Electrophysiological recordings investigating serotonin receptors in the retrosplenial cortex

The retrosplenial cortex (RSC) has been shown to be involved in processes of spatial navigation and fear conditioning. The underlying properties of retrosplenial neurons supporting these functions are not well understood. Here, we used ex-vivo electrophysiological recordings combined with pharmacological stimulation of serotonin receptors to investigate the intrinsic properties and their responses to the activation of serotonin receptors, respectively. We further applied optogenetic stimulation of the subiculum (SUB)-RSC projection to explore the role of serotonin receptors in modulating light-evoked synaptic events. We firstly classified retrosplenial neurons into three distinctive groups, low rheobase (LR), regular spiking (RS), and fast spiking (FS) neurons according to their different rheobase, input resistance and firing pattern. When we applied 30 µM serotonin neither of these three groups showed specific changes upon the application of the agonist, nor to the LED-induced excitatory postsynaptic potentials (EPSP). Besides serotonin, other neurotransmitter receptors were shown to be involved in the function of RSC. By using publicly available sequencing data, we performed a principal component analysis (PCA) on the expression level of 75 genes related to the receptors of γ- aminobutyric acid (GABA), glutamate, serotonin, and opioid from 697 neurons in both layer 2/3 and layer 5 of RSC. However, we did not find any neuronal subgroups based on the expression level of these genes, which led us to conclude that synaptic receptors included here may not be the determining factor of identifying subclasses of RSC neurons. In summary, we provide preliminary evidence that serotonin may unspecifically modulate synaptic input from the SUB to the RSC, but we so far failed to identify cell-type specific markers using publicly available RNA seq data.The retrosplenial cortex (RSC) has been shown to be involved in processes of spatial navigation and fear conditioning. The underlying properties of retrosplenial neurons supporting these functions are not well understood. Here, we used ex-vivo electrophysiological recordings combined with pharmacological stimulation of serotonin receptors to investigate the intrinsic properties and their responses to the activation of serotonin receptors, respectively. We further applied optogenetic stimulation of the subiculum (SUB)-RSC projection to explore the role of serotonin receptors in modulating light-evoked synaptic events. We firstly classified retrosplenial neurons into three distinctive groups, low rheobase (LR), regular spiking (RS), and fast spiking (FS) neurons according to their different rheobase, input resistance and firing pattern. When we applied 30 µM serotonin neither of these three groups showed specific changes upon the application of the agonist, nor to the LED-induced excitatory postsynaptic potentials (EPSP). Besides serotonin, other neurotransmitter receptors were shown to be involved in the function of RSC. By using publicly available sequencing data, we performed a principal component analysis (PCA) on the expression level of 75 genes related to the receptors of γ- aminobutyric acid (GABA), glutamate, serotonin, and opioid from 697 neurons in both layer 2/3 and layer 5 of RSC. However, we did not find any neuronal subgroups based on the expression level of these genes, which led us to conclude that synaptic receptors included here may not be the determining factor of identifying subclasses of RSC neurons. In summary, we provide preliminary evidence that serotonin may unspecifically modulate synaptic input from the SUB to the RSC, but we so far failed to identify cell-type specific markers using publicly available RNA seq data