Cholinergic responses of acoustically-characterized cochlear nucleus neurons: An in vivo iontophoretic study in Guinea pig

Highlights

• Cochlear nucleus neurons' responses to iontophoretic injection of cholinergic drugs were studied.

• Pauser-buildup neurons in dorsal cochlear nucleus were excited by cholinergic drugs.

• Transient choppers in ventral cochlear nucleus were excited by cholinergic drugs.

• Onset choppers in ventral cochlear nucleus were unaffected by cholinergic drugs.

• Findings are consistent with descending inputs directly to transient choppers.

Abstract

The responses of guinea pig cochlear nucleus neurons to in vivo iontophoretic application of various neurotransmitter agonists were recorded with extracellular multi-barrelled electrodes. Where possible, neurons were physiologically identified using strict criteria. Emphasis was placed on the action of cholinergic agonists in relation to the possible action of olivocochlear collateral innervation. Excitatory responses (increase in action potential firing) to glutamate were confirmed in a number of neuronal response types. Application of acetylcholine (ACh) or the broad spectrum cholinergic agonist carbachol produced reliable excitatory responses in about 47% of neurons (n = 29 out of 61 neurons). The remaining neurons were unresponsive to cholinergic agonists and no inhibitory responses were observed. Cholinergic responses were more common in dorsal cochlear nucleus (DCN) (73% of 30 neurons tested) than in ventral cochlear nucleus (VCN) (23% of 31 neurons). Of the total neuron sample in which cholinergic responses were investigated, 41 neurons were able to be categorized according to established acoustic response features. Excitatory responses to cholinergic agonists were seen in “Pauser-buildup” (Pb) and “Transient chopper” (Ct) response types. Primary-like neurons (PL and Pn) as well as “Onset chopper” (Oc) neurons (n = 6) were unresponsive to either ACh or carbachol. Oc neurons also did not show any effect on their acoustic responses. Robust cholinergic responses were also seen in several VCN and DCN neurons that were either unresponsive to sound, or had acoustic response properties that did not fit standard classification. The results suggest a relatively more robust cholinergic innervation of DCN compared to VCN. The excitatory cholinergic responses of some Ct neurons and the lack of effect on Oc neurons are consistent with previous results in mouse brain slice studies, but are in conflict with reports of medial olivocochlear collateral excitatory responses in onset-type neurons in vivo. The results also indicate that a number of neurons of unknown identity may also receive cholinergic input.

Introduction

The mammalian cochlear nuclear complex receives descending cholinergic innervation from several sources. Superior olivary complex inputs comprise the collateral projection of medial olivocochlear (MOC) efferent neurons (Benson and Brown, 1990; Brown and Benson, 1992; Brown et al., 1988, 1991), and a separate direct projection from the ventral nuclei of the trapezoid body (Sherriff and Henderson, 1994). Tegmental areas also are known to supply cholinergic input to both the dorsal (DCN) and ventral cochlear nucleus (VCN) (Mellott et al., 2011). The importance of these descending systems in the modulation of ascending auditory information is still a matter of conjecture and the specific neuronal targets that receive cholinergic input within the diverse populations of cell types in the cochlear nuclear complex, is not firmly established. Anatomical studies show that MOC collaterals make excitatory synaptic contacts with dendrites of multipolar neurons in VCN (Benson and Brown, 1990; Benson et al., 1996). Two major populations of multipolar cells in VCN are the d-stellate, thought to correspond to Onset Chopper (Oc) neurons and the T-stellate, thought to correspond to the transient chopper (Ct) and sustained chopper (Cs) neurons (Oertel and Fujino, 2001; Oertel et al., 1990; Smith and Rhode, 1989; Smith et al., 2005). In vivo electrophysiological studies using both intra and extracellular recordings, indicate that onset type and Oc neurons are excited by activation of olivocochlear collaterals (Mulders et al., 2002, 2003, 2007, 2009).

However, in mouse brain slices, excitatory responses to application of cholinergic agonists have only been reported in T-stellate cells and bushy cells (Fujino and Oertel, 2001; Oertel and Fujino, 2001). The latter authors specifically report a lack of cholinergic responses in d-stellate neurons, which evidence suggests correspond to Oc neurons (Oertel et al., 1990; Smith and Rhode, 1989). The present study attempts to resolve some of these conflicts by combining in vivo recording of cholinergic responses of cochlear nucleus (CN) neurons with detailed classification of their acoustic response types.