Nicotine self-administration and ERK signaling are altered in RasGRF2 knockout mice

Abstract

Ras/Raf/MEK/ERK (Ras-ERK) signaling has been demonstrated to play a role in the effects of drugs of abuse such as cocaine and alcohol, but has not been extensively examined in nicotine-related reward behaviors. We examined the role of Ras Guanine Nucleotide Releasing Factor 2 (RasGRF2), an upstream mediator of the Ras-ERK signaling pathway, on nicotine self-administration (SA) in RasGRF2 KO and WT mice. We first demonstrated that acute nicotine exposure (0.4 mg/kg) resulted in an increase in phosphorylated ERK1/2 (pERK1/2) in the striatum, consistent with previous reports. We also demonstrated that increases in pERK1/2 resulting from acute (0.4 mg/kg) and repeated (0.4 mg/kg, 10 daily injections) exposure to nicotine in WT mice were not present in RasGRF2 KO mice, confirming that RasGRF2 at least partly regulates the activity of the Ras-ERK signaling pathway following nicotine exposure. We then performed intravenous nicotine SA (0.03 mg/kg/infusion for 10 days) in RasGRF2 KO and WT mice. Consistent with a previous report using cocaine SA, RasGRF2 KO mice demonstrated an increase in nicotine SA relative to WT controls. These findings suggest a role for RasGRF2 in the reinforcing effects of nicotine, and implicate the Ras-ERK signaling pathway as a common mediator of the response to drugs of abuse.

Keywords: RasGRF2; extracellar signal-regulated kinase; nicotine; pERK; pERK1/2; self-administration (SA).

FIGURE 1

Acute nicotine increased striatal pERK in male C57Bl6/N mice. (A) An acute injection of nicotine (0.4 mg/kg IP) resulted in a significant increase in the number of pERK1/2-positive cells relative to saline controls in the striatum (n = 5/group). Data represent the mean number of pERK1/2-positive cells (±SEM) in each condition in the ventral and dorsal striatum. (B) Representative slices showing pERK-positive cells from the ventral and dorsal striatum of male C57Bl6N mice that received acute nicotine or saline injections. *p < 0.05, main effect of treatment (nicotine relative to saline).

FIGURE 2

Acute nicotine failed to increase striatal pERK in male RasGRF2 KO mice. (A) An acute injection of nicotine (0.4 mg/kg IP) resulted in a significant increase in the number of pERK1/2-positive cells relative to saline controls in the ventral striatum of male RasGRF2 WT, but not KO mice (n = 3–4/group). Data represent the mean number of pERK1/2-positive cells (±SEM) in each condition. (B) An acute injection of nicotine (0.4 mg/kg IP) resulted in a significant increase in the number of pERK1/2-positive cells relative to saline controls in the dorsal striatum of male RasGRF2 WT, but not KO mice (n = 3–4/group). Data represent the mean number of pERK1/2-positive cells (±SEM) in each condition. (C) Representative slices showing pERK-positive cells from the ventral and dorsal striatum of male RasGRF2 WT and KO mice that received acute nicotine or saline injections. *p < 0.025; ***p < 0.0005.

FIGURE 3

Repeated nicotine failed to increase striatal pERK in male RasGRF2 KO mice. (A) 10 days of nicotine (0.4 mg/kg IP) injections resulted in a significant increase in the number of pERK1/2-positive cells relative to saline controls in the ventral striatum of male RasGRF2 WT, but not KO mice (n = 4–5/group). Data represent the mean number of pERK1/2-positive cells (±SEM) in each condition. (B) 10 days of nicotine (0.4 mg/kg IP) injections resulted in a significant increase in the number of pERK1/2-positive cells relative to saline controls in the dorsal striatum of male RasGRF2 WT, but not KO mice (n = 4–5/group). Data represent the mean number of pERK1/2-positive cells (±SEM) in each condition. (C) Representative slices showing pERK-positive cells from the ventral and dorsal striatum of RasGRF2 WT and KO mice that received acute nicotine or saline injections. *p < 0.025.

FIGURE 4

Nicotine SA in male and female RasGRF2 KO mice and WT controls. (A) RasGRF2 KO mice demonstrated an increase in responding on the nicotine-associated lever relative to WT controls (n = 23/group). Data represent mean number of presses (±SEM) on the active and inactive levers during 10 daily 1 h sessions of nicotine SA (0.03 mg/kg/35 µl infusion). (B) RasGRF2 KO mice demonstrated an increase in the number of nicotine reinforcers achieved relative to WT controls. Data represent mean number of nicotine reinforcers (±SEM) achieved during 10 daily 1 h sessions of nicotine SA (0.03 mg/kg/35 µl infusion). **p < 0.005; ^## p < 0.005, main effect of genotype.