Fig. 4: Optical stimulation of CA2 axons evokes responses in caudal hippocampus.

a Intracranial injection of 500ul of a Cre recombinase dependent virus containing channelrhodopsin and an EYFP tag were delivered in the rostral CA2 of an Amigo2-icreERT2 mouse. b Image of the resulting channelrhoposin expression pattern in rostral CA2 of in a cre+ Amigo2-icreERT2 mouse (green). c (left) Channelrhodopsin-positive axons of rostral CA2 neurons extended caudally and were the densest in the dorsal and intermediate areas of cCA1 (scale bar = 500 µm). (inset) A biocytin filled pyramidal neuron in dorsal, cCA1 of the hippocampus (red, scale bar = 100 µm). d A 1 ms pulse of laser generated blue light evoked an inward, excitatory current in a cCA1 pyramidal neuron (scale bar = 50 pA x 50 ms). e Light-evoked synaptic responses from CA2 axons were significantly larger in amplitude in neurons found in dorsal and intermediate cCA1 neurons when compared to more ventral locations with the largest response generated in intermediate, cCA1 (Fig. 4d, e: Dorsal: −77.9 ± 14.0 pA vs. Intermediate: −199.42 ± 58.0 pA vs. Ventral −12.6 ± 14.4 pA, n = 18, 41 and 12 cells, Welch’s ANOVA W = 14.67 p < 0.0001; Dorsal vs. Intermediate unpaired t-test with Welch’s correction p = 0.048, Dorsal vs. Ventral unpaired t-test with Welch’s correction p = 0.0002, Intermediate vs. Ventral unpaired t-test with Welch’s correction p = 0.0026). f Two light-evoked pulses, delivered with a 500 ms interval, were used to evoke synaptic response in a cCA1 pyramidal neuron (scale bar = 25 pA x 50 ms). g Both dorsal and intermediate cCA1 neurons showed paired pulse ratios (PPR) of less than 1 indicating these are likely depressing synaptic connections. Dorsal CA1 neurons had a significantly lower PPR than intermediate neurons (Dorsal: 0.52 ± 0.031 vs. Intermediate: 0.65 ± 0.046, n = 12 and 25 cells, from 8 and 14 mice, respectively, unpaired t-test p = 0.012). *p < 0.05, **p < 0.01.