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Gentamicin (4%) was injected intratympanically into the middle ear of guinea pigs and the effects on the brainstem evaluated 28 days later by immunohistochemistry with an antibody against glial fibrillary acidic protein to detect astrocytes, and by cytochrome oxidase staining to detect metabolic activity. Astrocytes were observed in the cochlear nucleus indicating the possible presence of anterograde degeneration. Deficiencies in the intensity of cytochrome oxidase staining up to the level of the superior olivary complex indicated the presence of abnormal metabolic activity. Both of these observations support the conclusion that a single intratympanic injection of gentamicin may lead to neuronal degeneration along the central auditory pathway., (Copyright 1998 Elsevier Science B.V.)

The present study investigated the effect of seventh and eight cranial nerve lesions on the prominence of calcitonin gene-related peptide in the hypoglossal (XII), facial (VII), abducens (VI), and oculomotor (III) cranial nerve nuclei. Guinea pigs were anesthetized and subjected to unilateral cochlear removal, vestibular end organ ablation, and seventh nerve transection. After a survival period ranging from 4 h to 5 days, each animal was anesthetized and perfused intracardially. Frozen sections were collected through the brainstem and stained immunohistochemically for calcitonin gene-related peptide using a polyclonal antibody with the Vectastain ABC kit and protocol. Positive cells were counted in each nucleus bilaterally and analyzed for side to side differences. Nuclei XII and III showed no significant difference in the numbers of cells staining positively for calcitonin gene-related peptide between the ipsilateral and the contralateral sides to the lesion. However, nuclei VII and VI showed elevated numbers ipsilateral to the lesion on some days, but not all. For VII, there was no significant difference before 24 h, but there were significant differences 1-5 days after the lesion. Similarly, in VI, there was no difference before 24 h, but differences were significant beginning with day 1 and continuing through day 3, and finally disappearing by day 4. Changes in the numbers of CGRP positive cells in VII measurable 24 h after the lesion and continuing for at least 5 days afterward indicate a central nervous system retrograde response to peripheral motor nerve injury.(ABSTRACT TRUNCATED AT 250 WORDS)

Immunohistochemical and ligand-binding techniques were used to visualize the neurotransmitter serotonin and one of its receptors, the 5-HT1A subtype, in auditory nuclei of the brainstem. Serotonergic fibers and terminal endings were found in all auditory nuclei extending from the cochlear nucleus to the inferior colliculus, including the superior olivary complex and the nuclei of the lateral lemniscus. The density of the innervation varied between and within each nucleus. All serotonergic cell bodies were located outside the auditory nuclei. The 5-HT1A receptor subtype was found in the cochlear nucleus as well as in the inferior colliculus. With no serotonergic cell bodies present in the auditory nuclei, the present neuroanatomic and neurochemical findings support behavioral and neurophysiologic findings that the serotonergic system may modulate central auditory processing.

The efferent neural projections from posteroventral cochlear nucleus to the superior olivary complex in guinea pig were examined with the Phaseolus vulgaris-leucoagglutinin anterograde tract-tracing method. Light microscopic analysis demonstrated that the posteroventral cochlear nucleus of guinea pig bilaterally projects to the superior para-olivary nucleus and the rostral, medioventral, and lateroventral peri-olivary regions. Ipsilaterally, the posteroventral cochlear nucleus projects to the lateral superior olive, the caudal peri-olivary region, and areas immediately surrounding the capsule of the lateral superior olive. Contralaterally, the posteroventral cochlear nucleus projects to the medial nucleus of the trapezoid body. All of these projection axons travel in the trapezoid body and their terminals make, primarily, en passant endings upon their targets. Exclusively within the contralateral medial nucleus of the trapezoid body, some neurons terminate also in calyceal endings. The assumption that immunolabeled structures observed with light microscopy made actual synaptic contact in their respective target areas was confirmed with electron microscopy. With postembedding immunocytochemical procedures applied to thin sections, the electron microscope revealed labeled synaptic vesicles and pre- and postsynaptic membrane specializations. The projection pattern from posteroventral cochlear nucleus was found to be topographically organized in three distinct regions. The more dorsally located neurons of the posteroventral cochlear nucleus terminated dorsomedially in the ipsilateral lateral superior olive, ventromedially in the contralateral superior para-olivary nucleus, and medially in the contralateral medioventral peri-olivary region. The more ventrally located neurons of the posteroventral cochlear nucleus terminated dorsolaterally in the ipsilateral lateral superior olive, dorsolaterally in the contralateral superior para-olivary nucleus, and laterally in the contralateral medioventral peri-olivary region.

The presence of ascending auditory inputs from the posteroventral cochlear nucleus (PVCN) to olivocochlear neurons was examined in guinea pig by using the combination Phaseolus vulgaris-leucoagglutinin (PHA-L) anterograde and horseradish peroxidase (HRP) retrograde tract-tracing technique. By labeling the somata of olivocochlear neurons after injection of HRP into the cochlea and simultaneously labeling terminal endings of PVCN efferent neurons after injection of PHA-L into PVCN, we observed neuronal connections between these two elements within all regions of the superior olivary complex known to contain olivocochlear neurons. These regions include the superior paraolivary nucleus, medial nucleus of the trapezoid body, lateral superior olive, and periolivary regions. All possible projection patterns regarding side of input and output of both large (four combinations) and small (two combinations) olivocochlear neurons were observed. However, the most frequently observed pattern was the PVCN projection to a contralaterally located and contralaterally projecting, large olivocochlear neuron. Thus the most prevalent pattern demonstrated a feedback pathway that crossed the brainstem twice. Additional patterns demonstrated pathways that fed back to the same cochlea as well as pathways that fed forward to the opposite cochlea.

Centrifugal projections from the brain to the cochlea have been well described in rodents and cats. In order to gain a better understanding of the general mammalian features of this efferent projection system--the olivocochlear (OC) system--we have begun to extend its description to other mammalian orders, particularly primates. This report describes the origin, cellular morphology, and cholinergic nature of OC neurons in squirrel monkey. Olivocochlear neurons were identified after cochlear injection and subsequent retrograde transport of one of the tracers, horseradish peroxidase, True Blue, or Diamidino Yellow. One series of sections was processed to demonstrate the tracer and an adjacent series was processed to demonstrate acetylcholinesterase (AChE). In some cases, a series of sections was immunohistochemically processed to identify the presence of choline acetyltransferase (CAT), the synthesizing enzyme for acetylcholine. Approximately 1,700-1,800 OC neurons were contained in five distinct regions surrounding the major nuclei of the superior olivary complex (SOC), namely: dorsal to medial superior olive (MSO); between MSO and lateral superior olive (LSO); lateral to LSO; medial to SOC; and in the ventral nucleus of the trapezoid body (VTB). These neurons were larger in the regions dorsal to MSO, lateral to LSO, and within VTB; they tended to be smaller in the regions between MSO and LSO and medial to SOC. Neuronal shapes varied among regions and included oval, elongate, round, and multipolar cells. In further support of their cholinergic nature as implied by AChE reactivity, OC neurons also stained positively for the cholinergic marker, CAT.

Using an antibody against GABA, neurons within the guinea pig hindbrain, midbrain and forebrain auditory nuclei were identified which demonstrate GABA-like immunoreactivity. GABA-positive cells were localized in the cochlear nucleus, superior olivary complex, lateral lemniscus, inferior colliculus, and medial geniculate body. GABA-positive terminals could be seen surrounding globular and spherical cells in ventral cochlear nucleus and principal cells in medial nucleus of the trapezoid body. In addition, numerous positive, punctate terminals appeared throughout the hindbrain auditory nuclei and, although fewer in number, in midbrain and forebrain auditory nuclei.