Dopamine-depleting lesions of the striatum that mimic Parkinson's disease induce a profound pruning of spines and glutamatergic synapses in striatopallidal medium spiny neurons, leaving striatonigral medium spiny neurons intact. The mechanisms that underlie this cell type-specific loss of connectivity are poorly understood. The Kir2 K(+) channel is an important determinant of dendritic excitability in these cells. Here we show that opening of these channels is potently reduced by signaling through M1 muscarinic receptors in striatopallidal neurons, but not in striatonigral neurons. This asymmetry could be attributed to differences in the subunit composition of Kir2 channels. Dopamine depletion alters the subunit composition further, rendering Kir2 channels in striatopallidal neurons even more susceptible to modulation. Reduced opening of Kir2 channels enhances dendritic excitability and synaptic integration. This cell type-specific enhancement of dendritic excitability is an essential trigger for synaptic pruning after dopamine depletion, as pruning was prevented by genetic deletion of M1 muscarinic receptors.
Dendritically placed, voltage-sensitive ion channels are key regulators of neuronal synaptic integration. In several cell types, hyperpolarization/cyclic nucleotide gated (HCN) cation channels figure prominently in dendritic mechanisms controlling the temporal summation of excitatory synaptic events. In prefrontal cortex, the sustained activity of pyramidal neurons in working memory tasks is thought to depend on the temporal summation of dendritic excitatory inputs. Yet we know little about how this is accomplished in these neurons and whether HCN channels play a role. To gain a better understanding of this process, layer V-VI pyramidal neurons in slices of mouse prelimbic and infralimbic cortex were studied. Somatic voltage-clamp experiments revealed the presence of rapidly activating and deactivating cationic currents attributable to HCN1/HCN2 channels. These channels were open at the resting membrane potential and had an apparent half-activation voltage near Ϫ90 mV. In the same voltage range, K ϩ currents attributable to Kir2.2/2.3 and K ϩ -selective leak (K leak ) channels were prominent. Computer simulations grounded in the biophysical measurements suggested a dynamic interaction among Kir2, K leak , and HCN channel currents in shaping membrane potential and the temporal integration of synaptic potentials. This inference was corroborated by experiment. Blockade of Kir2/K leak channels caused neurons to depolarize, leading to the deactivation of HCN channels, the initiation of regular spiking (4 -5 Hz), and enhanced temporal summation of EPSPs. These studies show that HCN channels are key regulators of synaptic integration in prefrontal pyramidal neurons but that their functional contribution is dependent on a partnership with Kir2 and K leak channels.
The serotonin (5-HT) innervation of the prefrontal cortex (PFC) exerts a powerful modulatory influence on neuronal activity in this cortical region, although the mechanisms through which 5-HT modulates cellular activity are unclear. Voltage-dependent Na+ channels are one potential target of 5-HT receptor signaling that have wide-ranging effects on activity. Molecular and electrophysiological studies were used to test this potential linkage. Single cell RT-PCR profiling revealed that the vast majority of pyramidal neurons expressed detectable levels of 5-HT2a and/or 5-HT2c receptor mRNA with half of the cells expressing both mRNAs. Whole-cell voltage-clamp recordings of dissociated pyramidal neurons showed that 5-HT2a/c receptor activation reduced rapidly inactivating Na+ currents by reducing maximal current amplitude and shifting fast inactivation voltage dependence. These effects were mediated by G(q) activation of phospholipase C, leading to activation of protein kinase C (PKC). 5-HT2a/c receptor stimulation also reduced the amplitude of persistent Na+ current without altering its activation voltage dependence. This modulation was also mediated by PKC. Although 5-HT(2a,c) receptor activation did not affect somatic action potentials of layer V pyramidal neurons in PFC slices, it did reduce the amplitude of action potentials backpropagating into the apical dendrite. These findings show that 5-HT2a,c receptor activation reduces dendritic excitability and may negatively modulate activity-dependent dendritic synaptic plasticity.
The striatum is richly innervated by serotonergic afferents from the raphe nucleus. We explored the effects of this input on striatal cholinergic interneurons from rat brain slices, by means of both conventional intracellular and whole-cell patch-clamp recordings. Bathapplied serotonin (5-HT, 3-300 mM), induced a dose-dependent membrane depolarization and increased the rate of spiking. This effect was mimicked by the 5-HT reuptake blockers citalopram and fluvoxamine. In voltage-clamped neurons, 5-HT induced an inward current, whose reversal potential was close to the K + equilibrium potential. Accordingly, the involvement of K + channels was confirmed either by increasing extracellular K + concentration and by blockade of K + channels with barium. Single-cell reverse transcriptase-polymerase chain reaction (RT-PCR) profiling demonstrated the presence of 5-HT2C, 5-HT6, and 5-HT7 receptor mRNAs in identified cholinergic interneurons. The depolarization/inward current induced by 5-HT was partially mimicked by the 5-HT2 receptor agonist 2,5-dimethoxy-4-iodoamphetamine and antagonized by both ketanserin and the selective 5-HT2C antagonist RS102221, whereas the selective 5-HT3 and 5-HT4 receptor antagonists tropisetron and RS23597-190 had no effect. The depolarizing response to 5-HT was also reduced by the selective 5-HT6 and 5-HT7 receptor antagonists SB258585 and SB269970, respectively, and mimicked by the 5-HT7 agonist, 5-CT. Accordingly, activation of either 5-HT6 or 5-HT7 receptor induced an inward current. The 5-HT response was attenuated by U73122, blocker of phospholipase C, and by SQ22,536, an inhibitor of adenylyl cyclase. These results suggest that 5-HT released by serotonergic fibers originating in the raphe nuclei has a potent excitatory effect on striatal cholinergic interneurons.
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