Detailed description of the proposed use of the animals, including species, strains, ages, sex, and number to be used; Dissociated, primary cultures will be prepared from the cortex of new born mice of either sex (mus musculus, Postnatal day 0-1). These experiments will be performed using pups obtained from Vglut1-IRES2-Cre strains mated with Floxopatch (Lou et al., 2016) strains so that pyramidal cells will express channel rhodopsin CheRiff and voltage indicator QuasR2. Up to 5 cultures can be grown, which can be used for 5 experiments. Justification for the use of animals, choice of species, and numbers to be used; Description of procedures for minimizing discomfort, distress, pain, and injury; and Method of euthanasia and the reasons for its selection. Transgenic mice (category C) will be used because the experiments rely on the expression of CheRiff and QuasR2 for optical stimulation/recording. There are no alternative species. All experiments are terminal. Newborn mice will be cooled in ice prior to cervical dislocation and excision of the brain for culture preparation. All procedures are in accordance with guideleines of the NYU Animal Welfare Committee. The estimated number of animals below ensure that we have a continuous supply of cultures to perform experiments on a daily basis. Based on our experience in the past 3 years, each postnatal day 0-1 (P0-P1) mouse will generate at least 5 culture preparations. A viable culture will have a have a high density of neurons that express the Floxopatch construct, which varies from mouse to mouse. Thus, if one culture is not viable, all cultures derived from that mouse are also unusable. The success rate is approximately 50%. Because cultures can be made only form P0-1 day old mice and because it is not possible to determine whether expression is sufficient until the day of the experiment 2-3 weeks later, we will need to make 2 sets of cultures per week. Thus, we will need to maintain 2 breeding pairs per week or 8 breeding pairs per month to have a continuous supply of cultures. The breeding pairs will either consist of wildtype mice (for viral injection) or Vglut1-IRES2-Cre - Gt(ROSA)26Sor (floxopatch) mice. Breeding pairs will be replaced every six months. Total animals for experiments: 32 mice/year (=16 mice (8 breeding pairs) x 2/year replacement of breeding pairs). To maintain the 3 lines, 2 males and 2 females each of Vglut1-IRES2-Cre, Gt(ROSA)26Sor, and wildtype mice will be kept in separate cages (4+4+4=12 mice total). Every 6 months (twice per year), the mice of each type will be bred. From their offsprings, 4 males and 4 females from each line will be weaned and kept in separated cages as above (4+ 4 + 4 = 12) and the 4 original breeding pairs (12 mice) euthanized. Overall, 24 mice/year will be used to maintain the lines. Total mice/year: 56 (=24 mice/year to maintain lines + 32 mice/year for experiment). Total mice over 3 years = 168 mice. Reference Lou et al., (2016) Genetically Targ...