Selective degeneration of a putative cholinergic pathway in the chinchilla cochlea following infusion with ethylcholine aziridinium ion

Disruption of lateral olivocochlear neurons via a dopaminergic neurotoxin depresses sound-evoked auditory nerve activity

We applied the dopaminergic (DA) neurotoxin 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) to the guinea pig cochlear perilymph. Immunolabeling of lateral olivocochlear (LOC) neurons using antibodies against synaptophysin was reduced after the MPTP treatment. In contrast, labeling of the medial olivocochlear innervation remained intact. As after brainstem lesions of the lateral superior olive (LSO), the site of origin of the LOC neurons, the main effect of disrupting LOC innervation of the cochlea via MPTP was a depression of the amplitude of the compound action potential (CAP). CAP amplitude depression was similar to that produced by LSO lesions. Latency of the N1 component of the CAP, and distortion product otoacoustic emission amplitude and adaptation were unchanged by the MPTP treatment. This technique for selectively lesioning descending LOC efferents provides a new opportunity for examining LOC modulation of afferent activity and behavioral measures of perception.

Developmental mRNA expression of the alpha10 nicotinic acetylcholine receptor subunit in the rat cochlea

A recently discovered alpha10 subunit of the nicotinic acetylcholine receptor (nAChR) family is believed to form a heteromeric receptor with the alpha9 nAChR subunit in auditory hair cells. In the present study, the alpha10 nAChR subunit expression in the developing and adult rat inner ear was analyzed by PCR and localized using isotopic in situ hybridization. Unlike the alpha9 subunit, the alpha10 subunit was not detected at embryonic day 18 (E18). From E21 through postnatal day 15 (P15), the alpha10 subunit was localized over both inner hair cell (IHC) and outer hair cell (OHC) regions, but in the mature cochlea detectable levels of alpha10 mRNA were found only over the OHC region. From E21 through adult ages, there was also a small but consistent basal to apical gradient of alpha10 expression; that is, higher levels in basal regions and lower levels in apical regions. Previously, we detected the alpha9 nAChR subunit over IHCs as early as E18 and throughout adult ages with a clear basal-apical gradient of expression. Our studies raise the question of whether the alpha9 and alpha10 subunits are differentially regulated during embryonic and postnatal development.

Olivocochlear neurons in the chinchilla: a retrograde fluorescent labelling study

Although the chinchilla is widely used as a model for auditory research, little is known about the distribution and morphology of its olivocochlear neurons. Here, we report on the olivocochlear neurons projecting to one cochlea, as determined by single and double retrograde fluorescent tracer techniques. 10 adult chinchillas were anesthetized and given either unilateral or bilateral injections of a fluorescent tracer (either Fluoro-Gold or Fast Blue) into scala tympani or as a control, a unilateral injection into the middle ear cavity. The results indicate that there are similarities as well as significant differences between the chinchilla and other species of rodents in the distributions of their olivocochlear neurons. Based on three well-labelled cases, there was a mean total of 1168 olivocochlear neurons in the chinchilla. Of these, the majority (mean 787) were small, lateral olivocochlear neurons found almost exclusively within the ipsilateral lateral superior olivary nucleus. The next largest group consisted of a mean of 280 medial olivocochlear neurons virtually all of which were located in the dorsomedial peri-olivary nucleus. Chinchilla medial olivocochlear neurons were more predominantly crossed in their projections (4:1) than in any known species. The smallest group of olivocochlear neurons (mean 101) consisted of larger lateral olivocochlear neurons (shell neurons) which were located on the margins of the superior olivary nucleus and which projected mainly (2.2:1) ipsilaterally. Double retrograde labelling was observed only in medial olivocochlear neurons and occurred in only 1-2% of these cells. The results confirm previous findings which indicated a relative paucity of fibers belonging to the uncrossed as compared to the crossed olivocochlear bundle. This, together with the strong apical bias of the uncrossed projection reported previously, offers possible explanations for the apparent absence of efferent-mediated suppressive effects of contralateral acoustic stimulation in this species. Regarding the lateral olivocochlear system, the chinchilla is shown to possess both intrinsic and shell neurons, as in the rat.

Effects of AF64A on gene expression of choline acetyltransferase (ChAT) in the septo-hippocampal pathway and striatum in vivo

AF64A (ethylcholine mustard aziridinium ion) was stereotaxically administered bilaterally (1 nmol/side) into rat lateral cerebral ventricles. Choline acetyltransferase (ChAT) activity and ChAT mRNA levels were measured at predetermined time points in the septo-hippocampal pathway and striatum, both well identified as rich in cholinergic neurons. AF64A caused a rapid but transient increase in ChAT mRNA (167%, P < 0.05) and ChAT activity (164%, P < 0.01) in the septum. By day 7 post treatment, there was a significant decrease in ChAT mRNA (42.5% of control, P < 0.05) in the septum although the ChAT activity still stayed high. This decreased ChAT mRNA level in the septum lasted for at least four weeks, and was paralleled by a long-lasting decrease in ChAT activity in the hippocampus. In the striatum, on the other hand, there were no observed changes in either ChAT activity or ChAT mRNA. These data suggest that the long term effect of AF64A on the septo-hippocampal cholinergic pathway may, at least in part, be due to an action of AF64A on gene expression in the cholinergic neuron. The difference in the response to AF64A between the septo-hippocampal and striatal cholinergic systems might be due to their difference in neuron types.

Analysis of nicotinic acetylcholine receptor subunits in the cochlea of the mouse

The present study was designed to catalogue and compare nicotinic receptor subunit messages in the mammalian cochlea. Fourteen nicotinic acetylcholine receptor subunit messages were examined by polymerase chain reaction (PCR) analysis and nucleotide sequencing. Total RNA was extracted from the auditory organs of 14- to 18-day-old CBAJ mice, and mRNA was purified using oligo-dT cellulose. After reverse transcription, resulting cDNA was amplified by PCR with the use of primers specific for the nucleotide sequences representing the following nicotinic receptor subunits: muscle types alpha 1, beta 1, gamma, delta and epsilon and neuronal types alpha 2,3,4,5,6,7 and beta 2,3,4. cDNA from cochlear tissue corresponding to the muscle-type receptor subunit beta 1 and to neuronal-type receptor subunits alpha 2,4,5,6 and beta 2,3 was amplified, whereas cDNA for muscle types alpha 1, gamma, delta and epsilon and neuronal types alpha 3,7 and beta 4 was not. All PCR products were homologous in nucleotide sequence to the corresponding reference cDNAs from which the primers were designed. The current results indicate that nicotinic acetylcholine receptor (nAChR) subunits that are similar or identical to the stated muscle and neuronal types are expressed in the murine cochlea. The presence of messages corresponding to the muscle-type beta 1 and neuronal-type nAChR subunits may be correlated with the atypical cholinergic response of cochlear hair cells to agonists and antagonists.

Damage to cochlear efferents following AF64A intoxication

Damage to cochlear efferents in chinchillas was assessed using transmission electron microscopy following unilateral treatment with the cholinotoxin ethylcholine mustard aziridinium ion (AF64A). AF64A was diluted in artificial perilymph to concentrations ranging from 0.5 to 100 microM. Survival times ranged from 1 to 12 weeks. At concentrations above 10 microM, widespread damage was noted to efferent fibers within the inner spiral bundle (ISB), tunnel spiral bundle (TSB), tunnel radial fibers (TRF) and efferent terminals at the base of OHCs. This damage included degeneration of fibers and terminals, delamination of mitochondria, vacuolization, and loss of cell membrane. However, at high concentrations, non-specific damage was also noted as thinnings or discontinuities of the membrane of OHCs and afferent fibers. At concentrations between 3 and 10 microM, selective damage was observed to efferent fibers within the ISB, TSB, TRF, and to terminals at the base of the OHCs, with all other structures appearing normal. At concentrations of 0.5 and 1 microM, damage was limited to efferent fibers within the TSB and ISB below the inner hair cells. In general, insult was greatest to middle- and basal-turn efferents, and longer survival times did not produce greater damage to, or loss of, efferents. These data suggest that at low concentrations, AF64A produces a partial yet selective degeneration of cochlear efferents within both the medial and lateral tracts, and that at the lowest concentrations used in these studies, AF64A produces a preferential insult on lateral olivocochlear efferents.