Animal Preparation

All experiments were conducted on male Wistar rats (Japan SLC, Inc.¹) weighing 260–330 g at the time of the experiments. All the rats were maintained under standard laboratory conditions with a 12-h light/12-h dark normal cycle (lights on at 7 AM) at 25°C and had free access to water and food. On the day of the experiment, anesthesia was initially induced with isoflurane (Fujifilm Wako Pure Chemical Corporation), and the anesthetized state was maintained with urethane (1–1.25 g/kg body weight, i.p.; Tokyo Chemical Industry Co., Ltd.). The dose of the anesthetic agent was based on those of previous studies (Martin et al., 2006; Drew et al., 2011), and the depth of anesthesia was confirmed by a negative reflex on the paw-pinch test. A heparinized saline-filled polyethylene catheter (SP31; Natsume Seisakusho Co., Ltd.) was inserted into the left femoral artery and connected to a carrier amplifier (AP-621G; Nihon Kohden) to record the MAP. The HR was calculated from lead-I electrocardiography, which was amplified by the use of a bioelectrical amplifier (AB-651J; Nihon Kohden). The right femoral vein was cannulated with a saline-filled polyethylene catheter (SP31; Natsume Seisakusho Co., Ltd.) to apply the drugs intravenously. In the vagotomy experiments, described below, before the rats were positioned in a stereotaxic apparatus in the prone position, the bilateral vagus nerves were identified at the neck and threads were detained under the nerves as markers in the supine position.