Sedative and Motor Incoordination Effects of Ethanol in Mice Lacking CD14, TLR2, TLR4, or MyD88

BACKGROUND

In our companion article, we examined the role of MyD88-dependent signaling in ethanol (EtOH) consumption in mice lacking key components of this inflammatory pathway and observed differential effects on drinking. Here, we studied the role of these same signaling components in the acute sedative, intoxicating, and physiological effects of EtOH. Toll-like receptor 4 (TLR4) has been reported to strongly reduce the duration of EtOH-induced sedation, although most studies do not support its direct involvement in EtOH consumption. We examined TLR4 and other MyD88 pathway molecules to determine signaling specificity in acute EtOH-related behaviors. We also studied other GABAergic sedatives to gauge the EtOH specificity and potential role for GABA in EtOH's sedative and intoxicating effects in the mutant mice.

METHODS

Loss of righting reflex (LORR) and recovery from motor incoordination were studied following acute injection of EtOH or other sedative drugs in male and female control C57BL/6J mice versus mice lacking CD14, TLR2, TLR4 (C57BL/10ScN), or MyD88. We also examined EtOH-induced hypothermia and blood EtOH clearance in these mice.

RESULTS

Male and female mice lacking TLR4 or MyD88 showed reduced duration of EtOH-induced LORR and faster recovery from EtOH-induced motor incoordination in the rotarod test. MyD88 knockout mice had slightly faster recovery from EtOH-induced hypothermia compared to control mice. None of the mutants differed from control mice in the rate of blood EtOH clearance. All of the mutants showed similar decreases in the duration of gaboxadol-induced LORR, but only mice lacking TLR4 were less sensitive to the sedative effects of pentobarbital. Faster recovery from diazepam-induced motor impairment was observed in CD14, TLR4, and MyD88 null mice of both sexes.

CONCLUSIONS

TLR4 and MyD88 were key mediators of the sedative and intoxicating effects of EtOH and GABAergic sedatives, indicating a strong influence of TLR4-MyD88 signaling on GABAergic function. Despite the involvement of TLR4 in EtOH's acute behaviors, it did not regulate EtOH consumption in any drinking model as shown in our companion article. Collectively, our studies demonstrate differential effects of TLR-MyD88 components in the acute versus chronic actions of EtOH.

Ethanol Consumption in Mice Lacking CD14, TLR2, TLR4, or MyD88

BACKGROUND

Molecular and behavioral studies support a role for innate immune proinflammatory pathways in mediating the effects of alcohol. Increased levels of Toll-like receptors (TLRs) have been observed in animal models of alcohol consumption and in human alcoholics, and many of these TLRs signal via the MyD88-dependent pathway. We hypothesized that this pathway is involved in alcohol drinking and examined some of its key signaling components.

METHODS

Different ethanol (EtOH)-drinking paradigms were studied in male and female control C57BL/6J mice versus mice lacking CD14, TLR2, TLR4 (C57BL/10ScN), or MyD88. We studied continuous and intermittent access 2-bottle choice (2BC) and 1-bottle and 2BC drinking-in-the-dark (DID) tests as well as preference for saccharin, quinine, and NaCl.

RESULTS

In the 2BC continuous access test, EtOH intake decreased in male TLR2 knockout (KO) mice, and we previously reported reduced 2BC drinking in male and female CD14 KO mice. In the intermittent access 2BC test, EtOH intake decreased in CD14 KO male and female mice, whereas drinking increased in MyD88 KO male mice. In the 2BC-DID test, EtOH drinking decreased in male and female mice lacking TLR2, whereas drinking increased in MyD88 KO male mice. In the 1-bottle DID test, EtOH intake decreased in female TLR2 KO mice. TLR2 KO and CD14 KO mice did not differ in saccharin preference but showed reduced preference for NaCl. MyD88 KO mice showed a slight reduction in preference for saccharin.

CONCLUSIONS

Deletion of key components of the MyD88-dependent pathway produced differential effects on EtOH intake by decreasing (TLR2 KO and CD14 KO) or increasing (MyD88 KO) drinking, while deletion of TLR4 had no effect. Some of the drinking effects depended on the sex of the mice and/or the EtOH-drinking model.