Cochlear electrode reimplantation in the guinea pig.

Cochlear implants are being applied to an ever widening patient population, including children in whom lifetime use of these devices is anticipated. Replacement of implants can be expected for reasons of device failure as well as future upgrading. This investigation was undertaken to examine the effect of cochlear electrode explanation and reimplantation on spiral ganglion cell survival. Guinea pigs with normal ears were initially implanted and either explanted or explanted and reimplanted (at 2 months) with a single wire ball-tip intracochlear electrode or a silastic carrier (each remaining for an additional 2 months). Little loss of hair cells or auditory nerve was observed across experimental groups and normal controls. Restricted basal turn cochlear and spiral ganglion cell loss was observed in a few animals in each group and was likely associated with mechanical damage from initial implantation. Likewise the scattered organ of Corti damage and hair cell loss observed was noted in only a few cochleae in each experimental group. Therefore, no significant differences in the average pathology across experimental groups and controls were observed. Thus, explantation or explantation with subsequent reimplantation does not appear to constitute an additional significant pathological risk compared to implantation alone.     

Cholinergic innervation of the guinea pig tympanic membrane.

While cholinergic nerve fibers of the parasympathetic system have been demonstrated in the middle ear mucosa, such innervation of the tympanic membrane has never been shown. Such fibers may prove important since the tympanic membrane may be one of the initial sites of effusion production, and since parasympathetic innervation is thought to be involved with middle ear effusion. To demonstrate cholinergic innervation, we have used modified direct thiocholine histochemical staining. Anesthetized Hartley guinea pigs were killed, and the tympanic bullae were removed intact, fixed in 4% paraformaldehyde, and then stained whole. Following staining, the tympanic membrane was dissected from each bulla and whole-mounted for light microscopy. Numerous acetylcholinesterase-positive fibers were observed on the tympanic membrane. Some fibers appeared to be vessel associated, although the majority did not. This suggests that these fibers may act on the mucosa or vasculature of the tympanic membrane and contribute to the pathogenesis of middle ear effusion.     

The motor innervation of the tympanic muscles in the guinea pig

The number and the location of the motor neurons innervating the stapedius or tensor tympani muscles in the guinea pig were identified by retrograde axonal transport of the tracer horseradish peroxidase. Tracer injections were made either into the stapedius or tensor tympani muscle and effected the retrograde labeling of neurons in the ipsilateral brain stem. These findings showed that the stapedius motor neurons lie outside the traditionally recognized facial nucleus and are present in two cell columns: ventromedial and dorsomedial to the facial nucleus. These labeled neurons are dissimilar to cells within the facial nucleus, i.e. they are smaller and more fusiform in shape. The tensor tympani motor neurons were found outside the trigeminal motor nucleus. At a rostral level they were located in a region ventral and ventrolateral to the latter nucleus. These labeled neurons were smaller than the trigeminal motor neurons and polygonal in shape. In the animals studied there were about six times more tensor tympani motor neurons than stapedius motor neurons.     

Cochlear aqueduct flow resistance is not constant during evoked inner ear pressure change in the guinea pig

Inner ear fluid pressure was measured during 6.25 mHz square wave middle ear pressure manipulation, with a perforated tympanic membrane. After a negative-going middle ear pressure change the calculated flow resistance of the inner ear pressure release routes (mainly the cochlear aqueduct) was approximately constant, with a value of 12 Pa s/nl (averaged over two ears), when values for the inner ear window compliance are taken from the literature. After a positive-going middle ear pressure change the calculated flow resistance changed with round window position and with the pressure difference across the cochlear aqueduct. It reached an average maximum value of 114 Pa s/nl. The change of flow resistance during inner ear pressure variation can be explained by a permeability change of the cochlear aqueduct, caused by a change of structures filling the aqueduct and its entrance in scala tympani.     

The motor innervation of the tympanic muscles in the guinea pig

Summary The number and the location of the motor neurons innervating the stapedius or tensor tympani muscles in the guinea pig were identified by retrograde axonal transport of the tracer horseradish peroxidase. Tracer injections were made either into the stapedius or tensor tympani muscle and effected the retrograde labeling of neurons in the ipsilateral brain stem. These findings showed that the stapedius motor neurons lie outside the traditionally recognized facial nucleus and are present in two cell columns: ventromedial and dorsomedial to the facial nucleus. These labeled neurons are dissimilar to cells within the facial nucleus, i.e. they are smaller and more fusiform in shape. The tensor tympani motor neurons were found outside the trigeminal motor nucleus. At a rostral level they were located in a region ventral and ventrolateral to the latter nucleus. These labeled neurons were smaller than the trigeminal motor neurons and polygonal in shape. In the animals studied there were about six times more tensor tympani motor neurons than stapedius motor neurons.Supported by a grant from the Deutsche Forschungsgemeinschaft (Str./275)     

Cochlear mechanisms at low frequencies in the guinea pig

Summary The study of the cochlear microphonic and of the intracochlear sound pressure in guinea pigs shows that the behavior of the cochlea at very low frequencies is controlled by three discrete elements: (a) the compliance of the whole basilar membrane; (b) the acoustic resistance of the helicotrema; (c) the compliance of the round window. The part of each of these elements has been established. The compliance of the whole basilar membrane produces constant amplitudes at frequencies lower than the minimum frequency at which a travelling wave is present (130 Hz). In fact, this constant amplitude range is limited by connection of the two cochlear scalae through the helicotrema resistance. This protecting mechanism produces an attenuation slope for frequencies lower than 80 Hz. The compliance of the round window does not modify the slope of the cochlear microphonic, but it induces a constant sound pressure in scala tympani up to 200 Hz. Decreasing of the sound pressure in the scala vestibuli is, therefore, limited for frequencies less than 30 Hz by this constant value of the sound pressure in scala tympani.Presented at the 18th Workshop on Inner Ear Biology in Montpellier/La Grande Motte, September 14–16, 1981     

Cochlear responses to dynamic click patterns in the guinea pig

Summary Fast cochlear adaptation, expressed as percent inhibition of the summed action potential, is studied from the guinea pig cochlea responses to click trains, ramps, and steps in both directions. The transition time function of adaptation to constant click trains does not depend on click strength, but rather on click interval. On the background of steady state adaptation, click steps produce only transient changes of the adaptive state, opposite to step direction. Compared to constant train stimulation, adaptation is enhanced by descending click ramps, and diminished or reversed (near threshold) by ascending ramps.Presented at the 17th Workshop on Inner Ear Biology in Stockholm, June 23–25, 1980     

The sympathetic innervation of the vocal cord. An experimental study in the guinea pig

Formaldehyde-induced fluorescence and electron microscopy have been used to show the sympathetic innervation of the vocal cord in the guinea pig. The topographical distribution and morphology of the sympathetic fibres and endings are commented upon.     

Atrial natriuretic peptide-like immunoreactive cells in the guinea pig inner ear.

Using specific antibodies against cardiodilatin/atrial natriuretic peptide (CDD/ANP) in a conventional immuno-histochemical method (PAP) we located ANP/CDD-like immuno-reactive cells related to the secretory area, to the sensory and to the neuronal area in the compartments of the inner ear (cochlea, utricle/ampulla, and endolymphatic sac). Immunoreactive cells were unevenly distributed in the different compartments as well as within the cochlear space. Our findings suggest that ANP/CDD may play a role in the local control of fluid and electrolyte homeostasis of the inner ear. ANP/CDD-binding sites and ANP/CDD-like immunoreactivity in the inner ear may also indicate that the peptide has an additional paracrine and/or autocrine function in the organ.     

The effect of aerobic metabolism dissociation on the innervation of the guinea pig cochlea.

Fluorocitrate, a metabolic inhibitor of the Krebs' cycle at the aconitase hydrolase reaction (E.C. 4.2.1.3) and succinic dehydrogenase (E.C. 1.3.99.1) but not the respiratory carrier enzymes (NADH₂-D [E.C. 1.6.99.3] and the cytochromes [E.C. 1.9.3.1]), produces a time and dose related cochlear neural dystrophy by uncoupling aerobic from anaerobic metabolism. Neural degeneration appears to be the result of a dissociation of biochemical reactions which prevents proper utilization of organic fuel molecules for the generation of energy and a direct or indirect inhibition of neural respiration. Since fluorocitrate acts on nerve cell bodies and not directly on axons, it produces initially damage in the afferent nerves and in the spiral ganglia and later in the efferent nerves, whose cell bodies are in the pons. This study provides a biochemical model for selectively damaging the cochlear sensory neurons.     

The continuing search for outer hair cell afferents in the guinea pig spiral ganglion.

Antidromic stimulation of the stump of the VIIIth nerve was combined with microelectrode recording in the spiral ganglion of the guinea pig cochlea in an attempt to identify a sub-population of neurons with long-latency antidromic action potentials that might correspond to the thin unmyelinated afferent neurons emanating from the outer hair cells. The techniques used were similar but not identical to those employed in an earlier study by Brown (1994). By far the largest population of cells contacted had short antidromic latencies (0.58+/-0.12 ms, 76 units) and also responded to acoustic stimulation. These were assumed to be type I afferents emanating from inner hair cells. Eight cells had antidromic latencies larger than 1 ms, all but one of which had a zero spontaneous rate. All eight of these longer-latency cells were unresponsive to acoustic stimulation despite the fact that short-latency neurons in the same cochleas showed robust responses to sound before and after they were contacted. Four of these longer-latency cells had their antidromic thresholds accurately measured and two had significantly higher thresholds to electrical stimulation (0.1 ms duration) than type I cells in the same animal while two had similar electrical thresholds. Attempts to trace the eight long-latency neurons to the outer hair cells using intracellular injection of horseradish peroxidase were unsuccessful. On the basis of present evidence, we cannot conclude definitively that the long-latency neurons found in the spiral ganglion belong to the outer hair cell afferent population.     

Distribution of beta-tubulin in guinea pig inner ear

Our previous research had suggested that beta-tubulin might be an autoantigen for autoimmune inner ear disease. In this study, the expression of beta-tubulin in inner ears of normal and tubulin-immunized guinea pigs was examined by immunohistochemical staining. Strong immunoreactivity to beta-tubulin monoclonal antibody was found in stria vascularis, neurons of the spiral ganglion, cochlear nerve fibers and spiral ligament. Diffuse staining was found in the stria vascularis and the neurons of the spiral ganglion, while dense network staining was found in the spiral ligament, the nerve fibers and the vestibular end organs. The semicircular canals, endolymphatic duct and sac were also positively stained. In inner ears of guinea pigs challenged with beta-tubulin, staining intensity was diminished in the stria vascularis, the spiral ligament, and the neurons of the spiral ganglion. The results suggest that beta-tubulin is distributed to most structures of guinea pig inner ear. A challenge to the inner ear by tubulin could change the beta-tubulin distribution and cause degeneration in the spiral ganglion. The results support the hypothesis that beta-tubulin might be an autoantigen for autoimmune inner ear disease.     

Motor innervation of the eustachian tube muscles in the guinea pig

The brainstem location of motoneurons innervating eustachian tube-associated muscles in the adult guinea pig was determined using intramuscular injections of the neural tracer horseradish peroxidase (HRP). Following HRP injections into the tensor veli palatini and the eustachian tube belly of the medial pterygoid muscle, an ipsilateral column of HRP-labeled motoneurons was present medial to the dorsolateral division of the trigeminal motor nucleus. Following HRP injection into the levator veli palatini, labeled motoneurons were present in the ipsilateral dorsal division of nucleus ambiguus. The locations of the tensor veli palatini and levator veli palatini motoneurons are similar to those found in studies of other animals. A distinct eustachian tube belly of the medial pterygoid muscle was also identified. This sub-belly had a motoneuron pool distinct from the main medial pterygoid muscle group. The authors have provided the gross anatomical and neuroanatomical substrates upon which future studies of eustachian tube function in the guinea pig may be based.     

Vasopressin and isoproterenol activate adenylate cyclase in the guinea pig inner ear.

Cochleas from guinea pigs were perfused by isotonic buffer after punction of the carotid artery. The cochlea tissue was removed from the bony capsule and separated from the mediolus as band with a sharp needle under the microscope. Cell membranes were prepared subsequently from whole tissue. Purified membranes from the inner ear of guinea pigs contain adenylate cyclase which functionally is coupled with membrane receptors for vasopressin and beta-receptors for isoproterenol (epinephrine), respectively. Both hormones stimulate production of cyclic AMP at 37 degrees C. Furthermore, cyclase activity is increased by addition of Gpp (NH)p, a GTP analog. Possible relationships of these molecular events to cochlear events such as glycogenolysis, ionfluxes, transport and secretion mechanisms, and synaptic transmissions are discussed.     

Quisqualate excites spiral ganglion neurons of the guinea pig

The effects of quisqualate on primary afferent auditory neurons were examined by comparing the unit activity of guinea pig spiral ganglion neurons before, during, and after the infusion of artificial perilymph containing 1 mM quisqualate into the basal turn scala tympani. In 10 of 13 preparations quisqualate infusion increased unit activity above baseline rates established prior to infusion while control infusions of quisqualate-free artificial perilymph had no appreciable influence on the unit activity of four preparations. Postexcitatory depression typically followed peak evoked excitation, but no purely inhibitory responses were observed. Postexcitatory depression developed into a temporary cessation of spike production in three cases, suggesting depolarization blockade had developed; however, all of these preparations gradually regained some degree of spontaneous unit activity following the termination of quisqualate infusion. Quisqualate-induced excitation may be attributable to the activation of receptors for the afferent neurotransmitter released by hair cells. This interpretation is consistent with our working hypothesis that the primary afferent neurotransmitter is L-glutamate or a structural analog of this excitatory amino acid.     

Cochlear microinjection and its effects upon auditory function in the guinea pig

Microinjection through the round window membrane has been found to represent a method for vector delivery in intracochlear gene transfer in animal models but breaches the round window membrane, making it necessary to evaluate animals for possible postinjection hearing loss. In the present study healthy guinea pigs were evaluated for their baseline click auditory brainstem response (ABR) thresholds. Each animal was then injected with saline via the round window membrane. After 1 week auditory function was evaluated by click ABR. Animals with increased ABR thresholds were retested at 4 weeks. Animals with 1-week postoperative ABRs similar to baseline were not retested. Results showed that postoperative ABR thresholds in five animals (71%) remained unchanged from baseline, while two animals had increases of 20–¶25 dB in ABRs after 1 week but recovered baseline ABRs after 4 weeks. The mean baseline ABR threshold was ¶25.7 dB and was 27.9 dB after 1 week after injection. The difference between preoperative and 1-week postoperative averages was not significant (P = 0.707). In this preliminary study saline microinjection through the round window membrane did not cause permanent hearing loss in the guinea pigs tested, and any damage caused by microinjection appeared to be reversible.