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.

Sex-biased parasitism, seasonality and sexual size dimorphism in desert rodents

We investigated seasonality of gender differences in the patterns of flea infestation in nine rodent species to test if sex-biased parasitism in terms of mean abundance, species richness, prevalence and the level of aggregation (a) varies among hosts and between seasons, and (b) is linked to sexual size dimorphism. Sexual size differences were significant in both summer and winter in Acomys cahirinus, Gerbillus pyramidum and Meriones crassus, and in winter only in Acomys russatus, Gerbillus dasyurus, Gerbillus nanus and Sekeetamys calurus. Sexual size dimorphism was male biased except for A. russatus in which it was female biased. Manifestation of sexual differences in flea infestation was different among hosts between seasons. A significant effect of sex on mean flea abundance was found in six hosts, on mean flea species richness in five hosts and on prevalence in two hosts. Male-biased parasitism was found in summer in one host only and in winter in five hosts. Female-biased parasitism occurred in winter in A. russatus. Gender differences in the slopes of the regressions of log-transformed variances against log-transformed mean abundances occurred in three hosts. No relationship was found between sexual size dimorphism and any parasitological parameter in any season using both conventional regressions and the method of independent contrasts. Our results suggest that sex-biased parasitism is a complicated phenomenon that involves several different mechanisms.

Determination of the elongate spermatid-Sertoli cell ratio in various mammals

Criteria were devised for determining the elongate spermatid-Sertoli cell ratio in various mammalian species at the electron microscope level. When data from particular species were pooled, the values were: rabbit, 12.17:1, hamster, 10.75:1; gerbil, 10.64:1; rat, 10.32:1; guinea-pig, 10.10:1; vole, 9.75:1; and monkey, 5.94:1. The elongate spermatid-Sertoli cell ratio is a measure of the workload of the Sertoli cell and is a prime factor determining their efficiency. The higher the ratio, the higher the sperm output is likely to be per given weight of seminiferous tubule parenchyma for a particular species.

Ascending auditory projections to the inferior colliculus in the adult gerbil, Meriones unguiculatus

Ascending auditory projections to the inferior colliculus (IC) of the adult gerbil were studied using the retrograde transport of horseradish peroxidase. Our results indicate that in gerbils, the IC receives afferent projections from most brainstem auditory nuclei. A strong contralateral projection originates in the cochlear nuclear complex (CN). A smaller but consistent projection from all three divisions of ipsilateral CN is also present. The medial superior olive (MSO), superior parolivary nucleus, and ventral nucleus of the lateral lemniscus all maintain ipsilateral projections to the IC. Bilateral projections arise from the lateral superior olive, lateral nucleus of the trapezoid body, and dorsal nucleus of the lateral lemniscus. Previous investigations in other mammalian species provide conflicting data concerning the magnitude of a direct ipsilateral projection from CN to the IC. Our quantitative data indicate that the ipsilateral projection from CN in the gerbil is nearly one third as large as the projection from ipsilateral MSO. The projection from contralateral CN is six times larger than the MSO projection. The distribution of labeled cells across the rostrocaudal extent of MSO and the three divisions of the cochlear nuclear complex are presented.

Intragranular mossy fibers in rats and gerbils form synapses with the somata and proximal dendrites of basket cells in the dentate gyrus

Intragranular and supragranular mossy fibers arise from granule cells and are present in the dentate gyrus of hippocampi from kindled and epileptic animals. The intragranular fibers often appear as fibers perpendicular to the long axis of the granule cell layer at periodic intervals. Rats and gerbils were analyzed to determine whether such mossy fibers are also associated with nongranule cells (including the basket cells), which send their apical dendrites through this layer with a periodicity similar to that of mossy fibers. The results for rats and both epileptic and nonepileptic gerbils show that many intragranular mossy fibers are apposed to the surfaces of the somata and apical dendrites of basket cells where they form asymmetric synapses. This plexus of mossy fiber axons appears to follow the dendrites of these neurons into the inner molecular layer. Based on previous data indicating that basket cells are GABAergic inhibitory neurons, the present findings in normal rats and both types of gerbils suggest that intragranular and supragranular mossy fibers provide additional circuitry for feedback inhibition to granule cells. It is possible that under pathological conditions, such as denervation or kindling, these fibers sprout and form synapses with granule cells.

Functional organization of auditory cortex in the Mongolian gerbil (Meriones unguiculatus). III. Anatomical subdivisions and corticocortical connections

The auditory cortex of the Mongolian gerbil comprises several physiologically identified fields, including the primary (AI), anterior (AAF), dorsal (D), ventral (V), dorsoposterior (DP) and ventroposterior (VP) fields, as established previously with electrophysiological [Thomas et al. (1993) Eur. J. Neurosci., 5, 882] and functional metabolic techniques [Scheich et al. (1993) Eur. J. Neurosci., 5, 898]. Here we describe the cyto-, myelo- and chemoarchitecture and the corticocortical connections of the auditory cortex in this species. A central area of temporal cortex corresponding to AI and the rostrally adjacent AAF is distinguished from surrounding cortical areas by its koniocortical cytoarchitecture, by a higher density of myelinated fibres, predominantly in granular and infragranular layers, and by characteristic patterns of immunoreactivity for the calcium-binding protein parvalbumin (most intense staining in layers III/IV and VIa) and for the cytoskeletal neurofilament protein (antibody SMI-32; most intense staining in layers III, V and VI). Concerning the cortical connections, injections of the predominantly anterograde tracer biocytin into the four tonotopically organized fields AI, AAF, DP and VP yielded the following labelling patterns. (i) Labelled axons and terminals were seen within each injected field itself. (ii) Following injections into AI, labelled axons and terminals were also seen in the ipsilateral AAF, DP, VP, D and V, and in a hitherto undescribed possible auditory field, termed the ventromedial field (VM). Similarly, following injections into AAF, DP and VP, labelling was also seen in each of the noninjected fields, except in VM. (iii) Each field projects to its homotopic counterpart in the contralateral hemisphere. In addition, field AI projects to contralateral AAF, DP and VP, field DP to contralateral AI and VP, and field VP to contralateral AI and DP. (iv) Some retrogradely filled pyramidal neurons within the areas of terminal labelling indicate reciprocal connections between most fields, both ipsilateral and contralateral. (v) The labelled fibres within the injected and the target fields, both ipsilateral and contralateral, were arranged in continuous dorsoventral bands parallel to isofrequency contours. The more caudal the injection site in AI the more rostral was the label in AAF. This suggests divergent but frequency-specific connections within and, at least for AI and AAF, also across fields, both ipsilateral and contralateral. (vi) Projections to associative cortices (perirhinal, entorhinal, cingulate) and to other sensory cortices (olfactory, somatosensory, visual) from AAF, DP and VP appeared stronger than those from AI. These data support the differentiation of auditory cortical fields in the gerbil into at least 'core' (AI and AAF) and 'noncore' fields. They further reveal a complex pattern of interconnections within and between auditory cortical fields and other cortical areas, such that each field of auditory cortex has its unique set of connections.

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

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

Structural correlates of seizure behavior in the mongolian gerbil

Hippocampi of seizure-sensitive and seizure-resistant Mongolian gerbils were examined in search of structural correlates of seizure behavior. In animals with well-established seizure histories, differences were found in both presynaptic and postsynaptic structures. Seizing animals had less dense dendritic spines, a greater proportion of mossy tuft area devoted to presynaptic vesicles, and a smaller proportion devoted to spines. The possible relationship of these findings to epilepsy is discussed.

Descending projections from the auditory cortex to the inferior colliculus in the gerbil, Meriones unguiculatus

Corticofugal projections to the auditory midbrain, the inferior colliculus (IC), influence the way in which specific sets of IC neurons process acoustic signals. We used retrograde tracer (Fluorogold, Fluororuby, microbeads) injections in the IC to study the morphology and location of cortico-collicular projecting neurons and anterograde tracer (dextran biotin) injections in auditory cortical fields to describe the distribution of terminals in the IC. Nissl staining, cytochrome oxidase activity, and neurofilament SMI32 immunostaining were used to delimit the different auditory areas. We defined a primary or "core" auditory cortex and a secondary "caudal" auditory area containing layer V pyramidal neurons that project to the IC. These projections target the central nucleus of the IC (CNIC) ipsilaterally and the IC cortices bilaterally, with the ipsilateral component predominant. Other secondary auditory areas, dorsal and ventral to the core, do not directly participate in this projection. The ventral secondary cortex targets midbrain periaqueductal gray. The projection from the core cortex originates from two classes of layer V pyramidal cells. Cells presenting a tufted apical dendrite in layer I have dense terminal fields in the IC cortices. Pyramids lacking layer I dendritic tufts target the CNIC in a less dense but tonotopic manner. The caudal cortex projection originates from smaller layer V pyramids and targets the IC cortices with dense terminal fields. Descending auditory inputs from the core and caudal areas converge in the dorsal and external cortices of the IC. Descending connections to the gerbil IC form a segregated system in which multiple descending channels originating from different neuronal subpopulations may modulate specific aspects of ascending auditory information.

Projections from the lateral nucleus of the trapezoid body to the medial superior olivary nucleus in the gerbil

We made small injections of horseradish peroxidase into the medial superior olivary nucleus (MSO) of gerbils in order to examine the sources of input into that nucleus. As previously described, the MSO receives inputs from neurons in the rostral part of both anteroventral cochlear nuclei. In addition, we found evidence for a projection from the ipsilateral lateral nucleus of the trapezoid body (LNTB). Our results are also compatible with previous reports that the medical nucleus of the trapezoid body (NMTB) projects to the MSO. It is likely that these projections into the MSO from the LNTB and MNTB are sources of inhibitory synaptic inputs.

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.

Distribution of vestibular afferents that innervate the sacculus and posterior canal in the gerbil

The central distribution of afferents that innervate the macula of the saccule and the crista of the posterior canal was assessed in the gerbil following the direct injection of horseradish peroxidase (HRP) separately into the sensory neuroepithelia of each peripheral receptor organ. Ganglion cells innervating the posterior canal were located in the caudal part of the inferior ganglion, while those cells innervating the saccule were located in the rostral part of the inferior ganglion, scattered in the superior ganglion, and concentrated at the junction (isthmus) between the two. The paths of the central axons of these two groups of ganglion cells through the vestibular root and their division into ascending or descending pathways were similar. However, the distributions of their terminals were different. The posterior canal projected to medial parts of the vestibular nuclear complex. Terminals were found in the medial and superior vestibular nuclei. The posterior canal also projected to the uvula of the cerebellum. The saccule projected to more lateral-lying brainstem areas. Terminal fields were located in the lateral and descending vestibular nuclei and cell group y. Saccule projections outside the vestibular complex were observed to the lateral cuneate nucleus, the N. gigantocellularis, and the cerebellar cortex. Of the eight areas receiving primary afferent projections from these two organs, only within the medial and descending vestibular nuclei and the cerebellar cortex were overlapping projections observed.

Functional organization of auditory cortex in the Mongolian gerbil (Meriones unguiculatus). IV. Connections with anatomically characterized subcortical structures

The subcortical connections of the four tonotopically organized fields of the auditory cortex of the Mongolian gerbil, namely the primary (AI), the anterior (AAF), the dorsoposterior (DP) and the ventroposterior field (VP), were studied predominantly by anterograde transport of biocytin injected into these fields. In order to allow the localization of connections with respect to subdivisions of subcortical auditory structures, their cyto-, fibre- and chemoarchitecture was characterized using staining methods for cell bodies, myelin and the calcium-binding protein parvalbumin. Each injected auditory cortical field has substantial and reciprocal connections with each of the three subdivision of the medial geniculate body (MGB), namely the ventral (MGv), dorsal (MGd) and medial division (MGm). However, the relative strengths of these connections vary: AI is predominantly connected with MGv, AAF with MGm and MGv, and DP and VP with MGd and MGv. The connections of at least AI and MGv are topographic: injections into caudal low-frequency AI label laterorostral portions of MGv, whereas injections into rostral high-frequency AI label mediocaudal portions of MGv. All investigated auditory fields send axons to the suprageniculate, posterior limitans, laterodorsal and lateral posterior thalamic nuclei, with strongest projections from DP and VP, as well as to the reticular and subgeniculate thalamic nuclei. AI, AAF, DP and VP project to all three subdivisions of the inferior colliculus, namely the dorsal cortex, external cortex and central nucleus ipsilaterally and to the dorsal and external cortex contralaterally. They also project to the deep and intermediate layers of the ipsilateral superior colliculus, with strongest projections from DP and VP to the lateral and basolateral amygdaloid nuclei, the caudate putamen, globus pallidus and the pontine nuclei. In addition, AAF and particularly DP and VP project to paralemniscal regions around the dorsal nucleus of the lateral lemniscus (DNLL), to the DNLL itself and to the rostroventral aspect of the superior olivary complex. Moreover, DP and VP send axons to the dorsal lateral geniculate nucleus. The differences with respect to the existence and/or relative strengths of subcortical connections of the examined auditory cortical fields suggest a somewhat different function of each of these fields in auditory processing.

On the relationship of brain vasculature to production of neurological deficit and morphological changes following acute unilateral common carotid artery ligation in gerbils

The known susceptibility of the Mongolian gerbil to cerebral infarction following unilateral carotid artery ligation has been attributed in the past to the demonstrated absences of an anastomotic supply between the anterior and posterior cerebral circulations. In a study of 34 adult gerbils exposed to such a procedure, 11, or 33%, developed severe neurological sequelae and succumbed to the procedure in less than 30 hr, whereas 23 animals survived with only minor or transient neurological signs. All animals displayed the expected lack of an anastomosis between the anterior and posterior circulations, but in addition the animals which survived the procedure were found to have a prominent early cross-connection between the anterior cerebral arteries, whereas the animals which succumbed had no such connection. Neuropathological changes in susceptible animals were apparent as early as 3 and one-half hr after ligation and consisted of edema, initially perivascular and then intraneuronal, slowed by acute necrosis. A variety of other vascular anomalies was encountered. We conclude that the peculiar susceptibility of Mongolian gerbils to cerebral infarction following acute unilateral common carotid artery ligation is not related primarily to lack o adequate collaterals between the anterior and the anterior cerebral arteries, but to the degree of adequate adequacy of communication between the anterior cerebral arteries. The critical difference may be more one of degree, i.e. the point at which the medial branches of the anterior cerebral artery fust to become anazygos vessel, rather than an actual difference in the pattern of distribution of the anterior cerebral arteries. The presence of other variation in vascular supply in a relatively small series suggests that results of similar studies of infarction and response to treatment be interpreted with caution.

Seasonal adjustments in body mass and thermogenesis in Mongolian gerbils (Meriones unguiculatus): the roles of short photoperiod and cold

Seasonal adjustments in body mass and thermogenesis are important for the survival of small mammals during acclimatization in the temperate zone. To determine the contributions of short photoperiod and cold temperatures to seasonal changes in thermogenesis and body mass in Mongolian gerbils (Meriones unguiculatus), body mass, basal metabolic rate (BMR), nonshivering thermogenesis (NST), energy intake and energy digestibility were determined in seasonally acclimatized and laboratory acclimated animals. Body mass showed significant seasonal changes and decreased to a minimum in winter. Both BMR and NST increased in winter, and these changes were mimicked by exposing animals to short photoperiod or cold temperatures in the animal house. Digestible energy intake also increased significantly in winter, and also during exposure of housed animals to both short photoperiod and cold. These results suggest that Mongolian gerbils overcome winter thermoregulatory challenges by increasing energy intake and thermogenesis, and decreasing body mass to reduce total energy requirements. Short photoperiod and cold can serve as effective environmental cues during seasonal acclimatization.

Retinal afferents to the dorsal raphe nucleus in rats and Mongolian gerbils

A direct pathway from the retina to the dorsal raphe nucleus (DRN) has been demonstrated in both albino rats and Mongolian gerbils. Following intraocular injection of cholera toxin subunit B (CTB), a diffuse stream of CTB-positive, fine-caliber optic axons emerged from the optic tract at the level of the pretectum/anterior mesencephalon. In gerbils, CTB-positive axons descended ventromedially into the periaqueductal gray, moving caudally and arborizing extensively throughout the DRN. In rats, the retinal-DRN projection comprised fewer, but larger caliber, axons, which arborized in a relatively restricted region of the lateral and ventral DRN. Following injection of CTB into the lateral DRN, retrogradely labeled ganglion cells (GCs) were observed in whole-mount retinas of both species. In gerbils, CTB-positive GCs were distributed over the entire retina, and a nearest-neighbor analysis of CTB-positive GCs showed significant regularity (nonrandomness) in their distribution. The overall distribution of gerbil GC soma diameters ranged from 8 to 22 micrometer and was skewed slightly towards the larger soma diameters. Based on an adaptive mixtures model statistical analysis, two Gaussian distributions appeared to comprise the total GC distribution, with mean soma diameters of 13 (SEM +/-1.7) micrometer, and 17 (SEM +/-1.5) micrometer, respectively. In rats, many fewer CTB-positive GCs were labeled following CTB injections into the lateral DRN, and nearly all occurred in the inferior retina. The total distribution of rat GC soma diameters was similar to that in gerbils and also was skewed towards the larger soma diameters. Major differences observed in the extent and configuration of the retinal-DRN pathway may be related to the diurnal/crepuscular vs. nocturnal habits of these two species.

The cochlea in gerbilline rodents

The inner ears of 5 different gerbil species are compared on the basis of cochlear microphonic recordings, serial sections and computerized quantitative reconstructions of the cochleae and their specific morphological structures. The hearing range of most gerbils is below 20 kHz. Some species are extremely sensitive in the frequency range of 1-4 kHz. This special sensitivity is reflected in, among other features, the following cochlear structures and their suggested functions: (1) the rapid width increase of the basilar membrane in the basal portion of the cochlea provides additional space for the representation of lower frequencies at the expense of higher frequencies; (2) the large hyaline mass and the cells of Claudius and Hensen in the medial and apical portions of the cochlea influence the vibratory properties of the cochlear partition, and (3) the specialized structures of the cochlea may be an adaptation to the acoustical environment in arid habitats.

Sound-power collection by the auditory periphery of the Mongolian gerbil Meriones unguiculatus. I: Middle-ear input impedance

This is the first paper of a series dealing with sound-power collection by the auditory periphery of the gerbil. The purpose of the series is to quantify the physiological action of the gerbil's relatively large tympanic membrane and middle-ear air cavities. To this end the middle-ear input impedance ZT was measured at frequencies between 10 Hz and 18 kHz before and after manipulations of the middle-ear cavity. The frequency dependence of ZT is consistent with that of the middle-ear transfer function computed from extant data. Comparison of the impedance and transfer function suggests a middle-ear transformer ratio of 50 at frequencies below 1 kHz, substantially smaller than the anatomical value of 90 [Lay, J. Morph. 138, 41-120 (1972)]. Below 1 kHz the data suggest a low-frequency acoustic stiffness KT for the middle ear of 970 Pa/mm3 and a stiffness of the middle-ear cavity of 720 Pa/mm3 (middle-ear volume V MEC of 195 mm3); thus the middle-ear air spaces contribute about 70% of the acoustic stiffness of the auditory periphery. Manipulations of a middle-ear model suggest that decreases in V MEC lead to proportionate increases in KT but that further increases in middle-ear cavity volume produce only limited decreases in middle-ear stiffness. The data and the model point out that the real part of the middle-ear impedance at frequencies below 100 Hz is determined primarily by losses within the middle-ear cavity. The measured impedance is comparable in magnitude and frequency dependence to the impedance in several larger mammalian species commonly used in auditory research. A comparison of low-frequency stiffness and anatomical dimensions among several species suggests that the large middle-ear cavities in gerbil act to reduce the middle-ear stiffness at low frequencies. A description of sound-power collection by the gerbil ear requires a description of the function of the external ear.

Identification of vestibular efferent neurons in the gerbil: histochemical and retrograde labelling

The efferent neurons of the gerbil vestibular system were investigated by retrograde tracing techniques and cytochemical staining for acetylcholinesterase (AChE), choline acetyltransferase (ChAT) and a number of peptides. The location, bilateral distribution, cell area and number of neurons in two identified groups of retrogradely labelled cells were described and quantified. The larger of the two groups was located dorsolateral to the facial nerve genu, ventral and medial to the vestibular nuclei. Unilateral tracer injection in the vestibular end organs labelled cells bilaterally in this and the smaller group, which was located immediately ventral to the genu. No cells were found that individually projected bilaterally to both labyrinths. After injections of horseradish peroxidase (HRP) in the utricle or saccule, significantly more cells were located on the contralateral side of the brainstem. The average (+/- SD) cross sectional area of labelled cell bodies associated with the otolith organs was 259.8 (+/- 75.2) microns 2. ChAT immunoreactive and AChE positive cells were found in an area coextensive with the location of the dorsal efferent group. In double-labelling studies, cell bodies in the same group that had been retrogradely labelled with a utricular injection of HRP, were immunocytochemically stained for calcitonin gene-related peptide and met-enkephalin. In contrast, the ventral group of efferents did not have cells that were cytochemically stained for either of the acetylcholine-related enzymes or either peptide. The significance of the existence of peptidergic vestibular efferent neurons is discussed.

Projections from the sacculus to the cochlear nuclei in the Mongolian gerbil

The projections of the saccule, an otolith end organ, to the cochlear nuclei were studied using both transganglionic transport and intracellular injection techniques. Labeled fibers and terminals were observed in the anterior and posterior portions of the ventral cochlear nucleus and the dorsal cochlear nucleus. Most terminals were present in the granule cell domain, especially in the subpeduncular corner between the anteroventral cochlear nucleus and the floccular peduncle of the cerebellum. It has been hypothesized that the cochlea in mammals may have developed phylogenetically from the saccule. The projections from the saccule to the cochlear nuclei were investigated in a mammalian species, the Mongolian gerbil, in an attempt to obtain initial information supporting or refuting this hypothesis. The presence of an otolith end organ projection to the cochlear nuclei in rodents should encourage comparative studies in additional aspects of the evolution of the auditory system.

The pineal gland of the gerbil, Meriones unguiculatus. I. An ultrastructural study

Electron microscopy was employed in a study of the pineal gland of the Mongolian gerbil (Meiones unguiculatus). It was determined that the gerbil pineal gland contains pinealocytes and glial cells with the pinealocytes being the predominant cell type. The pinealocytes contain numerous organelles traditionally considered as being either synthetic or secretory in function such as an extensive Golgi region, smooth (SER) and rough (RER) endoplasmic reticulum, secretory vesicles and microtubules. Other cytoplasmic components are also present in the pinealocytes (synaptic ribbons, subsurface cisternae) for which no function has been assigned. Dense-cored vesicles are rare. Vacuolated pinealocytes are present and appear to be intimately associated with the formation of the pineal concertions. Evidence presented supports the proposal that the concretions form within the vacuoles. Once the concretions reach an enlarged state, the vacuolated pinealocytes break down and the concretions are thus extruded into the extracellular space where they apparently continue to increase in size. The morphology of the glial cells was interpreted as indicative of a high synthetic activity. The glial cells contain predominantly the rough variety of endoplasmic reticulum and form an expansion around the wide perivascular area.