Volume 24 Issue 11, November 2019

Axons expressing independently excitable opsins innervate retrovirally labeled granule neurons in the dentate gyrus of a 23-day-old Pten^flx/flx mouse. Perforant path axons expressing Chronos-GFP (green) fill the outer two-thirds of the molecular layer, while the axons of contralateral mossy cells expressing Chrimson-tdTomato (red) reside in the inner third of the molecular layer. Retrovirally labeled Pten knockout (red) and control (green) granule neurons receive input from both afferent populations. Independent excitation of the two pathways enables measurement of the relative proportions of input arising from each afferent onto a single granule neuron. Immature (16-day-old) wild-type neurons have highly variable inputs with an increased proportion arising from the perforant path. Immature Pten knockout neurons receive increased input from both pathways, in a ratio similar to mature wild-type neurons. This experiment supports the interpretation that Pten loss results in promiscuous excitatory synapse formation. For more information, see the article by Skelton et al. on pages 1627-1640.