Cannabinoid transmission in the prelimbic cortex bidirectionally controls opiate reward and aversion signaling through dissociable kappa versus μ-opiate receptor dependent mechanisms

Abstract

Cannabinoid, dopamine (DA), and opiate receptor pathways play integrative roles in emotional learning, associative memory, and sensory perception. Modulation of cannabinoid CB1 receptor transmission within the medial prefrontal cortex (mPFC) regulates the emotional valence of both rewarding and aversive experiences. Furthermore, CB1 receptor substrates functionally interact with opiate-related motivational processing circuits, particularly in the context of reward-related learning and memory. Considerable evidence demonstrates functional interactions between CB1 and DA signaling pathways during the processing of motivationally salient information. However, the role of mPFC CB1 receptor transmission in the modulation of behavioral opiate-reward processing is not currently known. Using an unbiased conditioned place preference paradigm with rats, we examined the role of intra-mPFC CB1 transmission during opiate reward learning. We report that activation or inhibition of CB1 transmission within the prelimbic cortical (PLC) division of the mPFC bidirectionally regulates the motivational valence of opiates; whereas CB1 activation switched morphine reward signaling into an aversive stimulus, blockade of CB1 transmission potentiated the rewarding properties of normally sub-reward threshold conditioning doses of morphine. Both of these effects were dependent upon DA transmission as systemic blockade of DAergic transmission prevented CB1-dependent modulation of morphine reward and aversion behaviors. We further report that CB1-mediated intra-PLC opiate motivational signaling is mediated through a μ-opiate receptor-dependent reward pathway, or a κ-opiate receptor-dependent aversion pathway, directly within the ventral tegmental area. Our results provide evidence for a novel CB1-mediated motivational valence switching mechanism within the PLC, controlling dissociable subcortical reward and aversion pathways.

Figure 1.

Histological analysis of intra-mPFC microinjection sites. A, Microphotograph of a representative injector placement within the PLC division of the mPFC. B, Schematic representation of select intra-PLC injector locations; ♦ = 500 ng WIN 55, 212-2 versus 5 mg/kg morphine group, ◊ = 500 ng AM 251 versus 0.05 mg/kg morphine group.

Figure 2.

Effects of intra-PLC CB1 receptor activation on morphine CPP conditioning. A, Bilateral intra-PLC micro-infusions of the CB1 receptor agonist WIN 55,212-2 (50 and 500 ng/0.5 μl), dose-dependently produced a morphine aversion against a sub-reward threshold conditioning dose of morphine (0.05 mg/kg, i.p.). Both vehicle controls and rats receiving a lower dose of WIN 55,212-2 (50 ng/0.5 μl; n = 7) display no significant preference for either environment. Conversely, animals receiving the higher dose of WIN 55,212-2 (500 ng/0.5 μl; n = 8) display a significant aversion to morphine-paired environments. B, Bilateral intra-PLC microinfusions of WIN 55,212-2 (500 ng/0.5 μl; n = 8) or vehicle (n = 7) versus a supra-reward threshold dose of morphine (5.0 mg/kg, i.p.) similarly switches the rewarding properties of morphine into aversion, with rats demonstrating robust CPA for morphine-paired environments. C, In control rats receiving intra-PLC WIN 55,212-2 (500 ng/0.5 μl; n = 8) versus vehicle, no preference for either environment is observed. *p < 0.05; **p < 0.01, for this and all subsequent figures.

Figure 3.

Effects of intra-PLC CB1 receptor blockade on morphine CPP conditioning. A, Bilateral intra-PLC microinfusions of the CB1 receptor antagonist, AM251 (50 ng/0.5 μl; n = 8, or 500 ng/0.5 μl; n = 7) dose-dependently potentiated the rewarding effects of morphine relative to vehicle controls that displayed no significant preference for either environment. B, Conversely, bilateral intra-PLC microinfusions of AM251 (500 ng/0.5 μl) versus a supra-reward threshold dose of morphine (5.0 mg/kg, i.p) has no effect on morphine reward conditioning, with both drug (n = 8) and vehicle control (n = 8) groups demonstrating robust morphine environment CPP. C, In control rats receiving intra-PLC AM251 (500 ng/0.5 μl; n = 8) versus vehicle, no preference for either environment is observed.

Figure 4.

Behavioral effects of intra-PLC CB1 receptor activation or blockade on morphine place conditioning. A, Summary of the bidirectional behavioral effects of intra-PLC AM-251 (50–500 ng/0.5 μl) or WIN 55,212-2 (50–500 ng/0.5 μl) on sub-threshold morphine (0.05 mg/kg, i.p.) reward or aversion effects, presented as difference scores (time in drug minus saline-paired environments). B, Summary of the effects of intra-PLC AM 251 (500 ng/0.5 μl) or WIN 55 212-2 (500 ng/0.5 μl) on supra-reward threshold (5.0 mg/kg, i.p.) morphine.

Figure 5.

Effects of DA receptor blockade on intra-PLC mediated modulation of opiate motivational processing. A, Rats treated with intra-PLC vehicle demonstrate robust CPP for environments paired with supra-reward threshold morphine (n = 8). Relative to vehicle pretreated controls (n = 8), pretreatment with the broad-spectrum DA receptor antagonist α-flu (0.8 mg/kg, i.p.) blocked the ability of intra-PLC WIN 55,212-2 (500 ng/0.5 μl; n = 7) to induce a behavioral morphine aversion to a supra-reward threshold conditioning dose of morphine (5.0 mg/kg, i.p.). B, Rats treated with intra-PLC vehicle demonstrated no preference for environments paired with sub-reward threshold morphine (n = 8). In contrast, rats treated with intra-PLC AM251 (n = 7) demonstrated a strong morphine CPP (p < 0.01; B). However, in rats pretreated with α-flu (n = 8), the ability of intra-PLC CB1 receptor blockade to potentiate the rewarding properties of morphine is blocked.

Figure 6.

Effects of intra-VTA κ- or μ-opiate receptor blockade on CB1 receptor-mediated modulation of opiate reward and aversion behaviors. A, Relative to rats receiving intra-PLC WIN 55,212-2 (500 ng/0.5 ml) versus supra-reward threshold morphine (n = 8), intra-PLC administration of WIN 55,212-2 (500 ng/0.5 μl) following intra-VTA administration of the KOR antagonist nor-BNI [50 (n = 8) or 500 (n = 6) ng/0.5 μl] dose-dependently blocks the ability of intra-PLC CB1 activation to switch morphine reward signaling into aversion. However, intra-VTA administration of a MOR antagonist, cyprodime (500 ng/0.5 μl; n = 6) fails to reverse the effects of intra-PLC CB1 activation. Intra-VTA administration of the highest effective dose of nor-BNI (500 ng/0.5 μl) does not produce any behavioral motivational effects in and of itself (n = 8; right). B, Intra-PLC administration of AM251 (500 ng/0.5 μl) following intra-VTA administration of the MOR antagonist cyprodime [50 (n = 7) or 500 (n = 7) ng/0.5 μl] dose-dependently blocks the ability of intra-PLC CB1 receptor blockade to potentiate sub-reward threshold morphine effects, relative to intra-PLC AM251 alone (n = 8). In contrast, intra-VTA administration of the KOR antagonist, nor-BNI (500 ng/0.5 μl; n = 6) fails to reverse the effects of intra-PLC CB1 blockade. Intra-VTA administration of the highest effective dose of cyprodime (500 ng/0.5 μl) does not produce any behavioral motivational effects in and of itself (n = 8; right).

Figure 7.

Histological analysis of intra-VTA microinfusion locations. A, Microphotograph showing representative bilateral intra-VTA infusion locations. B, Schematic summary of intra-VTA microinjector locations; ● = intra-VTA nor-BNI (500 ng/0.5 μl); □ = intra-VTA cyprodime (500 ng/05 μl). C, Schematic summary showing relative intra-VTA cannulae placement locations relative to behavioral CPP index score (total time in saline environment + total time in morphine environment/total time in morphine environment; de Jaeger et al., 2013) for rats receiving the behaviorally effective doses of either intra-VTA cyprodime or nor-BNI.