The action of picrotoxin and bicuculline on rat caudate neurons inhibited by GABA

Sequentially switching cell assemblies in random inhibitory networks of spiking neurons in the striatum

The striatum is composed of GABAergic medium spiny neurons with inhibitory collaterals forming a sparse random asymmetric network and receiving an excitatory glutamatergic cortical projection. Because the inhibitory collaterals are sparse and weak, their role in striatal network dynamics is puzzling. However, here we show by simulation of a striatal inhibitory network model composed of spiking neurons that cells form assemblies that fire in sequential coherent episodes and display complex identity-temporal spiking patterns even when cortical excitation is simply constant or fluctuating noisily. Strongly correlated large-scale firing rate fluctuations on slow behaviorally relevant timescales of hundreds of milliseconds are shown by members of the same assembly whereas members of different assemblies show strong negative correlation, and we show how randomly connected spiking networks can generate this activity. Cells display highly irregular spiking with high coefficients of variation, broadly distributed low firing rates, and interspike interval distributions that are consistent with exponentially tailed power laws. Although firing rates vary coherently on slow timescales, precise spiking synchronization is absent in general. Our model only requires the minimal but striatally realistic assumptions of sparse to intermediate random connectivity, weak inhibitory synapses, and sufficient cortical excitation so that some cells are depolarized above the firing threshold during up states. Our results are in good qualitative agreement with experimental studies, consistent with recently determined striatal anatomy and physiology, and support a new view of endogenously generated metastable state switching dynamics of the striatal network underlying its information processing operations.

Effects of local connectivity on striatal function: stimulation and analysis of a model

Neuronal population activity was investigated by computer simulation of a network model based on the neostriatum. Three network topologies were studied, based on different assumptions about the synaptic connectivity among medium spiny neurons. In all networks neurons were interconnected by inhibitory synapses. The connectivity was either symmetric, in which case all connections between cells were reciprocal and equal in strength; or asymmetric. Simulations showed that networks with symmetric connectivity receiving randomly distributed afferent excitation produced stationary spatial activity patterns. In contrast, asymmetric connectivity in homogeneous networks produced slow travelling-wave activity across the network. We suggest that the shape of the medium spiny neurons is an important determinant of synaptic connectivity and that changes in the shape of these neurons caused by Huntington's disease would result in asymmetric connectivity. Slow travelling-wave activity produced by asymmetric connectivity in the neostriatum could explain some aspects of the choreic movement and some electromyographic features seen in Huntington's patients.

GABA depolarizes neurons in the rat striatum: an in vivo study

GABA, applied by iontophoresis to striatal neurons of the rat in an in vivo preparation, depolarized the membrane potential and decreased the input resistance in a dose-dependent manner. The null potential of the GABA depolarization was about -50 mV. In addition, a fading of the GABA-induced response was observed for prolonged and relatively high amino acid application. We conclude that GABA has a depolarizing effect on striatal cells. This is in line with recent in vitro works describing a depolarizing effect of GABA in the rat neostriatum.

Late development of intrinsic excitation in the rat neostriatum: an in vitro study

Functional maturation of intrinsic circuitry in the neostriatum was studied by intracellular recording and intracellular staining with Lucifer yellow in slices obtained from rat pups at postnatal days (P)1-20 and from adult rats. The most striking observation was that intrastriatal stimulation elicited predominantly inhibitory responses in slices obtained from animals of P1-6. In contrast, intrastriatally evoked responses in slices obtained after P10 were predominantly excitatory. The inhibitory postsynaptic potentials (IPSPs) recorded in slices obtained from pups were blocked by bicuculline (50 microM) and exhibited a reversal potential at about -60 mV which shifted in depolarizing direction when intracellular Cl- activity increased. Thus, these IPSPs correspond to IPSPs observed in adult animals. It is concluded that maturation of excitatory synapses is the main change during postnatal development. The changes of postsynaptic potentials were paralleled by the appearance of spines on dendrites around P7 as revealed by intracellular staining. The apparent input resistances and time constants of young neurons were very high and responses to large current injections were often distorted by humps which could not be observed in adult neurons. Only young neurons responded with bursts to synaptic activation in the presence of bicuculline (50 microM). It appears that dendritic conductances have a stronger influence on somatic discharge in the electrotonically compact young neurons than in adult neurons.

The effects of chlormethiazole on single unit activity in rat brain; interactions with inhibitory and excitatory neurotransmitters

Extracellular recordings were made of single unit activity in the brainstem of urethane anaesthetized rats. Drugs were applied by microiontophoresis from multibarrelled micropipettes or administered intraperitoneally. Chlormethiazole (CMZ) caused a decrease in spontaneous firing rate when applied with high currents (greater than 40 nA). When applied with lower currents CMZ did not cause changes in firing rate, but enhanced the inhibitory effects of gamma-aminobutyric acid (GABA), muscimol and glycine in a dose-dependent manner. The inhibitory actions of acetylcholine were not affected. Excitatory responses to glutamate and acetylcholine were unaffected by applications of CMZ which caused potentiation of GABA, muscimol and glycine. When applied at higher currents CMZ caused a decrease in the response to glutamate. Intraperitoneal administration of CMZ (50-600 mumol kg-1) also enhanced responses to microiontophoretically applied GABA, muscimol and glycine. These results are compared with those reported for other anticonvulsant drugs and possible mechanisms of action of CMZ are discussed.

Disfacilitation and long-lasting inhibition of neostriatal neurons in the rat

Excitatory postsynaptic potentials evoked in rat neostriatal spiny projections neurons were followed by a long (100-300 ms) period of membrane hyperpolarization, followed in turn by a late depolarization. Concomitant with these changes in membrane potential were inhibition and subsequent excitation of spontaneous firing and excitatory activity evoked from substantia nigra and cerebral peduncle, but not from cortical stimulating sites. Thalamic-evoked excitatory activity was sometimes sensitive and sometimes insensitive to this inhibition, which has previously been believed to result from intrinsic inhibitory synaptic activity among neostriatal neurons. In intracellular recordings from neostriatal neurons in urethane anesthetized rats this long-lasting inhibitory response (1) exhibited alterations with intracellularly applied steady currents comparable to those of the EPSP, (2) failed to respond to intracellular injection of chloride ions, (3) was associated with either a decrease or no detectable change in the input conductance of the neurons, and (4) was abolished after lesions that interrupted polysynaptic pathways to neostriatum through intracortical and intrathalamic synaptic circuits. These findings indicate that the long lasting inhibitory portion of the responses of neostriatal neurons arises from a phasic inhibition of tonically active corticostriatal and thalamostriatal neurons and a concurrent decrease in the excitability of polysynaptic pathways converging on neostriatal neurons.

A theory of the functional organization of the neostriatum and the neostriatal control of voluntary movement

A theory of the intrinsic, functional organization of the neostriatum and the neostriatal control of voluntary movement is presented. The cell types of neostriatum are described in accordance with the classification scheme of Pasik, Pasik and DiFiglia (ref. 204) and their suspected neurotransmitters are identified. The functional relations between the afferent projections to neostriatum and the intrinsic elements of neostriatum are described. A division of the neostriatal efferents into two functional cell systems is conceived, in which one efferent system, the Spiny I cell matrix, is thought to be represented by a lateral inhibitory network comprised of the common Spiny I neurons which inhibit their targets, while another is termed the Spiny II cell cluster and consists of the far less numerous excitatory efferents of neostriatum (the Spiny II cells) and is represented by clusters of neurons, the members of each cluster consisting of one excitatory efferent neuron and three interneurons. The implications of this view for neostriatal input--output relations are discussed, and the theory is developed by reference to a variety of converging lines of empirical evidence, and is used to interpret the effects of various pharmacologic strategies which have been employed to achieve symptom management in disorders of voluntary movement. In the course of this review, there emerges an integrative theory of the principles governing the neostriatal control of voluntary movement.

Heterogeneous depression of glucose utilization in the caudate nucleus by GABA agonists

Local cerebral glucose utilization in the rat caudate nucleus has been examined following the intravenous administration of the GABA agonists muscimol and THIP. Although glucose utilization was reduced throughout the striatum following GABA agonists, the magnitude of the reductions in the lateral portion of the nucleus were markedly greater than in other portions of the caudate nucleus. This medial--lateral heterogeneity in response to GABA agonists contrasts with the known rostral--caudal heterogeneity in the distribution of biochemical markers for GABA.

Intracellular analysis of synaptic potentials in rat neostriatum following stimulation of the cerebral cortex, thalamus, and substantia nigra

Intracellular recordings were obtained from neostriatal neurons of unparalyzed male hooded rats anesthetized with urethane. Electrical stimulation of the cerebral cortex (Cx), centromedian-parafascicular area of the thalamus (CMP), and the substantia nigra (SN) elicited monosynaptic excitatory postsynaptic potentials (EPSPs) in neostriatal neurons. Response latencies were, on the average, 3.7 msec, 3.3 msec, and 3.8 msec, for Cx, CMP and SN stimulation, respectively. Over 85% of recorded neurons showed convergence of inputs from all three stimulation sites. The SN induced EPSP sometimes had two components, with the second component beginning 10-15 msec after the first. EPSPs from all three stimulation sites were often followed by inhibitory postsynaptic potentials (IPSPs) lasting from 50-250 msec. Double shock experiments indicated that SN induced EPSPs could be reduced in amplitude by 20-80% when preceded by conditioning stimulation to Cx, CMP or SN. In contrast, the EPSP elicited by Cx stimulation were unaffected by conditioning stimulation. Some recorded neurons were morphologically identified by means of intracellular injection of horseradish peroxidase. All were "medium spiny" neurons. The results of the present study agree well with those of previous studies of cat caudate neurons, and extend them to rat neostriatal neurons.

Recurrent inhibition in the rat neostriatum

During intracellular recording, in the neostriatum of rats anesthetized with urethane, the triggering of an action potential in the recorded neuron by a depolarizing pulse of current resulted in inhibition in that same neuron. This inhibition was evident through its ability to reduce the amplitude of EPSPs evoked from stimulation of substantia nigra. The shunting of SN EPSPs was shown not to be due to action potential currents. The inhibition is antagonized by the GABA blocking agent bicuculline. Intracellular labeling of recorded neurons revealed them as medium spiny neurons. It is concluded that the extensive axon collaterals of spiny projection neurons mediate recurrent inhibition, a portion of which involves autaptic synapses of a neuron back onto itself.

Effects of peripheral and local administration of picrotoxin on the release of newly synthesized 3H-dopamine in the caudate nucleus of the cat

The release of 3H-DA was estimated in superfusates of a superfusion cannula introduced into the caudate nucleus of "encéphale isolé" cats. 3H-DA was continuously formed from L-3,5-3H-tyrosine introduced into the superfusion medium. Injected at the periphery, picrotoxin (2.5 mg/kg, i.p.) markedly enhanced the release of 3H-DA, the effect being particularly pronounced 90 min after the drug injection. The effects of the local introduction of picrotoxin into the substantia nigra and into the caudate nucleus on 3H-DA release were also examined to ensure the specificity of the result. Picrotoxin (10(5) M) added into the superfusing medium of a second superfusion cannula introduced in to the substantia nigra enhanced by 2 to 5 times the release of 3H-DA during the drug application. No effect could be seen when the tip of the superfusion cannula was not exactly localized into the substantia nigra. Although less pronounced, a stimulation of 3H-DA release was also seen when picrotoxin(10(6) M) was added directly into the superfusion medium of the cannula introduced into the caudate nucleus. These results suggest that some gabaergic neurons are involved in the control of DA release by acting on dopaminergic cell bodies or dendrites into the substantia nigra. Other gabaergic neurons may directly or indirectly act on dopaminergic terminals within the caudate nucleus.