Acute restraint stress augments the rewarding memory of cocaine through activation of α1 adrenoceptors in the medial prefrontal cortex of mice.

Stress augments the rewarding memory of cocaine, which plays a critical role in inducing cocaine craving. However, the neurobiological mechanisms underlying the enhancing effect of stress remain unclear. Here, we show that noradrenaline (NA) transmission in the medial prefrontal cortex (mPFC) mediates stress-induced enhancement of cocaine craving. When mice were exposed to acute restraint stress immediately before the posttest session of the cocaine-induced conditioned place preference (CPP) paradigm, the CPP score was significantly higher than that in non-stressed mice. Because extracellular NA levels have been reported to be increased in the mPFC during stress exposure, we assessed the effects of NA on mPFC layer 5 pyramidal cell activity. Whole-cell recordings revealed that NA application induces depolarization and a concomitant increase in spontaneous excitatory postsynaptic currents (sEPSCs). The NA effects were inhibited by terazosin, but not by yohimbine or timolol, and the sEPSC increase was not observed in the presence of tetrodotoxin, suggesting the involvement of postsynaptic α1, but not α2 or ß, adrenoceptors in the NA effects. Additionally, intra-mPFC injection of terazosin before stress exposure attenuated the stress-induced increase in cocaine CPP. Intra-mPFC injection of phenylephrine, an α1 adrenoceptor agonist, before the posttest session without stress exposure significantly enhanced cocaine CPP. Furthermore, chemogenetic suppression of mPFC pyramidal cells with inhibitory DREADD (designer receptors exclusively activated by designer drugs) also suppressed the stress-induced CPP enhancement. These findings suggest that the stress-induced increase in NA transmission activates mPFC pyramidal cells via α1 adrenoceptor stimulation, leading to enhancement of cocaine craving-related behavior.

Glutamatergic neurons in the medial prefrontal cortex mediate the formation and retrieval of cocaine-associated memories in mice.

In drug addiction, environmental stimuli previously associated with cocaine use readily elicit cocaine-associated memories, which persist long after abstinence and trigger cocaine craving and consumption. Although previous studies suggest that the medial prefrontal cortex (mPFC) is involved in the expression of cocaine-addictive behaviors, it remains unclear whether excitatory and inhibitory neurons in the mPFC are causally related to the formation and retrieval of cocaine-associated memories. To address this issue, we used the designer receptors exclusively activated by designer drugs (DREADD) technology combined with a cocaine-induced conditioned place preference (CPP) paradigm. We suppressed mPFC neuronal activity in a cell-type- and timing-dependent manner. C57BL/6J wild-type mice received bilateral intra-mPFC infusion of an adeno-associated virus (AAV) expressing inhibitory DREADD (hM4Di) under the control of CaMKII promotor to selectively suppress mPFC pyramidal neurons. GAD67-Cre mice received bilateral intra-mPFC infusion of a Cre-dependent AAV expressing hM4Di to specifically silence GABAergic neurons. Chemogenetic suppression of mPFC pyramidal neurons significantly attenuated both the acquisition and expression of cocaine CPP, while suppression of mPFC GABAergic neurons affected neither the acquisition nor expression of cocaine CPP. Moreover, chemogenetic inhibition of mPFC glutamatergic neurons did not affect the acquisition and expression of lithium chloride-induced conditioned place aversion. These results suggest that the activation of glutamatergic, but not GABAergic, neurons in the mPFC mediates both the formation and retrieval of cocaine-associated memories.

Stress augments the rewarding memory of cocaine via the activation of brainstem-reward circuitry.

Effects of stress on the reward system are well established in the literature. Although previous studies have revealed that stress can reinstate extinguished addictive behaviors related to cocaine, the effects of stress on the rewarding memory of cocaine are not fully understood. Here, we provide evidence that stress potentiates the expression of rewarding memory of cocaine via the activation of brainstem-reward circuitry using a cocaine-induced conditioned place preference (CPP) paradigm combined with restraint stress in rats. The rats exposed to 30-minute restraint stress immediately before posttest exhibited significantly larger CPP scores compared with non-stressed rats. Intra-laterodorsal tegmental nucleus (LDT) microinjection of a ß or α2 adrenoceptor antagonist attenuated the stress-induced enhancement of cocaine CPP. Consistent with this observation, intra-LDT microinjection of a ß or α2 adrenoceptor agonist before posttest increased cocaine CPP. Additionally, intra-ventral tegmental area (VTA) microinjection of antagonists for the muscarinic acetylcholine, nicotinic acetylcholine or glutamate receptors attenuated the stress-induced enhancement of cocaine CPP. Finally, intra-medial prefrontal cortex (mPFC) microinjection of a D1 receptor antagonist also reduced the stress-induced enhancement of cocaine CPP. These findings suggest a mechanism wherein the LDT is activated by noradrenergic input from the locus coeruleus, leading to the activation of VTA dopamine neurons via both cholinergic and glutamatergic transmission and the subsequent excitation of the mPFC to enhance the memory of cocaine-induced reward value.

The Role of Dopaminergic Signaling in the Medial Prefrontal Cortex for the Expression of Cocaine-Induced Conditioned Place Preference in Rats.

The expression phase of cocaine-induced conditioned place preference (CPP) represents a cocaine-seeking behavior triggered by contextual cues associated with the rewarding effects of cocaine. However, the exact mechanisms underlying the cocaine CPP expression remain unclear. Here, we investigated the role of dopaminergic (DAergic) transmission in the medial prefrontal cortex (mPFC) for the expression of cocaine CPP. An intra-ventral tegmental area (VTA) injection of a cocktail of γ-aminobutyric acid (GABA)B and GABAA receptor agonists (baclofen and muscimol, respectively) immediately before the posttest inhibited the expression of cocaine CPP. An intra-mPFC injection of a dopamine D1 but not D2 receptor antagonist before the posttest significantly attenuated CPP expression. Moreover, after the posttest, the number of cFos-positive mPFC neurons in rats that were conditioned with cocaine was significantly larger than that with saline. Additionally, photostimulation of channelrhodopsin-2 expressing fibers derived from the VTA induced cFos expression in the mPFC, and this induction was reduced by a prior systemic injection of a D1 receptor antagonist. These findings indicate that during the expression of cocaine CPP, enhanced DAergic transmission from the VTA to the mPFC stimulates D1 receptors; this results in the activation of mPFC neurons, further leading to the expression of cocaine CPP.

Nitric oxide in the medial prefrontal cortex contributes to the acquisition of cocaine place preference and synaptic plasticity in the laterodorsal tegmental nucleus.

Nitric oxide (NO), a gaseous neurotransmitter, is involved in a variety of brain functions, including drug addiction. Although previous studies have suggested that NO plays an important role in the development of cocaine addiction, the brain region(s) in which NO acts and how it contributes to cocaine addiction remain unclear. In this study, we examined these issues using a cocaine-induced conditioned place preference (CPP) paradigm and ex vivo electrophysiological recordings in rats. Specifically, we focused on the medial prefrontal cortex (mPFC) and laterodorsal tegmental nucleus (LDT), brain regions associated with cocaine CPP development and cocaine-induced plasticity. Intra-mPFC injection of the non-selective NO synthase (NOS) inhibitor L-NAME or the neuronal NOS (nNOS) selective inhibitor L-NPA during the conditioning phase disrupted cocaine CPP. Additionally, intra-mPFC injection of L-NPA prior to each cocaine injection prevented the induction of presynaptic plasticity, induced by repeated cocaine administration, in LDT cholinergic neurons. These findings indicate that NO generated in the mPFC contributes to the acquisition of cocaine CPP and the induction of neuroplasticity in LDT cholinergic neurons. Together with previous studies showing that NO induces membrane plasticity in mPFC neurons, that mPFC neurons project to the LDT, and that LDT activity is critical for the acquisition of cocaine CPP, the present findings suggest that NO-mediated neuroplasticity induced in the mPFC-LDT circuitry is critical for the development of cocaine addiction.

The role of neuroplasticity in cholinergic neurons of the laterodorsal tegmental nucleus for cocaine addiction.

A large body of literature indicates that neural adaptations induced by cocaine in the mesocorticolibic system cause addictive behaviors. Emerging evidence suggests that the laterdorsal tegmental nucleus (LDT), which contains cholinergic, glutamatergic and GABAergic neurons and innervates the ventral tegmental area (VTA), might also contribute to the development of cocaine addiction. In this review, we summarize our recent findings showing that neuroplasticity elicited by cocaine administration in LDT cholinergic neurons is involved in the expression of addictive behaviors. Ex vivo electrophysiological recordings obtained from repeatedly cocaine administered rats revealed and increased excitatory synaptic transmission to and enhanced intrinsic membrane excitability in LDT cholinergic neurons. The former depended on enhanced glutamate release probability form presynaptic terminals and the latter was mediated by increased persistent sodium conductance. Additionally, intra-LDT administration of AMPA/HMDA receptor antagonists or a persistent sodium channel blocker attenuated the expression of cocaine-induced conditioned place preference. These findings suggest that chronic cocaine exposure-induced neuroplasticity in LDT cholinergic neurons may activate LDT cholinergic neurons, which in turn may enhance the activity of dopamine neurons in the VTA, leading to the development of cocaine addiction.

Intrinsic membrane plasticity via increased persistent sodium conductance of cholinergic neurons in the rat laterodorsal tegmental nucleus contributes to cocaine-induced addictive behavior.

The laterodorsal tegmental nucleus (LDT) is a brainstem nucleus implicated in reward processing and is one of the main sources of cholinergic afferents to the ventral tegmental area (VTA). Neuroplasticity in this structure may affect the excitability of VTA dopamine neurons and mesocorticolimbic circuitry. Here, we provide evidence that cocaine-induced intrinsic membrane plasticity in LDT cholinergic neurons is involved in addictive behaviors. After repeated experimenter-delivered cocaine exposure, ex vivo whole-cell recordings obtained from LDT cholinergic neurons revealed an induction of intrinsic membrane plasticity in regular- but not burst-type neurons, resulting in increased firing activity. Pharmacological examinations showed that increased riluzole-sensitive persistent sodium currents, but not changes in Ca(2+) -activated BK, SK or voltage-dependent A-type potassium conductance, mediated this plasticity. In addition, bilateral microinjection of riluzole into the LDT immediately before the test session in a cocaine-induced conditioned place preference (CPP) paradigm inhibited the expression of cocaine-induced CPP. These findings suggest that intrinsic membrane plasticity in LDT cholinergic neurons is causally involved in the development of cocaine-induced addictive behaviors.