Abstract
The inferior colliculus processes nearly all ascending auditory information. Most collicular cells respond to sound, and for a majority of these cells, the responses can be modulated by acetylcholine (ACh). The cholinergic effects are varied and, for the most part, the underlying mechanisms are unknown. The major source of cholinergic input to the inferior colliculus is the pedunculopontine tegmental nucleus (PPT), part of the pontomesencephalic tegmentum known for projections to the thalamus and roles in arousal and the sleep-wake cycle. Characterization of PPT inputs to the inferior colliculus has been complicated by the mixed neurotransmitter population within the PPT. Using selective viral-tract tracing techniques in a ChAT-Cre Long Evans rat, the present study characterizes the distribution and targets of cholinergic projections from PPT to the inferior colliculus. Following the deposit of viral vector in one PPT, cholinergic axons studded with boutons were present bilaterally in the inferior colliculus, with the greater density of axons and boutons ipsilateral to the injection site. On both sides, cholinergic axons were present throughout the inferior colliculus, distributing boutons to the central nucleus, lateral cortex, and dorsal cortex. In each inferior colliculus (IC) subdivision, the cholinergic PPT axons appear to contact both GABAergic and glutamatergic neurons. These findings suggest cholinergic projections from the PPT have a widespread influence over the IC, likely affecting many aspects of midbrain auditory processing. Moreover, the effects are likely to be mediated by direct cholinergic actions on both excitatory and inhibitory circuits in the inferior colliculus.
Highlights
Acetylcholine (ACh) plays a wide range of roles in normal auditory function, exerting influence from the cochlea to the auditory cortex
Additional labeled cells were present in the parabigeminal nucleus, a nucleus on the lateral edge of the rostral midbrain that includes a dense cluster of cholinergic cells
The cases that yielded the most labeled axons in the inferior colliculus (IC) were not those with the most labeled pontomesencephalic tegmentum (PMT) cells; we believe this reflects the fact that only a subset of PMT cells projects to the IC, and these cells are interspersed with those that project to other targets
Summary
Acetylcholine (ACh) plays a wide range of roles in normal auditory function, exerting influence from the cochlea to the auditory cortex. Cholinergic Inputs to Inferior Colliculus of the auditory system, ACh can increase spontaneous activity and excitability of auditory neurons and can alter tuning profiles of cells (Farley et al, 1983; Sarter and Bruno, 1997; Ji et al, 2001; Metherate, 2011; Suga, 2012). Much of the previous research on ACh in the auditory system has been done at the levels of forebrain and cochlea, the evidence is accumulating for widespread and varied effects of ACh in the inferior colliculus (IC), a midbrain hub for both ascending and descending auditory pathways (Winer and Schreiner, 2005; Schofield and Beebe, 2019)
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