Diurnal variation of c-Fos expression in subdivisions of the dorsal raphe nucleus of the Mongolian gerbil (Meriones unguiculatus).

Recent studies suggest that the dorsal raphe nucleus (DRN) of the brainstem contains several subdivisions that differ both anatomically and neurochemically. The present study examined whether variation of c-Fos expression across the 24-hour light-dark cycle may also be different in these subdivisions. Animals were kept on a 12:12 light-dark cycle, were perfused at seven different time points, and brain sections were processed by using c-Fos immunocytochemistry. At all coronal levels of the DRN, c-Fos expression reached a peak 1 hour after the light-dark transition (lights-off) and reached its lowest levels in the middle of the light period. In contrast to the light-dark transition, c-Fos levels did not change significantly after the dark-light transition (lights-on). One-way analysis of variance (ANOVA) revealed that the diurnal variation of c-Fos expression was highly significant in the caudal ventral DRN. Similar variation in c-Fos expression also was observed in the other DRN subdivisions, but this variation appeared to gradually diminish in the caudal-to-rostral and ventromedial-to-dorsomedial directions. Double-label immunocytochemistry revealed that, 1 hour after lights-off, only 11% of c-Fos-positive neurons in the caudal ventral DRN were serotonin (5-HT)-immunoreactive. These results suggest that DRN subdivisions may differ functionally with regard to the diurnal cycle, and that these differences may be reflected in the activity of nonserotonergic cells in the DRN.     

Grating acuity of the Mongolian gerbil (Meriones unguiculatus)

The grating acuity of the Mongolian gerbil was measured using a two-alternative forced-choice task in a jumping stand. Frequency-of-seeing curves generated by a modified method of limits indicated that the acuity of the gerbil for a horizontal square-wave grating is about 1.5-2.0 cycles/degree (c/deg) at 70 cd/m2, using the 70%- and 80%-correct levels of performance as criteria for defining acuity. The acuity for horizontal gratings is reliably better than that for vertical gratings, and this difference does not appear to be luminance-dependent. A corresponding orientation anisotropy is not seen in acquisition scores, nor is an 'oblique effect' evident. The acuity of the gerbil continues to increase with luminance into the (human) photopic range, suggesting the existence of a functional cone system. The gerbil visual system appears to be well adapted for detail vision in a diurnal lifestyle.     

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.     

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.     

Eye and brain growth in the Mongolian gerbil (Meriones unguiculatus)

Eye and brain growth were evaluated by measures of weight and surface dimensions at 11 ages spanning the period from birth to 150 days of age in the Mongolian gerbil (Meriones unguiculatus). Brain weight was found to increase very rapidly until day 18, followed by a more gradual increase to asymptote by 70 days of age. Eye growth follows overall body growth more closely than brain growth. Eye shape was found to change from ovoid to spherical over the first 70 days of life. The possible relationship between eye growth and retinal specialization is considered, and brain growth is compared across several rodent species.     

The effects of posterior cortical lesions on responses to visual threats in the Mongolian gerbil (Meriones unguiculatus).

Mongolian gerbils received aspiration lesions of either primary visual cortex (PVC), medial extrastriate visual cortex, retrosplenial cortex (RSC), or sham operations. The responses of gerbils to the presentation of an overhead visual stimulus were recorded in an open field. In all groups, presentation of the stimulus produced an increase in rearing. This suggests that the stimulus was detected by all animals. Gerbils with RSC or PVC lesions showed reduced levels of response to the stimulus. We suggest that some of the observed deficits can be explained as failures to produce responses to threat that are appropriate to the context in which the the threat was presented.     

The development of visual acuity in the Mongolian gerbil (Meriones unguiculatus)

The development of visual acuity for vertically oriented gratings was followed in gerbil pups from 34 days of age until adulthood using a jumping stand procedure. Acuity was found to improve gradually from an initial level of about 0.7 cycles/degree to asymptotic values of 1.2-1.6 cycles/degree by 70 days of age. Acuity did not vary as a function of viewing distance over the range tested (17.5-46 cm). Practice was found to play some role in the acuity changes seen but cannot alone explain the results. The contributions of other performance variables and of neural development are considered.     

Stereotaxic atlas of the hypothalamus of the Mongolian gerbil (Meriones unguiculatus)

A stereotaxic atlas of the hypothalamic area of the Mongolian gerbil (Meriones unguiculatus) is presented along with medical coordinates for all primary nuclei and the third ventricle.     

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.     

Certain efferent cortical fiber pathways of the Mongolian gerbil (Meriones unguiculatus)

Three areas of responsive cortex were demonstrated by electrical stimulation in the gerbil. Low voltage stimulation of frontal areas yielded a sequential pattern of discrete contralateral movements. Proceeding from a rostroventral to a caudodorsal position facial movements were obtained followed by upper extremity, trunk and lower extremity movements. This area includes primary motor cortex (area 4) in the gerbil. A comparable rostrocaudal motor pattern was obtained by stimulation above the rhinal fissure (insular cortex) and also from certain parietal areas. Although the sequential motor pattern was obvious in these additional areas of excitable cortex, the movements were more generalized and slightly higher voltages were required to obtain satisfactory results. A series of destructive lesions were made in each of these cortial areas. Subsequent degeneration studies, using reduced silver techniques, revealed that frontal and parietal cortex gave rise to corticospinal fibers. In addition all three areas gave rise to fibers which coursed to certain extrapyramidal nuclei of the basal ganglia, ventral thalamus, midbrain tegmentum, and medullary reticular formation.     

The ontogenesis of sensorimotor reflexes in the Mongolian gerbil Meriones unguiculatus.

The ontogeny of a number of sensorimotor reflexes has been studied in the Mongolian gerbil. In contrast to a number of other mammals, the gerbil has relatively long and strong hindlimbs but small forelimbs, indicating their different importance for a number of locomotor acts, and during the developmental period studied, the hindlimbs grow at a much faster rate than the forelimbs. The following sequence of appearance and maturation of the reflexes was observed: rooting, forelimb hopping, surface body righting, forelimb grasp, hindlimb hopping, chin tactile placing, visual placing, air body righting and, at the same time, forelimb and hindlimb tactile placing. This sequence concords with the general gradient of development and maturation of the spinal and brain centers subserving these reflexes, as evaluated from Nissl preparations. The results indicate that there is no clearcut rostro-caudal gradient of postnatal maturation of the spinal cord and the spinally mediated reflexes, but that there is a general caudo-rostral gradient of brain maturation and of the brain-mediated reflexes. Comparisons with other mammals are made.     

Subdivisions of the dorsal raphe nucleus projecting to the lateral geniculate nucleus and primary visual cortex in the Mongolian gerbil

The mammalian dorsal raphe nucleus (DRN) is composed of sub-divisions with different anatomical and functional properties. Using cholera toxin subunit B as a retrograde tracer, DRN subdivisions projecting to the lateral geniculate nucleus and to the primary visual cortex were examined in the Mongolian gerbil. DRN neurons projecting to the lateral geniculate nucleus were observed in the lateral DRN (rostrally) and in the ventromedial DRN (caudally), while DRN cells projecting to the primary visual cortex were observed at all rostral-caudal levels in the ventromedial DRN. These results demonstrate a significant overlap between the DRN projections to the lateral geniculate and superior colliculus, and show that only the caudal ventromedial DRN projects to all three major visual targets: the lateral geniculate nucleus, primary visual cortex, and superior colliculus. Since the DRN is involved in depression and other neuropsychiatric disorders, as well as is affected by many psychotropic substances, these data may help to develop new treatments and therapies targeting specific DRN subdivisions.     

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.     

Opiate receptor binding in the brain of the seizure sensitive Mongolian gerbil (Meriones unguiculatus)

Opiate receptor binding was studied in seizure sensitive (SS) and seizure resistant (SR) strains of the Mongolian gerbil. Cryostat sections of the brain were labeled with [3H]-dihydromorphine, subjected to autoradiography and analysed by microdensitometry. SS gerbils, prior to seizure induction, demonstrated overall greater brain opiate binding when compared to SR animals. Immediately following a seizure, binding in the interpeduncular nucleus fell to levels found in SR animals. The increased opiate binding in the SS (pre-seizure) compared to SR gerbils could reflect a deficit of endogenous ligand which could underlie the seizure diathesis in the gerbil.     

Ascending projections to the inferior colliculus following unilateral cochlear ablation in the neonatal gerbil,Meriones unguiculatus

We evaluated the consequences of neonatal cochlear destruction upon ascending projections to the inferior colliculi. Unilateral cochlear ablations were performed in both neonatal and adult gerbils. Four to 12 months later, the inferior colliculus (IC) was examined physiologically and injected uni-laterally with horseradish peroxidase (HRP). The number of labeled cells was determined bilaterally in all three divisions of cochlear nucleus (CN) and in the medial superior olive (MSO). In both experimental groups, trans-ncuronal changes within the deafferented CN were greater in the ventral divisions than in the dorsal division. On the unoperated side the magnitude of projections from CN to the inferior colliculi was altered in animals le-sioned as neonates. Following HRP injections into the IC on the unoperated side, the number of ipsi later ally labeled cells in CN (unoperated side) was significantly greater in the neonatal experimental group than in adult ex-perimental and control animals. These anatomical changes were accom-panied by increased ipsilaterally evoked excitatory activity recorded in the IC on the unoperated side. Following HRP injections into the IC on the ablated side, the number of contralaterally labeled cells in CN (unoperated side) was significantly reduced in animals lesioned as neonates as compared with control animals. The number of labeled cells in ipsilateral MSO was not significantly different across groups. Our interpretation is that unilat-eral cochlear ablation in neonatal gerbils results in an increase in the magnitude of ipsilateral projections and a decrease in the magnitude of contralateral projections from CN on the unoperated side to the inferior colliculi. These data suggest that the normal pattern of innervation of the IC results, in part, from interactions among afferent projections.     

Topographic organization of rat locus coeruleus and dorsal raphe nuclei: Distribution of cells projecting to visual system structures

Previous reports from this laboratory and elsewhere have provided evidence that the locus coeruleus (LC) and dorsal raphe (DR) nuclei are topographically organized with respect to their efferent targets. Whereas most of these previous studies have focused on relationships between these monoamine-containing brainstem nuclei and cerebral cortex, basal ganglia, and limbic structures, they have not systematically examined the distribution of LC and DR cells that project to multiple structures with common sensory or motor functions. The goal of the present study was to characterize and compare the distributions of LC and DR cells which project to different visual areas of the rat central nervous system. Long-Evans hooded rats received unilateral pressure injections of the retrograde tracer wheat germ agglutinin-horseradish peroxidase in either the dorsal lateral geniculate, ventral lateral geniculate, or lateral posterior nucleus of thalamus; superior colliculus, cortical area 17, cortical area 18a/b cerebellar vermis (lobules VI and VII); or paraflocculus. Transverse sections through the midbrain and pons were examined by light microscopy after performing routine tetramethyl benzidine histochemical procedures. For all cases studied, retrogradely labeled cells were observed throughout the rostrocaudal extent of the LC and DR; however, labeling patterns which were distinctive for different injection sites were noted in each of these brainstem nuclei. The major conclusion drawn from this work is that subsets of LC and DR cells which project to different target structures within the rat visual system are found in overlapping but not necessarily coextensive zones within these nuclei. These studies provide further evidence of a rough topographic ordering within both the LC and DR nuclei, as well as support a new hypothesis that the outputs from each of these nuclei are organized with respect to the sensory related functions of their efferent targets. © 1993 Wiley-Liss, Inc.     

Fetal asphyxia leads to a decrease in dorsal raphe serotonergic neurons

The aim of the present study was to determine the effects of fetal asphyxia (FA) on anxiety and serotonergic neurons in young adult and middle-aged rats. FA was induced at embryonic day 17 by clamping the uterine circulation for 75 min. Anxiety-related behavior was tested in an open field, and design-based stereology was used for counting serotonergic (5-hydroxytryptamine/serotonin, 5-HT) neurons in the dorsal raphe nucleus (DRN). The open field revealed increased anxiety in the 19-month-old FA rats in comparison to control animals. No significant differences were found in DRN 5-HT neuron numbers at 6 months. At 19 months, however, FA significantly lowered the mean density and volume of 5-HT neurons in the DRN as compared to controls. Further, an age-related reduction was found in the total number, the mean density and the mean volume of 5-HT neurons within the FA group. In conclusion, FA is associated with increased anxiety and age-related changes in 5-HT immunohistochemistry within the DRN. These results support the notion that insults caused by asphyxiation during critical periods of brain development could create a predisposition to serotonergic abnormalities and anxiety deficits in adulthood.     

Distribution and characterization of different molecular products of pro-somatostatin in the hypothalamus and posterior pituitary lobe of the Mongolian gerbil (Meriones unguiculatus).

Antisera raised against various synthetic peptide fragments of the pro-somatostatin molecule were used to visualize immunohistochemically the distributions of different pro-somatostatin fragments in the hypothalamus and posterior pituitary of the Mongolian gerbil. To define the nature of the immunoreactive somatostatin-related molecular forms, gel chromatography combined with radioimmunoassays of hypothalamic and posterior pituitary extracts was performed. Within the hypothalamus, only trace amounts of somatostatin-28 and somatostatin-28(1-12) were present, whereas pro-somatostatin(1-76), pro-somatostatin(1-64), and somatostatin-14 peptides were present in equimolar amounts. In contrast, the posterior pituitary lobe contained equal amounts of somatostatin-14, somatostatin-28, and somatostatin-28(1-12) but no pro-somatostatin(1-76), indicating that pro-somatostatin is further processed during the axonal flow to posterior pituitary nerve terminals. The gel chromatographic data were further substantiated by immunohistochemical data. Thus, perikarya containing all of these five immunoreactivities were strictly confined to the periventricular area and parvocellular subset of the paraventricular nucleus. However, the number of somatostatin-28- and somatostatin-28(1-12)-immunoreactive perikarya was approximately 20% of the number of somatostatin-14- and pro-somatostatin(1-64)-immunoreactive cells. In other hypothalamic areas only somatostatin-14 and pro-somatostatin(1-64) immunoreactivities were detectable in cell bodies. These cell bodies were encountered in the organum vasculosum laminae terminalis; the suprachiasmatic, ventromedial, arcuate, perifornical, and posterior hypothalamic nuclei; and the median preoptic and retrochiasmatic areas. In situ hybridization histochemistry revealed that the cellular distribution of pro-somatostatin mRNA corresponds to that of somatostatin-14 and pro-somatostatin immunoreactivity, suggesting that the immunoreactive material observed within the cell bodies is synthetized there and that the differences in density of immunoreactivities may be explained by intracellular processing of pro-somatostatin. Somatostatinergic nerve fibers and terminals in hypothalamic areas and the posterior pituitary lobe were immunoreactive to all of the employed antisera. From the present results, obvious differences between intrahypothalamic and hypothalamo-pituitary somatostatinergic neurons emerge. Within hypothalamic neurons not projecting to the median eminence and the posterior pituitary lobe, pro-somatostatin is posttranslationally processed in the cell body predominantly into pro-somatostatin(1-64) and somatostatin-14. Otherwise, within periventricular neurons projecting to the median eminence and the posterior pituitary lobe, pro-somatostatin is posttranslationally processed during the axonal flow into pro-somatostatin(1-64), somatostatin-14, somatostatin-28, and somatostatin-28(1-12).     

Retinal ganglion cells projecting to the dorsal raphe and lateral geniculate complex in Mongolian gerbils

Injections of rhodamine-B into the dorsal raphe nucleus (DRN) and Fluoro-Gold into the lateral geniculate nucleus (LGN) revealed double-labeled retinal ganglion cells (DL RGCs) projecting to both nuclei. The soma-size distribution of DL RGCs was compared with three other distributions: DRN-projecting RGCs, LGN-projecting RGCs, and a large sample of RGCs labeled via the optic nerve with DiI. DL RGC soma diameters fell primarily within the mid-to-upper size range of all three distributions. DL RGCs may provide information to both nuclei concerning comparable aspects of light and visual stimulation via collateralized axons.