Preferences for cocaine- or pup-associated chambers differentiates otherwise behaviorally identical postpartum maternal rats

Abstract

Rationale and objectives-Our previous work uncovered a differential preference of maternal female rats for cues associated with pups versus cues associated with cocaine at three different postpartum time points. Our current study examines the preference for these cues in conjunction with an assessment of the capacity to express the maternal behavior at one of these time points. We examined dams at day 10 postpartum using a procedure that included two additional controls, and a complete assessment of the expression of maternal behavior and locomotor activity.Methods-A conditioned place-preference procedure was used to determine the preference for cocaine-or pup-associated cues. The two controls were (1) a preconditioning test to verify no initial chamber preference and (2) a separate control group of postpartum day-10 dams exposed to chambers and cues but not to unconditioned stimuli. The expression of maternal behavior was determined by measurement of maternal nest building, retrieval of pups to the nest, grooming, crouching over pups, nursing, and maternal aggression. Locomotor activity was measured with an automated apparatus.Results-Dams conditioned with cocaine or pups showed a preference for either the cocaineassociated chamber or the pup-associated chamber, confirming the existence of two similar-sized preference groups at this time point. Regardless of preference, dams had equal and robust expression of maternal behavior and similar locomotor capacity. The pre-conditioning test showed no initial chamber preferences and did not alter the conditioned preference response. The use of unconditioned stimuli in the place-preference conditioning procedure was effective and necessary for the preference response.Conclusion-Our current study has revealed that differences in the motivational state of the maternal dam emerge even while the expression of maternal behavior is constant and substantial. The data suggest that the difference in preference is a very specific appetitive response that is not linked to expression of maternal behavior or locomotor capacity.

Plasma cocaine levels, metabolites, and locomotor activity after subcutaneous cocaine injection are stable across the postpartum period in rats

“…The stability of the values is remarkable considering that they are taken from day 4 to day 24 postpartum when the females have very different levels of circulating hormones. Therefore, the marked differences in the reward salience of cocaine across the postpartum period that we have uncovered with our place preference conditioning paradigm (Mattson et al, 2001(Mattson et al, , 2003Wansaw et al, 2003bWansaw et al, , 2004 are not likely due to differences in the peripheral processing or general accessibility of cocaine to the brain to activate motor systems. Hence, we next will investigate whether the differences in the reward salience of cocaine across the postpartum period are likely to be due to changes in the effect of cocaine on specific neural structures that mediate the perception and prioritization of rewards (Mattson and Morrell, in press;Morrell et al, 2005).…”

“…In our original work on the topic, we have shown that middle and late postpartum females prefer cocaine-associated cues, whereas early postpartum females prefer pup-associated cues (Mattson et al, 2001(Mattson et al, , 2003. In our subsequent work we have examined the salience of two hours of conditioning with subcutaneously injected cocaine (versus saline) independent of the pup-associated cues.…”

“…We recently investigated the effects of cocaine as a pharmacological challenge to natural motivational processes in the lactating, postpartum rat, which is known to be highly motivated to seek and care for her offspring (Fahrbach and Pfaff, 1982;Fleming et al, 1994;Hauser and Gandelman, 1985;Lee et al, 2000;Magnusson and Fleming, 1995;Mattson et al, 2001Mattson et al, , 2003Wilsoncroft, 1969). In a series of studies using place preference conditioning, we found that the motivation of the dam to seek pup-related cues or cocaine-related cues changes across the postpartum period.…”

“…In a series of studies using place preference conditioning, we found that the motivation of the dam to seek pup-related cues or cocaine-related cues changes across the postpartum period. When given a choice between cues associated with cocaine, pups, or neutrality/ novelty, late postpartum (day 16) dams prefer cocaine-associated cues, whereas early postpartum (day 8) dams prefer pupassociated cues (Mattson et al, 2001(Mattson et al, , 2003. We have further shown that the change in the preference response in our choice paradigm is due to underlying independent changes in the reward salience of each of these stimuli (pups and cocaine) across the postpartum period (Wansaw et al, 2003a,b).…”

“…We chose to study a dose of 10 mg/kg because we established that this dose is sufficient to impair the expression of maternal behavior when present in the blood (Vernotica et al, 1996a,b) and that this was a reinforcing dose for the late postpartum but not the early postpartum dam (Mattson et al, 2001;Wansaw et al, 2002Wansaw et al, , 2003a. Furthermore we used a subcutaneous route of administration because our group and others have used this approach in pregnant and postpartum females to avoid confounding influences on fetal development or nursing processes (Vernotica et al, 1996a,b;Mattson et al, 2001Mattson et al, , 2003Johns et al, 1997Johns et al, , 1998. Concentrations of cocaine and its two principal metabolites, BE and EME, were determined by liquid chromatography-tandem mass spectrometry in plasma collected up to 3 h after a subcutaneous cocaine injection; locomotor activity was measured in the same animals using automated activity monitors.…”