We study plasticity of neurotransmission and the events involved in cell death, particularly how the cortex, striatum, and dopamine system of the substantia nigra and ventral tegmental area work together to produce learning, memory, and decision making.
These circuits are involved in how habits are learned, chosen, and executed. At some level, decision making and habit execution occurs via activity in the nervous system causing some synapses to be activated and others to be depressed, and our new optical and electrochemical methods are being used to establish the rules by which this synaptic selection occurs.
These same synapses are abnormally selected during disease: rewarding and behavioral effects of psychostimulant drugs such as cocaine, amphetamine and alcoholare due to increased extraneuronal dopamine in the nucleus accumbens from the neurons of the ventral tegmental area. These neurons are also sites of action of the antipsychotic drugs administered for treatment of schizophrenia. Our studies on basic properties of neurotransmission provided several new insights in synaptic regulation in the brain. These mechanisms likely have important roles in the establishment of memories and behavior, as well as schizophrenia and drug dependence.
Our studies promise to lend insight into the etiology and treatment of Parkinson's disease, which is caused by the death of dopamine neurons of the substantia nigra. We have also adapted the use of postnatal culture systems for the study of methamphetamine-induced neurodegeneration and Huntington's disease. The modification of these synapses may underlie autistic disorders.
The quantal event is the fundamental unit of synaptic transmission. We developed the first techniques for direct presynaptic observation of quantal release in the brain, and are characterizing quantal release from ventral midbrain dopamine neurons. We are identifying mechanisms that modulate the number of molecules that can be released per synaptic vesicle exocytosis (quantal size) and the number of quanta released (quantal frequency). We have also developed the first optical methods to measure transmitter release from many individual synapses in the brain; these are in collaboration with Dalibor Sames's chemistry laboratory.
Effects of dopamine on cortical transmission
We are examining the effects of synaptically released dopamine in the striatum and its cortical innervation, which promises to elucidate why synaptic transmission associated with habit and decision making is so plastic.
We established the primary mode of action of L-DOPA, the most commonly used clinical treatment for Parkinson's Disease. The most relevant response is that this compound elevates the quantal size of dopamine release.
We established that the growth factor GDNF modulates quantal size and quantal frequency.
We find that D2 dopamine autoreceptors, primary targets of antipsychotic drugs, regulate quantal size. We are exploring how these receptors reduce quantal frequency by activating GIRK potassium channels.
An interesting outcome of these experiments, which use whole cell patch clamp and single cell RT-PCR, is that GIRK2 is the dominant form in substantia nigra, explaining its selective degeneration in the weaver mutation. We are currently studying these using D2 and D3 receptor knockout animals in collaboration with C. Schmauss (Columbia).
We find that overexpression of the dopamine vesicle transporter VMAT2 increases both quantal size and frequency, the latter by conversion of non-catecholaminergic synaptic vesicles to dopamine vesicles. We are also characterizing effects in VMAT knockouts in collaboration with R. Edwards (UCSF).
We find that prolonged depolarization profoundly potentiates dopamine release. This appears to be due to modulation of vesicular acidification and dopamine synthesis.
The vesicle fusion pore appears to share attributes with ion channels. We are studying modulation of fusion pore kinetics, which appears to regulate release during exocytosis.
To complement the effects in dissociated culture, we are examining real-time release in culture using cyclic voltammetry and whole cell patch clamp. Ongoing projects are to characterize modulation of release due to VMAT2 expression, alpha-synuclein expression, by PKA, and in D2/D3 knockout animals.
We have shown that dopamine neurons in culture release glutamate as a cotransmitter. We are currently using brain slice preparations to establish if this occurs in vivo in collaboration with S. Rayport (Columbia U.).
We introduced the contemporary model of how amphetamine initiates release of dopamine, the weak base hypothesis. We continue to study the compound's mechanism of action.
We find with A. Cuervo (Einstein U.) that mutant and modified forms of alpha-synuclein and LRRK2 that cause Parkinson's disease act to block lysosomal degradation by inhibiting chaperone mediated autophagy. This system further interacts with cytosolic dopamine to underlie specific neuronal death of some populations of central neurons.
We found that postnatally-derived substantia nigra neurons in culture can be induced by DOPA or Fe to produce neuromelanin indistinguishable from human neuromelanin by microscopy and electron paramagnetic resonance. We are collaborating with L. Zecca (U. Milan) and examining possible downstream modes of degeneration triggered by neuromelanin with L. Greene (Columbia. U.).
We suggest that elevated cytosolic DA provides a step in PD. We are examining the profound effect of VMAT2 regulation on amphetamine neurotoxicity. To establish the relationship with cytosolic dopamine, we have developed the first method to measure cytosolic dopamine, using a combined patch clamp /amperometric method in collaboration with Dr. M. Lindau (Cornell).
We are examining effects on neurodegeneration in the substantia nigra by altered expression of calbindin-D28K, VMAT2 (R. Edwards, UCSF), L-DOPA (M. Mena, U. Madrid), alpha-synuclein and LRRK2 ( William Dauer, U. Michigan), alcohol (N. Harrison, Columbia), and GDNF (R. Burke, Columbia).