FRACTIONATOR ANALYSIS SHOWS LOSS OF NEURONS IN THE LATERAL GENICULATE-NUCLEUS OF MACAQUES INFECTED WITH NEUROVIRULENT SIMIAN IMMUNODEFICIENCY VIRUS

Infection of macaques with neurovirulent strains of simian immunodefic iency virus (SIVmac) is an experimental model for the neurological man ifestations of AIDS. Loss of neurons has been reported in the cerebral cortex following immunodeficiency viral infection. but thalamic struc tures which may contribute to electrophysiological changes and neurolo gical deficits have not been examined. In this study, the lateral geni culate nucleus (LGN) of macaques inoculated with macrophage-tropic, ne urovirulent virus SIV(mac)239 (R71 and 17E) was examined for neuron lo ss using the optical fractionator method. Estimates of the number of n eurons in the P layers of the lateral geniculate nucleus of age-matche d control macaques ranged from 1.0 to 1.3 x 10(6), while the number of neurons in SIV infected macaques ranged from 0.8 to 1.1 x 10(6), refl ecting neuron loss of up to 28%. Neuron loss was not observed in the m agnocellular layers, The total number of glia and glial density were u nchanged. Loss of neurons in the lateral geniculate nucleus was correl ated with the pattern of neuropathological changes. Neuron loss was mo st severe in animals with encephalitis concentrated in the brain stem and subcortical white matter and was less apparent in animals with dif fuse encephalitis, Neuron loss in the lateral geniculate nucleus did n ot explain changes observed in the visual evoked potential, which was severely affected in two animals which showed a loss of 24 and 26%, wh ile it was normal in a third animal which showed neuron loss of 28%.