Noradrenaline blocks accommodation of pyramidal cell discharge in the hippocampus

An afterhyperpolarization recorded in striatal cells 'in vitro': effect of dopamine administration

Parahorizontal slices of the neostriatum from young adult male rats were maintained at the fluid interface of a perfusion chamber. Intracellular recordings were made with potassium acetate filled micropipettes. The mean resting membrane potential of the neurones was -84 mV and all produced action potentials which overshot 0 mV. All the cells were silent at resting membrane potentials and only fired in response to depolarizing pulses delivered through the intracellular electrode. When long pulses were applied, the firing rate within the train of action potentials evoked was slower at the end of the pulse. In a very few cells at resting membrane potential, but in all cells depolarised from a holding potential of -65 mV, the resulting train of action potentials was followed by a slow hyperpolarizing potential. The potential observed was smaller in amplitude than the afterhyperpolarization seen in hippocampal pyramidal cells in vitro, but its voltage sensitivity was similar. Iontophoresis of dopamine (DA) close to the recording pipette resulted in two distinct actions. Firstly, DA reduced the number of action potentials following a short pulse, by increasing the threshold for spike initiation. Secondly, it also inhibited the hyperpolarizing potential which followed trains of action potentials even when allowance was made for the increased firing threshold. The inhibition of this hyperpolarization was accompanied by an increase in the number of spikes per pulse. Whether these two actions of DA are mediated by a single receptor/membrane action is unclear. Nevertheless, they may help to reconcile the contradictory literature regarding the action of DA upon extra-cellular responses in striatal cells.(ABSTRACT TRUNCATED AT 250 WORDS)

Actions of serotonin recorded intracellularly in rat dorsal lateral septal neurons

The actions of serotonin (5HT) on passive and active membrane properties of neurons in the rat dorsal lateral septal nucleus (LSN) were studied by using intracellular recordings in transverse, septal slices. Superfusion with 10 microM 5HT induced a hyperpolarization of the membrane in almost all neurons tested in the dorsolateral part of the LSN. The hyperpolarization was accompanied by a decrease in membrane resistance. These effects of 5HT persisted in a low-Ca2+/high-Mg2+-containing medium or medium with tetrodotoxin, indicating a post-synaptic site of action for 5HT. The reversal potential for the hyperpolarizing effect was ca. -95 mV. If the extracellular K+-concentration was raised, the reversal potential became less negative. These data suggest that 5HT hyperpolarizes LSN neurons by increasing a K+-conductance. Spontaneous, synaptically evoked action potentials and action potentials induced in LSN neurons by a depolarizing current step typically display a fast Na+-spike with a subsequent K+-afterhyperpolarization, followed by a much slower Ca2+-dependent afterdepolarization. The amplitude of the K+-afterhyperpolarization was decreased by 5HT, while at the same time the afterdepolarization became more pronounced. The Ca2+-spike of LSN neurons was not affected by 5HT. Synaptic responses that were evoked in LSN neurons by stimulation of the dorsal part of the LSN consisted of a fast EPSP or spike, followed by a Cl(-)-dependent fast IPSP and a K+-dependent late IPSP. Of these synaptic responses, 5HT suppressed particularly the late IPSP. The present data indicate that 5HT affects the conductance for active and passive K+-channels in LSN neurons.(ABSTRACT TRUNCATED AT 250 WORDS)

Adrenaline depolarization in paravertebral sympathetic neurones of bullfrogs

Responses to adrenaline (Ad) and their ionic mechanisms were analysed using intracellular recording and voltage-clamp methods in neurones of bullfrog sympathetic ganglia. Ad (5 microM-1 mM) applied directly to sympathetic neurones by pressure ejection through a micropipette produced three types of depolarizing responses (2-20 mV). Under voltage-clamp conditions, Ad (100 microM) produced fast, slow and mixed types of inward currents (AdIs) with amplitude of 2.9 +/- 1.3 nA. beta-Adrenoceptors may be responsible for the generation of these AdDs. The slow AdI which lasted for 1-5 min was associated with a decreased membrane conductance. The slow AdI decreased at hyperpolarized potential level and eventually nullified at -70 mV. No reversal of the slow AdI polarity was observed in the Ringer solution. Injection of Cs2+ into the ganglion cells produced a marked depression of the amplitude of the slow AdI. The slow AdI was blocked by bath-applied Ba2+ but not by TEA. Ad reduced the slow current relaxation, the M current, associated with voltage jumps in the membrane potential range -35 to -55 mV. The fast Ad response was associated with an increase in membrane conductance. When the membrane was depolarized, the fast AdI decreased and reversed its polarity at -36 +/- 8.3 mV. Removal of Cl ion from superfusing solution depressed the fast AdI, suggesting that activation of Cl- conductances may be involved in the generation of the fast AdI. The mixed type of Ad response exhibited characteristics of both the fast and slow Ad responses. The results suggest that Ad increases the excitability of neurones in bullfrog sympathetic ganglia.

Pharmacological and physiological properties of the after-hyperpolarization current of bullfrog ganglion neurones

1. The slowly decaying, calcium-dependent after-hyperpolarization (a.h.p.) that follows action potentials in bullfrog ganglion B cells has previously been shown to be generated by a potassium current called IAHP. We have recorded IAHP using a switched, single-electrode hybrid clamp where current-clamp mode was changed to voltage-clamp mode immediately after repolarization of a spike or the last spike of a train. 2. Reduction of extracellular calcium reduced the decay time of IAHP following a single spike. At all levels of extracellular calcium tested (0.5-4 mM), the decay time of IAHP was longer following a train of action potentials than following a single action potential. Thus, the time course of IAHP evoked by action potentials is a function of the calcium load induced by the action potentials. Conversely, agents that reduce the amount of IAHP activated without affecting its rate of decay, probably do not affect calcium influx. 3. Muscarine (2 or 10 microM) inhibits IAHP following an action potential by at most 30% and has no effect on decay rate of IAHP. These results suggest that muscarine has little or no effect on either calcium influx or sequestration. Decay of the a.h.p. is accelerated by muscarine but this effect is due to an increased leak conductance. 4. Charybdotoxin (CTX) between 4 and 20 nM, prolongs action potential duration in a manner consistent with blockade of the voltage- and calcium-dependent potassium current (Ic) involved in spike repolarization in these cells. This action is consistent with its reported action on analogous channels in other systems. However, CTX also reduces IAHP. Thus, in bullfrog ganglion neurones, two distinct calcium-dependent potassium currents exhibit a comparable sensitivity to CTX. This cannot be due to a decreased influx of calcium because the decay rate of IAHP following an action potential is unchanged. The action of CTX was observed with both crude and purified preparations of CTX. 5. Apamin (25 nM) and (+)-tubocurarine (concentration giving 50% of maximal inhibition = 20 microM) block IAHP without affecting action potential duration. The action of (+)-tubocurarine is more readily reversible than apamin. Approximately 20% of IAHP is resistant to blockade by either apamin or (+)-tubocurarine. 6. Muscarine was used to block the M-current (IM) selectively and (+)-tubocurarine was used to inhibit IAHP selectively. Both currents were shown to contribute to spike frequency adaptation. Inhibition of both IM and IAHP has a synergistic action to increase repetitive firing.

Inhibition of calcium-dependent spike after-hyperpolarization increases excitability of rabbit visceral sensory neurones

1. Conventional intracellular recordings were made from rabbit nodose neurones in vitro. Prostaglandins D2 and E2, but not F2 alpha, produced a selective, concentration-dependent (1-100 nM) inhibition of a slow, Ca2+-dependent spike after-hyperpolarization (a.h.p.). Block of the slow a.h.p. was accompanied by an increased membrane resistance and a small (less than 10 mV) depolarization of the membrane potential. Inhibition of the slow a.h.p. produced no change in the voltage-current relationship other than the increased membrane resistance. 2. In C neurones with slow a.h.p.s, trains of brief depolarizing current pulses (2 ms duration, 0.1-10 Hz) could not elicit repetitive action potentials without failure at rates above 0.1 Hz. By contrast, C neurones without slow a.h.p.s could respond at stimulus frequencies up to 10 Hz. The frequency-dependent spike firing ability of slow a.h.p. neurones was eliminated by inhibition of the slow a.h.p. 3. Action potentials were also evoked by intrasomatic injection of paired, depolarizing current ramps (1 nA/10 ms, 0.1-5 s inter-ramp interval). For neurones without a slow a.h.p., the current threshold and number of evoked spikes were the same for both ramps, and the ramps were nearly superimposable. In neurones with a slow a.h.p., the current threshold for the first spike in the second ramp was greatly increased (300-500%) and the number of evoked spikes was reduced. Following inhibition of the slow a.h.p., the current threshold and number of evoked spikes was the same for both ramps. 4. Forskolin, a direct activator of the catalytic subunit of adenylate cyclase, also produced a concentration-dependent inhibition of the slow a.h.p., with 50% block at 30 nM. Prostaglandin D2 and forskolin produced identical enhancement of excitability in C neurones and neither substance produced any effect on C neurones that could not be attributed to inhibition of the Ca2+-dependent K+ conductance associated with the slow a.h.p. We propose that, in some visceral sensory neurones, the level of excitability is regulated by cyclic AMP-mediated control of the slow a.h.p.

Outward currents of single hippocampal cells obtained from the adult guinea-pig

1. Neurones were isolated from the hippocampus of adult guinea-pigs by enzymatic and mechanical treatment. The electrophysiological properties of these cells were examined immediately after dissociation by intracellular recordings using low-resistance electrodes (2-5 M omega). 2. Pyramidal-shaped cells were identified visually. Intracellular recordings showed that these cells have input resistances ranging from 200 to 1300 M omega. Passive voltage responses to hyperpolarizing current injection were fitted by single exponentials decaying with time constants ranging from 15 to 60 ms. This suggests that the electrotonic structure of these cells is compact such that injected current elicited isopotential intracellular responses. 3. Outward currents activated by depolarization were examined in these cells using voltage-clamp techniques. The amplitude and the time course of the outward currents were profoundly affected by the holding potential. For cells held at -50 mV or more positive, depolarizing steps produced a slowly rising outward current. At holding potentials negative to -55 mV depolarizing pulses produced an additional early transient outward current followed by a slowly rising component which decayed gradually during sustained depolarizations. 4. The outward currents were separated by their kinetic properties and their sensitivity to cobalt (Co2+), tetraethylammonium (TEA) and 4-aminopyridine (4-AP). 5. The transient current peaked within 6 ms of the onset of depolarizing pulses. It decayed exponentially with a time constant of 20-40 ms. The amplitude of the current activated by a fixed depolarization increased gradually as the duration or the amplitude of the hyperpolarizing pre-pulse increased. The current activated by a fixed depolarization reached its half-maximal level when the hyperpolarizing pre-pulse was at -83 mV. 6. 4-AP exerted two actions on the transient current. Firstly, the time constant of the falling phase decreased by about a factor of two. Secondly, the current was blocked in a time- and voltage-dependent manner: the block increased when the hyperpolarizing pre-pulse lengthened. TEA, up to 10 mM, did not affect the amplitude of the transient current. Co2+ suppressed this current. The effects of Co2+ consisted of a shift to the positive direction of the voltage dependence of the current. 7. The delayed currents can be divided into Ca2+-dependent and Ca2+-independent components. The component persistent in the Co2+ solution (K-current) decayed slowly with maintained depolarization (time constant greater than 3 s).(ABSTRACT TRUNCATED AT 400 WORDS)

Pertussis toxin blocks a late inhibitory postsynaptic potential in hippocampal CA3 neurons

These experiments show that a synaptic response, namely the late inhibitory postsynaptic potential (IPSP) of hippocampal CA3 neurons of rats, is blocked by pertussis toxin, an inactivator of several GTP-binding proteins (G-proteins) excluding the G-protein that stimulates adenylyl cyclase. This blockage occurred without a similar effect upon either the mossy fiber-evoked EPSP or the early (GABAa-mediated) IPSP. The toxin also blocked the response to baclofen, an agonist for a putative receptor (GABAb) mediating the late IPSP, but did not affect the response to THIP, an agonist for the receptor (GABAa) mediating the early IPSP. It is proposed that a pertussis toxin-sensitive G-protein controls the conductance of the late IPSP.

Differential effects of norepinephrine on hippocampal complex-spike and theta-neurons

The central noradrenergic system has long been postulated to modulate learning and memory. A brain structure known to be important in these functions is the hippocampus. Since the hippocampus receives a noradrenergic projection from the locus coeruleus, knowledge of norepinephrine's actions in the hippocampus may help determine its role in learning and memory. In the present study, the effects of norepinephrine were examined on two hippocampal cell types: complex-spike and theta-neurons. In the hippocampus, there is good evidence that complex-spike cells are pyramidal neurons, while theta-neurons are interneurons. Extracellular action potentials from hippocampal neurons were recorded using multibarrel glass micropipettes. Drugs were locally applied using pressure micro-ejection. Norepinephrine inhibited the spontaneous firing of complex-spike cells, while theta-neurons were excited. The inhibitory response of complex-spike neurons was mediated by an alpha 1-receptor. However, selective agonists for the alpha 2- and beta-noradrenergic receptors excited the complex-spike cells. The noradrenergic-induced excitatory response of theta-neurons was also mediated by alpha 2- and beta-receptors. This study provides evidence that locally applied norepinephrine produces different responses on two types of hippocampal neurons. Furthermore, these differential responses arise primarily from the activation of distinct populations of noradrenergic receptors.

The effects of axotomy on bullfrog sympathetic neurones

1. The effects of axotomy on the electrical properties of B cells in paravertebral sympathetic ganglia were studied using standard intracellular recording techniques. The effects were apparent after 1 week and persisted throughout the 47 days of study. 2. Action potential duration (spike width) and amplitude (spike height) were significantly increased in axotomized neurones. 3. The duration of the after-hyperpolarization which followed the action potential showed considerable scatter in control neurones (mean +/- S.E. of mean, 159.0 +/- 5.8 ms for 100 cells). Following axotomy, the duration was significantly reduced (50.9 +/- 2.3 ms for 97 cells). The amplitude of the after-hyperpolarization was also significantly smaller in axotomized neurones. 4. Changes in the characteristics of the action potential and the after-hyperpolarization in axotomized neurones were not due to alteration in resting membrane potential or input resistance which were unchanged after axotomy. Rheobase current was significantly increased. 5. There was neither a significant depression of the rate of rise or the amplitude of orthodromically evoked nicotinic e.p.s.p.s nor any obvious ultrastructural alteration following axotomy. 6. Despite the decrease in the duration of the after-hyperpolarization, the rate of discharge in response to constant current injection was little changed in axotomized neurones. 7. Although axotomy produces significant changes in several measurable electrophysiological parameters in bullfrog sympathetic ganglion cells, the present results imply that mature neurones are able to maintain relatively normal electrical activity despite injury.

Effects of phorbol dibutyrate on M currents and M current inhibition in bullfrog sympathetic neurons

1. Effects of bath-applied phorbol dibutyrate (PDBu) on M currents (IM) and on the inhibition of IM by muscarine and luteinizing hormone-releasing hormone (LHRH) were recorded in voltage-clamped bullfrog lumbar sympathetic ganglion cells. 2. PDBu (0.1-30 microM) produced a slowly developing, irreversible and partial (less than or equal to 60%) inhibition of IM. This effect was not replicated by 4-alpha-phorbol or by vehicle. 3. After treatment with PDBu, residual IM showed a reduced sensitivity to inhibition by muscarine or LHRH but not by Ba2+. The reduced response to muscarine appeared to result from a 10-fold shift in the concentration dependence for inhibition. 4. PDBu did not clearly reproduce the ability of muscarine to inhibit the slow, Ca-activated K current IAHP or to increase the leak conductance at hyperpolarized potentials. The latter effect of muscarine was enhanced, rather than inhibited, by PDBu. 5. IM and IAHP were not inhibited by 1 mM dibutyryl cyclic AMP or by 20 microM forskolin. 6. It is concluded that activation of protein kinase C, but not protein kinase A, partly replicates the effect of muscarine on frog sympathetic neurons.

Altered modulatory actions of serotonin on dentate granule cells of aged rats

The effects of serotonin (5-HT) on dentate granule (DG) neurons in hippocampal slices taken from young mature (6-8 months) and old (25-29 months) rats were compared. Intracellular measurements of membrane potential, cell input resistance and slow postspike afterhyperpolarization did not differ significantly between young and old neurons. Neurons recorded in slices taken from old animals responded with less hyperpolarization to increasing doses of the drug, and their responses were significantly reduced after repeated applications of 5-HT. Serotonin-mediated reduction of the slow afterhyperpolarization in young DG neurons was less prominent or totally absent in the old cells. It is concluded that serotonergic postsynaptic actions are impaired in old age.

Noradrenaline blocks potassium conductance in rat dentate granule cells in vitro

The actions of noradrenaline and the beta-adrenergic agonist, isoproterenol, were studied on the dentate gyrus in hippocampal slices from rats using extra- and intracellular recording. These agents facilitated field EPSPs (excitatory postsynaptic potentials) and population spikes evoked by perforant path stimulation. Intracellular recording revealed an attenuation of the long lasting afterhyperpolarization (AHP) and the accommodation of cell discharge in response to depolarizing current injection. It is suggested that beta-receptor activation blocks a calcium-dependent potassium current.

Cellular mechanisms of epilepsy: a status report

The cellular phenomena underlying focal epilepsy are currently understood in the context of contemporary concepts of cellular and synaptic function. Interictal discharges appear to be due to a combination of synaptic events and intrinsic currents, the exact proportion of which in any given neuron may vary according to the anatomic and functional substrate involved in the epileptic discharge and the epileptogenic agent used in a given model. The transition to seizure appears to be due to simultaneous increments in excitatory influences and decrements in inhibitory processes--both related to frequency-dependent neuronal events. A variety of specific hypotheses have been proposed to account for the increased excitability that occurs during epileptiform activity. Although each of the proposed mechanisms is likely to contribute significantly to the epileptic process, no single hypothesis provides an exclusive unifying framework within which all kinds of focal epilepsy can be understood. The spread of epileptic activity throughout the brain, the development of primary generalized epilepsy, the existence of "gating" mechanisms in specific anatomic locations, and the extrapolation of hypotheses derived from simple models of focal epilepsy to explain more complex forms of human epilepsy, all are not yet fully understood.

Repetitive inhibitory postsynaptic potentials evoked by 4-aminopyridine in hippocampal neurons in vitro

The effects of topical application of microdrops containing 4-aminopyridine (4-AP) on properties of CA1 neurons were examined in the hippocampal slice preparation. 4-AP triggered repetitive large (4-10 mV) hyperpolarizing potentials (HPs) having a short rise time and slow (3-4 s) decay. There was a marked decrease in input resistance during the HPs. The HPs are likely to be caused by an increase in potassium conductance; their reversal potential was 15-20 mV negative to rest, the reversal potential shifted in the depolarizing direction when the slice was bathed in high potassium medium, and it was the same with KCl or potassium acetate recording electrodes. The HPs were not generated by release of neurotransmitter substances from terminals of extrinsic afferents since they were present in slices taken from deafferented hippocampus but they were blocked by tetrodotoxin (TTX) or Cd and Mn, indicating that they are synaptic potentials of local origin. HPs were still present when Ca-dependent K currents were blocked by acetylcholine and noradrenaline. Three of 56 cells recorded in the hippocampus could be classified as interneurons. They emitted high frequency trains of action potentials in response to 4-AP, at a rate corresponding to the HPs recorded in all other neurons. It is suggested that 4-AP excites a specific type of interneuron which in turn generates large K-mediated inhibitory postsynaptic potentials in the pyramidal neurons of CA1 region of the hippocampus.

Effects of ethanol on CA1 and CA3 pyramidal cells in the hippocampal slice preparation: an intracellular study

Superfusion of ethanol (10-350 mM) sometimes caused weak hyperpolarization, but more often elicited weak depolarization or biphasic depolarizing, hyperpolarizing responses in CA1 and CA3 pyramidal neurons of the hippocampal slice. The occasional polarizations were sometimes accompanied by, but not always correlated with, small increases or decreases in input resistance. However, many cells in both areas showed no detectable change in membrane potential (36% of cells) or input resistance (57% of cells), even at very high ethanol concentrations (86-200 mM). Spontaneous spiking, when present, was occasionally accelerated or decelerated, although in CA3 a biphasic speeding-slowing sequence was often seen. The afterhyperpolarizations following bursts of action potentials evoked by current (CA1) or occurring spontaneously (CA3) were most often either slightly reduced in amplitude (CA3) or not affected (CA1) by ethanol superfusion. In contrast, synaptic potentials evoked by stimulation of the hilar mossy fiber pathway (for CA3) or the stratum radiatum (for CA1) were more sensitive to ethanol: excitatory postsynaptic potentials (EPSPs) and inhibitory postsynaptic potentials (IPSPs) were most often reduced in amplitude in both CA1 and CA2, even at low ethanol concentrations (10-50 mM). The action on IPSPs may be exerted presynaptically, because responses to locally applied GABA were little affected. These results suggest that hippocampal evoked synaptic activity may be more sensitive than postsynaptic membrane properties to physiologically relevant ethanol concentrations.

Calcium-dependent potassium conductance in guinea-pig olfactory cortex neurones in vitro

1. Guinea-pig olfactory cortex neurones in vitro (23-25 degrees C) were voltage clamped by means of a single-micro-electrode sample-and-hold technique. 2. Under current clamp at the resting potential (approximately -80 mV), brief depolarizing stimuli evoked trains of action potentials with little visible after-potential. However, in 90% of recorded cells held at membrane potentials between -70 and -45 mV, depolarizing current pulses evoked a slow after-hyperpolarization (a.h.p.) (approximately 8 mV) lasting several seconds and accompanied by an increase in input conductance. 3. The outward membrane current underlying the a.h.p. was revealed either by switching rapidly to voltage clamp at the end of a spike train ('hybrid' clamp) or by applying brief depolarizing commands from potentials between -60 to -45 mV. The tail current showed a distinct rising phase (time to peak approximately 1 s) and exponential decay (tau approximately 3 s) and was suppressed by removal of external Ca2+, or adding Co2+ (1-2 mM), Cd2+ (200 microM) or Mg2+ (6 mM). The a.h.p. current reversal potential was -96 mV in 3 mM-K+ medium. 4. Low concentrations (1-2 microM) of muscarine, carbachol, oxotremorine or the muscarinic ganglion stimulant, McN-A-343 (1-10 microM) reduced the a.h.p. current and leak conductance and induced a steady inward current, without affecting M-current (IM) relaxations. IM inhibition generally required higher (greater than 10 microM) agonist concentrations, although oxotremorine remained ineffective at up to 50 microM. 5. The a.h.p. current was reduced by noradrenaline and tetraethylammonium (TEA), but not by apamin or tubocurarine. Apart from TEA, these agents had no effect on IM. 6. Addition of tetrodotoxin (TTX, 1 microM) or removing external Na+ depressed the a.h.p. current amplitude recorded under voltage clamp. The residual tail current could be further reduced by adding Cd2+ or muscarinic agonists. 7. Repolarizing tail currents induced following positive voltage commands consisted mainly of IM and slow a.h.p. current with little evidence of a 'fast' Ca2+-activated K+ current (IC). 8. It is concluded that the slow a.h.p. current that underlies the post-burst after-hyperpolarization of olfactory neurones, is a Ca2+-dependent K+ current distinct from IM. It is suggested that the cholinergic modulation of this current (rather than IM) may provide a more subtle control of cell excitability in cortical neurones.

Serotonin decreases population spike amplitude in hippocampal cells through a pertussis toxin substrate

Activation of the serotonin1A receptor decreases CA1 population spike amplitude and inhibits forskolin-stimulated adenylate cyclase in rat hippocampus. Pretreatment of rats with pertussis toxin blocked both responses. Because the electrophysiological and biochemical responses to serotonin were correlated after pertussis toxin treatment, we conclude that both responses are mediated by a common regulatory protein, presumably Gi.

Action potential repolarization and a fast after-hyperpolarization in rat hippocampal pyramidal cells

1. The repolarization of the action potential, and a fast after-hyperpolarization (a.h.p.) were studied in CA1 pyramidal cells (n = 76) in rat hippocampal slices (28-37 degrees C). Single spikes were elicited by brief (1-3 ms) current pulses, at membrane potentials close to rest (-60 to -70 mV). 2. Each action potential was followed by four after-potentials: (a) the fast a.h.p., lasting 2-5 ms; (b) an after-depolarization; (c) a medium a.h.p., (50-100 ms); and (d) a slow a.h.p. (1-2 s). Both the fast a.h.p. and the slow a.h.p. (but not the medium a.h.p.) were inhibited by Ca2+-free medium or Ca2+-channel blockers (Co2+, Mn2+ or Cd2+); but tetraethylammonium (TEA; 0.5-2 nM) blocked only the fast a.h.p., and noradrenaline (2-5 microM) only the slow a.h.p. This suggests that two Ca2+-activated K+ currents were involved: a fast, TEA-sensitive one (IC) underlying the fast a.h.p., and a slow noradrenaline-sensitive one (IAHP) underlying the slow a.h.p. 3. Like the fast a.h.p., spike repolarization seems to depend on a Ca2+-dependent K+ current of the fast, TEA-sensitive kind (IC). The repolarization was slowed by Ca2+-free medium, Co2+, Mn2+, Cd2+, or TEA, but not by noradrenaline. Charybdotoxin (CTX; 30 nM), a scorpion toxin which blocks the large-conductance Ca2+-activated K+ channel in muscle, had a similar effect to TEA. The effects of TEA and Cd2+ (or Mn2+) showed mutual occlusion. Raising the external K+ concentration reduced the fast a.h.p. and slowed the spike repolarization, whereas Cl- loading of the cell was ineffective. 4. The transient K+ current, IA, seems also to contribute to spike repolarization, because: (a) 4-aminopyridine (4-AP; 0.1 mM), which blocks IA, slowed the spike repolarization; (b) depolarizing pre-pulses, which inactivate IA, had a similar effect; (c) hyperpolarizing pre-pulses speeded up the spike repolarization; (d) the effects of 4-AP and pre-pulses persisted during Ca2+ blockade (like IA); and (e) depolarizing pre-pulses reduced the effect of 4-AP. 5. Pre-pulses or 4-AP broadened the spike less, and in a different manner, than Ca2+-free medium, Cd2+, Co2+, Mn2+, TEA or CTX. The former broadening was uniform, with little effect on the fast a.h.p., whereas the latter affected mostly the last two-thirds of the spike repolarization and abolished the fast a.h.p.(ABSTRACT TRUNCATED AT 400 WORDS)

Neuroleptics decrease calcium-activated potassium conductance in hippocampal pyramidal cells

Intracellular recordings were made from pyramidal CA1-neurones of the hippocampal slice preparation. Bath application of a wide variety of neuroleptics was found to depress the slow afterhyperpolarization, which is mediated in these neurons by a calcium-dependent potassium conductance occurring following a burst of spikes. The depression of this conductance took place in the presence of calcium spikes of normal amplitude and duration, and except in the case of trifluoperazine, without alteration in resting membrane potential or input resistance.

Evidence for epinephrine-induced depolarization in neurons of bullfrog sympathetic ganglia

The response to epinephrine (EP) was determined for neurons in bullfrog sympathetic ganglia by intracellular and voltage-clamp recording techniques. EP (5 microM-1 mM) produced a concentration-dependent depolarization mediated through beta-adrenoceptors. The EP-induced depolarization (EPD) was associated with a decrease in the membrane conductance. The EP-induced current (EP1) was decreased at hyperpolarizing potential levels and nullified at -70 mV. No reversal of the EPI polarity was seen. It is concluded that the EPD is generated by the suppression of a voltage-dependent gK, probably the M-channel.

Adenosine antagonists increase spontaneous and evoked transmitter release from neuronal cells in culture

To examine the role played by endogenous adenosine in the modulation of transmitter release in the CNS, the effect of adenosine antagonists has been studied. Two systems have been used: EPSPs recorded from pyramidal cells in organotypic hippocampal cultures; and release of newly synthesized [3H]glutamate from cerebellar granule cells in dissociated culture. Bath application of 0.1-1 microM 8-phenyltheophylline (8-PT) reversibly increased both the number and size of spontaneous EPSPs and caused bursting activity in some cells. This effect was blocked by the glutamate antagonist gamma-D-glutamylglycine (DGG) (1 mM) but not by atropine (10 microM) or bicuculline (100 microM). Another adenosine antagonist isobutylmethylxanthine (IBMX, 10 microM) had a similar effect to 8-PT. Spontaneous activity in pyramidal cells and that induced by adenosine antagonists was blocked by the adenosine agonist 2-chloroadenosine (2-CA) (0.2-20 microM). 8-PT (10 microM) markedly potentiated K+-stimulated release of newly synthesized glutamate, and also enhanced basal glutamate release. The agonist (-)-phenylisopropyladenosine ((-)-PIA, 2 microM) which is relatively selective for A1 receptors, reduced by 19 +/- 5% the 8-PT-induced enhancement, and reduced K+-stimulated glutamate release in the absence of 8-PT to a similar extent. In contrast 5'-N-ethylcarboxamido adenosine (NECA, 2 microM), which is a relatively selective A2 agonist, slightly enhanced glutamate release. From these results it is likely that 8-PT potentiates glutamate release in both systems by blocking the effect of endogenous adenosine on presynaptic A1 receptors.

Beyond receptors: multiple second-messenger systems in brain

Second-messenger systems play a major role in mediating neurotransmitter actions. In recent years our understanding of the organization and function of two prominent second-messenger systems has progressed rapidly--the adenylate cyclase and phosphoinositide systems. Guanosine triphosphate-binding proteins, which are especially abundant in brain, couple transmitter receptors to the key second-messenger generating enzymes in both of these systems. Whereas activation of adenylate cyclase produces a single intracellular messenger, cyclic AMP, stimulation of the phosphoinositide system generates at least two, inositol trisphosphate and diacylglycerol. Inositol trisphosphate mobilizes calcium from intracellular stores, and diacylglycerol, like cyclic adenosine monophosphate, activates a phosphorylating enzyme, protein kinase C. These second-messenger systems are particularly enriched in the brain where they modulate many aspects of synaptic transmission.

Intracellular studies on the dopamine-induced firing inhibition of neostriatal neurons in vitro: evidence for D1 receptor involvement

Intracellular recordings were obtained from rat neostriatal slices. Bath-applied dopamine (1-10 microM) produced a reversible inhibition of the action potentials evoked by direct stimulation and a decrease in the amplitude of the intrastriatally evoked depolarizing postsynaptic potentials. No change in membrane potential was detected during the application of 1-10 microM dopamine. Dopamine application also produced a decrease in anomalous rectification in the depolarizing direction. This subthreshold inward rectification was abolished by tetrodotoxin, but not by calcium-free and cadmium (0.1-1 mM)-containing solutions. The dopamine-induced decrease in excitatory postsynaptic potential amplitude was evident at resting membrane potential or at more positive levels, but was absent at hyperpolarized values of the membrane potential. Addition of bicuculline (50-500 microM) to the medium did not affect the inhibitory action of dopamine. The inhibitory action of dopamine also persisted in calcium-free and cadmium-containing solutions. The adenosine 3',5'-cyclic monophosphate analogue, 8-bromo-adenosine 3',5'-cyclic monophosphate (0.1-1 mM), mimicked the effects produced by D1 receptor activation. Bath application of 2,3,4,5-tetrahydro-7,8-dihydroxy-1-phenyl-1H-3-benzazepine (SKF 38393) (1-10 microM), a selective D1 dopaminergic agonist, mimicked the effects of micromolar concentrations of dopamine. The D2 dopaminergic agonists, 4,4a,5,6,7,8,8a,9-octahydro-5-n-propyl-2H-pyrazolo-3,4-g-quinoline (LY 171555) and bromocriptine (both at 10 nM-10 microM), had no effects on neostriatal cells. The inhibition induced by micromolar doses of dopamine or SKF 38393 was antagonized by bath applications of R-(+)-8-chloro-2,3,4,5-tetrahydro-3-methyl-5-phenyl-1H-3-benzazepin++ +-7-ol (SCH 23390; 0.1-10 microM), a D1-selective antagonist, but not by sulpiride (10 nM-10 microM), a D2 antagonist. We conclude that the inhibitory effect of dopamine on rat striatal neurons is postsynaptically mediated by the activation of D1 dopaminergic receptors via the reduction of a voltage-dependent tetrodotoxin-sensitive inward conductance.

Low concentrations of GABA reduce accommodation in primary afferent neurons by an action at GABAB receptors

The pattern of accommodation of spike activity during sustained membrane depolarization was investigated in primary afferent neurons recorded intracellularly in vitro in the rat. We show that gamma-aminobutyric acid (GABA) and baclofen reduce accommodation in some fast conducting dorsal root ganglion neurons. This effect was restricted to those A delta cells with axons displaying a rather fast conduction velocity (15-25 m/s). GABA-induced blockade of accommodation was not observed in large A beta neurons. Pharmacological studies with baclofen, as opposed to isoguvacine, indicate that this effect is due to GABAB receptors activation. The effect is also shown to be resistant to bicuculline antagonism. In slow conducting afferents, GABAB receptor activation is known to shorten the CA2+ component of action potentials. By contrast, no such component was observed in the A delta cells studied. Furthermore, Ca2+-activated K+ conductances are not implicated in the reduction of accommodation caused by GABAB receptor activation. In conjunction with the actual knowledge about the distribution of GABA receptors on primary afferents, our result indicates that GABAA and GABAB receptors coexist on all categories of A delta and C primary afferents in the rat.

A role for norepinephrine in arousal, emotion and learning?: limbic modulation by norepinephrine and the Kety hypothesis

1. Kety hypothesized that forebrain norepinephrine acted to selectively enhance cell firing in neurons receiving environmental inputs during affectively important events. He further suggested that norepinephrine could act to induce a 'persistent facilitation' of the inputs which accompany affectively important events and would thus promote a memory for these events. 2. The electrophysiological actions of norepinephrine at the time Kety proposed his hypothesis were thought to be inhibitory. More recent evidence has demonstrated that norepinephrine in neocortex and cerebellum enhances both excitatory and inhibitory evoked activity much as Kety proposed. This has been termed norepinephrine neuromodulation. 3. The locus coeruleus-norepinephrine system which gives rise to neocortical and cerebellar norepinephrine innervation also innervates, even more densely, areas of the limbic system. A review of norepinephrine actions, particularly in limbic cortex, indicates that locus coeruleus-norepinephrine also enhances transmission of evoked inputs in these structures. 4. A long-lasting enhancement of evoked inputs by locus coeruleus-norepinephrine has been demonstrated in the hippocampus. This long-lasting enhancement of inputs is reviewed in detail since it appears to directly support Kety's hypothesized 'persistent facilitation' effect of norepinephrine. It is suggested that norepinephrine-induced long-lasting enhancement may underlie numerous demonstrations of norepinephrine-dependent memory and neural plasticity in the forebrain. 5. The relationship of norepinephrine neuromodulation to possible candidate mechanisms and to activation of specific norepinephrine receptors is briefly discussed.

Norepinephrine regulates long-term potentiation of both the population spike and dendritic EPSP in hippocampal dentate gyrus

Hippocampal slices from norepinephrine (NE)-depleted rats exhibited marked reductions in long-term potentiation (LTP) of both the population spike and dendritic EPSP in the dentate gyrus. In contrast, depletion of serotonin (5-hydroxytryptamine, 5-HT) had no effect on either population spike or EPSP-LTP. In addition, superfusion of slices with NE produced potentiation of both the granule cell population spike and dendritic EPSP which persisted long after NE washout. These data support a role for NE in regulating long-term plasticity of both granule cell action potential firing and dendritic EPSPs.

The cAMP cascade in the nervous system: molecular sites of action and possible relevance to neuronal plasticity

Many intercellular messages regulate the activity of their target cells by altering the intracellular level of cAMP and, as a consequence, the phosphorylation state of proteins which serve as substrates for cAMP-dependent protein kinase. Such regulation plays a crucial role in neuronal development, neuronal function, and neuronal plasticity (e.g., elementary learning mechanisms). Ample information has been accumulated in recent years on the enzymes that regulate the level of cAMP or respond to it, on the regulation of cAMP synthesis by neurohormones, neurotransmitters, ions, and toxins, on neuronal-specific substrate proteins that are phosphorylated by the cAMP-dependent kinase, and on the interaction of the cAMP-cascade with other second-messenger systems within neurons. Such data, obtained by a combination of molecular-biological, biochemical, and cellular approaches, shed light on the detailed mechanisms by which modulation of a ubiquitous molecular cascade leads to a great variety of short-term as well as long-term specific neuronal responses and alterations.

Kainate reduces two voltage-dependent potassium conductances in rat hippocampal neurons in vitro

The mechanisms of action of kainate were studied in CA1 hippocampal neurons using the single electrode voltage-clamp technique in vitro. Kainate (100-200 nM) reduced the potassium current which is responsible for the anomalous rectification (IQ). In 30% of the cells the drug reduced the calcium-dependent potassium current (IC) which is responsible for the afterhyperpolarization that follows calcium action potentials. The reduction of IC will contribute to the enhancement of the neural excitability by this drug.

Resistance of afterhyperpolarizations in hippocampal pyramidal cells to prostaglandins and vasoactive intestinal polypeptide (VIP)

The afterhyperpolarization (AHP) that follows repetitive stimulation was recorded intracellularly from CA1 pyramidal neurons in the guinea pig hippocampal slice preparation. Although the late AHP could be blocked by histamine (1-10 microM), forskolin (10 microM) and 8-bromo-cyclic AMP (100 and 500 microM), neither prostaglandins D2, E1 and F2 alpha (0.5 microM) nor vasoactive intestinal polypeptide (0.5 microM) had any effect on the AHP, membrane potential, membrane resistance or action potential properties.