Inhibition of neuronal nitric oxide synthase prevents alterations in medial prefrontal cortex excitability induced by repeated cocaine administration.

RATIONALE: The medial prefrontal cortex (mPFC), a forebrain region that regulates cognitive function and reward-motivated behaviors, has been implicated in the neuropathological mechanisms of drug addiction and withdrawal. In cocaine-abstinent human addicts, neuronal activity of the mPFC is increased in response to cocaine re-exposure or drug-associated cues. Additionally, repeated cocaine exposure alters the membrane properties and ion channel function of mPFC pyramidal neurons in drug-withdrawn rats, leading to an increased firing in response to excitatory stimuli. Nitric oxide (NO), a diffusible neuromodulator of neuronal excitability, may play a role in initiating and maintaining behavioral effects of psychostimulants. However, the role of NO in the mechanisms by which cocaine affects membrane excitability is not well clarified. OBJECTIVES: In this study, we attempted to determine whether inhibition of neuronal nitric oxide synthase (nNOS) altered the changes induced by repeated cocaine exposure and withdrawal. METHODS: Visualized whole-cell current clamp recordings in brain slices containing the mPFC of rats administered (once per day for 5 days) with either vehicle (10% Cremophor EL in saline 0.9%), cocaine (15 mg/kg, i.p.), or cocaine and the nNOS inhibitor 7-NI (50 mg/kg, i.p.) were employed. RESULTS: We found that nNOS inhibition prevented cocaine sensitization and the increased membrane excitability of pyramidal cells, evidenced by an increased number of evoked spikes and reductions in inward rectification observed after short-term withdrawal from cocaine. CONCLUSIONS: These findings suggest that NO plays an important role in chronic cocaine-induced deregulation of the mPFC activity that may contribute to the development of behavioral sensitization and cocaine withdrawal.

Fos regulates neuronal activity in the nucleus accumbens.

Repeated exposure to drugs of abuse induces a variety of persistent changes in the brain and the dopamine D1 receptor plays a major role in the process. To understand intracellular mechanisms contributing to cocaine-induced neuroadaptations, we previously examined the role of the immediate early gene Fos using a mouse in which Fos is disrupted primarily in D1 receptor-expressing neurons in the brain. We found that both dendritic remodeling of medium spiny neurons and behavioral sensitization induced by repeated exposure to cocaine are attenuated in the mutant mice. Moreover, the expression of genes encoding several transcription factors, neurotransmitter receptors and intracellular signaling molecules following repeated cocaine administration is altered in the mutant mice compared to that in wild-type mice. In the present study, we have investigated the role of Fos in regulating neuronal excitability at a cellular level and found that medium spiny nucleus accumbens neurons in the mutant mice exhibit increased excitability and attenuated inhibitory responses to stimulation of D1 receptors compared to those in wild-type mice. Our findings suggest that Fos functions in D1 receptor-bearing neurons to regulate neuronal activity which may contribute to the persistence of drug-induced changes.

Repeated cocaine administration increases voltage-sensitive calcium currents in response to membrane depolarization in medial prefrontal cortex pyramidal neurons.

The medial prefrontal cortex (mPFC) plays a critical role in cocaine addiction. However, evidence to elucidate how the mPFC is functionally involved in cocaine addiction remains incomplete. Recent studies have revealed that repeated cocaine administration induces various neuroadaptations in pyramidal mPFC neurons, including a reduction in voltage-gated K+ currents (VGKCs) and a possible increase in voltage-sensitive Ca2+ currents (I(Ca)). Here, we performed both current-clamp recordings in brain slices and voltage-clamp recordings in freshly dissociated cells to determine whether I(Ca) is altered in mPFC pyramidal neurons after chronic cocaine treatment with a short-term or long-term withdrawal. In addition, a critical role of VGKCs in regulating the generation of Ca2+ plateau potential was also studied in mPFC neurons. Repeated cocaine administration significantly prolonged the duration of evoked Ca2+ plateau potentials and increased the whole-cell I(Ca) in mPFC neurons after a 3 d withdrawal. Selective blockade of L-type Ca2+ channels by nifedipine not only significantly increased the threshold but also reduced the duration and amplitude of Ca2+ plateau potentials in both saline- and cocaine-withdrawn mPFC neurons. However, there was no significant difference in the increased threshold, reduced duration, and decreased amplitude of Ca2+ potentials between saline- and cocaine-withdrawn neurons after blockade of L-type Ca2+ channels. Moreover, an increase in amplitude was also observed, whereas the prolonged duration persisted, in Ca2+ potentials after 2-3 weeks of withdrawal. These findings indicate that chronic exposure to cocaine facilitates the responsiveness of I(Ca), particularly via the activated L-type Ca2+ channels, to excitatory stimuli in rat mPFC pyramidal neurons.

Cocaine-induced plasticity of intrinsic membrane properties in prefrontal cortex pyramidal neurons: adaptations in potassium currents.

Drug-induced adaptations in the prefrontal cortex (PFC) contribute to several core aspects of addictive behaviors, but the underlying neuronal processes remain essentially unknown. Here, we demonstrate that repeated in vivo exposure to cocaine persistently reduces the voltage-gated K+ current (VGKC) in PFC pyramidal neurons, resulting in enhanced membrane excitability. Analysis of dopamine D1-class receptor (D1R)-mediated modulation of VGKC indicates that, despite the absence of direct D1R stimulation, downstream D1 signaling (the cAMP/protein kinase A pathway) is increased during withdrawal from chronic cocaine treatment and plays a central role in the drug-induced membrane plasticity in PFC. This long-lasting, cocaine-induced plasticity of membrane excitability in PFC pyramidal neurons may contribute to the impaired decision making and drug craving that characterize cocaine withdrawal.

Repeated cocaine administration increases membrane excitability of pyramidal neurons in the rat medial prefrontal cortex.

Although the medial prefrontal cortex (mPFC) plays a critical role in cocaine addiction, the effects of chronic cocaine on mPFC neurons remain poorly understood. Here, we performed visualized current-clamp recordings to determine the effects of repeated cocaine administration on the membrane excitability of mPFC pyramidal neurons in rat brain slices. Following repeated cocaine administration (15 mg/kg/day i.p. for 5 days) with a 3-day withdrawal, alterations in membrane properties, including increased input resistance, reduced intensity of intracellular injected currents required for generation of Na(+)-dependent spikes (rheobase), and an increased number of spikes evoked by depolarizing current pulses were observed in mPFC neurons. The current-voltage relationship was also altered in cocaine-pretreated neurons showing reduced outward rectification during membrane depolarization and decreased inward rectification during membrane hyperpolarization. Application of the K(+) channel blocker Ba(2+) depolarized the resting membrane potential (RMP) and enhanced membrane potential response to injection of hyperpolarizing current pulses. However, the effects of Ba(2+) on RMP and hyperpolarized membrane potentials were significantly attenuated in cocaine-withdrawn neurons compared with saline-pretreated cells. These findings indicate that repeated cocaine administration increased the excitability of mPFC neurons after a short-term withdrawal, possibly via reducing the activity of the potassium inward rectifiers (K(ir)) and voltage-gated K(+) currents. Similar changes were also observed in cocaine-pretreated mPFC neurons after a long-term (2-3 weeks) withdrawal, revealing a persistent increase in excitability. These alterations in mPFC neuronal excitability may contribute to the development of behavioral sensitization and withdrawal effects following chronic cocaine exposure.