| Objective: To observe the behavior and cardiovascular effects of repeated electric foot-shock combined with noise in rats of different stress cycles; To investigate the cardiovascular and respiratory response to repeated administration of Baclofen in normotensive (non-stress) rats and stress rats of different cycles and to determine whether the response would be involved in different stress cycles.Methods: (1) The experiments were performed on male Wistar Rats. Rats were stimulated by repeated electric foot-shock combined with noise from a buzzer to establish conditional reflex, receiving stress twice daily, 2 hours per time, and the treatment lasted for 15 days. The BP and H.R were measured by tail cuff method 2 hours after stress respectively 1 day before and on the 1st, 3rd, 6th, 9th, 12th, 15th day of the stress. Their behavior was also observed. Stress models of different cycles, including 1-day, 3-day, 6-day, 9-day, 12-day, 15-day-stress, were made in this way. (2) Baclofen (Bac) was twice-repeated administered intraperitoneally into urethane-anesthetized non-stress rats and several-time-repeated into urethane-anesthetized stress rats with different cycles. Mean Blood Pressure (MBP), HeartRate (H.R) and Respiration Rate (R.R) were continuously recorded through Maclab/8S, by a light-electric energy conversion apparatus and two catheters, inserted into left femoral artery and the trachea.Results: (1) The stressed rats stagnated to grow, being restless, nervous and easy to be angry, accompanied by enhanced sympathetic action. Blood Pressure elevated significantly after stress (all P<0.01), from 121.6 ± 5.6 mmHg to the peak 158.3±7.5mmHg 6 days later, 152.8 mmHg±5.8 mmHg 9 days later, and maintained at the high level, still 150.5 ± 7.1mmHg after 15-day stress. Heart Rate increased only after 3-day stress, from 416±17 beats/min to 436±23 beats/min, (P<0.05), until the 6th day, increased to 442 ± 9 beats/min (P<0.05); thereafter decreased slowly. During the process, there was no significant changes in BP or H.R in control group rats.(2) Effects of twice-repeated injection into non-stress rats: In the experimental group (Bac-Bac), first treatment with Baclofen produced a marked and prolonged increase in MBP, from 113.2 ± 8.3 mmHg to 139.2±11.2 mmHg 30 minutes later (P<0.05), maintained at the level, 139.7 ± 21.7 mmHg 60 minutes later (P<0.05 ) .The pressor response was reversed to 120.3 ± 20.4 mmHg by second treatment (compared with the initial level, P>0.05), accompanied with no effects on H.R. In the control group (Bac-None) and the frequency control group (Bac-NS), the similar effects were observed with firsttreatment. But when the second administered, MBP was still different from its initial level (P<0.01), with no significance in PR (P>0.05). Responses of R.R appear descending at later time. That of experimental group fell from 116± 16 beats/min to 92±10 beats/min (P<(). 01) until 30 minutes after second injection. At the same time point, R.R of the control group and the frequency control group differed from that of respective baseline, P<0.01. In the volume control group, values were all unaltered in MBP, H.R, R.R, P>0.05.(3) Effects of several-time repeated injection into stressed rats: 30 minutes after the first Baclofen, similar pressor effects were observed. Except 6-day-stress increased 15.3 ±9.3 mmHg (not significant) and 9-day-stress groups (significant 60mins later), the increases were significant in other groups. The control group increased 26.0±9.5mmHg (P<0.05), 1-day-stress group 30.2 ± 6.9 mmHg (P<0.01), 3-day-stress group 26.0±6.6 mmHg (P<0.01), 12-day-stress 23.6±8.1mmHg (P<0.01), 15-day-stress group 27.9±10.2 mmHg (P<0.01). There was no significance between 30mins and 60mins after administration in all groups. Compared with 6-day-stress group, the increased amplitude of the control group(P<0.01), l-day-stress(P<0.01), 3-day-stress(P<0.05), 15-day-stress (P<0.01) groups were more marked. There was no diffirence in H.R between pre- and post-treatments.
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