Optical imaging of prefrontal deficits induced by cocaine: neurons versus dopamine D2-receptor expressing neurons

Deficits in the medial prefrontal cortex (mPFC) function play a crucial role in promoting compulsive cocaine use. This dysfunction might result from cocaine’s effects on neurons possibly including modification of the expression of the dopamine type 2 receptors (D2r). However, studies on D2r function have been hindered by relatively low receptor expression in mPFC and low detection sensitivity. To tackle this problem, we developed a workflow to optically image D2r through enhancing visualization of GFP fluorescence in a D2r-expressing transgenic mouse model. Two groups of animals were used: group 1 was treated daily by saline (0.1cc/100g, i.p.) and group 2 received cocaine (30mg/kg, i.p.) for 2 weeks. Following 1-day withdrawal, their brains were fixed and processed. The brain sections underwent immunostaining and ex vivo imaging using a confocal fluorescence microscope to capture co-registered fluorescence labeling of both neurons (NeuN) and D2r-expressing neurons (GFP). These images were montaged to cover a large field of view of the mPFC to quantify the changes in the count of neurons and D2r-expressing neurons. Our results show that chronic cocaine exposure results in 34.7%±5.7% decrease in neurons but a 44.7%±13.2% increase of D2r expressing neurons in the mPFC compared to saline treated group. Additionally, in-vivo Ca2+ imaging of D2r-expressing neurons in the mPFC revealed cocaine’s effects on the prefrontal response to the stimulation at the VTA. These changes might underlie the hypofrontality observed in human studies and exacerbate the loss of control associated with chronic cocaine use