Mutual assistance of nucleus accumbens cannabinoid receptor-1 and orexin receptor-2 in response to nicotine: a single-unit study

Abstract

Background and purpose: The nucleus accumbens (NAc) express both orexin-2 receptor (OX2R) and cannabinoid receptor type 1 (CB1R). Orexin and cannabinoid regulate the addictive properties of nicotine. In this study, the effect of the CB1R blockade on the electrical activity of NAc neurons in response to nicotine, and its probable interaction with the OX2R in this event, within this area, were examined via the single-unit recording.

Experimental approach: The spontaneous firing rate of NAc was initially recorded for 15 min, and then 5 min before subcutaneous injection of nicotine (0.5 mg/kg)/saline, AM251 and TCS-OX2-29 were injected into the NAc. Neuronal responses were recorded for 70 min, after nicotine administration.

Findings/results: Nicotine excited the NAc neurons significantly and intra-NAc microinjection of AM251 (25 and 125 ng/rat), as a selective CB1R antagonist, prevented the nicotine-induced increases of NAc neuronal responses. Moreover, microinjection of AM251 (125 ng/rat), before saline injection, could not affect the percentage of change of the neuronal response. Finally, simultaneous intra-NAc administration of the effective or ineffective doses of AM251 and TCS-OX2-29 (a selective antagonist of OX2R) prevented the nicotine- induced increases of NAc neuronal responses, so that there was a significant difference between the group received ineffective doses of both antagonists and the AM251 ineffective dose.

Conclusion and implications: The results suggest that the CB1R can modulate the NAc reaction to the nicotine, and it can be concluded that there is a potential interplay between the OX2R and CB1R in the NAc, in relation to nicotine.

Keywords: AM251; Cannabinoid system; Nicotine; Nucleus accumbens; Orexin system; Single-unit recording.

Fig. 1

(A) A representative image, showing the recording site in the NAc; (B) a representative pattern of baseline neuronal electrical activity recorded from the NAc; (C) an expanded waveform of a spike recorded from a NAc single neuron. AC, Anterior commissure; CPu, caudate putamen; NAc, nucleus accumbens.

Fig. 2

(A) Histograms represent the spike frequency of the entire recording (90 min) of all neurons. (A₁) Saline could not affect the firing rate of the NAc neurons; but (A₂) nicotine increased the firing frequency; (A₃) also, AM251 at 5 ng could not inhibit the nicotine-induced excitation; (A₄ and A₅) but 25 ng and 125 ng doses of AM251 could inhibit the nicotine-induced excitation; (A₆) the microinjection of the maximum dose of AM251 (125 ng), before the subcutaneous administration of saline could change the neuronal response, compared to the saline control group. (B) The effect of the CB1R blockade on the percentage of decrease/increase activity of the NAc neurons in response to nicotine. (C) The effect of CB1R blockade on the NAc neurons in response to nicotine, when the duration of the inhibition was taken into account Data is expressed as mean ± SEM. *P < 0.05 and **P < 0.01 indicate significant differences compared to the saline control group; ⁺P < 0.05 and ⁺⁺P < 0.01, and ⁺⁺⁺P < 0.001 different from the nicotine control group. Data are expressed as mean ± SEM. CB1R, Cannabinoid receptor type 1; NAc, nucleus accumbens.

Fig. 3

(A) Histograms representing the spike frequency of the entire recording (90 min) of all neurons. The concurrent administration of (A3) effective and (A4) ineffective doses of AM251 and TCS-OX2-29 could prevent the nicotine- induced excitation. (B) The effect of concurrent blockade of the OX2R and CB1R of NAc on the percentage of increase/decrease activity of the NAc neurons, in response to nicotine. (C) The effect of concurrent blockade of the OX2R and CB1R of NAc on neuronal activity in response to nicotine, when the duration of the inhibition was taken into account. Data are expressed as mean ± SEM. *P < 0.05 indicates significant differences compared to the saline control group; ⁺P < 0.05 and ⁺⁺P < 0.01, and ⁺⁺⁺P < 0.001 different from the nicotine control group. CB1R, cannabinoid receptor type 1; NAc, nucleus accumbens; OX2R, orexin-2 receptor.