Opioidergic interactions between striatal projection neurons.

Medium spiny striatal projection neurons (MSNs) release opioid neuropeptides, but the role of these neurotransmitters is still poorly understood. While presynaptic inhibition of corticostriatal axons by opioid receptors has been demonstrated using exogenous ligands, the action of synaptically released opioids in the striatum has not been investigated. We performed single and paired whole-cell recordings from rat MSNs while corticostriatal fibers were electrically activated. In single recording experiments, we also activated antidromically the axons of a population of MSNs. Corticostriatal fibers were stimulated once every 10 s and every other stimulation was preceded by 5 antidromic spikes (at 100 Hz). This burst of antidromic spikes produced robust inhibition of evoked corticostriatal responses. This inhibition was not affected by the δ-opioid receptor antagonist SDM25N, but was completely abolished by the µ-opioid receptor antagonist CTOP. Inhibitory effects were maximal (on average 29.6 ± 11.4%) when the burst preceded the corticostriatal stimulation by 500 ms and became undetectable for intervals >2 s. Paired recordings from MSNs located <100 µm apart revealed that, in 30 of 56 (54%) pairs, a burst of five action potentials in one of the MSNs caused significant inhibition (17.1 ± 5.7%) of evoked glutamatergic responses in the other MSN. In 5 of these pairs, reciprocal inhibition of corticostriatal inputs was present. These effects were maximal 500 ms after the burst and were completely blocked by CTOP. Thus, these results reveal a novel, strong opioid-mediated communication between MSNs and provide a new cellular substrate for competitive dynamics in the striatum.

Serotonin inhibits low-threshold spike interneurons in the striatum.

Low-threshold spike interneurons (LTSIs) are important elements of the striatal architecture and the only known source of nitric oxide in this nucleus, but their rarity has so far prevented systematic studies. Here, we used transgenic mice in which green fluorescent protein is expressed under control of the neuropeptide Y (NPY) promoter and striatal NPY-expressing LTSIs can be easily identified, to investigate the effects of serotonin on these neurons. In sharp contrast with its excitatory action on other striatal interneurons, serotonin (30 µM) strongly inhibited LTSIs, reducing or abolishing their spontaneous firing activity and causing membrane hyperpolarisations.These hyperpolarisations persisted in the presence of tetrodotoxin, were mimicked by 5-HT(2C) receptor agonists and reversed by 5-HT(2C) antagonists. Voltage-clamp slow-ramp experiments showed that serotonin caused a strong increase in an outward current activated by depolarisations that was blocked by the specific M current blocker XE 991. In current-clamp experiments,XE 991 per se caused membrane depolarisations in LTSIs and subsequent application of serotonin (in the presence of XE 991) failed to affect these neurons.We concluded that serotonin strongly inhibits striatal LTSIs acting through postsynaptic 5-HT(2C) receptors and increasing an M type current.

A simple method for characterizing passive and active neuronal properties: application to striatal neurons.

The study of active and passive neuronal dynamics usually relies on a sophisticated array of electrophysiological, staining and pharmacological techniques. We describe here a simple complementary method that recovers many findings of these more complex methods but relies only on a basic patch-clamp recording approach. Somatic short and long current pulses were applied in vitro to striatal medium spiny (MS) and fast spiking (FS) neurons from juvenile rats. The passive dynamics were quantified by fitting two-compartment models to the short current pulse data. Lumped conductances for the active dynamics were then found by compensating this fitted passive dynamics within the current-voltage relationship from the long current pulse data. These estimated passive and active properties were consistent with previous more complex estimations of the neuron properties, supporting the approach. Relationships within the MS and FS neuron types were also evident, including a graduation of MS neuron properties consistent with recent findings about D1 and D2 dopamine receptor expression. Application of the method to simulated neuron data supported the hypothesis that it gives reasonable estimates of membrane properties and gross morphology. Therefore detailed information about the biophysics can be gained from this simple approach, which is useful for both classification of neuron type and biophysical modelling. Furthermore, because these methods rely upon no manipulations to the cell other than patch clamping, they are ideally suited to in vivo electrophysiology.

Substance P mediates excitatory interactions between striatal projection neurons.

The striatum is the largest nucleus of the basal ganglia, and is crucially involved in motor control. Striatal projection cells are medium-size spiny neurons (MSNs) and form functional GABAergic synapses with other MSNs through their axon collaterals. A subpopulation of MSNs also release substance P (SP), but its role in MSN-MSN communication is unknown. We studied this issue in rat brain slices, in the presence of antagonists for GABA, acetylcholine, dopamine, and opioid receptors; under these conditions, whole-cell paired recordings from MSNs (located <100 microm apart) revealed that, in 31/137 (23%) pairs, a burst of five spikes in a MSN caused significant facilitation (14.2 +/- 8.9%) of evoked glutamatergic responses in the other MSN. Reciprocal facilitation of glutamatergic responses was present in 4 of these pairs. These facilitatory effects were maximal when spikes preceded glutamatergic responses by 100 ms, and were completely blocked by the NK1 receptor antagonist L-732,138. Furthermore, in 31/57 (54%) MSNs, a burst of 5 antidromic stimuli delivered to MSN axons in the globus pallidus significantly potentiated glutamatergic responses evoked 250 or 500 ms later by stimulation of the corpus callosum. These effects were larger at 250 than 500 ms intervals, were completely blocked by L-732,138, and facilitated spike generation. These data demonstrate that MSNs facilitate glutamatergic inputs to neighboring MSNs through spike-released SP acting on NK1 receptors. The current view that MSNs form inhibitory networks characterized by competitive dynamics will have to be updated to incorporate the fact that groups of MSNs interact in an excitatory manner.

Serotonin excites fast-spiking interneurons in the striatum.

Fast-spiking interneurons (FSIs) control the output of the striatum by mediating feed-forward GABAergic inhibition of projection neurons. Their neuromodulation can therefore critically affect the operation of the basal ganglia. We studied the effects of 5-hydroxytryptamine (5-HT, serotonin), a neurotransmitter released in the striatum by fibres originating in the raphe nuclei, on FSIs recorded with whole-cell techniques in rat brain slices. Bath application of serotonin (30 microm) elicited slow, reversible depolarizations (9 +/- 3 mV) in 37/46 FSIs. Similar effects were observed using conventional whole-cell and gramicidin perforated-patch techniques. The serotonin effects persisted in the presence of tetrodotoxin and were mediated by 5-HT(2C) receptors, as they were reversed by the 5-HT(2) receptor antagonist ketanserin and by the selective 5-HT(2C) receptor antagonist RS 102221. Serotonin-induced depolarizations were not accompanied by a significant change in FSI input resistance. Serotonin caused the appearance of spontaneous firing in a minority (5/35) of responsive FSIs, whereas it strongly increased FSI excitability in each of the remaining responsive FSIs, significantly decreasing the latency of the first spike evoked by a current step and increasing spike frequency. Voltage-clamp experiments revealed that serotonin suppressed a current that reversed around -100 mV and displayed a marked inward rectification, a finding that explains the lack of effects of serotonin on input resistance. Consistently, the effects of serotonin were completely occluded by low concentrations of extracellular barium, which selectively blocks Kir2 channels. We concluded that the excitatory effects of serotonin on FSIs were mediated by 5-HT(2C) receptors and involved suppression of an inwardly rectifying K(+) current.

Dual Nitrergic/Cholinergic Control of Short-Term Plasticity of Corticostriatal Inputs to Striatal Projection Neurons.

The ability of nitric oxide and acetylcholine to modulate the short-term plasticity of corticostriatal inputs was investigated using current-clamp recordings in BAC mouse brain slices. Glutamatergic responses were evoked by stimulation of corpus callosum in D1 and D2 dopamine receptor-expressing medium spiny neurons (D1-MSNs and D2-MSN, respectively). Paired-pulse stimulation (50 ms intervals) evoked depressing or facilitating responses in subgroups of both D1-MSNs and D2 MSNs. In both neuronal types, glutamatergic responses of cells that displayed paired-pulse depression were not significantly affected by the nitric oxide donor S-nitroso-N-acetylpenicillamine (SNAP; 100 µM). Conversely, in D1-MSNs and D2-MSNs that displayed paired-pulse facilitation, SNAP did not affect the first evoked response, but significantly reduced the amplitude of the second evoked EPSP, converting paired-pulse facilitation into paired-pulse depression. SNAP also strongly excited cholinergic interneurons and increased their cortical glutamatergic responses acting through a presynaptic mechanism. The effects of SNAP on glutamatergic response of D1-MSNs and D2-MSN were mediated by acetylcholine. The broad-spectrum muscarinic receptor antagonist atropine (25 µM) did not affect paired-pulse ratios and did not prevent the effects of SNAP. Conversely, the broad-spectrum nicotinic receptor antagonist tubocurarine (10 µM) fully mimicked and occluded the effects of SNAP. We concluded that phasic acetylcholine release mediates feedforward facilitation in MSNs through activation of nicotinic receptors on glutamatergic terminals and that nitric oxide, while increasing cholinergic interneurons' firing, functionally impairs their ability to modulate glutamatergic inputs of MSNs. These results show that nitrergic and cholinergic transmission control the short-term plasticity of glutamatergic inputs in the striatum and reveal a novel cellular mechanism underlying paired-pulse facilitation in this area.

Ethanol affects striatal interneurons directly and projection neurons through a reduction in cholinergic tone.

The acute effects of ethanol on the neurons of the striatum, a basal ganglia nucleus crucially involved in motor control and action selection, were investigated using whole-cell recordings. An intoxicating concentration of ethanol (50 mM) produced inhibitory effects on striatal large aspiny cholinergic interneurons (LAIs) and low-threshold spike interneurons (LTSIs). These effects persisted in the presence of tetrodotoxin and were because of an increase in potassium currents, including those responsible for medium and slow afterhyperpolarizations. In contrast, fast-spiking interneurons (FSIs) were directly excited by ethanol, which depolarized these neurons through the suppression of potassium currents. Medium spiny neurons (MSNs) became hyperpolarized in the presence of ethanol, but this effect did not persist in the presence of tetrodotoxin and was mimicked and occluded by application of the M1 muscarinic receptor antagonist telenzepine. Ethanol effects on MSNs were also abolished by 100 µM barium. This showed that the hyperpolarizations observed in MSNs were because of decreased tonic activation of M1 muscarinic receptors, resulting in an increase in Kir2 conductances. Evoked GABAergic responses of MSNs were reversibly decreased by ethanol with no change in paired-pulse ratio. Furthermore, ethanol impaired the ability of thalamostriatal inputs to inhibit a subsequent corticostriatal glutamatergic response in MSNs. These results offer the first comprehensive description of the highly cell type-specific effects of ethanol on striatal neurons and provide a cellular basis for the interpretation of ethanol influence on a brain area crucially involved in the motor and decisional impairment caused by this drug.