Reinstatement of cocaine seeking by hypocretin (orexin) in the ventral tegmental area: independence from the local corticotropin-releasing factor network

Abstract

Background: Hypocretin (Hcrt), an arousal- and feeding-associated peptide, is expressed in lateral hypothalamic neurons that project to the ventral tegmental area (VTA). Intra-VTA Hcrt reinstates morphine-conditioned place preferences, and intracerebroventricular and intra-VTA corticotropin-releasing factor (CRF) reinstate cocaine seeking. Each is presumed to act, at least in part, through actions local to the VTA. Here, we examined the possibility that VTA perfusion of Hcrt reinstates cocaine seeking and, if so, whether it does so through the VTA mechanism that is implicated in reinstatement by CRF.

Methods: Rats were trained to lever-press for intravenous cocaine (2 weeks) and then underwent extinction training (saline substituted for cocaine: 3 weeks). Reinstatement behavior was tested and VTA dialysates were collected and assayed for glutamate or dopamine following footshock or perfusion of Hcrt or CRF, with or without Hcrt or CRF antagonists, into the VTA.

Results: Ventral tegmental area perfusion of Hcrt-1 or footshock stress reinstated cocaine seeking and caused release of VTA glutamate and dopamine. The effects of Hcrt-1 were blocked by a selective Hcrt-1 antagonist, but not a CRF antagonist, and were not mimicked by Hcrt-2. The Hcrt-1 antagonist did not block CRF-dependent footshock-induced reinstatement or glutamate or dopamine release. The behavioral and neurochemical effects of Hcrt-1 were attenuated but not blocked by kynurenic acid, an ionotropic glutamate antagonist that blocks footshock-induced reinstatement and glutamate release.

Conclusions: While Hcrt and CRF are known to interact in some area of the brain, in the VTA proper they appear to have largely independent actions on the mesolimbic dopamine mechanisms of cocaine seeking.

Figure 1

VTA perfusion of Hcrt-1 (10 μM) reinstated cocaine-seeking and increased extracellular levels of dopamine and glutamate. VTA perfusion of SB-408124 (10 μM), a selective Hcrt-1 receptor antagonist, while having no effect itself on either the animal’s behavior or basal VTA neurotransmitter levels, blocked the reinstatement and the neurotransmitter increases induced by VTA Hcrt-1. * indicates difference from the other experimental conditions.

Figure 2

VTA perfusion of kynurenic acid (Kyn, 1 mM) attenuated the increase in VTA dopamine levels and the reinstatement of lever-pressing induced by VTA Hcrt-1 (10 μM) perfusion. VTA Kyn perfusion showed no effects on Hcrt-1-induced VTA glutamate levels. * indicates difference from the other experimental conditions. # indicates difference from the Hcrt-1 alone conditions.

Figure 3

VTA perfusion of α-helical CRF (1 μM), an antagonist at CRF receptors, had no effect on VTA Hcrt-1-induced reinstatement of cocaine-seeking or on VTA dopamine or glutamate levels. * indicate difference from the non-Hcrt-1 conditions.

Figure 4

VTA perfusion of the Hcrt-1 antagonist SB-408124, at the concentration (10 μM) that effectively blocks the actions of VTA Hcrt-1, had no effect on VTA dopamine or glutamate levels or on footshock-induced reinstatement of cocaine-seeking. * indicate difference from the no-shock conditions.