66 SW Park, et al the induction of LTP from young to adult rats are not avail- able yet. Thus, in the present study we studied the acute effect of 5-HT on the induction of LTP in visual cortex slices dur- ing development in young and adult rats. We found that, in contrast to the inhibitory effect of 5-HT on the induction of LTP in young rats, 5-HT facilitated the induction of LTP in adult rats at 8 and 10 weeks of age. Thus, the switching effect of 5-HT on the induction of LTP during postnatal de- velopment provides new insight on the role of 5-HT in the brain. METHODS Animal care and slice preparation Sprague-Dawley rats of both sexes (postnatal 3 to 10 weeks, Orientbio Inc., Korea) were maintained and raised under standard conditions (23±1oC, 12/12 hours light/dark cycle). Animal care and surgical procedures were approved by the Ethics Committee of the Catholic University of Korea and were consistent with the National Institutes of Health Guide for the Care and Use of Laboratory Animals. Visual cortex slices were prepared from 3- to 10-week-old rat as previously described [11]. Briefly, the brains were quickly removed after anesthetization with chloral hydrate (400 mg/kg, i.p.), and were submerged in ice-cold dissection medium. Coronal sections of the occipital cortex (400 μm in thickness) were prepared on a vibrotome (Campden Instruments, Leica, UK) and were allowed to recover in a storage chamber for 40 min at 37oC. The slices were main- tained at room temperature prior to recording. The dis- section and storage medium consisted of 125 mM NaCl, 2.5 mM KCl, 1 mM CaCl2, 2 mM MgSO4, 1.25 mM NaH2PO4, 25 mM NaHCO3, and 10 mM D-glucose, bubbled with 95% O2/5% CO2. The slices were transferred to the submerging chamber for recording and superfused continuously with ar- tificial cerebrospinal fluid (ACSF, 1.5∼2 ml/min) contain- ing 125 mM NaCl, 2.5 mM KCl, 2 mM CaCl2, 1 mM MgSO4, 1.25 mM NaH2PO4, 25 mM NaHCO3, and 10 mM D-glucose, bubbled with 95% O2/5% CO2 at 32∼33oC. In some experi- ments, the slices were incubated with para-chloroamphet- amine (PCA, 10 μM) for 2 hr to deplete endogenous 5-HT. Under these experimental conditions, PCA (10 μM) was also included in ACSF throughout the recording. Electrophysiological recording The recording electrode pulled from a glass pipette was filled with ACSF (1∼3 MΩ) and positioned in layer 2/3 of the primary visual cortex. Field potential (FP) was eli- cited by a rectangular current pulse (0.2 ms) at a site in the middle of the cortex, by means of a concentric bipolar stimulating electrode (100 μm in diameter, SNE-100, David Kopf). The baseline response was obtained at 30-s intervals for 10 min with a stimulus intensity that yielded half-maximal FP amplitude. Theta-burst stimulation (TBS, 5 bursts at 5 Hz of 10 pulses at 100 Hz) was applied 5 times at 10-s intervals to induce LTP. The FP was recorded for 50 min after the application of TBS. The signals were amplified 1,000-fold, filtered between 0.1 and 3 kHz, and saved to a Pentium PC using the LTP program (v2.3, www.ltp-program.com). The peak amplitude of the FP from the baseline was calculated and used for analysis of the effect of the conditioning stimulation. Chemicals D-AP5, NAN-190, 2-methyl-5-hydroxytryptamine (2-me-5- HT), and (±)-2,5-dimethoxy-4-iodoamphetamine (DOI) were purchased from Tocris (Bristol, UK). PCA, 5-HT, (±)-8-hy- droxy-2-dipropylaminotetralin (8-OH-DPAT), mesulergine and other chemicals were purchased from Sigma (St. Louis, MO, USA). Statistical analysis Data were expressed as the mean±SE. Group compar- isons were performed using either paired or unpaired two-tailed Student’s t-tests. The level of significance was set at p＜0.05. RESULTS 5-HT inhibits the induction of LTP in young rats In our previous study, the duration of the stimulation pulse was important for the recruitment of inhibitory cir- cuits and the induction of long-term synaptic plasticity in the supragranular layer of the rat visual cortex [22]. To investigate the inhibitory effect of 5-HT on the induction of LTP in the pathway from layer 4 to layer 2/3 in the pres- ent study, we induced LTP in layer 2/3 using a 0.2-ms stim- ulus pulse, which stably induced LTP during the critical period (postnatal 3∼5 weeks in rats). In accordance with our previous study, TBS evoked LTP in slices from 3-(125.6± 7.8% of the baseline FP amplitude, n=8, p＜0.05) and 5- week-old rats (133.8±3.7% of the baseline FP amplitude, n=10, p＜0.001) (Fig. 1A and C). However, although TBS induced a small LTP in 5-week-old rats in the presence of 5-HT (10 μM) (111.6±1.8% of the baseline FP amplitude, n=10, p＜0.001), 5-HT significantly inhibited the induction of LTP at this age (p＜0.001, compared with control) (Fig. 1C). The magnitudes of LTP and the inhibition by 5-HT were similar to those in our previous studies [11,22]. Thus, we confirmed the inhibitory effect of 5-HT on the induction of LTP, which was induced by the stimulation recruiting less inhibitory circuits, in the pathway from layer 4 to layer 2/3 in the visual cortex of young rats. 5-HT facilitates NMDA receptor-dependent LTP in adult rats In contrast with young rats, the magnitude of LTP in the pathway from layer 4 to layer 2/3 declined with age to 106.7±2.8% (n=12) of the baseline FP amplitude for 8-week- old rats (p＜0.05) and to 99.4±8.0% (n=5) of the baseline FP amplitude for 10-week-old rats (p=0.94) (Fig. 1B and C). Although it is known that LTP and LTD in the pathway from layer 4 to layer 2/3 is constantly elicited regardless of aging [23], our experimental conditions exhibited an age- dependent decline in the induction of LTP and LTD in this pathway, which might have resulted from the recruitment of a substantial amount of inhibitory components and their postnatal development [22,24]. However, the application of 5-HT into the bath solution reinstated the induction of LTP in 8-(131.1±5.9% of the baseline FP amplitude, n=12, p＜ 0.001) and 10-week-old rats (128.5±5.2% of the baseline FP