Heterogeneous neuronal activity in the lateral habenula after short‐ and long‐term cocaine self‐administration in rats

Cocaine addiction is thought to be the result of drug‐induced functional changes in a neural network implicated in emotions, learning and cognitive control. Recent studies have implicated the lateral habenula (LHb) in drug‐directed behavior, especially its aversive aspects. Limited cocaine exposure has been shown to alter neuronal activity in the LHb, but the impact of long‐term drug exposure on habenula function has not been determined. Therefore, using c‐fos as a marker, we here examined neuronal activity in LHb in rats that self‐administered cocaine for either 10 or 60 days. Both the density of labeled cells and the cellular labeling intensity were measured in the lateral (LHbL) and medial (LHbM) parts of LHb. After 10 days of cocaine self‐administration, both the density and intensity of c‐fos‐positive cells were significantly increased in LHbL, but not LHbM, while after 60 days, an increased density (but not intensity) of labeled neurons in both LHbL and LHbM was observed. Most c‐fos‐labeled neurons were glutamatergic. In addition, we found increased GAD65 expression after 10 but not 60 days of cocaine self‐administration in the rostral mesencephalic tegmental nucleus. These data shed light on the complex temporal dynamics by which cocaine self‐administration alters activity in LHb circuitry, which may play an important role in the descent to compulsive drug use as a result of prolonged cocaine‐taking experience. Ten days of cocaine self‐administration resulted in elevated levels of c‐fos mRNA in lateral habenula (LHb) compared with sucrose self‐administration or control. Increased activity depended on rostral to caudal level in Habenula and differed between lateral (LHbL) and medial (LHbM) LHb (A and C). When cocaine self‐administration sessions were extended from 10 to 60 days, major increases in the c‐fos‐positive cells persisted in LHbL (A and B) but not in LHbM (C and D).