Differential olivocochlear projections from lateral versus medial zones of the superior olivary complex

Single unit clues to cochlear mechanisms

In recent years studies on isolated hair cells have suggested that there is an inherent tuning of hair cells determined by their mechanical and electrical properties. However, tuning for mammalian cochleas appears to be much more complicated since there are typically two types of receptor cells (inner and outer hair cells) imbedded in a highly organized framework of supporting cells, membranes and fluids. The major neural output of the cochlea can be monitored by recording the activity of myelinated axons of spiral ganglion cells, not only under normal conditions, but also when the discharge patterns are altered by ototoxic drugs, acoustic trauma or olivocochlear bundle stimulation. A model system with two excitatory influences, one sharply tuned and highly sensitive, and a second, broadly tuned and relatively insensitive, can account for much of the existing data. Results from single-neuron marking studies support the notion that these two influences probably involve interactions between inner and outer hair cells. More global influences such as the endocochlear potential also can act on auditory-nerve fibers through the hair-cell systems. Thus, the inherent frequency selectivity of the receptor cell is only one of many factors that determine the tuning of mammalian auditory-nerve fibers.

Physiology and anatomy of single olivocochlear neurons in the cat

A surgical approach to the cat's VIIIth nerve has been developed which allows recordings to be made from efferent fibers of the olivocochlear bundle (OCB) as well as primary afferent fibers, without compromising the acoustic responsiveness of either ear. The designation of OCB fibers as those with regular interspike intervals was confirmed in five cases by intracellular labeling with horseradish peroxidase. Labeled fibers could be traced centrally to somata in the superior olivary complex and peripherally to large endings on outer hair cells. The locations of the labeled neurons are consistent with a classification as cells of the medial olivocochlear system [Warr and Guinan (1979): Brain Res. 173, 152-155]. Within the cochlea, efferent neurons branched profusely to innervate as many as 84 outer hair cells over as much as 2.8 mm of the organ of Corti. Efferent units had tuning curves which were similar to those of primary afferents, although most were somewhat more broadly-tuned than afferents from the same animal. The cochlear region innervated by an efferent neuron was always close to the place where afferent fibers of the same characteristic frequency (CF) would be found. Most efferents (89%) were excited by only one ear and showed no spontaneous activity. Neurons with an ipsilateral response (n = 3) had cell bodies in the contralateral brainstem and vice versa (n = 2). Binaural units (none of which were labeled) often had spontaneous discharge and were generally restricted to low-CF regions. Differences between low- and high-CF units, which cut across the monaural/binaural distinction, were seen in the dynamic range and minimum latency. Interanimal differences seen in the responses of the efferent neurons may be related to differences in the level of anesthesia.

Effects of section of the medial efferent tracts (crossed and uncrossed) on cochlear frequency selectivity

The cochlear innervation of guinea pigs was sectioned midway between the midline and the external side of the cochlear nucleus, in a rostrocaudal direction at the level of the floor of the fourth ventricle, to study the effects of medial efferent pathways on cochlear frequency selectivity estimated with tuning curves of single auditory nerve fibers. Single-unit tuning curves were affected by this type of efferent sectioning. Thus, the ipsi-lateral efferent system seems to be involved, through a tonic action, in the mechanisms responsible for high frequency cochlea selectivity.

Central organization of the efferent supply to the labyrinthine and lateral line receptors of the dogfish

Neurons that provide the efferent innervation of the inner ear and lateral line were located in the brain by applying horseradish peroxidase to appropriate cranial nerves. The efferent neurons are found in a rhombencephalic nucleus, called here the octavolateralis efferent nucleus, which lies at the rostral pole of the visceromotor column. Up to 40% of these neurons are contralateral. The location of efferent cells is not topographically related to the sense organs they innervate. Their axons leave the brain together with other motor axons in the facial and glossopharyngeal nerves and there is evidence that some neurons innervate both the ear and the lateral line. The efferent nucleus receives direct sensory input from the labyrinth, but not from the lateral line. The organization of the efferent neurons and the distribution of their axons indicates that their effect on the sense organs is probably widespread and nonspecific.

Efferent tracts and cochlear frequency selectivity

The cochlear innervation of guinea pigs was sectioned medially in a rostrocaudal direction at the level of the floor of the fourth ventricle, to study the effects of efferent pathways on cochlear microphonic (CM) suppression, the compound action potential (CAP) masking phenomenon, the input-output CAP function, and cochlear frequency selectivity estimated with tuning curves of single auditory nerve fibers. Sectioning reduced CM suppression without having any effect on absolute CM amplitude; it also reduced CAP masking. The input-output CAP function was not changed at intensities below 75 dB, and the single-unit tuning curves recorded before and after nerve sectioning were unaffected. Thus, the crossed efferent tracts (i.e., mainly the medial system) seems to be involved in the masking function itself, rather than one of the mechanisms responsible for high frequency cochlear selectivity.

Projections from the cochlear nucleus to the inferior colliculus in normal and neonatally cochlea-ablated gerbils

The distribution of the projection from one cochlear nucleus (CN) within each inferior colliculus (IC) was studied in adult, normal gerbils and adult gerbils subjected to unilateral ablation of the contralateral cochlea at 2 days of age. The projection was studied by using the Fink-Heimer technique for impregnating degenerating axons and their terminal processes with silver. Following an extensive, unilateral lesion of the CN, degeneration was seen in both ICs of all animals. In normal animals, degeneration was both more widespread and heavier in the contralateral than in the ipsilateral central nucleus of IC (ICC). Degeneration was most widespread in the rostral and lateral parts of both ICCs and in the ventral part of the contralateral ICC. Degeneration was observed in 26% of the area examined in ipsilateral ICC and in 73% of the area examined in contralateral ICC. In cochlea-ablated animals there was a much greater similarity in the area of degeneration in the ICC ipsilateral (57%) and contralateral (67%) to the CN lesion. The same regional distributions of degeneration were observed as in the normal animals except that the distribution of degeneration in the ipsilateral ICC more closely resembled the normal contralateral than the normal ipsilateral profile. We conclude that the normal distribution of projections from the CN within the ipsilateral ICC is substantially modified by neonatal ablation of the contralateral cochlea.

Immunocytochemical localization of choline acetyltransferase-like immunoreactivity in the guinea pig cochlea

The immunocytochemical localization of the enzyme choline acetyltransferase (ChAT) was examined in the guinea pig organ of Corti to determine if both lateral and medial systems of efferents would show immunoreactive labeling for this specific enzyme marker of cholinergic neurons. Cochleae were also examined after lesion of efferents to determine if ChAT-like immunoreactivity is confined to efferents. ChAT-like immunoreactivity was seen in the inner spiral bundle, tunnel spiral bundle and by the bases of inner hair cells corresponding to the lateral system of efferents. ChAT-like immunoreactivity was also seen in crossing fibers and puncta at the bases and by the nuclei of outer hair cells corresponding to the medial system of efferents. With the use of video enhanced contrast microscopy more than 9 ChAT-like immunoreactive puncta at the bases of outer hair cells could be resolved. In cochleae examined 6 weeks after ipsilateral lesion of efferents, no ChAT-like immunoreactivity was observed. These results add strong evidence that acetylcholine is a transmitter of both the medial and lateral systems of efferents.

Enzymes of Acetylcholine Metabolism in the Rat Cochlea

The distributions within the rat cochlea of choline acetyltransferase and acetylcholinesterase activities were measured to evaluate the prominence of cholinergic mechanisms in cochlear function. Samples obtained by microdissection of freeze-dried bony labyrinths were assayed radiometrically. Activities of both enzymes were highest in regions containing olivocochlear fibers and terminals, especially the organ of Corti and spiral ganglion. Within the organ of Corti, activities of both enzymes were consistently higher in the vicinity of the inner hair cells than in that of the outer hair cells and were much lower in the apical turn than in middle or basal turns. Surgical cuts in the brain stem transecting the olivocochlear pathway on one side led within seven days to total loss of choline acetyltransferase activity in the ipsilateral organ of Corti. It is concluded that all cholinergic structures in the rat organ of Corti derive from the brain stem and that synapses on or near both inner and outer hair cells are cholinergic.

Localization of dynorphin B-like and alpha-neoendorphin-like immunoreactivities in the guinea pig organ of Corti

Antiserum to dynorphin B and antiserum to alpha-neoendorphin were used in an immunocytochemical examination of the guinea pig organ of Corti. Immunoreactive staining for these two proenkephalin B (prodynorphin)-derived peptides was seen in the lateral system of olivocochlear efferents in the organ of Corti: the inner spiral bundle, the tunnel spiral bundle and by the bases of inner hair cells. Immunoreactive staining with both antisera was also seen in efferent terminals on outer hair cells at or above the level of the nucleus, which may represent terminals of either the lateral or the medial system. No immunoreactive staining was seen in tunnel crossing fibers and at bases of outer hair cells corresponding to the medial system of efferents. The staining seen with antiserum to dynorphin B and to alpha-neoendorphin has similar distribution to that seen with antisera to methionine enkephalin; there may be co-localization of these neuropeptides in the lateral system of efferents. Choline acetyltransferase-like immunoreactivity (co-localized with enkephalin-like immunoreactivity in the lateral system in the brainstem) and glutamic acid decarboxylase (GAD)-like immunoreactivity have also been found in olivocochlear efferents. Further studies will be necessary to determine if the dynorphins are co-localized with other neurotransmitter candidates and what their interactions may be.

Neuron-specific enolase-like immunoreactivity in inner hair cells but not outer hair cells in the guinea pig organ of Corti

Neuron-specific enolase (NSE) has been localized only in neurons and cells with characteristics of neurons. The immunocytochemical localization of NSE was examined in guinea pig cochleae to determine if hair cells, which have some neuronal characteristics, would show NSE-like immunoreactive labeling. NSE-like immunoreactivity was seen in inner hair cells but not in outer hair cells. This is the first report of NSE-like immunoreactivity in a receptor cell. NSE-like immunoreactivity was also seen in efferent fibers and terminals and in both type I and type II spiral ganglion cells. The finding of NSE-like immunoreactivity in inner but not outer cells adds to the number of differences found between them and may be related to differences in function and action.

Physiological and morphological characterization of efferent neurones in the guinea pig cochlea

Efferent neurones within the intraganglionic spiral bundle of the guinea pig cochlea were characterized in terms of their response properties, and their pattern of termination within the receptor organ revealed by intracellular labelling with horseradish peroxidase. All neurones subsequently identified as efferent neurones had clear features of their response properties which distinguished them from primary auditory afferents. They had long latency, low maximum discharge rate and low levels of spontaneous activity under Nembutal/Innovar anaesthesia. The pattern of discharge was extremely regular, revealed by symmetrical interspike interval histograms. 49.4% responded best to ipsilateral, 43.3% to contralateral sound and a third group of 7.3% responded equally well to either ipsilateral or contralateral sound. In cochleae in good physiological condition, these efferents were as sensitive and as sharply tuned as primary afferents with the same characteristic frequencies (CFs). All efferents fully traced in histological processing terminated on the outer hair cells. Several efferents showed extensive branching beneath the inner hair cells which might represent en passant synapses with other neuronal elements. There was clear evidence of tonotopic organization of the efferent projection. The site of termination on the outer hair cells in most instances was very close to the region of the cochlea predicted from the fibres' CF and the known place-frequency map for primary afferent neurones in the guinea pig.

The neocortical projection to the inferior colliculus in the albino rat

The purpose of the present study was to define the field of termination of the neocortical projection to the inferior colliculus in rat. The study was based on fiber degeneration following large lesions of the cerebral cortex, and anterograde transport of wheat germ agglutinin horseradish peroxidase ejected iontophoretically into more restricted neocortical loci. Neocortical fibers were found to supply the dorsal and external cortices of the inferior colliculus. The central nucleus, in contrast, did not receive such fibers. The results speak in favor of three separate projections, one partly bilateral to the deeper part of the dorsal collicular cortex, a second ipsilateral to the superficial part of this subdivision, and a third ipsilateral to the external collicular cortex.

Numerical estimations of structures in the cochlear nuclei and cochlear afferents and efferents

The cochlear nuclei and the cochlear afferents and efferents have been studied by light and electronmicroscopy. Numerical estimations have been performed concerning number of neurons and fibre population. Ultrastructural elements such as myelin lamellae, neurofilaments and neurotubuli were counted and measurements of axonal diameter and circumference performed. The results, seen in conjunction with other morphological and neurophysiological data indicate that it may be realistic to speak about two auditory nervous systems: one innervating the outer hair cells and one innervating the inner hair cells.

Fibre population of the vestibulocochlear anastomosis in humans

The vestibulocochlear anastomosis in the adult human was studied by light and electron microscopy. It was found to contain both myelinated and unmyelinated axons. The number of myelinated axons ranged from 223 to 695, with a mean of 360, while the unmyelinated axons varied from 638 to 1453, with a mean of 1005. The ratio of unmyelinated to myelinated axons varied from 2.1 to 4.4, with a mean of 3.0.

Enkephalin-like immunoreactivity in the guinea pig organ of Corti: ultrastructural and lesion studies

Enkephalin-like immunoreactivity (ELI) was examined in a light and electron microscopic study of the normal guinea pig cochlea and of cochlea de-efferented through evulsion of the vestibular nerve. Antiserum to methionine enkephalin, 164, which gives immunoreactive labeling of only the lateral system of efferents, and antiserum 163, which gives immunoreactive labeling of lateral and medial efferents, were used. In de-efferented cochleae no immunoreactive labeling was seen with either antiserum, confirming that in the organ of Corti ELI is confined to efferents. At the ultrastructural level antiserum 163 but not 164 showed ELI in efferent terminals at the base of outer hair cells. ELI with 164 was seen in efferents ending on outer hair cells at the level of the nucleus. Medially, ELI was seen with both antisera in the inner and tunnel spiral bundles. Efferent terminals containing ELI were seen apposing afferent dendrites, other efferents and the inner hair cell. However, only rarely could synaptic contacts be unambiguously identified and then only with afferent dendrites.

Fibre population of the vestibulocochlear anastomosis in the cat

Olivocochlear fibres were studied by light and electron microscopy in the microdissected vestibulocochlear anastomosis of adult cats. The anastomosis was found to contain at least two types of fibres--one myelinated variety with mean axonal diameter 1.7 micron and numbering 498; the other, an unmyelinated variety, with mean axonal diameter 0.4 micron and numbering 868. The average total number of axons was 1356. The ratio of unmyelinated to myelinated axons was usually about 1.5, with a single cat having a bilateral ratio close to 3.5.

Efferent control of cochlear inner hair cell responses in the guinea-pig

The efferent crossed olivocochlear bundle (c.o.c.b.) was electrically stimulated during intracellular recordings from cochlear inner hair cells in anaesthetized guinea-pigs. The effect of c.o.c.b. stimulation was to decrease the magnitude of the inner hair cell depolarizing component (d.c.) and alternating component (a.c.) receptor potentials evoked by tone bursts at the characteristic frequency. At low sound pressure levels, the decrease in receptor potentials caused by c.o.c.b. stimulation was equivalent to decreasing the sound intensity by 9-24 dB. C.o.c.b. stimulation usually had a similar effect on the compound action potential of the auditory nerve. The change in inner hair cell membrane resistance during moderate-level sound was measured for sound alone and when sound was accompanied by c.o.c.b. stimulation. Sound alone produced a greater membrane resistance change than sound with c.o.c.b. stimulation, in proportion to the d.c. receptor potential during the same conditions. The time course of the c.o.c.b. effect was slow, with 50-250 ms required for a full effect and for recovery. The effects of varying the frequency and voltage of electrical stimulation were similar for the d.c. receptor potential and for the compound action potential. For sounds of high level and for frequencies well below the characteristic frequency, c.o.c.b. stimulation was less effective in reducing receptor potentials. Frequency tuning curves for the d.c. receptor potential taken during intervals of continuous c.o.c.b. stimulation showed decreases in sensitivity primarily in the tip segment of the tuning curve. When no sound stimulus was present, the resting membrane potential was relatively unaltered during c.o.c.b. stimulation. The resting membrane resistance did not change during c.o.c.b. stimulation. Since the c.o.c.b. innervates mainly the outer hair cells, these results strongly suggest that changes in outer hair cell activity can influence the receptor potentials of inner hair cells and thus alter the transmission of acoustic responses to the central nervous system.

Glutamic acid decarboxylase immunoreactivity of olivocochlear neurons in the organ of Corti of guinea pig and rat

The distribution of glutamic acid decarboxylase (GAD)-like immunoreactivity in the organ of Corti of guinea pig and rat was studied under the light microscope. Indirect immunohistochemical techniques were used. Cochleae were first incubated with a specific antiserum to rat brain GAD and then stained through an avidin-biotin-horseradish peroxidase (HRP) procedure. GAD-like immunoreactivity was visualized as staining with HRP reaction product. Surface preparations were prepared from the immunoreacted cochleae. GAD-like immunoreactivity was found in the inner spiral bundle, tunnel spiral bundle, tunnel crossing fibers, outer hair cell synaptic regions and outer spiral bundles. Little staining was seen in the basal turn. Most of the immunoreactivity was seen in the third and lower fourth turn of the guinea pig cochlea, but even there many efferent fibers and endings were unstained. It is concluded that GAD-like immunoreactivity is present in a subpopulation of cochlear efferents that contains elements from both the medial and the lateral olivocochlear system. Future studies are needed to determine whether this subpopulation is GABA-ergic (i.e. uses gamma-aminobutyric acid as a neurotransmitter) and/or cholinergic.

Furosemide selectively reduces one component in rate-level functions from auditory-nerve fibers

Furosemide, an ototoxic diuretic, was administered intravenously while rate- and phase-level functions of auditory nerve fibers were measured in the cat. Normal level functions can demonstrate two components distinguished by an abrupt shift in the phase of the response as the sound level is increased. Furosemide, administered at doses that decrease the endocochlear potential, selectively reduces the discharge rate in response to tones at sound levels below that of the abrupt phase shift.

Topographic organization of the olivocochlear projections from the lateral and medial zones of the superior olivary complex

By anterograde tracing using autoradiography, we have found topographic organizations in the projections of both medial and lateral olivocochlear (OC) systems in the cat. Lateral-zone injections show an ipsilateral cochleotopic projection pattern with more medial injections projecting more basally in the cochlea. In the contralateral cochlea, in contrast, the projections from all of the lateral-zone injections were predominantly to the apex. However, detailed analysis suggests the possibility that the contralateral lateral-zone projections may have the same cochleotopic organization as the ipsilateral projections but with a heavy bias toward the apex. Medial-zone injections show a pattern in which more dorsal regions project more basally in both cochleas. The ipsilateral projections of lateral OC neurons appear to connect regions with similar best frequencies but the projections of medial OC neurons do not. Summation of data from all of the injections in each zone indicates that lateral OC projections are relatively evenly distributed throughout the ipsilateral cochlea but are predominantly to the apex in the contralateral cochlea. Medial OC projections are predominantly to the middle and basal parts of the cochlea on both sides with contralateral projections somewhat more basal than ipsilateral projections.

Choline acetyltransferase and acetylcholinesterase in centrifugal labyrinthine bundles of rats

Activities of choline acetyltransferase and acetylcholinesterase were measured for the acetylcholinesterase-positive fiber bundles containing axons projecting from the brainstem to the labyrinth of the rat. These activities were compared to those of a well-established cholinergic tract: the facial motor root. The choline acetyltransferase activities were roughly similar between the tracts, consistent with a conclusion that the centrifugal labyrinthine fibers are all cholinergic. The acetylcholinesterase activities were much higher in the centrifugal labyrinthine bundle than in the facial motor root, probably relating to the smaller diameters of the labyrinthine fibers. Transection of the centrifugal labyrinthine bundle led to virtually total loss of its choline acetyltransferase activity lateral to the cut, consistent with a centrifugal direction of all the fibers, but loss of only half its acetylcholinesterase activity, even after 34 days. These results agree with those for well-established cholinergic pathways, including the facial motor root in the present study, and with previous suggestions that a component of the acetylcholinesterase in cholinergic tracts might be synthesized by cells other than the neurons in the tract.