Tolerance to Ethanol or Nicotine Results in Increased Ethanol Self-Administration and Long-Term Depression in the Dorsolateral Striatum

Abstract

Ethanol (EtOH) and nicotine are the most widely coabused drugs. Tolerance to EtOH intoxication, including motor impairment, results in greater EtOH consumption and may result in a greater likelihood of addiction. Previous studies suggest that cross-tolerance between EtOH and nicotine may contribute to the abuse potential of these drugs. Here we demonstrate that repeated intermittent administration of either EtOH or nicotine in adult male Sprague Dawley rats results in tolerance to EtOH-induced motor impairment and increased EtOH self-administration. These findings suggest that nicotine and EtOH cross-tolerance results in decreased aversive and enhanced rewarding effects of EtOH. Endocannabinoid signaling in the dorsolateral striatum (DLS) has been implicated in both EtOH tolerance and reward, so we investigated whether nicotine or EtOH pretreatment might modulate endocannabinoid signaling in this region. Using similar EtOH and nicotine pretreatment methods resulted in increased paired-pulse ratios of evoked EPSCs in enkephalin-positive medium spiny neurons in DLS slices. Thus, EtOH and nicotine pretreatment may modulate glutamatergic synapses in the DLS presynaptically. Bath application of the CB1 receptor agonist Win 55,2-212 increased the paired-pulse ratio of evoked EPSCs in control slices, while Win 55,2-212 had no effect on paired-pulse ratio in slices from either EtOH- or nicotine-pretreated rats. Consistent with these effects, nicotine pretreatment occluded LTD induction by high-frequency stimulation of the corticostriatal inputs to the dorsolateral striatum. These results suggest that nicotine and EtOH pretreatment modulates striatal synapses to induce tolerance to the motor-impairing effects of EtOH, which may contribute to nicotine and EtOH coabuse.

Keywords: addiction; endocanabinoid; motor impairment; reward; self-administration.

Fig. 1.

EtOH pretreatment results in tolerance to EtOH-induced motor impairment and increased EtOH self-administration. A, Timeline for the rotarod experiment in which rats were pretreated in the home cage with either EtOH or vehicle. B, Animals previously trained on the accelerating rotarod and pretreated for 3 d with either EtOH (1 g/kg, s.c., twice per day) or vehicle were tested for the effects of EtOH (1 g/kg, i.p.) on rotarod performance. Arrow indicates the time at which acute EtOH was administered. EtOH-pretreated animals displayed less impairment on the rotarod after acute EtOH administration than did the vehicle-treated animals (ANOVA, p = 0.0664; Holm–Sidak post hoc test, *p < 0.05; EtOH, n = 6; vehicle, n = 7). C, Timeline for the EtOH two-bottle choice self-administration experiment in which rats were pretreated in the home cage with either EtOH or vehicle. D, Animals pretreated with EtOH (1 g/kg, s.c., twice per day) or vehicle were given 24 h access to both a bottle of water and a bottle of 20% EtOH for 20 d. EtOH pretreated animals self-administered more EtOH than vehicle-pretreated animals (ANOVA, p < 0.05; Holm–Sidak post hoc test, *p < 0.05; n = 8 for both groups).

Fig. 2.

Acute nicotine administration causes motor impairment, but repeated administration results in tolerance to EtOH-induced motor impairment. Rats that were previously trained on the rotarod were tested for motor performance over 4 d. For the first 3 d, rats were administered either nicotine (0.1 mg/kg, s.c.) or vehicle. Gray arrows indicate the time point at which nicotine was administered. On the fourth day, both nicotine- and vehicle-treated groups were challenged with EtOH and tested for motor performance on the rotarod. The black arrow indicates the time at which EtOH was given. Acute nicotine administration resulted in significant motor impairment over all 3 d compared with vehicle administration (ANOVA, p < 0.0001 days 1, 2, 3; Holm–Sidak post hoc test, *p < 0.05; nicotine: n = 8, vehicle: n = 7). Repeated nicotine administration, however, resulted in tolerance to EtOH-induced motor impairment compared with vehicle administration (ANOVA, p < 0.001; Holm–Sidak post hoc test, *p < 0.05).

Fig. 3.

Nicotine pretreatment results in tolerance to EtOH-induced motor impairment and increased EtOH self-administration. A, Timeline for the rotarod experiment in which rats were pretreated with either nicotine or vehicle. B, Animals previously trained on the accelerating rotarod and pretreated for 3 d with either nicotine (0.1 mg/kg, s.c., once per day) or vehicle were tested for the effects of EtOH (1 g/kg, i.p.) on rotarod performance. Arrow indicates time at which acute EtOH was administered. Nicotine pretreated animals displayed less impairment on the rotarod after acute EtOH administration than did the vehicle-treated animals (ANOVA, p < 0.05; Holm–Sidak post hoc test, *p < 0.01; n = 14 for each group). C, Timeline for the EtOH two-bottle choice self-administration experiment in which rats were pretreated with either nicotine or vehicle. D, Animals pretreated with nicotine (0.1 mg/kg, s.c., once per day) or vehicle were given 24 h access to both a bottle of water and a bottle of 20% EtOH for 20 d. Nicotine-pretreated animals self-administered more EtOH than vehicle-pretreated animals (ANOVA, p < 0.01; Holm–Sidak post hoc test, *p < 0.05; n = 8 for both groups).

Fig. 4.

EtOH and nicotine pretreatment increase the paired-pulse ratio in enkephalin-positive (putative D₂ receptor-containing) MSNs in the DLS. A, Timeline for electrophysiology experiments in which rats were pretreated with EtOH (1 g/kg, s.c., twice per day), nicotine (0.1 mg/kg, s.c., once per day), or vehicle. B, Schematic showing the positioning of recording and stimulating electrodes in the DLS. C, Images of MSNs in the DLS. Arrowheads show the neuron that was recorded from. From left to right: representative biocytin-filled MSN (red); immunohistochemical labeling of enkephalin-positive MSNs (green); merged image (scale bar, 50 μm). All neurons included were putative D₂ MSNs. D, Representative traces showing the effects of vehicle, EtOH, or nicotine pretreatments on the paired-pulse ratio of EPSCs. E, Summary of the effects of vehicle, EtOH, or nicotine pretreatment on paired-pulse ratios in putative D₂-containing MSNs. EtOH and nicotine pretreatment both increased the paired-pulse ratio compared with vehicle pretreatment (ANOVA, p < 0.0001; Holm–Sidak post hoc test, #p < 0.0001, *p < 0.05), and EtOH pretreatment increased the paired-pulse ratio significantly more than nicotine pretreatment (**p < 0.01; vehicle: n = 19; EtOH: n = 16; nicotine: n = 23; one cell/slice/rat).

Fig. 5.

Nicotine or EtOH pretreatment occludes the effects of the CB1 receptor agonist on synaptic transmission in enkephalin-positive MSNs in the DLS. A, Representative traces of evoked EPSCs in putative D₂-containing MSNs from rats that had been pretreated with vehicle, EtOH, or nicotine during baseline and during application of the CB1 receptor agonist Win 55,2-212 (5 μm). B, Time course for the effects of Win 55, 2-212 on evoked EPSC amplitudes in each of the pretreatment groups. C, Summary data showing that the inhibitory effect of Win 55, 2-212 was only present in vehicle pretreated animals (paired t test baseline vs Win 55, 2-212, *p < 0.05; vehicle, n = 6; EtOH, n = 7; nicotine, n = 5). D, Time course for the effects of Win 55, 2-212 on PPR in each of the pretreatment groups. E, Summary data showing that Win 55, 2-212 only increases the PPR in vehicle-pretreated rats (paired t test baseline vs Win 55, 2-212, *p < 0.05; vehicle, n = 6; EtOH, n = 7; nicotine, n = 5).

Fig. 6.

Nicotine pretreatment occludes HFS-induced LTD in enkephalin-positive MSNs in the DLS. A, Representative EPSCs from putative D₂-containing MSNs from vehicle- and nicotine-pretreated rats during baseline and after HFS. B, Time course of the effects HFS on evoked EPSC amplitudes in vehicle- (open symbols) and nicotine- (filled symbols) pretreated groups. C, Summary data showing that HFS induces LTD of the excitatory inputs to DLS MSNs in vehicle-pretreated rats, but not in nicotine-pretreated rats (unpaired t test baseline vs HFS, *p < 0.05; vehicle, n = 9; nicotine, n = 7).