α3β4* Nicotinic acetylcholine receptors strongly modulate the excitability of VIP neurons in the mouse inferior colliculus

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 cholinergic excitation is intrinsic to VIP neurons and does not require activation of presynaptic inputs. 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.Key points summaryThe inferior colliculus (IC), the midbrain hub of the central auditory system, receives extensive cholinergic input and expresses a variety of nicotinic acetylcholine receptor (nAChR) subunits.In vivo activation of nAChRs alters the input-output functions of IC neurons and influences performance in auditory tasks. However, how nAChR activation affects the excitability of specific IC neuron classes remains unknown.Here we show in mice that cholinergic terminals are located adjacent to the somas and dendrites of VIP neurons, a class of IC principal neurons.We find that acetylcholine elicits surprisingly strong, long-lasting excitation of VIP neurons and this is mediated mainly through activation of α3β4* nAChRs, a subtype that is rare in the brain.Our data identify a role for α3β4* nAChRs in the central auditory pathway and reveal a mechanism by which cholinergic input can influence auditory processing in the IC and the postsynaptic targets of VIP neurons.

Optogenetic cholinergic modulation of the mouse superior colliculus in vivo

The superior colliculus (SC) plays a critical role in orienting movements, in part by integrating modulatory influences on the sensorimotor transformations it performs. Many species exhibit a robust brain stem cholinergic projection to the intermediate and deep layers of the SC arising mainly from the pedunculopontine tegmental nucleus (PPTg), which may serve to modulate SC function. However, the physiological effects of this input have not been examined in vivo, preventing an understanding of its functional role. Given the data from slice experiments, cholinergic input may have a net excitatory effect on the SC. Alternatively, the input could have mixed effects, via activation of inhibitory neurons within or upstream of the SC. Distinguishing between these possibilities requires in vivo experiments in which endogenous cholinergic input is directly manipulated. Here we used anatomical and optogenetic techniques to identify and selectively activate brain stem cholinergic terminals entering the intermediate and deep layers of the awake mouse SC and recorded SC neuronal responses. We first quantified the pattern of the cholinergic input to the mouse SC, finding that it was predominantly localized to the intermediate and deep layers. We then found that optogenetic stimulation of cholinergic terminals in the SC significantly increased the activity of a subpopulation of SC neurons. Interestingly, cholinergic input had a broad range of effects on the magnitude and timing of SC responses, perhaps reflecting both monosynaptic and polysynaptic innervation. These findings begin to elucidate the functional role of this cholinergic projection in modulating the processing underlying sensorimotor transformations in the SC.