Estrogen-dependent enhancement of NO production in the nucleus tractus solitarius contributes to ethanol-induced hypotension in conscious female rats

Abstract

Background: Our previous pharmacological and cellular studies showed that peripheral (cardiac and vascular) nitric oxide synthase (NOS)-derived NO is implicated in the estrogen (E(2))-dependent hypotensive action of ethanol in female rats. The objective of this study was to test the hypothesis that enhanced NO production in the nucleus tractus solitarius (NTS) is implicated in the E(2)-dependent hypotensive action of ethanol.

Methods: To achieve this goal, we utilized in vivo electrochemistry to measure real time changes in neuronal NO to investigate the acute effects of intragastric ethanol (0, 0.5, or 1 g/kg) on NO in NTS neurons, blood pressure (BP), and heart rate (HR) in conscious female rats in the absence (ovariectomized, OVX, rats) or presence of E(2).

Results: In sham operated (SO) rats, ethanol elicited dose-related increase in NTS NO and reduction in BP. These neurochemical and BP effects of ethanol were absent in OVX rats. Whether the neurochemical effect of ethanol and the associated hypotension are dependent on rapid E(2) signaling was investigated. In OVX rats pretreated, 30 minutes earlier, with E(2) (1 microg/kg), intragastric ethanol (1 g/kg) increased NTS NO and reduced BP and these responses were comparable to those obtained in SO rats.

Conclusions: The present findings suggest that increased production of NO in NTS neurons contributes to ethanol-evoked hypotension in female rats. Further, ethanol enhancement of neuronal NO production in the brainstem is dependent on rapid E(2) signaling.

Fig. 1

Representative tracings depicting the changes in nucleus tractus solitarius (NTS) nitric oxide (NO), blood pressure (BP) and heart rate (HR) elicited by intragastric ethanol (1g/kg), compared with equal volume of vehicle (water), in conscious freely moving sham-operated SD female rats.

Fig. 2

Effect of intragastric ethanol (Eth 0.5 or 1.0 g/kg), compared with vehicle (water), on nucleus tractus solitarius (NTS) nitric oxide (NO) level, blood pressure (MAP), and heart rate (HR) in conscious freely moving sham-operated female SD rats. The bar graphs show the Area Under the Curve for the measured variables. Values are mean ± SEM. P<0.05 compared with corresponding control, #P<0.05 compared with ethanol 0.5g/kg group.

Fig. 3

Effect of intragastric ethanol (Eth 0.5 or 1.0 g/kg), compared with vehicle (water), on nucleus tractus solitarius (NTS) nitric oxide (NO) level, blood pressure (MAP), and heart rate (HR) in conscious freely moving ovariectomized SD rats. The bar graphs show the Area Under the Curve for the measured variables. Values are mean ± SEM.

Fig. 4

Effect of acute (30 min) estrogen (E₂, 1µg/kg, i.v.) pretreatment in conscious ovariectomized rats on the neurochemical (NTS NO), blood pressure (MAP), and heart rate (HR) responses elicited by intragastric ethanol (Eth 0.5 or 1.0 g/kg) or the vehicle (water). The bar graphs show the Area Under the Curve for the measured variables. Values are mean ± SEM. *P<0.05 compared with corresponding control, #P<0.05 compared with ethanol 0.5g/kg group.

Fig. 5

Picture (A) and schematic diagrams (B) of coronal sections of rat brainstem showing the electrochemical recording sites in the NTS. The rostro-caudal coordinates related to interaural line are in the upper right corner of each diagram (B). Py, pyramidal tract; ROb, raphe obscurus nu; IOD, inferior olive, dorsal nu; cc, central canal; SP5I, spinal 5 nu, interpolar part; Amb, ambiguus nu; 12, hypoglossal nu; IOA, IO, subnu Aof medial nu. According to Paxinos and Watson (Paxinos & Watson, 1998).