Extended access self-administration of methamphetamine is associated with age- and sex-dependent differences in drug taking behavior and recognition memory in rats
Introduction
Most drug use begins during adolescence and those who initiate their use earlier in life appear to experience worse outcomes compared to those who start late in adolescence or during adulthood. For example, earlier onset of cocaine and other drug use has been associated with greater deficits in cognition in a battery of neuropsychological tests, a higher risk for psychosocial problems, and an increased risk for substance use disorder (SUD; [[1], [2], [3]]). Female users also tend to experience worse outcomes, including a more rapid transition from initial to problematic drug use [[4], [5], [6]]. Use of amphetamines, and especially the methylated derivative, methamphetamine (METH), may be particularly problematic for these populations. Compared to males, females tend to initiate METH use earlier, are more sensitive to its acute behavioral and subjective effects, are more likely to have psychiatric problems associated with their drug use, and have worse treatment outcomes [[7], [8], [9], [10]]. In laboratory rats, females [11] and those beginning drug use during adolescence [12] develop compulsive-like METH seeking more readily as evidenced by greater and more rapid escalation of METH intake during extended access self-administration sessions. Together, these studies suggest that age-of-onset and sex may be factors that confer vulnerability to adverse outcomes of METH use, including a greater likelihood to develop compulsive METH-taking behavior and a heightened susceptibility to METH-induced cognitive dysfunction.
One hypothesized explanation for this heightened vulnerability is that drug use early in life may induce delays or other significant perturbations in normal brain development. METH, like nearly all other drugs of abuse, has potent effects in corticolimbic brain regions, such as the nucleus accumbens and prefrontal cortex (PFC). These regions continue to reorganize and mature throughout late childhood and adolescence, with the PFC not reaching its mature, adult-like state until individuals are in their mid- to late twenties [13]. Moreover, this continued development is protracted compared to subcortical regions such as the nucleus accumbens [14]. Studies using rodent models of adolescence have revealed a developmental shift in dopamine and glutamate signaling that occurs during adolescence. Specifically, dopamine D1 receptor (D1R) expression on PFC projections to the accumbens peaks during adolescence before pruning and relative decreases in expression occur as rats reach adulthood [15]. This D1R remodeling may precede the late adolescent emergence of GluN2B-containing NMDA receptor transmission in the PFC, which has been shown to be mediated by D1R signaling [16]. These ontogenetic changes are likely important mechanisms for developing adult-like cognition. In adults, intact D1R and NMDAR transmission in the medial prefrontal cortex (mPFC) are needed for certain forms of recognition memory. Pharmacological blockade of D1Rs in the mPFC impaired object-in-place (OiP) recognition memory, while sparing both novel object (NOR) and object location recognition memory [17]. Non-selective NMDAR blockade in the mPFC impairs OiP [18]. Although the role of different NMDAR subunits in OiP memory is not entirely clear, GluN2B function in the PFC has been implicated in working memory [19]. Thus, drugs of abuse taken during adolescence may disrupt the ontogeny of D1R and/or GluN2B signaling in the developing PFC leading to greater deficits in OiP memory compared to adult-onset drug use, while sparing NOR memory.
Exposure to amphetamines during adolescence has been shown to influence the development of certain aspects of dopamine and glutamate signaling, and in turn cognitive functioning, though most of the published work to date has investigated age-of-onset or sex separately. In male rodents exposed to amphetamine non-contingently during adolescence, presynaptic sites on dopamine fiber inputs into the PFC are significantly reduced [20,21], and dopamine-mediated inhibition of pyramidal cells in the PFC is significantly impaired at four- and twelve weeks after the last drug injection [22,23]. We have previously reported that these drug-induced neuroadaptations in the dopamine system are region specific, with reduced expression of D1Rs in the mPFC but no change in the NA after adolescent AMPH exposure [24]. Moreover, the drug-induced changes in pyramidal cell function are associated with significant disruptions in cognitive function, including impaired working memory [25], reduced impulse control [26] and reductions in behavioral flexibility [27]. A more recent study that examined the potential for age of exposure-dependent effects of METH on conditioned fear learning and extinction used only male rats and found adult-exposed animals to have deficits in extinction retrieval that were not apparent in their adolescent-exposed counterparts [28]. The impact of adolescent amphetamine exposure on glutamate signaling has not been published to date, but a recent study demonstrated reduced expression of phosphorylated GluN2B in the infralimbic PFC after adolescent cocaine exposure in male rats [29]. Notably, most of the aforementioned studies employed non-contingent experimenter administered injections, and it is currently unknown whether psychostimulant-induced reductions in PFC D1Rs and GluN2B expression occur with contingent drug-taking during adolescence. Moreover, since most of these studies assessed adolescent drug exposure without including an adult-exposed comparison group, it is unclear whether observed adaptations are due to disruptions in the developmentally regulated processes specific to adolescence, or if they occur regardless of age of drug exposure.
The current study sought to address these gaps by using a methamphetamine (METH) self-administration paradigm to investigate the hypothesis that adolescent-onset METH-taking would disrupt the ontogenetic trajectories of D1R and GluN2B in the PFC in a sex-dependent fashion. To this end, we trained Sprague-Dawley rats of both sexes to self-administer METH or a non-drug reinforcer, saccharin, under short access (ShA) conditions beginning during adolescence or adulthood, followed by an extended period of long access (LgA) METH self-administration. One and two weeks following cessation of self-administration, rats were tested on object recognition memory tasks to assess METH-induced memory impairments. Seven days later, tissue was collected to assess NMDAR subunits and D1R protein expression in the PFC and NA. In line with previous work [11,12], we hypothesized that females and adolescent-onset rats would escalate their METH intake more rapidly during LgA compared to their male and adult-onset counterparts. Importantly, if METH-taking during adolescence indeed disrupted the ontogeny of D1R and GluN2B function in the PFC, we expected that adolescent-onset rats would experience greater deficits in PFC-dependent recognition memory and display greater reductions in D1R and GluN2B protein expression in the PFC, with no changes in the NA, as we found previously [24]. We further predicted that these METH-induced neuroadaptations may be more pronounced in females. Finally, we hypothesized that the effects would be specific to METH as a reinforcer, such that these patterns would not be evident in rats that self-administered the non-drug reinforcer, saccharin.
Section snippets
Subjects
Subjects were a total of 81 male and 84 female Sprague-Dawley rats that were born in-house on postnatal day (P) 1 from breeders originally obtained from Envigo (Indianapolis, IN, USA). Several rats were lost from the study due to issues with catheter patency (adolescent: males = 5, females = 3; adult: males = 3, females = 1), illness (adolescent: males = 3, females = 5), or other technical problems (adolescent females n = 2; adult: male n = 1; females = 1), yielding final subject totals of 69
Body weights
Separate two-way ANOVAs by sex and age-of-onset revealed significant main effects of postnatal day [Adolescent-onset: females F(49,1262) = 90.08, p < 0.0001; Adult-onset: males F(45,1211) = 9.12, p < 0.0001, females F(45,1435) = 5.27, p < 0.0001], which was due to the expected weight gain as Sprague-Dawley rats age under ad libitum access to food (Fig. 2). In adolescent-onset males, two-way ANOVA revealed a significant reinforcer by postnatal day interaction [F(49,1167) = 1.58, p = 0.0076,
Discussion
Adolescence is characterized by considerable development of D1R and GluN2B neurotransmission in the PFC [15,16,43], which may constitute a window of vulnerability to drug-induced neuroadaptations [44,45]. This vulnerability may partly explain why adolescent-onset drug users suffer worse outcomes of their drug use compared to adult-onset users. We tested this hypothesis in the current study by investigating PFC-dependent cognition and receptor expression in rats with a history of METH or
Funding
This work was supported by funding from the National Institutes of Health (DA 029815), the University of Illinois Campus Research Board, and an National Science Foundation Research Experiences for Undergraduates award (NSF REU award) (1559908/1559929).
Declaration of Competing Interest
All authors report they have no conflicts of interest.
Acknowledgements
The authors thank Jessica Alvarez (supported by an NSF/REU award), Erika Carlson, Qingrou (JoJo) Gu, Kate Hamblen, Kristen Hughes, Adrianna Jelen, Shawn Kurian, Karen Lai, Jacob O’Russa, Sarah Rahman, Brittany Rhed, Tugba Serbest, and Ashley Wehrheim for excellent technical assistance. Experimental data and representative band images for all groups can be found on our project portal on the Open Science Framework (https://osf.io/bv7yk/).
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