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.

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.

The laterodorsal tegmental nucleus (LDT), which sends cholinergic efferent connections to dopaminergic (DA) neurons in the ventral tegmental area (VTA), plays a critical role in the development of addictive behavior and the reinstatement of cocaine-seeking behavior. Although repeated cocaine exposure elicits plastic changes in excitatory synaptic transmission and intrinsic membrane excitability in LDT cholinergic neurons, it remains unclear whether inhibitory synaptic transmission is modulated by cocaine exposure. The LDT receives fibers containing noradrenaline (NA), a neurotransmitter whose extracellular levels increase with cocaine exposure. Therefore, it is hypothesized that repeated cocaine exposure induces plastic changes in LDT cholinergic neurons via NA. Ex vivo electrophysiological recordings in LDT cholinergic neurons were obtained from rats repeatedly exposed to cocaine. Bath-application of NA induced similar levels of hyperpolarization in both saline- and cocaine-treated neurons. However, NA attenuated the amplitude of inhibitory postsynaptic currents (IPSCs) in cocaine- but not saline-treated neurons through α2 adrenoceptors. This NA-induced IPSC attenuation was observed in the presence of strychnine, but not gabazine, indicating that NA modulated GABAergic but not glycinergic neurotransmission. NA increased the paired-pulse ratios of evoked IPSCs and decreased the frequencies of miniature IPSCs (mIPSCs) without affecting their amplitudes, suggesting a presynaptic mechanism. These findings suggest that repeated cocaine exposure induces neuroplasticity in GABAergic synaptic transmission onto LDT cholinergic neurons by probably modulating presynaptic α2 adrenoceptors. This potentially increases the activity of LDT cholinergic neurons, which might contribute to the development of addictive behavior by enhancing VTA DA neuronal activity.

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.