Dissociation of the IPSP and response to GABA during spreading depression-like depolarizations in hippocampal slices

We have compared the sensitivity of CA1 and CA3 hippocampal pyramidal cells, in mature and immature tissue, to spreading depression-like depolarization episodes. Using hippocampal slices from rabbit, we have found that mature and immature tissue, and CA1 and CA3 neurons, were differentially prone to depolarization episodes, depending on the method used to produce the depolarization. CA1 region was generally more sensitive than CA3. Spontaneous and stimulus-evoked depolarizations were seen more frequently in immature tissue than in mature slices, but anoxia-induced depolarizations were much more likely to occur in mature tissue. Synaptic transmission and responses to somatic gamma-aminobutyric acid (GABA) ejection were compared during anoxia-induced depolarizations in mature slices. The early component of the inhibitory postsynaptic potential (IPSP) normally had the same reversal potential as the GABA response. During anoxia-induced depolarization, both the drug response and the PSPs were lost. Synaptic transmission generally disappeared before the response to exogenous GABA application; the GABA response reappeared before synaptic function was restored. During the recovery of resting potential (RMP) following depolarization, the reversal potential of the early IPSP differed significantly from that of the GABA response; when the cell had recovered to RMP, the IPSP was depolarizing, whereas GABA application produced a 'normal' cell hyperpolarization. IPSPs and GABA-mediated responses attained their pre-depolarization form within a few minutes of RMP recovery. These observations suggest that, at least under special circumstances, the early component of the IPSP and GABA-mediated hyperpolarizations can be dissociated. Therefore, the early IPSP may be mediated by more complex mechanisms than a simple alteration in chloride conductance due to GABA-receptor interactions.     

Effects of GABA on CA3 pyramidal cell dendrites in rabbit hippocampal slices

Using the in vitro rabbit hippocampal slice preparation, we have investigated the effects of gamma-aminobutyric acid (GABA) iontophoresis on CA3 pyramidal cell dendrites. The predominant response (70% of the cells tested) was a hyperpolarization associated with a 30% decrease in cell input resistance (Rm). These hyperpolarizations displayed a very pronounced voltage dependency: they were decreased by cell depolarization and flattened by hyperpolarization. Bicuculline methiodide (BMI, 50 microM) did not abolish this response, nor did intracellular iontophoresis of chloride ions. In 5% of the cells, an additional hyperpolarization was obtained with longer ejection times; it reversed close to the reversal potential of the early component of the IPSP. In 25% of the cells, dendritic GABA application produced a depolarization. This response was reversed with cell membrane depolarization and was associated with a large (80%) decrease in Rm. The depolarizations were abolished by BMI (50 microM) and greatly increased by increasing the intracellular chloride concentration. None of the responses to GABA were affected by blockade of synaptic transmission. We conclude that the predominant response of CA3 pyramidal cell dendrites to GABA application is a hyperpolarization mediated by GABAB receptors and probably carried by potassium ions. The depolarizing responses are mediated via GABAA receptors and depend on an increase in chloride permeability.     

GABA-induced responses in Purkinje cell dendrites of the rabbit cerebellar slice.

Pressure applications of GABA localized to Purkinje cell somas in a rabbit cerebellar slice produced uniphasic hyperpolarizing responses, whereas applications of GABA that were directed at the Purkinje cell dendrites produced complex, triphasic responses with hyperpolarizing and depolarizing components. Both somatic and dendritic application of GABA elicited fast hyperpolarization (GABAhf), but dendritic application also elicited a slower depolarization (GABAd) and a later, long-lasting hyperpolarization (GABAhl). All three types of responses were accompanied by increased conductance. Use of either GABA antagonist, bicuculline or picrotoxin, eliminated the GABAhf and GABAd responses but left the GABAhl response intact. Pressure delivery of the GABA agonist, baclofen, to the dendrites but not the soma elicited a GABAhl response. Application of baclofen paired with membrane depolarization sufficient to elicit local, calcium-dependent dendritic spiking produced a persistent reduction in the GABAhl response, whereas alternating presentations of baclofen and membrane depolarization or presentations of baclofen alone could not. The fact that GABA and baclofen inhibited Purkinje cell activity in the rabbit cerebellar slice and that picrotoxin and bicuculline eliminated some, but not all of the components of the GABA response suggests the presence of both GABAA and GABAB receptors. The ability of baclofen to inhibit Purkinje cells if it was applied to the dendrites but not if applied to the soma suggests that GABAB receptors are located predominantly on Purkinje cell dendrites. The pairing-specific change in the baclofen response suggests the existence of GABAB-mediated modifiability of Purkinje cell dendrites.(ABSTRACT TRUNCATED AT 250 WORDS)     

Effects of acute and chronic ethanol treatment on pre- and postsynaptic responses to baclofen in rat hippocampus

Interactions between the GABA_B receptor and acute or chronic ethanol treatment were studied using extracellular and intracellular electrophysiological recording techniques. Bath application of the GABA_B receptor agonist, (−)-baclofen (0.1–100 μM) induced concentration-dependent inhibition of extracellularly recorded dendritic excitatory postsynaptic potentials (EPSPs) in the CA1 region of hippocampal slices. Responses to baclofen were unchanged relative to control either by simultaneous application of ethanol (10–60 mM) or by previous chronic ethanol exposure. The membrane potential of CA1 pyramidal neurons was reversibly hyperpolarized an average of 5 mV by pressure ejection of baclofen (1 mM). Bath application of ethanol (30 mM) alone occasionally caused a small depolarization of resting membrane potentials in CA1 neurons but failed to increase hyperpolarizing responses to pressure-ejected baclofen. However, in slices from chronic ethanol-treated animals hyperpolarizing responses to bath-applied baclofen (10 μM) were reduced by approximately 30% relative to controls. These results suggest that GABA_B-mediated responses in CA1 hippocampal pyramidal neurons are relatively resistant to the acute effects of ethanol, but that continuous exposure to ethanol sufficient to induce physical dependence may evoke an adaptive reduction in some GABA_B receptor mediated responses.     

Responses to γ-aminobutyric acid applied to cell bodies and dendrites of rat visual cortical neurons

The effects of pressure-applied γ-aminobutyric acid (GABA) on the soma and dendrites of pyramidal and non-pyramidal neurons of rat visual cortical slices were recorded intracellularly. When applied close to the soma, GABA produced hyperpolarizations and depolarizations, but when GABA was applied more than 250 μm from the soma only depolarizations were recorded. The results suggest that most visual cortical cells respond to GABA and that the responses of pyramidal and non-pyramidal cells to GABA are similar.     

Postnatal changes in somatic gamma-aminobutyric acid signalling in the rat hippocampus

During postnatal development of the rat hippocampus, γ-aminobutyric acid (GABA) switches its action on CA3 pyramidal cells from excitatory to inhibitory. To characterize the underlying changes in the GABA reversal potential, we used somatic cell-attached recordings of GABA(A) and N -methyl- d -aspartate channels to monitor the GABA driving force and resting membrane potential, respectively. We found that the GABA driving force is strongly depolarizing during the first postnatal week. The strength of this depolarization rapidly declines with age, although GABA remains slightly depolarizing, by a few millivolts, even in adult neurons. Reduction in the depolarizing GABA driving force was due to a progressive negative shift of the reversal potential of GABA currents. Similar postnatal changes in GABA signalling were also observed using the superfused hippocampus preparation in vivo , and in the hippocampal interneurons in vitro . We also found that in adult pyramidal cells, somatic GABA reversal potential is maintained at a slightly depolarizing level by bicarbonate conductance, chloride-extrusion and chloride-loading systems. Thus, the postnatal excitatory-to-inhibitory switch in somatic GABA signalling is associated with a negative shift of the GABA reversal potential but without a hyperpolarizing switch in the polarity of GABA responses. These results also suggest that in adult CA3 pyramidal cells, somatic GABAergic inhibition takes place essentially through shunting rather than hyperpolarization. Apparent hyperpolarizing GABA responses previously reported in the soma of CA3 pyramidal cells are probably due to cell depolarization during intracellular or whole-cell recordings.     

Reduction of GABAB inhibitory postsynaptic potentials by serotonin via pre- and postsynaptic mechanisms in CA3 pyramidal cells of rat hippocampus in vitro.

The action of serotonin (5-HT) on GABAergic synaptic transmission was investigated with intracellular recordings in CA3 pyramidal cells of rat hippocampal slices. Local application of 5-HT (500 microM) hyperpolarized CA3 pyramidal cells, decreased cellular input resistance, and reduced slow afterhyperpolarizations. Serotonin attenuated the late (GABAB) component of polysynaptic inhibitory postsynaptic potentials (IPSPs; 47% of control) without affecting the early (GABAA) component. During bath application of the excitatory amino acid antagonists 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX) (20 microM) and 2-amino-5-phosphonovalerate (AP-5) (40 microM), 5-HT similarly decreased the amplitude of the late (GABAB) component (17% of control) of monosynaptic IPSPs but did not affect the early (GABAA) component. The mean reversal potentials of poly- and monosynaptic IPSPs were unaffected by 5-HT. The conductance increases associated with the late component of poly- and monosynaptic IPSPs were reduced by 5-HT. Hyperpolarizing responses evoked in CA3 pyramidal cells by somatic applications of gamma-aminobutyric acid (GABA) were unaffected by 5-HT. During bath application of bicuculline (20-50 microM), hyperpolarizing responses elicited by dendritic GABA application were reduced by 5-HT (71% of control). The effect of 5-HT on these direct GABAB hyperpolarizations (29% decrease in response) does not appear sufficient to fully account for the effect of 5-HT on late GABAB IPSPs (53-83% decrease in response). Therefore, 5-HT may reduce GABAB IPSPs in CA3 pyramidal cells 1) by a postsynaptic action on pyramidal cells and 2) by a selective presynaptic action on GABAergic interneurons mediating the GABAB IPSP. This presynaptic action of 5-HT does not appear to involve excitatory afferents onto inhibitory interneurons.     

Effects of cholinergic agonists on immature rat hippocampal neurons

We have investigated the effects of acetylcholine (ACh) and the cholinergic agonist carbachol on several cell types in the developing rat hippocampus. Pyramidal cells were responsive to cholinergic applications on the first day examined (postnatal day 2), indicating that postsynaptic cholinoceptivity develops early, perhaps before functional cholinergic innervation is present. These drugs, which induce a membrane depolarization and a conductance decrease in mature pyramidal cells, had similar effects (both magnitude and pharmacology) on most immature neurons. However, a minority of cells in immature tissue exhibited decreased input resistance (Rin) during the cholinergic-induced depolarization. This response is likely a product of cholinergic action on local circuit neurons: non-pyramidal-type cells from animals as young as 8 days demonstrated excitatory responses to application of cholinergic agonists. The study revealed a number of other features of immature cells which may have functional significance. Lucifer yellow injections showed significant dye coupling among CA3 (but not CA1) pyramidal cells in immature tissue, suggesting close metabolic and/or electrotonic coupling between those cells during development. Mature CA3 cells showed less dye coupling, but increased anomalous rectification, and longer time constant. Developmental changes in intrinsic cell properties, coupled to alterations in local circuit interactions, may alter tissue responsiveness to neurotransmitters such as acetylcholine, even if the receptor-mediated drug action remains stable.     

Distinct types of ionic modulation of GABA actions in pyramidal cells and interneurons during electrical induction of hippocampal seizure-like network activity

It has recently been shown that electrical stimulation in normal extracellular fluid induces seizure-like afterdischarge activity that is always preceded by GABA-dependent slow depolarization. These afterdischarge responses are synchronous among mature hippocampal neurons and driven by excitatory GABAergic input. However, the differences in the mechanisms whereby the GABAergic signals in pyramidal cells and interneurons are transiently converted from hyperpolarizing to depolarizing (and even excitatory) have remained unclear. To clarify the network mechanisms underlying this rapid GABA conversion that induces afterdischarges, we examined the temporal changes in GABAergic responses in pyramidal cells and/or interneurons of the rat hippocampal CA1 area in vitro. The extents of slow depolarization and GABA conversion were much larger in the pyramidal cell group than in any group of interneurons. Besides GABA(A) receptor activation, neuronal excitation by ionotropic glutamate receptors enhanced GABA conversion in the pyramidal cells and consequent induction of afterdischarge. The slow depolarization was confirmed to consist of two distinct phases; an early phase that depended primarily on GABA(A)-mediated postsynaptic Cl- accumulation, and a late phase that depended on extracellular K+ accumulation, both of which were enhanced by glutamatergic neuron excitation. Moreover, extracellular K+ accumulation augmented each oscillatory response of the afterdischarge, probably by further Cl- accumulation through K+-coupled Cl- transporters. Our findings suggest that the GABA reversal potential may be elevated above their spike threshold predominantly in the pyramidal cells by biphasic Cl- intrusion during the slow depolarization in GABA- and glutamate-dependent fashion, leading to the initiation of seizure-like epileptiform activity.     

Pre- and postsynaptic effects of baclofen in the rat hippocampal slice

CA1 pyramidal cells responded to baclofen with a hyperpolarization. This response was found in the apical and basal dendrites and, like the hyperpolarizing response of the dendrites to GABA, appeared to be Ca2+-dependent since it was blocked or reduced by intracellular injection of EGTA or extracellular application of cadmium. Baclofen also reduced the excitatory and inhibitory postsynaptic potentials produced by stimulation of the Schaffer collaterals. The pre- and postsynaptic effects on the synaptic waveform could be distinguished.     

Taurine activates GABA(A) but not GABA(B) receptors in rat hippocampal CA1 area

We investigated if taurine, an endogenous GABA analog, could mimic both hyperpolarizing and depolarizing GABA(A)-mediated responses as well as pre- and postsynaptic GABA(B)-mediated actions in the CA1 region of rat hippocampal slices. Taurine (10 mM) perfusion induced changes in membrane potential and input resistance that are compatible with GABA(A) receptor activation. Local pressure application of taurine and GABA from a double barrel pipette positioned along the dendritic shaft of pyramidal cells revealed that taurine evoked a very small change of membrane potential and resistance compared with the large changes induced by GABA in these parameters. Moreover, in the presence of GABA(A) antagonists, local application of GABA on the dendrites evoked a GABA(B)-mediated hyperpolarization while taurine did not induce any change. Taurine neither mimicked baclofen inhibitory actions on presynaptic release of glutamate and GABA as judging by the lack of taurine effect on paired-pulse facilitation ratio and slow inhibitory postsynaptic potentials, respectively. These results show that taurine mainly activates GABA(A) receptors located on the cell body, indicating therefore that if taurine has any action on the dendrites it will not be mediated by either GABA(A) or GABA(B) receptors activation.     

Contrasting properties of K+ conductances induced by baclofen and gamma-aminobutyric acid in slices of the guinea pig hippocampus

Properties of membrane K+ conductances induced by baclofen and gamma-aminobutyric acid (GABA) in the hippocampus were investigated by using guinea-pig brain slices. Baclofen hyperpolarized the membrane and decreased the input resistance of pyramidal cells through the activation of a membrane K+ conductance. GABA caused a biphasic response in pyramidal cells, consisting of hyperpolarizing and depolarizing components. Combined application of picrotoxin and bicuculline eliminated the major part of the depolarizing component of the biphasic response and produced a relatively pure hyperpolarizing response which was also mediated by an increase in K+ conductance. The K+ conductance change induced by baclofen showed prominent inward rectification. However, the K+ conductance induced by GABA did not show an obvious rectifying property. The K+ conductance activated by baclofen was strongly antagonized by a low concentration (5 x 10(-6) M) of 4-aminopyridine (4-AP). In contrast, the K+ conductance activated by GABA was insensitive to 4-AP even at a high concentration of 10(-3) M. The slow inhibitory postsynaptic potential (slow i.p.s.p.) evoked by stimulation of the mossy fibres was totally suppressed by a low concentration of baclofen (5 x 10(-6) M). Whereas GABA (10(-3) M) decreased the amplitude of the slow i.p.s.p., the reduction of the amplitude was proportional to the decrease in the amplitude of the electrotonic potentials produced by constant inward current injections. These results suggest that the hyperpolarizations induced by GABA and baclofen may be generated by K+ conductances of different kinetic and pharmacologic properties.     

Differential effects of zinc on hyperpolarizing and depolarizing GABAA synaptic potentials in hippocampal slice cultures

We have examined the changes in GABA_A-mediated synaptic potentials recorded from CA3 pyramidal neurons in hippocampal slice cultures following application of zinc (Zn²⁺). Unlike 4-AP, Zn²⁺ did not enhance fast hyperpolarizing potentials but primarily enhanced depolarizing GABA_A potentials. Zn²⁺ did not alter the postsynaptic response of pyramidal neurons to pressure applied GABA, consistent with previous reports that Zn²⁺ enhances the release of GABA from presynaptic terminals. To examine the role of local circuitry in the production of Zn²⁺ responses, we recorded from cultures maintained for 7–10 days following removal of the dentate and hilus to allow complete degeneration of the mossy fibers (DGX cultures). Zn²⁺ produced giant depolarizing potentials (GDPs) in DGX cultures that were identical to those in intact cultures. In contrast, the 4-AP response was dramatically altered in DGX cultures. In DGX cultures, Zn²⁺ co-applied with 4-AP appeared to inhibit the production of fast hyperpolarizing GABA_A synaptic potentials produced by 4-AP alone. This inhibition of fast hyperpolarizing potentials suggests that Zn²⁺ may reduce the release of GABA onto pyramidal cell somata. These observations suggest that Zn²⁺ enhances GABA release from local circuit neurons that synapse onto pyramidal cell dendrites, and inhibits GABA release onto pyramidal cell somata.     

Loss of inhibition in the CA1 region of the kainic acid lesioned hippocampus is not associated with changes in postsynaptic responses to GABA

Somatic and dendritic responses to gamma-aminobutyric acid (GABA) were recorded intracellularly from CA1 pyramidal cells in slices of the hippocampus ipsilateral and contralateral to a unilateral kainic acid lesion of the CA3 region. Ipsilateral CA1 cells show a loss of GABA-mediated synaptic inhibition. However, somatic GABA responses and the sensitivity of cells to GABA were very similar in ipsilateral and contralateral cells. This was also true for dendritic applications of GABA.     

Immature Neurons and GABA Networks May Contribute to Epileptogenesis in Pediatric Cortical Dysplasia

Summary:  Cortical dysplasia (CD), a frequent pathological substrate of pediatric epilepsy surgery patients, has a number of similarities with immature cortex, such as reduced Mg²⁺ sensitivity of N-methyl-D-aspartate (NMDA) receptors and the persistence of subplate-like neurons and undifferentiated cells. Because γ-aminobutyric acid (GABA) is the main neurotransmitter in early cortical development, we hypothesized increased GABA receptor-mediated synaptic function in CD tissue. Infrared videomicroscopy and whole-cell patch clamp recordings were used to characterize the morphology and electrophysiological properties of immature and normal-appearing neurons in slices from cortical tissue samples resected for the treatment of pharmacoresistant epilepsy in children (0.2–14 years). In addition, we examined spontaneous and evoked synaptic activity, as well as responses to exogenous GABA application. We demonstrate both the presence of immature pyramidal neurons and networks in young CD tissue and the predominance of GABA synaptic activity. In addition, spontaneous GABA depolarizations frequently induced action potentials, supporting a potential excitatory role of GABA in CD. Evoked synaptic responses mediated by GABA were also prominent, and bath application of 4-aminopyridine induced rhythmic depolarizations that were blocked by bicuculline. Finally, responses to exogenous application of GABA had depolarized reversal potentials in severe compared to mild and non-CD cases. The present data support the hypothesis that CD shares features of immature cortex, with predominant and potentially excitatory GABA_A receptor-mediated neurotransmission. These results could partially explain the increased excitability of the cortical network in pediatric CD.     

Modulation by dopamine of population responses and cell membrane properties of hippocampal CA1 neurons in vitro

Dopamine (DA) was applied to rat hippocampal slices maintained in vitro. Extracellular and intracellular recording techniques were used to study the effect of DA on population responses, membrane potentials, and membrane responses to hyperpolarizing current pulses in CA1 pyramidal cells. Temporary exposure of hippocampal slices to DA has a dual effect. The initial action of DA is to produce a suppression of the extra-cellularly recorded population responses. In individual neurons, this initial effect is seen as a membrane hyperpolarization accompanied by a decrease in the amplitude of responses to hyperpolarizing current pulses. The frequency of occurrence of spontaneous depolarizations and spikes is reduced. The early action of DA is followed by a profound potentiation of the population responses that can last for hours. This long-lasting potentiation of the population response, induced by DA, is depressed by spiroperidol, a DA antagonist. In individual neurons, the late effect of DA is a long-lasting membrane depolarization associated with an increase in the amplitude of responses to hyperpolarizing current pulses. During this late phase, spontaneous activity is increased, as are single cell responses to stimulation of afferents. The evidence presented here indicates that DA is able to induce a long-lasting modification of the excitability of CA1 hippocampal neurons. This modulation of excitability by DA may be similar in nature to previously described DA-modulatory actions in the peripheral nervous system.     

Intracellular correlates of morphine excitation in the hippocampal slice preparation

The influences of morphine and opioid peptides on hippocampal CA1 pyramidal cells were investigated using intracellular recordings from the in vitro slice preparation. Morphine applied to somal and basal dendritic areas of CA1 cells via a pressure ejection system confirmed a number of excitatory actions of opiates and opioid peptides in this brain region. These included an increase in the amplitude and duration of orthodromically (radiatum) elicited EPSPs and a decrease in amplitude of the following IPSP. The increase in EPSP amplitude was accompanied by a reduction in stimulus intensity necessary for eliciting the action potential. Morphine delivered to the slice in this manner induced synaptically elicited and spontaneous multiple spike burst discharges. In slice maintained in 10⁻⁴ M pentobarbital¹, morphine reversed the presumably GABA mediated long-duration depolarization following orthodromic stimulation. Finally, depending on the specific site of application (apical or basal dendritic region) of the opiate, morphine produced two different effects on the resting membrane potential and input resistance of CA1 pyramidal cells. These findings are discussed as to whether opiates act directly excitatory influences in the hippocampus, or via blockade of GABA mediated inhibitory mechanisms.     

Pyramidal neurons in immature rat hippocampus are sensitive to beta-adrenergic agents.

The development of hippocampal neuronal sensitivities to the beta-noradrenergic agent, isoproterenol, was examined in tissue from immature rats. The in vitro hippocampal slice preparation was used to assess intracellularly recorded responses from hippocampal neurons to pressure-pulse and bath application of noradrenergic drugs. Effects of the drug on individual hippocampal CA3 pyramidal neurons were compared across several stages of development, ranging from postnatal day 4-5 (P4-5) to maturity. Isoproterenol, pressure-pulse applied to CA3c pyramidal cells, produced a depolarization of membrane potential and an increase in cell input resistance in tissue as young as P7. Spike frequency adaptation (in trains of action potentials triggered by depolarizing pulses) was reduced, as were the slow after-hyperpolarizations following the spike trains. All agonist effects were blocked by timolol, a beta-antagonist. Drug-induced changes in cell membrane and firing properties in immature tissue were qualitatively similar to beta-receptor-mediated noradrenergic effects in adult tissue. These results indicate that the beta-receptor-mediated component of the noradrenergic effect in rat hippocampus is physiologically functional by the seventh day of postnatal life; at earlier times (P4-5) these beta-receptor-mediated noradrenergic actions are, at best, equivocal.     

Differences in the Cs block of baclofen and 4-aminopyridine induced potassium currents of guinea pig CA3 neurons in vitro

Single-electrode current- and voltage-clamp techniques were employed to study responses elicited by (?)baclofen or γ-aminobutyric acid (GABA) and 4-aminopyridine (4-AP) induced inhibitory postsynaptic potentials in CA3 pyramidal neurons in guinea pig hippocampal slices. All drugs were applied by the bath to submerged slices in which fast synaptic transmission was blocked by 6-cyano-7-nitroquinoxaline-2,3-dione (10 μM), bicuculline (50 μM), and picrotoxin (50 μM). (?)Baclofen (0.5 μM) and GABA (1 μM) induced equivalent-sized hyperpolarizations and input resistance decreases. The agonist induced hyperpolarization or current and 4-AP induced hyperpolarizations or currents (4-AP induced K-IPSPs or IPSCs) reversed in sign near the K-equilibrium potential (E_K). The GABA_B receptor antagonists, OH-saclofen (500 μM) and CGP 35348 (100 μM), reduced (?)baclofen responses, and 4-AP induced K-IPSPs, suggesting that they were mediated by GABA_B receptors. Intracellular tetraethylammonium-, and extracellular barium-ions (1 μM) diminished the (?)baclofen induced current and 4-AP induced K-IPSCs. Intracellular Cs-ions blocked the (?)baclofen induced outward current at resting membrane potential but did not grossly affect the inward current recorded at membrane potentials negative to E_K. 4-AP induced inwardly or outwardly directed KIPSCs were not blocked by intracellular Cs-ions. Extracellular Cs-ions (5 μM) blocked the (?)baclofen induced inward K-current, but did not block 4-AP induced inwardly directed K-IPSCs. In conclusion, we found differences in the Cs block of K-channels activated by (?)baclofen or the endogenous transmitter GABA. One reason could be that (?)baclofen predominantly activated extra synaptic GABA_B receptors provided that extrasynaptic and subsynaptic receptors couple to different potassium channels. © 1994 Wiley-Liss, Inc.     

Theta oscillations in somata and dendrites of hippocampal pyramidal cells in vivo: Activity-dependent phase-precession of action potentials

Theta frequency field oscillation reflects synchronized synaptic potentials that entrain the discharge of neuronal populations within the ≈100–200 ms range. The cellular-synaptic generation of theta activity in the hippocampus was investigated by intracellular recordings from the somata and dendrites of CA1 pyramidal cells in urethane-anesthetized rats. The recorded neurons were verified by intracellular injection of biocytin. Transition from non-theta to theta state was characterized by a large decrease in the input resistance of the neuron (39% in the soma), tonic somatic hyperpolarization and dendritic depolarization. The probability of pyramidal cell discharge, as measured in single cells and from a population of extracellularly recorded units, was highest at or slightly after the negative peak of the field theta recorded from the pyramidal layer. In contrast, cyclic depolarizations in dendrites corresponded to the positive phase of the pyramidal layer field theta (i.e. the hyperpolarizing phase of somatic theta). Current-induced depolarization of the dendrite triggered large amplitude slow spikes (putative Ca²⁺ spikes) which were phase-locked to the positive phase of field theta. In the absence of background theta, strong dendritic depolarization by current injection led to large amplitude, self-sustained oscillation in the theta frequency range. Depolarization of the neuron resulted in a voltage-dependent phase precession of the action potentials. The voltage-dependent phase-precession was replicated by a two-compartment conductance model. Using an active (bursting) dendritic compartment spike phase advancement of action potentials, relative to the somatic theta rhythm, occurred up to 360 degrees. These data indicate that distal dendritic depolarization of the pyramidal cell by the entorhinal input during theta overlaps in time with somatic hyperpolarization. As a result, most pyramidal cells are either silent or discharge with single spikes on the negative portion of local field theta (i.e., when the somatic region is least polarized). However, strong dendritic excitation may overcome perisomatic inhibition and the large depolarizing theta rhythm in the dendrites may induce spike bursts at an earlier phase of the extracellular theta cycle. The magnitude of dendritic depolarization is reflected by the timing of action potentials within the theta cycle. We hypothesize that the competition between the out-of-phase theta oscillation in the soma and dendrite is responsible for the advancement of spike discharges observed in the behaving animal. Hippocampus 1998;8:244–261. © 1998 Wiley-Liss, Inc.