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

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.

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.

GABA-receptor stimulation enhances norepinephrine-induced polyphosphoinositide metabolism in rat hippocampal slices

The effect of gamma-aminobutyric acid (GABA)receptor stimulation on norepinephrine (NE)-induced metabolism of polyphosphoinositides (PIPs) was studied in rat hippocampal slices. Inositol phosphates (IPs), PIPs, and phosphatidic acid were measured. NE induced formation of IPs and phosphatidic acid in a dose-dependent manner with an EC50 of 4.5 microM. GABA, 3-aminopropanesulphonic acid (3APS) and muscimol did not affect PIPs breakdown, but they strongly increased (greater than 100%) PIPs metabolism induced by 1 microM NE. Their action was antagonized by bicuculline (10 microM). We discuss the implications of these findings in hippocampal neurotransmission.

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.

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.

Presynaptic action of neuropeptide Y in area CA1 of the rat hippocampal slice

1. Neuropeptide tyrosine (neuropeptide Y, NPY), a recently isolated endogenous brain peptide, reduces the extracellular population spike evoked by stimulation of stratum radiatum in area CA1 of the in vitro rat hippocampal slice, without reducing the antidromically evoked population spike. To test the hypothesis that NPY acts presynaptically, intracellular recordings were made of pyramidal neurones of area CA1 in vitro. 2. Bath application of 10(-6) M-NPY causes a long-lasting (1-1.5 h), reversible reduction of the orthodromically evoked excitatory post-synaptic potential (e.p.s.p.) recorded intracellularly from CA1 pyramidal neurones. This effect on the e.p.s.p. was dependent upon the concentration of NPY. 3. The resting membrane potential, slope input resistance, and action potential threshold, amplitude and duration of the CA1 pyramidal neurones were not affected by NPY. 4. The responses of CA1 pyramidal neurones to ionophoretic pulses of glutamate, applied to the dendrites during synaptic blockade, was also unaffected by NPY. 5. The evidence supports the hypothesis that NPY acts presynaptically in the CA1 region of hippocampus to reduce excitatory input to the pyramidal neurones.

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.

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.

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.

Phorbol esters block a voltage-sensitive chloride current in hippocampal pyramidal cells

The importance of second-messenger systems in controlling the excitability of neurones and other cells, through modulation of voltage- and calcium-dependent ionic conductances, has become increasingly clear. Cyclic AMP, acting via protein kinase A, has been identified as the second messenger for several neurotransmitters, and recent studies have suggested that activation of protein kinase C may have similar modulatory actions on neurones. Calcium and potassium currents have so far been shown to be the major ionic conductances modified by kinase activation. We now report that hippocampal pyramidal cells contain a previously undescribed voltage-dependent chloride current which is active at resting potential and is turned off either by membrane depolarization or by activation of protein kinase C by phorbol esters. We propose that this current may reside predominantly in the cell's dendritic membrane and thereby may regulate dendritic excitability.

The control of retinogeniculate transmission in the mammalian lateral geniculate nucleus

In the mammalian visual system, the lateral geniculate nucleus is commonly thought to act merely as a relay for the transmission of visual information from the retina to the visual cortex, a relay without significant elaboration in receptive field properties or signal strength. However, many morphological and electrophysiological observations are at odds with this view. Only 10-20% of the synapses found on geniculate relay neurons are retinal in origin. Roughly half of all synapses derive from cells in layer VI of visual cortex; roughly one third are inhibitory and GABAergic, derived either from interneurons or from cells of the nearby perigeniculate nucleus. Most of the remaining synapses probably derive from cholinergic, noradrenergic, and serotonergic sites within the brainstem reticular formation. Moreover, recent biophysical studies have revealed several ionic currents present in virtually all thalamic neurons. One is a Ca2+-dependent K+ current underlying the afterhyperpolarization (or the IAHP), which may last up to 100-200 ms following an action potential. Activation of the IAHP leads to spike frequency adaptation in response to a sustained, suprathreshold input. Intracellular recordings from other neuronal preparations have shown that the IAHP can be blocked by noradrenaline or acetylcholine, leading to an increased cellular excitability. Another ionic current results from a voltage- and time-dependent Ca2+ conductance that produces a low threshold spike. Activation of this conductance transforms a geniculate neuron from a state of faithful relay of information to one of bursting behavior that bears little relationship to the activity of its retinal afferents. We propose that state-dependent gating of geniculate relay cells, which may represent part of the neuronal substrate involved in certain forms of selective visual attention, can be effected through at least three different mechanisms: conventional GABAergic inhibition, which is largely controlled via brainstem and cortical afferents through interneurons and perigeniculate cells; the IAHP, which is controlled via noradrenergic and cholinergic afferents from the brainstem reticular formation; and the low threshold spike, which may be controlled by GABAergic inputs, cholinergic inputs, and/or the corticogeniculate input, although other possibilities also exist. Furthermore, it seems likely that gating functions involving the corticogeniculate pathway are suited to attentional processes within the visual domain (e.g., saccadic suppression), whereas brainstem inputs seem more likely to have more global effects that switch attention between sensory systems.(ABSTRACT TRUNCATED AT 400 WORDS)

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.