68 SW Park, et al Fig. 3. Effect of 5-HT receptor antagonists and agonists on the induction of LTP in 8-week-old rats. (A) The effect of 5-HT receptor antagonists on layer 2/3 LTP induced by TBS to layer 4 (indicated with arrow) in 8-week-old rats. NAN-190 (10 μM, n=7, open trian- gle), mesulergine (10 μM, n=6, closed circle) and Y-25130 (10 μM, n=5, open square) were co-applied into the bath with 5-HT (10 μM). 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 5-HT receptor agonists on layer 2/3 LTP induced by TBS to layer 4 (indicated with arrow) in 8-week-old rats. 8-OH-DPAT (10 μM, n=7, open triangle), DOI (10 μM, n=7, closed circle) and 2-me-5-HT (10 μM, n=7, open square) were applied into the bath throughout the experiment. 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) Summary plot of the effect of anta- gonists for 5-HT receptor subtypes. *p＜0.05 and ***p＜0.001 com- pared with the baseline. (D) Summary plot of the effect of agonists for 5-HT receptor subtypes. *p＜0.05 compared with the baseline. LTP (103.8±3.8% of the baseline FP amplitude, n=6, p=0.36). These results suggest that the activation of 5-HT2 receptors might be involved in the facilitation of LTP in the visual cortex of aged rats. In another set of experiments, DOI (10 μM), a specific Fig. 4. Effect of 5-HT2 receptor antagonists and a agonist on the 5-HT facilitation of LTP induction in 8-week-old rats. (A) The effect of 5-HT2 receptor antagonists and agonists on layer 2/3 LTP induced by TBS to layer 4 (indicated with arrow) in 8-week-old rats. MDL 11939 (10 μM, n=5, closed circle), MDL 11939 (10 nM, n=11, open circle), and SB 242084 (5 μM, n=5, closed square) were co-applied into the bath with 5-HT (10 μM). WAY 161503 (10 μM, n=8, gray triangle) was applied without 5-HT into the bath. 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) Summary plot of the effect of anta- gonists and a agonist for 5-HT2 receptors. *p＜0.05 and ***p＜ 0.001 compared with the baseline. 5-HT2 receptor agonist, mimicked the induction of LTP fa- cilitated by the application of 5-HT in 8-week-old rats (128.4±8.1% of the baseline FP amplitude, n=7, p＜0.05), while 8-OH-DPAT (10 μM), a specific 5-HT1A receptor ago- nist (103.3±2.4% of the baseline FP amplitude, n=7, p=0.21), or 2-me-5-HT, a specific 5-HT3 receptor agonist (106.2±3.4% of the baseline FP amplitude, n=7, p=0.12), failed to induce LTP (Fig. 3B). These results also support the hypothesis that the activation of the 5-HT2 receptor might be involved in the 5-HT facilitation of the induction of LTP in adult rats. Effect of specific 5-HT2 receptor antagonists on the facilitated induction of LTP Both 5-HT2A and 5-HT2C receptors are expressed in pyr- amidal neurons in the neocortex of the rat [21,28]. Thus, we further investigated the involvement of these receptor subtypes in the 5-HT facilitation of LTP (Fig. 4). Co-applica- tion of the specific 5-HT2A/C receptor blocker MDL 11939 at a high concentration (10 μM) consistently abolished LTP facilitated by 5-HT (107.2±3.9% of the baseline FP ampli- tude, n=5, p=0.14). However, MDL 11939 as a selective 5-HT2A receptor antagonist at a low concentration (10 nM)