Schizophrenia is a chronic, devastating, psychiatric disorder characterized attributed to abnormalities in dopamine and glutamate signaling. Excitatory circuits control the activity of dopamine neurons, and it is thought that abnormalities in these circuits produce the positive, negative and cognitive features of schizophrenia. Glutamate homeostasis refers to the control of glutamate levels in and around excitatory synapses, and we now have evidence that glutamate homeostasis might be important in controlling the circuits involved in schizophrenia. Glutamate transporters control brain glutamate homeostasis, and the major glutamate transporter in the brain is GLT-1, primarily expressed in astrocytes. Others and we have found that GLT-1 is also expressed in excitatory presynaptic terminals. To understand the function of GLT-1 expressed in neurons, we generated a conditional GLT-1 knockout mouse in which we have used synapsin-cre to accomplish the selective inactivation of GLT-1 in neurons. We have performed extensive behavioral phenotyping of this mouse, including testing responses to amphetamine, which are highly modulated by excitatory signaling and therefore likely, we thought, to be affected by glutamate dyshomeostasis. Previous work by many groups has demonstrated the phenomenon of sensitization to amphetamine, in which behavioral or neural (i.e. dopamine release) effects increase with repeated administration. Amphetamine sensitization is thought to model the cellular processes that underlie the positive symptoms of schizophrenia. Remarkably, we found that inactivation of GLT-1 in neurons produced significant decrease in the acute and sensitized locomotor responses to amphetamine. In addition, we have found improved performance of the nGLT-1 KO in novel object recognition and light-dark emergence. Defects on NOR and LDE may reflect impaired working memory and increased anxiety, components of the cognitive and negative domain of symptoms of schizophrenia. These observations have led us to hypothesize that the nGLT-1 KO may demonstrate resilience to the biochemical and circuit disturbances associated with schizophrenia. We hypothesize further that the phenotype that we observe in the nGLT-1 KO partially stems from changes in ambient glutamate within regions of the brain dependent upon neuronal GLT-1 for glutamate homeostasis. We propose to characterize these phenotypes further to establish whether GLT-1 may be valid target for therapeutic drug discovery. Toward that end, we will: 1) characterize the behavioral phenotype of the nGLT-1 KO mouse subjected to subchronic PCP administration to model symptom domains observed in schizophrenia; 2) characterize the biochemical phenotype of nGLT-1 KO mice; 3) determine the effect of neuronal knockout of GLT-1 on glutamate homeostasis in the nucleus reticularis slice preparation.
Abnormal glutamate mediated synaptic transmission is an important feature of schizophrenia suggesting that abnormal glutamate clearance might be important in the pathogenesis of this disorder. We found that in a conditional knockout of the glutamate transporter GLT-1, in which GLT-1 is deleted in CNS neurons, amphetamine produces significantly decreased locomotor response compared to littermate controls. Understanding the molecular mechanisms underlying this phenotype may help us to understand the role of glutamate homeostasis in the pathophysiology of schizophrenia.
