Long‐lasting tagging of neurons activated by seizures or cocaine administration in Egr1‐CreERT2 transgenic mice
Egr1‐CreERT2 mice express CreERT2 recombinase, which in the presence of 4‐hydroxytamoxifen triggers permanent activation of Cre‐dependent reporter gene(s) (tagging). Neuronal activity during pilocarpine‐ or pentylenetetrazol‐induced seizures triggers the permanent tagging of neurons in the dentate gyrus and other regions. Cocaine injection triggers tagging of striatal projection neurons that could be part of the neural network involved in the long‐term effects of cocaine.
Permanent tagging of neuronal ensembles activated in specific experimental situations is an important objective to study their properties and adaptations. In the context of learning and memory, these neurons are referred to as engram neurons. Here, we describe and characterize a novel mouse line, Egr1‐CreERT2, which carries a transgene in which the promoter of the immediate early gene Egr1 drives the expression of the CreERT2 recombinase that is only active in the presence of tamoxifen metabolite, 4‐hydroxy‐tamoxifen (4‐OHT). Egr1‐CreERT2 mice were crossed with various reporter mice, Cre‐dependently expressing a fluorescent protein. Without tamoxifen or 4‐OHT, no or few tagged neurons were observed. Epileptic seizures induced by pilocarpine or pentylenetetrazol in the presence of tamoxifen or 4‐OHT elicited the persistent tagging of many neurons and some astrocytes in the dentate gyrus of hippocampus, where Egr1 is transiently induced by seizures. One week after cocaine and 4‐OHT administration, these mice displayed a higher number of tagged neurons in the dorsal striatum than saline/4‐OHT controls, with differences between reporter lines. Cocaine‐induced tagging required ERK activation and tagged neurons were more likely than others to exhibit ERK phosphorylation or Fos induction after a second injection. Interestingly neurons tagged in saline‐treated mice also had an increased propensity to express Fos, suggesting the existence of highly responsive striatal neurons susceptible to be re‐activated by cocaine repeated administration, which may contribute to the behavioral adaptations. Our report validates a novel transgenic mouse model for permanently tagging activated neurons and studying long‐term alterations of Egr1‐expressing cells.