Developmental Switching of Serotonin on LTP 67 Fig. 1. Switching effect of 5-HT on the induction of LTP in the rat visual cortex. (A) The effect of the TBS of layer 4 (indicated with arrow) on layer 2/3 FP in control slices (open circle, n=10) and in 5-HT-perfused slices (closed circle, n=10) from 5-week-old rats. The right panel shows the representative average traces in a slice marked with corresponding symbols, which were taken at the time period indicated with corresponding numbers. Scales represent 0.4 mV and 2 ms, respectively. (B) The effect of the TBS of layer 4 (indicated with arrow) on layer 2/3 FP in control slices (open circle, n=11) and in 5-HT-perfused slices (closed circle, n=12) from 8-week-old rats. The right panel shows the representative average traces in a slice marked with corresponding symbols, which were taken at the time period indicated with corresponding numbers. Scales represent 0.4 mV and 2 ms, respectively. (C) Age-dependent inhibition and facilitation of layer 2/3 LTP induction induced by TBS to layer 4 in the rat visual cortex. Numbers of the experiments are from 5 to 12. **p＜0.01 and ***p＜0.001 compared with the corresponding control. amplitude, n=7, p＜0.01) (Fig. 1C). Thus, these results showed that 5-HT exerts a facilitating effect on the in- duction of LTP in the pathway from layer 4 to layer 2/3 in the visual cortex of adult rats. In our previous study, 5-HT specifically inhibited NMDA receptor-dependent LTP in young rats, but not metabo- tropic glutamate receptor (mGluR)-dependent LTP [11]. We investigated whether the reinstated LTP was dependent on the activation of NMDA receptors. Bath application of the NMDA receptor antagonist D-AP5 (50 μM) in the presence of 5-HT abolished the induction of LTP in 8-week-old rats Fig. 2. Effect of endogenous 5-HT on the induction of LTP in 8- week-old rats. (A) Average response to TBS applied at the indicated time (arrow) in PCA-incubated (open circle, n=5) and citalopram- treated slices (closed circle, n=6). Insets show the average FP recording taken from a representative experiment at the indicated time period with corresponding numbers. Scales represent 0.4 mV and 2 ms, respectively. (B) Summary plot of the LTP induction in PCA- and citalopram-treated slices from 8-week-old rats (n=5 and n=6, respectively). *p＜0.05 compared with the baseline. (106.4±7.6% of the baseline FP amplitude, n=8, p=0.43). Thus, 5-HT specifically facilitated the induction of NMDA receptor-dependent LTP in adult rats. Endogenous 5-HT facilitates the induction of LTP in adult rats In the next experiment, we tested the effect of pharmaco- logical manipulation of endogenous 5-HT in adult rats. Initially, endogenous 5-HT was depleted by the incubation of slices from 8-week-old rats with PCA (10 μM, 2 hr) [11]. TBS did not induce LTP in 5-HT-depleted slices (104.6± 2.8% of the baseline FP amplitude, n=5, p=0.17) (Fig. 2). The SSRI citalopram was used in order to determine the effect of an increased tone of serotonin in the slices [25] on LTP induction. Bath application of citalopram (10 μM) mimicked the effect of 5-HT on the induction of LTP in 8-week-old rats (132.5±10.5% of the baseline FP amplitude, n=6, p＜0.05) (Fig. 2). These results suggest that endoge- nously released 5-HT could facilitate the induction of LTP in the visual cortex of aged rats. Effect of specific 5-HT receptor agonists and antagonists on the induction of LTP Membrane receptors for 5-HT comprise at least 14 sub- types from 5-HT1 to 5-HT7 receptors [26], among which 5-HT1A, 5-HT2 and 5-HT3 receptors are reportedly expressed in the rat visual cortex [21,25,27]. Thus, in the following experiment we tested which 5-HT receptor subtypes are in- volved in the facilitation of LTP in adult rats using specific antagonists (Fig. 3A and C) and agonists (Fig. 3B and D) for 5-HT1A, 5-HT2 and 5-HT3 receptors. Co-application of the specific 5-HT1A receptor antagonist NAN-190 (10 μM) with 5-HT did not inhibit the induction of LTP (129.5±11.8% of the baseline FP amplitude, n=7, p＜0.05). The specific 5-HT3 receptor antagonist Y-25130 (10 μM) did not change the magnitude of LTP facilitated by 5-HT (122.9±2.6% of the baseline FP amplitude, n=5, p＜0.001). In contrast, co-application of the 5-HT2 receptor antagonist mesulergine (10 μM) with 5-HT abolished the induction of