Role of the median preoptic nucleus in the chronic hypotensive effect of losartan in sodium-replete normal rats

1. We have shown previously that the chronic hypotensive effect of the angiotensin II AT₁ receptor antagonist losartan is mediated, in part, by the subfornical organ (SFO). However, the neural pathway(s) mediating this central effect of losartan downstream from the SFO has not been completely elucidated.
2. The present study was designed to test the hypothesis that the median preoptic nucleus (MnPO) is a crucial part of the neural pathway necessary for the chronic hypotensive effect of losartan. To test this hypothesis, male Sprague-Dawley rats were subjected to either Sham or electrolytic lesion of the MnPO (MnPOx). Rats were instrumented with radiotelemetric transducers and aortic flow probes for the continuous measurement of mean arterial pressure (MAP) and heart rate and cardiac output (CO), respectively. Total peripheral resistance (TPR) was calculated as MAP/CO. After 3 days of baseline measurements, rats were infused intraperitoneally with losartan (10 mg/kg per day) via an osmotic minipump at a rate of 5 μL/min.
3. The data revealed that, by Day 9 of losartan treatment, MAP had decreased 34 ± 2 mmHg in MnPOx rats ( n  = 9), whereas the MAP of Sham-lesioned rats ( n  = 8) had only decreased 24 ± 3 mmHg. These findings were accompanied by a greater decrease in TPR in MnPOx compared with Sham rats (?0.464 vs ?0.237 mmHg/mL per min, respectively), whereas CO remained unchanged throughout the study protocol.
4. These results do not support the hypothesis that an intact MnPO is necessary to mediate the full chronic hypotensive effect of losartan in normal rats. Instead, they appear to suggest that the MnPO may play an important role in buffering the profound hypotension induced by losartan.