Fernando J. Nasif

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 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 (ICa). Here, we performed both current-clamp recordings in brain slices and voltage-clamp recordings in freshly dissociated cells to determine whether ICa 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 ICa 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the threshold but also reduced the duration and amplitude of Ca2+ plateau potentials in both saline- and cocaine-withdrawn mPFC neurons.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 ICa, particularly via the activated L-type Ca2+ channels, to excitatory stimuli in rat mPFC pyramidal neurons.

Dopamine Modulates Inwardly Rectifying Potassium Currents in Medial Prefrontal Cortex Pyramidal Neurons

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 μm) reduced IRKC amplitude, as did selective stimulation of DA D1 or D2 class receptors (D1Rs and D2Rs). D1Rs activate, whereas D2Rs inhibit, the adenylyl cyclase–cAMP–protein kinase A (PKA) signaling pathway. Suppression of IRKC by D2R 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 D1R 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: D1R activation of cAMP and direct interactions of the nucleotide with IRK channels and D2R-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.

Effects of chronic risperidone on central noradrenergic transmission.

In the present work, we investigated the effects of chronic risperidone administration on the activity of locus coeruleus noradrenergic neurons. In addition, the effect of chronic risperidone administration on the basal level of norepinephrine in the prefrontal cortex was evaluated. Results of this research showed that chronic risperidone administration increased the activity of locus coeruleus noradrenergic neurons. The sensitivity of alpha(2)-adrenoceptors in the somatodendritic region of the locus coeruleus was assessed by using the ID(50) of clonidine. Results indicated that the firing rate of locus coeruleus noradrenergic neurons was the same in risperidone-treated rats and controls. Similarly, the ID(50) for (+/-)-2,5-dimetoxy-4-iodoamphetamine (DOI), an agonist of 5-HT(2) receptors which inhibits the activity of locus coeruleus neurons by acting on these receptors, did not show any differences between the firing rate of these neurons in risperidone treated rats and controls. Unlike controls, chronically treated rats showed a significant decrease in norepinephrine levels in the prefrontal cortex. The decreased release of norepinephrine following continuous risperidone administration could be explained by the sustained increase in locus coeruleus neuronal activity after chronic risperidone administration. This low norepinephrine level in the prefrontal cortex may contribute to the relief of certain negative schizophrenic symptoms and to the improvement of cognitive function.

Permanent alteration of central noradrenergic system by prenatally administered amphetamine

Amphetamine-induced psychosis is frequently associated with a chronic, high-dose, daily pattern of amphetamine exposure. In the present study we investigate the effects of prenatal exposure to amphetamine during the development of the central noradrenergic (NA) system in adult rats. Pregnant Wistar rats were given 4 mg/kg/day of d-amphetamine (AMPH), subcutaneously, from gestational day 8 to 21. No additional drug treatment was given to the animals until the beginning of the experiments, in adult, control and prenatally amphetamine treated rats. Since we study the electrophysiology and neurochemistry of the central NA system, we investigated the electric activity of locus coeruleus (LC) norepinephrine (NE) neurons and the levels of NE on prefrontal cortex. What we found, was a decreased number of spontaneously active cells in the LC nucleus with a lower pattern of discharge whereas, the basal levels of NE in the prefrontal cortex, was greatly increased. The increased cortical NE levels, observed in the present study may account for the proposed hyperactive NA system being responsible for some psychotic symptoms observed in paranoid schizophrenia. Besides, our results concerning the permanent alteration observed in the central NA system, in rats prenatally exposed to amphetamine, raise the possibility that this animal model may be useful to further study the neurobiologic alterations underlying certain clinical features involved in some psychosis such as schizophrenia.

Hippocampus and locus coeruleus activity on rats chronically treated with diazepam

The neural mechanisms underlying benzodiazepine (BZD) dependence remain equivocal. The present studies tested the hypothesis that similar neural circuitry might be involved in the effects of chronic 7-chloro-1-methyl-5-phenyl-3H-1,4-benzodiazepine-2(1H)-one, diazepam (DZ, Roche), administration and withdrawal. The results of our study showed an increased hippocampal synaptic plasticity in slices from rats chronically treated with DZ (5 mg/kg/18 days), assessed as a decrease of the threshold in the stimulation rate for long-term potentiation (LTP) elicitation. Rats with the same schedule of DZ administration but without signs of withdrawal behaved similarly to vehicle-treated ones (VEH), in the threshold to induce LTP. Furthermore, the activity of locus coeruleus (LC) norepinephrine (NE) neurons in rats tested 24 h after the last DZ injection showed a significant increase. On the other hand, rats that after chronic DZ administration did not develop signs of withdrawal and exhibited a similar pattern of discharge on LC-NE nucleus compared with their controls. We conclude that chronic DZ administration enhances both hippocampal synaptic plasticity and activity of LC-NE neurons. This neural system could be the biological substrate underlying the behavioral alterations accompanying chronic DZ administration and withdrawal.

Increased neuronal activity in locus coeruleus from adult rats undernourished at perinatal age: Its reversal by desipramine

The spontaneous activity of locus coeruleus (LC) noradrenergic neurons was assessed by single unit recording in adult recovered rats undernourished at perinatal age as compared with wellnourished animals. Locus coeruleus activity, measured by the firing rate of noradrenergic neurons and the number of spontaneously active cells/track was significantly higher in deprived rats than in controls. In addition, dose-response curves for the inhibitory LC activity of clonidine showed a shift to the right in deprived animals indicating a subsensitivity of alpha2-adrenergic autoreceptors. This fact suggests an alteration in the negative feedback mechanism mediated by somatodentritic alpha2 autoreceptors that modulate the activity of LC neurons, and may account for the behavioral alterations attributed to early undernutrition. Repeated desipramine (DMI) administration to deprived rats reduced LC activity to values comparable to controls, which were not affected after a similar treatment. These data extend to previous reports on long-lasting or permanent plastic changes in the CNS induced by early undernutrition, which may be reverted by pharmacological manipulations. In addition, these results support the hypothesis that alterations induced by early undernutrition are in the same direction as and resemble those described for patients with panic disorders. Furthermore, together with behavioral alterations and selective anxiolytic effect of DMI and other drugs with antipanic effects described in early malnourished rats, the present data support the proposal that perinatally deprived rats may be a useful model for screening drugs with potential antipanic activity.

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.