Three dimensional morphometric analyses of axon terminals early changes induced by methylmercury intoxication in the adult cat striate cortex.

The aim of the present report is to investigate in detail morphometric changes of axon terminals of area 17 of adult cat induced by methylmercury intoxication. Six adult male cats (Felix catus), with 12 h day-light cycle and ad libitum water and food regimen, received a single dose of MeHgCl (6.4 mg/kg) dissolved in milk, whereas control subjects (n=6) received only milk. After 30 days, biocytin iontophoretic injections were done into the area 17, (Horsley-Clark coordinates between AP 3.0-6.0) on the crown of the lateral gyrus, near the border with area 18. MeHg and inorganic Hg (Hgi) concentrations were measured in the brain parenchyma of intoxicated cats and corresponded on average to 1.39+/-0.3 and 6.79+/-0.6 ppm (mean+/-s.e.m.) respectively. Twenty four hours after iontophoresis, aldehyde fixed brain sections (200 microm thick), were processed to reveal biocytin labeled terminals. Axonal microscopic 3D reconstructions using Neurolucida software (Microbright Systems Inc.) allowed estimations of boutons, branching points and segment densities for each terminal. Cluster analysis of morphometric axonal features from control and intoxicated group revealed, two distinct axon families (Type I and II) as described elsewhere. Total density values of boutons, branching points and segment densities of intoxicated group, decreased 81, 59 and 91% respectively, as compared to the control group (ANOVA two-way, Bonferroni a priori test p<0.05). Altered axonal morphology associated with MeHg, appeared early in the disease (30 days after contamination), revealing new aspects of the neuronal pathology of the methylmercury intoxication in the visual cortex.

Specialization of pyramidal cell structure in the visual areas V1, V2 and V3 of the South American rodent, Dasyprocta primnolopha.

Marked phenotypic variation has been reported in pyramidal cells in the primate cerebral cortex. These extent and systematic nature of these specializations suggest that they are important for specialized aspects of cortical processing. However, it remains unknown as to whether regional variations in the pyramidal cell phenotype are unique to primates or if they are widespread amongst mammalian species. In the present study we determined the receptive fields of neurons in striate and extrastriate visual cortex, and quantified pyramidal cell structure in these cortical regions, in the diurnal, large-brained, South American rodent Dasyprocta primnolopha. We found evidence for a first, second and third visual area (V1, V2 and V3, respectively) forming a lateral progression from the occipital pole to the temporal pole. Pyramidal cell structure became increasingly more complex through these areas, suggesting that regional specialization in pyramidal cell phenotype is not restricted to primates. However, cells in V1, V2 and V3 of the agouti were considerably more spinous than their counterparts in primates, suggesting different evolutionary and developmental influences may act on cortical microcircuitry in rodents and primates.