Insulin restores the neurochemical effects of nicotine in the mesolimbic pathway of diabetic rats

Abstract

This study examined whether insulin modulates the neurochemical effects of nicotine in the mesolimbic pathway of diabetic rats. The rats received vehicle or streptozotocin (STZ) to induce hypoinsulinemia. A subset of STZ-treated rats was implanted with insulin pellets that rapidly normalized glucose levels. Two-weeks later, dialysis probes were implanted into the nucleus accumbens (NAc) and ipsilateral ventral tegmental area (VTA). The next day, dialysate samples were collected during baseline and then following systemic administration of nicotine. Samples were also collected following intra-VTA administration of the gamma-aminobutyric acid (GABA)_A receptor antagonist, bicuculline. Dopamine, GABA, glutamate, and acetylcholine (ACh) levels were assessed using liquid chromatography/mass spectrometry (LC/MS). The results revealed that vehicle-treated rats displayed a nicotine-induced increase in NAc dopamine levels. In contrast, STZ-treated rats did not display any changes in NAc dopamine following nicotine administration, an effect that was likely related to a concomitant increase in GABA and decrease in glutamate levels in both the NAc and VTA. Intra-VTA administration of bicuculline increased NAc dopamine in vehicle-treated rats, and this effect was absent in STZ-treated rats. Vehicle-treated rats displayed a nicotine-induced increase in ACh levels in the NAc (but not VTA), an effect that was lower in the NAc of STZ-treated rats. Insulin supplementation normalized the neurochemical effects of nicotine in the NAc and VTA of STZ-treated rats, suggesting that insulin modulates the neurochemical effects of nicotine in the mesolimbic pathway of diabetic rats.

Keywords: amino acids; diabetes; dopamine; insulin; micro-dialysis; tobacco use.

Figure 1.

Experimental design and dialysis regimen. On day 1, rats were given vehicle or STZ administration. A subset of STZ-treated rats was implanted with insulin pellets. The experimental groups were as follows, vehicle-treated (n=9), STZ-treated (n=11), and STZ-treated+insulin (n=10). On day 17, three rats were eliminated from the original cohort of animals that displayed glucose levels higher than 650 mg/dL. Table 1 illustrates the total group size and number of samples that were included in our final analysis for each neurotransmitter.

Figure 2.

Schematic illustration of probe placements in the NAc and VTA in vehicle-treated (white probe tip), STZ-treated (black probe tip), and STZ-treated+insulin rats gray probe tip). The left panel reflects placements in the NAc (2 mm in length), and the right panel reflects placements in the VTA (1 mm in length). The diameter of the probe membrane was 0.24 mm wide, such that the estimated drawings of the probe span a range of sections wider than what is noted on a single plate in the diagram. These images were adapted from the Paxinos and Watson rat brain atlas (2004).

Figure 3.

Mean (±SEM) body weight (top panel) and glucose levels (bottom panel) in vehicle-treated (open circles), STZ-treated (black circles), and STZ-treated+insulin rats (gray circles). The daggers (†) denotes a difference from vehicle-treated and STZ-treated+insulin rats (p ≤ 0.05).

Figure 4.

NAc dialysate levels of dopamine, GABA, glutamate, and ACh levels during baseline and following nicotine administration. The panels on the left reflect each 20-min sampling period, and the panels on the right reflect sampling conditions (baseline versus nicotine) in vehicle-treated (open circles), STZ-treated (black circles), and STZ-treated+insulin (gray circles) rats. Data are expressed in nM concentrations (±SEM). The asterisk (*) denotes a significant difference from vehicle-treated rats, the number sign (#) denotes significant difference from STZ-treated+insulin rats, the () denotes a significant difference from baseline within-group, and the daggers (†) denote a significant difference from both vehicle-treated and STZ-treated+insulin rats (p ≤ 0.05). Table 1 illustrates the total group size and number of samples that were included in our final analysis for each neurotransmitter.

Figure 5.

VTA dialysate levels of dopamine, GABA, glutamate, and ACh levels during baseline and following nicotine administration. The panels on the left reflect each 20-min sampling period, and the panels on the right reflect sampling conditions (baseline versus nicotine) in vehicle-treated (open circles), STZ-treated (black circles), and STZ-treated+insulin (gray circles) rats. Data are expressed in nM concentrations (±SEM). The () denotes a significant difference from baseline within-group, and the dagger (†) denotes a significant difference from both vehicle-treated and STZ-treated+insulin rats (p ≤ 0.05). Table 1 illustrates the total group size and number of samples that were included in our final analysis for each neurotransmitter.

Figure 6.

NAc dopamine levels during baseline and following intra-VTA infusion of bicuculline (100 μM) in vehicle-treated (open circles), STZ-treated (black circles), and STZ-treated+insulin (gray circles) rats. Data are expressed in nanomolar (nM) concentrations (±SEM). The number sign (#) denotes significant difference from STZ-treated+insulin rats, and the dagger (†) denotes a significant difference from both vehicle-treated and STZ-treated+insulin rats (p ≤ 0.05). Table 1 illustrates the total group size and number of samples that were included in our final analysis for each neurotransmitter.

Figure 7.

The image summarizes the neurochemical effects of nicotine in the mesolimbic pathway of vehicle- and STZ-treated rats. In STZ-treated rats, nicotine-induced increases in NAc dopamine are blunted as compared to vehicle-treated rats. It is hypothesized that the latter effect is mediated via greater GABA inhibition (larger minus sign) and reduced glutamate excitation (smaller plus sign) in STZ-treated rats. Importantly, STZ-induced neurochemical changes were restored to vehicle-treated control levels following insulin supplementation.