Fig. 5: Activation of CA2 neurons engages corticolimbic brain regions, likely through caudal CA1 projection neurons.

a Coronal sections from the anterior cingulate cortex of cre- (top) and cre+ Amigo2-icreERT2 (bottom) mice immunohistochemically stained for the IEG cFos (scale bar = 500 µm). b in vivo activation of area CA2 via bilateral expression of Gq-DREADDs causes a significant increase in cFos expression in the ACC of cre+ mice compared to cre- mice (cre-: 72.3 ± 15.1 cFos+ cells vs. cre + : 239.1 ± 8.5 cFos+ cells, 8 hemispheres from 4 animals and 6 hemispheres from 3 animals, respectively, unpaired t-test p < 0.0001). c Sections from the prelimbic cortex (mPFC) of cre- Amigo2-icreERT2 (top) and cre+ Amigo2-icreERT2 (bottom) mice immunohistochemically stained for the IEG cFos (dashes outline the more dorsal prelimbic and the more ventral infralimbic cortices, scale bar = 500 µm). d In vivo activation of area CA2 via bilateral expression Gq-DREADDs drives a significant increase in cFos expression in both prelimbic (cre-: 6.1 ± 1.5 cFos+ cells/area⁻⁵ µm vs. cre + : 18.7 ± 1.8 cFos+ cells/area⁻⁵ µm, 7 sections from 3 mice and 5 sections from 3 mice, respectively, unpaired t-test p = 0.0004) and infralimbic (cre-: 3.9 ± 1.2 cFos+ cells/area⁻⁵ µm vs. cre + : 18.7 ± 5.8 cFos+ cells/area⁻⁵ µm, 7 sections from 3 mice and 5 sections from 3 mice, respectively, unpaired t-test p = 0.0004) cortex of Cre+ mice compared to Cre- mice. e (left, top) 250–300 µl intracranial injection of AAV-retro-mCherry virus injected into the prelimbic cortex of a wildtype mouse resulted in mCherry expression in neurons along the dorsal-ventral axis of the ipsilateral, cHPC (left, bottom). (right, top) 250–300 µl intracranial injection of AAV-retro-mCherry virus injected into the ACC of a wildtype mouse results in mCherry expression in neurons along the dorsal-ventral axis of the ipsilateral cHPC (right, bottom) (scale bars = 500 µm). f Dual intracranial viral injections result in rostral CA2 specific expression of a channelrhodopsin-EYFP (left) and AAV-retro-mCherry labeled neurons in the cHPC neurons that sent axonal projections to the ACC (right). g Rostral CA2 > cCA1 light evoked, excitatory synaptic responses in cHPC neurons that project to the ACC. Responses from neurons that send axonal projections to ACC (mCherry + ) were significantly larger in amplitude than the synaptic responses from mCherry- neurons that likely project elsewhere (non-labeled cells: −10.28 ± 4.3 pA vs. −75.8 ± 25.7 pA, n = 5 and 10 cells, respectively, p = 0.034, unpaired t-test with Welch’s correction, scale bars = 50 pA x 50 ms). *p < 0.05, **p < 0.01, ***p < 0.001, ****p < 0.0001.