Deconstructing craving: dissociable cortical control of cue reactivity in nicotine addiction

Abstract

Background: Cue reactivity, the ability of cues associated with addictive substances to induce seeking and withdrawal, is a major contributor to addiction. Although human imaging studies show that cigarette-associated cues simultaneously activate the insula and the orbitofrontal cortex and evoke craving, how these activities functionally contribute to distinct elements of cue reactivity remains unclear. Moreover, it remains unclear whether the simultaneous activation of these cortical regions reflects coordinated functional connectivity or parallel processing.

Methods: We selectively lesioned the insula or orbitofrontal cortex with the excitotoxin ibotenic acid in mice, and their approach to nicotine-associated cues (n = 6-13/group) and avoidance of withdrawal-associated cues (n = 5-12/group) were separately examined in place conditioning paradigms. We additionally tested the role of these two cortical structures in approach to food-associated cues (n = 6-7/group) and avoidance of lithium chloride-associated cues (n = 6-7/group).

Results: Our data show a double dissociation in which excitotoxic lesions of the insula and orbitofrontal cortex selectively disrupted nicotine-induced cue approach and withdrawal-induced cue avoidance, respectively. These effects were not entirely generalized to approach to food-associated cues or avoidance of lithium chloride-associated cues.

Conclusions: Our data provide functional evidence that cue reactivity seen in addiction includes unique neuroanatomically dissociable elements and suggest that the simultaneous activation of these two cortical regions in response to smoking-related cues does not necessarily indicate functional connectivity.

Figure 1

Experimental protocol. A) Nicotine-induced conditioned cue approach (0 and 0.2 mg free base kg⁻¹, s.c.) and LiCl-induced conditioned cue avoidance (LiCl salt, 0 and 240 mg kg⁻¹, i.p.). An acute nicotine or LiCl injection was not given on the test days. B) Food (sweetened condensed milk)-induced conditioned cue approach. Food and an empty cup were alternately given in each compartment during conditioning sessions. C) Mecamylamine-induced conditioned cue avoidance (0 and 2.5 mg free base kg⁻¹, s.c.). The black horizontal line indicates exposure to oral nicotine (200 μg free base ml⁻¹) in the home cage. Black box, 15-min drug-free testing session; P, pre-conditioning test; T, post-conditioning test; grey box, 30-min conditioning (C) session.

Figure 2

Schematic of ibotenic acid-induced lesions of the A) insula and B) orbitofrontal cortex. Red-toned areas represent individual lesions. The darker red areas represent areas lesioned in more than one mouse. Grey areas represent individual lesions of “dorsal control” groups. Representative photographs of coronal sections showing MAP2 staining. Lesioned areas are devoid of staining in the insula (C) and the orbitofrontal cortex (D). INS, insula; OFC, orbitofrontal cortex; M1, primary motor cortex; M2, secondary motor cortex; S1, primary somatosensory cortex; Cg, cingulate cortex; PrL, prelimbic cortex; FrA, frontal association cortex; CL, claustrum; MO, medial orbitofrontal cortex; VO, ventral orbitofrontal cortex; LO, lateral orbitofrontal cortex. Scale bar: 500 μm.

Figure 3

Impact of cortical lesions on cue approach and avoidance. A) Nicotine-induced cue approach. N=6–13/group. B) Food-induced cue approach. N=7 (Sham) and 6 (Insula). C) Withdrawal-induced cue avoidance. N=5–12/group. D) Total nicotine intake of groups used for withdrawal-induced cue avoidance. OFC, orbitofrontal group; INS, insula group; Cont, control group that received lesions dorsal to the orbitofrontal cortex. E) LiCl-induced cue avoidance. N=7 (Sham) and 6 (orbitofrontal). X axis in A-C and E, pre-conditioning (P) and 10 postconditioning test days (Test day). Y axis in A, C and E, difference in time spent in the drug-paired and saline-paired compartments (sec). Y axis in B, difference in time spent in the food-paired and unpaired compartments (sec). # and ## (p<0.05 and p<0.01) represent statistically significant difference from sham group; * and ** represent a statistically significant difference from the pre-conditioning test at 5 and 1% levels, respectively. NS indicates no difference from the pre-conditioning test day.