Low-dose sevoflurane promotes hippocampal neurogenesis and facilitates the development of dentate gyrus-dependent learning in neonatal rats

Abstract

Huge body of evidences demonstrated that volatile anesthetics affect the hippocampal neurogenesis and neurocognitive functions, and most of them showed impairment at anesthetic dose. Here, we investigated the effect of low dose (1.8%) sevoflurane on hippocampal neurogenesis and dentate gyrus-dependent learning. Neonatal rats at postnatal day 4 to 6 (P4-6) were treated with 1.8% sevoflurane for 6 hours. Neurogenesis was quantified by bromodeoxyuridine labeling and electrophysiology recording. Four and seven weeks after treatment, the Morris water maze and contextual-fear discrimination learning tests were performed to determine the influence on spatial learning and pattern separation. A 6-hour treatment with 1.8% sevoflurane promoted hippocampal neurogenesis and increased the survival of newborn cells and the proportion of immature granular cells in the dentate gyrus of neonatal rats. Sevoflurane-treated rats performed better during the training days of the Morris water maze test and in contextual-fear discrimination learning test. These results suggest that a subanesthetic dose of sevoflurane promotes hippocampal neurogenesis in neonatal rats and facilitates their performance in dentate gyrus-dependent learning tasks.

Keywords: dentate gyrus; neonate; neurogenesis; pattern separation; sevoflurane; spatial learning.

Figure 1.

Low-dose sevoflurane promotes neuronal proliferation and increases the survival of newborn cells in the dentate gyrus. (a) Protocol of BrdU inject ions for testing the proliferation of progenitor cells in the dentate gyrus. (b) Positive signals of cells for both the proliferative marker BrdU (red) and DAPI (blue). Scale bar represents 100 µm. (c) Summary data for the experiment are presented in (b). (d) Protocol of BrdU injection for testing the survival of newborn cells. (e) Positive signals of cells for both BrdU and DAPI. Scale bar represents 100 µm. (f) Summary data for the experiment are presented in (e) control group, sevoflurane group; *p < .05. Ctrl = control group; SEVO = sevoflurane group.

Figure 2.

Low dose of sevoflurane promotes neuronal proliferation and increases the survival of newborn cells in the dentate gyrus. (a) Input resistance (R _in) distribution of two type granular cells in the dentate gyrus. Continuous curves represent the sum of two Gaussian functions fitted to the histogram. (b) The resting membrane potential of mature granular cells is more negative than in immature cells. (c) Action potential properties of mature and immature granular cells in response to the current step. Scale bars represent 50 mV and 50 ms. (d) Amplitudes of the first action potential from mature and immature granular cells. (e) The proportion of immature granular cells in outer half (left), inner half (middle), and total (right) of granular cell layer. **p < .01, ***p < .001. Ctrl = control group; SEVO = sevoflurane group.

Figure 3.

Low dose of sevoflurane facilitates performance in the Morris water maze test. (a) Time to find the platform on five training days. (b) The probe test after five days of training. *p < .05. Ctrl = control group; SEVO = sevoflurane group.

Figure 4.

Low dose of sevoflurane does not influence motor system and anxiety level of rats. (a) Swimming speeds during the training days for the Morris water maze. Open field test for total path length (b), path length in center (c), and time in center (d). Ctrl = control group; SEVO = sevoflurane group.

Figure 5.

Low dose of sevoflurane facilitates pattern separation in rats. (a) Protocol for a single trial contextual fear-conditioning test. (b) Freezing level for the training context A. (c) Freezing level for the distinct context C. (d) Protocol for the contextual fear-discrimination learning test. (e) Freezing level for training context A and similar context B on day 5. (f) Freezing level for training context A and similar context B on day 7. *p < .05. Ctrl = control group; SEVO = sevoflurane group.