Project Summary/Abstract
The peptide neurotensin (NTS) is known to be a potent modulator of dopamine neuron activity, and NTS
signaling has been linked to various modalities of behavioral reinforcement and reward, as well as to the
behavioral response to drugs of abuse and the motivation for drug taking. Pharmacological studies have found
that in the ventral tegmental area (VTA), NTS binding to its receptors depolarizes dopamine neurons through a
variety of second messenger cascades, resulting in increased dopamine neuron firing and downstream
dopamine release. However, much less is known about the physiological release of NTS from endogenous
brain circuits and what role these circuits may play in regulating motivated behavior. Using retrograde mapping
we identified 23 brain regions that send NTS input to the VTA; however, few of these inputs are well-studied for
their role in NTS signaling, and little is known about what stimuli or behavioral actions may activate NTS
neurons. Furthermore, though peptidergic neurons typically co-release a fast neurotransmitter, such as
glutamate or GABA, current methods for circuit-specific activation fail to distinguish between downstream
effects triggered by peptides versus those evoked by fast transmitters. We hypothesize that different NTS
projections to the VTA may play distinct roles in modulating motivated behavior, and that NTS acts
synergistically with co-released fast transmitters to govern the dynamics of downstream dopamine neuron
activation. Here we propose to determine the neurotransmitter co-expression of select NTS inputs to the VTA
and map their synaptic connectivity to dopamine and non-dopamine neurons. We also propose to use viral
CRISPR gene mutagenesis techniques in combination with optogenetics to isolate the peptide and fast
transmitter components of NTS inputs to the VTA. We will dissect the role of these components in regulating
different modalities of behavioral reinforcement and will measure the response of dopamine neurons in vivo to
stimulation of specific inputs. Finally, we will use fiber photometry to measure the activity profiles of NTS
neurons during learning and performance of a cued reinstatement food reward task. Together, these
experiments will add unprecedented circuit-specific precision to our understanding of how these critical
peptidergic inputs influence the activity state of dopamine neurons and govern reward and reinforcement
learning.