Amphetamine modulation of long-term potentiation in the prefrontal cortex: dose dependency, monoaminergic contributions, and paradoxical rescue in hyperdopaminergic mutant.

Amphetamine can improve cognition in healthy subjects and patients with schizophrenia, attention-deficit hyperactivity disorder, and other neuropsychiatric diseases; higher doses, however, can impair cognitive function, especially those mediated by the prefrontal cortex. We investigated how amphetamine affects prefrontal cortex long-term potentiation (LTP), a cellular correlate of learning and memory, in normal and hyperdopaminergic mice lacking the dopamine transporter. Acute amphetamine treatment in wild-type mice produced a biphasic dose-response modulation of LTP, with a low dose enhancing LTP and a high dose impairing it. Amphetamine-induced LTP enhancement was prevented by pharmacological blockade of D(1) - (but not D(2)-) class dopamine receptors, by blockade of ß-adrenergic receptors, or by inhibition of cAMP-PKA signaling. In contrast, amphetamine-induced LTP impairment was prevented by inhibition of post-synaptic protein phosphatase-1, a downstream target of PKA signaling, or by blockade of either D(1) - or D(2)-class dopamine, but not noradrenergic, receptors. Thus, amphetamine biphasically modulates LTP via cAMP-PKA signaling orchestrated mainly through dopamine receptors. Unexpectedly, amphetamine restored the loss of LTP in dopamine transporter-knockout mice primarily by activation of the noradrenergic system. Our results mirror the biphasic effectiveness of amphetamine in humans and provide new mechanistic insights into its effects on cognition under normal and hyperdopaminergic conditions.

D1 and D2 dopamine receptors in separate circuits cooperate to drive associative long-term potentiation in the prefrontal cortex.

Dopamine release associated with motivational arousal is thought to drive goal-directed learning and consolidation of acquired memories. This dopamine hypothesis of learning and motivation directly suggests that dopamine is necessary for modifications of excitatory synapses in dopamine terminal fields, including the prefrontal cortex (PFC), to "stamp in" posttrial memory traces. It is unknown how such enabling occurs in native circuits tightly controlled by GABAergic inhibitory tone. Here we report that dopamine, via both D1-class receptors (D1Rs) and D2-class receptors (D2Rs), enables the induction of spike timing-dependent long-term potentiation (t-LTP) in layer V PFC pyramidal neurons over a "window" of more than 30 ms that is otherwise closed under intact inhibitory constraint. Dopamine acts at D2Rs in local GABAergic interneurons to suppress inhibitory transmission, gating the induction of t-LTP. Moreover, dopamine activates postsynaptic D1Rs in excitatory synapses to allow t-LTP induction at a substantially extended, normally ineffective, timing interval (+30 ms), thus increasing the associability of prepost coincident stimuli. Although the D2R-mediated disinhibition alone is sufficient to gate t-LTP at a normal timing (+10 ms), t-LTP at +30 ms requires concurrent activation of both D1Rs and D2Rs. Our results illustrate a previously unrecognized circuit-level mechanism by which dopamine receptors in separate microcircuits cooperate to drive Hebbian synaptic plasticity across a significant temporal window under intact inhibition. This mechanism should be important in functioning of interconnected PFC microcircuits, in which D1Rs and D2Rs are not colocalized but their coactivation is necessary.

Hyperdopaminergic tone erodes prefrontal long-term potential via a D2 receptor-operated protein phosphatase gate.

Dopamine (DA) plays crucial roles in the cognitive functioning of the prefrontal cortex (PFC), which, to a large degree, depends on lasting neural traces formed in prefrontal networks. The establishment of these permanent traces requires changes in cortical synaptic efficacy. DA, via the D(1)-class receptors, is thought to gate or facilitate synaptic plasticity in the PFC, with little role recognized for the D(2)-class receptors. Here we show that, when significantly elevated, DA erodes, rather than facilitates, the induction of long-term potentiation (LTP) in the PFC by acting at the far less abundant cortical D(2)-class receptors through a dominant coupling to the protein phosphatase 1 (PP1) activity in postsynaptic neurons. In mice with persistently elevated extracellular DA, resulting from inactivation of the DA transporter (DAT) gene, LTP in layer V PFC pyramidal neurons cannot be established, regardless of induction protocols. Acute increase of dopaminergic transmission by DAT blockers or overstimulation of D(2) receptors in normal mice have similar LTP shutoff effects. LTP in mutant mice can be rescued by a single in vivo administration of D(2)-class antagonists. Suppression of postsynaptic PP1 mimics and occludes the D(2)-mediated rescue of LTP in mutant mice and prevents the acute erosion of LTP by D(2) agonists in normal mice. Our studies reveal a mechanistically unique heterosynaptic PP1 gate that is constitutively driven by background DA to influence LTP induction. By blocking prefrontal synaptic plasticity, excessive DA may prevent storage of lasting memory traces in PFC networks and impair executive functions.

PSD-95 uncouples dopamine-glutamate interaction in the D1/PSD-95/NMDA receptor complex.

Classical dopaminergic signaling paradigms and emerging studies on direct physical interactions between the D(1) dopamine (DA) receptor and the NMDA glutamate receptor predict a reciprocally facilitating, positive feedback loop. This loop, if not controlled, may cause concomitant overactivation of both D(1) and NMDA receptors, triggering neurotoxicity. Endogenous protective mechanisms must exist. Here, we report that PSD-95, a prototypical structural and signaling scaffold in the postsynaptic density, inhibits D(1)-NMDA receptor subunit 1 (NR1) NMDA receptor association and uncouples NMDA receptor-dependent enhancement of D(1) signaling. This uncoupling is achieved, at least in part, via a disinhibition mechanism by which PSD-95 abolishes NMDA receptor-dependent inhibition of D(1) internalization. Knockdown of PSD-95 immobilizes D(1) receptors on the cell surface and escalates NMDA receptor-dependent D(1) cAMP signaling in neurons. Thus, in addition to its role in receptor stabilization and synaptic plasticity, PSD-95 acts as a brake on the D(1)-NMDA receptor complex and dampens the interaction between them.

Divalent cation modulation of a-type potassium channels in acutely dissociated central neurons from wide-type and mutant Drosophila.

Drosophila mutants provide an ideal model to study channel-type specificity of ion channel regulation in situ. In this study, the effects of divalent cations on voltage-gated K+ currents were investigated in acutely dissociated central neurons of Drosophila third instar larvae using the whole-cell patch-clamp recording. Our data showed that micromolar Cd2+ enhanced the peak inactivating current (I(A)) without affecting the delayed component (I(K)). The same results were obtained in Ca(2+)-free external solution, and from slo1 mutation, which eliminates transient Ca(2+)-activated K+ current. Micromolar Cd2+ and Zn2+, and millimolar Ca2+ and Mg2+ all shifted the steady-state inactivation curve of I(A) without affecting the voltage-dependence of I(A) activation, whereas millimolar Cd2+ markedly affected both the activation and steady-state inactivation curves for I(A). Divalent cations affected I(A) with different potency; the sequence was: Zn2+ > Cd2+ > Ca2+ > Mg2+. The modulation of I(A) by Cd2+ was partially inhibited in Sh(M), a null Shaker (one of I(A)-encoding genes) mutation. Taken together, the channel-type specificity, the asymmetric effects on I(A) activation and inactivation kinetics, and the diverse potency of divalent cations all strongly support the idea that physiological divalent cations modulate A-type K+ channels through specific binding to extracellular sites of the channels.

Inhibitors of GlyT1 and GlyT2 differentially modulate inhibitory transmission.

The chronic effects of glycine transporter 1 and 2 inhibitors (sarcosine and ALX-1393, respectively) on miniature inhibitory postsynaptic currents were studied in cultured spinal neurons. We found that sarcosine increased the frequency of overall miniature inhibitory postsynaptic currents without affecting the ratio of glycinergic, mixed and GABAergic miniature inhibitory postsynaptic currents, whereas ALX-1393 changed the ratio by increasing the proportions of GABAergic and mixed miniature inhibitory postsynaptic currents without affecting overall mIPSC frequency. We propose that inhibition of glycine transporter 1 by sarcosine increased overall mIPSC frequency via the activation of presynaptic glycine receptors, while inhibition of glycine transporter 2 by ALX-1393 changed the ratio of glycinergic, mixed and GABAergic miniature inhibitory postsynaptic currents by shifting the balance of inhibitory transmitters in vesicles towards gamma-aminobutyric acid.

[Properties of whole-cell potassium currents in mechanically dissociated Drosophila larval central neurons].

By electrophysiological methods, cultured Drosophila embryonic and larval central neurons have been widely used to study ion channels, neurotransmitter release and intracellular message regulation. Voltage-activated K(+) channels play a crucial role in repolarizing the membrane following action potentials, stabilizing membrane potentials and shaping firing patterns of cells. In this study, a mechanical vibration-isolation system was used to produce a sufficient number of acutely dissociated larval central neurons, of which the majority were type II neurons (2~5 microm in diameter). Using patch clamp technique, the whole-cell K(+) currents in type II neurons were characterized by containing a transient 4-AP-sensitive current (I(A)) and a more slowly inactivating, TEA-sensitive component (I(K)). According to their kinetic properties, five types of whole-cell K(+) currents were identified. Type A current exhibited primarily fast transient K(+) currents that activated and inactivated rapidly. The majority of the neurons, however, slowly inactivated K(+) currents with variable inactivation time course (type B current). Type C current, being present in a small number of the cells, was mainly composed of noninactivating components. Some of the neurons expressed both transient and slow inactivating components, but the slowly inactivating components could reach more than 50% of the peak current (type D current). Type E current showed distinct voltage-dependent activation properties, characterized by its bell-shaped activation curve. Type E current was inhibited by application of Ca(2+)-free solution or 0.1 mmol/L Cd(2+). Moreover, this novel current ran down much more rapidly than other types. These results indicate that different K(+) channels, which have different kinetic and pharmacological properties, underlie the whole-cell K(+) currents in type II neurons of Drosophila larval central nervous system.