Biomedical subjects

J Bargas

Publications and source records attributed to J Bargas.

At least 37 records · Page 2Linked to original sources

PubMed1997

Dopamine facilitates striatal EPSPs through an L-type Ca2+ conductance.

When synaptic activity is evoked from relatively depolarized membrane potentials, D1 receptor agonists enhance the depolarization level and slow the decay of synaptic responses recorded from neostriatal spiny neurons. The population spikes' amplitude is also increased. These D1 actions facilitate firing and are evident in the presence of both NMDA and GABA selective blockers. Thus, dopaminergic D1 receptor activation facilitates the AMPA-mediated EPSP in these conditions. This facilitatory effect could be suppressed by L-type Ca2+ channel antagonists (200 nM calciseptine and 5 microM nicardipine), suggesting that it is mediated by an increase in L-current. D1-receptor activation thus mediates orthodromic facilitation of neostriatal neurons when evoked from depolarized membrane potentials. This reinforces the dopamine facilitation mediated through NMDA responses.

Animals↗

PubMed1997

D1 receptor activation enhances evoked discharge in neostriatal medium spiny neurons by modulating an L-type Ca2+ conductance.

Most in vitro studies of D1 dopaminergic modulation of excitability in neostriatal medium spiny neurons have revealed inhibitory effects. Yet studies made in more intact preparations have shown that D1 receptors can enhance or inhibit the responses to excitatory stimuli. One explanation for these differences is that the effects of D1 receptors on excitability are dependent on changes in the membrane potential occurring in response to cortical inputs that are seen only in intact preparations. To test this hypothesis, we obtained voltage recordings from medium spiny neurons in slices and examined the impact of D1 receptor stimulation at depolarized and hyperpolarized membrane potentials. As previously reported, evoked discharge was inhibited by D1 agonists when holding at negative membrane potentials (approximately -80 mV). However, at more depolarized potentials (approximately -55 mV), D1 agonists enhanced evoked activity. At these potentials, D1 agonists or cAMP analogs prolonged or induced slow subthreshold depolarizations and increased the duration of barium- or TEA-induced Ca2+-dependent action potentials. Both effects were blocked by L-type Ca2+ channel antagonists (nicardipine, calciseptine) and were occluded by the L-type channel agonist BayK 8644-arguing that the D1 receptor-mediated effects on evoked activity at depolarized membrane potential were mediated by enhancement of L-type Ca2+ currents. These results reconcile previous in vitro and in vivo studies by showing that D1 dopamine receptor activation can either inhibit or enhance evoked activity, depending on the level of membrane depolarization.

Animals↗

PubMed1997

Dopamine selects glutamatergic inputs to neostriatal neurons.

Glutamatergic synaptic potentials induced by micromolar concentrations of the potassium conductance blocker 4-aminopyridine (4-AP) were recorded intracellularly from rat neostriatal neurons in the presence of 10 microM bicuculline (BIC). These synaptic potentials originate from neostriatal cortical and thalamic afferents and were completely blocked by 10 microM 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX) plus 100 microM D-2-amino-5-phosphonovaleric acid (2-APV). Their inter-event time intervals could be fitted to exponential distributions, suggesting that they are induced randomly. Their amplitude distributions had most counts around 1 mV and fewer counts with values up to 5 mV. Since input resistance of the recorded neurons is about 40 M omega, the amplitudes agree to quantal size measurements in mammalian central neurons. The action of a D2 agonist, quinpirole, was studied on the frequency of these events. Mean amplitude of synaptic potentials was preserved in the presence of 2-10 microM quinpirole, but the frequency of 4-AP-induced glutamatergic synaptic potentials was reduced in 35% of cases. The effect was blocked by the D2 antagonist sulpiride (10 microM). Input resistance, membrane potential, or firing threshold did not change during quinpirole effect, suggesting a presynaptic site of action for quinpirole in some but not all glutamatergic afferents that make contact on a single cell. The present experiments show that dopaminergic presynaptic modulation of glutamatergic transmission in the neostriatum does not affect all stimulated afferents, suggesting that it is selective towards some of them. This may control the quality and quantity of afferent flow upon neostriatal neurons.

Animals↗

PubMed1996

Dopamine modulates the afterhyperpolarization in neostriatal neurones.

Intracellular techniques were used to study the actions of dopaminergic D1 agonists on the afterhyperpolarization (AHP) that follows action potentials in rat neostriatal neurones. Dopamine or Cl-APB (10 microM), or 1-10 microM 6-Cl-PB all increased AHP amplitude. This effect was blocked by 1 microM SCH-23390, a D1 antagonist, but not by 1 microM sulpiride, a D2 antagonist. Both 500 microM dibutyryl cAMP and 5 microM BayK 8644 induced a similar AHP increase. BayK 8644 occluded the effect of agonists. The results suggest that the action of dopamine is mediated via the recently described protein kinase A enhancement of L-type Ca2+ channels. The results partially explain the decrease in firing frequency induced by dopamine and a possible site of antagonism with cholinergic modulation.

3-Pyridinecarboxylic acid, 1,4-dihydro-2,6-dimethy↗

PubMed1996

Inhibitory action of dopamine involves a subthreshold Cs(+)-sensitive conductance in neostriatal neurons.

Intracellular recordings in in vitro slice preparations of rat brain were used to compare the actions of dopamine and dopamine receptor agonists on the subthreshold membrane properties of neostriatal neurons. A reproducible response for dopaminergic agonists was evoked after firing produced by current ramp injections that induced a subthreshold voltage displacement. Dopamine (10-100 microM) decreased both firing rate and membrane slope input resistance in virtually all cells tested. Input resistance change appeared as an increase in inward rectification. Approximate reversal potential was around -87 mV. The D1 receptor agonists SKF 38393 and Cl-APB (1-10 microM) mimicked both dopamine effects with a reversal potential around -89 mV. The effects were blocked by the presence of 5-10 mM caesium (Cs+) but not by 1 microM tetrodotoxin, suggesting that main D1 effects on input resistance are due to subthreshold Cs(+)-sensitive conductances. cAMP analogues mimicked the actions of D1 receptor agonists. The D2 agonist, quinpirole (1-10 microM), did not produce any input resistance change, nonetheless, it still produced a decrease in firing rate. This suggests that the main D2 effect on firing is due to actions on suprathreshold ion conductances. All effects were blocked by D1 and D2 antagonists, respectively. D1 or D2 effects were found in the majority of cells tested.

2,3,4,5-Tetrahydro-7,8-dihydroxy-1-phenyl-1H-3-ben↗

PubMed1995

Muscarinic receptors modulate the afterhyperpolarizing potential in neostriatal neurons.

The actions of carbachol were studied on the firing response of neostriatal neurons recorded intracellularly from in vitro slice preparations of the rat brain. Carbachol (1-10 microM) reversibly reduced the afterhyperpolarization in neostriatal neurons. This effect was accompanied by an increase in both firing frequency and input resistance in the subthreshold voltage range. Atropine (1-10 microM) reversibly blocked carbachol effects, suggesting muscarinic receptor modulation. Pirenzepine (up to 1 microM), but not AF-DX 384 (10 microM) or gallamine (30 microM), blocked the effects of carbachol on the afterhyperpolarization. The protein kinase C activator, phorbol 12,13 dibutyrate, but not the inactive phorbol ester, 4 alpha-phorbol 12-myristate 13-acetate, mimicked carbachol effects. The results suggest that muscarinic receptors, probably of the M1 type, regulate neostriatal excitability by modulating afterhyperpolarization.

Action Potentials↗

PubMed1995

Modulation of calcium currents by a D1 dopaminergic protein kinase/phosphatase cascade in rat neostriatal neurons.

In rat neostriatal neurons, D1 dopamine receptors regulate the activity of cyclic AMP-dependent protein kinase (PKA) and protein phosphatase 1 (PP1). The influence of these signaling elements on high voltage-activated (HVA) calcium currents was studied using whole-cell voltage-clamp techniques. The application of D1 agonists or cyclic AMP analogs reversibly reduced N- and P-type Ca2+ currents. Inhibition of PKA antagonized this modulation, as did inhibition of PP1, suggesting that the D1 effect was mediated by a PKA enhancement of PP1 activity directed toward Ca2+ channels. In a subset of neurons, D1 receptor-mediated activation of PKA enhanced L-type currents. The differential regulation of HVA currents by the D1 pathway helps to explain the diversity of effects this pathway has on synaptic integration and plasticity in medium spiny neurons.

1-Methyl-3-isobutylxanthine↗

PubMed1995

Firing patterns in substantia nigra compacta identified neurons in vitro.

Dopamine-containing neurons of the midbrain are required to control voluntary movements, behavior and motivation. Damage of these neurons in the substantia nigra zona compacta (SNc) is considered as the cause of Parkinson's disease. In this work, the firing characteristics of SNc neurons were studied in vitro. Neurons were identified by intracellular labeling tyrosine hydroxylase (TH) immunocytochemistry, and their electrophysiological characteristics. TH-positive neurons displayed three different firing modes. At around -55 mV neurons exhibited a regular single spike firing mode (pacemaking). At around -65 mV, neurons discharged in an irregular single spike firing mode (idling). At around -75 mV, many neurons exhibited a mode of discharge characterized by low threshold spikes (LTS), oscillations and, on some occasions, bursting. When present, bursts were of short duration and commonly made up of incomplete somatodendritic action potentials. TH-negative neurons also exhibited single spikes and bursting modes of firing. Both types of SNc neurons generated LTS oscillations following a hyperpolarization. However, LTS in TH-negative neurons could be evoked from less negative membrane potentials and produced more prolonged bursts. LTS from both types of neurons could be differentiated. LTS were blocked by 100 microM Ni2+ and became prolonged in the presence of TEA.

Action Potentials↗

PubMed1994

Subthreshold rectification in neostriatal spiny projection neurons.

Intracellular recordings from slice preparations were used to assess the subthreshold electrophysiological behavior of rat neostriatal projection neurons. Both current steps and ramp currents were used to estimate the current-voltage relationship (I-V plot). Inward rectification in the subthreshold range was a characteristic of most neurons. The amount of rectification varied greatly, and it was complex: membrane voltage trajectories in response to ramps were made up by almost piece-wise changes in the rate of voltage rise, suggesting that multiple conductances contribute to the subthreshold range. Inward current blockers such as tetrodotoxin (TTX) or Cd2+ decreased inward rectification, whereas outward current blockers such as tetraethylammonium (TEA) or 4-aminopyridine (4-AP) increased inward rectification. However, most inward rectification was due to TEA- and Cs(+)-sensitive conductances and not to TTX- or Cd(2+)-sensitive conductances. Cs(+)-sensitive conductances predominated at more negative membrane potentials, whereas 4-AP-sensitive conductances predominated at just +/- 10 mV below the firing threshold. In spite of a very slow activation, there was evidence for transient outward currents modulating the response, i.e., 4-AP-sensitivity, and voltage-sensitivity for firing frequency and threshold. TEA-sensitive conductances also contributed toward fixing the firing threshold. These results imply the contribution of various ion conductances on the shaping of the characteristic physiological firing recorded in vivo. Modulation of these responses by transmitters or peptides may help to understand neural processing in the neostriatum.

Action Potentials↗

PubMed1994

Patterns of excitatory and inhibitory synaptic transmission in the rat neostriatum as revealed by 4-AP.

1. Synaptic potentials induced by 4-aminopyridine (4-AP) were recorded intracellularly from rat neostriatal neurons in an in vitro slice preparation. EC50 for this 4-AP action was approximately 120 microM. The threshold for activation of synaptic potentials was 5 microM. 2. 4-AP-induced synaptic potentials appeared stochastically. Most were blocked by 1 microM tetrodotoxin or 400 microM Cd2+. Therefore they reflect a release of neurotransmitters dependent on both Ca2+ entry to the terminals and action potential firing. 3. Bicuculline (BIC) (< or = 10 microM), a gamma-aminobuturic acid-A (GABAA) antagonist, blocked about half of the 4-AP-induced synaptic potentials. This suggests that intrinsic inhibitory connections within the neostriatum are activated by 4-AP administration. 4. 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX; < or = 10 microM) plus D-2-amino-5-phosphonovaleric acid (D-APV; < or = 100 microM) blocked most of the BIC-resistant 4-AP-induced synaptic potentials. This suggests that 4-AP induced release of glutamate (GLU) from extrinsic glutamatergic afferents. As most glutamatergic afferents are extrinsic, these afferents then would be able to fire spikes and release transmitter for several hours after they are cut from their somata. 5. If CNQX plus D-APV were administered before BIC, neostriatal neurons responded in different ways. In one half of the neurons, all induced synaptic potentials were blocked. This suggests that most GABAergic intrinsic connections between neostriatal neurons are activated indirectly by 4-AP. 4-AP would first activate extrinsic glutamatergic afferents and these in turn would activate GABAergic intrinsic neurons and connections. 6. In the remaining half of the recorded neurons, administration of CNQX plus D-APV blocked most, but not all of the 4-AP-induced synaptic potentials. The synaptic potentials that remained had a characteristic pattern: they were high amplitude, rhythmic, bursts of synaptic potentials. They were blocked by BIC (5 microM) but not by mecamylamine (> 10 microM). This suggests that these bursts of synaptic potentials were GABAergic and generated by intrinsic neurons. Therefore these neurons would not innervate all neostriatal neurons equally but just a subset of them. 7. Records from an identified aspiny neostriatal interneuron, obtained from the same preparation, are shown. This interneuron fired in bursts and its morphologically and physiologically similar to the recently described, fast spiking, parvalbumin immunoreactive, GABAergic, aspiny interneuron is functional in the slice preparation.(ABSTRACT TRUNCATED AT 400 WORDS)

4-Aminopyridine↗

PubMed1994

Cellular and molecular characterization of Ca2+ currents in acutely isolated, adult rat neostriatal neurons.

Ca2+ currents in acutely isolated, adult rat neostriatal neurons were studied with whole-cell voltage-clamp techniques. In the vast majority of neurons (approximately 90%, n > 250), currents were exclusively of the high-voltage-activated (HVA) type. HVA currents activated near -40 mV and reached their maximum amplitude near 0 mV. Quasi-steady-state inactivation curves in many neurons were well fitted only with a sum of Boltzmann functions, suggesting that the HVA currents were heterogeneous. Although the block of whole-cell current by Cd2+ was well fitted with a single isotherm having an IC50 of near 1 microM, experiments with organic channel antagonists suggested that at least four types of HVA channels were expressed by most cells. On average, the L-channel antagonist nifedipine (5-10 microM) blocked 31 +/- 10% of the whole-cell current (n = 20), the N-channel antagonist omega-conotoxin GVIA (omega-CgTx) (2-5 microM) blocked 27 +/- 11% (n = 20), and the P-channel antagonist omega-agatoxin IVA (100-500 nM) blocked 21 +/- 10% (n = 18). In many neurons, the block by omega-CgTx was partially or completely reversible. In cells tested with a combination of these antagonists, 34 +/- 17% of the peak Ca2+ current remained unblocked (n = 13). Single-cell expression profiling of medium-sized neurons revealed the presence of rbA and rbB Ca2+ channel alpha 1 subunit mRNAs but low or undetectable levels of rbC mRNA (n = 12). These findings suggest that although adult neostriatal projection neurons do not express significant levels of LVA Ca2+ current, they do express a pharmacologically and structurally heterogeneous population of HVA currents.

Animals↗

PubMed1992

Hyperexcitability of hippocampal CA1 region in brain slices after GABA withdrawal.

The interruption of GABA infusion in the cerebral cortex and in the hippocampus produces electrographic seizures in rats. Here, we have used the hippocampal slice preparation to induce a 'GABA withdrawal syndrome (GWS)'. With the stimulation parameters used (0.2 Hz, 200 microseconds), activation of the Schaffer afferents produced one population spike in the CA1 subfield, while multiple population spikes were observed in the slices previously incubated in GABA. Also, we recorded an increase in the amplitude of the population spike when compared to its control value. Paired pulse test showed absence of recurrent inhibition in these slices. These results suggest a dysfunction in GABAergic neurotransmission.

Animals↗

PubMed1992

Charybdotoxin and apamin sensitivity of the calcium-dependent repolarization and the afterhyperpolarization in neostriatal neurons.

1. Intracellular recordings from neostriatal neurons in an in vitro slice preparation of the rat brain were used to analyze the pharmacological sensitivity of the action potential (AP) repolarization and the afterhyperpolarization (AHP) that follows a single action potential. The interspike voltage trajectory and the AHP could be divided into two main parts: a fast component lasting a few milliseconds and better observed during a train of spikes, and a slow component lasting approximately 250 ms and that comprises the main portion of the AHP. In some cells, a slow (up to 1 s) component of low amplitude was also detected. 2. Single APs were elicited at two imposed membrane potentials (around -60 and around -80 mV). The AP amplitude was larger, the repolarization rate was faster, and the duration was shorter when spikes were evoked at -80 mV. When measured from the -60 mV holding potential, the afterpotential was an AHP with peak amplitude of -5 mV. The afterpotential became a delayed depolarization (DD) at -80 mV. 3. Firing frequency adaptation was voltage sensitive. The firing of APs induced by long intracellular current pulses from a holding potential of -80 mV exhibited only a slow-frequency adaptation (time constant of seconds). However, at -60 mV, an initial and faster frequency adaptation was evident (time constant of tens of milliseconds). 4. The Ca2+ channel blocker Cd2+ retarded AP repolarization rate. This effect correlated with a significant block of the fast and slow components of the AHP. In contrast, Ni2+ had no significant effects on the same parameters.(ABSTRACT TRUNCATED AT 250 WORDS)

Action Potentials↗

PubMed1991

High- and low-voltage activated calcium currents are expressed by neurons cultured from embryonic rat neostriatum.

Current-clamp studies have shown that voltage-dependent Ca currents are present in rat neostriatal neurons. Although these studies have provided evidence for the presence of high-voltage activated Ca channels, it has been unclear whether low-voltage activated channels are also present. Using the whole-cell variant of the patch-clamp technique, we have studied isolated Ca currents in an attempt to answer this question. We have found that both high- and low-voltage activated calcium currents are expressed by neostriatal neurons cultured from embryonic rat brain. These currents are similar in voltage-dependence and pharmacology to those found in other brain neurons.

Animals↗

PubMed1991

Dendritic activity on neostriatal neurons as inferred from somatic intracellular recordings.

Orthodromic responses after local field stimulation were studied in a neostriatal slice preparation. Single suprathreshold stimuli evoked slow plateau-like orthodromic responses with repetitive firing and duration proportional to the stimulus intensity. Its persistence and apparent threshold upon hyperpolarization, as well as the actions of tetraethylammonium, QX-314 and Sr2+, suggest that this orthodromic response and associated phenomena such as fast prepotentials or Ca-dependent action potentials, may be generated on dendrites.

Action Potentials↗

PubMed1989

Two types of A-current differing in voltage-dependence are expressed by neurons of the rat neostriatum.

Transient potassium currents of the A type are thought to be important in a number of physiological processes of excitable cells, including spike repolarization and synaptic integration. This functional diversity may reflect the contribution of distinct subtypes of A channel to cellular behavior. Using the whole-cell variant of the patch clamp technique, we have found that two types of A-current are expressed in rat neostriatal neurons, one that is similar to previous descriptions in mammals and a second that is activated at considerably more depolarized potentials.

Animals↗

PubMed1989

An early outward conductance modulates the firing latency and frequency of neostriatal neurons of the rat brain.

An in vitro slice preparation was used to obtain intracellular recordings of neostriatal neurons. Indirect evidence for the presence of an early outward conductance in neostriatal neurons is presented. With near threshold stimulation neostriatal neurons fired very late during the pulse. The long firing latency was associated with a slow (ramp-like) depolarization. In the presence of TTX the slow depolarization was lost and outward-going rectification dominated the subthreshold response. This finding demonstrated that both, outward- and inward-going conductances play a role during the ramp-like depolarization. Outward-going rectification during depolarizing responses could be further augmented if the depolarizing stimulus was preceded by a conditioning hyperpolarization. A conditioning hyperpolarization prolonged the firing latency and slowed the firing frequency. A conditioning depolarization had opposite effects. After TTX treatment, the response showed a hyperpolarizing "sag" when depolarizing stimulation was preceded by conditioning hyperpolarization. 4-AP (0.5-2.5 mM) blocked the effects of the conditioning hyperpolarization on the firing latency and on the voltage trajectory. 4-AP also disclosed a slow depolarization which could produce neuronal firing very early during the pulse. This depolarization was TTX-sensitive and Co++-insensitive. In contrast to 4-AP, TEA (20 mM) did not produce a reduction in the firing latency but disclosed a membrane oscillatory behavior most probably produced by the interplay of these opposing conductances: the slow inward (probably Na+) and the transient outward (probably K+). Repetitive firing during 4-AP treatment was of the "phasic-tonic" type with an initial burst riding on the initial Co++-insensitive slow depolarization and a somehow irregular train of spikes during the remainder of the stimulation. Action potentials during 4-AP treatment were followed by an afterdepolarization which dominated the initial part of the interspike interval.

Action Potentials↗

PubMed1989

The role of calcium in the repetitive firing of neostriatal neurons.

The Ca++ -dependence of the repetitive firing of neostriatal neurons was studied in an in vitro slice preparation of the rat neostriatum. Neuronal firing was evoked by injecting depolarizing currents of 100-200 ms duration. In normal conditions, the mode of firing was tonic and showed very little adaptation. The frequency-current relation was linear over a wide range of frequencies. The repetitive firing was first enhanced and later suppressed by Co++, Mn++ and Cd++. These effects on the repetitive firing by the Ca++ -channel blockers paralleled the suppression of the slow afterhyperpolarizing potential. The lowering (0.2 mM) of Ca++ had similar effects. In the presence of TEA (up to 10 mM), the cell fired both Na+ and Ca+ action potentials. The results suggest that, as in other CNS neurons of the vertebrate, in neostriatal neurons the slow afterhyperpolarizing potential (AHP) is due to a Ca++ -activated K+ -conductance, and that the AHP plays a crucial role in the repetitive firing of these neurons.

Action Potentials↗