Abstract
The hippocampus is a medial temporal lobe structure in the brain and is widely studied for its role in memory and learning, in particular, spacial memory and emotional responses. It was thought to be a homogenous structure but emerging evidence shows differentiation along the dorsoventral axis and even microdomains for functional and cellular markers. We have examined in two cell-types of the hippocampal projection neurons, the dentate gyrus (DG) granule cells and CA3 pyramidal neurons, if the GABA-activated tonic current density varied between the dorsal (septal) and the ventral (temporal) poles of the male mouse hippocampus. Tonic synaptic currents, arising from spontaneous and miniature inhibitory postsynaptic currents (sIPSC, mIPSC), and extrasynaptic tonic currents were evaluated. The results revealed different levels of sIPSC but not mIPSC density between the dorsal and ventral hippocampal neurons for both the DG granule cells and the CA3 pyramidal neurons. The extrasynaptic tonic current density was larger in the DG granule cells as compared to the CA3 pyramidal neurons but did not vary between the dorsal and ventral regions. IPSC bursting was observed in both cell-types in the ventral hippocampus but was more common in the CA3 pyramidal neurons. Only in the dorsal DG granule cells was the level of the sIPSC and mIPSC density similar. The results indicate that the tonic GABAergic inhibition is particularly strong in the ventral hippocampal DG granule cells and enhanced in the dorsal as compared to the ventral hippocampal CA3 pyramidal neurons.
Highlights
The hippocampus is a medial temporal lobe structure critically involved in spatial memory formation and navigation, emotional responses, and regulation of body physiology (Lathe, 2001; Papatheodoropoulos, 2018; Risold & Swanson, 1996; Strange, Witter, Lein, & Moser, 2014)
To gain insight into the inhibitory regulation of the local hippocampal circuitry, we examined the functional characteristic of the IPSCs in dentate gyrus (DG) granule cells (Figure 1) and CA3 pyramidal neurons (Figure 2) in the dorsal and ventral hippocampus
The results indicate that the tonic GABAergic inhibition is strong in the ventral hippocampal DG granule cells and enhanced in the dorsal as compared to the ventral hippocampal CA3 pyramidal neurons (Figure 5)
Summary
The hippocampus is a medial temporal lobe structure critically involved in spatial memory formation and navigation, emotional responses, and regulation of body physiology (Lathe, 2001; Papatheodoropoulos, 2018; Risold & Swanson, 1996; Strange, Witter, Lein, & Moser, 2014). Several studies have revealed variations in electrophysiological properties of hippocampal neurons along the longitudinal axis (Maggio & Segal, 2007; Malik, Dougherty, Parikh, Byrne, & Johnston, 2016; Milior et al, 2016; Papatheodoropoulos, 2015; Petrides, Georgopoulos, Kostopoulos, & Papatheodoropoulos, 2007; Schreurs, Sabanov, & Balschun, 2017) It is well-established that there is an increasing degree of neuronal excitability from the dorsal to the ventral pole of the hippocampus (Bragdon, Taylor, & Wilson, 1986; Dougherty, Islam, & Johnston, 2012; Gilbert, Racine, & Smith, 1985; Malik et al, 2016; Papatheodoropoulos, 2018; Petrides et al, 2007). The results are consistent with cell-type, DG granule cell, or CA3 pyramidal neuron, specific variation in the tonic inhibitory level along the dorsoventral axis of the mouse hippocampus
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