasoactive intestinal peptide (VIP) neurons as a distinct class of glutamatergic principal neurons in the IC. Here, in experiments using male and female mice, we show that cholinergic terminals are routinely located adjacent to the somas and dendrites of VIP neurons. Using whole-cell electrophysiology in brain slices, we found that acetylcholine drives surprisingly strong and long-lasting excitation and inward currents in VIP neurons. This excitation was unaffected by the muscarinic receptor antagonist atropine. Application of nAChR antagonists revealed that acetylcholine excites VIP neurons mainly via activation of α3β4∗ nAChRs, a nAChR subtype that is rare in the brain. Furthermore, we show that acetylcholine excites VIP neurons directly and does not require intermediate activation of presynaptic inputs that might express nAChRs. Lastly, we found that low frequency trains of acetylcholine puffs elicited temporal summation in VIP neurons, suggesting that in vivo-like patterns of cholinergic input can reshape activity for prolonged periods. These results reveal the first cellular mechanisms of nAChR regulation in the IC, identify a functional role for α3β4∗ nAChRs in the auditory system, and suggest that cholinergic input can potently influence auditory processing by increasing excitability in VIP neurons and their postsynaptic targets.

Cholinergic boutons are distributed along the dendrites and somata of VIP neurons in the inferior colliculus

“…Thus, in a minority of VIP neurons, the α 3 subunit presumably forms functional nAChRs by pairing with non-β 4 subunits. This extends the interpretation of our recent pharmacological study, which showed that cholinergic responses in VIP neurons were largely blocked by SR16584 (Rivera-Perez et al 2021), an antagonist reported to be selective for α 3 β 4 * nAChRs (Zaveri et al 2010). A likely explanation is that SR16584 blocks not only α 3 β 4 * nAChRs but also other α 3 -containing nAChRs.…”

“…Although α 3 and β 4 nAChR subunits have limited expression in the brain, radioligand-binding and in situ-hybridization studies indicate that the IC is one of the few brain regions where these subunits are expressed (Wada et al 1989; Whiteaker et al 2002; Marks et al 2002; Salas et al 2003; Gahring et al 2004; Marks et al 2006). Consistent with this, we recently found that excitation of VIP neurons by acetylcholine is largely blocked by SR16584, an antagonist selective for α 3 β 4 * nAChRs (Rivera-Perez et al 2021). Since the specificity of receptor antagonists is rarely perfect, especially for antagonists against receptors that can exist in many heteromeric combinations, we tested whether these pharmacological results were supported by the expression of α 3 and β 4 subunits in VIP neurons.…”

“…Based on results from our previous pharmacological study and the present study, we therefore propose that nearly all VIP neurons express α 3 -containing nAChRs and that the majority of VIP neurons express α 3 β 4 * nAChRs (Rivera-Perez et al 2021). This parallels results from another recent in situ hybridization study, which reported that putative glutamatergic neurons in the IC could express α4 and β2 subunits together, β2 alone, or neither subunit (Sottile et al 2017).…”

“…Although VIP neurons express nAChR subunits ( Figure 1 ) and depolarize in response to exogenous applications of acetylcholine (Rivera-Perez et al 2021), it is not clear whether cholinergic input to VIP neurons is common or how cholinergic inputs are distributed along the somatic and dendritic compartments of VIP neurons. These are important questions to address because the anatomical arrangement of cholinergic input to VIP neurons will constrain hypotheses about how cholinergic signaling influences VIP neurons.…”

“…We recently showed that VIP neurons in the IC are strongly excited by acetylcholine (Rivera-Perez et al 2021). VIP neurons are a molecularly defined class of glutamatergic stellate neurons found throughout the major subdivisions of the IC.…”