Properties of ipsilateral retinogeniculate afferents in albino and hooded rats

Structural changes of the macula and optic nerve head in the remaining eyes after enucleation for retinoblastoma: an optical coherence tomography study

BACKGROUND

To describe objectively the possible structural changes of the macula and optic nerve head in the free eyes of unilateral cured retinoblastoma patients and, also after enucleation using spectral domain optical coherence tomography.

METHODS

A cross sectional study involving 60 patients subdivided into three groups; 15 unilateral RB patients in whom enucleation was indicated as a sole treatment performed earlier in life [(study group (I)], 15 unilateral RB patients who had completely regressed disease with a preserved eye [(study group (II)] and 30 age and sex matched healthy controls. The remaining and free eyes in study groups and right eyes of control group had full ophthalmological examination, static automated perimetry and optical coherence tomography of the macula and optic nerve head.

RESULTS

In study group (II); a significant thinning of total macula, central fovea, ganglion cell layer (GCL), ganglion cell complex (GCC), and some sectors of outer nuclear layer (P- values ≤0.05) was found with no significant difference in peripapillary nerve fiber layer (pRNFL) thickness and optic nerve head parameters compared to the control group and the study group (I). A significantly thickened total macula, GCL, GCC, and pRNFL in study group (I) compared to study group (II). Thickened pRNFL was significantly correlated to standard automated perimetry pattern deviations. No significant difference was found between study group (I) and control group.

CONCLUSION

Retinoblastoma eyes characterized by thinning of central fovea, GCL, GCC compared to the control group. After unilateral enucleation, increased GCC and pRNFL thicknesses were detected compared to retinoblastoma group.

Activity-dependent changes in eye influence during monocular blockade: increases in the effects of visual stimulation on 2-DG uptake in the adult rat geniculostriate system

We examined the effects of loss of monocular retinal activity on 2-deoxyglucose (2-DG) uptake in the adult rat geniculostriate system. Of particular interest was whether the influence of the normally functioning eye changed during long-term contralateral retinal silence. Group 1 rats were subjected to short-term (24 hours) and group 2 rats to long-term (21-90 days) monocular tetrodotoxin (TTX) blockade, and metabolic activity was assessed during exposure to square-wave gratings. Group 1 rats exhibited patterns of cortical glucose utilization commensurate with complete monocular loss of retinal activity: minimal 2-DG uptake in contralateral monocular area 17 and dorsal lateral geniculate nucleus (LGN), and a bilateral depression in the binocular regions; 2-DG uptake was highest in the monocular regions fed by the stimulated normal eye (in both area 17 and the LGN) and these regions appeared unaffected by the monocular blockade. After repeated injections of TTX (group 2), metabolic activity in binocular area 17 and binocular LGN increased bilaterally relative to the metabolically active monocular regions contralateral to the normal eye. Group 3 rats were monocularly TTX-injected for 30 or 60 days, and, 24 hours before 2-DG, all retinal activity was eliminated by means of binocular TTX injections or binocular enucleation. Glucose utilization in the binocular regions of both area 17 and the LGN in these rats was depressed to levels seen in monocular area 17 after complete and recent loss of activity from the contralateral eye, indicating that the metabolic increase which occurred in the binocular regions during long-term monocular retinal blockade was dependent upon the neuronal processing of retinal information from the non-TTX eye. We conclude that, in the adult rat, an activity-dependent, physiologically based shift in ocular influence occurred in the binocular geniculostriate system during long-term monocular retinal inactivation.

Metabolic activity in striate and extrastriate cortex in the hooded rat: contralateral and ipsilateral eye input

The extent of changes in glucose metabolism resulting from ipsilateral and contralateral eye activity in the posterior cortex of the hooded rat was demonstrated by means of the C-14 2-deoxyglucose autoradiographic technique. By stimulating one eye with square wave gratings and eliminating efferent activation from the other by means of enucleation or intraocular TTX injection, differences between ipsilaterally and contralaterally based visual activity in the two hemispheres were maximized. Carbon-14 levels in layer IV of autoradiographs of coronal sections were measured and combined across sections to form right and left matrices of posterior cortex metabolic activity. A difference matrix, formed by subtracting the metabolic activity matrix of cortex contralateral to the stimulated eye from the ipsilateral "depressed" matrix, emphasized those parts of the visual cortex that received monocular visual input. The demarcation of striate cortex by means of cholinesterase stain and the examination of autoradiographs from sections cut tangential to the cortical surface aided in the interpretation of the difference matrices. In striate cortex, differences were maximal in the medial monocular portion, and the lateral or binocular portion was shown to be divided metabolically into a far lateral contralaterally dominant strip along the cortical representation of the vertical meridian, and a more medial region of patches of more or less contralaterally dominant binocular input. Lateral peristriate differences were less than those of striate cortex, and regions of greater and lesser monocular input could be distinguished. We did not detect differences between the two hemispheres in either anterior or medial peristriate areas, thus indicating either completely binocular input (which seems unlikely given the retinotopic organization of these regions), or a greater dependence than in the lateral peristriate on inputs that were not affected by the visual manipulations.

'Hidden lamination' in the dorsal lateral geniculate nucleus: the functional organization of this thalamic region in the rat

The cyto-and myeloarchitecture of the rat's dorsal lateral geniculate nucleus (dLGN) display none of the laminar features characteristic of this thalamic region in carnivores and primates. Despite this, the rodent's nucleus contains a segregation of functionally and ocularly distinct afferents--organizational properties manifested in the prominent lamination of these other mammalian forms. The rat's dLGN can be divided into two main regions: an inner core and an outer shell. The inner core contains two ocular laminae receiving direct retinotopic projections from the contralateral nasal and ipsilateral temporal retinae, mapping the contralateral visual hemifield. The outer shell receives a retinotopic projection from the complete contralateral retina only, the representation of the ipsilateral hemifield being extremely compressed at the medial edge of this lamina. The retinotopic maps in these three ocular laminae (contra, ipsi, contra) are in conjugate register, so that lines of projection course rostro-ventro-medially from the optic tract at the thalamic surface through these laminae. Three morphologically distinct retinal ganglion cell types project to the dLGN, and the axons of these ganglion cells are partially segregated within the optic tract in anticipation of their segregation within the nucleus, where they terminate at distinct locations along the lines of projection. Type I and III cells terminate in the inner core of the nucleus, while type II and III cells terminate in the outer shell. The outer shell also receives a direct projection from the superior colliculus. These characteristics of the afferent termination within the rat's dLGN support the view of a general mammalian plan for the organization of this thalamic region, and provide a basis for further experimentation to test speculations about potentially homologous subdivisions of this nucleus. Conclusions regarding functionally analogous pathways are proposed with less confidence, due to the paucity of definitive evidence for physiologically distinct cell classes. The type I cells in the rat's retina are the likely homologues of the cat's alpha-cell. Geniculocortical relay cells driven by them have properties similar to the cat's Y-cell. The inner core of the nucleus then may transmit information of a Y-like nature onto striate cortex. The outer shell of the rat's nucleus, a portion of which receives collicular as well as retinal innervation, may convey W-like information onto striate cortex. The rat's retinogeniculate projection appears to be lacking a beta-cell-like pathway that may subserve X-cell function altogether.

Dependence of cytochrome oxidase activity in the rat lateral geniculate nucleus on retinal innervation

Patterns of cytochrome oxidase (CO) activity were examined histochemically in the dorsal lateral geniculate nucleus (LGNd) and retina of pigmented rats. CO staining was not uniform and was distributed in a pattern similar to that of retinal afferents. Portions of the LGNd receiving an exclusively crossed projection were moderately reactive whereas regions receiving an uncrossed or overlapping crossed and uncrossed projection were darkly reactive. The dependence of oxidative metabolic activity in the LGNd on retinal innervation was verified in animals with unilateral enucleation. In adults, chronic monocular enucleation led to a decrease in CO staining in portions of the LGNd deprived of retinal input; in animals enucleated at birth, normal patterns of CO reactivity failed to develop and both LGNds had a more uniform pattern of moderate CO staining. Most neurons in the ganglion cell layer of the retina were moderately reactive for CO. However, there were approximately 3,000 darkly reactive cells, most of which appear to be ganglion cells. The darkly reactive cells were more numerous in the peripheral temporal retina. The laminar pattern of CO staining in the retina was similar to that described previously for carnivores and primates. The most reactive laminae were the inner and outer plexiform layers and the photoreceptor inner segments. Within the inner plexiform layer, sublamina a was more darkly stained than sublamina b. These results suggest that the physiological properties of crossed and uncrossed visual pathways in rats are functionally dissimilar at the level of both the retina and the LGNd.

Plastic changes in the distribution and soma size of retinal ganglion cells after neonatal monocular enucleation in rats

Using the method of retrograde labeling of ganglion cells with HRP, we studied in adult rats the plastic changes in the retinogeniculate projections due to monocular enucleations shortly after birth. Four normal and 6 neonatally enucleated rats were used. In two of the normal and 4 of the enucleated rats a small amount of HRP was injected into the dorsal lateral geniculate nucleus (LGd) and in 4 other rats massive injections were made into the optic tract near the LGd. Neonatally unilaterally eye-enucleated rats were characterized by an expanded distribution of ipsilaterally projecting ganglion cells all over the retina of the remaining eye and by a densely packed distribution of these cells in the lower temporal retina in which area these cells have only a moderate density in normal rats. On the contrary, in the lower temporal retina of monocularly enucleated rats the incidence of contralaterally projecting ganglion cells was decreased. Soma areas of ipsi- and contralaterally projecting ganglion cells were measured for the peripheral crescent in lower temporal and lower nasal retinas. As compared with normal rats, neonatally enucleated rats had a larger mean soma area of ipsilaterally projecting cells and a smaller mean soma area of contralaterally projecting cells. This result was interpreted as suggesting that after neonatal monocular enucleation medium to large cells had changed their side of axonal projection from the contralateral to ipsilateral LGd.

A visual pathway that mediates fear-conditioned enhancement of acoustic startle

Eighty rats received 10 light-shock pairings on two successive days. Seventy-two h after the final training session, subjects received lesions directed at the primary visual areas (deep and superficial layers of the superior colliculus, dorsal lateral geniculate nucleus, pretectal nuclei, visual cortex and thalamic reticular nucleus) and at the nuclei of the lateral lemniscus and reticularis pontis caudalis, proposed components of a primary acoustic startle circuit in the rat. Control animals were sham operated. One day later, all animals were tested for startle by presenting noise bursts of 3 different intensities in the presence or absence of the light conditioned stimulus. Potentiated startle (the difference between light-noise vs noise-alone trials) was significantly attenuated or eliminated by lesions directed at the dorsal nucleus of the lateral geniculate, deep layers of the superior colliculus, visual cortex, and the posteroventral region of the nucleus of the lateral lemniscus. Lesions directed at pretectal nuclei, superficial layers of the superior colliculus, thalamic reticular nucleus, nucleus reticularis pontis caudalis or dorsal nucleus of the lateral lemniscus did not attenuate potentiated startle. The results suggest that the visual pathway that mediates potentiated startle goes from the retina to the dorsal lateral geniculate nucleus to visual cortex to deep layers of superior colliculus and down to the postero-ventral region of the lateral lemniscus where acoustic startle is modulated.