Rewarding stimuli and addictive drugs both enhance the activity of dopamine neurons in the ventral tegmental area (VTA) of the brain. Drugs of abuse, such as cocaine and amphetamine, can also produce long-lasting strengthening of synaptic communication between glutamate-secreting neurons and their dopamine neuron targets in the VTA. Such plasticity may play a role in the behavioral process of addiction.
Now, in a recent issue of PLoS ONE (1), Ya-Chun Huang and Neal Hessler at the RIKEN Brain Science Institute in Wako have reported that social courtship behavior in male zebra finches increases synaptic plasticity in VTA dopamine neurons in a similar fashion as previous studies have reported for drugs of abuse.
The researchers examined three groups of male birds: those who sang without a female present (‘undirected singing’), those who sang with a female present (‘directed singing’), and those who did not sing with a female present. An hour later, they prepared brain slices from these birds and measured glutamate-induced synaptic activity in both dopaminergic and non-dopaminergic VTA neurons.
Male birds that exhibited undirected singing did not show any augmentation of synaptic activity in their VTA. However, the researchers found that both groups of male birds that had a female present—whether they sang or not—demonstrated synaptic potentiation in dopaminergic, but not in non-dopaminergic, neurons in the VTA.
Huang and Hessler determined that the strengthening of synaptic communication they observed after exposure to females was not due to an increase in glutamate release onto the dopaminergic neurons. Rather, the increase in dopamine neuron synaptic plasticity in the VTA reflected changes within the dopaminergic neurons themselves. Further experiments are needed to determine the exact molecular mechanism by which social interactions with females render VTA dopamine neurons in male birds more responsive to glutamate.
The similarities in synaptic plasticity in the VTA in response to both drugs of abuse and to social interactions with females indicate that courtship behavior is rewarding for male zebra finches. In addition, because dopamine neuron activity is required for pair bonding in mammals, these findings suggest that the synaptic plasticity observed in VTA dopamine neurons in response to female exposure may also be involved in formation and maintenance of the monogamous relationships that characterize the zebra finch species.
“As female choice is critical during courtship, we next plan to test whether brain reward systems are also activated when females are courted by a desirable male,” says Hessler.
1. Huang, Y.-C. & Hessler, N.A. Social modulation during songbird courtship potentiates midbrain dopaminergic neurons. PLoS ONE 3, e3281 (2008).
The corresponding author for this highlight is based at the RIKEN Laboratory for Vocal Behavior Mechanisms
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