Insulin modulates cocaine-sensitive monoamine transporter function and impulsive behavior

Abstract

Because insulin acutely enhances the function of dopamine transporters, the tyrosine kinase receptors activated by this hormone may modulate transporter-dependent neurochemical and behavioral effects of psychoactive drugs. In this respect, we examined the effects of insulin on exocytotic monoamine release and the efficacy of the monoamine transporter blocker cocaine in rat nucleus accumbens. Whereas insulin reduced electrically evoked exocytotic [(3)H]dopamine release in nucleus accumbens slices, the hormone potentiated the release-enhancing effect of cocaine thereon. The phosphatidylinositol 3-kinase inhibitor LY294002 abolished these effects, indicating the involvement of insulin receptors. Similar insulin effects were observed on the release of [(3)H]norepinephrine in nucleus accumbens slices, but not on that of [(3)H]serotonin, and were also apparent in medial prefrontal cortex slices. As might then be expected, insulin also potentiated the dopamine and norepinephrine release-enhancing effects of the selective monoamine uptake inhibitors GBR12909 and desmethylimipramine, respectively. In subsequent behavioral experiments, we investigated the role of insulin in motor impulsivity that depends on monoamine neurotransmission in the nucleus accumbens. Intracranial administration of insulin in the nucleus accumbens alone reduced premature responses in the five-choice serial reaction time task and enhanced the stimulatory effect of peripheral cocaine administration on impulsivity, resembling the observed neurochemical effects of the hormone. In contrast, cocaine-induced locomotor activity remained unchanged by intra-accumbal insulin application. These data reveal that insulin presynaptically regulates cocaine-sensitive monoamine transporter function in the nucleus accumbens and, as a consequence, impulsivity. Therefore, insulin signaling proteins may represent targets for the treatment of inhibitory control deficits such as addictive behaviors.

Figure 1.

A, B, The effect of insulin on the electrically evoked [³H] DA release (A) and on the release-enhancing effect of cocaine (B) in NAc slices. Slices, incubated with [³H] DA (in the presence of 3 μm desmethylimipramine), were superfused and stimulated electrically at t = 40 min for 10 min. Insulin and cocaine were added to the superfusion medium 20 min before depolarization and were present until the end of the experiment. Data are expressed as percentage of electrically evoked [³H]DA release observed in the absence of insulin (A) and as percentage of electrically evoked [³H]DA release in the absence (closed circles) or presence (open circles) of 10 nm insulin (B). Data represent means ± SEM of 24–30 observations. *p < 0.05, significantly different from control release; **p < 0.05, significantly different from effect of cocaine in the absence of insulin. resp. control, Respective control; conc., concentration.

Figure 2.

The PI3K inhibitor LY294002 blocks the insulin-induced increase of the DA release-enhancing effect of cocaine in NAc slices. Slices, incubated with [³H]DA (in the presence of 3 μm desmethylimipramine), were superfused and stimulated electrically at t = 40 min for 10 min. Insulin (10 nm), 1 μm cocaine (COC) and 10 μm LY294002 (LY) were added to the superfusion medium 20 min before depolarization and were present until the end of the experiment. Data are expressed as percentage of respective control (resp. control) [³H]DA release in excess of spontaneous tritium efflux, i.e., evoked release in the absence and presence of insulin. Data represent means ± SEM of 24 observations. *p < 0.01, significantly different from the effect of cocaine in the absence of insulin.

Figure 3.

Region and neurotransmitter specificity of the regulatory role of insulin receptors. Slices of the NAc, CP, mPFC, or OFC were incubated with [³H]DA (in the presence of 3 μm desmethylimipramine) or [³H]NE or [³H]5-HT (both in the presence of 10 μm GBR12909). Radiolabeled slices were superfused and stimulated electrically at t = 40 min for 10 min. Insulin (10 nm) and cocaine (1 μm) were added to the superfusion medium 20 min before depolarization and were present until the end of the experiment. The release-enhancing effects of cocaine are expressed as percentage of respective electrically evoked release in the absence or presence of insulin. Data represent means ± SEM of 24–30 observations. *p < 0.05, significantly different from control release; **p < 0.05, significantly different from the effect of cocaine in the absence of insulin. resp. control, Respective control.

Figure 4.

The effect of leptin on the electrically evoked [³H]DA release and cocaine efficacy in NAc slices. Slices, incubated with [³H]DA (in the presence of 3 μm desmethylimipramine), were superfused and stimulated electrically at t = 40 min for 10 min. Leptin (10 nm) and cocaine (COC; 1 μm) were added to the superfusion medium 20 min before depolarization and were present until the end of the experiment. The data are expressed as percentage of control [³H]DA release in excess of spontaneous tritium efflux observed in the absence of insulin and cocaine. Data represent means ± SEM of 24 observations. *p < 0.05, significantly different from control release.

Figure 5.

The effect of insulin on the efficacy of monoamine transporter inhibitors in the NAc. Slices of the NAc were incubated with [³H]DA (in the presence of 3 μm desmethylimipramine), or [³H]NE or [³H]5-HT (both in the presence of 10 μm GBR12909). Radiolabeled slices were superfused and stimulated electrically at t = 40 min for 10 min. Insulin (10 nm; INS), GBR12909 (1 μm; GBR), desmethylimipramine (1 μm; DMI) or fluvoxamine (1 μm; FLU) were added to the superfusion medium 20 min before depolarization and were present until the end of the experiment. The release-enhancing effects of the uptake inhibitors were expressed as percentage of respective control (resp. control) release in excess of spontaneous tritium efflux in the absence or presence of insulin. Data represent means ± SEM of 24 observations. *p < 0.05, significantly different from control release in the absence of insulin; **p < 0.01, significantly different from the effect of cocaine in the absence of insulin.

Figure 6.

Effect of NAc insulin on motor impulsivity in the 5-CSRTT. Left, Schematic drawing of coronal sections depicting cannula placement into the NAc [adopted from Paxinos and Watson (1998)]. Right, Effects of intracranial application of insulin into the NAc on motor impulsivity and on the stimulatory effect of cocaine (5 mg/kg i.p.; COC). Note that the effects of cocaine on premature responding amounted to ∼60% and 280% of respective control values in the absence (open bars) and presence (closed bars) of insulin. *p < 0.05 and ** p < 0.005, significantly different from vehicle/saline; ^##significantly different from insulin/saline (p < 0.001).

Figure 7.

NAc insulin does not affect locomotor activity. Left, Schematic drawing of coronal sections depicting cannula placement into the NAc [adopted from Paxinos and Watson (1998)]. Right, Lack of effects of intracranial application of insulin into the NAc on locomotor activity and on the stimulatory effect of cocaine (10 mg/kg i.p.). VEH, Vehicle; SAL, saline; COC, cocaine.