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Schizophrenia is a complex psychiatric disorder, affecting about 1 % of the world population. Recently, functional imaging studies demonstrated an increase in the density and occupancy of dopamine (DA) D2 receptors (D2R) in the striatum of untreated schizophrenia patients. This D2R overexpression is genetically determined in a subpopulation of the patients. Mice, which reversibly overexpress D2Rs selectively in striatal medium spiny neurons (D2R OE mice) exhibit deficits in working memory, behavioural flexibility and incentive motivation, and thus endophenotypes which resemble cognitive and negative symptoms of schizophrenia. Cognitive and motivational processes are modulated by DA release in the prefrontal cortex (PFC) and nucleus accumbens (NAc), respectively. Interestingly, the behavioural phenotypes of D2R OE mice are accompanied by an increased ex vivo DA tissue content and enhanced DA receptor sensitivity in PFC, indicating a dysfunction of the DA midbrain system in these mice.
In this study, the D2R OE schizophrenia model was used to investigate the influence of an increased density of striatal postsynaptic D2Rs on the activity of DA midbrain neurons, which modulates DA release in PFC and NAc. For this purpose, the in vivo activity of PFC and NAc projecting DA neurons in the ventral tegmental area (VTA) was recorded. In addition, DA neurons in the substantia nigra (SN), which modulate voluntary movement by DA release in the dorsal striatum, were examined. In vivo single-unit recordings of midbrain neurons in anaesthetized D2R OE mice and control littermates were combined with juxtacellular single-cell labelling and immunohistochemical procedures to identify and localize DA neurons unequivocally. DA VTA neurons from D2R OE mice displayed significantly reduced mean firing rates and fired less spikes in phasic bursts. In contrast, the electrophysiological properties of DA SN neurons were unchanged. Switching off transgenic D2R overexpression in adulthood for five weeks normalized mean firing rates in DA VTA neurons from D2R OEs to control levels. However, the reduction of phasic burst activity persisted irreversibly.
To investigate whether modifications of intrinsic cellular properties might be responsible for the observed in vivo activity changes of DA VTA neurons in D2R OE mice, projection-specific electrophysiological in vitro analyses of DA subpopulations were performed. Therefore, in vivo retrograde tracings were combined with in vitro patch clamp recordings in acute brain slices and subsequent immunohistochemical staining to confirm DA identity. Mesocortical and mesolimbic DA VTA neurons from D2R OEs displayed higher in vitro spontaneous firing rates compared to controls. This increased intrinsic excitability persisted after switching off transgenic overexpression in adulthood. Similar to the in vivo observations, the spontaneous in vitro activity of DA SN neurons did not differ between D2R OE and control mice. A detailed analysis of voltage-gated ion channels responsible for intrinsic membrane oscillations, which drive spontaneous DA neuron firing, did not reveal any differences between DA VTA neurons from D2R OEs and controls. However, spontaneous firing frequencies of mesolimbic DA VTA neurons were normalized to control levels in complete synaptic isolation by pharmacological blockade of NMDA receptors. Further, evoked NMDA receptor currents in mesolimbic DA VTA neurons from D2R OEs had slowed decay kinetics, which persisted after switching of transgenic overexpression. Bath application of the selective NR2B antagonist ifenprodil restored the slowed decay kinetics in neurons from D2R OE mice to control levels without affecting NMDA receptor currents in control animals. Indeed, NR2B containing NMDA receptors proved to be responsible for the increased in vitro pacemaker rates of mesolimbic DA VTA neurons from D2R OE mice, since the spontaneous activity was normalized to control levels by ifenprodil.
Incentive motivation critically depends on tonic DA release in NAc. Thus, the transient decrease of DA VTA neuron mean firing rates in vivo during acute D2R overexpression might be correlated with the reversible motivational deficit in D2R OE mice. This effect is likely to be mediated by an altered afferent drive onto DA VTA neurons, since mesolimbic projecting DA neurons displayed identical in vitro pacemaker frequencies in complete synaptic isolation after NMDA receptor blockade. On the other hand, the observed persistent reduction in burst activity of DA VTA neurons might be responsible for the irreversible cognitive endophenotypes in D2R OE mice, since working memory performance relies on phasic DA release in PFC. Synaptic NMDA receptors are key players for cellular burst induction in DA VTA neurons. The atypical NMDA receptors with NR2B subunits in DA VTA neurons from D2R OE mice might indicate a developmental disorder of synaptic function, which could cause the irreversibly reduced burst activity. Further experiments aiming at an explicit definition of the molecular composition of NMDA receptors in DA VTA neurons from D2R OE mice could lead to the identification of new pharmacological targets for largely therapy resistant cognitive and negative symptoms in schizophrenia.