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 ϩ currents (VGKCs) and a possible increase in voltage-sensitive Ca 2ϩ 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 Ca 2ϩ plateau potential was also studied in mPFC neurons. Repeated cocaine administration significantly prolonged the duration of evoked Ca 2ϩ plateau potentials and increased the whole-cell I Ca in mPFC neurons after a 3 d withdrawal. Selective blockade of L-type Ca 2ϩ channels by nifedipine not only significantly increased the threshold but also reduced the duration and amplitude of Ca 2ϩ plateau potentials in both saline-and cocaine-withdrawn mPFC neurons.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in the increased threshold, reduced duration, and decreased amplitude of Ca 2ϩ potentials between saline-and cocaine-withdrawn neurons after blockade of L-type Ca 2ϩ channels. Moreover, an increase in amplitude was also observed, whereas the prolonged duration persisted, in Ca 2ϩ 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 Ca 2ϩ channels, to excitatory stimuli in rat mPFC pyramidal neurons.

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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 ϩ -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 ϩ channel blocker Ba 2ϩ depolarized the resting membrane potential (RMP) and enhanced membrane potential response to injection of hyperpolarizing current pulses. However, the effects of Ba 2ϩ 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 ϩ 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.

Dopamine (DA) modulation of excitability in medial prefrontal cortex (mPFC) pyramidal neurons has attracted considerable attention because of the involvement of mPFC DA in several neuronal disorders. Here, we focused on DA modulation of inwardly rectifying K(+) current (IRKC) in pyramidal neurons acutely dissociated from rat mPFC. A Cs(+)-sensitive whole-cell IRKC was elicited by hyperpolarizing voltage steps from a holding potential of -50 mV. DA (20 microm) reduced IRKC amplitude, as did selective stimulation of DA D(1) or D(2) class receptors (D(1)Rs and D(2)Rs). D(1)Rs activate, whereas D(2)Rs inhibit, the adenylyl cyclase-cAMP-protein kinase A (PKA) signaling pathway. Suppression of IRKC by D(2)R stimulation was attributable to decreased PKA activity because similar inhibition was observed with PKA inhibitors, whereas enhancing PKA activity increased IRKC. This suggests that the DA D(1)R suppression of IRKC occurred through a PKA phosphorylation-independent process. Using outside-out patches of mPFC pyramidal neurons, which preclude involvement of cytosolic signaling molecules, we observed a Cs(+)-sensitive macroscopic IRKC that was suppressed by the membrane-permeable cyclic nucleotide Sp-cAMP but was unaffected by non-nucleotide modulators of PKA, suggesting direct interactions of the cyclic nucleotides with IRK channels. Our results indicate that DA suppresses IRKC through two mechanisms: D(1)R activation of cAMP and direct interactions of the nucleotide with IRK channels and D(2)R-mediated dephosphorylation of IRK channels. The DA modulation of IRKC indicates that ambient DA would tend to increase responsiveness to excitatory inputs when PFC neurons are near the resting membrane potential and may provide a mechanism by which DA impacts higher cognitive function.

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.