Paul Kramer



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G protein-coupled receptors (GPCRs) modulate the activity of neurons in many

ways on both short and long timescales. These receptors initiate intracellular

signaling cascades that can activate ion channels, suppress neurotransmitter

release, mobilize calcium release from stores, promote gene transcription,

increase or decrease adenylyl cyclase activity, and perform many other functions.

Therefore the comprehensive result of activating a single GPCR is not

straightforward, and will depend on the context in which that receptor was


Dopamine neurons synthesize and release dopamine throughout the

brain, and contribute to behaviors like movement, learning, and reward; as well

as many diseases of the central nervous system such as addiction and

Parkinson’s Disease. These neurons express a wide variety of diverse GPCRs

that couple to different intracellular signaling networks. Group I metabotropic

glutamate receptors, expressed by dopamine neurons, couple to Gq and mediate

acute inhibition through activation of the calcium-sensitive potassium channel SK,

though they also activate a non-selective cation conductance. These receptors

modulate the development of cellular changes induced by systemic in vivo

cocaine exposure, but it is not known if or how the receptors are modulated by

that same treatment. Moreover, other GPCRs on dopamine neurons, such as

GABAB receptors, have altered signaling properties following cocaine

administration, but the upstream mediators of these changes are not well


The goal of the work presented here was to gain a better understanding of

how the intracellular signaling cascade generated by group I metabotropic

glutamate receptors functions physiologically and pathologically within dopamine

neurons. Physiologically, the interaction between the mGluR signaling pathway

and Gi-coupled GPCRs was tested. Pathologically, the effect of a single injection

of cocaine on the signaling of mGluRs on dopamine neurons was tested. For the

course of this dissertation, electrophysiological recordings were made from

dopamine neurons in acutely prepared brain slices. Through the use of

pharmacological tools such as specific agonists, antagonists, and multiple caged

compounds, data were gathered on isolated GPCR signaling. These data

included coupling to ion channels, functional intracellular interactions, and

activation of relevant downstream signaling molecules.

Physiologically, activation of Group I metabotropic receptors produced an

unexpected rapid, and reversible, inhibition of the inwardly rectifying potassium

current mediated by GABAB receptors. This inhibition of GIRK was mediated by

calcium release from stores. Pathologically, a single injection of cocaine reduced

mGlu1 receptor signaling in dopamine neurons. The reduction in mGluR1

signaling was prevented by injecting the animal with an mGlu5 receptor negative

allosteric modulator, MPEP, prior to cocaine.

The investigation of the signaling cascades generated by metabotropic

glutamate receptor activation in dopamine neurons has furthered our

understanding of how these receptors affect the balance of neuronal inhibition

and excitation. Not only do mGluRs directly activate both excitatory and inhibitory

channels, they also mediate a secondary disinhibition of the neuron by transiently

inhibiting GIRK. These results offer new insights for predicting how mGluR

activation will alter dopamine neuron output within the context of synaptic

transmission and modulation by cocaine.




School of Medicine



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