Geniculo-collicular descending projections in the gerbil

Axonal projections were labeled using a combination of tract-tracing and intracellular staining to examine the connection between the auditory thalamus and tectum. We report descending projections from the medial and dorsal divisions of the medial geniculate body to the external nucleus of the ipsilateral inferior colliculus and lower brainstem of the gerbil. These were studied in quasi-parasagittal brain slice preparations containing auditory thalamus, tectum and brainstem. Slices were cut in a curved, roughly parasagittal plane from the level of the thalamus to the brainstem. When properly cut, a single thick slice included most of the ipsilateral auditory cell groups and pathways at these levels. The geniculo-collicular projections were revealed after extracellular injections of Biocytin and rhodamine-conjugated biotinylated dextran amine in the medial geniculate body were applied to these slices. Furthermore, cells in the medial geniculate body that had been retrogradely prelabeled with a fluorescent tracer, Fluoro-Gold, injected in the inferior colliculus were intracellularly labeled to confirm the presence of the descending axons. In these preparations, single cells with descending axons were traced through their entire extent to their terminals in the inferior colliculus. Our results clearly demonstrate the geniculo-collicular descending projection and suggest a thalamo-tectal feedback loop.

Local collateral projections from the medial superior olive to the superior paraolivary nucleus in the gerbil

Local collateral projections from the medial superior olivary nucleus in the gerbil auditory brainstem were examined to study the possible communication of this nucleus with periolivary cell groups. The projections were investigated using intracellular and extracellular labeling with Biocytin in the medial superior olive (MSO) in brainstem tissue slices. Collateral axons were found to branch from the main axons of the central cells of the MSO as the latter passed through a dorsally neighboring periolivary nucleus, the superior paraolivary nucleus (SPN), toward the ipsilateral inferior colliculus (IC), their traditionally accepted target. Bouton-like endings and en passant varicosities of these collaterals appeared to contact the somata and proximal dendrites of cells within the SPN. Furthermore, close observation revealed that these collaterals terminate on at least two types of SPN cells. Intracellular labeling of the collateral axons of the MSO neurons combined with retrograde prelabeling of their target cells, however, revealed that the collaterals selectively contact the cells of the SPN that project to the ipsilateral IC. A link between the MSO and SPN has not been reported previously. This connection is of interest since SPN cells themselves project either to the cochlear nuclei (CN) or the IC. The MSO-SPN projection identified here raises the possibility that the latter may serve as an ancillary channel to convey MSO information to the IC.

Projections to the medial superior olive from the medial and lateral nuclei of the trapezoid body in rodents and bats

In this study we present direct evidence of axonal projections from both the medial and lateral nuclei of the trapezoid body to the medial superior olive. Projections were traced by intracellularly labeling cells and axons in a tissue slice preparation of two rodent species, Mus musculus and Meriones unguiculatus and two bat species, Eptesicus fuscus and Pteronotus parnellii. The main axon of most principal cells in the medial nucleus of the trapezoid body gives off one or more collateral branches which arborize within the medial superior olive. These collateral axons form small bouton-like swellings which primarily contact somata within the central cell column in the medial superior olive. Likewise, labeled elongate and multipolar cells of the lateral nucleus of the trapezoid body send axons to both the medial and lateral superior olives. These axons also form perisomatic contacts in both target nuclei. These two sets of projections may relay ascending input to the medial superior olive and the lateral superior olive; the medial nucleus of the trapezoid body is known to relay input from the contralateral ventral cochlear nucleus, and the lateral nucleus of the trapezoid body may relay input from the ipsilateral ventral cochlear nucleus. These projections offer two routes for indirect, possibly inhibitory input to reach the medial superior olive from both cochlear nuclei. These indirect, inhibitory pathways may parallel the direct excitatory projections the medial superior olive receives from each cochlear nucleus.

Classification of the principal cells of the medial nucleus of the trapezoid body

Cells in the medial nucleus of the trapezoid body were intracellularly labeled in brainstem tissue slices of two bat and two rodent species. The main cell type found in this nucleus, the principal cell, is an important link in the relay of ascending projections from the contralateral cochlear nucleus to the lateral superior olive, completing an essential pathway for sound localization. Principal cells are often viewed as a highly homogeneous group with a consistent morphology as well as a common function. Intracellular labeling has revealed a number of new axonal and dendritic features of principal cells. Some of these features vary widely from cell to cell, suggesting that the population of principal cells contains several morphologically distinct subgroups. Similar subsets of principal cells were recognized in all species examined. Five subgroups were distinguished on the basis of the position of dendritic fields. Although the dendrites of most labeled cells were confined to the medial nucleus of the trapezoid body, some principal cells had dendrites that spread outside the nucleus to one of several adjacent periolivary cell groups. Cells were also found that had dendrites that spread medially across the midline and into the contralateral medial nucleus of the trapezoid body. Axonal projections were used to distinguish two additional subgroups of principal cells. All principal cells project to the lateral superior olive and virtually all have one or more secondary projections. There are two subgroups with unusual collateral projections: one with collaterals that extended to the lateral lemniscus and one with recurrent collateral axons.

Afferents to the medial nucleus of the trapezoid body and their collateral projections

Cells and axons that supply direct afferent input to the medial nucleus of the trapezoid body are described. Afferents were intracellularly labeled in brainstem tissue slices of two rodent and two bat species. The main afferents are calyciferous axons from globular bushy cells of the ventral cochlear nucleus. Calyciferous axons were highly consistent across species, projecting directly from the cochlear nucleus, across the midline in the trapezoid body, to the contralateral medial nucleus of the trapezoid body. Within the target nucleus, a typical axon turned sharply away from horizontal to form a large ending, the calyx of Held, around the soma of a single principal cell. Three groups of calyciferous axons were classified based on the path taken from bend to calyx. In subjects younger than four weeks, single axons often formed two calyces, each on a different cell. These calyx pairs were often found on adjacent or vertically aligned cells. In older animals, calyx pairs were more closely aligned, but fewer double calyx axons were seen. A secondary focus of this study was the system of thin collateral branches that characterizes calyciferous axons in all species. The projection patterns of these collaterals suggest that calyciferous axons may provide ascending input to periolivary cell groups with descending projections. In addition to calyciferous afferents, labeled cells that provide input to the medial nucleus of the trapezoid body from adjacent periolivary cell groups are described. Also described is a type of afferent that descends from the level of the lateral lemniscus to the medial nucleus of the trapezoid body.

Connections and frequency representation in the auditory brainstem of the mustache bat, Pteronotus parnellii

The goals of this study were to describe the cochlear frequency map of the mustache bat, Pteronotus parnellii, and to relate the organization of cochlear primary afferents to that of the second-order projections from the cochlear nucleus to the superior olivary complex. Small deposits of horseradish peroxidase (HRP) were placed in the cochlear nucleus at sites that were physiologically characterized with respect to unit-best frequency. From the deposits, labeled fibers were traced in the retrograde direction to the cochlea and in the anterograde direction to the superior olive. Cochleas from both experimental and control animals were examined with light and electron microscopy. The peripheral axons of spiral ganglion neurons were counted in order to quantify the unusual variation in the innervation density along the cochlear spiral of the mustache bat. Regions of increased innervation density were found at the beginning and end of the basal turn of the cochlea. The highest cochlear innervation density consistently occurred in the upper basal turn. In horseradish peroxidase tracing experiments, this region contained labeled fibers only when HRP deposits were made at sites within the cochlear nucleus with unit-best frequencies around 60 kHz. These findings support the suggestion (Kössl and Vater, '85) that the cochlear sites of increased innervation density are related to the neural and behavioral emphasis that this echolocating bat places upon the analysis of the 60 kHz frequency band. The general arrangement of tonotopic maps within the cochlea, cochlear nucleus, and superior olive was consistent with previous observations in this bat and other mammalian species. At all three levels, there was evidence of a disproportionately large representation of frequencies around 60 kHz, similar to the enlarged representation reported within the inferior colliculus and auditory cortex of the mustache bat. In all cases there was a consistent relation between the size of the HRP deposit and the number and distribution of retrogradely labeled neurons in the cochlea. For most cases there was a similar relation between the size of the deposit and the terminal arborization field of anterogradely labeled fibers in the superior olive. However, in cases with deposits associated with the 60 kHz frequency band, the size of the labeled arborization field was more than twice as large as expected from the size of the deposits and from the extent of labeling in the cochlea. These cases suggest that the representation of frequencies around 60 kHz, already overrepresented in both the cochlea and cochlear nucleus, may be further expanded at the level of the superior olivary complex.

Intracellular labeling of afferents to the lateral superior olive in the bat, Eptesicus fuscus

An in vitro tissue slice preparation of the bat brain stem was used to label intracellularly individual axons projecting to the lateral superior olive from two different sources: the medial nucleus of the trapezoid body (MNTB) and the anteroventral cochlear nucleus (AVCN). The tracing of individually labeled MNTB axons into the lateral superior olive reaffirms the long accepted indirect route by which information from the contralateral ear reaches the lateral superior olive. While the MNTB appears to relay input from the contralateral AVCN, information from the ipsilateral ear reaches the lateral superior olive via a direct projection from the ipsilateral AVCN. Axons from the contralateral and ipsilateral pathways have different distribution patterns upon the fusiform cells of the lateral superior olive. Axon terminals of MNTB principal cells have a perisomatic and proximal dendritic distribution pattern. Axon terminal varicosities from the ipsilateral anteroventral cochlear nucleus are distributed primarily to more distal dendrites.

Origin of ascending projections to an isofrequency region of the mustache bat's inferior colliculus

The inferior colliculus of the mustache bat is similar in most respects to the inferior colliculus of more commonly studied mammals, but one isofrequency contour, the dorsoposterior division, is greatly overrepresented. The present study utilizes this specialization of the auditory system in the mustache bat to determine the total set of ascending projections to a single isofrequency contour of the inferior colliculus. Within the dorsoposterior division, neurons are all very narrowly tuned to 60 kHz, the major component of this bat's echolocation call. The afferent projections to this isofrequency contour were identified by making deposits of horseradish peroxidase (HRP) within the dorsoposterior division after physiologically defining its borders. Two other frequency representations are present in the central nucleus of the inferior colliculus of the mustache bat, the anterolateral division in which there is an orderly progression of frequencies from 59 down to 20 kHz, and the medial division in which frequencies from 63-120 kHz are represented. In additional experiments, the afferent projections to the medial and anterolateral divisions were examined, providing an anatomical description of the tonotopicity of the lower auditory nuclei. Deposits of HRP in the DPD labeled cells in each of the lower brainstem auditory nuclei that have previously been shown to project to the entire central nucleus of the inferior colliculus. The ascending projections to the dorsoposterior division include contralateral projections from the cochlear nucleus and inferior colliculus, ipsilateral projections from the medial superior olive, ventral and intermediate nuclei of the lateral lemniscus, and bilateral projections from the lateral superior olive and dorsal nucleus of the lateral lemniscus. In most of the nuclei, labeled cells were confined to specific portions of the nuclei, often forming "slabs" of labeled cells across the rostrocaudal extent of most nuclei. These slabs presumably represent the 60 kHz representation in each of the lower nuclei. When deposits of HRP were made into other frequency band representations of the inferior colliculus, in either the medial or anterolateral division, labeled cells again formed slabs in each lower nucleus. However, the location of the slab varied as a function of the best frequency of neurons at the deposit site, and labeled cells were not present within the 60 kHz representation. These results show the general tonotopy of the mustache bat's brainstem auditory nuclei, and with respect to the dorsoposterior division, clearly reveal the total set of projections to a single isofrequency region.(ABSTRACT TRUNCATED AT 400 WORDS)

Convergence of ascending pathways at the inferior colliculus of the mustache bat, Pteronotus parnellii

To compare patterns of projections to the inferior colliculus from different sources, injections of [3H]-leucine were placed in the cochlear nuclei, superior olivary complex, and nuclei of the lateral lemniscus of Pteronotus parnellii. The results show that the target of the anteroventral cochlear nucleus (AVCN) is the ventral and lateral two thirds of the central nucleus of the inferior colliculus. The binaural pathways from the medial and lateral superior olives (MSO and LSO) project to the same target. The dorsal cochlear nucleus (DCN) projects to the entire central nucleus of the inferior colliculus and does so in a more diffuse manner than does the AVCN. The DCN also sends sparse projections beyond the central nucleus into dorsal parts of the pericentral area. The intermediate (INLL) and ventral (VNLL) nuclei of the lateral lemniscus are relays in pathways that originate in the cochlear nucleus and terminate in the contralateral inferior colliculus. These nuclei also receive indirect input from the contralateral AVCN via the medial nucleus of the trapezoid body. Although nuclei of the lateral lemniscus project most densely to those areas of the inferior colliculus that are also the targets of the AVCN, MSO, and LSO, the nuclei of the lateral lemniscus also send spare projections outside these areas. Many of the pathways just described project in bands, a finding that raises the possibility that the projections parallel the orientation of disk-shaped cells in the inferior colliculus and raises the question of whether the bands from one source overlap or interdigitate with the bands from another source.

Variability in hand surface representations in areas 3b and 1 in adult owl and squirrel monkeys

Detailed microelectrode maps of the hand representation were derived in cortical areas 3b and 1 from a series of normal adult owl and squirrel monkeys. While overlap relationships were maintained, and all maps were internally topographic, many map features varied significantly when examined in detail. Variable features of the hand representations among different monkeys included a) the overall shapes and sizes of hand surface representations; b) the actual and proportional areas of representations of different skin surfaces and the cortical magnifications of representations of specific skin surfaces, which commonly varied severalfold in area 3b and manyfold in area 1; c) the topographic relationships among skin surface representations, with skin surfaces that were represented adjacently in some monkeys represented in locations many hundreds of microns apart in others; d) the internal orderliness of representations; e) the completeness of representations of the dorsal hand surfaces; and f) the skin surfaces represented along the borders of the hand representation. Owl monkey maps were, in general, internally more strictly topographic than squirrel monkey maps. In both species, area 3b was more strictly topographic and less variable than was area 1. The degree of individual variability revealed in these experiments is difficult to reconcile with the hypothesis that details of cortical maps are ontogenetically specified during a period in early life. Instead, we propose that differences in the details of cortical map structure are the consequence of individual differences in lifelong use of the hands. This conclusion is consistent with earlier studies of the consequences of peripheral nerve transection and digital amputation, which revealed that cortical maps are dynamically maintained and are alterable as a function of use or nerve injury in these monkeys (Merzenich et al., '83a,b, '84a; Merzenich, '86; Jenkins et al., '84; Jenkins and Merzenich, '87).

Projections from the cochlear nuclei in the mustache bat, Pteronotus parnellii

Ascending projections of the cochlear nuclei in the mustache bat were analyzed by anterograde transport of [3H]-leucine and by retrograde transport of HRP. We were particularly interested in pathways to two parts of the system: (1) to the medial superior olive, because this nucleus is missing in most echolocating bats, but appears to be present in the mustache bat, and (2) to the intermediate and ventral nuclei of the lateral lemniscus, because these nuclei are hypertrophied and highly differentiated in all echolocating bats that we have examined. The results show a highly systematic projection from the anteroventral cochlear nucleus to all of the auditory nuclei in the brain stem. After an injection of [3H]-leucine in the anterior and dorsal part of the anteroventral cochlear nucleus, presumably in a region sensitive to low frequencies, label is seen in the following locations: ipsilateral to the injection in the lateral part of the lateral superior olive; bilaterally in the dorsal part of the medial superior olive; contralateral to the injection in the dorsal parts of the intermediate and ventral nuclei of the lateral lemniscus; and in the anterolateral part of the central nucleus of the inferior colliculus. After an injection of [3H]-leucine in a posterior part of the anteroventral cochlear nucleus, presumably in a region sensitive to high frequencies, labeling is in the same set of nuclei, but within each nucleus the label is now in a different location: medially in the lateral superior olive, ventrally in the medial superior olive, ventrally in each division of the ventral and intermediate nuclei of the lateral lemniscus, and medially in the central nucleus of the inferior colliculus. Projections from the entire anteroventral cochlear nucleus to the inferior colliculus are confined to the ventral two-thirds of the central nucleus. The dorsal one-third of the central nucleus of the inferior colliculus is the principal target of the dorsal cochlear nucleus and may be a target of the posteroventral cochlear nucleus. Both of these nuclei appear to project sparsely to the ventral parts of the inferior colliculus. We conclude first that the bilateral input to the medial superior olive in the mustache bat is similar to the input seen in other mammals. Thus this bat has a neural structure which is associated with the analysis of binaural time differences and which usually is seen only in animals with heads large enough to create interaural time differences greater than those available to Pteronotus.(ABSTRACT TRUNCATED AT 400 WORDS)

Topology of the central nucleus of the mustache bat's inferior colliculus: correlation of single unit properties and neuronal architecture

The central nucleus of the mustache bat's inferior colliculus was studied in Golgi, Nissl, and fiber stained preparations; the neuronal organization and cytoarchitecture were correlated with the tonotopic maps revealed by single cell recordings. Three divisions of the central nucleus were defined by anatomical and physiological criteria: the anterolateral, medial, and dorsoposterior divisions. In horizontal sections, the anterolateral division has pronounced, semicircular fibrodendritic laminae. The dendritic fields of adjacent neurons form rostro-caudally-oriented laminae related to the tonotopic sequence. The neurons in the medial division are similar in size and arrangement, but here the laminar orientation follows the lateral-to-medial axis, with less curvature. The dorsoposterior division has many small disc-shaped and stellate neurons and a different, somewhat less fully expressed, laminar orientation. Each division has a unique frequency representation and tonotopic organization which conform to the pattern of dendritic orientation. In the anterolateral division, frequencies from about 10 kHz to about 59 kHz are represented, whereas the frequency representation in the medial division ranges from about 65 kHz to 110 kHz, and higher. The dorsoposterior division has an isofrequency organization in which the best frequency is characteristic for each bat, ranging from 60 to 64 kHz and varying by only +/- 300 Hz. This frequency corresponds to the dominant echo frequency of the bat's echolocation signals. We suggest that the dorsoposterior division is a hypertrophied isofrequency lamina, with many neurocytological features comparable to the isofrequency laminae in the central nucleus of other mammals.

Somatosensory cortical map changes following digit amputation in adult monkeys

The cortical representations of the hand in area 3b in adult owl monkeys were defined with use of microelectrode mapping techniques 2-8 months after surgical amputation of digit 3, or of both digits 2 and 3. Digital nerves were tied to prevent their regeneration within the amputation stump. Successive maps were derived in several monkeys to determine the nature of changes in map organization in the same individuals over time. In all monkeys studied, the representations of adjacent digits and palmar surfaces expanded topographically to occupy most or all of the cortical territories formerly representing the amputated digit(s). With the expansion of the representations of these surrounding skin surfaces (1) there were severalfold increases in their magnification and (2) roughly corresponding decreases in receptive field areas. Thus, with increases in magnification, surrounding skin surfaces were represented in correspondingly finer grain, implying that the rule relating receptive field overlap to separation in distance across the cortex (see Sur et al., '80) was dynamically maintained as receptive fields progressively decreased in size. These studies also revealed that: the discontinuities between the representations of the digits underwent significant translocations (usually by hundreds of microns) after amputation, and sharp new discontinuous boundaries formed where usually separated, expanded digital representations (e.g., of digits 1 and 4) approached each other in the reorganizing map, implying that these map discontinuities are normally dynamically maintained. Changes in receptive field sizes with expansion of representations of surrounding skin surfaces into the deprived cortical zone had a spatial distribution and time course similar to changes in sensory acuity on the stumps of human amputees. This suggests that experience-dependent map changes result in changes in sensory capabilities. The major topographic changes were limited to a cortical zone 500-700 micron on either side of the initial boundaries of the representation of the amputated digits. More distant regions did not appear to reorganize (i.e., were not occupied by inputs from surrounding skin surfaces) even many months after amputation. The representations of some skin surfaces moved in entirety to locations within the former territories of representation of amputated digits in every monkey studied. In man, no mislocation errors or perceptual distortions result from stimulation of surfaces surrounding a digital amputation.(ABSTRACT TRUNCATED AT 400 WORDS)

Intrinsic organization of the cat's medial geniculate body identified by projections to binaural response-specific bands in the primary auditory cortex

The area of the cat's primary auditory cortex (AI) within which high frequency sounds are represented can be subdivided using functional criteria. Within each subdivision, or "binaural interaction band," all recorded neurons display similar responses to binaural stimulation. The current study distinguishes the thalamic sources of input to these subdivisions of AI and characterizes the topography within the thalamic projection to each class of bands. The borders of binaural bands in AI were mapped using microelectrode recording with diotic tonal stimulation, then injections of one to three retrograde tracers were introduced into identified bands. Within the ventral division (V) of the medial geniculate body (the major thalamic source of input to AI), the neuronal populations that projected to different classes of binaural bands were strictly segregated from each other. This segregation of class-specific thalamic sources constitutes a laminar organization within an axis of V that is orthogonal to the previously described tonotopic organization. Excitatory/excitatory (EE) binaural neurons in AI were found to be segregated from excitatory/inhibitory (EI) neurons in alternating "bands." We consistently identified: (1) a ventral pair of rostrocaudally continuous EI and EE bands, (2) a middle area within which the pattern of binaural subdivisions was more variable and within which bands often were discontinuous rostrocaudally, and (3) a dorsal zone (DZ) within which the responses of neurons differed in binaural properties and in frequency specificity from the response patterns that were characteristic of neurons elsewhere in AI. Each EI band apparently derived input that converged from three thickened laminae of cells in V that were oriented approximately horizontally. The most ventral of these laminae encompassed the ovoidal part of V (Vo), suggesting that EI bands are the only recipients in AI of a projection from Vo. All of the EE bands and DZ derived their input from a single continuous structure which included the dorsal two-thirds of the rostral pole of V and a horizontal lamina interposed between the two dorsalmost EI-projecting laminae. Restricted portions of the complex EE-projecting structure in V projected preferentially to particular EE subdivisions of AI. The V-to-AI thalamocortical topography exhibited a high degree of convergence and divergence within the projections to each cortical binaural band and within the projections to each class of bands. These observations indicate that the high frequency representation in AI and its principal thalamic source of input, the ventral division of the medial geniculate body, may be thought of as assemblies of spatially discrete, functionally distinguishable subunits. The significance of this intrinsic organization is discussed in regard to the requirements for analysis of sound stimuli.

Origin of ascending projections to inferior colliculus in the mustache bat, Pteronotus parnellii

The origins of pathways to the inferior colliculus of the mustache bat were identified by retrograde transport of horseradish peroxidase (HRP). A specific goal of this study was to obtain evidence that would help determine whether the nuclei, shown in the previous paper to have unusual cytoarchitectural features, are unique to bats, or whether they are homologous to areas that are not well differentiated in other mammals. The auditory pathways in the lower brain stem of Pteronotus appear to conform to the same basic organization as in other mammals: After injection of HRP into one inferior colliculus, labeled cells are located contralaterally in the cochlear nucleus, ipsilaterally in the medical superior olive, bilaterally in the lateral superior olive, ipsilaterally in the ventral and intermediate nuclei of the lateral lemniscus, and bilaterally in the dorsal nucleus of the lateral lemniscus. These patterns of labeling provide a basis for understanding how the specialized auditory areas of the bat may be organized within a basic plan of mammalian auditory systems. In the anteroventral cochlear nucleus the unusually small spherical cells seem to be homologous to stellate cells in the anteroventral cochlear nucleus of the cat. In the superior olive, differences in patterns of labeled cells distinguish the medial from the lateral superior olive. In the lateral lemniscus the pattern of labeled cells shows clear differences between the two special parts, intermediate and ventral nuclei, as well as between these and the dorsal nucleus of the lateral lemniscus. The results are consistent with the hypothesis that the unusual auditory nuclei of the bat have homologues in mammals whose auditory systems are not specialized for echolocation.

Cytoarchitecture of auditory system in lower brainstem of the mustache bat, Pteronotus parnellii

To begin an investigation of the auditory pathways in the brainstem of the mustache bat, we examined the cytoarchitecture of the cochlear nuclei, superior olivary complex, nuclei of the lateral lemniscus, and inferior colliculus. Although all of these auditory centers are hypertrophied in this echolocating bat, only some areas have unusual cytoarchitectural features: 1) In the anterior part of the anteroventral cochlear nucleus we do not find the large spherical cells seen in other mammals; instead, very small spherical cells are found in this area. 2) In the posterior part of the anteroventral cochlear nucleus there is a region containing a homogeneous population of very large multipolar cells. 3) The medial superior olive is unusually large for an animal with a small head. 4) The most striking observations are seen in the lateral lemniscus. The ventral nucleus of the lateral lemniscus has a distinct columnar organization. The intermediate area of the lateral lemniscus contains a large and very distinct nucleus. Each of these cytoarchitectural features distinguishes the auditory system of this bat from that of other mammals. The results raise questions about whether or not there are unique subdivisions in the auditory pathways of echolocating bats. The results also identify these unusual nuclei as candidates to play a role in the special auditory functions related to echolocation.

Alterations in activity at auditory nuclei of the rat induced by exposure to microwave radiation: autoradiographic evidence using [14C]2-deoxy-D-glucose

Autoradiographic maps of brain activity in rats exposed to pulsed or continuous-wave (CW) microwave radiation were made using [14C]2-deoxy-D-glucose ([14C]2-DG). Special emphasis was given to measurements of activity in the auditory system because previous work had shown that pulsed microwave radiation can elicit auditory responses in man and other animals. In particular, one middle ear was ablated in nine rats to attenuate the transmission of air-borne sound to one cochlea. The resulting imbalance in auditory input for four animals not exposed to microwave radiation was reflected as a bilateral asymmetry of [14C]2-DG uptake at the inferior colliculus and medial geniculate body. In contrast, a symmetrical pattern of uptake at these structures in an animal exposed to pulsed microwave radiation showed that this stimulus bypasses the middle ear in eliciting auditory responses. This result established the utility of the [14C]2-DG method for demonstrating a known effect of microwave radiation on brain activity. The results also revealed responses at auditory nuclei in 4 animals exposed to CW microwave radiation. These responses, which have not been observed with other methods, were evident at the power densities of 2.5 and 10 mW/sq. cm. To exclude the possibility that CW microwave radiation produced this result by direct action on brain tissue, additional data were obtained from two rats with one cochlea destroyed. In both animals, the uptake of [14C]2-DG at the inferior colliculus and medial geniculate body was virtually identical to the uptake in animals not exposed to microwave radiation, i.e. greatest on the side of the brain contralateral to the intact cochlea. This finding, coupled with the finding of a bilateral symmetry of [14C]2-DG uptake in the auditory pathways of animals with one middle ear ablated, confirmed the hypothesis that auditory responses to CW microwave radiation originate within the cochlea. Effects on brain activity outside of the auditory system were not found in qualitative analyses of autoradiographs for the conditions of exposure to CW microwave radiation noted above or for exposure to pulsed microwave radiation at the average power density of 2.5 mW/sq. cm.

Competitive fighting in the rat

Competitive fighting was obtained in pairs of like-sexed laboratory rats by placing a single piece of food into the food hopper following 48 hr. of food deprivation. The fighting was characterized by offensive sideways posture, full aggressive posture, and bite and kick attack. Tests were conducted at 110-120 days of age on pairs of animals that had been housed together since weaning. Fighting was more frequent in pairs consisting of nonlittermates than in pairs of littermates, and it was equally frequent in male and female pairings. Probability of fighting was enhanced by prior experience with food deprivation, and attack was most often initiated by the heavier animal of the pair.