Connections between neuroscientists from Europe and Latin America reinforced.
Explore the source record for details and available documents.
Communication↗
Biomedical subjects
E Galarraga
Publications and source records attributed to E Galarraga.
Firing frequency modulation of substantia nigra reticulata neurons by 5-hydroxytryptamine.
Unitary extracellular recordings were made in in vitro rat brain slices to explore the effects of serotoninergic analogues on the spontaneous activity of substantia nigra reticulata (SNr) neurons. Most SNr neurons exhibited regular spontaneous firing (23.4 +/- 8.9 Hz, mean +/- S.E.M., n = 30) similar to that found in vivo. The most reproducible effect of serotonin (5-HT) was an increase in firing frequency found in 53% of the cells. The effect was concentration dependent and blocked by the 5-HT1/2 antagonist methysergide (1-10 microM) but unaffected by the 5-HT4- and 5-HT1-preferring antagonists DAU 6285 (5 microM) and metiothepin (5 microM), respectively. However, 5-HT also decreased the firing frequency in several neurons. In 19% of the neurons an inhibition was found alone but a biphasic response (inhibition and excitation) was found in another 28% of the neurons. Interestingly, the effect of the 5-HT-uptake inhibitor, duloxetine (100-400 nM), was frequency inhibition. Agonists that mimicked the 5-HT-induced inhibition were of the 5-HT1B-class (25 microM CP 93129 and 25 microM TFMPP). Neither the 5-HT2-antagonist ritanserin (5 microM) nor the GABA(A)-antagonist, bicuculline (30 microM) were able to block the inhibition suggesting that some SNr neurons may be directly inhibited by 5-HT.
Animals↗
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↗
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↗
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↗
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↗
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↗
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↗
Somatostatin- and substance P-like immunoreactivity in rat neostriatal cultures.
In the normal adult neostriatum, somatostatin immunoreactive interneurons constitute a few percent of the total neuronal population whereas substance P immunoreactive neurons, which project to the substantia nigra, constitute nearly half of the total. Primary monolayer neostriatal cultures derived from E17 rat brains displayed both somatostatin-like and substance P-like immunoreactivity (SOM-IR and SP-IR). However, the proportions of SOM-IR and SP-IR neurons in vitro were significantly different from those in vivo. At 4 days in vitro (DIV), SOM-IR neurons comprised 19% of all neurons and this percentage increased to 30% at 25 DIV. In contrast, SP-IR neurons were less common than expected at 4 DIV (20%) and declined in percentage to 13% at 27 DIV. These results suggest that survival in target-deprived neostriatal cultures is favored for SOM-IR interneurons.
Animals↗
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↗
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↗
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↗
Quinolinate and kainate neurotoxicity in neostriatal cultures is potentiated by co-culturing with neocortical neurons.
It has been suggested that a disorder in the regulation of excitatory amino acids (EAA) may underlie the loss of neostriatal neurons seen in Huntington's disease. The role of neocortical afferent fibers in determining the EAA sensitivity of neostriatal neurons was assessed by comparing EAA toxicity in co-cultures of neocortex and neostriatum with that of neostriatum alone. In cultures of neostriatum alone, EAAs produced only modest neuronal losses. Kainate, which tended to be the most potent excitotoxin, produced a loss of approximately 30% of the neurons after a 5-min exposure at a 1-mM concentration. In co-cultures, the sensitivity of neostriatal neurons to EAA toxicity was dramatically enhanced; toxicity was increased about two-fold for kainate and quinolinate at millimolar concentrations and as much as 8-fold for quinolinate at micromolar concentrations. The effects of EAA co-incubation with the N-methyl-D-aspartate (NMDA) receptor antagonist, 2-amino-5-phosphonovaleric acid, suggested that the toxic actions of quinolinate, but not kainate, were mediated largely by NMDA receptors.
2-Amino-5-phosphonovalerate↗
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↗
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↗
Electrotonic properties of neostriatal neurons are modulated by extracellular potassium.
In order to assess the effects of [K+]o on the passive membrane properties of neostriatal neurons, the cable properties of these cells were determined at two extracellular potassium concentrations (6.25 and 3.0 mM). The effect of tetraethylammonium (TEA) on cable properties was also studied at 6.25 [K+]o. At 6.25 mM [K+]o, the mean input resistance at the resting membrane potential (RMP), and the mean membrane time constant (tau o) were 27 +/- 1.5 M omega and 6.9 +/- 0.5 ms respectively (n = 17), while at 3 mM [K+]o they were 62.9 +/- 4.8 M omega and 14.3 +/- 0.6 ms (n = 15) (mean +/- SEM). With one of the methods used to calculate the electronic parameters, the total electrotonic length of the dendrites (L) and the dendritic to somatic conductance ratio (rho) were 1.3 +/- 0.05 and 5 +/- 0.8 at the higher [K+]o respectively, while they were 0.95 +/- 0.04 and 3 +/- 0.7 at the lower [K+]o. Cells were depolarized in 6.25 as compared to 3 mM [K+]o (RMP = -66 +/- 1.3 mV vs RMP = -80.5 +/- 1.4 mV). After one hour exposure to TEA (10 mM), the input resistance and time constant tripled at 6.25 mM [K+]o. TEA slightly depolarized the cells bathed in 6.25 mM [K+]o. The results suggest that changes in [K+]o, within the physiological range, markedly affect the cable properties of neostriatal neurons, possibly modifying subthreshold, voltage-dependent K+-conductances. TEA seems to block some of these channels.
Animals↗
Spontaneous synaptic potentials in dopamine-denervated neostriatal neurons.
Intracellular spontaneous activity was recorded in neostriatal slices from rats with 6-hydroxydopamine-induced lesion of the left nigrostriatal dopaminergic system. Recordings were made at different times after denervation. Dopaminergic denervation caused the appearance of spontaneous synaptic potentials, which were present even after 8 months. The results suggest a tonic inhibitory influence of the dopaminergic innervation on the synaptic input of neostriatal neurons.
Action Potentials↗
Reduction in paradoxical sleep after L-dopa administration in rats.
The EEG and EMG activity of two groups of rats (N = 8 each) was recorded during one experimental and 5 control days. On the experimental day, one group received 125.0 mg/kg L-dopa preceded by 50.0 mg/kg benserazide hydrochloride. The other group received 1.0 mg/kg haloperidol, immediately before the recording session. After L-dopa plus benserazide administration the total time and the number of episodes of paradoxical sleep and slow wave sleep decreased significantly compared to control sessions.
Animals↗