Visually-induced NGFI-A protein expression in the calbindin-, parvalbumin- and nitric oxide synthase-neuronal populations of the rat superior colliculus

The expression of the immediate early gene NGFI-A in the nervous system is induced by sensory stimulation and seems to be related to long-term synaptic plasticity. We have used double-labeling immunohistochemistry to identify calbindin (CB)+, parvalbumin (PV)+ and neuronal nitric oxide synthase (nNOS)+ neurons that also expressed the protein encoded by this immediate early gene after light-exposure on in the superficial layers of the rat superior colliculus (sSC). The majority of the NGFI-A+ cells were not double-labeled for the tested markers. In the stratum zonale + stratum griseum superficiale (SZ/SGS), only 17.8%, 8.0% and 12.1% of NGFI-A+ cells were also labeled for CB, PV or nNOS, respectively. In the stratum opticum (SO), only 10.5% of the NGFI-A+ cells were also CB+. Furthermore, only a small subset of each population expressed the NGFI-A protein after light-exposure. In the SZ/SGS, 35.7% of the CB+, 32.1% of the PV+ and 26.6% of the nNOS+ neurons also expressed the NGFI-A. In the SO, 31.7% of the CB+ neurons also expressed the NGFI-A. The proportional distribution of the nNOS+/NGFI-A+ neurons throughout the SZ/SGS layers showed a slight but significant rostro-caudal gradient. No significant difference was observed for the other markers, indicating homogeneous activation of these populations throughout the retinotopic map. Our results suggest that the visually-driven NGFI-A expression is not restricted to a specific population of the sSC and that visual processing in this structure, as assessed by the expression of this candidate-plasticity protein, involves the activation of subsets of ascending and non-ascending projection neurons.