Changes in the Retinal Ganglion Cell Layer and Optic Nerve of Rats Exposed Neonatally to Cocaine

Abstract

The effects of neonatal exposure to cocaine upon the structure of the visual system are poorly understood despite the evidence of eye abnormalities in infants exposed in utero to cocaine. We previously demonstrated alterations in the optic nerve of rats exposed neonatally to cocaine, although no changes were detected in the number of its axons. This study was undertaken to investigate the retinal ganglion cell layer and the size distribution of the optic axons, in an attempt to assess further changes in the visual pathways.Groups of rats (Wistar strain) were given subcutaneous injections of cocaine hydrochloride (15 mg kg-1 body weight day-1) divided into two daily doses, from the day following birth until postnatal day 30. Controls were given subcutaneous saline throughout the same experimental period. Per group, five animals from three different litters were evaluated morphometrically. Following perfusion with aldehydes, samples from the median ventral and dorsal parts of the retina and the optic nerves were processed for electron microscopy. Morphometric techniques at light and electron microscopic levels allowed us to determine the following. (a) In the optic nerve: ( 1 ) frequency size-distribution of myelinated nerve fibres; and (2) number of myelin sheaths per fibre. (b) In the retina; (1) thickness of the layers: (2) frequency size-distribution of ganglion layer neurons; (3) mean cell nuclear size; and (4) packing density of ganglion cell layer neurons. Cocaine4reated rats had marked alterations in the ventral part of the retina, including reduction of the thickness of the ganglion cell layer, higher proportion of small ganglion layer neurons with reduced nuclear diameters and higher packing density, along with the presence of degenerate cell profiles. The changes in the optic nerve closely matched those of the retina, including a higher proportion of small myelinated nerve fibres.These results demonstrate that the changes found in the optic nerve mirror those of the ganglion call layer, supporting the view that these changes represent morphological changes in the developing visual pathways induced by neonatal cocaine exposure.