Activation of 5-HT7 receptors in the mouse dentate gyrus selectively enhances GABAergic inhibition of hilar mossy cells without affecting plasticity at the perforant path synapse
Background: The study examined the effects of 5-HT7 receptor activation on GABAergic transmission within the dentate gyrus and plasticity at the glutamatergic perforant path input. Methods: Immunofluorescence imaging was performed using transverse hippocampal slices from transgenic mice expressing GFP under the Htr7 promoter. This was followed by whole-cell patch clamp electrophysiological recordings of spontaneous inhibitory postsynaptic currents recorded from dentate granule cells and hilar mossy cells -- two glutamatergic neuron types present in the dentate gyrus. Extracellular recordings of field excitatory postsynaptic potentials were then performed to assess whether 5-HT7 receptor activation influenced theta burst stimulation-evoked plasticity of the perforant path synaptic input. Results: It was found that parvalbumin and somatostatin interneurons in the dentate gyrus expressed GFP. Activation of 5-HT7 receptors increased GABAergic transmission targeting mossy cells but not granule cells. However, there was no effect of 5-HT7 receptor activation on perforant path plasticity either with intact or blocked GABAA receptor signaling. Conclusion: The presence of 5-HT7 receptors in a subset of parvalbumin and somatostatin interneurons in the mouse dentate gyrus could mean that they are involved in the inhibitory control of dentate gyrus activity, although the effect seems to be confined to mossy cells and did not translate to changes in perforant path plasticity. Further experiments are needed to fully elucidate the functional role of these receptors in the dentate gyrus.