Endogenous Melatonin Decreases Place Preference For Methamphetamine Through A Learning And Memory Independent Mechanism In C57BL/6 Mice
Authors: Clough Shannon J. , Kasahara Takaoki , Veros Kaliana M. , Hudson Randall L. , Dubocovich Margarita L. 
Published: November 2023

The pineal molecule melatonin is synthesized via two key enzymes (AANAT and HIOMT) (Reiter, Mol Cel Endocrinol 1991;79:C153–C158). Melatonin has been linked to an inhibition of conditioned place preference (CPP) for methamphetamine (METH), mediated through its two G protein‐coupled receptors (Clough et al., Physiol Behav 2014,132:79–86) Additionally, CPP for METH has been shown to follow a diurnal variation, with a complete abrogation of CPP occurring during the dark phase when melatonin levels are at their peak. This suggests a potential role for endogenous melatonin in modulating METH‐induced CPP. In order to further examine the role of endogenous melatonin in METH‐induced CPP we utilized a mouse line developed by Kasahara and his team. These congenic C57BL/6J mice were developed through backcrossing with MSM mice, to produce melatonin proficient (Aanat(+/+);Hiomt(+/+): AH+/+) and melatonin deficient (Aanat(−/−);Hiomt(−/−): AH−/−) mice on the same genetic background. AH−/− mice lack any significant production of melatonin, while AH+/+ mice have a constant elevated level of melatonin in the blood (2–5 fold > AH−/−) and pineal gland (10–15 fold > AH−/−). Male AH−/− and AH+/+ mice were conditioned with alternating treatments of METH (1.2mg/kg, ip) or vehicle (VEH) and tested for CPP after the last conditioning session. Testing for all experiments occured during the day (Zeitgeber Time [ZT] 6–8; 12h:12h light‐dark cycle, ZT 0 = lights on). The CPP for METH was significantly higher than VEH in both AH−/− (VEH: −100±42s, n=12; METH: 284±32s, n=16, p<0.05) and AH+/+ (VEH: 14±31s n=13; METH: 181±30s, n=16, p<0.05) mice. However, AH+/+ mice displayed a significantly lower CPP score for METH compared to the AH−/− mice (p<0.05). The CPP paradigm requires reward related learning and memory to occur, a process which relies on long‐term potentiation (Keleta & Martinez, Brain Behav 2012,2:128–141). Melatonin has been linked to an inhibition of long‐term potentiation (Wang et al., Eur J Neurosci 2005,22:2231–7). Therefore we sought to determine whether the decreased CPP observed in AH+/+ mice was due to a learning and memory deficit. To accomplish this we utilized a hippocampal dependent task (attentional set shift) and a hippocampal independent task (novel object recognition). For the attentional set shift task mice were subjected to a series of behavioral tasks to test discrimination and reversal learning. A significant difference between AH−/− and AH+/+ mice was observed in the simple discrimination task (AH−/− [n=6]: 14±3 trials vs. AH+/+ [n=9]: 8±1 trials; p<0.05). No other differences were observed between the two genotypes in terms of compound discrimination, intradimensional shift, extradimensional shift, or reversal tasks. Similarly no significant differences were observed between AH−/− and AH+/+ mice in the novel object recognition task, with both genotypes displaying strong object recognition. Taken together, these data suggest a role for endogenous melatonin in regulating reinforcing behaviors occurring independently of a learning and memory related mechanism. Therefore, suggesting melatonin is exerting its effects independently of its known inhibition of long‐term potentiation and may be acting directly within the reward pathway.Support or Funding InformationUB JSMBS Funds to MLD.