Recurrent inhibition in the rat neostriatum

Neuronal diversity in the caudate nucleus: A comparative study between camel and human brains

Caudate nucleus (CN) neurons in camels and humans were examined using modified Golgi impregnation methods. Neurons were classified based on soma morphology, dendritic characteristics, and spine distribution. Three primary neuron types were identified in both species: rich-spiny (Type I), sparsely-spiny (Type II), and aspiny (Type III), each comprising subtypes with specific features. Comparative analysis revealed significant differences in soma size, dendritic morphology, and spine distribution between camels and humans. The study contributes to our understanding of structural diversity in CN neurons and provides insights into evolutionary neural adaptations.

Distinct neurochemical influences on fMRI response polarity in the striatum

The striatum, known as the input nucleus of the basal ganglia, is extensively studied for its diverse behavioral roles. However, the relationship between its neuronal and vascular activity, vital for interpreting functional magnetic resonance imaging (fMRI) signals, has not received comprehensive examination within the striatum. Here, we demonstrate that optogenetic stimulation of dorsal striatal neurons or their afferents from various cortical and subcortical regions induces negative striatal fMRI responses in rats, manifesting as vasoconstriction. These responses occur even with heightened striatal neuronal activity, confirmed by electrophysiology and fiber-photometry. In parallel, midbrain dopaminergic neuron optogenetic modulation, coupled with electrochemical measurements, establishes a link between striatal vasodilation and dopamine release. Intriguingly, in vivo intra-striatal pharmacological manipulations during optogenetic stimulation highlight a critical role of opioidergic signaling in generating striatal vasoconstriction. This observation is substantiated by detecting striatal vasoconstriction in brain slices after synthetic opioid application. In humans, manipulations aimed at increasing striatal neuronal activity likewise elicit negative striatal fMRI responses. Our results emphasize the necessity of considering vasoactive neurotransmission alongside neuronal activity when interpreting fMRI signal.

Neuronal glutamate transporters control reciprocal inhibition and gain modulation in D1 medium spiny neurons

Understanding the function of glutamate transporters has broad implications for explaining how neurons integrate information and relay it through complex neuronal circuits. Most of what is currently known about glutamate transporters, specifically their ability to maintain glutamate homeostasis and limit glutamate diffusion away from the synaptic cleft, is based on studies of glial glutamate transporters. By contrast, little is known about the functional implications of neuronal glutamate transporters. The neuronal glutamate transporter EAAC1 is widely expressed throughout the brain, particularly in the striatum, the primary input nucleus of the basal ganglia, a region implicated with movement execution and reward. Here, we show that EAAC1 limits synaptic excitation onto a population of striatal medium spiny neurons identified for their expression of D1 dopamine receptors (D1-MSNs). In these cells, EAAC1 also contributes to strengthen lateral inhibition from other D1-MSNs. Together, these effects contribute to reduce the gain of the input-output relationship and increase the offset at increasing levels of synaptic inhibition in D1-MSNs. By reducing the sensitivity and dynamic range of action potential firing in D1-MSNs, EAAC1 limits the propensity of mice to rapidly switch between behaviors associated with different reward probabilities. Together, these findings shed light on some important molecular and cellular mechanisms implicated with behavior flexibility in mice.

Functional Autapses Form in Striatal Parvalbumin Interneurons but not Medium Spiny Projection Neurons

Autapses selectively form in specific cell types in many brain regions. Previous studies have also found putative autapses in principal spiny projection neurons (SPNs) in the striatum. However, it remains unclear whether these neurons indeed form physiologically functional autapses. We applied whole-cell recording in striatal slices and identified autaptic cells by the occurrence of prolonged asynchronous release (AR) of neurotransmitters after bursts of high-frequency action potentials (APs). Surprisingly, we found no autaptic AR in SPNs, even in the presence of Sr2+. However, robust autaptic AR was recorded in parvalbumin (PV)-expressing neurons. The autaptic responses were mediated by GABAA receptors and their strength was dependent on AP frequency and number. Further computer simulations suggest that autapses regulate spiking activity in PV cells by providing self-inhibition and thus shape network oscillations. Together, our results indicate that PV neurons, but not SPNs, form functional autapses, which may play important roles in striatal functions.

Impacts of autapse on chaotic resonance in single neurons and small-world neuronal networks

Chaotic resonance (CR) is a new phenomenon induced by an intermediate level of chaotic signal intensity in neuronal systems. In the current study, we investigated the effects of autapse on the CR phenomenon in single neurons and small-world (SW) neuronal networks. In single neurons, we assume that the neuron has only one autapse modelled as electrical, excitatory chemical and inhibitory chemical synapse, respectively. Then, we analysed the effects of each one on the CR, separately. Obtained results revealed that, regardless of its type, autapse significantly increases the chaotic resonance of the appropriate autaptic parameter's values. It is also observed that, at the optimal chaotic current intensity, the multiple CR emerges depending on autaptic time delay for all the autapse types when the autaptic delay time or its integer multiples match the half period or period of the weak signal. In SW networks, we investigated the effects of chaotic activity on the prorogation of pacemaker activity, where pacemaker neurons have different kinds of autapse as considered in single neuron cases. Obtained results revealed that excitatory and electrical autapses prominently increase the prorogation of pacemaker activity, whereas inhibitory autapse reduces or does not change it. Also, the best propagation was obtained when the autapse was excitatory. This article is part of the theme issue 'Vibrational and stochastic resonance in driven nonlinear systems (part 2)'.

Contribution of AMPA and NMDA receptors in the spontaneous firing patterns of single neurons in autaptic culture

Single neurons in an autaptic culture exhibit various types of firing pattern with different firing durations and rhythms. However, a neuron with autapses has often been modeled as an oscillator providing a monotonic firing pattern with a constant periodicity because of the lack of a mathematical model. In the work described in this study, we use computational simulation and whole-cell patch-clamp recording to elucidate and model the mechanism by which such neurons generate various firing pattens. In the computational simulation, three types of spontaneous firing pattern, i.e., short, long-lasting, and periodic burst firing patterns are realized by changing the combination ratio of N-methyl-d-aspartate (NMDA) to α-amino-3-hydroxyl-5-methyl-4-isoxazole-propionate (AMPA) conductance. These three types of firing patterns are also observed in the experiments where neurons are cultured in isolation on micropatterned substrates. Using the AMPA and NMDA current models, we discuss that, in principle, autapses can regulate rhythmicity and information selection in neuronal networks.

Autaptic Cultures: Methods and Applications

Neurons typically form daisy chains of synaptic connections with other neurons, but they can also form synapses with themselves. Although such self-synapses, or autapses, are comparatively rare in vivo, they are surprisingly common in dissociated neuronal cultures. At first glance, autapses in culture seem like a mere curiosity. However, by providing a simple model system in which a single recording electrode gives simultaneous access to the pre- and postsynaptic compartments, autaptic cultures have proven to be invaluable in facilitating important and elegant experiments in the area of synaptic neuroscience. Here, I provide detailed protocols for preparing and recording from autaptic cultures (also called micro-island or microdot cultures). Variations on the basic procedure are presented, as well as practical tips for optimizing the outcomes. I also illustrate the utility of autaptic cultures by reviewing the types of experiments that have used them over the past three decades. These examples serve to highlight the power and elegance of this simple model system, and will hopefully inspire new experiments for the interrogation of synaptic function.

Autapses enhance bursting and coincidence detection in neocortical pyramidal cells

Autapses are synaptic contacts of a neuron’s axon onto its own dendrite and soma. In the neocortex, self-inhibiting autapses in GABAergic interneurons are abundant in number and play critical roles in regulating spike precision and network activity. Here we examine whether the principal glutamatergic pyramidal cells (PCs) also form functional autapses. In patch-clamp recording from both rodent and human PCs, we isolated autaptic responses and found that these occur predominantly in layer-5 PCs projecting to subcortical regions, with very few in those projecting to contralateral prefrontal cortex and layer 2/3 PCs. Moreover, PC autapses persist during development into adulthood. Surprisingly, they produce giant postsynaptic responses (∼5 fold greater than recurrent PC-PC synapses) that are exclusively mediated by AMPA receptors. Upon activation, autapses enhance burst firing, neuronal responsiveness and coincidence detection of synaptic inputs. These findings indicate that PC autapses are functional and represent an important circuit element in the neocortex.

While autapses are synapses made by a neuron onto itself, its functional significance in pyramidal cells are not clear. Here, the authors show that in the mammalian neocortex, autapses of pyramidal cells can enhance burst firing and coincidence detection from other inputs.

Striatal Local Circuitry: A New Framework for Lateral Inhibition

This Perspective will examine the organization of intrastriatal circuitry, review recent findings in this area, and discuss how the pattern of connectivity between striatal neurons might give rise to the behaviorally observed synergism between the direct/indirect pathway neurons. The emphasis of this Perspective is on the underappreciated role of lateral inhibition between striatal projection cells in controlling neuronal firing and shaping the output of this circuit. We review some classic studies in combination with more recent anatomical and functional findings to lay out a framework for an updated model of the intrastriatal lateral inhibition, where we explore its contribution to the formation of functional units of processing and the integration and filtering of inputs to generate motor patterns and learned behaviors.

Autapse-induced multiple stochastic resonances in a modular neuronal network

This study investigates the nontrivial effects of autapse on stochastic resonance in a modular neuronal network subjected to bounded noise. The resonance effect of autapse is detected by imposing a self-feedback loop with autaptic strength and autaptic time delay to each constituent neuron. Numerical simulations have demonstrated that bounded noise with the proper level of amplitude can induce stochastic resonance; moreover, the noise induced resonance dynamics can be significantly shaped by the autapse. In detail, for a specific range of autaptic strength, multiple stochastic resonances can be induced when the autaptic time delays are appropriately adjusted. These appropriately adjusted delays are detected to nearly approach integer multiples of the period of the external weak signal when the autaptic strength is very near zero; otherwise, they do not match the period of the external weak signal when the autaptic strength is slightly greater than zero. Surprisingly, in both cases, the differences between arbitrary two adjacent adjusted autaptic delays are always approximately equal to the period of the weak signal. The phenomenon of autaptic delay induced multiple stochastic resonances is further confirmed to be robust against the period of the external weak signal and the intramodule probability of subnetwork. These findings could have important implications for weak signal detection and information propagation in realistic neural systems.

Autapse-induced multiple coherence resonance in single neurons and neuronal networks

We study the effects of electrical and chemical autapse on the temporal coherence or firing regularity of single stochastic Hodgkin-Huxley neurons and scale-free neuronal networks. Also, we study the effects of chemical autapse on the occurrence of spatial synchronization in scale-free neuronal networks. Irrespective of the type of autapse, we observe autaptic time delay induced multiple coherence resonance for appropriately tuned autaptic conductance levels in single neurons. More precisely, we show that in the presence of an electrical autapse, there is an optimal intensity of channel noise inducing the multiple coherence resonance, whereas in the presence of chemical autapse the occurrence of multiple coherence resonance is less sensitive to the channel noise intensity. At the network level, we find autaptic time delay induced multiple coherence resonance and synchronization transitions, occurring at approximately the same delay lengths. We show that these two phenomena can arise only at a specific range of the coupling strength, and that they can be observed independently of the average degree of the network.

Regulation of Irregular Neuronal Firing by Autaptic Transmission

The importance of self-feedback autaptic transmission in modulating spike-time irregularity is still poorly understood. By using a biophysical model that incorporates autaptic coupling, we here show that self-innervation of neurons participates in the modulation of irregular neuronal firing, primarily by regulating the occurrence frequency of burst firing. In particular, we find that both excitatory and electrical autapses increase the occurrence of burst firing, thus reducing neuronal firing regularity. In contrast, inhibitory autapses suppress burst firing and therefore tend to improve the regularity of neuronal firing. Importantly, we show that these findings are independent of the firing properties of individual neurons, and as such can be observed for neurons operating in different modes. Our results provide an insightful mechanistic understanding of how different types of autapses shape irregular firing at the single-neuron level, and they highlight the functional importance of autaptic self-innervation in taming and modulating neurodynamics.

Autaptic activity-induced synchronization transitions in Newman-Watts network of Hodgkin-Huxley neurons

In this paper, we numerically study the effect of autapse on the synchronization of Newman-Watts small-world Hodgkin-Huxley neuron network. It is found that the neurons exhibit synchronization transitions as autaptic self-feedback delay is varied, and the phenomenon becomes strongest when autaptic self-feedback strength is optimal. This phenomenon also changes with the change of coupling strength and network randomness and become strongest when they are optimal. There are similar synchronization transitions for electrical and chemical autapse, but the synchronization transitions for chemical autapse occur more frequently and are stronger than those for electrical synapse. The underlying mechanisms are briefly discussed in quality. These results show that autaptic activity plays a subtle role in the synchronization of the neuronal network. These findings may find potential implications of autapse for the information processing and transmission in neural systems.

Firing statistics of inhibitory neuron with delayed feedback. II: Non-Markovian behavior

The instantaneous state of a neural network consists of both the degree of excitation of each neuron the network is composed of and positions of impulses in communication lines between the neurons. In neurophysiological experiments, the neuronal firing moments are registered, but not the state of communication lines. But future spiking moments depend essentially on the past positions of impulses in the lines. This suggests, that the sequence of intervals between firing moments (inter-spike intervals, ISIs) in the network could be non-Markovian. In this paper, we address this question for a simplest possible neural "net", namely, a single inhibitory neuron with delayed feedback. The neuron receives excitatory input from the driving Poisson stream and inhibitory impulses from its own output through the feedback line. We obtain analytic expressions for conditional probability density P(tn+1|tn, …, t1, t0), which gives the probability to get an output ISI of duration tn+1 provided the previous (n+1) output ISIs had durations tn, …, t1, t0. It is proven exactly, that P(tn+1|tn, …, t1, t0) does not reduce to P(tn+1|tn, …, t1) for any n≥0. This means that the output ISIs stream cannot be represented as a Markov chain of any finite order.

A pause in nucleus accumbens neuron firing is required to initiate and maintain feeding

Nucleus accumbens (NAc) inactivation increases food intake, indicating that NAc neurons exert ongoing inhibition of feeding. We previously described a subpopulation of NAc neurons that pause during sucrose licking and proposed that the pause permits consumption. We tested this hypothesis by first recording NAc neurons during sucrose consumption, and then electrically stimulating through the same electrodes. A large proportion of NAc shell and core neurons were inhibited during sucrose consumption, and local electrical stimulation abruptly interrupted licking. Effective stimulation sites were more anterior than ineffective sites in NAc. At low stimulus intensities, licking resumed immediately on stimulation offset. The latency to lick resumption from NAc neuron inhibition onset ( approximately 460 ms) was very similar to that after electrical stimulation offset ( approximately 440 ms). These results directly support the hypothesis that a significant subpopulation of NAc neurons inhibit palatable food consumption and that a pause in their firing is required to initiate and maintain consumption.

Dopamine, schizophrenia, mania, and depression: Toward a unified hypothesis of cortico-striatopallido-thalamic function

Considerable evidence from preclinical and clinical investigations implicates disturbances of brain dopamine (DA) function in the pathophysiology of several psychiatric and neurologic disorders. We describe a neural model that may help organize theseindependent experimental observations. Cortical regions classically associated with the limbic system interact with infracortical structures, including the nucleus accumbens, ventral pallidum, and dorsomedial nucleus of the thalamus. In our model, overactivity in forebrain DA systems results in the loss of lateral inhibitory interactions in the nucleus accumbens, causing disinhibition of pallidothalamic efferents; this in turn causes rapid changes and a loss of focused corticothalamic activity in cortical regions controlling cognitive and emotional processes. These effects might be manifested clinically by some symptoms of psychoses. Underactivity of forebrain DA results in excess lateral inhibition in the nucleus accumbens, causing tonic inhibition of pallidothalamic efferents; this perpetuates tonic corticothalamic activity and prevents the initiation of new activity in other critical cortical regions. These effects might be manifested clinically by some symptoms of depression. This model parallels existing explanations for the etiology of several movement disorders, and may lead to testable inferences regarding the neural substrates of specific psychopathologies.

Feedforward and feedback inhibition in neostriatal GABAergic spiny neurons

There are two distinct inhibitory GABAergic circuits in the neostriatum. The feedforward circuit consists of a relatively small population of GABAergic interneurons that receives excitatory input from the neocortex and exerts monosynaptic inhibition onto striatal spiny projection neurons. The feedback circuit comprises the numerous spiny projection neurons and their interconnections via local axon collaterals. This network has long been assumed to provide the majority of striatal GABAergic inhibition and to sharpen and shape striatal output through lateral inhibition, producing increased activity in the most strongly excited spiny cells at the expense of their less strongly excited neighbors. Recent results, mostly from recording experiments of synaptically connected pairs of neurons, have revealed that the two GABAergic circuits differ markedly in terms of the total number of synapses made by each, the strength of the postsynaptic response detected at the soma, the extent of presynaptic convergence and divergence and the net effect of the activation of each circuit on the postsynaptic activity of the spiny neuron. These data have revealed that the feedforward inhibition is powerful and widespread, with spiking in a single interneuron being capable of significantly delaying or even blocking the generation of spikes in a large number of postsynaptic spiny neurons. In contrast, the postsynaptic effects of spiking in a single presynaptic spiny neuron on postsynaptic spiny neurons are weak when measured at the soma, and unable to significantly affect spike timing or generation. Further, reciprocity of synaptic connections between spiny neurons is only rarely observed. These results suggest that the bulk of the fast inhibition that has the strongest effects on spiny neuron spike timing comes from the feedforward interneuronal system whereas the axon collateral feedback system acts principally at the dendrites to control local excitability as well as the overall level of activity of the spiny neuron.

Dopaminergic modulation of short-term synaptic plasticity at striatal inhibitory synapses

Circuit properties, such as the selection of motor synergies, have been posited as relevant tasks for the recurrent inhibitory synapses between spiny projection neurons of the neostriatum, a nucleus of the basal ganglia participating in procedural learning and voluntary motor control. Here we show how the dopaminergic system regulates short-term plasticity (STP) in these synapses. STP is thought to endow neuronal circuits with computational powers such as gain control, filtering, and the emergence of transitory net states. But little is known about STP regulation. Employing unitary and population synaptic recordings, we observed that activation of dopamine receptors can modulate STP between spiny neurons. A D(1)-class agonist enhances, whereas a D(2)-class agonist decreases, short-term depression most probably by synaptic redistribution. Presynaptic receptors appear to be responsible for this modulation. In contrast, STP between fast-spiking interneurons and spiny projection neurons is largely unregulated despite expressing presynaptic receptors. Thus, the present experiments provide an explanation for dopamine actions at the circuit level: the control of STP between lateral connections of output neurons and the reorganization of the balance between different forms of inhibitory transmission. Theoretically, D(1) receptors would promote a sensitive, responsive state for temporal precision (dynamic component), whereas D(2) receptors would sense background activity (static component).

Changes in activity of the striatum during formation of a motor habit

To examine experience-dependent plasticity of striatal neurons during habit learning in awake, freely moving animals, single neurons in the dorsolateral striatum (54 neurons related specifically to vertical head movement and 14 unresponsive neurons, i.e. not related to any body movement) were recorded and tracked off-line to assess changes in firing rate over sessions as performance of instrumental head movement became automatic and habitual. Rats were trained to emit operant vertical head movements that triggered water delivery for 14 sessions (2 h per session, one session per day). Rats significantly increased the number and efficiency of head movements over sessions until reaching asymptotic behaviour. Habit formation was indicated by significantly higher levels of instrumental responding exhibited by rats during a late, relative to an early, session in which the reward was devalued. As head movements became habitual across sessions, most head movement-related neurons (89%) exhibited decreased firing rate, while a small population (11%) exhibited increased or maintained firing rate. The rate of decrease in firing by the majority correlated with the rate of improvement in movement efficiency over sessions. All unresponsive neurons, though not apparently related to movement, exhibited decreased firing rate over sessions. Our findings suggest that, during habit learning, the striatum may shift from facilitating acquisition of efficient movement with a large population of neurons to modulating or maintaining habitual movement with stronger firing of fewer movement-related neurons.

Higher magnitude accumbal phasic firing changes among core neurons exhibiting tonic firing increases during cocaine self-administration

Studies using i.v. cocaine self-administration in rats have documented rapid-phasic changes in the firing rate of nucleus accumbens neurons within seconds of cocaine-reinforced lever presses, as well as changes that occur over the course of the cocaine self-administration experiment, i.e. tonic changes in firing rate. During the self-administration period of the experiment, individual neurons exhibit either a tonic increase, a tonic decrease, or no tonic change in firing rate, relative to the neuron's firing rate during the pre-drug period. We evaluated whether rapid-phasic changes in firing were differentially associated with tonically reduced or tonically elevated firing of nucleus accumbens core and shell neurons in cocaine self-administering rats. Rapid-phasic firing patterns within seconds of the cocaine-reinforced lever press were exhibited predominantly by core neurons that also exhibited tonic increases in firing. Conversely, core neurons that did not exhibit such rapid-phasic firing patterns were more likely to show tonically reduced firing. Moreover, core neurons were more likely than shell neurons to exhibit: 1) tonic increases in firing and 2) rapid-phasic increases in firing preceding the cocaine-reinforced lever press. These differences between accumbens subterritories may be related to their distinct involvement in operant responding; the present findings are consistent with an emerging literature which implicates shell in contextual stimulus-induced responding, and core in processing the instrumental response via its discrete output to classic basal ganglia structures. The distinct tendency of the core to exhibit increased firing, coupled with its dichotomous firing outputs (i.e. tonic decreases without rapid phasic responses or tonic increases with rapid phasic responses), may reflect particular sensitivity of these neurons to excitatory limbic afferent signaling involved in instrumental responding. Enhanced phasic responsivity in the core may be an integral component of the mechanism inherent in normal reward processing which is subverted by chronic drug exposure.

GABAergic microcircuits in the neostriatum

The vast majority of neostriatal neurons and intrinsic intrastriatal synapses are GABAergic, the latter arising from axon collaterals of spiny projection neurons and from GABAergic interneurons. An important feature of the functional organization of the neostriatum has long been assumed to be the existence of a widespread lateral inhibitory network mediated by the axon collaterals of spiny projection neurons. However, these collateral connections have recently been demonstrated electrophysiologically to be relatively weak--in contrast to feedforward interneuronal inhibition, which exerts strong effects on spike timing in spiny neurons. These new data are incompatible with current "winner-take-all" models of lateral inhibitory function in the neostriatum, and they force a modification of established concepts of the functional roles of feedback inhibition in this nucleus.

Regulation of affect by the lateral septum: implications for neuropsychiatry

Substantial evidence indicates that the lateral septum (LS) plays a critical role in regulating processes related to mood and motivation. This review presents findings from the basic neuroscience literature and from some clinically oriented research, drawing from behavioral, neuroanatomical, electrophysiological, and molecular studies in support of such a role, and articulates models and hypotheses intended to advance our understanding of these functions. Neuroanatomically, the LS is connected with numerous regions known to regulate affect, such as the hippocampus, amygdala, and hypothalamus. Through its connections with the mesocorticolimbic dopamine system, the LS regulates motivation, both by stimulating the activity of midbrain dopamine neurons and regulating the consequences of this activity on the ventral striatum. Evidence that LS function could impact processes related to schizophrenia and other psychotic spectrum disorders, such as alterations in LS function following administration of antipsychotics and psychotomimetics in animals, will also be presented. The LS can also diminish or enable fear responding when its neural activity is stimulated or inhibited, respectively, perhaps through its projections to the hypothalamus. It also regulates behavioral manifestations of depression, with antidepressants stimulating the activity of LS neurons, and depression-like phenotypes corresponding to blunted activity of LS neurons; serotonin likely plays a key role in modulating these functions by influencing the responsiveness of the LS to hippocampal input. In conclusion, a better understanding of the LS may provide important and useful information in the pursuit of better treatments for a wide range of psychiatric conditions typified by disregulation of affective functions.

Adenosine A2A receptor-induced inhibition of NMDA and GABAA receptor-mediated synaptic currents in a subpopulation of rat striatal neurons

The function of adenosine A(2A) receptors, localized at the enkephalin-containing GABAergic medium spiny neurons of the striatum, has been discussed controversially. Here we show that, in the absence of external Mg(2+), the adenosine A(2A) receptor agonist CGS 21680 postsynaptically depressed the NMDA, but not the non-NMDA (AMPA/kainate) receptor-mediated fraction of the electrically evoked EPSCs in a subpopulation of striatal neurons. Current responses to locally applied NMDA but not AMPA were also inhibited by CGS 21680. However, in the presence of external Mg(2+), the inhibition by CGS 21680 of the GABA(A) receptor-mediated IPSCs led to a depression of the EPSC/IPSC complexes. The current response to the locally applied GABA(A) receptor agonist muscimol was unaltered by CGS 21680. Whereas, the frequency of spontaneous (s)IPSCs was inhibited by CGS 21680, their amplitude was not changed. Hence, it is suggested that under these conditions the release rather than the postsynaptic effect of GABA was affected by CGS 21680. In conclusion, under Mg(2+)-free conditions, CGS 21680 appeared to postsynaptically inhibit the NMDA receptor-mediated component of the EPSC, while in the presence of external Mg(2+) this effect turned into a presynaptic inhibition of the GABA(A) receptor-mediated IPSC.

Direct Physiological Evidence for Synaptic Connectivity Between Medium-Sized Spiny Neurons in Rat Nucleus Accumbens In Situ

Dual whole cell patch-clamp recordings in rat nucleus accumbens, the main component of the ventral striatum, were made to assess the presence of synaptic interconnections between medium-sized spiny neurons, a group of GABAergic and peptidergic neurons that constitute the principal cells of the striatum. Neurons were stained with biocytin for subsequent morphological analysis. Electrical activity of cells was recorded in current- and voltage-clamp mode; the characteristics of medium-sized spiny neurons were confirmed by electrophysiological and morphological properties. Thirteen of 38 medium-sized spiny neuron pairs (34%) showed a synaptic connection. In these pairs, suprathreshold stimulation with current injection evoked a train of action potentials in the presynaptic cell, which in turn elicited depolarizing postsynaptic potentials (dPSPs) in the postsynaptic cell. Twelve of these 13 pairs were connected unilaterally. The onset latency of the postsynaptic response was 1.7 ± 0.7 ms. dPSPs were blocked by 12.5 μM bicuculline, suggesting they were mediated by GABAA receptors. A linear fit of the current-voltage relationship of GABAergic currents crossed the voltage axis near the value of -20 mV, in agreement with the Cl- equilibrium potential predicted from the composition of the artificial cerebrospinal fluid and pipette medium. No evidence for electrotonic coupling was found. Paired-pulse facilitation and depression were induced when the amplitude of the first IPSC of a pair was relatively small and large, respectively. No clear dependence of paired-pulse facilitation or depression was found on the width of the spike interval, which ranged between 100 and 380 ms. Conversely, 1- to 2-s trains of dPSPs showed marked frequency facilitation at low presynaptic frequencies, but frequency depression at high firing rates. These data show that intra-accumbens synaptic communication between medium-sized spiny neurons exists, is mediated by GABAA receptors, and exhibits spike train-dependent short-term dynamics.

Dopaminergic modulation of axon collaterals interconnecting spiny neurons of the rat striatum

Dopamine is a critical modulator of striatal function; its absence produces Parkinson's disease. Most cellular actions of dopamine are still unknown. This work describes the presynaptic actions of dopaminergic receptor agonists on GABAergic transmission between neostriatal projection neurons. Axon collaterals interconnect projection neurons, the main axons of which project to other basal ganglia nuclei. Most if not all of these projecting axons pass through the globus pallidus. Thus, we lesioned the intrinsic neurons of the globus pallidus and stimulated neostriatal efferent axons antidromically with a bipolar electrode located in this nucleus. This maneuver revealed a bicuculline-sensitive synaptic current while recording in spiny cells. D1 receptor agonists facilitated whereas D2 receptor agonists depressed this synaptic current. In contrast, a bicuculline-sensitive synaptic current evoked by field stimulation inside the neostriatum was not consistently modulated, in agreement with previous studies. The data are discussed in light of the most recent experimental and modeling results. The conclusion was that inhibition of spiny cells by axon collaterals of other spiny cells is quantitatively important; however, to be functionally important, this inhibition might be conditioned to the synchronized firing of spiny neurons. Finally, dopamine exerts a potentially important role regulating the extent of lateral inhibition.

Synaptic transmission: functional autapses in the cortex

Autapses--synapses made by a neuron onto itself--have often been observed anatomically in vivo, but their purpose has remained uncertain. A recent paper describes functional autapses in the cerebral cortex and gives clues to their physiological role.

Functional autaptic neurotransmission in fast-spiking interneurons: a novel form of feedback inhibition in the neocortex

Autapses are synapses made by a neuron onto itself. Although morphological evidence for existence of autapses has been reported in several brain areas, it is not known whether such self-innervation in the neocortex is functional and robust. Here we report that GABAergic autaptic activity is present in fast-spiking, but not in low-threshold spiking, interneurons of layer V in neocortical slices. Recordings made with the perforated-patch technique, in which physiological intracellular chloride homeostasis was unperturbed, demonstrated that autaptic activity has significant inhibitory effects on repetitive firing and increased the current threshold for evoking action potentials. These results show that autapses are not rudimentary nonfunctional structures, but rather they provide a novel and powerful form of feedback inhibitory synaptic transmission in one class of cortical interneurons.

Modulation of inhibitory autapses and synapses on rat CA1 interneurones by GABA(A) receptor ligands

To determine whether autaptic inhibition plays a functional role in the adult hippocampus, the action potential afterhyperpolarisations (spike AHPs) of CA1 interneurones were investigated in 25 basket, three bistratified and eight axo-axonic cells. The spike AHPs showed two minima in all regular-spiking (5), burst-firing (3) and in many fast-spiking cells (17:28). The fast component had a time-to-peak (TTP) of 1.2 +/- 0.5 ms, the slower TTP was very variable (range of 3.3-103 ms). The AHP width at half-amplitude (HW) was 12.5 +/- 5.7 ms in fast-spiking, 29.3 +/- 18 ms in regular-spiking and 99.7 +/- 42 ms in burst-firing cells. Axo-axonic cells never establish autapses, and the fast-spiking variety showed narrow (HW: 3.9 +/- 0.7 ms) spike AHPs with only one AHP minimum (TTP: 0.9 +/- 0.1 ms). When challenged with GABA(A) receptor modulators, spike AHPs in basket and bistratified cells were enhanced by zolpidem (HW by 18.4 +/- 6.2 % in 10:15 cells tested), diazepam (45.2 +/- 0.5 %, 6:7), etomidate (43.9 +/- 36 %, 6:8) and pentobarbitone sodium (41 %, 1:1), and were depressed by bicuculline (-41 +/- 5.7 %, 5:8) and picrotoxin (-54 %, 1:1), and the enhancement produced by zolpidem was reduced by flumazenil (-31 +/- 13 %, relative to the AHP HW during exposure to zolpidem, 3:4). Neuronal excitability was modulated in parallel. The spike AHPs of three axo-axonic cells tested showed no sensitivity to etomidate, pentobarbitone or diazepam. Interneurone-to-interneurone inhibitory postsynaptic potentials (IPSPs), studied with dual intracellular recordings, had time courses resembling those of the spike AHPs. The IPSP HW was 13.4 +/- 2.8 ms in fast-spiking (n = 16) and 28.7 +/- 5.8 ms in regular-spiking/burst-firing cells (n = 6), and the benzodiazepine1-selective modulator zolpidem strongly enhanced these IPSPs (45 +/- 28 %, n = 5). Interneurones with spike AHPs affected by the GABA(A) receptor ligands exhibited 3.8 +/- 1.9 close autaptic appositions. In three basket cells studied at the ultrastructural level 6 of 6, 1 of 2 and 1 of 2 close appositions were confirmed as autapses. Therefore, in the hippocampus autaptic connections contribute to spike AHPs in many interneurones. These autapses influence neuronal firing and responses to GABA(A) receptor ligands.

Inhibitory interactions between spiny projection neurons in the rat striatum

The spiny projection neurons are by far the most numerous type of striatal neuron. In addition to being the principal projection neurons of the striatum, the spiny projection neurons also have an extensive network of local axon collaterals by which they make synaptic connections with other striatal projection neurons. However, up to now there has been no direct physiological evidence for functional inhibitory interactions between spiny projection neurons. Here we present new evidence that striatal projection neurons are interconnected by functional inhibitory synapses. To examine the physiological properties of unitary inhibitory postsynaptic potentials (IPSPs), dual intracellular recordings were made from pairs of spiny projection neurons in brain slices of adult rat striatum. Synaptic interactions were found in 9 of 45 pairs of neurons using averages of 200 traces that were triggered by a single presynaptic action potential. In all cases, synaptic interactions were unidirectional, and no bidirectional interactions were detected. Unitary IPSPs evoked by a single presynaptic action potential had a peak amplitude ranging from 157 to 319 microV in different connections (mean: 277 +/- 46 microV, n = 9). The percentage of failures of single action potentials to evoke a unitary IPSP was estimated and ranged from 9 to 63% (mean: 38 +/- 14%, n = 9). Unitary IPSPs were reversibly blocked by bicuculline (n = 4) and had a reversal potential of -62.4 +/- 0.7 mV (n = 5), consistent with GABA-mediated inhibition. The findings of the present study correlate very well with anatomical evidence for local synaptic connectivity between spiny projection neurons and suggest that lateral inhibition plays a significant role in the information processing operations of the striatum.

Direct examination of local regulation of membrane activity in striatal and prefrontal cortical neurons in vivo using simultaneous intracellular recording and microdialysis

Slice preparations are typically used to study the effects of pharmacological manipulations on the electrophysiological activity of mature neurons. However, the severing of afferent inputs is known to significantly change the natural membrane activity of the neuron. To study the effects of local pharmacological manipulations on neurons in the intact brain, we combined the methods of microdialysis and intracellular recording in vivo. After implantation of a microdialysis probe into the prefrontal cortex (PFC) or striatum, intracellular recordings were conducted within approximately 500 microm of the active surface of the probe. The spontaneous membrane activity, passive membrane properties, and intracellularly and synaptically evoked responses of striatal and cortical neurons recorded during perfusion of artificial cerebral spinal fluid were not different from that of neurons recorded in intact animals. Moreover, in the PFC, local perfusion with glutamate or N-methyl-D-aspartate depolarized neurons and increased spike activity. Conversely, local perfusion of tetrodotoxin hyperpolarized neurons while markedly reducing spontaneous membrane depolarizations and eliminating spike activity. In the striatum, local perfusion of the gamma-aminobutyric acid(A) receptor antagonist bicuculline rapidly depolarized neurons and increased spontaneous spike activity. Given that striatal and PFC neurons recorded in animals undergoing microdialysis in the current study exhibited electrophysiological properties similar to those recorded in intact controls, it is likely that the effects of local microdialysis on ongoing synaptic activity, neuronal excitability, and endogenous neurotransmitter levels are minimal. We conclude that the use of local microdialysis with intracellular recording is a powerful method for studying local receptor regulation of synaptic activity in vivo.

Differential changes of synaptic transmission in spiny neurons of rat neostriatum following transient forebrain ischemia

Spiny neurons in neostriatum are vulnerable to cerebral ischemia. To reveal the mechanisms underlying the postischemic neuronal damage, the spontaneous activities, evoked postsynaptic potentials and membrane properties of spiny neurons in rat neostriatum were compared before and after transient forebrain ischemia using intracellular recording and staining techniques in vivo. In control animals the membrane properties of spiny neurons were about the same between the left and right neostriatum but the inhibitory synaptic transmission was stronger in the left striatum. After severe ischemia, the spontaneous firing and membrane potential fluctuation of spiny neurons dramatically reduced. The cortically evoked initial excitatory postsynaptic potentials were suppressed after ischemia indicated by the increase of stimulus threshold and the rise time of these components. The paired-pulse facilitation test indicated that such suppression might involve presynaptic mechanisms. The inhibitory postsynaptic potentials in spiny neurons were completely abolished after ischemia and never returned to the control levels. A late depolarizing postsynaptic potential that was elicited from approximately 5% of the control neurons by cortical stimulation could be evoked from approximately 30% of the neurons in the left striatum and approximately 50% in the right striatum after ischemia. The late depolarizing postsynaptic potential could not be induced after acute thalamic transection. The intrinsic excitability of spiny neurons was suppressed after ischemia evidenced by the significant increase of spike threshold and rheobase as well as the decrease of repetitive firing rate following ischemia. The membrane input resistance and time constant increased within 6 h following ischemia and the amplitude of fast afterhyperpolarization significantly increased after ischemia. These results indicate the depression of excitatory monosynaptic transmission, inhibitory synaptic transmission and excitability of spiny neurons after transient forebrain ischemia whereas the excitatory polysynaptic transmission in neostriatum was potentiated. The facilitation of excitatory polysynaptic transmission is stronger in the right neostriatum than in the left neostriatum after ischemia. The suppression of inhibitory component and the facilitation of excitatory polysynaptic transmission may contribute to the pathogenesis of neuronal injury in neostriatum after transient cerebral ischemia.

A possible functional organization of the corticostriatal input within the weakly-correlated striatal activity: a modeling study

Recently, it was reported in an in vivo study that pairs of the striatal projection neurons (medium-sized spiny neurons) of the basal ganglia show asynchronous spiking within weakly-correlated subthreshold depolarized states. In this computational study, we investigate a possible functional organization of corticostriatal inputs that accounts for the experimental observations within known anatomical and physiological constraints. In a pair of medium-sized spiny neurons, a small fraction of corticostriatal fibers is common to both neurons. To explain the weak correlations in sub- and supra-threshold activities of the neuron pair, we postulate that the two input channels, common or specific to the individual neurons, have distinct functional roles. The common input channel delivers random spike trains and is primarily responsible for the initiation and maintenance of the depolarized states. In contrast, the input through the neuron-specific channels elicit postsynaptic spikes by delivering intermittently-synchronized spikes. The results of this model were compared with those derived from a newly-performed analysis of the previous double-intracellular recording data. We show that the behavior of this model agrees qualitatively and quantitatively with that of the medium-sized spiny neurons observed in the experiments in a certain range of the coincident time window.

Ultrastructural localization of adenosine A2A receptors suggests multiple cellular sites for modulation of GABAergic neurons in rat striatum

Activation of adenosine A2A receptors (A2AR) has been shown to antagonize the function of D2 dopaminergic regulation of striatal gamma-aminobutyric acid (GABA)-ergic output and, thus, locomotor activity. Adenosine A2A receptor immunoreactivity (A2A-LI) has been localized to rat striatum by light microscopy by using a previously characterized human A2AR monoclonal antibody. In this study, we evaluated the localization of A2A-LI and its colocalization with GABA immunoreactivity (GABA-LI) in dorsolateral rat striatum by immunoelectron microscopy to further characterize the potential mechanism of purinergic control of striatal output. Ultrastructural analysis demonstrated A2A-LI associated with the plasma membrane and cytoplasmic membranous structures of striatal neurons. A2A-LI was prevalent in dendrites and dendritic spines ( approximately 70% of total A2A-profiles counted) and less prevalent in axons and axon terminals (23%), soma (3%), and glia (3%). Cellular elements exhibiting both A2A-LI and GABA-LI comprised 23% of the total profiles counted; colabeling was most common in dendrites. A2A-LI was observed primarily at asymmetric synapses (n = 70) (both pre- and postsynaptically but predominantly in the postsynaptic element) and less frequently at symmetric synapses (n = 17). Of the 714 A2A-immunoreactive profiles examined, 37% were apposed to GABA-labeled profiles. The most common appositions were A2A-labeled dendrites apposed to GABA-immunoreactive dendrites (n = 132), axon terminals (n = 28), and somata (n = 22) and A2A-labeled axons apposed to GABA-labeled dendrites (n = 58), axon terminals (n = 14), and somata (n = 9). Our findings suggest that adenosine may play an important role in modulating excitatory input to striatal neurons and that A2AR may modulate GABAergic signaling at several cellular sites within the rat striatum.

Direct actions of cannabinoids on synaptic transmission in the nucleus accumbens: a comparison with opioids

The nucleus accumbens (NAc) represents a critical site for the rewarding and addictive properties of several classes of abused drugs. The medium spiny GABAergic projection neurons (MSNs) in the NAc receive innervation from intrinsic GABAergic interneurons and glutamatergic innervation from extrinsic sources. Both GABA and glutamate release onto MSNs are inhibited by drugs of abuse, suggesting that this action may contribute to their rewarding properties. To investigate the actions of cannabinoids in the NAc, we performed whole cell recordings from MSNs located in the shell region in rat brain slices. The cannabinoid agonist WIN 55,212-2 (1 microM) had no effect on the resting membrane potential, input resistance, or whole cell conductance, suggesting no direct postsynaptic effects. Evoked glutamatergic excitatory postsynaptic currents (EPSCs) were inhibited to a much greater extent by [Tyr-D-Ala(2), N-CH(3)-Phe(4), Gly-ol-enkephalin] (DAMGO, approximately 35%) than by WIN 55,212-2 (<20%), and an analysis of miniature EPSCs suggested that the effects of DAMGO were presynaptic, whereas those of WIN 55,212-2 were postsynaptic. However, electrically evoked GABAergic inhibitory postsynaptic currents (evIPSCs), were reduced by WIN 55,212-2 in every neuron tested (EC(50) = 123 nM; 60% maximal inhibition), and the inhibition of IPSCs by WIN 55,212-2 was completely antagonized by the CB1 receptor antagonist SR141716A (1 microM). In contrast evIPSCs were inhibited in approximately 50% of MSNs by the mu/delta opioid agonist D-Ala(2)-methionine(2)-enkephalinamide and were completely unaffected by a selective mu-opioid receptor agonist (DAMGO). WIN 55,212-2 also increased paired-pulse facilitation of the evIPSCs and did not alter the amplitudes of tetrodotoxin-resistant miniature IPSCs, suggesting a presynaptic action. Taken together, these data suggest that cannabinoids and opioids differentially modulate inhibitory and excitatory synaptic transmission in the NAc and that the abuse liability of marijuana may be related to the direct actions of cannabinoids in this structure.

Third group of neostriatofugal neurons: neurokinin B-producing neurons that send axons predominantly to the substantia innominata

Neostriatal neurons that produce neurokinin B were investigated immunocytochemically in the rat brain with an antibody against the C-terminal portion of the precursor prepropeptide of neurokinin B, preprotachykinin B (PPTB). PPTB-immunoreactive neurons were scattered throughout the neostriatum and constituted 5.1% of neostriatal neurons. They were immunopositive for projection neuron markers, such as precursor peptides of substance P, enkephalins, and dynorphins, but negative for intrinsic neuron markers, suggesting that PPTB was expressed in neostriatal projection neurons. However, PPTB-immunoreactive neurons were immunonegative for dopamine- and cyclic AMP-regulated phosphoprotein, which is known to be produced by striatopallidal and striatonigral neurons. Furthermore, almost no PPTB-immunoreactive axon terminals were observed in the substantia nigra or globus pallidus. The authors then made large kainic acid lesions in the neostriatum to reveal the target areas of PPTB-producing neurons and observed a decrease in PPTB-immunoreactive fibers in the sublenticular portion of the substantia innominata and, to much lesser extent, in the bed nucleus of the stria terminalis and central nucleus of the amygdala. After injection of wheat germ agglutinin into the substantia innominata, PPTB immunoreactivity was detected in many retrogradely labeled neostriatal neurons. In contrast, no PPTB immunoreactivity was observed in striatonigral or striatopallidal neurons after injection of retrograde tracers into the substantia nigra or globus pallidus. Thus, neurokinin B-producing neostriatal neurons were considered to send projection fibers predominantly to the substantia innominata. Furthermore, PPTB-immunoreactive axonal swellings were closely apposed to neurokinin B receptor-immunoreactive dendrites in the substantia innominata. Overall, the present results indicate that the rat brain possesses a chemically and hodologically unique neostriatofugal pathway in addition to the direct and indirect pathways.

Inhibition by adenosine A(2A) receptors of NMDA but not AMPA currents in rat neostriatal neurons

Whole-cell patch clamp experiments were used to investigate the transduction mechanism of adenosine A(2A) receptors in modulating N-methyl-D-aspartate (NMDA)-induced currents in rat striatal brain slices. The A(2A) receptor agonist 2-p-(2-carboxyethyl)phenethylamino-5'-N-ethylcarboxamidoadenosine (CGS 21680) inhibited the NMDA, but not the (S)-alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) current in a subset of striatal neurons. Lucifer yellow-filled pipettes in combination with immunostaining of A(2A) receptors were used to identify CGS 21680-sensitive cells as typical medium spiny striatal neurons. Dibutyryl cyclic AMP and the protein kinase A activator Sp-cyclic AMPs, but not the protein kinase A inhibitors Rp-cyclic AMPS or PKI(14 - 24)amide abolished the inhibitory effect of CGS 21680. The phospholipase C inhibitor U-73122, but not the inactive structural analogue U-73343 also interfered with CGS 21680. The activation of protein kinase C by phorbol 12-myristate 13-acetate or the blockade of this enzyme by staurosporine did not alter the effect of CGS 21680. Heparin, an antagonist of inositol 1, 4,5-trisphosphate (InsP(3)) and a more efficient buffering of intracellular Ca(2+) by BAPTA instead of EGTA in the pipette solution, abolished the CGS 21680-induced inhibition. The calmodulin antagonist W-7 and cytochalasin B which enhances actin depolymerization also prevented the effect of CGS 21680; the calmodulin kinase II inhibitors CaM kinase II(281 - 309) and KN-93 but not the inactive structural analogue KN-92 were also effective. The calcineurin inhibitor deltamethrin did not interfere with CGS 21680. It is suggested that the transduction mechanism of A(2A) receptors to inhibit NMDA receptor channels is the phospholipase C/InsP(3)/calmodulin and calmodulin kinase II pathway. The adenylate cyclase/protein kinase A and phospholipase C/protein kinase C pathways do not appear to be involved.

Sequence generation in arbitrary temporal patterns from theta-nested gamma oscillations: a model of the basal ganglia-thalamo-cortical loops

A computational model that is able to generate sequences at arbitrary rates in a given serial order is presented for the cortico-basal ganglia (BG)-thalamic neural circuitry. Upon generating a sequence, this model stores information on the serial order of components in a cortical buffer by means of theta-nested gamma frequency oscillations observed experimentally in cortico-striatal neurons. This model assumes the existence of at least two functionally different classes of striatal spiny neurons. One class of striatal projection neurons (S-cells) select the first component in the cortical buffer through a temporal winner-take-all mechanism implemented by lateral inhibition. The inhibition should last for at least a few hundred milliseconds. In reality, it may be mediated by GABA(B) receptors at the presynaptic terminals of the cortico-striatal projection. The other class of striatal projection neurons (M-cells) retain the currently executed component in a cortico-BG-thalamic loop, for which the strong nonlinearity in transitions between up and down states of striatal neurons is crucial. For sequence generation at the level of striatum, the cortical neurons encoding the component selected for execution are inactivated by the feedback from the activated cortico-BG-thalamic loop. This model predicts that the transition to next component is triggered by a single external signal, i.e. the subthalamic input to the globus pallidum. This input gives a neural substrate for adjusting the rate of sequence generation.