Passive properties of neostriatal neurons during potassium conductance blockade

Functional comparison of corticostriatal and thalamostriatal postsynaptic responses in striatal neurons of the mouse

Synaptic inputs from cortex and thalamus were compared in electrophysiologically defined striatal cell classes: direct and indirect pathways' striatal projection neurons (dSPNs and iSPNs), fast-spiking interneurons (FS), cholinergic interneurons (ChINs), and low-threshold spiking-like (LTS-like) interneurons. Our purpose was to observe whether stimulus from cortex or thalamus had equivalent synaptic strength to evoke prolonged suprathreshold synaptic responses in these neuron classes. Subthreshold responses showed that inputs from either source functionally mix up in their dendrites at similar electrotonic distances from their somata. Passive and active properties of striatal neuron classes were consistent with the previous studies. Cre-dependent adeno-associated viruses containing Td-Tomato or eYFP fluorescent proteins were used to identify target cells. Transfections with ChR2-eYFP driven by the promoters CamKII or EF1.DIO in intralaminar thalamic nuclei using Vglut-2-Cre mice, or CAMKII in the motor cortex were used to stimulate cortical or thalamic afferents optogenetically. Both field stimuli in the cortex or photostimulation of ChR2-YFP cortical fibers evoked similar prolonged suprathreshold responses in SPNs. Photostimulation of ChR2-YFP thalamic afferents also evoked suprathreshold responses. Differences previously described between responses of dSPNs and iSPNs were observed in both cases. Prolonged suprathreshold responses could also be evoked from both sources onto all other neuron classes studied. However, to evoke thalamostriatal suprathreshold responses, afferents from more than one thalamic nucleus had to be stimulated. In conclusion, both thalamus and cortex are capable to generate suprathreshold responses converging on diverse striatal cell classes. Postsynaptic properties appear to shape these responses.

Morphological characterization of respiratory neurons in the pre-Bötzinger complex

Although the pre-Bötzinger complex (preBötC) was defined as the inspiratory rhythm generator long ago, the functional-anatomical characterization of its neuronal components is still being achieved. Recent advances have identified the expression of molecular markers in the preBötC neurons that, however, are not exclusive to specific respiratory neuron subtypes and have not always been related to specific cell morphologies. Here, we evaluated the morphology and the axonal projections of electrophysiologically defined respiratory neurons in the preBötC using whole-cell recordings and intracellular biocytin labeling. We found that respiratory pacemaker neurons are larger than expiratory neurons and that inspiratory neurons are smaller than pacemaker and expiratory neurons. Other morphological features such as somata shapes or dendritic branching patterns were not found to be significantly different among the preBötC neurons sampled. We also found that both pacemaker and inspiratory nonpacemaker neurons, but not expiratory neurons, show extensive axonal projections to the contralateral preBötC and show signs of electrical coupling. Overall, our data suggest that there are morphological differences between subtypes of preBötC respiratory neurons. It will be important to take such differences in consideration since morphological differences would influence synaptic responses and action potential propagation.

Dopaminergic modulation of striatal neurons, circuits, and assemblies

In recent years, there has been a great deal of progress toward understanding the role of the striatum and dopamine in action selection. The advent of new animal models and the development of optical techniques for imaging and stimulating select neuronal populations have provided the means by which identified synapses, cells, and circuits can be reliably studied. This review attempts to summarize some of the key advances in this broad area, focusing on dopaminergic modulation of intrinsic excitability and synaptic plasticity in canonical microcircuits in the striatum as well as recent work suggesting that there are neuronal assemblies within the striatum devoted to particular types of computation and possibly action selection.

Different corticostriatal integration in spiny projection neurons from direct and indirect pathways

The striatum is the principal input structure of the basal ganglia. Major glutamatergic afferents to the striatum come from the cerebral cortex and make monosynaptic contacts with medium spiny projection neurons (MSNs) and interneurons. Also: glutamatergic afferents to the striatum come from the thalamus. Despite differences in axonal projections, dopamine (DA) receptors expression and differences in excitability between MSNs from “direct” and “indirect” basal ganglia pathways, these neuronal classes have been thought as electrophysiologically very similar. Based on work with bacterial artificial chromosome (BAC) transgenic mice, here it is shown that corticostriatal responses in D₁- and D₂-receptor expressing MSNs (D₁- and D₂-MSNs) are radically different so as to establish an electrophysiological footprint that readily differentiates between them. Experiments in BAC mice allowed us to predict, with high probability (P > 0.9), in rats or non-BAC mice, whether a recorded neuron, from rat or mouse, was going to be substance P or enkephalin (ENK) immunoreactive. Responses are more prolonged and evoke more action potentials in D₁-MSNs, while they are briefer and exhibit intrinsic autoregenerative responses in D₂-MSNs. A main cause for these differences was the interaction of intrinsic properties with the inhibitory contribution in each response. Inhibition always depressed corticostriatal depolarization in D₂-MSNs, while it helped in sustaining prolonged depolarizations in D₁-MSNs, in spite of depressing early discharge. Corticostriatal responses changed dramatically after striatal DA depletion in 6-hydroxy-dopamine (6-OHDA) lesioned animals: a response reduction was seen in substance P (SP)+ MSNs whereas an enhanced response was seen in ENK+ MSNs. The end result was that differences in the responses were greatly diminished after DA depletion.

Alterations in cortical excitation and inhibition in genetic mouse models of Huntington's disease

Previously, we identified progressive alterations in spontaneous EPSCs and IPSCs in the striatum of the R6/2 mouse model of Huntington's disease (HD). Medium-sized spiny neurons from these mice displayed a lower frequency of EPSCs, and a population of cells exhibited an increased frequency of IPSCs beginning at approximately 40 d, a time point when the overt behavioral phenotype begins. The cortex provides the major excitatory drive to the striatum and is affected during disease progression. We examined spontaneous EPSCs and IPSCs of somatosensory cortical pyramidal neurons in layers II/III in slices from three different mouse models of HD: the R6/2, the YAC128, and the CAG140 knock-in. Results revealed that spontaneous EPSCs occurred at a higher frequency, and evoked EPSCs were larger in behaviorally phenotypic mice whereas spontaneous IPSCs were initially increased in frequency in all models and subsequently decreased in R6/2 mice after they displayed the typical R6/2 overt behavioral phenotype. Changes in miniature IPSCs and evoked IPSC paired-pulse ratios suggested altered probability of GABA release. Also, in R6/2 mice, blockade of GABA(A) receptors induced complex discharges in slices and seizures in vivo at all ages. In conclusion, altered excitatory and inhibitory inputs to pyramidal neurons in the cortex in HD appear to be a prevailing deficit throughout the development of the disease. Furthermore, the differences between synaptic phenotypes in cortex and striatum are important for the development of future therapeutic approaches, which may need to be targeted early in the development of the phenotype.

Electrical synapses in basal ganglia

The basal ganglia (BG) provide a major integrative system of the forebrain involved in the organization of goal-directed behaviour. Pathological alteration of BG function leads to major motor and cognitive impairments such as observed in Parkinson's disease. Recent advances in BG research stress the role of neural oscillations and synchronization in the normal and pathological function of BG. As demonstrated in several brain structures, these patterns of neural activity can emerge from electrically coupled neuronal networks. This review aims at addressing the presence, functionality and putative role of electrical synapses in BG, with a particular emphasis on the striatum and the substantia nigra pars compacta (SNc), two main BG nuclei in which the existence and functional properties of neuronal coupling are best documented.

Delta opioids reduce the neurotransmitter release probability by enhancing transient (KV4) K+-currents in corticostriatal synapses as evaluated by the paired pulse protocol

Field recordings were used to determine the influence of delta-opioid receptor activation on corticostriatal synaptic transmission. Application of the selective delta-opioid receptor agonist, [Tyr-D-Pen-Gly-Phe-D-Pen]-enkephalin (DPDPE, 1 microM), decreased the amplitude of the field-excitatory synaptic potential and at the same time increased the paired pulse ratio (PPR) suggesting a presynaptic site of action. This response reversed rapidly when DPDPE was washed and blocked by 1 nM of the selective delta-receptor antagonist naltrindole. Neither omega-conotoxin GVIA (1 microM) nor omega-agatoxin TK (400 nM), blockers of N- and P/Q-type Ca2+-channels, respectively, nor TEA (1 mM), blocker of some classes of K+-channels, occluded the effects of DPDPE. Instead, 1 mM 4-AP or 400 microM Ba2+ occluded completely the effects of DPDPE. Therefore, the results suggest that the modulation by delta opioids at corticostriatal terminals is mediated by transient (KV4) K+-conductances.

GABAergic inhibition in the neostriatum

In the neostriatum, GABAergic inhibition arises from the action of at least two classes of inhibitory interneurons, and from recurrent collaterals of the principal cells. Interneurons receive excitatory input only from extrinsic sources, and so act in a purely feedforward capacity. Feedback inhibition arises from the recurrent collaterals of the principal cells. These two kinds of inhibition have functionally distinct effects on the principal cells. Inputs from interneurons are not very convergent. There are few inhibitory neurons, and so each principal cell receives inhibitory synaptic input from very few interneurons. But, they are individually powerful, and a single interneuron can substantially delay action potentials in a group of nearby principal cells. Recurrent inhibition is highly convergent, with each principal cell receiving inhibitory input from several hundred other such cells. Feedback inhibitory synaptic inputs individually have very weak effects, as seen from the soma. The differences in synaptic strength are not caused by differences in the release of transmitter or in sensitivity of the postsynaptic membrane. Rather, they arise from differences in the number of synaptic contacts formed on individual principal cells by feedforward or feedback axons, and from differences in synaptic location. Interneurons form their powerful synapses near the somata of principal cells, while most feedback synapses are more distal, where they interact with the two-state nonlinear properties of the principal cells' dendrite. This arrangement suggests that feedforward inhibition may serve in the traditional role for inhibition, adjusting the excitability of the principle neuron near the site of action potential generation. Feedback inhibitory synapses may interact with voltage-sensitive conductances in the dendrite to alter the electrotonic structure of the spiny cell.

A comparative voltage and current-clamp analysis of feedback and feedforward synaptic transmission in the striatal microcircuit in vitro

Striatal spiny projection (SP) neurons control movement initiation by integrating cortical inputs and inhibiting basal ganglia outputs. Central to this control lies a "microcircuit" that consists of a feedback pathway formed by axon collaterals between GABAergic SP neurons and a feedforward pathway from fast spiking (FS) GABAergic interneurons to SP neurons. Here, somatically evoked postsynaptic potentials (PSPs) and currents (PSCs) were compared for both pathways with dual whole cell patch recording in voltage- and current-clamp mode using cortex-striatum-substantia nigra organotypic cultures. On average, feedforward inputs were 1 ms earlier, more reliable, and about twice as large in amplitude compared with most feedback inputs. On the other hand, both pathways exhibited widely varying, partially overlapping amplitude distributions. This variability was already established for single FS neurons targeting many SP neurons. In response to precisely timed action potential bursts, feedforward and feedback inputs consistently showed short-term depression < or =50-70% in voltage-clamp, although feedback inputs also displayed strong augmentation in current-clamp in line with previous reports. The augmentation of feedback inputs was absent in gramicidin D perforated-patch recording, which also showed the natural reversal potential for both inputs to be near firing threshold. Preceding depolarizing feedback inputs during the down state did not consistently change subsequent postsynaptic action potentials. We conclude that feedback and feedforward inputs have their dominant effect during the up-state. The reversal potential close to the up-state potential, which supports shunting operation with millisecond precision and the strong synaptic depression, should enable both pathways to carry time-critical information.

Neuronal vulnerability in mouse models of Huntington's disease: membrane channel protein changes

Huntington's disease (HD) is caused by a polyglutamine expansion that results in atrophy of the striatum and frontal cortex during disease progression. HD-susceptible striatal neurons are affected chronologically with initial degeneration of the striatopallidal neurons then the striatonigral projections, whereas large aspiny striatal interneurons (LAN) survive. Two classes of critical membrane proteins were evaluated in transgenic mouse models to determine their association with HD susceptibility, which leads to dysfunction and death in selected striatal neuron populations. We examined potassium (K+) channel protein subunits that form membrane ionophores conducting inwardly and outwardly rectifying K+ currents. K+ channel protein staining was diminished substantially in the HD striatal projection neurons but was not expressed in the HD-resistant LAN. Because loss of K+ channel subunits depolarizes neurons, other voltage-gated ionophores will be affected. N-methyl-D-aspartate (NMDA) receptors and their phosphorylation by cyclic AMP were studied as a mechanism contributing to excitotoxic vulnerability in striatal projection neurons that would lose voltage regulation after diminished K+ channels. NR1 subunits showed significant elevation in the HD transgenic projection systems but were expressed at very low levels in LAN. NR1 subunit phosphorylation by cyclic AMP also was enhanced in striatal projection neurons but not in LAN. Cyclic AMP-driven phosphorylation of NMDA receptors increases the channel open time and elevates neuronal glutamate responsiveness, which may lead to excitotoxicity. Together our data suggest that changes in these proteins and their modification may predispose striatal projection neurons to dysfunction and then degeneratation in HD and provide a mechanism for LAN resistance in the disease.

Comparison of IPSCs evoked by spiny and fast-spiking neurons in the neostriatum

Most neurons in the neostriatum are GABAergic spiny projection neurons with extensive local axon collaterals innervating principally other spiny projection neurons. The other source of GABAergic inputs to spiny neurons derives from a small number of interneurons, of which the best characterized are the parvalbumin-containing, fast-spiking interneurons. Spiny neuron collateral inhibition was not demonstrated until recently, because the IPSPs recorded at the soma are surprisingly small. In contrast, interneuronal inhibition was readily detected, comprising much larger IPSPs. Here, we report the application of quantal analysis and compartmental modeling to compare and contrast IPSCs in spiny neurons originating from axon collaterals and interneurons. The results indicate that individual release sites at spiny and interneuron synapses have similar quantal sizes and baseline release probabilities. Interneuronal unitary IPSCs are several times larger because of their proximal location on the neuron and because they have a larger number of transmitter release sites. Despite the small amount of current they can deliver to the soma, spiny cell collateral synapses had moderately high baseline release probabilities (0.5-0.9), suggesting that they are not weak because of some form of depression or modulation. The size of unitary collateral synaptic currents increased monotonically during development. These results argue against models of competitive inhibition in neostriatum, including those in which competitive inhibition is transiently effective during development and learning, and suggest a different role for the spiny cell axon collaterals.

Striatal potassium channel dysfunction in Huntington's disease transgenic mice

Huntington's disease (HD) is a neurodegenerative disorder that mainly affects the projection neurons of the striatum and cerebral cortex. Genetic mouse models of HD have shown that neurons susceptible to the mutation exhibit morphological and electrophysiological dysfunctions before and during development of the behavioral phenotype. We used HD transgenic mouse models to examine inwardly and outwardly rectifying K+ conductances, as well as expression of some related K+ channel subunits. Experiments were conducted in slices and dissociated cells from two mouse models, the R6/2 and TgCAG100, at the beginning and after full development of overt behavioral phenotypes. Striatal medium-sized spiny neurons (MSNs) from symptomatic transgenic mice had increased input resistances, depolarized resting membrane potentials, and reductions in both inwardly and outwardly rectifying K+ currents. These changes were more dramatic in the R6/2 model than in the TgCAG100. Parallel immunofluorescence studies detected decreases in the expression of K+ channel subunit proteins, Kir2.1, Kir2.3, and Kv2.1 in MSNs, which contribute to the formation of the channel ionophores for these currents. Attenuation in K+ conductances and channel subunit expression contribute to altered electrophysiological properties of MSNs and may partially account for selective cellular vulnerability in the striatum.

Spontaneous voltage oscillations in striatal projection neurons in a rat corticostriatal slice

In a rat corticostriatal slice, brief, suprathreshold, repetitive cortical stimulation evoked long-lasting plateau potentials in neostriatal neurons. Plateau potentials were often followed by spontaneous voltage transitions between two preferred membrane potentials. While the induction of plateau potentials was disrupted by non-NMDA and NMDA glutamate receptor antagonists, the maintenance of spontaneous voltage transitions was only blocked by NMDA receptor and L-type Ca2+ channel antagonists. The frequency and duration of depolarized events, resembling up-states described in vivo, were increased by NMDA and L-type Ca2+ channel agonists as well as by GABAA receptor and K+ channel antagonists. NMDA created a region of negative slope conductance and a positive slope crossing indicative of membrane bistability in the current-voltage relationship. NMDA-induced bistability was partially blocked by L-type Ca2+ channel antagonists. Although evoked by synaptic stimulation, plateau potentials and voltage oscillations could not be evoked by somatic current injection--suggesting a dendritic origin. These data show that NMDA and L-type Ca2+ conductances of spiny neurons are capable of rendering them bistable. This may help to support prolonged depolarizations and voltage oscillations under certain conditions.

Muscarinic receptors involved in the subthreshold cholinergic actions of neostriatal spiny neurons

Administration of the peptide MT-1 (48 nM), a selective agonist of muscarinic M(1)-type receptors, mimicked the subthreshold actions of muscarine (1 microM) on neostriatal neurons, i.e., it produced a reduction in subthreshold inward rectification leading to an enhancement in input resistance (R(N)) and evoked discharge. In all recorded cells, MT-1 effects remained in the presence of the specific peptidergic antagonist of the M(4)-type receptor, MT-3 (10 nM), but were blocked by the specific M(1)-type receptor antagonist MT-7 (5 nM). These results suggest that most muscarinic facilitatory actions in the subthreshold voltage range occur through M(1)-type receptors. However, in a fraction of cells (40%) muscarine produced an excitability enhancement not blocked by MT-7. This additional facilitatory action, not present when using MT-1, was blocked by MT-3, suggesting it was mediated by M(4)-type receptor activation. This facilitation could not be blocked by Cs(+), TTX, or Cd(2+), but only by a reduction in extracellular sodium. This result is the first evidence that M(4)-type receptor activation enhances a cationic inward current in a fraction of neostriatal projection neurons.

Role of potassium conductances in determining input resistance of developing brain stem motoneurons

The role of potassium conductances in determining input resistance was studied in 166 genioglossal (GG) motoneurons using sharp electrode recording in brain stem slices of the rats aged 5-7 days, 13-15 days, and 19-24 days postnatal (P). A high magnesium (Mg(2+); 6 mM) perfusate was used to block calcium-mediated synaptic release while intracellular or extracellular cesium (Cs(+)) and/or extracellular tetraethylammonium (TEA) or barium (Ba(2+)) were used to block potassium conductances. In all cases, the addition of TEA to the high Mg(2+) perfusate generated a larger increase in both input resistance (R(n)) and the first membrane time constant (tau(0)) than did high Mg(2+) alone indicating a substantial nonsynaptic contribution to input resistance. With intracellular injection of Cs(+), GG motoneurons with lower resistance (<40 MOmega), on the average, showed a larger percent increase in R(n) than cells with higher resistance (>40 MOmega). There was also a significant increase in the effect of internal Cs(+) on R(n) and tau(0) with age. The largest percent increase (67%) in the tau(0) due to intracellular Cs(+) occurred at P13-15, a developmental stage characterized by a large reduction in specific membrane resistance. Addition of external Cs(+) blocked conductances (further increasing R(n) and tau(0)) beyond those blocked by the TEA perfusate. Substitution of external calcium with 2 mM barium chloride produced a significant increase in both R(n) and tau(0) at all ages studied. The addition of either intracellular Cs(+) or extracellular Ba(2+) created a depolarization shift of the membrane potential. The amount of injected current required to maintain the membrane potential was negatively correlated with the control R(n) of the cell at most ages. Thus low resistance cells had, on the average, more Cs(+)- and Ba(2+)-sensitive channels than their high resistance counterparts. There was also a disproportionately larger percent increase in tau(0) as compared with R(n) for both internal Cs(+) and external Ba(2+). Based on a model by Redman and colleagues, it might be suggested that the majority of these potassium conductances underlying membrane resistance are initially located in the distal dendrites but become more uniformly distributed over the motoneuron surface in the oldest animals.

Action of substance P (neurokinin-1) receptor activation on rat neostriatal projection neurons

Substance P (SP) acts as a neurotransmitter in the neostriatum through the axon collaterals of spiny projection neurons. However, possible direct or indirect actions of SP on the neostriatal output neurons have not been described. Targets of SP terminals within the neostriatum include interneurons, spiny neurons, afferent fibers and boutons. SP induces the release of both dopamine (DA) and acetylcholine (ACh). Since some postsynaptic actions of both DA and ACh on spiny neurons are known, we asked if activation of neostriatal NK1-class receptors is able to reproduce them. The SP NK1-receptor agonist, GR73632 (1 microM), had both excitatory and inhibitory actions on virtually all spiny neurons tested at resting potential. The excitatory action was blocked by atropine and coursed with an increase in firing rate and input resistance (R(N)). The inhibitory action was blocked by haloperidol and coursed with a reduction in firing rate and R(N). Therefore, the release of both DA and ACh induced by NK1-receptor activation modulates indirectly the excitability of the projection neurons. SP facilitates the actions of these transmitters on the spiny neuron. A residual excitatory response to the NK1-receptor agonist was observed in 30% of a sample of neurons tested in the presence of both haloperidol and atropine. The increase in R(N) that accompanied this response could be observed in the presence of 1 microM TTX or 100 microM Cd2+, suggesting a direct effect. Double labeling showed that only SP-immunoreactive neurons were facilitated by NK1-receptor activation in these conditions.

Cholinergic modulation of neostriatal output: a functional antagonism between different types of muscarinic receptors

It is demonstrated that acetylcholine released from cholinergic interneurons modulates the excitability of neostriatal projection neurons. Physostigmine and neostigmine increase input resistance (RN) and enhance evoked discharge of spiny projection neurons in a manner similar to muscarine. Muscarinic RN increase occurs in the whole subthreshold voltage range (-100 to -45 mV), remains in the presence of TTX and Cd2+, and can be blocked by the relatively selective M1,4 muscarinic receptor antagonist pirenzepine but not by M2 or M3 selective antagonists. Cs+ occludes muscarinic effects at potentials more negative than -80 mV. A Na+ reduction in the bath occludes muscarinic effects at potentials more positive than -70 mV. Thus, muscarinic effects involve different ionic conductances: inward rectifying and cationic. The relatively selective M2 receptor antagonist AF-DX 116 does not block muscarinic effects on the projection neuron but, surprisingly, has the ability to mimic agonistic actions increasing RN and firing. Both effects are blocked by pirenzepine. HPLC measurements of acetylcholine demonstrate that AF-DX 116 but not pirenzepine greatly increases endogenous acetylcholine release in brain slices. Therefore, the effects of the M2 antagonist on the projection neurons were attributable to autoreceptor block on cholinergic interneurons. These experiments show distinct opposite functions of muscarinic M1- and M2-type receptors in neostriatal output, i.e., the firing of projection neurons. The results suggest that the use of more selective antimuscarinics may be more profitable for the treatment of motor deficits.