The purpose of this study was to compare the responsiveness of the rostral and caudal ventrolateral medulla in spontaneously hypertensive (SH) and normotensive Wistar-Kyoto (WKY) rats to microinjection of l-glutamate, and to estimate tonic output of these areas by microinjecting the neurotoxin tetrodotoxin. Rats were anesthetized with 1.25 g/kg urethane s.c., implanted with arterial (femoral) and venous (femoral) catheters, artificially ventilated and paralyzed with gallamine triethiodide (10 mg/kg). Using a ventral approach to the brainstem, the mean arterial pressure and heart rate responses to microinjection (30 nl) of l-glutamate (1, 10 and 100 mM) and tetrodotoxin (10 μM) into the rostral and caudal ventrolateral medulla were compared in SH (n = 7) and WKY (n = 7) groups. Microinjection of l-glutamate into the rostral ventrolateral medulla produced equivalent increases in mean arterial pressure (maximum +33 ± 3 and +36 ± 6 mm Hg, SH and WKY groups respectively) and minimal changes in heart rate. Similar administration of l-glutamate into the caudal ventrolateral medulla caused decreases in mean arterial pressure and heart rate; changes in mean arterial pressure were significantly greater in the SH group than in the WKY group (-52.3 ± 2.9 mm Hg for SH, -22.6 ± 2.6 mm Hg for WKY). Bilateral microinjection of tetrodotoxin into the caudal ventrolateral medulla produced significantly larger increases of mean arterial pressure in WKY rats (+8 ± 4 vs +46 ± 8 mm Hg for SH vs WKY). These data indicate that SH rats may have a lower tonic activity of neurons in the caudal ventrolateral medulla, resulting in a lower restraining influence on sympathetic outflow in the SH rat. (Supported by: HL 36552 and KY-THRI 5-42078).
- Heart rate
- Mean arterial pressure
- Spontaneously hypertensive rat
- Ventrolateral medulla
- Wistar-Kyoto rat