Neuronal regulation of glutamate homeostasis
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. We have evidence that glutamate homeostasis, the control of glutamate levels in and around excitatory synapses, might be important in controlling the circuits involved in schizophrenia. Glutamate transporters control brain glutamate homeostasis. GLT-1, the major glutamate transporter in the brain, is primarily expressed in astrocytes and 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 used synapsin-cre to accomplish the selective inactivation of GLT-1 in neurons. We 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. Impairment of working memory is a hallmark of the cognitive symptoms of schizophrenia. We have found improved performance in the nGLT-1 KO compared to littermate controls in the novel object recognition task, which is commonly used to characterize working memory in rodents. These observations have led us to propose that the nGLT-1 KO may demonstrate resilience to the biochemical and circuit disturbances associated with schizophrenia. We hypothesize 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 impact of neuronal knockout of GLT-1 on expression of interneuron biomarkers; 3) determine the effect of neuronal knockout of GLT-1 on glutamate homeostasis in the nucleus reticularis slice preparation.
GLT1 is expressed in excitatory terminals in regions outside the hippocampus. The expression of GLT1 in excitatory terminals has important implications for our understanding of the physiology of excitatory synaptic transmission, synaptic plasticity, and dopamine signaling. Using mouse lines that will be produced for this project, we hope to gain important insights into the role of neuronal expression of GLT1 into the normal and abnormal regulation of glutamate at central synapses.