Induction of NGF after zinc exposure. Immunocytochemical staining with anti-NGF antibody of a sham-washed control culture (A) and a sister culture 8 hr after 15 min exposure to 300 μm zinc (B). After zinc exposure, NGF levels were increased in the majority of neurons. Scale bar, 200 μm.C, Bars represent NGF concentrations (picograms per milliliter) attained in the bathing medium (n = 4; mean + SEM) in cortical cultures 8 or 20 hr after sham wash or 15 min exposure to 300 μm zinc. Asterisk denotes difference from control (p < 0.01; two-tailed t test with Bonferroni correction for two comparisons).