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Herpes virus technology involving manipulation of GAD65 was used to study effects on audiogenic seizures (AGS). Audiogenic seizure behaviors were examined following injections of replication-defective herpes simplex virus (HSV-1) vectors incorporating sense or antisense toward GAD65 along with 10% lac -Z into the central nucleus of inferior colliculus (CNIC) of Long–Evans rats. In seizure-sensitive animals developmentally primed by intense sound exposure, injection of GAD65 in the sense orientation increased wild running latencies and reduced incidence of clonus compared with lac -Z only, unoperated, and vehicle seizure groups. In contrast, infection of CNIC with GAD65 antisense virus resulted in 100% incidence of wild running and clonus behaviors in AGS animals. Unprimed animals not operated continued to show uniform absence of seizure activity. Administration of GAD65 antisense virus into CNIC produced novel wild running and clonus behaviors in some unprimed animals. Staining for β-galactosidase in all vector animals revealed no differences in pattern or numbers of immunoreactive cells at injection sites. Qualitatively, typical small and medium multipolar/stellate and medium fusiform neurons appeared in the CNIC of vector animals. These results demonstrate that HSV-1 vector constructs implanted into the CNIC can predictably influence incidence and severity of AGS and suggest that viral vectors can be useful in studying GABA mechanisms with potential for therapeutic application in epilepsy. This article is part of a Special Issue entitled “Genetic and Reflex Epilepsies, Audiogenic Seizures and Strains: From Experimental Models to the Clinic”.

The effect of priming for audiogenic seizures (AGS) on the development of epileptiform activity in the hippocampus was studied using in vitro kindling (IVK) in Long-Evans rats. AGS priming consists of intense auditory stimulation during a critical period of auditory development, resulting in sound-induced clonic convulsions upon subsequent testing. Between postnatal day (PND) 28 and 50, slices from subjects primed and sham-primed for AGS on PND 18 were used for recording responses in area CA1 of hippocampus following Schaffer collateral stimulation from stratum radiatum of area CA2/CA3. The developmental priming procedure, which enhances auditory brainstem excitability, resulted in fewer afterdischarges in slices from primed subjects across initial IVK stimulation sequences. These results suggest that changes in excitability that occur with acoustic priming can initially diminish selective epileptiform response characteristics in forebrain areas such as the hippocampus.

Audiogenic seizure susceptibility in the normally seizure-resistant Long-Evans rat may result from altered processing in the auditory pathway. Representative waveform latencies of the auditory brainstem responses (ABR) were recorded to examine generator alterations at different levels of the auditory neuraxis. Male Long-Evans rats primed for audiogenic seizures (AGS) on PND 14 with a 10 kHz pure tone at 120 dB SPL for 8 min were tested for AGS on PND 28 with 120 dB SPL continuous white noise. Primed subjects displayed wild running culminating in clonic convulsions. Following behavioral testing at 4-6 months, vertex recordings of ABR waves Ia-VI were made in anesthetized subjects using pure tone stimulus bursts. AGS subjects showed marginally elevated ABR thresholds. Shorter ABR wave latencies were elicited in AGS subjects for peripheral and central auditory components with stimulus intensities above 50 dB PeSPL at 8 and 40 kHz. Interpeak intervals were reduced for waves Ia-V and III-V in AGS subjects. These results reveal that intense sound stimulation during a sensitive period of development later reduces processing time at higher intensity levels.

The ability of Long-Evans hooded rats (n = 10) to detect sounds presented from sources in the horizontal plane at 0 degrees elevation and the effects of bilateral lesions of the inferior colliculus on these abilities were examined. Rats were trained on a directional detection task which required animals to suppress licking responses in a conditioned avoidance paradigm when 100-ms noise bursts were presented at random from speakers at 45 degrees intervals beginning at azimuth (0 degrees). A task performance rate was determined by reducing the correct lick suppression rate for signal trials by the proportion of incorrect suppression responses on non-signal trials. Higher performance rates were observed for stimuli presented from 0-90 degrees than for stimuli presented in the caudal hemifield prior to surgical procedures. Bilateral lesions restricted to the inferior colliculus reduced detection performance (p0.05) and shifted the best performance rates from sounds presented at 0-45 degrees to stimuli emitted from a 90 degrees source (p0.05). These results demonstrate that pigmented rats show differential detection levels for noise bursts presented from different locations throughout the horizontal interaural plane, and suggest that the inferior colliculus is involved in this aspect of directional hearing.

Previous work has suggested possible influences of ovarian hormones on evoked potentials in the auditory system. The aim of this project was to study the effects of ovariectomy and subsequent administration of estrogen replacement on the auditory brainstem response and the middle latency response. Groups of 90 day-old Long-Evans hooded rats were anesthetized for bilateral ovariectomies (ovex) and recordings made 3 weeks later. During the week prior to recordings some ovaricctomized groups received subcutaneous injections of 10, 100 or 500 μg/kg Premarin in peanut oil, and other unoperated animals received vehicle injections. Recordings from vertex/chin using needle electrodes and pure tone stimulus parameters were made under Rompun/Ketamine. The results using 40 kHz tone stimuli showed that mean latencies for ovex animals were longer than animals in the 100 μg/kg Premarin group for waves 1a, 1an, 1b, 11, 111, 111n, and 1V/V. Other posthoc comparisons at 40 kHz stimulation revealed differences between control and 100 μg/kg Premarin groups for latencies of waves 1b, 1bn, 11 and 111. Latency reduction appeared for waves 1b, 1bn, 11 and 111 for the 10 ovex group, but only at wave 11 for the 500 ovex group, compared to ovex-only animals. Data from 8 kHz stimulation also demonstrated significant differences between the ovex and ovex 100 groups at waves 1bn and Vn. Observations of interpeak latency differences, especially between waves 1a and 11, suggested central as well as cochlear involvement in hormone action. Examination of the middle-latency responses revealed considerable variability of latencies and amplitudes among subjects with latencies of ovariectomy animals significantly lengthened for the third component of the middle latency response at 40 kHz stimulation. These results suggest that changes in neurophysiological processing of auditory information followed by ovariectomy arc reversed by estrogen treatments which may alter activity in peripheral and central auditory structures. Modification of processing in the auditory system thus provides dependent measures for study in the surgical model of menopause.

The cytoarchitecture, myeloarchitecture, and neocortical connectivity of the rat medial geniculate body (MGB) were comprehensively studied in adult and immature rats to define major anatomical divisions and nuclei. The MGB is a highly intricate structure composed of the ventral (MGv), dorsal (MGd), and medial (MGm) divisions and component nuclei, each having reciprocal connections with auditory neocortex. The MGv lies inferior to the midgeniculate bundle and extends to the rostral, but not caudal MGB tip. The MGv is composed of ventral and ovoid nuclei bounded by a marginal zone, each region containing dark staining small and medium sized, densely packed neurons shown to have tufted dendritic morphology; in contrast to the MGd, but similar to the dorsal lateral geniculate nucleus, only the perikarya of MGv neurons stain for Nissl in early postnatal material. Ventral nucleus cells align with afferent brachial axons, which penetrate the nucleus in a dorsoventral direction, whereas rostrocaudal cellular arrays are retrogradely labeled after injections of horseradish peroxidase (HRP) into auditory cortex. The ovoid nucleus is a double spiraled structure encircled and penetrated by afferent fibers that determine the orientation of constituent perikarya. Neurons in the transition zone align with a spray of axons emanating from the juncture of the ovoid and midgeniculate bundles. Marginal zone neurons are oriented in parallel to the free geniculate wall. The MGd resides within and superior to the midgeniculate bundle, and is composed of several nuclei that stain palely for myelin. In immature material, both dendritic processes and somata in the MGd stain for Nissl with our protocol; many of these cells show a stellate arborization pattern that distinguishes this region from the MGv, but is similar to the staining pattern of immature neurons of the lateral posterior nucleus. The adult dorsal nucleus has medium-sized, loosely packed neurons. The deep dorsal nucleus is situated among the fibers of the midgeniculate bundle and contains loosely packed round and fusiform cells; the latter cell type constitutes a minor proportion of the adult neuronal population but the major cell type in immature animals. The caudodorsal nucleus, which occupies the caudal tip of the MGB and rostrally courses superior to the dorsal nucleus, contains small, dark staining multipolar cells; the ventrolateral nucleus courses inferior to the MGv. The suprageniculate and limitans nuclei are included in the auditory thalamus on the basis of connections with auditory neocortex; the former has medium to dark staining mixed-sized cells, and the latter has densely packed cells which form a vertical column. HRP-labeled cells of dorsal division nuclei typically show either stellate or tufted dendritic morphologies. The MGm is traversed by dense myelinated fiber bundles emanating from the brachium of the inferior colliculus and includes very large “magnocellular” neurons, which predominate caudally, as well as medium-sized and small cells. “Magnocellular” neurons are elongate, triangular, or round, have loose, nonuniform packing, and tend to orient vertically; many of these cells have sparse, stellate dendritic arbors.

The medial geniculate body (MGB) of the rat was studied with Golgi methods to determine the distribution of neurons identified by dendritic morphology. These findings were compared with major divisions and constituent nuclei established by somatic and fiber architectonics, and by connections with temporal neocortex (Clerici et al.: Society of Neuroscience Abstracts 12:1272, 1986; 13:325, 1987; Anatomical Record 218:23, 1987; Winer and Larue: Journal of Comparative Neurology 257:282–315, 1987; Clerici and Coleman: Journal of Comparative Neurology 297:14–31, 1990). It was found that an elaboration of the prototypical scheme proposed by Morest (Journal of Anatomy 98:611–630, 1964) for partitioning the mammalian MGB is valid for characterizing the rat MGB. Two predominant categories of principal neuron dendritic patterning were identified: a bushy cell having tufted dendritic fields and a stellate cell with a radiate dendritic domain. Tufted neurons have large caliber dendritic trunks that divide profusely into daughter branches close to the soma with intertwining higher order branches that maintain a relatively restricted dendritic field. Stellate neurons typically emit primary dendrites in all directions that then divide dichotomously at wide angles at subsequent orders of branching to produce a somewhat spheroidal dendritic field. In the present study, the rat MGB is found to be a tripartite structure composed of ventral (MGv), dorsal (MGd), and medial (MGm) divisions, each uniquely characterized by constituent dendritic morphology. The paramount neuronal class of the MGv is the tufted principal cell. In the ventral and ovoid nuclei of the MGv the neuronal orientation of highly oriented bitufted cells is in register with afferent brachial axons. In the ventral nucleus, this arrangement approximates vertical with a dorsomedial tilt most prominent rostrally; in the ovoid nucleus, tufted cells adhere to the double spiraled course of afferent axons. The transition zone between ventral and ovoid nuclei contains tufted neurons that align with radially oriented fibers issuing from the junction of the ovoid and midgeniculate bundles. Bitufted neurons of the marginal zone parallel fibers at the lateral margin of the geniculate. Within the MGd the dorsal and caudodorsal nuclei are characterized by stellate cells with extensive dendritic arbors and bushy neurons with dendritic branches less tufted than those observed in the MGv. The deep dorsal nucleus contains bitufted neurons that polarize with the long axis of the midgeniculate bundle and intermingle with stellate neurons. The suprageniculate nucleus includes neurons with large somata and long, sparsely branched and dorsoventrally oriented dendrites orthagonal to corticothalamic axons, as well as smaller neurons and classical stellate cells. Neurons of the limitans nucleus have elongated dendritic fields directed along the medial border of the auditory thalamus. Among the variety of cell types of the MGm are ones with the largest somata in the MGB, the prominent “magnocellular” neurons. These cells emit only a few gradually tapering dendrites, which are encrusted with appendages; dendrites branch at irregular intervals and lie oblique to incoming axons from the brachium of the inferior colliculus. The MGm also contains smaller neurons, both tufted and radiate in typology, along with more classical stellate cells. The distribution of cells characterized by morphological features of dendrites corresponds to anatomical regions of the MGB identified by cytoarchitecture, myeloarchitecture, and necortical connectivity. The structural of the rat MGB are mostly typical of mammalian forms, which suggests the rat provides a use for the auditory forebrain.

Introduction: M cells are located in the epithelial layer covering the gut-associated lymphoid tissue and are responsible for delivery of macromolecules and microorganisms to the underlying lymphoid cells. It has been shown that the human colonic cell line Caco-2 can be converted to M cells in vitro following coculture with isolated lymphocytes from murine Peyer's patches. Studies were undertaken to evaluate and characterize the transepithelial transport of select macromolecules across these in vitro derived M cells. Methods: Caco-2 cells were converted to M cells as reported previously. The morphology of Caco-2 cells and M cells was compared by transmission electron microscopy (TEM). The transport properties of macromolecules such as horseradish peroxidase, FITC-conjugated polystyrene beads, and radiolabeled dextrans were examined. The activation of murine antigen-specific T cells following transport of the antigen ovalbumin across the M-cell barrier was assessed by measuring cytokine production. Results: M cells were shown to be irregular in shape and have fewer and shorter microvilli compared to the Caco-2 cell progenitors. These cells were still able to form tight junctions and monolayers on polycarbonate membranes. Time-course studies demonstrated that the transport of polystyrene beads and large-molecular-weight dextrans at physiological temperature across M-cell-containing monolayers was size dependent and more rapid than across Caco-2 cell monolayers. The transport of dextrans was also shown to be temperature and concentration dependent. Befitting the role of the M cell in mucosal defense, protein antigen could be delivered by these cells in order to be processed and presented to antigen-specific CD4+ T lymphocytes. Discussion: The M-cell permeability model is a functional and practical system for evaluating the transport properties of macromolecules and assessing the potential for intestinal mucosal antigen sampling to elicit immunological responses.

The purpose of the present study was to examine the effects of GABA-producing cell transplants on audiogenic seizures (AGS). The M213-2O cell line was derived from fetal rat striatum and has GABAergic properties. This cell line was further modified to express human GAD 67 and produce elevated levels of GABA. The present study compares the effects of parent M213-2O cell transplants with those of GAD 67 -modified M213-2O cells in AGS-prone Long–Evans rats. Two weeks following implantation of engineered cells, latency to AGS-typical wild running was increased compared to nonimplanted subjects. Survival of the transplanted cells was confirmed by immunochemical labeling of GAD 67 and Epstein–Barr virus nuclear antigen. These findings support the use of GABA-producing cell lines to modify seizure activity.

Audiogenic seizure (AGS) activity can be induced in the seizure-resistant Long-Evans rat by postnatal priming. This study examined the effects of unilateral lesions of the inferior colliculus (IC) and implantation of tectal grafts on AGS components. Animals were primed with a 10-kHz tone burst at 120 dB on postnatal day 14 and tested for AGS susceptibility on day 28, and then two groups were unilaterally lesioned including animals receiving embryonic day 16-17 grafts of caudal tectum. Subsequently, animals were repeatedly tested for wild running and clonic-tonic convulsion components of AGS. The results demonstrate that unilaterally grafted animals with partial IC lesions showed significant reduction in the incidence of clonus expression with greater terminal uniphasic wild running behavior. These effects were stronger than in animals with comparable unilateral lesions alone. Many neurons in graft cases were in direct contact with host tissues to provide a substrate for tissue interactions previously demonstrated to promote neuron survival and remediate IC functions.

The auditory brainstem response (ABR) was recorded from 20-month-old Long Evans hooded female rats to determine if latency reductions occur from estrogen replacement. The ABR in these post-breeding age rats was also examined for reductions in response latencies as a function of adult age. Tone pip stimuli (8 and 40 kHz) were presented at 21, 51, or 81 s(-1). Aging control and ovariectomized animals showed slower response latencies for waves Ib-VI than young adults for 8 and 40 kHz stimulation at 21 s(-1). Increased stimulus rate resulted in longer latencies for all waves at 20 months. In contrast to hormone treatment effects in young adults, ABR latencies in post-breeding age estrogen-treated animals were not reduced, consistent with a general decrease in CNS responsiveness to estrogen steroids associated with age. The results also suggest that sensorineural modifications in the auditory system which prolong ABR latencies can occur early in the aging process of adult female subjects.

The goals of this research were to examine the role of the inferior colliculus (IC) in mediating sound localization behavior and the ability of tectal grafts to restore function after IC ablation in the Long-Evans rat. Previous work has suggested that the IC is a major center for processing of information used in localizing sound sources in space. Adult rats were trained on a lick suppression paradigm to discriminate the location of the second pulse in a noise burst pair presented in the horizontal interaural plane. Following baseline testing, rats received bilateral IC lesions, bilateral lesions followed in 1 week by bilateral tectal grafts, or were sham operated. Sound localization ability was then tested 15 to 30 days and 40 to 50 days following surgical procedures. Performance across experimental groups was statistically the same during baseline testing. During the first operative test period lesion-only and grafted animals showed deficits in sound localization ability relative to controls. By the second postoperative test period control and grafted animals did not differ statistically in sound localization ability and performance of both groups was superior to that of lesion-only animals. Histology revealed a similar extent of IC damage in lesion-only and lesion-graft animals and revealed the presence of implanted tectal tissue in all grafted animals. There was significant neuron loss in the dorsal nucleus of lateral lemniscus (DNLL) in lesion-only animals relative to grafted rats and sham controls. Behavioral results suggest that the IC of pigmented rat is important for sound localization ability. The sparing of DNLL neurons in grafted animals suggests that the tectal grafts may directly integrate into or aid intrinsic recovery of the host auditory pathway.

Tone pip stimuli were used at different repetition rates to examine influences upon auditory brainstem response components. Rate increases to 80.1/s result in significant latency increases in waves IV and V at all test frequencies. Rate effects occur in rostral brainstem response components which show frequency related latencies different from wave I. Amplitude measures decline at higher repetition rates for waves I and IV at most test frequencies. Rate effects are most pronounced at 8 kHz stimulation which is around the optimal behavioral frequency in rat. At higher rates the second peak of wave I is enhanced at 8 kHz, whereas the third peak of wave I at 40 kHz disappears. These results demonstrate that tone-specific stimuli provide sensitive measures of latency, amplitude and wave morphology of components of the auditory brainstem response as a function of stimulus repetition.

The inferior colliculus was selected as a brain stem site for study of neural grafting and identification of calcium binding proteins. Unilateral ablation sites of eight midbrain inferior colliculus in adult Long-Evans rats were implanted with E17-18 caudal tectum. After 2 to 9 months animals were sacrificed and sections reacted using antibodies for calbindin and parvalbumin. The central nucleus of normal inferior colliculus shows high density of neuronal and fiber staining for parvalbumin. Typical graft cores had similar staining distributions including discoid and stellate neuron populations. Graft cores showed low densities of reactivity for calbindin comparable to central nucleus. In surrounding graft regions there was substantive-neuronal and fiber labeling for calbindin and parvalbumin including stellate neuron populations normally found in the dorsal and lateral nuclei of inferior colliculus. These results demonstrate that the expression of calcium binding proteins in tectal grafts resembles that of inferior colliculus.

The inferior colliculus (IC) is a midbrain structure that receives ascending auditory input from brainstem nuclei via the lateral lemniscus, sends efferent fibers to the medial geniculate body of thalamus and receives descending projections from auditory cortex. In the rat, the IC consists of dorsal and external cortices surrounding the central nucleus of IC (CNIC) which is populated by discoid and stellate neurons; the CNIC has a laminar appearance arising from organization of lemniscal fibers and processes of discoid cells. The IC of adult rats was chosen for implantation of whole grafts of E16-17 caudal tectum into unilateral lesion sites. Dendritic and somal architecture of graft neurons was examined 1 to 4.5 months following implantation using rapid Golgi, HRP and Nissl methods. The CNIC of rat is dominated by principal neurons with relatively flattened dendritic fields. In grafts of caudal tectum the most common neuron class observed possesses flattened dendritic arbors which often parallel one another. These neurons also resemble CNIC neurons of host tissue adjacent to the graft border. Spine formations appear on both proximal and distal dendrites of this neural type in both normal and implanted tissues. In addition, comparable somal features of graft neurons include ovoid or fusiform shapes with regular nuclear membranes as found in the normal colliculus. In Golgi stained material fewer stellate class neurons appear as in the normal CNIC, although stellate cell classes are more abundant in the pericentral areas of normal tissue. Both neuron populations are retrogradely labelled in graft and normal IC after HRP injection into the medial geniculate body. These features suggest that the graft core typically consists of prototypic CNIC cells. Other features of neuron and glial cell density vary in graft material which also shows a complex network of vasculature. These results demonstrate that whole grafts of caudal tectum placed into the inferior colliculus can form organized neural architecture similar to the normal CNIC. The somal, dendritic and spine features of these neurons form a potential substrate for connectional and functional properties which establish this preparation as suitable for further investigation as a model for development and recovery of function in the central auditory system.

The known structural and functional features of the inferior colliculus provided an advantageous substrate for examining the outcome of neural transplantation. In the present study, tissue from the midbrain tectum of Sprague-Dawley rat fetuses was removed at 15, 16 and 19 days of gestation (E15, E16 and E19), incubated as a dissociated cell suspension for 12 to 15 h in a medium containing True Blue for prelabeling, and injected into the central nucleus of the inferior colliculus (CNIC) of 28 normal male adult conspecifics. Viable tectal grafts were found in 57.5% of host animals with 65.2% located in CNIC. Grafts of E16 tissue had a survival rate of 84.2%, which was twice that of E15 or E19 donors, and there was no difference in graft survival rate in hosts sacrificed up to 6 months after the implantation procedure. Neurons prelabeled with True Blue and counterstained with thionin were identified as uniquely green stained cells in brightfield illumination and brilliant opaque cells in darkfield microscopy. Grafts were organized in tightly packed clusters of cells including large rounded polygonal and smaller cells similar in size to normal CNIC large and small multipolar neurons. Cell morphology of the larger ovoid and fusiform grafted neurons most closely resembled that of typical adult CNIC cell populations; other implanted cell types appeared immature even when host post-implant survival was extended. At longer post-implant intervals graft and host cells further intermingled, and both cellular and fibrous bridges were observed at the interface between host and graft tissue. The present work demonstrates the viability and development of homotypic cells grafted into a central auditory structure utilizing a simple method of prelabeling donor tissue with a fluorescent dye.

Metabolic responses from three sectors of the inferior colliculus (IC) divided according to frequency representation were examined bilaterally in young (YA) and aged adult (AA) rats using the 2-deoxyglucose method under quiescent or pure tone stimulus conditions. Resting IC metabolism in YA rats is characterized by elevated glucose uptake in the ventromedial (high frequency) relative to intermediate and dorsolateral sectors. AA resting incorporation is not reduced in any sector and is more uniform than YA uptake. Monaural high frequency stimulation (50 kHz) in YA animals evokes discrete contralateral banding along the ventromedial IC border, while stimulation in the optimal auditory sensitivity range of the rat at 8 kHz activates the intermediate sector of the contralateral IC. Although 50 kHz evoked ventromedial uptake is slightly reduced in AA rats contralaterally, the overall uptake within the three sectors does not differ significantly between ages under either of the monaural stimulus conditions; therefore senescent stability is exhibited. However, in AA rats the focus to 8 kHz stimulation has less clearly defined borders even though uptake within the intermediate sector is undiminished compared to YA animals, while 50 kHz elicited activity is typically reduced to a discontinuous band in the ventromedial sector of AA animals. These senescent changes may represent compensation in IC metabolism to changes in cochlear processing which are reflected in the activity of ascending auditory pathways.

Development of resting and high-frequency-evoked metabolic patterns in the central nucleus of the inferior colliculus (ICC) in rat was investigated using the 2-deoxyglucose (2-DG) method. The effects of early short-term acoustic deprivation were studied by unilateral blockade of the external auditory meatus on postnatal day 12. Quantitative and qualitative analyses were performed on 2-DG uptake in 5 regional sectors of ICC. Adult resting ICC uptake is robust and exhibits regional differentiation with the ventromedial region preferentially labelled. At 17 days of age, resting uptake is substantially lower and regionally more uniform than in the adult. This age-dependent change is likely a function of the immature stage of neuronal and cochlear development at 17 days. Unilateral 50 kHz stimulation in the adult results in bilaterally enhanced 2-DG incorporation in ventromedial ICC, with a discrete band of elevated uptake along the contralateral ventromedial border. 50 kHz stimulation evokes ventromedial banding in the contralateral, but not ipsilateral, ICC of undeprived 17-day-olds, though at a much reduced level relative to the adult. Following monaural conduction blockade from 12 to 17 days of age, stimulation through the reopened deprived ear elicits a band of uptake along the lateral ICC border, oblique to the normal tonotopic gradient. Cochlear mechanisms may account for modified patterning of high-frequency-evoked 2-DG uptake in ICC subsequent to conductive hearing loss during a period of elevated susceptibility to experiential factors.

The course of development of the auditory brainstem response (ABR) in rat was examined. Pure tone pip stimuli of 3, 8 and 40 kHz at intensities up to 106 dB sound pressure level peak equivalent were presented to anesthetized rats on postnatal days (PND) 9, 10, 12, 14, 16, 20, 24, 36, 70 and 96. The peak-to-peak amplitude and the order of appearance of each wave was examined at each stimulus parameter. The first response to presentation of an auditory stimulus was observed on PND 12. This response was demonstrated to be of non-neural origin and presumed to be a summating potential representing hair cell function in the cochlea. The first neural responses, observed on PND 14, to 3- and 8-kHz tones only, appeared as two or more waves. High-frequency (40 kHz) tones did not elicit any substantive neural response on PND 14, but neural activity to this stimulus was observed consistently by PND 16. By PND 20, 4 waves were present consistently at all test frequencies, and by PND 36 the full adult complement of 5 vertex positive waves was recorded. The frequency-dependent sequential appearance of waves during development lends support to data charting the development of frequency mapping in the cochlea. The amplitude of waves I and II at 8 kHz decreased as the animal matured, while waves III-V generally showed an initial increase in amplitude at all test frequencies through PND 24 followed by a gradual amplitude decrease to adult levels. Possible sources for developmental changes in amplitude are discussed.

Patterns of normal development of the rat dorsal and ventral cochlear nuclei (DCN, VCN) and the changes induced in the adult by postnatal acoustic deprivation were examined using volumetric determinations. VCN and DCN showed striking increases in size during the interval from postnatal days 10 to 16 which includes meatus opening. Sound deprivation from postnatal day 10 resulted in the most pronounced reduction in both DCN and VCN. The findings suggest a sensitive period during which structural development in the cochlear nuclei is influenced by acoustic stimuli.

Tonotopic organization is a major feature of central auditory structures. The tritiated 2-deoxyglucose metabolic marking technique is demonstrated to be useful for visualizing isofrequency contours in the rat inferior colliculus. Following pure-tone stimulation, the central nucleus of the inferior colliculus shows discrete, prominent banding patterns and a well-organized tonotopic arrangement.

The effects of monaural deprivation by ligation of the external auditory meatus on spherical cells in the anteroventral cochlear nucleus of the rat were examined. Sound deprivation from postnatal days 10, 16, 24, and 36 resulted in a reduction in spherical cell size, with the most dramatic reduction produced by deprivation at 10 and 16 days after birth. The anteroventral cochlear nucleus was subdivided into dorsal, middle, and ventral zones which generally correspond to different frequency representations, and the effects of deprivation on cells in these regions were examined. Cells in the dorsal zone of the anteroventral cochlear nucleus were significantly smaller than cells in the middle or ventral zones, thus indicating a position-dependent effect of acoustic deprivation. Our results suggest a sensitive period during which alterations in normal acoustic input can significantly and differentially affect the development of cells in the anteroventral cochlear nucleus.

Brainstem and forebrain projections to major subdivisions of the rat inferior colliculus were studied by using retrograde and anterograde transport of horseradish peroxidase. Retrograde label from injection into the external cortex of the inferior colliculus appears bilaterally in cells of the inferior colliculus, as well as in other brainstem auditory groups including the ipsilateral dorsal nucleus of the lateral lemniscus and contralateral dorsal cochlear nucleus. The external cortex is the only collicular subdivision where an injection labels cells in the contralateral cuneate nucleus, gracile nucleus, and spinal trigeminal nucleus. Other projecting cells to the external cortex are found in the lateral nucleus of substantia nigra, the parabrachial region, the deep superior colliculus, the midbrain central gray, the periventricular nucleus, and area 39 of auditory cortex. Injection of the dorsal cortex of inferior colliculus heavily labels pyramidal cells of areas 41, 20, and 36 of the ipsilateral neocortex. Anterograde label from a large injection of auditory cortex is densely distributed in the dorsal cortex, lesser so in the external cortex, and only slightly in the central nucleus. Labelled cells appear in the central nucleus, dorsal cortex, and external cortex, primarily ipsilaterally, following dorsal cortex injection. Relatively few cells from other brainstem auditory groups show projections to the dorsal cortex. Injection of the central nucleus of the inferior colliculus results in robust labelling of nuclei of the ascending auditory pathway including the anteroventral, posteroventral, and dorsal cochlear nuclei (mainly contralaterally), and bilaterally the lateral superior olive, lateral nucleus of the trapezoid body, dorsal nucleus of the lateral lemniscus, and the central nucleus, dorsal cortex, and external cortex of the colliculus. The medial superior olive, superior paraolivary nucleus, and ventral nucleus of the trapezoid body essentially show ipsilateral projections to the central nucleus. The differential distribution of afferents to the inferior colliculus provides a substrate for functional parcellation of collicular subdivisions.

The sizes of large spherical and fusiform cells of the anteroventral cochlear nucleus were studied in rats reared after unilateral middle ear ossicle removal on postnatal days 10 or 16, or after bilateral removal on postnatal day 16. In animals deprived monaurally from 10 days after birth, large spherical cell areas of the deprived side were reduced by 21% compared to the nondeprived side; the deprived cells were 17% smaller than cells of control animals. Large spherical cells contralateral to the deprived ear were slightly larger than those of controls. Large spherical cells monaurally deprived from the 16th day showed effects similar to those deprived from the onset of hearing, although cell reduction on the deprived side was only 10% relative to controls. In contrast to the consequences of monaural sound deprivation, binaural deprivation from the 16th day resulted in no reduction in size of large spherical cells. Fusiform cells of animals in monaurally and binaurally deprived groups were not significantly smaller than the fusiform cells of controls. Ear histologies confirmed that each cochlea and spiral ganglion was normal in experimental and control animals. Because large spherical cells receive unilateral input directly from the acoustic nerve and are known to project bilaterally to the medial superior olive, the bilateral differences observed in large spherical cell area following rearing under monaural deprivation conditions may reflect a form of imbalanced binaural interactions between axon terminals of spherical cells.