Serotonin Activates Presynaptic and Postsynaptic Receptors in Rat Globus Pallidus

Connectivity and Functionality of the Globus Pallidus Externa Under Normal Conditions and Parkinson's Disease

The globus pallidus externa (GPe) functions as a central hub in the basal ganglia for processing motor and non-motor information through the creation of complex connections with the other basal ganglia nuclei and brain regions. Recently, with the adoption of sophisticated genetic tools, substantial advances have been made in understanding the distinct molecular, anatomical, electrophysiological, and functional properties of GPe neurons and non-neuronal cells. Impairments in dopamine transmission in the basal ganglia contribute to Parkinson's disease (PD), the most common movement disorder that severely affects the patients' life quality. Altered GPe neuron activity and synaptic connections have also been found in both PD patients and pre-clinical models. In this review, we will summarize the main findings on the composition, connectivity and functionality of different GPe cell populations and the potential GPe-related mechanisms of PD symptoms to better understand the cell type and circuit-specific roles of GPe in both normal and PD conditions.

The 5-HT1B receptor - a potential target for antidepressant treatment

Major depressive disorder (MDD) is the leading cause of disability worldwide. The serotonin hypothesis may be the model of MDD pathophysiology with the most support. The majority of antidepressants enhance synaptic serotonin levels quickly, while it usually takes weeks to discern MDD treatment effect. It has been hypothesized that the time lag between serotonin increase and reduction of MDD symptoms is due to downregulation of inhibitory receptors such as the serotonin 1B receptor (5-HT1BR). The research on 5-HT1BR has previously been hampered by a lack of selective ligands for the receptor. The last extensive review of 5-HT1BR in the pathophysiology of depression was published 2009, and based mainly on findings from animal studies. Since then, selective radioligands for in vivo quantification of brain 5-HT1BR binding with positron emission tomography has been developed, providing new knowledge on the role of 5-HT1BR in MDD and its treatment. The main focus of this review is the role of 5-HT1BR in relation to MDD and its treatment, although studies of 5-HT1BR in obsessive-compulsive disorder, alcohol dependence, and cocaine dependence are also reviewed. The evidence outlined range from animal models of disease, effects of 5-HT1B receptor agonists and antagonists, case-control studies of 5-HT1B receptor binding postmortem and in vivo, with positron emission tomography, to clinical studies of 5-HT1B receptor effects of established treatments for MDD. Low 5-HT1BR binding in limbic regions has been found in MDD patients. When 5-HT1BR ligands are administered to animals, 5-HT1BR agonists most consistently display antidepressant-like properties, though it is not yet clear how 5-HT1BR is best approached for optimal MDD treatment.

The external globus pallidus: progress and perspectives

The external globus pallidus (GPe) of the basal ganglia is in a unique and powerful position to influence processing of motor information by virtue of its widespread projections to all basal ganglia nuclei. Despite the clinical importance of the GPe in common motor disorders such as Parkinson's disease, there is only limited information about its cellular composition and organizational principles. In this review, recent advances in the understanding of the diversity in the molecular profile, anatomy, physiology and corresponding behaviour during movement of GPe neurons are described. Importantly, this study attempts to build consensus and highlight commonalities of the cellular classification based on existing but contentious literature. Additionally, an analysis of the literature concerning the intricate reciprocal loops formed between the GPe and major synaptic partners, including both the striatum and the subthalamic nucleus, is provided. In conclusion, the GPe has emerged as a crucial node in the basal ganglia macrocircuit. While subtleties in the cellular makeup and synaptic connection of the GPe create new challenges, modern research tools have shown promise in untangling such complexity, and will provide better understanding of the roles of the GPe in encoding movements and their associated pathologies.

Posttetanic enhancement of striato-pallidal synaptic transmission

The striato (Str)-globus pallidus external segment (GPe) projection plays major roles in the control of neuronal activity in the basal ganglia under both normal and pathological conditions. The present study used rat brain slice preparations to characterize the enhancement of Str-GPe synapses observed after repetitive conditioning stimuli (CS) of Str with the whole cell patch-clamp recording technique. The results show that 1) the Str-GPe synapses have a posttetanic enhancement (PTE) mechanism, which is considered to be a combination of an augmentation and a posttetanic potentiation; 2) the degree of PTE observed in GPe neurons had a wide range and was positively correlated with a wide range of paired-pulse ratios assessed before application of CS; 3) a wide range of CS, from frequencies as low as 2 Hz with as few as 5 pulses to as high as 100 Hz with 100 pulses, could induce PTE; 4) the decay time constant of PTE was dependent on the strength of CS and was prolonged greatly, up to 120 s, when strong CS were applied; and 5) the level of postsynaptic Cl(-) became a limiting factor for the degree of PTE when strong CS were applied. These results imply that Str-GPe synapses transmit inhibitions in a nonlinear activity-weighted manner, which may be suited for scaling timing and force of repeated or sequential body movements. Other possible factors controlling the induction of PTE and functional implications are also discussed.

Interaction between the 5-HT system and the basal ganglia: functional implication and therapeutic perspective in Parkinson's disease

The neurotransmitter serotonin (5-HT) has a multifaceted function in the modulation of information processing through the activation of multiple receptor families, including G-protein-coupled receptor subtypes (5-HT1, 5-HT2, 5-HT4-7) and ligand-gated ion channels (5-HT3). The largest population of serotonergic neurons is located in the midbrain, specifically in the raphe nuclei. Although the medial and dorsal raphe nucleus (DRN) share common projecting areas, in the basal ganglia (BG) nuclei serotonergic innervations come mainly from the DRN. The BG are a highly organized network of subcortical nuclei composed of the striatum (caudate and putamen), subthalamic nucleus (STN), internal and external globus pallidus (or entopeduncular nucleus in rodents, GPi/EP and GPe) and substantia nigra (pars compacta, SNc, and pars reticulata, SNr). The BG are part of the cortico-BG-thalamic circuits, which play a role in many functions like motor control, emotion, and cognition and are critically involved in diseases such as Parkinson's disease (PD). This review provides an overview of serotonergic modulation of the BG at the functional level and a discussion of how this interaction may be relevant to treating PD and the motor complications induced by chronic treatment with L-DOPA.

Anticataleptic effects of 5-HT(1B) receptors in the globus pallidus

The globus pallidus occupies an important position in the indirect pathway of the basal ganglia. Being a monoamine neurotransmitter, 5-HT is involved in mediating many physiological functions and pathophysiological processes in several movement disorders. Morphological studies have revealed that the globus pallidus receives serotonergic innervation arising from the raphe nuclei, mainly the dorsal raphe nucleus. A high level of 5-HT and 5-HT(1B) receptors were detected in the globus pallidus. In the present study, bilateral microinjection of 5-HT or 5-HT(1B) receptor agonist, CP-93129, into the globus pallidus significantly alleviated the symptoms of rigidity caused by haloperidol. To further elucidate 5-HT(1B) receptor-induced anticatalepsy, in vivo extracellular recordings were performed to examine the effects of 5-HT(1B) receptor activation on the firing activity of the globus pallidus neurons under the presence of haloperidol. Micro-pressure ejection of 5-HT or CP-93129 increased the spontaneous firing rate of the pallidal neurons. Furthermore, by using immunohistochemistry, positive staining of 5-HT(1B) receptor was observed in the globus pallidus neurons. Taken together, the present findings provide evidence that activation of 5-HT(1B) receptor may exert anticataleptic effects by increasing the activity of pallidal neurons.

Multiple controls exerted by 5-HT2C receptors upon basal ganglia function: from physiology to pathophysiology

Serotonin2C (5-HT2C) receptors are expressed in the basal ganglia, a group of subcortical structures involved in the control of motor behaviour, mood and cognition. These receptors are mediating the effects of 5-HT throughout different brain areas via projections originating from midbrain raphe nuclei. A growing interest has been focusing on the function of 5-HT2C receptors in the basal ganglia because they may be involved in various diseases of basal ganglia function notably those associated with chronic impairment of dopaminergic transmission. 5-HT2C receptors act on numerous types of neurons in the basal ganglia, including dopaminergic, GABAergic, glutamatergic or cholinergic cells. Perhaps inherent to their peculiar molecular properties, the modality of controls exerted by 5-HT2C receptors over these cell populations can be phasic, tonic (dependent on the 5-HT tone) or constitutive (a spontaneous activity without the presence of the ligand). These controls are functionally organized in the basal ganglia: they are mainly localized in the input structures and preferentially distributed in the limbic/associative territories of the basal ganglia. The nature of these controls is modified in neuropsychiatric conditions such as Parkinson's disease, tardive dyskinesia or addiction. Most of the available data indicate that the function of 5-HT2C receptor is enhanced in cases of chronic alterations of dopamine neurotransmission. The review illustrates that 5-HT2C receptors play a role in maintaining continuous controls over the basal ganglia via multiple diverse actions. We will discuss their interest for treatments aimed at ameliorating current pharmacotherapies in schizophrenia, Parkinson's disease or drugs abuse.

Functional anatomy of 5-HT2A receptors in the amygdala and hippocampal complex: relevance to memory functions

The amygdaloid complex and hippocampal region contribute to emotional activities, learning, and memory. Mounting evidence suggests a primary role for serotonin (5-HT) in the physiological basis of memory and its pathogenesis by modulating directly the activity of these two areas and their cross-talk. Indeed, both the amygdala and the hippocampus receive remarkably dense serotoninergic inputs from the dorsal and median raphe nuclei. Anatomical, behavioral and electrophysiological evidence indicates the 5-HT2A receptor as one of the principal postsynaptic targets mediating 5-HT effects. In fact, the 5-HT2A receptor is the most abundant 5-HT receptor expressed in these brain structures and is expressed on both amygdalar and hippocampal pyramidal glutamatergic neurons as well as on γ-aminobutyric acid (GABA)-containing interneurons. 5-HT2A receptors on GABAergic interneurons stimulate GABA release, and thereby have an important role in regulating network activity and neural oscillations in the amygdala and hippocampal region. This review will focus on the distribution and physiological functions of the 5-HT2A receptor in the amygdala and hippocampal region. Taken together the results discussed here suggest that 5-HT2A receptor may be a potential therapeutic target for those disorders related to hippocampal and amygdala dysfunction.

Short-term plasticity shapes activity pattern-dependent striato-pallidal synaptic transmission

The cortico-striato (Str)-globus pallidus external segment (GPe) projection plays major roles in the control of neuronal activity in the basal ganglia under both normal and pathological conditions. The present study used rat brain-slice preparations to address our hypothesis that the gain of this disynaptic projection is dynamically controlled by activations of short-term plasticity mechanisms of Str-GPe synapses. The Str-GPe projection neurons fire with very different frequency and firing patterns in vivo depending on the condition of the animal. The results show that the Str-GPe synapses have very strong short-term enhancement mechanisms and that repetitive burst activation of the Str-GPe synapses, which mimic oscillatory burst firing of Str neurons, can sustain enhanced states of synaptic transmission for tens of seconds. The results reveal that the short-term enhancement of Str-GPe synapses contributes to the generation of pauses in the firing of GPe neurons and that signal transfer function in the Str-GPe projection is highly dependent on the firing pattern of Str neurons.

Reinforcement learning: computing the temporal difference of values via distinct corticostriatal pathways

Midbrain dopamine neurons supposedly encode reward prediction error, but how error signals are computed remains elusive. Here, we propose a mechanism based on recent findings regarding corticostriatal circuits. Specifically, we propose that two distinct subpopulations of corticostriatal neurons differentially represent the animal's current and previous states/actions through unidirectional connectivity from one subpopulation to the other and strong recurrent excitation that exists only within the recipient subpopulation. These corticostriatal subpopulations selectively connect to the direct and indirect pathways of the basal ganglia, such that the temporal difference between the values of current and previous states/actions--the core of the error signal--can be computed. Our hypothesis suggests a unified view of basal ganglia functions and has important clinical implications.

Neuronal localization of 5-HT2A receptor immunoreactivity in the rat hippocampal region

The 5-HT2A receptor subtype (5-HT2Ar) plays an important role in the modulation of the hippocampal region activity and it has been associated with learning and memory processes. In the present study, the 5-HT2Ar was immunohistochemically localized in the rat hippocampal region, which includes the hippocampal formation and the parahippocampal region. In the hippocampal formation (dentate gyrus, hippocampus proper and subiculum) and entorhinal cortex, the colocalization of the 5-HT2Ar with the inhibitory transmitter γ-aminobutyric acid (GABA) was studied using double immunofluorescence confocal microscopy. The patterns of immunostaining were very different in non-injected and colchicine-injected rats. In untreated rats, the immunoreactivity could be attributed especially to neuropil. Interestingly, in non-injected rats, the 5-HT2Ar immunoreactivity was located in the mossy fibers, suggesting that serotonin acts presynaptically via this receptor subtype directly on glutamate axons. Pretreatment with colchicine increased the number of 5-HT2Ar-immunoreactive somata. Morphological and double immunofluorescence analyses indicated that the 5-HT2Ar was located on both the excitatory and the inhibitory neurons of the rat hippocampal region. The results of the present study suggest that the 5-HT2Ar could participate in the hippocampal neurotransmission by acting on different neuronal populations.

Serotonin and prefrontal cortex function: neurons, networks, and circuits

Higher-order executive tasks such as learning, working memory, and behavioral flexibility depend on the prefrontal cortex (PFC), the brain region most elaborated in primates. The prominent innervation by serotonin neurons and the dense expression of serotonergic receptors in the PFC suggest that serotonin is a major modulator of its function. The most abundant serotonin receptors in the PFC, 5-HT1A, 5-HT2A and 5-HT3A receptors, are selectively expressed in distinct populations of pyramidal neurons and inhibitory interneurons, and play a critical role in modulating cortical activity and neural oscillations (brain waves). Serotonergic signaling is altered in many psychiatric disorders such as schizophrenia and depression, where parallel changes in receptor expression and brain waves have been observed. Furthermore, many psychiatric drug treatments target serotonergic receptors in the PFC. Thus, understanding the role of serotonergic neurotransmission in PFC function is of major clinical importance. Here, we review recent findings concerning the powerful influences of serotonin on single neurons, neural networks, and cortical circuits in the PFC of the rat, where the effects of serotonin have been most thoroughly studied.

Dendritic sodium channels promote active decorrelation and reduce phase locking to parkinsonian input oscillations in model globus pallidus neurons

Correlated firing among populations of neurons is present throughout the brain and is often rhythmic in nature, observable as an oscillatory fluctuation in the local field potential. Although rhythmic population activity is believed to be critical for normal function in many brain areas, synchronized neural oscillations are associated with disease states in other cases. In the globus pallidus (GP in rodents, homolog of the primate GPe), pairs of neurons generally have uncorrelated firing in normal animals despite an anatomical organization suggesting that they should receive substantial common input. In contrast, correlated and rhythmic GP firing is observed in animal models of Parkinson's disease (PD). Based in part on these findings, it has been proposed that an important part of basal ganglia function is active decorrelation, whereby redundant information is compressed. Mechanisms that implement active decorrelation, and changes that cause it to fail in PD, are subjects of great interest. Rat GP neurons express fast, transient voltage-dependent sodium channels (NaF channels) in their dendrites, with the expression level being highest near asymmetric synapses. We recently showed that the dendritic NaF density strongly influences the responsiveness of model GP neurons to synchronous excitatory inputs. In the present study, we use rat GP neuron models to show that dendritic NaF channel expression is a potential cellular mechanism of active decorrelation. We further show that model neurons with lower dendritic NaF channel expression have a greater tendency to phase lock with oscillatory synaptic input patterns like those observed in PD.

Serotonin-dependent depression in Parkinson's disease: a role for the subthalamic nucleus?

Depression is the most common neuropsychiatric co-morbidity in Parkinson's disease (PD). The underlying mechanism of depression in PD is complex and likely involves biological, psychosocial and therapeutic factors. The biological mechanism may involve changes in monoamine systems, in particular the serotonergic (5-hydroxytryptamine, 5-HT) system. It is well established that the 5-HT system is markedly affected in the Parkinsonian brain, with evidence including pathological loss of markers of 5-HT axons as well as cell bodies in the dorsal and median raphe nuclei of the midbrain. However, it remains unresolved whether alterations to the 5-HT system alone are sufficient to confer vulnerability to depression. Here we propose low 5-HT combined with altered network activity within the basal ganglia as critically involved in depression in PD. The latter hypothesis is derived from a number of recent findings that highlight the close interaction between the basal ganglia and the 5-HT system, not only in motor but also limbic functions. These findings include evidence that clinical depression is a side effect of deep brain stimulation (DBS) of the subthalamic nucleus (STN), a treatment option in advanced PD. Further, it has recently been demonstrated that STN DBS in animal models inhibits 5-HT neurotransmission, and that this change may underpin depressive-like side effects. This review provides an overview of 5-HT alterations in PD and a discussion of how these changes might combine with altered basal ganglia network activity to increase depression vulnerability.

Functional connectivity and integrative properties of globus pallidus neurons

The globus pallidus consists of the external (GPe) and the internal (GPi) segments. The GPe and GPi have different functional roles. The GPe is located centrally within multiple basal ganglia feedforward and feedback connections. The GPi is an output nucleus of the basal ganglia. A complex interplay between intrinsic pacemaking conductances and the balance of glutamatergic and GABAergic input largely determines the rate and pattern of firing of pallidal neurons. The initial part of this article introduces recent findings made in vivo that are related to the roles of glutamatergic and GABAergic inputs in the control of pallidal activity. The latter part describes the roles of intrinsic mechanisms of GPe neurons in the integration of the synaptic inputs. The presence of dendritic voltage-gated sodium channels may allow the initiation of dendritic spikes, giving distal inputs on the long and thin GPe dendrites an opportunity to strongly shape spiking activity. Basal ganglia disorders including Parkinson's disease, hemiballismus, and dystonias are accompanied by increased irregularity and synchronized bursts of pallidal activity. These changes may be in part due to changes in the GABA release in the GPe and GPi, but also involve intrinsic cellular changes in pallidal neurons.

Behavioral and electrophysiological effects of 5-HT in globus pallidus of 6-hydroxydopamine lesioned rats

Anatomical studies have shown that the globus pallidus receives abundant 5-hydroxytryptamine (5-HT) innervations from raphe nuclei. 5-HT may occupy an important position in the modulation of motor function through its affect on the activity of globus pallidus. In the present study, intrapallidal microinjection of 5-HT (0.1 mM) alone did not induce any motor behavior or postural asymmetry in the unilateral 6-hydroxydopamine (6-OHDA)-lesioned rats. However, when infused concomitantly with a low dose of 3, 4-dihydroxyphenylalanine (L-DOPA, 3 mg/kg i.p.), which itself can induce modest contralateral rotational behavior, 5-HT significantly potentiated the number of contralateral rotations. To elucidate the cellular mechanism, in vivo extracellular recordings were performed to examine the effects of 5-HT on globus pallidus neurons. In normal rats, the predominant effect of micropressure ejection of 5-HT on pallidal neurons was excitation. In 6-OHDA-lesioned rats, although 5-HT increased the firing rate in most pallidal neurons, 5-HT-induced inhibitory effects was stronger than that on the unlesioned side as well as normal rats. Furthermore, 5-HT(1B) receptors are mainly involved in 5-HT-induced excitation while 5-HT(1A) receptors are involved in 5-HT-induced inhibition. The results suggest that 5-HT may potentiate the antiparkinsonian effect of L-DOPA through modulating the activity of globus pallidus.

Presynaptic 5-HT(1B) receptor-mediated serotonergic inhibition of glutamate transmission in the bed nucleus of the stria terminalis

Activation of neurons in the bed nucleus of the stria terminalis (BNST) plays a critical role in stress and anxiety-related behaviors. Previously, we have shown that serotonin (5-HT) can directly modulate BNST neuronal excitability by an action at postsynaptic receptors. In this study we built upon that work to examine the effects of 5-HT on excitatory neurotransmission in an in vitro rat BNST slice preparation. Bath application of 5-HT reversibly reduced the amplitude of evoked excitatory postsynaptic currents (eEPSCs). These effects were mimicked by the 5-HT(1B/D) receptor agonist, sumatriptan, and by the 5-HT(1B) receptor selective agonist, CP93129. Conversely, the effects of 5-HT and sumatriptan could be blocked by the 5-HT(1B) receptor-selective antagonist, GR55562. In contrast, the 5-HT(1A) receptor agonist 8-OH DPAT or antagonist WAY 100635 could not mimic or block the effect of 5-HT on eEPSCs. Together, these data suggest that the 5-HT-induced attenuation of eEPSCs was mediated by 5-HT(1B) receptor activation. Moreover, sumatriptan had no effect on the amplitude of the postsynaptic current elicited by pressure applied AMPA, suggesting a possible presynaptic locus for the 5-HT(1B) receptor. Furthermore, 5-HT, sumatriptan and CP93129 all increased the paired pulse ratio of eEPSCs while they concomitantly decreased the amplitude of eEPSCs, suggesting that these agonists act to reduce glutamate release probability at presynaptic locus. Consistent with this observation, sumatriptan decreased the frequency of miniature EPSCs, but had no effect on their amplitude. Taken together, these results suggest that 5-HT suppresses glutamatergic neurotransmission in the BNST by activating presynaptic 5-HT(1B) receptors to decrease glutamate release from presynaptic terminals. This study illustrates a new pathway by which the activity of BNST neurons can be indirectly modulated by 5-HT, and suggests a potential new target for the development of novel treatments for depression and anxiety disorders.

The sigma-1 antagonist BMY-14802 inhibits L-DOPA-induced abnormal involuntary movements by a WAY-100635-sensitive mechanism

RATIONALE

Levodopa (L-DOPA), the gold standard treatment for Parkinson's disease (PD), eventually causes L-DOPA-induced dyskinesia (LID) in up to 80% of patients. In the 6-hydroxydopamine (6-OHDA) rat model of PD, L-DOPA induces a similar phenomenon, which has been termed abnormal involuntary movement (AIM). We previously demonstrated that BMY-14802 suppresses AIM expression in this model.

OBJECTIVES

Although BMY-14802 is widely used as a sigma-1 antagonist, it is also an agonist at serotonin (5-HT) 1A and adrenergic alpha-1 receptors. The current study was conducted to determine which of these mechanisms underlies BMY-14802's AIM-suppressing effect. This characterization included testing the 5-HT1A agonist buspirone and multiple sigma agents. When these studies implicated a 5-HT1A mechanism, we subsequently undertook a pharmacological reversal study, evaluating whether the 5-HT1A antagonist WAY-100635 counteracted BMY-14802's AIM-suppressing effects.

RESULTS

Buspirone dose-dependently suppressed AIM, supporting past findings. However, no AIM-suppressing effects were produced by drugs with effects at sigma receptors, including BD-1047, finasteride, SM-21, DTG, trans-dehydroandrosterone (DHEA), carbetapentane, and opipramol. Finally, we show for the first time that the AIM-suppressing effect of BMY-14802 was dose-dependently prevented by WAY-100635 but not by the alpha-1 antagonist prazosin.

CONCLUSIONS

BMY-14802 exerts its AIM-suppressing effects via a 5-HT1A agonist mechanism, similar to buspirone. Other 5-HT1A agonists have failed clinical trials, possibly due to submicromolar affinity at other receptors, including D2, which may exacerbate PD symptoms. BMY-14802 is a promising candidate for clinical trials due to its extremely low affinity for the D2 receptor and lack of extrapyramidal effects during prior clinical trials for schizophrenia.

Effects of 5-HT in globus pallidus on haloperidol-induced catalepsy in rats

Histological, behavioral and electrophysiological studies have suggested that 5-HT may regulate motor function by affecting globus pallidus neurons activity. In this study, the effects of 5-HT in globus pallidus on haloperidol-induced catalepsy and its possible receptor mechanisms were examined in rats using bar tests. Bilateral microinjection of 5-HT (10microM) into globus pallidus significantly attenuated haloperidol-induced catalepsy. This anticataleptic effect was completely counteracted by selective 5-HT(1B) receptors antagonist SB-224289 (10microM), while partly reversed by selective 5-HT(4) receptors antagonist GR-113808 (1microM). In addition, the selective 5-HT(7) receptors antagonist SB-269970 (1microM) partly reversed the anticataleptic effect of 5-HT only at the incipient period after the intrapallidal injection. In conclusion, 5-HT in globus pallidus could attenuate haloperidol-induced catalepsy via multiple receptor mechanisms.

Presynaptic actions of D2-like receptors in the rat cortico-striato-globus pallidus disynaptic connection in vitro

The cerebral cortex, the neostriatum (Str), and the external segment of the globus pallidus (GPe) form a cortico-Str-GPe disynaptic connection, which is one of the major connections in the basal ganglia circuitries and a target of dopamine modulation. The aim of this study was to examine the actions of D2-like dopamine receptors (D2LRs) in this connection using rat brain slice preparations. Electrical stimulation of the frontal cortex evoked disynaptic inhibitory postsynaptic currents (IPSCs) in cesium-filled GPe neurons voltage-clamped at 0 mV. The IPSCs evoked by threshold stimulation were small, <10 pA. Bath or local applications of the D2LR agonist quinpirole to Str decreased the amplitude of the cortical stimulation-induced IPSCs. Electrical stimulation of Str evoked monosynaptic IPSCs in GPe neurons. Local application of quinpirole to GPe decreased the Str stimulation-induced IPSCs. Bath application of quinpirole decreased the frequency of large miniature IPSCs (mIPSCs) that were considered to be evoked by local collateral axons of GPe neurons. These results suggested that activation of D2LRs decrease the gain of the cortico-Str-GPe disynaptic connection, with the decrease attributed to activation of D2LRs in Str and GPe, and that both Str-GPe and GPe-GPe GABAergic inhibitions are under the control of presynaptic D2LRs.

Channel density distributions explain spiking variability in the globus pallidus: a combined physiology and computer simulation database approach

Globus pallidus (GP) neurons recorded in brain slices show significant variability in intrinsic electrophysiological properties. To investigate how this variability arises, we manipulated the biophysical properties of GP neurons using computer simulations. Specifically, we created a GP neuron model database with 100,602 models that had varying densities of nine membrane conductances centered on a hand-tuned model that replicated typical physiological data. To test the hypothesis that the experimentally observed variability can be attributed to variations in conductance densities, we compared our model database results to a physiology database of 146 slice recordings. The electrophysiological properties of generated models and recordings were assessed with identical current injection protocols and analyzed with a uniform set of measures, allowing a systematic analysis of the effects of varying voltage-gated and calcium-gated conductance densities on the measured properties and a detailed comparison between models and recordings. Our results indicated that most of the experimental variability could be matched by varying conductance densities, which we confirmed with additional partial block experiments. Further analysis resulted in two key observations: (1) each voltage-gated conductance had effects on multiple measures such as action potential waveform and spontaneous or stimulated spike rates; and (2) the effect of each conductance was highly dependent on the background context of other conductances present. In some cases, such interactions could reverse the effect of the density of one conductance on important excitability measures. This context dependence of conductance density effects is important to understand drug and neuromodulator effects that work by affecting ion channels.