Noradrenaline blocks accommodation of pyramidal cell discharge in the hippocampus

Dopamine decreases the calcium-activated afterhyperpolarization in hippocampal CA1 pyramidal cells

The effect of dopamine (DA) on the calcium-activated potassium conductance underlying the slow afterhyperpolarization (AHP) which follows a train of action potentials in hippocampal pyramidal cells was studied utilizing the in vitro hippocampal slice preparation. Bath-applied DA (1-100 microM) significantly reduced the AHP in a reversible, dose-dependent manner. Neither the amount of current injected to elicit the AHP nor its initial amplitude had an effect on the reduction of the AHP by DA. DA did not depress calcium spikes, suggesting that the blockade of the AHP likely occurs at a step subsequent to the entry of calcium. Since DA's actions on the AHP closely mimicked those of norepinephrine, we examined the effect of beta-adrenergic antagonists on DA's actions. At concentrations which in other systems have been shown not to block DA stimulated adenylate cyclase, beta-adrenergic antagonists completely inhibited the reduction of the AHP by DA. In some cells DA also elicited small hyperpolarizations which were not blocked by application of dopamine receptor antagonists. These findings strongly suggest that a major electrophysiological action of DA in the hippocampus (i.e. blockade of the AHP) is due to its cross reactivity with beta-adrenergic receptors and that rigid pharmacologic criteria must be used before attributing an action of DA unambiguously to its interaction with DA receptors.

Norepinephrine enhances stimulus-evoked calcium and potassium concentration changes in dentate granule cell layer

Changes in extracellular Ca2+ and K+ concentrations were measured in the dentate gyrus with ion-selective/reference electrodes during high-frequency perforant path stimulation. Bath application of norepinephrine (NE, 50 microM) markedly enhanced both evoked decreases in extracellular calcium and increases in extracellular potassium concentration. These effects of NE were observed in the granule cell layer (stratum granulosum), but not 200 microns away in the dendritic layer (stratum moleculare). The beta-antagonist propranolol (1 microM) completely blocked the NE-induced enhancement of Ca2+ signals in the dentate. In contrast to the dentate, NE did not enhance evoked Ca2+ of K+ concentration changes in the CA1 pyramidal cell layer. These results indicate that NE markedly enhanced both Ca2+ and K+ fluxes, probably by a beta-receptor-mediated mechanism, in the dentate gyrus during high-frequency stimulation of a type able to elicit long-term potentiation (LTP). These increases may underly the action of NE in modulating LTP in the dentate gyrus.

Characterization of opioid receptors modulating noradrenaline release in the hippocampus of the rabbit

Noradrenaline (NA) release and its modulation via presynaptic opioid receptors were studied in rabbit hippocampal slices, which were preincubated with [3H]NA, continuously superfused in the presence of 30 microM cocaine and stimulated electrically. The evoked release of [3H]NA was strongly reduced by the preferential kappa-agonists ethylketocyclazocine, dynorphin A1-13, dynorphin A, trans-3,4-dichloro-N-methyl-N-[2-(1-pyrrolidinyl)-cyclohexyl] -benzeneacetamide (U-50,488), and (-)-5,9-dimethyl-2'-OH-2-tetrahydrofurfuryl-6,7-benzomorphan [(-)-MR 2034], whereas (+)-MR 2035 [the (+)-enantiomer of (-)-MR 2034] was ineffective. In contrast, the preferential delta-agonists Leu-enkephalin, Met-enkephalin, and D-Ala2-D-Leu5-enkephalin (DADLE) as well as the mu-agonists morphine, normorphine, D-Ala2-Gly-ol5-enkephalin (DAGO), and beta-casomorphin 1-4 amide (morphiceptin) were much less potent. However, in similar experiments on rat hippocampal slices DAGO (1 microM) was much more potent than ethylketocyclazocine (1 microM) or DADLE (1 microM). (-)-N-(3-furylmethyl)-alpha-noretazocine [(-)-MR 2266], 1 microM, a preferential kappa-antagonist, antagonized the effect of ethylketocyclazocine more potently than (-)-naloxone or (+)-MR 2267 [the (+)-enantiomer of (-)-MR 2266]. Given alone, (-)-MR 2266 slightly and (+)-MR 2267 (1 microM each) greatly enhanced NA release, apparently due to alpha 2-adrenoceptor blockade since their effects were completely abolished in the presence of yohimbine (0.1 microM).(ABSTRACT TRUNCATED AT 250 WORDS)

Tubocurarine suppresses slow calcium-dependent after-hyperpolarization in guinea-pig inferior mesenteric ganglion cells

Intracellular recordings were made from neurones of the isolated guinea-pig inferior mesenteric ganglia. Single-spike potentials evoked by either depolarizing current pulses applied through the recording micro-electrode or stimulation of the hypogastric nerves were followed by an after-hyperpolarization (a.h.). The spike a.h. in 40% of the neurones, referred to herein as type I, had a relatively short duration (less than 50 ms) and exhibited a monophasic decay with a mean time constant (tau) of 11.4 ms. In the remaining cells (type II), the spike was followed by a long a.h. (greater than 100 ms) having a double-exponential decay; the fast and slow components of the a.h. are termed a.h.f and a.h.s, respectively, and they had mean tau values of 11.4 and 74 ms, respectively. A.h.f and a.h.s of type II neurones were reduced by membrane hyperpolarization and reversed their polarities between -80 and -90 mV. The reversal potentials shifted in a manner closely predicted by the Nernst equation as external K+ concentration was increased. Superfusion of low-Ca2+ high-Mg2+ solution to type II neurones reduced the a.h.f and a.h.s by 32 and 82%, respectively, indicating that a.h.s is largely Ca2+-dependent. Application of (+)-tubocurarine (10-100 microM) reversibly suppressed the a.h.s without affecting a.h.f in a concentration-dependent manner. Following a short train of action potentials evoked from type II neurones, the post-tetanic hyperpolarization (p.t.h.) was similarly depressed by (+)-tubocurarine in a dose-dependent manner. (+)-tubocurarine did not significantly change the amplitude of Ca2+-dependent spike potentials evoked in neurones bathed in Na+-free high-Ca2+ plus tetraethylammonium (5-10 mM) solution. The results indicate that (+)-tubocurarine selectively suppresses a.h.s, a slow Ca2+-dependent a.h., the consequence of which is a facilitation of repetitive discharges of the cells.

Prolongation of calcium action potentials by gamma-aminobutyric acid in primary sensory neurones of lamprey

Intracellular recordings from primary mechanosensory neurones (dorsal cells) in the lamprey spinal cord were used to test the membrane effects of a variety of putative neuromodulatory agents. gamma-Aminobutyric acid (GABA) produced a dose-dependent increase in the duration of mixed Na-Ca or pure Ca action potentials in these cells. L-Glutamate and glycine produced minimal broadening of Ca action potentials. Acetylcholine, noradrenaline, serotonin, met-enkephalin, D-glutamate and dopamine had no effect. The pharmacology of GABA's action appeared to be complex. While the GABAA receptor antagonists, bicuculline, picrotoxin and curare, did not block GABA's effect, both the GABAA receptor agonist, muscimol, and the GABAB-receptor agonist, baclofen, occasionally broadened Ca action potentials in these cells. GABA had no effect on the resting potential, passive current-voltage (I-V) characteristics and pure Na action potential of dorsal cells, ruling out an action on passive membrane channels, transmitter-activated channels, or on those voltage-dependent channels activated during the Na action potential. Thus, GABA affected dorsal cells only when a significant Ca current was evident. GABA appeared not to increase the conductance of the Ca channels since its action was accompanied by an increase in input resistance, suggesting an inhibition of Ca-dependent conductance that normally acts to repolarize the membrane during a Ca action potential. An inhibitory effect of GABA on a Ca-dependent Cl conductance was ruled out in experiments where the Cl gradient was altered by removal of extracellular Cl without affecting GABA-induced Ca action potential prolongation. Dorsal cells have a prominent Ca-dependent K conductance (gK(Ca], and it is this conductance that GABA may inhibit. Consistent with this was the observation that the hyperpolarizing after-potential that follows Ca action potentials in dorsal cells, which reflects gK(Ca) in these cells and whose duration is normally increased when the Ca action potential duration increases, was not prolonged when the Ca action potential was broadened by GABA. Further, the failure of GABA to prolong Ba action potentials was consistent with this proposed mechanism of action, since Ba apparently does not activate gK(Ca) in these cells. Forskolin, a specific adenylate cyclase activator, caused broadening of Ca action potentials in lamprey dorsal cells comparable in magnitude to that of GABA. Thus, an increase in intracellular cyclic AMP is a candidate for the intracellular mediator of GABA's effect on these cells.

Depression of spike adaptation and afterhyperpolarization by 4-aminopyridine in hippocampal neurons

In guinea pig hippocampal slices, 4-aminopyridine (4-AP) in concentrations of 100-500 microM reduced the adaptation of CA3 pyramidal neurons to depolarizing stimuli, resulting in a prolongation of repetitive firing during injection of long-lasting depolarizing currents. Concurrently, there was a decrease in the 'sag' of potential after spike bursts. Furthermore, 4-AP decreased or abolished the hyperpolarizing potential (the afterhyperpolarization) which normally followed repetitive firing of the neurons. The findings suggest that 4-AP could interfere with the Ca2+-activated K+ current in hippocampal CA3 pyramidal neurons.

Noradrenaline-stimulated inositol phospholipid breakdown in rat dorsal lateral geniculate nucleus neurones

Noradrenaline-stimulated inositol phospholipid breakdown in matched vibratome sections through the rat dorsal lateral geniculate nucleus (dLGN). The response was measured as a large accumulation of [3H]inositol labelled inositol monophosphate and was mediated via activation of alpha 1-adrenergic receptors. Accumulation of [3H]inositol phosphates was reduced in kainic acid-lesioned animals, indicating that this response occurred within dLGN neurones and not afferent terminals. The results implicate inositol phospholipid breakdown as part of the mechanism of noradrenergic neurotransmission within the dLGN.

Excitatory synaptic interactions between CA3 neurones in the guinea-pig hippocampus

Excitatory synaptic interactions between CA3 neurones in slices from guinea-pig hippocampus were examined. Recurrent excitatory post-synaptic potentials (e.p.s.p.s) were evoked by action potentials in a single presynaptic neurone or by the antidromic activation of part of the CA3 pyramidal cell population. The peak amplitude of unitary e.p.s.p.s was 1-2 mV at potentials between -64 and -70 mV. Their time to peak was 7-12 ms and the initial phase of their decay was slower than that of a somatically injected voltage pulse. Recurrent e.p.s.p.s were often followed by a small (0.3 mV) hyperpolarization, or undershoot. Recurrent e.p.s.p.s were compared with e.p.s.p.s evoked by stimulating mossy fibres, which terminate proximally on apical dendrites of CA3 pyramidal cells. They were of slower time course and reversed at a more positive potential than mossy fibre e.p.s.p.s. Some synaptic terminals made by recurrent axon collaterals apparently terminate at distant locations on apical dendrites. The decay of both recurrent e.p.s.p.s and dendritic voltage pulses was prolonged by membrane depolarization within a 10-15 mV subthreshold potential range. Voltage-dependent inward currents activated by the synaptic depolarization may contribute to the slow initial decay of these synaptic events. The undershoot did not occur when transmission of a unitary e.p.s.p. failed and was of slower time course than the hyperpolarization due to an inhibitory post-synaptic potential (i.p.s.p.). It was suppressed by intracellular application of K+ channel blockers and probably reflects an intrinsic outward current activated as a consequence of the synaptic depolarization. Considerable temporal summation of synaptic potentials occurred when recurrent synapses were activated twice at an interval of 5-10 ms, typical of the spontaneous burst firing pattern of CA3 neurones. The mean facilitation of a second e.p.s.p. at this interval was about 0.6. The efficacy of a third and subsequent e.p.s.p.s at similar interval was reduced. Presynaptic bursts of three to five action potentials evoked summed e.p.s.p.s of amplitude 2-4 mV, with time to peak 20-40 ms and decaying phase of similar duration. Their rising phase was relatively smooth and summed events were succeeded by an undershoot. Presynaptic bursts could cause a post-synaptic neurone to discharge.

Glutamate-induced activation of rat locus coeruleus increases CA1 pyramidal cell excitability

In anesthetized rats, injections of a 0.5 mM glutamate solution into the locus coeruleus (LC) reversibly increased the amplitude of the population spike evoked in CA1 by stimulation of the Schaffer-commissural fiber tract. This effect was absent in N-(2-chloroethyl)-N-ethyl-2-bromobenzylamine (DSP-4)-treated, noradrenaline (NA)-depleted animals. The excitatory postsynaptic potential recorded in the stratum radiatum was unaffected following the glutamate injections. Systemic administration of the NA-uptake inhibitor desipramine also produced an increase in population spike amplitude. The findings demonstrate that activation of LC neurons increases pyramidal cell excitability in vivo.

Modulation of visual cortical plasticity by acetylcholine and noradrenaline

During a critical period of postnatal development, the temporary closure of one eye in kittens will permanently shift the ocular dominance (OD) of neurones in the striate cortex to the eye that remains open. The OD plasticity can be substantially reduced if the cortex is infused continuously with the catecholamine neurotoxin 6-hydroxydopamine (6-OHDA) during the period of monocular deprivation, an effect that has been attributed to selective depletion of cortical noradrenaline. However, several other methods causing noradrenaline (NA) depletion leave the plasticity intact. Here we present a possible explanation for the conflicting results. Combined destruction of the cortical noradrenergic and cholinergic innervations reduces the physiological response to monocular deprivation although lesions of either system alone are ineffective. We also find that 6-OHDA can interfere directly with the action of acetylcholine (ACh) on cortical neurones. Taken together, our results suggest that intracortical 6-OHDA disrupts plasticity by interfering with both cholinergic and noradrenergic transmission and raise the possibility that ACh and NA facilitate synaptic modifications in the striate cortex by a common molecular mechanism.

Actions of noradrenaline recorded intracellularly in rat hippocampal CA1 pyramidal neurones, in vitro

CA1 pyramidal neurones were studied in rat in vitro hippocampal slices using standard intracellular and single-electrode voltage-clamp recording techniques to examine the actions of noradrenaline (NA). NA had two different effects on the resting membrane potential of pyramidal neurones; either a hyperpolarization accompanied by a decrease in membrane input resistance, or less commonly, a depolarization accompanied by an increase in input resistance. In many cells, both effects, a hyperpolarization followed by a depolarization were observed. The depolarization was mediated by a noradrenergic beta-receptor. The hyperpolarization was more difficult to characterize, but may result from alpha-receptor activation. NA reduced the amplitude and duration of the slow calcium-activated potassium after-hyperpolarization (a.h.p.) that follows depolarization-induced action potentials. This action of NA was mediated by beta 1-noradrenergic receptors. NA, in the presence of tetrodotoxin and tetraethylammonium, reduced the a.h.p. without reducing the size of the calcium action potential which preceded it. This was unlike the action of the calcium channel blocker, cadmium, which reduced the calcium action potential and the a.h.p. in parallel. Furthermore, NA did not reduce the amplitude of calcium or barium currents recorded under voltage clamp after blockade of potassium currents. A functional consequence of this blockade of the calcium-activated a.h.p. was a reduction of the accommodation of action potential discharge such that the excitatory responses of the neurone to depolarizing stimuli, such as glutamate application or current passed through the recording electrode, were enhanced. We conclude that the effects of NA on calcium-activated potassium conductance and on resting membrane potential can interact to increase the signal-to-noise ratio of hippocampal pyramidal neurone responsiveness.

Cyclic adenosine 3',5'-monophosphate mediates beta-receptor actions of noradrenaline in rat hippocampal pyramidal cells

Intracellular recordings were made from rat hippocampal CA1 pyramidal neurones in the in vitro slice preparation to study the actions of cyclic adenosine 3',5'-monophosphate (cyclic AMP). Application of the membrane permeant analogue of cyclic AMP, 8-Br cyclic AMP caused a small depolarization of the resting membrane potential accompanied by an increase in membrane input resistance and also reduced the amplitude of depolarization-evoked calcium-activated potassium after-hyperpolarizations (a.h.p.s.). 8-Br cyclic AMP reduced calcium-activated a.h.p.s but did not reduce calcium action potentials in these cells. 8-Br cyclic AMP also reduced action potential frequency accommodation. The effects of 8-Br cyclic AMP were not mimicked by cyclic AMP applied extracellularly but were imitated by intracellular injections of cyclic AMP. Activation of the endogenous adenylate cyclase of pyramidal cells either by intracellular injection of the stable guanosine 5'-triphosphate (GTP) analogue guanylyl-imidodiphosphate, or by extracellular application of forskolin, reduced the a.h.p. and accommodation. Reducing phosphodiesterase activity with application of either 3-isobutyl-1-methylxanthine or Ro20-1724 reduced the amplitude of the a.h.p. and potentiated the a.h.p.-blocking action of noradrenaline. Reducing adenylate cyclase activity by application of SQ22,536 slightly increased the amplitude of the (a.h.p.) and reduced the a.h.p.-blocking action of noradrenaline. We conclude that the beta-receptor actions of NA on hippocampal CA1 pyramidal cells are mediated by intracellularly produced cyclic AMP.

Electrophysiological properties of sympathetic preganglionic neurons in the cat spinal cord in vitro

Intracellular recordings were obtained from sympathetic preganglionic neurons of the intermedio-lateral nucleus of the adult cat in slices of upper thoracic spinal cord maintained in vitro. The neurons were identified by their antidromic responses to stimulation of various ipsilateral sites. Sites from which antidromic responses could be evoked were the white ramus, the ventral root, the ventral root exit zone, the white matter between the latter and the outer edge of the tip of the ventral horn, the lateral edge of the ventral horn. Resting membrane potential was --61.3 +/- 1.6 mV (mean +/- SEM), input resistance 67.5 +/- 3.7 M omega, time constant 11.5 +/- 1.2 ms. The amplitude of the action potential generated by antidromic or direct stimulation was 77.4 +/- 2.3 mV. Threshold for direct spikes was 18.2 +/- 1.8 mV. The action potential had an average duration of 3.03 +/- 0.16 ms. It showed a prominent "hump" on the falling phase. The action potential had a tetrodotoxin (TTX)-sensitive and a TTX-resistant component. The latter was abolished by cobalt. Tetraethylammonium, cesium and barium prolonged the action potential duration which acquired a plateau-shape. A prolonged after-hyperpolarization (AHP) followed the sympathetic preganglionic neuron spike. Following a single spike, AHP duration and peak amplitude were 2.8 +/- 0.3 s and 16.6 +/- 0.7 mV, respectively. The AHP was abolished by cesium or barium, but enhanced by tetraethylammonium. An AHP followed the TTX-resistant spike. EPSPs and IPSPs could be generated by focal stimulation. The EPSP triggered spikes when threshold (15.0 +/- 2.0 mV) was reached.(ABSTRACT TRUNCATED AT 250 WORDS)

Noradrenaline modifies sympathetic preganglionic neuron spike and afterpotential

Sympathetic preganglionic neurons were antidromically identified in the slice of the upper thoracic spinal cord of the adult cat, maintained in vitro. In normal Krebs solution, spikes evoked by intracellular stimulation had a marked 'hump' on the repolarization phase and were followed by an afterhyperpolarization of 2.8 s duration and 16.6 mV peak amplitude. Superfusion with Krebs solution containing noradrenaline 10-50 micron reversibly abolished the 'hump' of the spike and the late component of the afterhyperpolarization. In addition, it caused the appearance of a depolarizing afterpotential of 100-600 ms duration. This depolarization could result in repetitive firing of the neuron in response to a single intracellular current pulse.

Ionic mechanisms of 3 types of functionally different neurons in the lamprey spinal cord

Action potentials and afterpotentials were compared in giant interneurons, sensory dorsal cells and large intraspinal axons in the lamprey spinal cord. Afterpotentials of giant interneurons and dorsal cells consisted of two hyperpolarizing phases, an early and a late one, which were separated by a delayed depolarization. The afterpotentials of axons had a single hyperpolarizing phase also followed by a delayed depolarization. Tetraethyl ammonium chloride (TEA+) eliminated the early phase of the afterhyperpolarization in giant interneurons, only partially reduced the early phase in dorsal cells and did not affect the single phase of axons. The delayed depolarization of dorsal cells was attenuated by TEA+ but in axons it was unaltered. The heavy metal ions Mn2+ and Co2+ (2 mM) eliminated the late phase in giant interneurons but did not reduce the late phase in dorsal cells. The delayed depolarization remained in both types of cell in the presence of these ions. Action potentials of giant interneurons and dorsal cells, but not those of axons, were broadened by TEA+. The TEA-prolonged action potentials were narrowed by Mn2+ applied in combination with TEA+. The afterhyperpolarizations of all 3 cells were reduced by injection of negative current and enhanced by positive current. Repetitive stimulation resulted in summation of the afterhyperpolarization in giant interneurons and dorsal cells. The results suggest that different sets of potassium channels are responsible for the afterhyperpolarizations in each type of cell. In giant interneurons fast channels which are sensitive to TEA+ may underlie the early phase and slow channels activated by calcium entry may underlie the slow phase. The early phase of dorsal cells may be caused by two types of fast channel, one similar to that in giant interneurons and another less sensitive to external TEA+. This latter type may also cause the afterhyperpolarization in axons. Although calcium channels appear to contribute to the action potentials of giant interneurons and dorsal cells, the late phase of the latter neurons does not seem to be activated by calcium entry. The delayed depolarizations of the neurons appear to be due to an inward current which is not carried by calcium.

The action of norepinephrine in the dentate gyrus: beta-mediated facilitation of evoked potentials in vitro

The effects of superfusion of norepinephrine (NE) on perforant path (PP) evoked potentials in the dentate gyrus were evaluated in the rat hippocampal slice preparation. Superfusion of NE (10 microM) produced a facilitation of the PP evoked responses. Facilitation of the synaptically-evoked responses was expressed in the field potential as an increase in extracellular excitatory postsynaptic potential (EPSP) (117% of control), a decrease in population spike onset latency (94% of control) and an increase in population spike amplitude (131% of control). In 24% of the slices the facilitation of the population spike amplitude lasted longer than 30 min. Isoproterenol, a beta-agonist, mimicked NE effects while timolol, a beta-antagonist, blocked them. Facilitation of the population spike amplitude by NE could not be accounted for solely by the increase in EPSP slope also produced by NE. Superfusion of NE did not produce facilitation of the antidromically evoked field potentials, but in 4 of 8 slices produced a small decrease. NE effects were activity-independent, since the subsequently evoked PP responses were facilitated even when the PP was not concurrently stimulated during superfusion with NE.

Synaptic block of a transmitter-induced potassium conductance in Aplysia neurones

A voltage-clamp study was made of a slow excitatory post-synaptic potential (slow e.p.s.p.) that can be elicited in the medial cells of the left pleural ganglion of Aplysia californica by the firing of at least three different presynaptic neurones (labelled I, II and III). Each of these three neurones elicits other permeability changes in addition to the slow e.p.s.p., and all elements of these synaptic responses were shown to be mediated monosynaptically. The slow e.p.s.p., associated with an increase in membrane resistance, was shown to be due to a decrease in K permeability. When the slow e.p.s.p. was present spontaneously, it could be blocked by three compounds (tetraethylammonium (TEA), phenyltrimethylammonium (PTMA), or methylxylocholine (beta-TM 10], all previously shown to block the cholinergic receptor that mediates an increase in K conductance in the medial cells (see Kehoe, 1972b). Furthermore, in ganglia in which no slow e.p.s.p. was seen in response to firing of the neurones I, II, and III, such a response became manifest when agonists capable of activating the cholinergic receptor were applied (e.g. acetylcholine (ACh), carbachol, arecoline, or F2268). The slow e.p.s.p. thus appears to result from the reduction, induced by any one of three 'blocking neurones', of a cholinergically controlled K conductance. Finally, when presynaptic neurone I (the only neurone tested) was fired shortly before or during the activation of presynaptic neurone IV, previously shown to be cholinergic (Kehoe, 1972b), the K component of the cholinergic post-synaptic inhibitory potential was markedly reduced. The concentration at which a given agonist caused the manifestation of the synaptic diminution in K conductance (i.e. the slow e.p.s.p.) was found to be the same as that at which it caused a reduction in the synaptically activated, cholinergic, K-dependent conductance elicited by presynaptic neurone IV. Intracellularly injected adenosine 3',5'-cyclic monophosphate (cyclic AMP) imitated the effect of the 'blocking neurones' on the K conductance activated by bath-applied cholinomimetics. This effect was superimposed on a cyclic-AMP-induced, voltage-dependent inward current that disappeared when the cell was bathed in Na-free sea water, or when the extracellular Ca concentration was increased to 60 mM. The effect of cyclic AMP on the cholinergic K conductance remained even after this cyclic-AMP-activated inward current was eliminated.(ABSTRACT TRUNCATED AT 400 WORDS)

Synaptic block of a calcium-activated potassium conductance in Aplysia neurones

In the preceding paper (Kehoe, 1985) it was shown that the firing of any one of three neurones (I, II, III) presynaptic to the medial cells of the pleural ganglion of Aplysia californica causes a diminution of the cholinergically controlled K conductance in those cells. Firing of the same three presynaptic neurones was shown here to cause a similar diminution in a depolarization-induced K-dependent conductance in the same post-synaptic cells. The depolarization-induced K conductance was found to disappear when Ca ions were removed from the sea water bathing the ganglion or when the cell was injected with the Ca chelator ethyleneglycol-bis-(beta-aminoethylether)N,N'-tetra-acetic acid (EGTA). The diminution in this Ca-activated, K-dependent current occurred even when the presynaptic neurone was fired a few seconds after the end of the depolarizing voltage step to the post-synaptic neurone, showing that the diminution in K conductance was not an indirect effect of a transmitter-induced diminution in Ca influx during the depolarizing pulse. The two K conductances affected by the 'blocking neurones' could be selectively eliminated. The cholinergic conductance could be blocked by receptor-specific cholinergic antagonists (e.g. 1 mM concentrations of phenyltrimethylammonium (PTMA), choline and tetraethylammonium (TEA]. Even at 10 mM concentrations, none of these compounds (including TEA, which is known to block certain Ca-activated K conductances) had an effect on the depolarization-induced, Ca-activated K conductance studied here. This latter conductance, on the other hand, was selectively blocked by an intracellular injection of EGTA. The three blocking neurones continued to diminish the K conductance (cholinergic or depolarization induced) that remained intact under these different experimental conditions. The depolarization-induced influx of Ca was shown to block the cholinergically controlled K conductance, but Ca was excluded as the possible mediator of the diminution in K conductance caused by the three blocking neurones. An intracellular injection of Ca ions into the medial cells was shown to activate a variety of changes in membrane conductance; in particular, two K-conductance increases: an early, TEA-sensitive one, and a slowly developing, TEA-insensitive one. Both the permeant cyclic AMP analogue p-chlorophenylthioadenosine 3',5'-monophosphate (CPT-cyclic AMP) and the phosphodiesterase inhibitors amino-phylline and isobutyl-1-methylxanthine (IBMX) were shown to block the depolarization-induced K conductance, and to reduce, though not eliminate, the slowly developing K conductance activated by an intracellular injection of Ca.(ABSTRACT TRUNCATED AT 400 WORDS)

Prostaglandins block a Ca2+-dependent slow spike afterhyperpolarization independent of effects on Ca2+ influx in visceral afferent neurons

The blockade of a slow Ca2+-activated K+-dependent afterhyperpolarization (AHPs) in rabbit visceral sensory neurons by the prostaglandins, PGE1 and PGD2, was investigated to determine whether the blockade was indirectly due to a reduction in Ca2+ influx. The prostaglandins (PGs) could block the AHPs in the absence of any change in Ca2+-dependent spikes elicited in the presence of tetrodotoxin and tetraethylammonium bromide. A PG-induced decrease in Ca2+-dependent spike width observed in some neurons was temporally dissociated from the PG-induced block of the AHPs. In addition, a slow afterhyperpolarization produced by the application of the Ca2+ ionophore, A23187, was blocked by the PGs. It is concluded that a reduction in Ca2+ influx is not responsible for the PG-induced blockade of the AHPs.

Effects of noradrenaline on some potassium currents in CA1 neurones in rat hippocampal slices

Pyramidal (CA1) cells in rat hippocampal slices were voltage clamped using a single electrode voltage clamp. In the presence of tetrodotoxin (TTX), depolarizing pulses from holding potentials of -60 to -70 mV elicited a slow inward calcium (Ca2+) current and two outward potassium (K+) currents: an A current and a slower, Ca2+-dependent K+ current. Noradrenaline (NA) (20 microM) depressed the amplitude of the K+ currents without affecting the Ca2+ current. The effect of NA could be blocked with propranolol and was mimicked by isoprenaline, suggesting that NA depresses the K+ currents by binding to beta-receptors.

Effects of norepinephrine on rat neocortical neurons in dissociated cell culture

Intracellular recordings were made from neurons in dissociated cell culture of neocortex during application of norepinephrine (NE) or other adrenergic agonists. In the population of neurons generally studied, greater than 18 micron in diameter, adrenergic agonists from 1 nM to 50 microM produced no change in membrane potential or input resistance 120 cells). Adrenergic agonists increased synaptic activity impinging on the impaled cell in 25/120 neurons (21%). In neurons in cocultures of locus coeruleus and cerebral cortex, again the same synaptic response to perfusion with NE was noted in 13/93 neurons (14%). In addition, direct effects of NE were noted on 6/93 neurons recorded from in cocultures, all close to the explant. In these cells, NE hyperpolarized the membrane in association with a small decrease in input resistance (11%). These responsive cells may have originated within the explant. A paradigm was used for testing the possibility of a responsive element in the cultures distinct from the impaled soma. 'Hot spots' were found using concentrations of isoproterenol as low as 10 nM.

Modulation of a transient outward current in serotonergic neurones by alpha 1-adrenoceptors

The excitability of various neurones in the mammalian central nervous system (CNS), ranging from motoneurones to serotonergic neurones, is enhanced by alpha 1-adrenoceptor agonists. Excitations mediated via alpha 1-adrenoceptors are associated with a slow depolarization and an increase in input resistance, probably resulting from a decrease in resting potassium conductance. However, the involvement of voltage-dependent transient currents in mediating alpha 1 excitatory effects has not been evaluated. An early transient outward current has been described which is important in regulating the frequency of repetitive firing; it is activated by depolarizing voltage steps from potentials more negative than rest and blocked by 4-aminopyridine. This current, which has been termed 'IA', was found originally in invertebrates and subsequently in various vertebrate neurones. The present single-electrode voltage-clamp study demonstrates an early transient outward current (IA) in serotonergic neurones which is suppressed by noradrenaline and the alpha 1-agonist phenylephrine; a suppression of IA may account in part for the acceleration of pacemaker activity induced by alpha 1-agonists in serotonergic neurones.

Regulation of cardiac vulnerability by the cerebral defense system

Psychosocial stressors are risk factors for sudden cardiac death. A theoretical model of the brain mechanism that links defined environmental events (stressors) to cardiac vulnerability (initiation of ventricular fibrillation) has been developed. In the model, a stressor event evokes a set of electrochemical responses in the frontal cortex. Depending on the state of acquisition of these electrochemical responses to the stressor, activity will or will not be initiated in the frontocortical-brainstem pathway. Activity in this pathway, either alone or in combination with myocardial ischemia, triggers a state of increased vulnerability of the heart to the initiation of ventricular fibrillation. Three independent interventions have been shown to prevent the initiation of ventricular fibrillation after acute coronary artery occlusion in the psychologically stressed pig: 1) learned behavioral adaptation to the stressor, 2) cryogenic blockade of the frontocortical-brainstem pathway, and 3) intracerebral (but not intravenous) injection of a beta-receptor blocking agent (levo-propranolol).

A threshold sodium current in pyramidal cells in rat hippocampus

Maintained, inward currents were activated by small depolarizations from the resting membrane potential (-50 to -60 mV) in voltage-clamped, pyramidal neurons in rat hippocampal slices. The currents were apparently Na currents as they were blocked by tetrodotoxin or removal of extracellular Na and were not affected by Cd. They showed little decrease in amplitude during prolonged depolarizations. The increase in Na conductance with depolarization was sigmoidal, with half-maximum conductance at about -50 mV, and saturated at -20 to -30 mV. This 'threshold' Na current may be involved in setting patterns of repetitive firing of action potentials.

Effects of caffeine on hippocampal pyramidal cells in vitro

The effects of caffeine on the electrophysiological properties of CA1 pyramidal neurones were investigated in the rat hippocampal slice preparation in vitro. A concentration-dependent increase in both the extracellularly recorded excitatory postsynaptic potential (e.p.s.p.) and the population spike resulting from stimulation of the stratum radiatum could be evoked by caffeine with a threshold concentration of 10 microM. Intracellular recordings demonstrate a caffeine-evoked decrease in resting membrane potential, an increase in input resistance, a reduction of the long afterhyperpolarization (a.h.p.) and a decrease in accommodation. The interaction between caffeine and adenosine was investigated on the extracellularly recorded e.p.s.p. The maximal response evoked by caffeine was increased in the presence of adenosine and the adenosine concentration-response curve was shifted to the right in a parallel fashion in the presence of caffeine. It is suggested that the effects of caffeine on hippocampal neurones may be mediated by a decrease of one or more potassium conductance(s), and that adenosine and caffeine may compete for the same electrophysiologically active receptor site on these cells.

Histamine may act through cyclic AMP on hippocampal neurones

Bath applied histamine and 8-bromo-cyclic AMP and intracellularly injected cyclic AMP block-long-lasting after-hyperpolarizations and the accommodation of firing in CA1 pyramidal cells recorded in rat hippocampal slices. This action is due to reduction of a calcium-activated potassium conductance (gK(Ca] and leads to potentiation of excitatory signals including epileptiform discharges. The effects are further potentiated and prolonged by a phosphodiesterase inhibitor (Ro 20-1724).

Protein kinase C regulates ionic conductance in hippocampal pyramidal neurons: electrophysiological effects of phorbol esters

The vertebrate central nervous system contains very high concentrations of protein kinase C, a calcium- and phospholipid-stimulated phosphorylating enzyme. Phorbol esters, compounds with inflammatory and tumor-promoting properties, bind to and activate this enzyme. To clarify the role of protein kinase C in neuronal function, we have localized phorbol ester receptors in the rat hippocampus by autoradiography and examined the electrophysiological effects of phorbol esters on hippocampal pyramidal neurons in vitro. Phorbol esters blocked a calcium-dependent potassium conductance. In addition, phorbol esters blocked the late hyperpolarization elicited by synaptic stimulation even though other synaptic potentials were not affected. The potencies of several phorbol esters in exerting these actions paralleled their affinities for protein kinase C, suggesting that protein kinase C regulates membrane ionic conductance.

Comparison of the action of baclofen with gamma-aminobutyric acid on rat hippocampal pyramidal cells in vitro

Intracellular recordings from CA1 pyramidal cells in the hippocampal slice preparation were used to compare the action of baclofen, a gamma-aminobutyric acid (GABA) analogue, with GABA. Ionophoretic application of GABA or baclofen into stratum (s.) pyramidale evoked hyperpolarizations associated with reductions in the input resistance of the cell. Baclofen responses were easier to elicit in the dendrites than in the cell body layer. Blockade of synaptic transmission, with tetrodotoxin or cadmium, did not reduce baclofen responses, indicating a direct post-synaptic action. (+)-Bicuculline (10 microM) and bicuculline methiodide (100 microM) had little effect on baclofen responses but strongly antagonized somatic GABA responses of equal amplitude. The bicuculline resistance of the baclofen response was not absolute, as higher concentrations of these compounds did reduce it. Pentobarbitone (100 microM) enhanced somatic GABA responses without affecting baclofen responses. (-)-Baclofen was approximately 200 times more potent than (+)-baclofen. The reversal potentials for the somatic GABA and baclofen responses were -70 mV and -85 mV respectively. When the membrane was depolarized, the baclofen response was reduced. This apparent voltage sensitivity was not seen with somatic GABA responses. Altering the chloride gradient across the cell membrane altered the reversal potential of the somatic GABA response but not that of the baclofen response. It was extrapolated that a tenfold shift in the extracellular potassium concentration would cause a 48 mV shift in the reversal potential of the baclofen response. Barium ions reduced the baclofen response, but not the GABA response. Orthodromic stimulation produced a fast inhibitory post-synaptic potential (i.p.s.p.) and a slow i.p.s.p. The properties of the fast and slow i.p.s.p.s were remarkably similar to those of the somatic GABA and baclofen responses, respectively. Application of GABA to the pyramidal cell dendrites evoked, in addition to a depolarization, two types of hyperpolarization. One type of hyperpolarization was bicuculline sensitive, had a reversal potential of about -65 mV and appeared to be chloride dependent. The other hyperpolarization was more easily observed in bicuculline methiodide (100 microM). This response was similar to that evoked by baclofen since it had a high reversal potential (about -90 mV), was relatively insensitive to changes in the chloride gradient across the cell membrane and was reduced by barium. The bicuculline-sensitive hyperpolarization could be evoked by the dendritic or somatic ionophoresis of muscimol and THIP (4,5,6,7-tetrahydroisoxazolo-[5,4-c]pyridin-3(2H)-one.(ABSTRACT TRUNCATED AT 400 WORDS)

Loss of accommodation in sympathetic neurons from spontaneously hypertensive rats

Synaptic transmission and membrane properties of sympathetic neurons in superior cervical ganglia of spontaneously hypertensive rats (SHR), normotensive Wistar-Kyoto rats (WKY), and Sprague-Dawley rats (SD) were investigated in vitro by extracellular and intracellular recording. The sympathetic neurons of SHR showed an atypical loss of spike accommodation. The spike discharge was insensitive to the sodium channel blocker tetrodotoxin, but it was reversibly blocked by a variety of calcium antagonists. The loss of accommodation in the neurons of SHR was not due to a loss of M-current, a potassium current involved in controlling spike frequency adaptation in sympathetic neurons. Superfusion of ganglia of SHR with muscarine (10 microM), which suppresses M-current and leads to a loss of accommodation, potentiated the repetitive discharge. In the presence of muscarine the current-voltage curves in neurons of SHR and SD were shifted to similar extents. Resting membrane potentials of neurons of SHR and WKY were consistently depolarized as compared with neurons of SD. Synaptic efficacy through the ganglia of SHR, assessed by extracellular recordings of presynaptic and postsynaptic compound action potentials at 0.25 Hz stimulation, was elevated when compared with the ganglia of WKY, but was similar to that of the ganglia of SD. These results indicate that strain differences should be considered when attempting to attribute changes in sympathetic neuron membrane properties to hypertension. The sympathetic neurons of SHR appear to have lost their accommodative properties and might possess an exaggerated calcium conductance. This calcium conductance may explain the augmented calcium-dependent release of norepinephrine during sympathetic nerve stimulation in the SHR.

Functional innervation of cultured hippocampal neurones by cholinergic afferents from co-cultured septal explants

The rat hippocampus receives a strong cholinergic innervation from the medial septum; information about the development and function of this pathway could help to elucidate the mechanisms of memory functions. Previous electrophysiological studies have shown that septal stimulation in vivo facilitates commissural and perforant path inputs and that stimulation of intrahippocampal cholinergic fibres in vitro produces a slow depolarization of rat hippocampal CA3 pyramidal neurones and increases their excitability. We describe here a different approach to the investigation of this system, by co-culturing slices of young rat hippocampus and septum, then recording the effects of septal nucleus stimulation on single voltage-clamped hippocampal CA3 pyramidal neurones. Under these conditions acetylcholinesterase-staining (presumed cholinergic) fibres grow out from the septum into the hippocampus. Single septal stimuli produce a short-latency non-cholinergic fast excitatory postsynaptic current, whereas trains of stimuli produce a slow inward current augmented by neostigmine and suppressed by atropine; hence this has a cholinergic origin. Our experiments provide both the first demonstration that functional synapses can be established between explanted cholinergic and cholinoceptive neuronal systems from the mammalian brain in organotypic culture and the first description of cholinergic slow excitatory postsynaptic currents in the mammalian central nervous system.

Significance of slow synaptic potentials for transmission of excitation in guinea-pig myenteric plexus

Intracellular recordings were made from neurones in myenteric ganglia of the guinea-pig ileum in vitro. Synaptic potentials were evoked by electrically stimulating presynaptic fibres as they entered the ganglion, using a small focal electrode. Slow synaptic depolarizations (excitatory postsynaptic potentials) were evoked in most myenteric neurones of both types. A single stimulus was more likely to evoke a slow excitatory postsynaptic potential in cells with nicotinic synaptic input (S cells; 50%) than in cells with long-lasting after-hyperpolarizations following the soma action potential (AH cells; 20%). Two pulses often evoked a slow excitatory postsynaptic potential in AH cells when one pulse was ineffective. The optimally effective time between the pulses was about 100 ms. Ten pulses resulted in slow excitatory postsynaptic potentials even when delivered at frequencies as low as 0.5 Hz. For the same frequency of presynaptic stimulation, the duration of the slow excitatory postsynaptic potential was greater in AH cells than in S cells and the amplitude of the slow excitatory postsynaptic potential was slightly greater in S than AH cells. Spontaneous depolarizations were observed which had time-courses and amplitudes similar to the evoked slow excitatory postsynaptic potential. They were not blocked by tetrodotoxin or atropine. The calcium-dependent after-hyperpolarization which follows one or more action potentials in AH cells was reduced or even abolished during the slow excitatory postsynaptic potential. Presynaptic nerve stimulation at intensities lower than those required to cause a slow excitatory postsynaptic potential caused a reduction in the calcium dependent after-hyperpolarization. It is concluded that the slow excitatory postsynaptic potential is generated by an intracellular intermediate process which is sensitive to the intracellular calcium concentration. The results suggest that the postsynaptic action of the synaptic transmitter is to interfere with the intracellular process which couples the entry of calcium to the increase in potassium conductance.