Adolescent rats are resistant to adaptations in excitatory and inhibitory mechanisms that modulate mesolimbic dopamine during nicotine withdrawal

Abstract

Adolescent smokers report enhanced positive responses to tobacco and fewer negative effects of withdrawal from this drug than adults, and this is believed to propel higher tobacco use during adolescence. Differential dopaminergic responses to nicotine are thought to underlie these age-related effects, as adolescent rats experience lower withdrawal-related deficits in nucleus accumbens (NAcc) dopamine versus adults. This study examined whether age differences in NAcc dopamine during withdrawal are mediated by excitatory or inhibitory transmission in the ventral tegmental area (VTA) dopamine cell body region. In vivo microdialysis was used to monitor extracellular levels of glutamate and gamma-aminobutyric acid (GABA) in the VTA of adolescent and adult rats experiencing nicotine withdrawal. In adults, nicotine withdrawal produced decreases in VTA glutamate levels (44% decrease) and increases in VTA GABA levels (38% increase). In contrast, adolescents did not exhibit changes in either of these measures. Naïve controls of both ages did not display changes in NAcc dopamine, VTA glutamate, or VTA GABA following mecamylamine. These results indicate that adolescents display resistance to withdrawal-related neurochemical processes that inhibit mesolimbic dopamine function in adults experiencing nicotine withdrawal. Our findings provide a potential mechanism involving VTA amino acid neurotransmission that modulates age differences during withdrawal.

Fig. 1

Timeline of experimental procedures and postnatal day ages of animals.

Fig. 2

Effect of mecamylamine on transmitter overflow in naïve adult and adolescent rats. Sham-operated rats that did not receive nicotine were subjected to microdialysis as described in the methods. NAcc dopamine (top panel) and VTA glutamate (middle panel) and GABA (bottom panel) levels were measured during baseline and following administration of saline and then mecamylamine in naïve animals. Data are expressed as % change from baseline (±SEM). Mecamylamine did not produce alterations in any of these neurotransmitters relative to baseline in naive adult (n=5–6) or adolescent (n=6–7) rats.

Fig. 3

Effect of mecamylamine on glutamate overflow in adult and adolescent rats that were treated with nicotine and were then subjected to microdialysis procedures. VTA glutamate levels were measured during baseline and following administration of saline and then mecamylamine in nicotine-treated animals. Data are expressed as % change from baseline VTA glutamate levels (±SEM). Mecamylamine decreased glutamate levels relative to baseline in adults (n=14; *p≤0.05), and this effect was higher than adolescents (n=7; †p≥0.05). The figure on the right reflects the total mean AUC (±SEM) following both doses of mecamylamine. These data show that adults displayed lower glutamate levels following mecamylamine versus adolescents (†p≤0.05).

Fig. 4

Effect of mecamylamine on GABA overflow in adult and adolescent rats that were treated with nicotine and were then subjected to microdialysis procedures. VTA GABA levels were measured during baseline and following administration of saline and then mecamylamine in nicotine-treated animals. Data are expressed as % change from baseline VTA GABA levels (±SEM). Mecamylamine increased GABA levels relative to baseline in adults (n=14; *p≤0.05), and this effect was higher than adolescent rats (n=7; †p≤0.05). The figure on the right reflects the total mean AUC (±SEM) following both doses of mecamylamine. These data show that adults displayed higher GABA levels following mecamylamine versus adolescents (†p≤0.05).

Fig. 5

Relationship between VTA glutamate and NAcc dopamine before and after mecamylamine administration in nicotine-treated adult and adolescent rats. Data reflect % change from baseline in nicotine-treated adolescent (n=7) and adult (n=7) rats. Each point reflects an average value across all rats for dialysate fractions that were collected before (top panel) and after (bottom panel) mecamylamine administration. Thus, the data points per age group reflect an average of the 12 points collected before mecamylamine (i.e., baseline and saline) and 12 points after the 2 doses of mecamylamine. Dopamine and glutamate were positively correlated in adults (R²=0.82; p≤0.05) following mecamylamine administration, whereas this relationship was not observed in adolescents (R²=0.30). The correlation was stronger in adults as compared to adolescents (p≤0.05).

Fig. 6

Relationship between VTA GABA and NAcc dopamine before and after mecamylamine administration in nicotine-treated adult and adolescent rats. Data reflect % change from baseline in nicotine-treated adolescent (n=7) and adult (n=7) rats. Each point reflects an average value across all rats for dialysate fractions that were collected before (top panel) and after (bottom panel) mecamylamine administration. Thus, the data points per age group reflect averages of the 12 points collected before mecamylamine (i.e., baseline and saline) and 12 points after the 2 doses of mecamylamine. Dopamine and GABA were negatively correlated in adults (R²=0.42; p≤0.05) following mecamylamine administration, whereas this relationship was not observed in adolescents (R²=0.14). The correlation was stronger in adults as compared to adolescents (p≤0.05).

Fig. 7

Illustration of hypothesized mechanisms mediating age differences during nicotine withdrawal. In adults, nicotine withdrawal produces a decrease in NAcc dopamine that is larger as compared to adolescents. It is suggested that the decrease in dopamine in adults is mediated via reduced excitation and increased inhibition of the dopamine cell bodies in the VTA. However, in adolescents, reduced changes in NAcc dopamine (denoted as a larger dopamine neuron as compared to adults) during withdrawal are mediated via enhanced excitation (denoted by the larger glutamate neuron) and reduced inhibition (denoted by the smaller GABA neuron) of the dopamine cell bodies in the VTA as compared to adults.

Fig. 8

Schematic illustrating the microdialysis probe placements (adapted from Paxinos and Watson 1998). The left panel reflects placement of the 2 mm dialysis membrane tips in the NAcc and the right panel reflects placement of the 1 mm membrane tips in the VTA. Although the hemisphere that was implanted with the probe was randomized across animals, the schematic shows all of the adult placements on the left hemisphere and the adolescent placements on the right hemisphere.