Chronic methamphetamine exposure exerts few effects on the iTat mouse model of HIV, but blocks Tat expression-induced slowed reward retrieval

Abstract

Human immunodeficiency virus (HIV) continues to infect millions worldwide, negatively impacting neurobehavioral function. Further understanding of the combined effects of HIV and methamphetamine use is crucial, as methamphetamine use is prevalent in people with HIV. The HIV-associated protein Tat may contribute to cognitive dysfunction, modeled preclinically in mice using doxycycline (DOX)-inducible Tat expression (iTat). Tat may exert its effects on cognitive function via disruption of the dopamine transporter, similar to the action of methamphetamine. Additionally, Tat and methamphetamine both decrease interneuron populations, including those expressing calbindin. It is important to understand the combined effects of Tat and methamphetamine in preclinical models of HIV infection. Here, we used iTat transgenic mice and a chronic binge regimen of methamphetamine exposure to determine their combined impact on reward learning and motivation. We also measured calbindin expression in behavior-relevant brain regions. Before induction with DOX, iTat mice exhibited no differences in behavior. Chronic methamphetamine exposure before Tat induction impaired initial reward learning but did not affect motivation. Furthermore, DOX-induced Tat expression did not alter behavior, but slowed latencies to retrieve rewards. This effect of Tat, however, was not observed in methamphetamine-treated mice, indicative of a potential protective effect. Finally, Tat expression was associated with an increase in calbindin-expressing cells in the VTA, while methamphetamine exposure did not alter calbindin numbers. These findings may indicate a protective role of methamphetamine in HIV neuropathology, which in turn may help in our understanding of why people with HIV use methamphetamine at disproportionately higher rates.

Keywords: ITAT HIV model; Methamphetamine; Motivation; Mouse; Operant; Reinforcement.

Fig. 1.

Timeline of methamphetamine (METH) exposure and Tat expression. Prior to methamphetamine exposure, mice were trained to nose poke in illuminated holes in an operant chamber (FR1) and were subsequently tested in the Probabilistic Learning Task (PLT) and Progressive Ratio Breakpoint Task (PRBT). During the methamphetamine exposure period, mice were administered drug or saline control four times per day (10:00, 12:00, 14:00, and 16:00). Initial methamphetamine administration was low (1 mg/mL) and steadily increased to a higher dose (6 mg/mL) over four days. During subsequent exposure days, the 10:00 dose was set at 3 mg/mL on the first day and increased to 6 mg/mL by the third day. Mice were tested on FR1 during the third day of each exposure regimen to maintain responding. Each of the four exposure blocks was separated by a 3-day period where mice did not receive injections and were provided additional FR1 training to maintain responding. 24 days after the initial methamphetamine exposure, mice were treated with 100 mg/kg doxycycline for 7 days and retested on the PLT and the PRBT. After the 7-day doxycycline exposure, mice were tested for the final time on the PLT and PRBT, after which point mice were sacrificed and their brains collected for molecular analysis (9 days after last methamphetamine exposure).

Fig. 2.

Mice possessing the Tat promotor transgene perform no different from their wildtype (Tat-) littermate mice. Both genotypes performed at chance during baseline PLT (A). Both groups of mice exhibited similar latencies to make a correct response (B) and retrieve rewards (C). In the PRBT, both Tat- and Tat+ mice had similar breakpoints (D) and displayed similar reaction times (E) and reward latencies (F). Data presented as mean + SEM.

Fig. 3.

Methamphetamine (METH) exposure reduces responding in FR1 maintenance sessions. Across genotypes, mice in both drug groups performed similarly in the absence of METH (A). METH exposure, however, decreased responding across all maintenance sessions, with the strongest effect seen in sessions closest to METH treatment (B). Data presented as mean ± SEM. * denotes p < 0.05 vs. saline within each testing session.

Fig. 4.

Chronic methamphetamine (METH) exposure does not affect overall reward learning or motivation. In the PLT, neither genotype nor methamphetamine exposure affected overall learning performance (despite higher scores in saline-treated mice; A), latency to make correct responses (B), or latency to retrieve rewards (C). Additionally, genotype and methamphetamine did not affect breakpoint (D), reaction time (E), or reward latency (F) in the PRBT. Data presented as mean ± SEM.

Fig. 5.

Tat induction from doxycycline (DOX), impairs a secondary measure of motivation, rescued by previous chronic methamphetamine (METH) exposure. Genotype and methamphetamine exposure did not affect accuracy (A) or target latency (B) in the PLT. Tat expression did slow latency to retrieve rewards however (C), which was rescued by previous METH exposure. Additionally, neither genotype nor drug exposure affected breakpoint (D), reaction time (E), or reward latency (F) in the PRBT. Data presented as mean ± SEM. * = p < 0.05 vs. Saline; p < 0.05. # = main effect of genotype; p < 0.05. $ = main effect of drug; p < 0.05.

Fig. 6.

Limited probabilistic learning impaired by methamphetamine treatment. %Correct performance of mice across trial blocks during baseline (A), methamphetamine (METH) treatment (B), and after METH and Tat induction via doxycycline (DOX; C). These data reveal that mice exhibit some within-session learning of the probabilistic learning task that is stronger with repeated testing. Importantly, both during- and post-METH treatment revealed that initial performance was worse in METH-treated mice but that improved to saline-treated levels over trials blocks. Data presented as mean ± S.E.M. * = p < 0.05 vs. methamphetamine-treated mice. & = p < 0.05 vs. trialblock 1–10 in methamphetamine-treated mice. @ = p < 0.05 vs. trialblock 1–10 in Tat+ mice. $ = p < 0.05 vs. trialblock 1–10 in Tat- mice irrespective of treatment. # = p < 0.1 vs. trialblock 1–10 in Tat- mice irrespective of treatment.

Fig. 7.

Tat expression increases calbindin expression in the ventral tegmental area. Neither genotype nor drug exposure altered calbindin expression in the prefrontal cortex (A), hippocampus (B), or nucleus accumbens (C). In the ventral tegmental area, Tat expression increased calbindin expression, not otherwise influenced by prior methamphetamine (METH) exposure (D). Data presented as individual data points as well as mean ± S.E.M., * denotes p < 0.05 in Tat+ vs. Tat- mice after doxycycline treatment to both groups.