Role Of Vagus Nerve Stimulation On Heroin-seeking Behavior Induced By Condition Cues In Rats

Objective Vagus nerve stimulation (VNS) has been used for the treatment of neuropsychiatric disorders, such as epilepsy, depression, anxiety, Alzheimer's disease . However, little is known whether it is also effective for the treatment of heroin dependence, in particular for relapse to heroin seeking. Recent work by our group has found VNS inhibits heroin-seeking behavior induced by heroin priming or heroin-associated cues in rats. Our aim in this study was to further confirm the effect of acute and chronic VNS on heroin-seeking behavior and underlying molecular neurobiological mechanisms focusing on the expression of alpha-2A adrenergic receptor (alpha-2AR), in order to provide experimental proof for anti-relapse treatment for heroin addiction.Part 1. Effects of single VNS on cue-induced heroin-seeking.Methods The SD rats were trained to self-administration heroin by nose-poking under a FR1 schedule in a daily 4-h session for consecutive 14 days, then the electrode was implanted into the left VN and followed by 3d recovery. After 10 daily extinction session, cue induced heroin-seeking were measured by a single session. A group of rats (n=10) was given VNS only before the last extinction session (The VNS parameters are: 30HZ, 500 us pulse width, 1 mA, 30s on and 5 min off, continuous cycle for 2h). Cue-induced drug-seeking were measured immediately or 24h after VNS. Sham rats served as control (n=10), which is same with VNS group except that the stimulating current was set to 0 mA.Results Single VNS could decrease active nose-poking response if the cue-induced heroin-seeking test were conducted immediately after VNS (P=0.001) in comparison with sham controls, no difference was found on the active nose-poking if the heroin-seeking test were conducted after 24h (P=0.188). No difference were found for inactive nose-poking responses (P=0.696, P=0.875 respectively for the two tests)Part 2. Effects of repeated VNS on cue-induced heroin-seeking and the expression of alpha-2AR.Methods The experimental procedure is same with Part 1. The rats were divided into 2 groups : sham group (n=10) and VNS group (n=10). The rats in the VNS group were given VNS 2h before each extinction session (The VNS parameters are: 30HZ, 500 us pulse width, 1 mA, 30s on and 5 min off, continuous cycle for 2h). Then Cue-induced drug-seeking were measured immediately or 24h after VNS. After behavioral testing, Alpha-2A levels in the LC and PFC were measured by Western blotting.Results Compared with sham, repeated VNS could decrease nose-poking response if the cue-induced heroin-seeking test were conducted either immediately or 24h after VNS (P=0.000, P=0.000). Western blotting showed that the Alpha-2A levels were increased dramatically in LC, PFC of the rats after repeated VNS (P=0.007, P=0.002 ).Part 3. Effect of vagus nerve-ectomy on cue-induced heroin-seeking.Methods Using the same experimental procedure as in Part 1 and Part 2, after self-administration training, a surgery was made by cutting the vagus nerve, and the electrodes were implanted either in the proximal (descending stimulation) or distal (ascending stimulation) end from the cutting point. The rats were divided into 4 groups (n=5 for each group): sham, no-VNS group, descending VNS group, ascending VNS group. VNS were delivered before each extinction session. Cue-induced heroin-seeking test were conducted after 5 days stimulation.Results Either ascending or descending VNS could inhibit cue-induced drug-seeking response (P=0.000, P=0.000), while the inhibiting effect was stronger in the ascend group compared with the descend group (P=0.012), and VN cutting alone had no effect (P=0.293).Part 4. Role of LC NE neurons in the inhibiting effect of VNS on cue-induced heroin-seeking.Methods The experimental procedure was same with Part 1. The rats were divided into 4 groups: sham, DSP group, VNS group, DSP+VNS group. DSP-4, a noradrenergic toxin, were injected (50 mg/kg, ip) 5d before VN surgery for the rats of DSP group and DSP+VNS group. VNS was given 2h before each extinction session for consecutive 10 days, then cue induced heroin-seeking were measured.Results The active nose-poking was significantly higher in DSP group and DSP+VNS group (P=0.000, P=0.000) compared with VNS group, but still lower that the sham control. Conclusion:1. Both single and repeated VNS could inhibit heroin-seeking behavior induced by condition cues in the rats, and the inhibiting effect after repeated VNS could also be observed after 24h.2. The inhibition effect of VNS the drug-seeking behavior could be abolished by lesioning NE neurons in locus coeruleus, suggesting that LC NE system might be involved in the role of VNS, presumably was mediated by the alpha-2A receptor.3. Both the ascending and descending part of the VN be involved in the role of VNS on drug-seeking, while the ascending pathways might play a major role.