O2.1. CIRCUIT MECHANISM MEDIATES THE EFFECTS OF SUB-CHRONIC KETAMINE ON STRIATAL DOPAMINE SYNTHESIS CAPACITY AND LOCOMOTOR ACTIVITY: A COMBINED CHEMOGENETICS/PET STUDY

Abstract

Background: Ketamine, an N-methyl-D-aspartate (NMDA) glutamate receptor antagonist, induces psychotic-like effects in healthy human and exacerbates symptoms in patients with schizophrenia (Krystal et al., 1994, Lahti et al., 1991). Therefore, ketamine could be used to simulate the neurochemical alterations seen in patients in the mouse to understand the underlying circuitry. Here, we combined chemogenetics with pre-clinical neuroimaging and the open field test to investigate the effect of sub-chronic ketamine on striatal dopamine synthesis capacity and locomotor activity and to probe the circuit implementation of these effects. Specifically, we aimed to investigate the role of midbrain dopamine neurons in mediating ketamine's effects on striatal dopamine synthesis capacity and locomotor activity. Methods: All procedures were conducted under licence in accordance with the UK Animals (Scientific Procedures) Act of 1986. Male mice received 30mg/kg ketamine or saline for five consecutive days. Locomotor activity was recorded using the open field test. Moreover, mice received a dynamic 3, 4-dihydroxy-6-[(18f)F]-fluoro-L-phenylalanine ([18F] -FDOPA) Positron Emission Tomography (PET) scan to assess striatal dopamine synthesis capacity in vivo. Data were analysed using an extended Patlak graphical analysis approach and the outcome measure was the uptake rate constant of [18F] -FDOPA, namely the Ki mod (min-1) which is representative of the striatal dopamine synthesis capacity. Furthermore, dopamine neurons in the ventral tegmental area and substantia nigra pars compacta in DAT:Cre mice were transduced with the Cre-dependent hM4Di-mCherry designer receptors exclusively activated by designer drug (DREADDs) vectors. Two weeks following the stereotaxic delivery of the DREADDs, mice received clozapine N-oxide (CNO) or saline before the administration of ketamine to investigate the effect of inhibiting dopamine neuron firing on striatal dopamine synthesis capacity and on locomotor activity in the sub-chronic ketamine model. Data were analysed using two-way repeated-measures ANOVA followed by Bonferroni post hoc tests. P<0.05 was considered statistically significant. Results: Sub-chronic ketamine administration significantly increased striatal dopamine synthesis capacity (p<0.05) and induced locomotor sensitization (p<0.05). Standard immunohistochemistry showed successful transduction of the hM4Di DREADDs in dopamine neurons in the midbrain in DAT:Cre mice with over 98% specificity respectively. Inhibiting midbrain dopamine neurons prevented the ketamine-induced increase in dopamine synthesis capacity and locomotor sensitization (p<0.05). Discussion: Our data show that sub-chronic ketamine administration significantly increases striatal dopamine synthesis capacity and induces locomotor sensitization in the mouse. Moreover, our data show that the effects of sub-chronic ketamine on dopamine synthesis capacity and on locomotor activity are prevented by the inhibition of midbrain dopamine neurons. Our results are consistent with the hypothesis that increased striatal dopamine function is mediated via NMDA receptor hypofunction on GABAergic interneurons, disinhibiting glutamatergic projections to the midbrain consequently increasing midbrain dopamine neuron activity and striatal dopamine synthesis capacity.