Ethanol selectively counteracts hypotension evoked by central I(1)-imidazoline but not alpha2-adrenergic receptor activation in spontaneously hypertensive rats

Differential central NOS-NO signaling underlies clonidine exacerbation of ethanol-evoked behavioral impairment

BACKGROUND

The molecular mechanisms that underlie clonidine exacerbation of behavioral impairment caused by ethanol are not fully known. We tested the hypothesis that nitric oxide synthase (NOS)-derived nitric oxide (NO) signaling in the locus coeruleus (LC) is implicated in this phenomenon.

METHODS

Male Sprague-Dawley rats with intracisternal (i.c.) and jugular vein cannulae implanted 6 days earlier were tested for drug-induced behavioral impairment. The latter was assessed as the duration of loss of righting reflex (LORR) and rotorod performance every 15 minutes until the rat recovered to the baseline walk criterion (180 seconds). In a separate cohort, we measured p-neuronal NOS (nNOS), p-endothelial NOS (eNOS), and p-ERK1/2 in the LC following drug treatment, vehicle, or NOS inhibitor.

RESULTS

Rats that received clonidine [60 Ig/kg, i.v. (intravenous)] followed by ethanol (1 or 1.5 g/kg, i.v.) exhibited synergistic impairment of rotorod performance. Intracisternal pretreatment with nonselective NOS inhibitor N(omega)-nitro-L-arginine methyl ester (L-NAME, 0.5 mg) or selective nNOS inhibitor N-propyl-L-arginine (1 microg) exacerbated the impairment of rotorod performance caused by clonidine-ethanol combination. Exacerbation of behavioral impairment was caused by L-NAME enhancement of the effect of ethanol, not clonidine. L-NAME did not influence blood ethanol levels; thus, the interaction was pharmacodynamic. LORR caused by clonidine (60 microg/kg, i.v.)-ethanol (1 g/kg, i.v.) combination was abolished by selective inhibition of central eNOS (L-NIO, 10 microg i.c.) but not by nNOS inhibition under the same conditions. Western blot analyses complemented the pharmacological evidence by demonstrating that clonidine-ethanol combination inhibits phosphorylation (activation) of nNOS (p-nNOS) and increases the level of phosphorylated eNOS (p-eNOS) in the LC; the change in p-nNOS was paralleled by similar change in LC p-ERK1/2. NOS inhibitors alone did not affect the level of nitrate/nitrite, p-nNOS, p-eNOS, or p-ERK1/2 in the LC.

CONCLUSIONS

Alterations in NOS-derived NO in the LC underlie clonidine-ethanol induced behavioral impairment. A decrease in nNOS activity, due at least partly to a reduction in nNOS phosphorylation, mediates rotorod impairment, while enhanced eNOS activity contributes to LORR, elicited by clonidine-ethanol combination.

Direct evidence for imidazoline (I1) receptor modulation of ethanol action on norepinephrine-containing neurons in the rostral ventrolateral medulla in conscious spontaneously hypertensive rats

BACKGROUND

Enhancement of the rostral ventrolateral medulla (RVLM) presympathetic (norepinephrine, NE) neuronal activity represents a neurochemical mechanism for the pressor effect of ethanol. In this study, we tested the hypothesis that ethanol action on RVLM presympathetic neurons is selectively influenced by the signaling of the local imidazoline (I1) receptor. To support a neuroanatomical and an I1-signaling selectivity of ethanol, and to circumvent the confounding effects of anesthesia, the dose-related neurochemical and blood pressure effects of ethanol were investigated in the presence of selective pharmacological interventions that cause reduction in the activity of RVLM or nucleus tractus solitarius (NTS) NE neurons via local activation of the I1 or the alpha2-adrenergic receptor in conscious spontaneously hypertensive rats.

RESULTS

Local activation of the I1 receptor by rilmenidine (40 nmol) or by the I1/alpha2 receptor mixed agonist clonidine (1 nmol), and local activation of the alpha2-adrenergic receptor (alpha2AR) by the pure alpha2AR agonist alpha-methylnorepinephrine (alpha-MNE, 10 nmol) caused reductions in RVLM NE, and blood pressure. Intra-RVLM ethanol (1, 5, or 10 microg), microinjected at the nadir of the neurochemical and hypotensive responses, elicited dose-dependent increments in RVLM NE and blood pressure in the presence of local I1--but not alpha2-receptor activation. Only intra-NTS alpha-MNE, but not rilmenidine or clonidine, elicited reductions in local NE and blood pressure; ethanol failed to elicit any neurochemical or blood pressure responses in the presence of local activation of the alpha2AR within the NTS.

CONCLUSION

The findings support the neuroanatomical selectivity of ethanol, and support the hypothesis that the neurochemical (RVLM NE), and the subsequent cardiovascular, effects of ethanol are selectively modulated by I1 receptor signaling in the RVLM.

Mitogen-activated protein kinase phosphorylation in the rostral ventrolateral medulla plays a key role in imidazoline (i1)-receptor-mediated hypotension

Our previous study showed that rilmenidine, a selective I(1)-imidazoline receptor agonist, enhanced the phosphorylation of mitogen-activated protein kinase (MAPK)(p42/44), via the phosphatidylcholine-specific phospholipase C pathway in the pheochromocytoma cell line (PC12). In the present study, we tested the hypothesis that enhancement of MAPK phosphorylation in the rostral ventrolateral medulla (RVLM) contributes to the hypotensive response elicited by I(1)-receptor activation in vivo. Systemic rilmenidine (600 microg/kg i.v.) elicited hypotension and bradycardia along with significant elevation in MAPK(p42/44), detected by immunohistochemistry, in RVLM neurons. To obtain conclusive evidence that the latter response was I(1)-receptor-mediated, similar hypotensive responses were elicited by intracisternal (i.c.) rilmenidine (25 microg/rat) or the highly selective alpha(2)-agonist alpha-methylnorepinephrine (4 microg/rat). An increase in RVLM MAPK(p42/44) occurred only after rilmenidine. Furthermore, pretreatment with efaroxan (0.15 microg/rat i.c.), a selective I(1)-imidazoline receptor antagonist, or with PD98059 (2'-amino-3'-methoxyflavone) (5 microg/rat i.c.), a selective extracellular signal-regulated kinase 1/2 inhibitor, significantly attenuated the hypotensive response and the elevation in RVLM MAPK(p42/44) elicited by i.c. rilmenidine. The findings suggest that MAPK phosphorylation in the RVLM contributes to the hypotensive response induced by I(1)-receptor activation and presents in vivo evidence that distinguishes the neuronal responses triggered by the I(1)-receptor from those triggered by the alpha(2)-adrenergic receptor.

Chronic ethanol administration attenuates imidazoline I1 receptor- or alpha 2-adrenoceptor-mediated reductions in blood pressure and hemodynamic variability in hypertensive rats

Our previous studies have demonstrated that acute ethanol administration counteracts imidazoline I(1) receptor but not alpha(2)-adrenoceptor-mediated hypotension in spontaneously hypertensive rats (SHR). In the present study, we investigated the effect of chronic ethanol administration on hypotensive responses elicited by acute administration of selective imidazoline I(1) receptor (rilmenidine) or alpha(2)-adrenoceptor (alpha-methyldopa) agonist along with ethanol effects on: (i) locomotor activity and (ii) time-domain indices of variability in blood pressure (standard deviation of mean arterial pressure) and heart rate (standard deviation of beat-to-beat intervals and root mean square of successive differences in R-R intervals). Hemodynamic and locomotor responses elicited by rilmenidine or alpha-methyldopa were assessed in radiotelemetered ethanol-fed (2.5% or 5% w/v, 12 week) and control SHR. In control SHR, i.p. rilmenidine (600 microg/kg) or alpha-methyldopa (100 mg/kg) significantly reduced blood pressure. Rilmenidine had no effect on heart rate whereas alpha-methyldopa elicited a biphasic response (tachycardia followed by bradycardia). Blood pressure and heart rate oscillations were also reduced by both drugs, which may conform to sympathoinhibition. The hypotensive effect of rilmenidine or alpha-methyldopa was significantly attenuated by ethanol feeding (2.5% or 5%) in a concentration-dependent manner. In addition, ethanol attenuated alpha-methyldopa-evoked reduction in heart rate, but not blood pressure, variability in marked contrast to attenuating rilmenidine-evoked reductions in blood pressure, but not heart rate, variability. These findings demonstrate that, unlike its acute effects, chronic ethanol attenuates both imidazoline I(1) receptor and alpha(2)-adrenoceptor-mediated hypotension whereas its effect on hemodynamic variability depended on the nature of the hypotensive stimulus.

Effect of ethanol on reductions in norepinephrine electrochemical signal in the rostral ventrolateral medulla and hypotension elicited by I1-receptor activation in spontaneously hypertensive rats

BACKGROUND

The mechanism of the antagonistic hemodynamic interaction between ethanol and centrally acting sympatholytics is not known. In this study, we tested the hypothesis that the imidazoline (I1)-receptor modulation of norepinephrine (NE) release within the rostral ventrolateral medulla (RVLM) plays a pivotal role in this clinically relevant hemodynamic interaction. METHOD In anesthetized spontaneously hypertensive rats, the effects of centrally acting sympatholytics on RVLM NE electrochemical signal were investigated by in vivo electrochemistry along with cardiovascular responses in the absence and presence of ethanol. In vivo microdialysis in conscious spontaneously hypertensive rats was used to confirm the electrochemical findings. RESULTS Clonidine (30 microg/kg, intravenously) or rilmenidine (400, 600, or 800 microg/kg) significantly reduced RVLM NE electrochemical signal (index of neuronal activity) and mean arterial pressure; rilmenidine effects were dose-related, and ethanol (1 g/kg) counteracted these responses. Ethanol (1 g/kg) pretreatment increased both RVLM NE electrochemical signal and blood pressure but did not influence the reductions in both variables elicited by subsequently administered clonidine. The alpha2-adrenergic antagonist 2-methoxyidazoxan (30 microg/kg) counteracted rilmenidine (800 microg/kg)-evoked responses. In vivo microdialysis in conscious spontaneously hypertensive rats confirmed the electrochemical findings since clonidine- (30 microg/kg, intravenously) evoked reductions in RVLM NE and the associated hypotension were counteracted by ethanol (1 g/kg). CONCLUSIONS (1) Ethanol counteracts centrally mediated hypotension, at least in part, by increasing RVLM NE; (2) the interaction involves the I1 receptor modulation of RVLM neuronal activity; (3) the alpha2-adrenergic receptor contributes to the electrochemical and cardiovascular effects of high doses of rilmenidine, and (4) the RVLM is a neuroanatomical target for systemically administered ethanol.

Evidence for the involvement of central I1 imidazoline receptor in ethanol counteraction of clonidine hypotension in spontaneously hypertensive rats

Our previous studies have shown that ethanol counteracts centrally mediated hypotensive responses to clonidine. In this study, we investigated the relative roles of central alpha2-adrenergic and I1 imidazoline receptors in the antagonistic ethanol-clonidine hemodynamic interaction. The effects of selective blockade of alpha2- or I1 receptor by 2-methoxyidazoxan and efaroxan, respectively, on the blood pressure and heart rate responses to clonidine and subsequent ethanol administration were evaluated in conscious spontaneously hypertensive rats. Intracisternal administration of clonidine (1.5 microg/kg) produced significant (30 mm Hg; p < 0.05) and sustained (at least 60 min) decreases in blood pressure and heart rate. Systemic ethanol (1 g/kg), administered 10 min after clonidine, counteracted the hypotensive response and restored blood pressure to the preclonidine levels. Treatment with 2-methoxyidazoxan (0.16 microg/kg, intracisternal) or efaroxan (0.45 microg/kg, intracisternal) produced similar attenuation of the hypotensive and bradycardic responses to clonidine. The ability of ethanol to counteract the hypotensive action of clonidine was significantly (p < 0.05) attenuated in rats pretreated with efaroxan. The pressor response to ethanol lasted only 10 min compared with at least 60 min in the absence of efaroxan. In contrast, ethanol counteraction of clonidine-evoked hypotension was not altered when alpha2-adrenoceptors were blocked by 2-methoxyidazoxan. These findings suggest that centrally mediated hypotensive and bradycardic effects of clonidine in conscious spontaneously hypertensive rats involve activation of both alpha2-adrenergic and I1 imidazoline receptors. Furthermore, the findings suggest the dependence of a fully expressed ethanol counteraction of the hypotensive action of clonidine on functional I1 receptor within the central nervous system.