Noradrenaline enhances the excitatory effects of dopamine on medial prefrontal cortex pyramidal neurons in rats.

AIM: Our previous studies showed that exposure to acute restraint stress enhanced cocaine-induced conditioned place preference (cocaine-CPP) and suggested the possibility that co-activation of adrenergic transmission boosts the increase in medial prefrontal cortex (mPFC) neuronal activity by the activation of dopaminergic transmission. To examine this possibility, the effects of the co-treatment with dopamine (DA) and noradrenaline (NA) on mPFC neurons were compared with those of treatment with DA alone using whole-cell patch-clamp recordings. METHODS: The effects of DA alone and a mixture of DA and NA on the membrane potentials and spontaneous excitatory postsynaptic currents (sEPSCs) were examined by electrophysiological recordings of mPFC pyramidal neurons in brain slices of male Sprague Dawley rats. Extracellular DA and NA levels in the mPFC during and after restraint stress exposure were also examined by in vivo microdialysis. RESULTS: Dopamine significantly produced depolarizing effects on mPFC neurons and tended to increase sEPSC frequency. Co-administration of NA with DA produced stronger depolarizing effects and significantly increased sEPSC frequency. The findings suggest that the additional depolarizing effect of NA on DA-responsive neurons, rather than the excitation of DA-nonresponsive neurons by NA, contributes to the stronger effect of co-treatment of NA with DA. CONCLUSION: The present study suggests that NA released by restraint stress exposure cooperates with DA to stimulate DA-responsive neurons in the mPFC, thereby causing the stress-induced enhancement of cocaine-CPP.

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.

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.

[Involvement and plasticity of brainstem cholinergic neurons in cocaine-induced addiction].

Although the involvement and plasticity of the mesocorticolimbic dopamine (DA) system in cocaine-induced addiction have been studied extensively, the role of the brainstem cholinergic system in cocaine addiction remains largely unexplored. The laterodorsal tegmental nucleus (LDT) contains cholinergic neurons that innervate the ventral tegmental area (VTA) and is crucial for regulating the activity of VTA DA neurons, implying that LDT may also be associated with cocaine addiction. In this review, we summarize our recent findings showing that cholinergic transmission from the LDT to the VTA is involved in acquisition and expression of cocaine-induced conditioned place preference and that, after repeated cocaine exposures, these neurons exhibit synaptic plasticity, which is dependent on NMDA receptor activation, nitric oxide production, and the activity of medial prefrontal cortex. The findings strongly suggest that LDT cholinergic neurons may critically contribute to developing cocaine-induced addiction.

Critical role of cholinergic transmission from the laterodorsal tegmental nucleus to the ventral tegmental area in cocaine-induced place preference.

Conditioned place preference (CPP) is widely used to investigate the rewarding properties of cocaine. Various brain regions and neurotransmitters are involved in developing cocaine CPP. However, the contribution of cholinergic transmission in the ventral tegmental area (VTA) to cocaine CPP remains largely unexplored. Here, we examined the role of cholinergic input arising from the laterodorsal tegmental nucleus (LDT) to the VTA in the acquisition and expression of cocaine CPP in rats. Intra-LDT injection of carbachol, which hyperpolarizes LDT neurons, and of NMDA and AMPA receptor antagonists before cocaine conditioning blocked and attenuated cocaine CPP, respectively, indicating the necessity of LDT activity for acquiring the CPP. Additionally, intra-VTA injection of scopolamine or mecamylamine before cocaine conditioning also attenuated cocaine CPP, demonstrating the contribution of cholinergic transmission via muscarinic and nicotinic acetylcholine receptors in CPP acquisition. Furthermore, intra-VTA injection of scopolamine or mecamylamine immediately before the test attenuated cocaine CPP, indicating that cholinergic signaling is also associated with the expression of CPP. These results suggest that cholinergic transmission from the LDT to the VTA is critically involved in both acquiring and retrieving cocaine-associated memories in cocaine CPP.

Cocaine exposure enhances excitatory synaptic drive to cholinergic neurons in the laterodorsal tegmental nucleus.

Accumulating evidence indicates that the laterodorsal tegmental nucleus (LDT) is associated with reward processing and addiction. The cholinergic projection from the LDT to the ventral tegmental area is essential for a large dopamine release in the nucleus accumbens, which is critically involved in the reinforcing effects of addictive drugs, including cocaine. In contrast to the large number of studies on plasticity induced after cocaine exposure in the mesocorticolimbic dopaminergic system, it remains unknown whether LDT cholinergic neurons exhibit plastic changes following cocaine administration. To address this issue, we performed ex vivo whole-cell recordings in LDT cholinergic neurons obtained from rats following cocaine administration. Neurons obtained from 1 day after 5-day cocaine-treated rats showed significantly smaller paired-pulse ratios of evoked EPSCs and higher miniature EPSC frequencies than those from saline-treated rats, indicating an induction of presynaptic plasticity of increased glutamate release. This plasticity seemed to recover after a 5-day withdrawal from repeated cocaine exposure, and required NMDA receptor stimulation and nitric oxide production. Additionally, pharmacological suppression of activity of the medial prefrontal cortex inhibited the presynaptic plasticity in the LDT. On the other hand, AMPA/NMDA ratios were not different between saline- and cocaine-treated groups, revealing an absence of postsynaptic plasticity. These findings provide the first direct evidence of cocaine-induced synaptic plasticity in LDT cholinergic neurons and suggest that the presynaptic plasticity enhances the activity of LDT cholinergic neurons, contributing to the expression of cocaine-induced addictive behaviors through the dysregulation of the mesocorticolimbic system.