Pathological astrocyte morphology and dysregulated glutamate homeostasis in a preclinical phenotyping model of methamphetamine addiction

Methamphetamine (METH) remains one of the most abused illicit substances inAustralia with only 10-20% of METH-users developing METH-addiction. Glutamate homeostasis and astrocyte-mediated neuroinflammation may be responsible for addiction behaviours. Therefore, this study aimed to distinguish between the role of astrocyte hypertrophy and glutamate activity in the neurocircuitry of addiction-vulnerable, compared to addiction-resistant animals. The study employed an intravenous self-administration (IVSA)addiction-phenotyping paradigm. Male Sprague-Dawley rats (n = 30) were scored on four addictive behaviours, cue-induced METH-seeking, motivation to take-METH, habitual METH-seeking and resistance-to-extinction to characterise rats as either addiction-vulnerable (AVul) or addiction-resistant(ARes). AVul, ARes and control (n = 5) rats then had their brains stained for calmodulin-kinase-II-alpha (CaMKIIa), glial-fibrillary acidic-protein (GFAP), and synapsin-I. Glutamate activity, astrocyte quantity and morphology, and relationship between astrocyte-synapse connections and glutamate activity, at each level of each region investigated, were then measured. Results revealed decreased glutamate activity in the nucleus accumbens core (NAcc), central amygdala (CeA) and basolateral amygdala (BLA) in AVul rats, compared to controls. There was no difference in astrocyte proliferation in AVul or ARes rats compared to controls, except for a decrease in the CeA of AVul rats. There was a widespread increase in astrocyte hypertrophy in AVul compared to ARes and control rats, with METH-induced hypertrophy, independent of phenotype. Correlational analyses suggested changes in glutamate homeostasis throughout addiction neurocircuitry. Findings from this study suggested dysregulation of glutamate homeostasis in the rostral NAcc and rostral-central BLA in AVul rats, regions involved in cue-induced relapse in humans and animals. Likewise, widespread astrocyte hypertrophy was found to be generally more severe in AVul, compared to ARes and control, rats. Therefore, it was posited that suppressed glial-glutamate release and increased neuroinflammatory mechanisms may be a factor in METH-addiction behaviours. These findings therefore highlight the potential of glutamatergic and astrocytic targets for relapse-prevention.