The current report is the first to examine the effects of intracranial ALLO on the responding for and self-administration of ethanol in mice, and to discern ALLO’s influence on ethanol-reinforced responding versus consumption. ALLO did not influence the appetitive measures examined. In contrast, the lowest ALLO dose (50 ng) significantly enhanced mean bout size and duration, indicating that ALLO modulated the micro-architecture of consumption maintenance. The same ALLO dose promoted consumption onset by significantly augmenting the first bout size as well as the cumulative lick frequency during the initial 10-min of the session. The absence of ALLO effects on locomotor behavior and the frequency of total responses indicate that ALLO’s impact on drinking patterns was not attributable to a non-selective effect on activity.
The effects of intracranial ALLO on drinking patterns closely resemble the previously reported influence of systemically-administered neurosteroid assessed within a 2-bottle choice procedure in mice (see [14
]). The 50 ng (ICV) and 10 mg/kg (intraperitoneally; IP) doses similarly augmented both the mean and first bout sizes. Furthermore, a low systemic dose of ALLO (3.2 mg/kg, IP) significantly elevated the maintenance of ethanol licks, whereas higher doses (17 and 24 mg/kg, IP) suppressed licks [14
]. This cumulative lick profile mirrored the current finding that 50 ng ALLO enhanced licks during the initial 10-min of the self-administration session whereas 400 ng led to a significant attenuation in licks. The acute application of the 5α-reductase inhibitor finasteride, which blocks the synthesis of endogenous ALLO, also affected mean bout size, first bout size, and bout duration components of bout micro-architecture (see [13
]), but the effects were exactly the opposite to those observed following 50 ng ALLO in the current study. Thus, ALLO modulates ethanol intake patterns in a consistent manner regardless of the route of neurosteroid administration or the self-administration procedure utilized. The current observations, taken in concert with earlier findings following finasteride treatment, suggest that ALLO may be a physiologically-relevant endogenous modulator of regulatory processes underlying ethanol consumption.
Although the current work did not evaluate the specificity of ALLO’s effect on ethanol self-administration, previous work from our laboratory and others have reported disparate findings on this issue. In a study evaluating concurrent responding for 10% ethanol and a 1% sucrose solution in male rats, a 5.6 mg/kg dose of ALLO selectively enhanced responding for ethanol while leaving sucrose responding unaltered [20
]. Another study revealed that a 3-day regimen of 10 mg/kg ALLO administered to male mice augmented either 10% ethanol or a 0.033% saccharin solution provided under a limited access 2-bottle preference procedure in the home cage [42
]. Consistent with the latter observation, ALLO (0.5 to 2 mg/kg) has been shown to elicit a hyperphagic feeding response in male mice [31
]. Clearly, additional effort is needed to tease apart the specificity of ALLO’s influence on ethanol reinforcement and consumption under both operant and non-operant conditions.
ALLO predominantly elicited its impact on consumption patterns at the lowest dose administered (i.e., 50 ng) whereas higher doses exhibited no difference from the vehicle control. The occurrence of a narrow effective dose range as well as a bimodal dose-response function in the current study is consistent with numerous earlier observations in rodent models and humans. A 3 mg/kg ALLO dose, but not 1 or 10 mg/kg, significantly enhanced ethanol-reinforced responding in rats [21
]. Similarly, our laboratory previously reported a significant enhancement in ethanol intake following systemic pre-treatment with 3.2 mg/kg ALLO, no change with a 10 mg/kg dose, and a significant decrease with 24 mg/kg [14
]. Intra-accumbal infusion of 0.55 and 1.0 ng ALLO (~1.5 and 3 pmol) partially substituted for a systemic ethanol injection in a drug discrimination procedure, whereas lower and higher doses were without effect [19
]. Similarly, in a mouse model of aggression, 10 and 17 mg/kg ALLO elevated sideway threats and attack bites, whereas 3 mg/kg had no effect, and 30 mg/kg decreased these measures [11
]. A recent clinical report also demonstrated that the ALLO concentrations stemming from 30 mg progesterone, but not 0, 60 or 200 mg were associated with enhanced negative mood scores in postmenopausal women [1
]. Lastly, administration of 100 pmol ALLO into the lateral ventricle was found to elevate extracellular dopamine content in the NAc [32
] whereas 45 nmol ALLO suppressed this measure [24
]. Notably, ALLO’s peak effect (50 ng; equivalent to 150 pmol) on bout micro-architecture in the present study closely resembled the concentration and route of ALLO administration (100 pmol; ICV) that led to enhanced dopamine content in the NAc. These observations collectively indicate that the behavioral and neurochemical manifestations of ALLO’s CNS effects occur within a limited dose range. Multiple interpretations of ALLO’s biphasic dose-response function have been offered. It is believed that higher ALLO doses reverse the influence of low to moderate doses by a mechanism that may involve disinhibition [1
], shift from modulation to direct activation of chloride flux through GABAA
], and loss of specificity for GABAA
Because systemically-administered ALLO has been previously shown to produce locomotor stimulation in B6 mice [27
], it was necessary to demonstrate a dissociation between ALLO’s effects on ethanol self-administration versus general locomotor activity. The absence of locomotor effects with 100 and 400 ng ALLO is congruent with the unpublished observation that doses in excess of 1 μg were required to elicit locomotor stimulation when administered intra-VTA (Phillips & Finn, unpub). When taken in conjunction with the lack of ALLO-induced changes in the number of total responses on both levers, it is unlikely that the altered drinking patterns following treatment with the 50 ng dose or higher doses were attributable to a general locomotor effect of ALLO.
As previously predicted [4
], the procedural separation of appetitive and consummatory phases of ethanol self-administration in rodents has proven to be an effective means to dissociate the influence of pharmacological interventions on processes underlying ethanol seeking versus drinking. It has been demonstrated that systemic and brain site-specific pharmacological interventions can selectively impact appetitive versus consummatory processes [5
] or influence both concomitantly [40
]. In the present work, ALLO influenced consummatory processes. However, the absence of an effect of ALLO on appetitive processes associated with ethanol intake should be interpreted cautiously. The response requirement (RR) employed in the current work was modest (i.e., 8 lever presses) when compared to earlier work by Samson and colleagues in rats (i.e., RR30) using a similar operant procedure [6
]. The lower RR in the present work was dictated by a general lower level of responding in mice when compared to rats. A more robust appetitive component (i.e., greater RR) may be necessary to reveal an effect of ALLO on ethanol-seeking behavior in mice.
In conclusion, the present findings provide evidence that centrally-active ALLO modulates ethanol intake patterns associated with the onset and maintenance of self-administration. When taken in conjunction with the recent observations that ethanol consumption increased endogenous ALLO concentrations in B6 mice [10
] and human adolescents [43
], and that the inhibition of ALLO biosynthesis attenuated ethanol intake [13
], the current findings are consistent with the hypothesis that alterations in endogenous ALLO levels may influence the regulatory processes governing ethanol consumption and reinforcement. The development of pharmacological interventions that manipulate endogenous ALLO concentrations may prove to be a beneficial treatment strategy.