Arecoline Induces an Excitatory Response in Ventral Tegmental Area Dopaminergic Neurons in Anesthetized Rats

Abstract

Arecoline is the principle psychoactive alkaloid in areca nuts. Areca nuts are chewable seeds of Areca catechu L., which are epidemic plants that grow in tropical and subtropical countries and cause dependency after long-term use. However, the mechanisms underlying such dependency remain largely unclear, and therefore, no effective interventions for its cessation have been developed. The present study aimed to examine the effects of arecoline on neurons of the ventral tegmental area (VTA). After rats were anesthetized and craniotomized, electrophysiological electrodes were lowered into the VTA to obtain extracellular recordings. The mean firing rate of dopaminergic and GABAergic neurons were then calculated and analyzed before and after arecoline treatment. The burst characteristics of the dopaminergic neurons were also analyzed. The results showed that arecoline evoked a significant enhancement of the firing rate of dopaminergic neurons, but not GABAergic neurons. Moreover, arecoline evoked remarkable burst firings in the dopaminergic neurons, including an increase in the burst rate, elongation in the burst duration, and an enhancement in the number of spikes per burst. Collectively, the findings revealed that arecoline significantly excited VTA dopaminergic neurons, which may be a mechanism underlying areca nut dependency and a potential target for areca nut cessation therapy.

Keywords: GABAergic neuron; arecoline; dopaminergic neuron; electrophysiological activity; ventral tegmental area.

FIGURE 1

Arecoline increases the firing rates of VTA dopaminergic neurons. (A) Representative recordings from a dopaminergic neuron before and after saline (A1) and arecoline (A2) treatment, and then with quinpirole administration. (B) Representative recordings from a GABAergic neuron before and after saline (B1) and arecoline (B2) treatment, and then with quinpirole administration. The arrowheads indicate the time of injections. The left and middle insets (above A2) represent the traces of dopaminergic neuron firings before and after arecoline administration, and the right inset (above A2) represents the firing response to quinpirole. The small dots indicate putative bursts. (C,D) Histograms of the firing rates of dopaminergic neurons (C) and GABAergic neurons (D) before and after saline/arecoline injections. (E,F) Histograms of the firing rates of dopaminergic neurons (E) and GABAergic neurons (F) before and after quinpirole injections. The numbers presented in the parentheses denote the number of neurons measured, while the overlaid dots are the individual data points. DA neuron: dopaminergic neuron. ^** p < 0.01; NS, not significant.

FIGURE 2

Arecoline induces burst firings in VTA dopaminergic neurons. (A–C) Histograms of the burst parameters in dopaminergic neurons before and after saline/arecoline treatments. (A) Burst rate, (B) the burst duration, and (C) the number of spikes per bursts. The numbers presented in the parentheses denote the number of neurons measured, while the overlaid dots are the individual data points. DA neuron: dopaminergic neuron. ^** p < 0.01; NS, not significant.

FIGURE 3

Identification of the recording site in the VTA. (A) A representative histological section from a rat brain showing the electrode site (red arrowhead). (B) Histological reconstructions showing the recording sites in VTAs (between -4.92 and -5.20 mm from bregma) according to the rat brain atlas.