(T1230-04-26) Mice Deficient in Interleukin 6 (IL-6) Show Improved Short-Term versus Long Term Memory following Methamphetamine Treatment

Purpose: Methamphetamine use disorder (MUD) develops progressively over time from a mere recreational use to a full-blown compulsive drug-seeking behavior, during which stage there are changes in the arousal, reward, and cognition functional systems of the brain [1]. Recreational drug use causes cortical arousal to evoke anxiety and pleasurable feelings [2]. The dopamine reward system is then activated to “reward the behavior” (feeling of a high) or “punish abstinence” (negative emotions of withdrawal) [3]. Progressively over time, this becomes a learned behavior and sets up the vicious cycle that ultimately leads to MUD. Similar to other drug addictions, this is a chronic and debilitating mental disorder characterized by a compulsive relapsing drug seeking and use behavior despite the severe adverse effects associated with the substance use [4]. In the central nervous system, methamphetamine (METH) induces the release of excitatory neurotransmitters (dopamine, serotonin and norepinephrine) and their accumulation in the synaptic cleft by inhibiting the degradation and/or reuptake for sustained effect [5]. METH has a high potential for abuse and dependence (psychological and physical) and hence regulated under the Controlled Substances Act as a Schedule II drug. There is a restricted medical use of METH hydrochloride (Desosyn®) available as oral tablets for the treatment of attention deficit hyperactivity disorder (ADHD) and refractory obesity [6]. METH use remains a major public health concern in the US. The 2021 National Survey on Drug Uses and Health reported that 0.9 % of the population aged 12 years and older had used METH in the preceding year. Between 2012 and 2018, there was a 5-fold increase in METH-associated deaths in the US [7]. MUD may result in neuropsychiatric and neurocognitive disorders as a result of molecular mechanisms that involve oxidative stress processes. The early stage of addiction is driven by the dopamine reward system, but progression to MUD is largely a memory-related process whereby the urge to use the substance is driven by audio-visual cues that have been associated with past drug use [8]. Current pharmacotherapies that are based on known pathophysiological mechanisms do not fully mitigate the condition. This suggests that there may be less known or explored pathways that underlie MUD. Interleukin 6 (IL-6) is a pleiotropic cytokine that mediates both pro- and anti-inflammatory processes and has been implicated in memory processes. A null mutation of IL-6 (IL6KO) mice has been shown to attenuate METH-induced neurotoxicity [9]. In this present study, we investigated the mediator role (IL-6) plays in both short- and long-term memory effects of METH use.

Methods: Male and Female two-months-old treatment-naïve IL6KO mice and their C57BL/6J wild type background control (n=5) were used for the behavioral study. Mice received a low dose intraperitoneal injection of METH at 1 mg/kg every other day for 2 weeks prior to the start of behavioral assessments. Mice then underwent behavioral assessments which comprised of rotor-rod, open field test, light-dark box, social interaction test, Y maze and Morris water maze. To obviate the acute effects of METH, injections were done in the late afternoons prior to behavioral testing in the mornings. The dose frequency ensured reduced infections and injection site pathologies. Treatment regimen was maintained throughout the experimental period.

Results: The dose of METH used did not induce anxiolytic effects and did not affect ambulatory distance travelled in the IL6KO or WT mice the light-bark box or open field test respectively (Fig 1). IL6KO mice showed impairment in motor coordination with a shorter latency to fall off the rotor rod. METH-treated IL6KO mice showed higher spontaneous alternations in the Y-Maze test indicating an improvement in the short-term memory. In the social interaction test, METH treatment resulted in improved social affiliation and social memory in the IL-6 deficient mice. The IL6KO mice exhibited better social for novelty by their preference for the unfamiliar mouse compared to the previously introduced stranger mouse (Fig 2). There was, however, a contrast observed with long-term memory assessment. Following 5 days of training, IL6KO mice showed higher latency to escape, and less time spent in the target zone during the probe trial (Fig 3). This indicated an impaired spatial memory effect.

Conclusion: IL6KO mice exhibited better short-term memory and compared to long-term memory when treated with METH in our study. This indicated that IL-6 has mediatory roles in the formation and consolidation of the long-term memory associated with the compulsive drug use in MUD. Further studies are warranted to understand this disparate role of IL-6 to help develop improved therapeutics for MUD.

References: 1. Everitt, B.J. and T.W. Robbins, Neural systems of reinforcement for drug addiction: from actions to habits to compulsion. Nature neuroscience, 2005. 8(11): p. 1481-1489.
2. Vetulani, J., Drug addiction. Part II. Neurobiology of addiction. Polish journal of pharmacology, 2001. 53(4): p. 303-318.
3. Koob, G.F., Drug addiction: hyperkatifeia/negative reinforcement as a framework for medications development. Pharmacological Reviews, 2021. 73(1): p. 163-201.
4. Nestler, E.J. and C. Lüscher, The molecular basis of drug addiction: linking epigenetic to synaptic and circuit mechanisms. Neuron, 2019. 102(1): p. 48-59.
5. Ahuja, N., et al., Online narratives of methamphetamine use and risky sexual behavior: can Shame-Free guilt aid in recovery? Archives of Sexual Behavior, 2021. 50(1): p. 323-332.
6. Abbruscato, T.J. and P.C. Trippier, DARK classics in chemical neuroscience: methamphetamine. ACS chemical neuroscience, 2018. 9(10): p. 2373-2378.
7. Han, B.H. and J.J. Palamar, Multimorbidity Among US Adults Who Use Methamphetamine, 2015–2019. Journal of General Internal Medicine, 2021: p. 1-3.
8. Shukla, M. and B. Vincent, Methamphetamine abuse disturbs the dopaminergic system to impair hippocampal-based learning and memory: An overview of animal and human investigations. Neuroscience & Biobehavioral Reviews, 2021. 131: p. 541-559.
9. Ladenheim, B., et al., Methamphetamine-induced neurotoxicity is attenuated in transgenic mice with a null mutation for interleukin-6. Molecular pharmacology, 2000. 58(6): p. 1247-1256.

Acknowledgements: This research was funded in part by by the UMKC School of Graduate Studies Research Grant awarded to Leslie Essel.

Figure represents total distance traveled expressed as Mean ± SEM by mice treated with PBS (A) and METH (B) in the open field. No statistically significant difference was seen in the METH treated groups with one-way ANOVA followed by Tukey post hoc test. *p≤0.05

Figure 2 shows number of entries (A) and time spent (B) between stranger 1 and 2 mice by the the test mouse. Data was analyzed by two-way ANOVA followed by Tukey post hoc test. *p≤0.05

Figure 3 shows latency to escape during the 5-day training in PBS (A) and METH (B) treated mice in the Morris Water Maze. C and D show results obtained in the probe trial for PBS and METH-treated mice respectively. Data was analyzed by two-way ANOVA followed by Tukey post hoc test. *p≤0.05