The release of acetylcholine from the cochlear nucleus upon stimulation of the crossed olivo-cochlear bundle

The final stage of cholinergic differentiation occurs below inner hair cells during development of the rodent cochlea

To gain further insights into the cholinergic differentiation of presynaptic efferent terminals in the inner ear, we investigated the expression of the high-affinity choline transporter (ChT1) in comparison to other presynaptic and cholinergic markers. In the adult mammalian cochlea, cholinergic axons from medial olivocochlear (OC) neurons form axosomatic synapses with outer hair cells (OHCs), whereas axons from lateral OC neurons form axodendritic synapses on afferent fibers below inner hair cells (IHCs). Mouse brain and cochlea homogenates reveal at least two ChT1 isoforms: a nonglycosylated approximately 73 kDa protein and a glycosylated approximately 45 kDa protein. In mouse brain, ChT1 is preferentially expressed by neurons in periolivary regions of the superior olive consistent with the location of medial OC neurons. In the adult mouse cochlea, ChT1-positive terminals are located almost exclusively below OHCs consistent with a medial OC innervation. Between postnatal day 2 (P2) and P4, ChT1-positive terminals are below IHCs and occur after the expression of growth-associated protein 43, synapsin, and the vesicular acetylcholine transporter. By P15, ChT1-positive terminals are mostly on OHCs. Accounting for differences in gestational age, the developmental expression of ChT1 in the rat cochlea is similar to the mouse. However, in older rats ChT1-positive terminals are below IHCs and OHCs. In both rat and mouse, our observations indicate that the onset of ChT1 expression occurs after efferent terminals are below IHCs and express other presynaptic and cholinergic markers. In the mouse, but not in the rat, ChT1 may preferentially identify medial OC neurons.

Modulation of spontaneous activity by acetylcholine receptors in the rat dorsal cochlear nucleus in vivo

In vitro studies have implicated muscarinic acetylcholine receptors (mAChRs) in the modulation of spontaneous activity (SA) of neurons in the rat dorsal cochlear nucleus (DCN) (Chen et al., 1994,1998). Early studies suggest that cholinergic pathways also modulate SA in vivo, but these effects have not been investigated pharmacologically. The purpose of the present study was to determine whether multiunit SA can be modulated in vivo by application of cholinergic agents to the surface of the DCN. Sprague Dawley rats were used in the current experiment. The influence of cholinergic activation on SA was tested by applying carbachol (5-500 microM) to the DCN surface while recording multiunit SA at a depth of 250 microm. Out of a total of 32 sites tested, all but 2 (94%) showed well-defined responses to carbachol, characterized by suppression, activation or a combination of both (two-component responses). The most common responses were pure suppression and suppression accompanied by transient activation. Both the proportion of sites showing suppressive responses and the magnitude of suppression averaged across sites increased with dose. Although the proportion of sites showing pure activation in response to carbachol decreased with dose, there was no clear trend in the magnitude of activation with dose. The suppressive responses to high doses of carbachol were blocked by pre-application of atropine. These results extend previous work by suggesting that muscarinic receptors play an important role in the modulation of SA in vivo.

The hair cell acetylcholine receptors: a synthesis

In this article the evidence concerning the nature of the acetylcholine (ACh) receptors on hair cells is reviewed. A schematic organization of these receptors is offered, based on the evidence as follows. (1) There are two kinds of ACh receptors on hair cells: muscarinic-like and nicotinic-like. (2) The nicotinic-like receptor mediates a hyperpolarizing response to ACh and a consequent reduction in afferent firing. (3) The muscarinic-like receptors mediate both a depolarization and a hyperpolarization of hair cells. (4) The hyperpolarization results in a reduction in afferent firing and (5) the depolarization results in an increase in afferent firing.

Immunohistochemistry of muscarinic acetylcholine receptors in rat cochlear nucleus

Neurons of the cochlear nucleus (CN) receive extrinsic and intrinsic cholinergic inputs, the effects of which appear to be mediated primarily by muscarinic acetylcholine receptors (mAChRs). To investigate the distribution of mAChRs and the correlation of pre- and post-synaptic cholinergic markers in CN, we used a monoclonal anti-mAChR antibody (M35) and a monoclonal antibody against choline acetyltransferase (ChAT) to perform immunohistochemistry on rat brain sections, and we also carried out histochemistry for acetylcholinesterase (AChE) activity. The density distributions of ChAT immunohistochemistry and AChE activity histochemistry agreed well with previous quantitative measurements of the activity distributions of these enzymes. A generally close correlation between densities of M35 and ChAT immunoreactivities was found across subregions of rat CN. The localizations of M35 and ChAT immunoreactivities suggest that cholinergic synapses are mostly axosomatic in ventral CN and possibly mostly axodendritic in dorsal CN. Prominent differences in density and appearance between M35 immunoreactivity and AChE activity were found in the regions of the rat CN where granule cells predominate.

Muscarinic receptors in the cochlear nucleus and auditory nerve of the guinea pig

The specific-binding properties of l-[3H]quinuclidinyl benzilate, a muscarinic acetylcholine-receptor antagonist, were investigated in synaptic and other membrane preparations of the guinea pig cochlear nucleus and auditory nerve. Binding parameters for all experiments were consistent with a single binding site with a Hill coefficient of 1.0. The binding of the ligand was specific and of high affinity, with values of KD in the range of 30-80 pM. Bmax was 0.352 +/- 0.023 pmol/mg protein for the dorsal cochlear nucleus and 0.215 +/- 0.011 pmol/mg protein for the ventral cochlear nucleus. The dorsal cochlear nucleus/ventral cochlear nucleus ratio for density of muscarinic receptors (1.6/1.0) was maintained across two different buffer systems, which varied with respect to the inclusion of proteolysis inhibitors. The results for auditory nerve indicated a level of binding much below that of the cochlear nucleus, with Bmax = 0.052 +/- 0.011 pmol/mg protein. The results of specific-binding experiments for l-[3H]quinuclidinyl benzilate support a role for acetylcholine as a neurotransmitter in the cochlear nucleus. The greater density of muscarinic receptors in the dorsal cochlear nucleus may indicate greater cholinergic activity in the dorsal relative to the ventral cochlear nucleus.

Effects of acetylcholine on cochlear nucleus neurons

Iontophoretic application of acetylcholine (ACh) onto neurons in the dorsal cochlear nucleus (DCN) resulted in an inhibition of the tone-evoked responses of 85% of neurons which were affected. That effect in the DCN contrasts with the predominance of excitatory effects of ACh seen in ventral cochlear nucleus (VCN) neurons. The ACh-induced inhibition in the DCN had a considerably slower onset and time course of recovery than that seen with glycine-induced inhibition. The degree of ACh effects was constant with increasing intensity or attained a maximum effect at 20 to 30 dB above best-frequency threshold in contrast to glycine, which had effects that were relatively greater at low intensities. These findings suggested a modulatory role for ACh at DCN and VCN synapses.

Effects of large brain stem lesions on the cholinergic system in the rat cochlear nucleus

Large lesions were made medial to one cochlear nucleus in rats, in order to cut virtually centrifugal pathways to it. To estimate the contribution of these centrifugal pathways to cholinergic synapses in the cochlear nucleus, choline acetyltransferase and acetylcholinesterase activities were mapped, by quantitative histochemical procedures, in lesion and control side cochlear nuclei. Choline acetyltransferase activities were reduced by 85-90% in most regions of the lesion side cochlear nucleus and by 65-75% in granular regions. Acetylcholinesterase activities were reduced by 50% or less in the same regions. The choline acetyltransferase results are consistent with a conclusion that by far most cholinergic synapses in the rat cochlear nucleus derive from centrifugal pathways. Additionally, the effects of the lesions on enzyme activities in the lateral superior olivary nucleus and ventral nucleus of the trapezoid body, and in the facial, motor trigeminal, and spinal trigeminal nuclei were examined. In the lesion side facial nucleus, 60% and 40% decreases in choline acetyltransferase and acetylcholinesterase activities, respectively, were apparently consequences of facial root transection. Lesion-control enzyme activity differences in the other nuclei were much smaller.

Neurochemistry of the auditory system

Two areas of auditory biochemistry are reviewed: the identification and characterization of neurotransmitters in the auditory system and the biochemical approach to the study of genetic hearing disorders. Studies to identify neurotransmitters at major auditory synapses are outlined. Evidence supporting glutamate or aspartate as the neurotransmitter for the auditory nerve is presented. The application of biochemistry to the study of genetic hearing disorders is discussed.

[Physiological basis for a cochlear prosthesis (author's transl)]

For the attempt to develop a cochlear prosthesis, which allows some understanding of speech, it seems--at least for the first attempts--to be appropriate to mimic natural conditions as far as possible. The auditory nerve contains about 30,000 afferent fibres. Qualitatively, their behavior is similar but quantitative measures show considerably differences (2.3). Nothing certain can be said at present however about the spiral fibres originating from the outer hair cells. The quantitative differences between single afferents concern tuning, frequency selectivity, thresholds, intensity functions and--of particular interest for electrical stimulation--differences in timing of the activity pattern, brought about by differences in travelling time along the cochlear duct (2.3). The time differences seen in the activity pattern of different fibres are in the order of several ms (2.3.6;2.3.7). Actionpotentials elicited by natural acoustic stimuli show probabilistic behavior, that is they are not strictly determined. It is obvious that with artificial electrical stimulation not every surviving single fibre can be selectively stimulated. An electrode will always stimulate a group of fibres simultaneously. With any conceivable electrical stimulation all fibres in the suprathreshold region of the electrode will be synchronously activated (3.2); a fundamental difference to the natural situation. To estimate the number of channels, necessary to stimulate the auditory nerve with sufficient accuracy to allow speech perception we consider some psychoacoustic data. These have shown that the auditory system possesses the ability to differentiate a great number of different pitches, but on the other hand it is capable of integrating different frequency areas to a so called critical bandwidth. Sound energy falling into one critical bandwidth is integrated to a uniform auditory sensation. If one is to integrate various fibres of the auditory nerve to one channel of stimulation it seems to be adequate to use the critical bandwidth as a measure (3.1). According to this criterion 15 channels would have to be introduced into the speech region of the cochlea. This would allow 1.2 mm of cochlear length for each channel. Perfect electrical separation of the channels is required. Considering the severe distortions in neuronal activity pattern, introduced by electrical stimulation in comparison to the natural conditions it is not clear even whether the number given would be sufficient. On the other hand, current spreading would appear to prohibit any higher electrode density. As far as coding of sound parameters within one channel is concerned it is proposed that full use should be made of frequency analysis according to the place principle. In respect to coding of periodicity and loudness it is proposed to approach natural conditions as far as possible (3.3). Here delay times between the individual channels and a probabilistic character of the stimuli should be introduced to avoid dominance of periodicity pitch...

GABA and its related enzymes in the lower auditory system of the guinea pig

The possible existence of GABA-transmitter neurons in the lower auditory system of the guinea pig has been investigated by means of three different experimental approaches: (1) the regional distribution of GABA and its related enzymes, (2) the subcellular distribution of glutamate decarboxylase, and (3) the effect of selected nerve lesions on glutamate decarboxylase concentrations in the auditory nuclei. Within the regions investigated considerable variations in glutamate decarboxylase activity and GABA concentration were found, with the highest values observed in the inferior colliculus. The dorsal cochlear nucleus also contained significant amounts of both glutamate decarboxylase and GABA, in addition to high concentrations of GABA transaminase. The subcellular distribution of glutamate decarboxylase was bimodal in both the cochlear nucleus and inferior colliculus with most enzyme activity recovered in the soluble and synaptosomal fractions. Neither end organ (cochlea) nor trapezoid body lesions induced a significant loss of glutamate decarboxylase activity in either the cochlear nucleus or inferior colliculus. The results suggest the presence of short axon GABAergic interneurons in the cochlear nucleus, most of which appear to terminate within the dorsal cochlear nucleus.