Activity-dependent expression of Egr1 mRNA in somatosensory cortex of developing rats

The rat barrel field in somatosensory cortex is a well-characterized model of neocortical development, with activity-dependent and activity-independent components. Egr1 encodes an inducible transcription factor that is required for certain forms activity-dependent plasticity. This study examines Egr1 mRNA expression in the developing barrel field under basal conditions and after short-term deprivation or stimulation of whiskers. Egr1 mRNA was measured with in situ hybridization at postnatal Day (P) 6, P9, P12, P15, and P21. For short-term deprivation, whiskers were trimmed close to the skin and Egr1 mRNA was examined 3 hr later. For controlled stimulation of a single whisker, surrounding whiskers were trimmed, a wire was glued to the designated whisker, and animals were placed in an AC magnetic field pulsed at 2 Hz, 10 mT rms for 15 min. Egr1 mRNA was examined 30 min later. At P6, basal Egr1 mRNA in the barrel field was very low and was increased only slightly by stimulation (P < 0.05). At each of the later ages, there was a large increase in Egr1 mRNA in stimulated versus deprived barrels (P < 0.001). Egr1 mRNA expression after whisker stimulation increased exponentially with age through P15 (P < 0.001) and then declined between P15 and P21. The onset of Egr1 responses to whisker stimulation at P9 and the striking increase in activity-dependent Egr1 mRNA expression in the second postnatal week suggest that this transcription factor may play a role in activity-dependent processes that occur in this developmental period, such as maturation of barrel cortex circuitry.