Influence of sex, estrous cycle, and drug-onset age on cocaine self-administration in rats (Rattus norvegicus).

The influence of sex, phase of the estrous cycle, and age of drug onset on cocaine self-administration was examined. Adult male, adult female, and adolescent male rats (Rattus norvegicus) were evaluated using low fixed-ratio (FR) schedules of drug delivery with a single fixed cocaine unit dose or a range of cocaine unit doses with a single FR schedule. Sex differences in adults were observed for mg/kg consumption of the 3.0-mg/kg unit dose, with consumption being significantly less in estrus females than in males. Over the estrous cycle, mg/kg consumption of this unit dose was significantly less during estrus than during metestrus-diestrus. Differences due to age of drug onset were also observed, with mg/kg consumption of the 3.0-mg/kg unit dose being significantly less in adolescent males than adult males or adult females during metestrus-diestrus. In contrast, these various groups did not have significantly different mg/kg intakes of cocaine unit doses <3.0 mg/kg, nor did they significantly differ in the rates and patterns of responding and number of infusions earned as a function of FR schedule or unit dose of cocaine available. The role of sex, estrus cycle, and drug-onset age on cocaine self-administration appears to be minimal under these experimental conditions. Experimental conditions that favor no sex or age differences in cocaine intake (1.0-mg/kg unit dose and low FR) may be useful for evaluating potential sex or age differences in the consequences of cocaine self-administration more reliably, as cocaine intake would not be an uncontrolled factor.