Voltage-sensitive K-currents in sympathetic neurons and their modulation by neurotransmitters

Control of Neuronal Excitability by Cell Surface Receptor Density and Phosphoinositide Metabolism

Phosphoinositides are members of a family of minor phospholipids that make up about 1% of all lipids in most cell types. Despite their low abundance they have been found to be essential regulators of neuronal activities such as action potential firing, release and re-uptake of neurotransmitters, and interaction of cytoskeletal proteins with the plasma membrane. Activation of several different neurotransmitter receptors can deplete phosphoinositide levels by more than 90% in seconds, thereby profoundly altering neuronal behavior; however, despite the physiological importance of this mechanism we still lack a profound quantitative understanding of the connection between phosphoinositide metabolism and neuronal activity. Here, we present a model that describes phosphoinositide metabolism and phosphoinositide-dependent action potential firing in sympathetic neurons. The model allows for a simulation of activation of muscarinic acetylcholine receptors and its effects on phosphoinositide levels and their regulation of action potential firing in these neurons. In this paper, we describe the characteristics of the model, its calibration to experimental data, and use the model to analyze how alterations of surface density of muscarinic acetylcholine receptors or altered activity levels of a key enzyme of phosphoinositide metabolism influence action potential firing of sympathetic neurons. In conclusion, the model provides a comprehensive framework describing the connection between muscarinic acetylcholine signaling, phosphoinositide metabolism, and action potential firing in sympathetic neurons which can be used to study the role of these signaling systems in health and disease.

Functional plasticity of cardiac efferent neurons contributes to traumatic brain injury-induced cardiac autonomic dysfunction

Traumatic brain injury (TBI) frequently causes cardiac autonomic dysfunction (CAD), irrespective of its severity, which is associated with an increased morbidity and mortality in patients. Despite the significance of probing the cellular mechanism underlying TBI-induced CAD, animal studies on this mechanism are lacking. In the current study, we tested whether TBI-induced CAD is associated with functional plasticity in cardiac efferent neurons. In this regard, TBI was induced by a controlled cortical impact in rats. Assessment of heart rate variability and baroreflex sensitivity indicated that CAD was developed in the sub-acute period after moderate and severe TBI. The cell excitability was increased in the stellate ganglion (SG) neurons and decreased in the intracardiac ganglion (ICG) neurons in TBI rats, compared with the sham-operated rats. The transient A-type K+ (KA) currents, but not the delayed rectifying K+ currents were significantly decreased in SG neurons in TBI rats, compared with sham-operated rats. Consistent with these electrophysiological data, the transcripts encoding the Kv4 α subunits were significantly downregulated in SG neurons in TBI rats, compared with sham-operated rats. TBI causes downregulation and upregulation of M-type K+ (KM) currents and the KCNQ2 mRNA transcripts, which may contribute to the hyperexcitability of the SG neurons and the hypoexcitability of the ICG neurons, respectively. In conclusion, the key cellular mechanism underlying the TBI-induced CAD may be the functional plasticity of the cardiac efferent neurons, which is caused by the regulation of the KA and/or KM currents.

Diabetes mellitus alters electrophysiological properties in neurons of superior cervical ganglion of rats

Diabetic neuropathy is the most prevalent complication associated with diabetes mellitus (DM). The superior cervical ganglion (SCG) is an important sympathetic component of the autonomic nervous system. We investigated the changes in cellular electrophysiological properties and on Na⁺K⁺-ATPase activity of SCG neurons of rats with DM induced by streptozotocin (STZ). Three types of action potentials (AP) firing pattern were observed in response to a long (1 s) depolarizing pulse. Whilst some neurons fired a single AP (single firing phasic, SFP), others fired few APs (multiple firing phasic, MFP). A third type fired APs during more than 80% of the stimulus duration (tonic-like, TL). The occurrence of SFP, MFP and TL was 84.5, 13.8, and 1.7%, respectively. SFP and MFP differed significantly in their membrane input resistance (R_in). At the end of the 4th week of its time course, DM differently affected most types of neurons: DM induced depolarization of resting membrane potential (RMP), decreased AP amplitude in SFP, and decreased R_in in MFP. DM decreased spike after-hyperpolarization amplitude in MFP and the duration in SFP. Based on the RMP depolarization, we investigated the Na⁺K⁺-ATPase action and observed that DM caused a significant decrease in Na⁺K⁺-ATPase activity of SCG. In conclusion, we have demonstrated that DM affects several parameters of SCG physiology in a manner likely to have pathophysiological relevance.

Nerve growth factor sensitizes adult sympathetic neurons to the proinflammatory peptide bradykinin

Levels of nerve growth factor (NGF) are elevated in inflamed tissues. In sensory neurons, increases in NGF augment neuronal sensitivity (sensitization) to noxious stimuli. Here, we hypothesized that NGF also sensitizes sympathetic neurons to proinflammatory stimuli. We cultured superior cervical ganglion (SCG) neurons from adult male Sprague Dawley rats with or without added NGF and compared their responsiveness to bradykinin, a proinflammatory peptide. The NGF-cultured neurons exhibited significant depolarization, bursts of action potentials, and Ca(2+) elevations after bradykinin application, whereas neurons cultured without NGF showed only slight changes in membrane potential and cytoplasmic Ca(2+) levels. The NGF effect, which requires trkA receptors, takes hours to develop and days to reverse. We addressed the ionic mechanisms underlying this sensitization. NGF did not alter bradykinin-induced M-current inhibition or phosphatidylinositol 4,5-bisphosphate hydrolysis. Maxi-K channel-mediated current evoked by depolarizations was reduced by 50% by culturing neurons in NGF. Application of iberiotoxin or paxilline, blockers of Maxi-K channels, mimicked NGF treatment and sensitized neurons to bradykinin application. A calcium channel blocker also mimicked NGF treatment. We found that NGF reduces Maxi-K channel opening by decreasing the activity of nifedipine-sensitive calcium channels. In conclusion, culture in NGF reduces the activity of L-type calcium channels, and secondarily, the calcium-sensitive activity of Maxi-K channels, rendering sympathetic neurons electrically hyper-responsive to bradykinin.

Activation of TREK currents by the neuroprotective agent riluzole in mouse sympathetic neurons

Background K2P channels play a key role in stabilizing the resting membrane potential, thereby modulating cell excitability in the central and peripheral somatic nervous system. Whole-cell experiments revealed a riluzole-activated current (I(RIL)), transported by potassium, in mouse superior cervical ganglion (mSCG) neurons. The activation of this current by riluzole, linoleic acid, membrane stretch, and internal acidification, its open rectification and insensitivity to most classic potassium channel blockers, indicated that I(RIL) flows through channels of the TREK [two-pore domain weak inwardly rectifying K channel (TWIK)-related K channel] subfamily. Whole-ganglia and single-cell reverse transcription-PCR demonstrated the presence of TREK-1, TREK-2, and TRAAK (TWIK-related arachidonic acid-activated K(+) channel) mRNA, and the expression of these three proteins was confirmed by immunocytochemistry in mSCG neurons. I(RIL) was enhanced by zinc, inhibited by barium and fluoxetine, but unaffected by quinine and ruthenium red, strongly suggesting that it was carried through TREK-1/2 channels. Consistently, a channel with properties identical with the heterologously expressed TREK-2 was recorded in most (75%) cell-attached patches. These results provide the first evidence for the expression of K2P channels in the mammalian autonomic nervous system, and they extend the impact of these channels to the entire nervous system.

Ionic basis of the resting membrane potential in cultured rat sympathetic neurons

The conductances which determine the resting membrane potential of rat superior cervical ganglia (SCG) neurons were investigated using perforated voltage- and current-clamp whole-cell techniques. The resting potential of SCG cells varied from -47 to -80 mV (-58.3 +/- 0.8 mV, n = 55). Blockade of M and h currents induced a depolarisation (7.4 +/- 0.7 mV, n = 22) and a hyperpolarisation (7.2 +/- 0.7 mV, n = 20) respectively; however, no correlation between the amplitude of these currents and the resting potential was found. The inhibition of the Na/K pump also induced membrane depolarisation (3.2 +/- 0.2 mV, n = 8). Inhibition of voltage-gated currents unmasked a voltage-independent resting conductance reversing at -50 mV. The reversal potential of the voltage-independent conductance, which included the electrogenic contribution of the Na/K pump, was strongly correlated with the resting potential (R = 0.87, p < 0.0001, n = 30). Ionic substitution experiments confirmed the existence of a voltage-independent conductance (leakage) with four components, a main potassium conductance, two minor sodium and chloride conductances and a small contribution of the Na/K pump. It is concluded that the resting potential of SCG cells strongly depends on the reversal potential of the voltage-independent conductance, with voltage-activated M and h currents playing a prominent stabilising role.

Development of electrophysiological and morphological diversity in autonomic neurons

The generation of neuronal diversity requires the coordinated development of differential patterns of ion channel expression along with characteristic differences in dendritic geometry, but the relations between these phenotypic features are not well known. We have used a combination of intracellular recordings, morphological analysis of dye-filled neurons, and stereological analysis of immunohistochemically labeled sections to investigate the development of characteristic electrical and morphological properties of functionally distinct populations of sympathetic neurons that project from the celiac ganglion to the splanchnic vasculature or the gastrointestinal tract of guinea pigs. At early fetal stages, neurons were significantly more depolarized at rest compared with neurons at later stages, and they generally fired only a single action potential. By mid fetal stages, rapidly and slowly adapting neurons could be distinguished with a topographic distribution matching that found in adult ganglia. Most rapidly adapting neurons (phasic neurons) at this age had a long afterhyperpolarization (LAH) characteristic of mature vasomotor neurons and were preferentially located in the lateral poles of the ganglion, where most neurons contained neuropeptide Y. Most early and mid fetal neurons showed a weak M current, which was later expressed only by rapidly-adapting and LAH neurons. Two different A currents were present in a subset of early fetal neurons and may indicate neurons destined to develop a slowly adapting phenotype (tonic neurons). The size of neuronal cell bodies increased at a similar rate throughout development regardless of their electrical or neurochemical phenotype or their topographical location. In contrast, the rate of dendritic growth of neurons in medial regions of the ganglion was significantly higher than that of neurons in lateral regions. The apparent cell capacitance was highly correlated with the surface area of the soma but not the dendritic tree of the developing neurons. These results demonstrate that the well-defined functional populations of neurons in the celiac ganglion develop their characteristic electrophysiological and morphological properties during early fetal stages of development. This is after the neuronal populations can be recognized by their neurochemical and topographical characteristics but long before the neurons have finished growing. Our data provide strong circumstantial evidence that the development of the full phenotype of different functional classes of autonomic final motor neurons is a multi-step process likely to involve a regulated sequence of trophic interactions.

M-type K+ currents in rat cultured thoracolumbar sympathetic neurones and their role in uracil nucleotide-evoked noradrenaline release

Cultured sympathetic neurones are depolarized and release noradrenaline in response to extracellular ATP, UDP and UTP. We examined the possibility that, in neurones cultured from rat thoracolumbar sympathetic ganglia, inhibition of the M-type potassium current might underlie the effects of UDP and UTP. Reverse transcriptase-polymerase chain reaction indicated that the cultured cells contained mRNA for P2Y(2)-, P2Y(4)- and P2Y(6)-receptors as well as for the KCNQ2- and KCNQ3-subunits which have been suggested to assemble into M-channels. In cultures of neurones taken from newborn as well as from 10 day-old rats, oxotremorine, the M-channel blocker Ba(2+) and UDP all released previously stored [(3)H]-noradrenaline. The neurones possessed M-currents, the kinetic properties of which were similar in neurones from newborn and 9 - 12 day-old rats. UDP, UTP and ATP had no effect on M-currents in neurones prepared from newborn rats. Oxotremorine and Ba(2+) substantially inhibited the current. ATP also had no effect on the M-current in neurones prepared from 9 - 12 day-old rats. Oxotremorine and Ba(2+) again caused marked inhibition. In contrast to cultures from newborn animals, UDP and UTP attenuated the M-current in neurones from 9 - 12 day-old rats; however, the maximal inhibition was less than 30%. The results indicate that inhibition of the M-current is not involved in uracil nucleotide-induced transmitter release from rat cultured sympathetic neurones during early development. M-current inhibition may contribute to release at later stages, but only to a minor extent. The mechanism leading to noradrenaline release by UDP and UTP remains unknown.

Cloning of a mammalian elk potassium channel gene and EAG mRNA distribution in rat sympathetic ganglia

1. Three new members of the EAG potassium channel gene family were identified in rat and the complete coding sequence of one of these genes (elk1) was determined by cDNA cloning. 2. The elk1 gene, when expressed in Xenopus oocytes, encodes a slowly activating and slowly deactivating potassium channel. 3. The elk1 gene is expressed in sympathetic ganglia and is also expressed in sciatic nerve. 4. Six of the seven known EAG genes were found to be expressed in rat sympathetic ganglia, suggesting an important functional role for these channels in the sympathetic nervous system.

Passive and active membrane properties of isolated rat intracardiac neurons: regulation by H- and M-currents

The electrical characteristics of isolated neonatal rat intracardiac neurons were examined at 22 and 37 degrees C using the perforated-patch whole cell recording technique. The mean resting membrane potential was -52.0 mV at 37 degrees C and exhibited no temperature dependence. Lowering the temperature from 37 to 22 degrees C decreased the mean input resistance from 854 to 345 Momega, respectively, and reduced the membrane time constant approximately threefold yielding a Q10 of 2.1. Hyperpolarizing current pulses induced time-dependent rectification of the voltage response in all neurons at both temperatures. This behavior was previously not observed in dialyzed neurons and was reversibly blocked by external Cs+ (2 mM) but not Ba2+ (1 mM). Voltage-clamp studies of isolated neurons revealed a hyperpolarization-activated inward current. This inwardly rectifying conductance was isolated from other membrane currents using external Cs+. The time and voltage dependence of this current is consistent with Ih and contributes to the passive electrical properties of rat intracardiac neurons. In >90% of the neurons studied, depolarizing currents evoked firing of multiple, adapting, action potentials at 22 degrees C. The number of action potentials increased with current strength producing a mean discharge of 5.1 (+100 pA, 1 s pulse), which was attenuated at 37 degrees C to a mean of 1.4. The amplitude and kinetics of the slow, muscarine-sensitive inward and outward currents (IM) were highly temperature dependent. Lowering the temperature from 37 to 22 degrees C reduced the steady-state current amplitude by approximately one-third and the rate of deactivation of IM by six- to ninefold at all voltages examined. The average Q10 for the time constant of deactivation of IM was 3.7 +/- 0.3 (mean +/- SE). Acetylcholine (ACh) induced tonic discharges in response to depolarizing currents (+100 pA, 1 s pulse) at both temperatures. This effect of ACh was inhibited by the muscarinic receptor antagonists, pirenzepine (100 nM), and mL-toxin (60 nM). At 37 degrees C, a mean discharge of 1.5 was increased to 23.5 in the presence of ACh. A similar switch from phasic to tonic discharge was also produced by the potassium channel inhibitors, Ba2+ (1 mM) and uridine-5'-triphosphate (UTP; 100 microM), whereas cadmium, 4-aminopyridine, apamin, charybdotoxin, and dendrotoxin did not alter discharge activity. The pharmacological sensitivity profile and temperature dependence of the active membrane properties are consistent with the muscarine-sensitive potassium current (IM) regulating the discharge activity in rat intracardiac neurons.

Cardiac neurones of autonomic ganglia

The properties of the postganglionic sympathetic neurones supplying the heart and arising in the stellate and adjacent paravertebral ganglia of various species are discussed with respect to their location, morphology, synaptic input and membrane characteristics. Results from our laboratory on the morphology of rat stellate neurones projecting to the heart were obtained either by intracellular injection of hexammine cobaltic (III) chloride or by retrograde labelling of cells using cobalt-lysine complex. Intracellular recordings were made from cells using electrodes filled either with potassium chloride plus hexammine cobaltic chloride or potassium acetate. Neurones which projected axons into cardiac nerve branches arising from the stellate ganglion were termed putative cardiac neurones, because of the possibility that some supply pulmonary targets. Putative cardiac neurones had unbranched axons and were ovoid or polygonal in shape, but showed considerable variation in soma size and in the complexity of dendritic trees. The mean two-dimensional surface area was 463 microns2 and the mean number of primary dendrites was seven. Other studies have found that the morphology of rat stellate ganglion neurones is similar to that of superior cervical ganglion cells. However, in strains of rat displaying spontaneous hypertension, dendritic length may be increased. Histochemical studies do not, as yet, seem to have demonstrated a distinctive neurochemical profile for stellate cardiac neurones, but various types of peptide-containing intraganglionic nerve fibres have been identified in the guinea pig. In our electrophysiological studies, putative cardiac neurones were found to receive a complex presynaptic input arising from the caudal sympathetic trunk and from T1 and T2 thoracic rami. In addition, 16% of cardiac neurones received a synaptic input from the cardiac nerve. The properties of postganglionic parasympathetic neurones distributed in the cardiac plexus and termed intrinsic cardiac neurones are discussed, including the results of studies on cultures of these neurones.

Reduction of spike frequency adaptation and blockade of M-current in rat CA1 pyramidal neurones by linopirdine (DuP 996), a neurotransmitter release enhancer

1. Linopirdine (DuP 996) has been shown to enhance depolarization-induced release of several neurotransmitters in the CNS through a mechanism which may involve K+ channel blockade. The electrophysiological effects of linopirdine were therefore investigated directly, by use of conventional voltage recording and single electrode voltage-clamp. 2. Linopirdine (10 microM) reduced spike frequency adaptation (SFA) in rat hippocampal CA1 pyramidal neurones in vitro. The reduction of SFA comprised an increase in number of spikes and a reduction in inter-spike intervals after the first, but with no effect on time to first spike. Linopirdine also caused a voltage-dependent depolarization of resting membrane potential (RMP). 3. M-current (IM), a current known to underlie SFA and to set RMP, was blocked by linopirdine in a reversible, concentration-dependent manner (IC50 = 8.5 microM). This block was not reversed by atropine (10 microM). 4. Linopirdine did not affect IQ, the slow after-hyperpolarization following a spike train, or spike duration. 5. Linopirdine may represent a novel class of K+ blocker with relative selectivity for the M-current. This block of IM is consistent with the suggestion from a previous study that linopirdine may affect a tetraethylammonium-sensitive channel, and it could be speculated that IM blockade may be involved with the enhancement of neurotransmitter release by linopirdine.

Properties of putative cardiac and non-cardiac neurones in the rat stellate ganglion

Intracellular recordings were made from isolated left or right stellate ganglia of Wistar rats and the morphology of neurones studied after intracellular injection of hexammine cobaltic chloride or back-filling from the post-ganglionic nerve with cobalt lysine complex. The experiments attempted to identify the location, electrophysiological properties, morphology and chemosensitivity of putative cardiac neurones in the ganglion. These were identified by antidromic activation of the axon in a cardiac nerve and compared with neurones projecting towards the brachial plexus (non-cardiac neurones). Putative cardiac neurones were localized in the ganglion around the postganglionic nerve entry zone and showed considerable morphological diversity. They had complex dendritic trees with, on average, seven dendrites. They included both phasic and tonic neurones and were depolarized by muscarinic agonists, angiotensin and substance P; they invariably had a synaptic input from the sympathetic trunk and from a T1 or T2 ramus and, in 16% of cells, from a cardiac nerve. Non-cardiac neurones were more widely scattered through the stellate ganglion but were not clearly different in morphology, resting membrane potential or the proportion of phasic and tonic cells from putative cardiac neurones. They also showed depolarizing responses to muscarinic agonists, angiotensin and substance P. Angiotensin responses of stellate ganglion cells were blocked by the peptide antagonist, saralasin (1 microM).

Dequalinium, a selective blocker of the slow afterhyperpolarization in rat sympathetic neurones in culture

The actions of dequalinium have been investigated in cultured rat sympathetic neurones. It produced a rapid and reversible inhibition of the slow apamin-sensitive component of the afterhyperpolarization (AHP) which follows a single action potential in these cells. The IC50 for this effect was 1.1 microM and in voltage clamp experiments, 1 microM dequalinium produced 45% inhibition of the underlying current IAHP. When the small conductance Ca(2+)-activated K+ channels were blocked by 20 nM apamin the slow component of the AHP was abolished, and dequalinium (10 microM) produced no further change in the residual AHP. Dequalinium (10 microM) had no effect on the voltage-activated Ca2+ current in these cells, suggesting that the inhibition of the AHP was the result of a direct interaction with the K+ channels. The A-current as well as a composite current made up of IK and IC were all unaffected by 10 microM dequalinium. However, at this concentration it did produce 18% inhibition of the M-current. These results show dequalinium to be a potent and selective non-peptide blocker of the apamin-sensitive small conductance Ca(2+)-activated K+ channel in rat sympathetic neurones.

Nicotinic and muscarinic synaptic transmission in canine intracardiac ganglion cells innervating the sinoatrial node

Nicotinic and muscarinic mediated synaptic mechanisms were investigated in isolated, canine intracardiac ganglia taken from the right atrial fat pad. Using conventional intracellular microelectrode recording techniques on 216 neurons, fast and slow synaptic potentials were evoked by single or trains of stimulation of presynaptic fibers in interganglionic nerves. By varying the stimulus intensity, single or multiple fast excitatory postsynaptic potentials (f-EPSPs) were evoked, indicating the convergence of synaptic inputs on these cells. These f-EPSPs often reached the action potential threshold, were enhanced by the acetylcholinesterase inhibitor physostigmine and were blocked by the nicotinic antagonist hexamethonium. The f-EPSPs were accompanied by a decreased input resistance and had an extrapolated reversal potential of -7.1 mV, suggesting increased conductances to more than one cation. Repetitive presynaptic stimulation evoked slow excitatory postsynaptic potentials (s-EPSPs) in 41% of the cells while slow inhibitory postsynaptic potentials (s-IPSPs) or s-IPSPs followed by s-EPSPs were evoked in 19% of the cells. All slow potentials were abolished by atropine and low Ca2+/high Mg2+ solutions and enhanced by physostigmine. Hexamethonium and adrenergic receptor antagonists had no effects on s-EPSP and s-IPSP. The M1 receptor antagonist pirenzepine reversibly blocked the s-EPSP but not the s-IPSP. On the other hand, the M2 receptor blocker 4-diphenyl-acetoxy-N-methyl piperidine methiodide (4-DAMP) had no effects on the s-EPSP. These observations suggest that s-EPSPs and s-EPSPs are mediated by distinct muscarinic receptors. The amplitude of the s-EPSP and the depolarization evoked by the muscarinic agonist, bethanechol were accompanied by increased input resistance. These responses were decreased in amplitude by membrane hyperpolarization and either reversed polarity or declined to zero amplitude at about -80 mV, suggesting the inhibition of a potassium conductance.

Adenosine modulation of potassium currents in postganglionic neurones of cultured avian ciliary ganglia

1. Potassium currents in cultured postganglionic neurones of avian ciliary ganglia were analysed under whole-cell voltage clamp and their modulation by adenosine determined. 2. In the presence of tetrodotoxin (200 nM), and with moderate holding potentials (Vh = -40 mV), the steady-state current-voltage (I/V) curve was N-shaped over the range from -70 mV to +155 mV. CsCl (1 M) blocked the current, indicating that it was carried by K+. If Ca2+ influx was blocked by CdCl2 (500 microM) then the outward current was reduced and the N-shaped I-V curve lost, indicating the presence of a calcium-activated potassium current (IK(Ca)); the remaining current, due to the delayed rectifier (IK), increased with depolarization up to about a conductance of 10 nS near + 50 mV. This IK was 50% activated at about +20 mV and 50% inactivated at about -50 mV. Adenosine (10 microM) had similar affects on the N-shaped I/V curve as did CdCl2, indicating that it blocked IK(Ca). However, adenosine had little affect on the steady-state current in the presence of CdCl, indicating that it did not much affect IK. 3. In the presence of tetrodotoxin (200 nM), a large inward current occurred for large hyperpolarizations from a Vh = -50 mV. This inward rectifying current (IIR) had a reversal potential near EK and showed 50% activation at about -130 mV. Adenosine (10 microM) reduced IIR, by as much as 50% at large hyperpolarizations beyond -80 mV. 4. Relaxations of the outward current on hyperpolarization from Vh = -30mV were blocked by carbachol (10 microM), had a reversal potential near EK, and an I/V curve typical of 1M currents. These currents were little affected by adenosine (10 microM). 5. A fast transient outward current, due to depolarizing pulses from a large Vh = -110mV was observed in the presence of tetrodotoxin (200 nM). This had the characteristics of an IA current as it could be blocked with 4-aminopyridine (5 mM) and was 50% activated at about -20 mV and 50% inactivated at about -94 mV. The IA current was reduced by 42% at a depolarization of -20 mV by adenosine (10 microM). 6. Many neurones possessed a fast transient outward current that was blocked by tetrodotoxin (200nM). This current could be blocked with 4-aminopyridine (5mM); it therefore has the characteristics of a sodium-activated potassium current ('K(Na)). This IK(Na) was unaffected by adenosine (1O microM). 7. These results are discussed in relation to the role of adenosine in blocking Ca2 + channels and thereby modifying calcium-dependent components of K+ currents.

Ion channels involved in the presynaptic hyperexcitability induced by herpes virus suis in rat superior cervical ganglion

Rat superior cervical ganglia infected with herpes virus suis (pseudorabies virus) display a spontaneous bursting activity of still unknown origin. Previous intracellular recordings from the ganglionic neurons combined with pharmacological studies showed that the postganglionic action potentials are induced by acetylcholine release spontaneously from the preganglionic nerve. In this study we investigated whether the acetylcholine release is caused by mechanisms which are dependent on action potentials spontaneously generated on the preganglionic nerve or by mechanisms which occur without any changes in the excitability of presynaptic fibers. Simultaneous intra- and extracellular recordings from the ganglion cells and from the preganglionic nerve, respectively, were performed 32-38 h after the inoculation of herpes virus suis (strain Aujeszky) into the anterior chamber of one eye of the rat. Tetrodotoxin, well known to prevent the generation of action potentials by blocking the fast sodium channels, completely and reversibly abolished, whereas the potassium channel blockers 4-aminopyridine and apamin, enhanced the spontaneous, bursting activity at pre- and postsynaptic levels. The nicotinic receptor antagonist hexamethonium abolished the postsynaptic discharges and reduced the preganglionic activity by 50%. Pre- and postsynaptic electrical activities were suppressed in low calcium Krebs' solution, demonstrating that extracellular calcium is required not only for acetylcholine release but also for triggering the presynaptic action potentials. It is concluded that in the infected ganglia the spontaneous acetylcholine release is due to the generation of action potentials in the preganglionic nerve. Voltage-gated sodium and calcium channels contribute to the presynaptic electrogenesis, while the latter appears to be damped by the activation of voltage- and calcium-dependent potassium channels. Possible factors as well as mechanisms inducing such an increase in excitability are discussed.

Electrophysiology of parasympathetic neurones isolated from the interatrial septum of bull-frog heart

1. Whole-cell voltage-clamp techniques were used to study the voltage-dependent membrane conductances in parasympathetic neurones enzymatically isolated from the interatrial septum of bull-frog heart and maintained in short-term (1-10 day) tissue culture. 2. The resting potential of the isolated neurones averaged -55.4 +/- 1.1 mV (+/- S.E.M., n = 11). Action potentials evoked in the isolated cells by brief (1-2 ms) current injections were similar to those recorded from neurones in the 'intact' septum. The amplitude of action potentials of isolated neurones averaged about 113 mV, with a peak depolarization of +32.8 +/- 2.8 mV and after-hyperpolarization of -80.0 +/- 2.8 mV. 3. The pattern of membrane currents recorded using voltage clamp with 'normal' external (containing 110 mM-Na+) and internal (110 mM-K+) solutions consisted of a rapidly activating and inactivating inward current followed by a slower, sustained outward current. 4. The inward components of current were isolated by using an internal solution in which Cs+ and TEA+ (tetrathylammonium) ions replaced K+. Depolarizations from holding potentials of -50 to -70 mV produced inward currents which had an initial transient phase followed by a maintained, or very slowly inactivating, component. The current-voltage relation for the initial transient phase reached a peak at membrane potentials near 0 mV, while the maintained phase, measured, for example, at the end of 50 ms voltage-clamp steps, had its peak near +10 mV. 5. The transient component of inward current was carried primarily by Na+ ions, as replacement of Na+ by TEA+ in the external solution abolished the transient. This current was thus identified as a voltage-dependent Na+ current, INa. The maintained component was greatly attenuated by removing 80-90% of the external Ca2+ ions, and it was abolished by divalent cations such as Cd2+ (0.2-0.4 mM), Ni2+ (0.5 mM) and La3+ (10-100 microM). This maintained component was thus a voltage-dependent calcium current, ICa. 6. About 80% of INa recorded in the presence of low (0.2-0.5 mM) external Ca2+ and 2 microM-LaCl3 was blocked by tetrodotoxin (TTX) with an apparent Kd of about 8 nM. The remaining 20% of INa was resistant to block by 2-10 microM-TTX. However, the 'TTX-resistant' component of INa was blocked by Cd2+ (0.2-0.4 mM). 7. The voltage-dependent calcium current, ICa, measured in saline in which Na+ was replaced by N-methyl-D-glucamine, activated near -40 mV and reached a peak near +10 to +15 mV.(ABSTRACT TRUNCATED AT 400 WORDS)

Effects of repetitive activation and changes in external ionic environment on hippocampal CA1 pyramidal cell afterhyperpolarizations

Afterhyperpolarizations (AHPs) in hippocampal CA1 pyramidal neurons known to be generated predominantly by a Ca-dependent K conductance were examined to see if they could be inverted by changes in extracellular potassium of the magnitude observed during interictal or ictal discharges and for their liability during repetitive activation under normal ionic conditions or in the presence of elevated extracellular potassium and decreased extracellular calcium. Under all circumstances tested, the AHP remained hyperpolarizing and was associated with a conductance increase. Thus, the very liable hyperpolarizing event that follows a depolarizing shift in hippocampal pyramidal neurons in various epileptic foci (which disappears early during the transition between interictal and ictal activity) probably is not due to the same mechanism as that which underlies the AHP.

Properties of K-currents in unmyelinated presynaptic axons of brain revealed revealed by extracellular polarisation

Thin pial surface slices of guinea-pig olfactory cortex contain unmyelinated axons derived from the lateral olfactory tract (LOT). The severed ends of the groups of these axons were drawn into a suction electrode to record the currents resulting from propagated action potentials. On stimulating these unmyelinated axons, a tetrodotoxin-sensitive positive current of 22.5 +/- 3.0 nA was recorded by the suction electrode. The positive current was often followed by a small negative current. A 100 mV positive polarisation applied to the suction electrode revealed a large negative after-current (25.8 +/- 3.5 nA). In contrast, the positive current was slightly reduced by the polarisation. The early phases of the negative after-current induced by the electrode polarisation were blocked by 3,4-diaminopyridine, 4-aminopyridine or 2,3-diaminopyridine (in order of potency). The entire negative after-current was blocked by prolonged (3 h) equilibration in a medium containing 3.5 mmol/l Cs and 1.5 mmol/l K. Tetraethylammonium (TEA) or Ba2+ by themselves had little effect. In aminopyridine, the residual negative after-current was blocked by TEA (10 mmol/l) or 1 mmol/l Ba2+. Muscarinic agonists had no effect on these currents. These experiments show that some axonal currents can be revealed by extracellular polarisation and that these axons rely on an 'A' type of current for the rapid repolarisation of the membrane although slower K-channels are present.

Characteristics of synaptic input to three classes of sympathetic neurone in the coeliac ganglion of the guinea-pig

1. Intracellular recordings from sympathetic neurones in the isolated coeliac ganglion of guinea-pigs have been used to define the synaptic input to three subtypes of neurone, classified on the basis of their discharge during maintained depolarizing current as phasic neurones, neurones with prolonged after-hyperpolarizations (LAH), and tonic neurones. 2. The three classes of neurone were distributed characteristically in different parts of the ganglion. 3. Passive membrane properties differed between the three neurone types. Mean input resistance was highest in phasic neurones and was inversely related to the size of the prolonged calcium-activated potassium conductance in LAH neurones. Mean input time constant was highest in tonic neurones, because of significantly higher cell capacitance. 4. Phasic and LAH neurones usually received one suprathreshold ('strong') as well as several subthreshold excitatory synaptic potentials (ESPs) from the ipsilateral splanchnic nerve. In general, the amplitude and number of splanchnic inputs were greater, and the occurrence of two strong inputs more common, in phasic than in LAH neurones. The input to tonic neurones was small and usually subthreshold, even with supramaximal splanchnic stimulation. In a few (mostly tonic) neurones lying close to the midline, small ESPs were evoked by contralateral splanchnic stimulation. 5. Antidromic action potentials were evoked in more than half of all neurones by high voltage coeliac nerve stimulation. In addition, multiple small subthreshold ESPs were recorded in virtually all tonic neurones (99%) on coeliac nerve stimulation. In contrast, coeliac stimulation rarely evoked a few very small ESPs in LAH neurones (9%), but no synaptic response in phasic neurones. 6. In about half of the tonic neurones tested (but no phasic or LAH neurones), small ESPs were evoked by stimulation of the intermesenteric nerve. 7. Slow depolarization elicited by repetitive activation of splanchnic and coeliac nerve trunks, at voltages supramaximal for the fast cholinergic responses, were recorded from about half of both phasic and tonic neurones, but only one of twenty-four LAH neurones. These responses commonly faded during subsequent trials, so that it was difficult to characterize them. 8. The data indicate that the three broad groups of coeliac neurone, classified on the basis of their voltage- and calcium-dependent potassium conductances, receive different patterns of synaptic input. The differences may be related to the three major functions of vasoconstriction, motility and mucosal secretion in the small intestine.

Afterspike-hyperpolarization of neurons in the inferior mesenteric ganglion in guinea-pig

Intracellular recordings were made from neurons (n = 121) in the inferior mesenteric ganglion (IMG) in guinea-pig. The afterspike hyperpolarization (ASH) following a single action potential was studied in IMG cells which received an excitatory, cholinergic innervation from mechanosensory nerves in the gastrointestinal tract. The amplitude of ASH was dependent on the membrane potential of IMG cells and the concentration of K+ in the bathing solution. The reversal potential of ASH (-80- -90 mV, in normal Krebs solution) appeared to follow the equilibrium potential for K+, as [K+]o was changed, suggesting that ASH was the product of K+-efflux. Further evidence suggested that a major component of the K+-efflux was dependent on the concentration of Ca2+ in the bathing medium. Elevation and reduction of [Ca2+]o increased and decreased, respectively, the amplitude and duration of ASH. In the presence of tetrodotoxin, depolarizing current pulses elicited spike-like events which (1) were dependent on [Ca2+]o and the degree of depolarization by current-clamp and (2) were followed by afterhyperpolarizations that were also dependent on [Ca2+]o and degree of depolarization by current-clamp. In the combined presence of tetrodotoxin and tetraethylammonium, depolarizing current pulses elicited prolonged action potentials (up to 100 ms in duration) followed by prolonged ASH (up to 3 s in duration). Spike-like events, prolonged action potentials and their afterhyperpolarizations were reduced in amplitude and duration when the calcium-channel blocking ion, Co2+, or blocking drug, verapamil, was present in the bathing medium. In normal Krebs solution, the ASH of action potentials produced by nerve stimulation was reduced but not abolished in the presence of Co2+. These results suggested that Ca2+ entered IMG cells during depolarization and activated the K+-conductance mechanisms responsible for the ASH. However, an initial component of the ASH may have involved other voltage-dependent K+-currents known to be activated during the excitation of sympathetic neurons. The amplitude and duration of ASH differed during non-synaptic and synaptic excitation of IMG cells, and differed when action potentials resulted from fast and slow EPSPs. In addition, the amplitude and duration of ASH were altered by noradrenaline, by the cholinomimetic, carbachol, and by 3 neuropeptides present in the IMG, namely leucine-enkephalin, substance P and vasoactive intestinal polypeptide.(ABSTRACT TRUNCATED AT 400 WORDS)

Expression of membrane currents in rat neocortical neurons in serum-free culture. II. Outward currents

The timing of expression and properties of outward membrane currents in cultured neocortical pyramidal-shaped neurons have been investigated using the gigaseal whole-cell voltage clamp and single-channel recording techniques. Dissociated primary cultures of synchronized (same cell cycle), growth arrested (G1 phase) and birth-dated cells from fetal rat (E18) were maintained in a serum-free medium. The earliest appearing membrane current in the soma is a voltage-dependent outward current carried by K+. The current consists of two components, one rapidly rising component, resembling those associated with the transient outward current (IA) and the other similar to the delayed rectifier current (IK). The ratio between the peak IA and IK was about 0.3 at all membrane voltages. The magnitude of both IA and IK increased with time in culture but the ratio remained unchanged. Direct measurements of unitary currents showed the presence of two voltage-activated outward conductances, 32 pS and 120 pS. The small conductance channel was sparse. The large conductance channel is K+-selective and was sensitive to both voltage and internal Ca2+.

Transient calcium-dependent potassium current in magnocellular neurosecretory cells of the rat supraoptic nucleus

1. Magnocellular neurosecretory neurones were impaled in the supraoptic nucleus of perfused explants of rat hypothalamus. Membrane currents were studied at 35 degrees C using the single-microelectrode voltage-clamp technique. 2. Depolarizing voltage steps applied from -100 mV evoked a transient outward current (TOC) from a threshold of -75 mV. From this potential, the amplitude of the current increased non-linearly with voltage. 3. Following its activation TOC reached a peak within 7 ms and subsequently decayed monotonically with a time constant of 30 ms. This time constant did not vary significantly with voltage between -75 and -55 mV. 4. The TOC showed complete steady-state inactivation at potentials positive to -55 mV. Inactivation was removed by hyperpolarization, with a mid-point near -80 mV. The removal of inactivation followed a complex time course with distinct fast (tens of milliseconds) and slow (hundreds of milliseconds) components. 5. Tail current measurements revealed that the TOC equilibrium potential (ETOC) lies near -97 mV in the presence of 3 mM [K+]o. Increasing [K+]o caused a decrease of TOC amplitude and a shift in ETOC of 57 mV/log [K+]o. The TOC is therefore predominantly a K+ current. 6. The TOC was unaffected by tetraethylammonium (up to 12 mM) but was reversibly reduced by 4-aminopyridine (ca. 50% block at 1.0 mM) and dendrotoxin (ca. 50% block at 4 nM). 7. The TOC was strongly inhibited (greater than 70%) by adding Co2+ or Mn2+ (1-3 mM) or Cd2+ (50-400 microM) to Ca-containing solutions, or by removal of Ca2+ from the perfusate. These effects were not accompanied by detectable changes in threshold voltage. The amplitude of TOC was also depressed by the organic Ca2+ channel blocker methoxyverapamil (D600). Finally replacement of Ca2+ by Ba2+ in the perfusate completely and reversibly abolished the TOC. 8. These findings suggest that neurosecretory neurones of the rat supraoptic nucleus display a transient K+ current which is strongly dependent on the presence of external Ca2+. The possible role of this current is briefly discussed.

Long-duration spike afterhyperpolarizations in neurons from the guinea pig superior cervical ganglion

Intracellular recordings from guinea pig superior cervical ganglia maintained in vitro have revealed a protracted spike afterhyperpolarization in approximately 18% of the principal neurons that is substantially longer (greater than 1 s duration) than those previously reported. This long afterhyperpolarization is distinct from shorter duration afterpotentials in these cells because it can be selectively and reversibly blocked by cooling and by bath applied histamine. Cellular excitability is increased when the long-duration afterhyperpolarization is abolished and thus it deserves consideration as a site for modulation of synaptic transmission through the superior cervical ganglion.

Electrophysiological characterization of functionally distinct 5-hydroxytryptamine receptors on guinea-pig submucous plexus

Intracellular recordings were made from neurons of the guinea-pig submucous plexus and the actions of 5-hydroxytryptamine on the postsynaptic membrane and on evoked synaptic potentials were examined. 5-Hydroxytryptamine produced two types of direct postsynaptic responses: (1) A depolarization associated with a fall in input resistance was observed in all cells. Voltage-clamp and ion substitutions showed that this depolarization resulted primarily from an inward sodium current. This response could be as brief as 30 ms; it showed desensitization and was selectively abolished by 0.2-2 microM ICS 205-930. (2) A depolarization (or inward current) associated with a decreased conductance was observed in about 50% of neurons, usually after the first response was blocked by ICS 205-930. This response was due to a decreased potassium conductance; the minimum time course of this response was 8-10 s. It did not show desensitization and was not sensitive to blockade by currently available antagonists of 5-hydroxytryptamine, nicotinic and/or muscarinic receptors. Higher concentrations of 5-hydroxytryptamine were required to produce the sodium conductance increase than the potassium conductance decrease; 2-methyl-5-hydroxytryptamine was equally effective in producing these responses. 5-Hydroxytryptamine also caused a barrage of "spontaneous" nicotinic excitatory post-synaptic potentials which were sensitive to tetrodotoxin. This response desensitized, was blocked by ICS 205-930 and is presumed to reflect excitation of other cholinergic cell bodies in the plexus by the sodium conductance increase mechanism described. The evoked nicotinic excitatory postsynaptic potential and the adrenergic inhibitory postsynaptic potential were decreased by 5-hydroxytryptamine; a portion of this inhibition showed desensitization and was blocked by ICS 205-930 as well as by the muscarinic receptor antagonists, atropine and pirenzepine. The ICS 205-930-insensitive portion of this inhibition could not be attributed to activation of 5-hydroxytryptamine-1 or 5-hydroxytryptamine-2 receptors. Thus, the following conclusions are drawn: 5-hydroxytryptamine excites submucous plexus neurons by activating two distinct 5-hydroxytryptamine receptors. Activation of the 5-hydroxytryptamine-3 receptor (sensitive to ICS 205-930) produces a depolarization mediated by an increased sodium conductance. The same effect occurring in other cholinergic cell bodies initiates action potentials which are responsible for the 5-hydroxytryptamine-induced release of acetylcholine.(ABSTRACT TRUNCATED AT 400 WORDS)

The Ca2+-sensitive K+-currents underlying the slow afterhyperpolarization of bullfrog sympathetic neurones

Ca2+-sensitive K+ currents involved in the slow afterhyperpolarization (a.h.p.) of an action potential of bullfrog sympathetic neurones were studied with a single-electrode voltage clamp method. The outward tail current (IAH) generated after the end of a depolarizing command pulse (from the holding potential of -60 mV to 0 mV, 5-20 ms in duration), mimicking an action potential, was separated into at least two exponential components (IAHf and IAHs). They were identified as K+ currents, since their reversal potentials were close to the K+ equilibrium potential and they were sensitive to external K+. The time constant of IAHf (tf; 44 ms at -60 mV) was decreased by membrane hyperpolarization from -40 to -80 mV, while that of IAHs (ts; 213 ms) remained constant. Removal of external Ca2+ or addition of Cd2+ significantly decreased the IAHs amplitude (As) and tf without a change in ts and the IAHf amplitude (Af). On the other hand, increasing Ca2+ influx by applying repetitive command pulses enhanced both Af and As with negligible effects on tf and ts, and produced a much slower component. Intracellular injection of EGTA reduced Af with no effect on tf, and increased As with a decreased ts. Both muscarine and (+/-)-tubocurarine, which reduced IAHs, hardly affected IAHf. These results indicate that a.h.p. is induced by the activation of two distinct Ca2+-dependent K+ channels, which differ in voltage sensitivity, Ca2+-dependence and pharmacology.

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.

Acetylcholine as a neurotransmitter in the vertebrate retina

The evidence for the existence of acetylcholine as a neurotransmitter in the vertebrate retina is reviewed. There is evidence for the existence of a cholinergic system in every retina studied to date; therefore, it appears that acetylcholine is both essential and ubiquitous at this level of the visual system. Particular attention is directed to descriptions of the possible functions of acetylcholine in the retina, and formation of testable models which will serve to elucidate some of the details of cholinergic neurotransmission in the retina.

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.

Cholecystokinin octapeptide depolarizes guinea pig inferior mesenteric ganglion cells and facilitates nicotinic transmission

Cholecystokinin octapeptide (CCK-8) applied either by superfusion (0.1-10 microM) or by pressure ejection elicited a slow depolarization in a portion of inferior mesenteric ganglion cells studied in vitro. The depolarization which persisted in a low Ca2+/high Mg2+ solution, or solution containing cholinergic antagonists, was often associated with a small to moderate increase in neuronal input resistance, and the response was reduced by conditioning hyperpolarization. Nicotinic excitatory postsynaptic potentials were consistently augmented during the course of CCK-8-induced depolarization. Our results, together with findings of the presence of CCK-immunoreactive fibers in the prevertebral ganglia, suggest that the peptide may serve to facilitate nicotinic transmission.

Responses of morphologically identified cortical neurons to intracellularly injected cyclic GMP

Cyclic nucleotides are thought to act as second messengers of neurotransmission inside central neurons, and cyclic guanosine monophosphate (cGMP) has been postulated to act as a messenger for muscarinic, cholinergic transmission. Nonetheless, the action of cGMP has not yet been established in identified cortical neurons. We injected cGMP and horseradish peroxidase (HRP) intracellularly in neurons of the motor cortex of awake cats. Fifty-four percent of injected cells responded to cGMP and HRP with an increase in input resistance within 30 s after injection. None of a control group of cells injected with HRP without cGMP so responded. In cells receiving intracellular depolarizing current sufficient to produce repeated spike discharge at the time of injection, the increase in input resistance after cGMP persisted for as long as the cells could be held. There was no significant increase in firing rate after injection of cGMP. Cells responding to cGMP with an increased input resistance were identified as pyramidal cells of layer V. One inverted pyramidal cell of layer VI also showed an increase in input resistance in response to cGMP. Previous studies have suggested that muscarinic cholinergic agents produce an increased input resistance (thought to reflect a decreased potassium conductance) underlying an increased rate of discharge in neocortical neurons. Our results favor a dual action of muscarinic cholinergic transmission in mammalian cortical neurons--the increase in input resistance in layer V pyramidal cells mediated by cGMP, and the increase in rate of discharge mediated by other means.

Characteristics of phasic and tonic sympathetic ganglion cells of the guinea-pig

Intracellular recording techniques have been used to determine the electrophysiological properties of sympathetic neurones in ganglia of the caudal lumbar sympathetic chain (l.s.c.) and in the distal lobes of inferior mesenteric ganglia (i.m.g.) isolated from guinea-pigs. Passage of suprathreshold depolarizing current initiated transient bursts of action potentials in 97% of l.s.c. neurones, but only 13% of i.m.g. cells ('phasic' neurones). Most i.m.g. neurones fired continuously during prolonged depolarizing pulses ('tonic' neurones). Passive membrane properties varied; mean cell input resistance was similar between groups, but phasic neurones had smaller major input time constants on average than had tonic cells. Current-voltage relations determined under both current clamp and voltage clamp were linear around resting membrane potential (approximately 60 mV), where membrane conductance was lowest. Instantaneous and time-dependent rectification varied in the different neurone types. The current underlying the after-hyperpolarization following the action potential was significantly larger on average in tonic i.m.g. cells than in phasic neurones, although its time course (tau = 100 ms) was similar. Phasic neurones fired tonically when depolarized after adding the muscarinic agonist, bethanechol (10(-5) M to 10(-4) M), to the bathing solution. Bethanechol blocked a proportion of the maintained outward current (presumably the M-current, IM, Adams, Brown & Constanti, 1982) in phasic neurones; this current was small or absent in tonic neurones. Transient outward currents resembling the A-current (IA, Connor & Stevens, 1971 a) were evoked in tonic but not in phasic neurones by depolarization from resting membrane potential. IA could only be demonstrated in phasic neurones after a period of conditioning hyperpolarization. After a step depolarization to approximately --50 mV, IA reached peak amplitude at about 7 ms and then decayed with a time constant of about 25 ms in both neurone types. Activation characteristics of IA were similar for phasic and tonic neurones, but inactivation curves, although having the same shape, were shifted to more depolarized voltages in tonic neurones. That is, IA was largely inactivated at resting membrane potential in phasic, but not tonic neurones. It is concluded that the discharge patterns of the two populations of sympathetic neurones result from differences in the voltage-dependent potassium channels present in their membranes. The anatomical occurrence of the different cell types suggests that phasic neurones are vasoconstrictor and tonic neurones are involved with visceral motility.

Excitation of lateral horn neurons of the neonatal rat spinal cord by 5-hydroxytryptamine

The effects of 5-hydroxytryptamine (5-HT) on lateral horn cells contained in thin in vitro slices of neonatal rat spinal cord were investigated by means of intracellular recording techniques. Superfusion of 5-HT (1-100 microM) to lateral horn cells caused a concentration-dependent membrane depolarization leading to, in some instances, repetitive cell discharges. A number of lateral horn cells could be activated antidromically by stimulating the ventral rootlets. The conduction velocity of the antidromic spikes was estimated to be 0.3-2 m/s which corresponds to that of the axons of rat sympathetic preganglionic neurons reported by others. The 5-HT depolarization evoked in neurons that could be activated antidromically was similar to that elicited from unidentified lateral horn cells. The depolarization induced by 5-HT could be partially eliminated by low Ca/high Mg solution or tetrodotoxin in a portion of lateral horn cells and was accompanied by an increase in membrane resistance. The response was nullified near the membrane potential at which the spike after hyperpolarization was abolished; a clear reversal of polarity was not observed at a more negative potential level. The 5-HT depolarization was reversibly blocked by methysergide and cyproheptadine and enhanced by fluoxetine, a 5-HT-uptake inhibitor. The results indicate that the indoleamine primarily exerted an excitatory action on lateral horn cells, including those tentatively identified as sympathetic preganglionic neurons, by a direct depolarization which appears to be mediated by decrease of a voltage-sensitive K conductance and partly by an indirect effect via the release of an excitatory substance(s).

Blockade of Ca-activated K conductance by apamin in rat sympathetic neurones

Effects of apamin on rat sympathetic neurones were investigated by means of intracellular and extracellular recording. Apamin (50 nM) significantly shortened the after-hyperpolarization (AH) following the spike evoked by current injection and slightly decreased its peak amplitude without affecting the time course of the spike. The AH following the synaptically-evoked spike was also blocked by apamin. This effect was dose- and time-dependent (ID50 estimated by extracellular recording approximately 15 nM, 20 min after application) and poorly reversible. Transmission of a single volley was not affected by 50 nM apamin. Though a long depolarizing current caused one or two spikes in the cell, greater repetitive firing was observed in the presence of apamin. Spontaneous repetitive firing, however, was not observed except for anodal-break spikes. Resting potential and input membrane resistance were essentially unchanged by apamin. The maximum rate of rise of the Ca spike was not decreased by 50 nM apamin but the duration of the spike was lengthened by 60%. The AH following the Ca spike was also blocked by apamin. These results suggest that apamin suppressed the slow AH without any inhibition of the Ca flux into the cell and is useful as a blocker of GK(Ca) in the rat sympathetic neurone.

Spontaneous muscarinic suppression of the Ca-activated K-current in bullfrog sympathetic neurons

Neurons in bullfrog sympathetic ganglia were voltage-clamped using a single microelectrode. A prolonged outward current which was identified as the Ca-activated K-current secondary to a transient Ca entry through voltage-operated channels was shortened by oxotremorine. An inward Ca-current was not significantly depressed by oxotremorine. It was suggested that muscarinic agonists accelerate the re-closure of K-channels either directly or secondarily via their effects on an intracellular sequestration process of Ca ions. It was also suggested that a small amount of acetylcholine only sufficient to cause a miniature synaptic current via nicotinic receptors could shorten the Ca-activated K-current via muscarinic receptors.

An intracellular study of synaptic transmission and dendritic morphology in sympathetic neurons of the chick embryo

The characteristics of synaptic transmission in whole embryonic avian sympathetic ganglia have been examined by intracellular recording. Neurons in lumbar paravertebral ganglia of chick embryos exhibit both fast nicotinic excitatory postsynaptic potentials (EPSPs) and non-cholinergic slow EPSPs. Fast nicotinic transmission is mediated by at least 3-5 convergent preganglionic inputs and can be detected at the earliest embryonic stage examined (Stage 38; 12 days of incubation). Two types of non-cholinergic slow EPSPs have been observed and distinguished by their time course and the resulting changes in input resistance. One of these slow synaptic potentials is mimicked by exogenously applied substance P, but not by exogenous luteinizing hormone-releasing hormone (LH-RH). Muscarinic agonists also evoke slow depolarizations in the ganglia, mediated at least in part by inhibition of the M-current. Intracellular labeling with horseradish peroxidase reveals cells with 5-10 primary dendrites at Stage 42 (16 days of incubation), the earliest stage to exhibit slow EPSPs. The active and passive membrane properties of avian sympathetic neurons, including the presence of the M-current, generally resemble those of adult mammalian and amphibian sympathetic neurons. Functional activity in chick sympathetic neurons is present at a developmental stage where both biochemical and morphological indices of synapse maturation are at low levels. Since this progression has also been observed in the avian ciliary ganglion, it may be of general relevance to neuronal development.

Membrane currents underlying the cholinergic slow excitatory post-synaptic potential in the rat sympathetic ganglion

Non-nicotinic slow synaptic currents were recorded from voltage-clamped neurones in isolated rat superior cervical ganglia bathed in a solution containing d-tubocurarine and (usually) 1 microM-neostigmine. Three components of slow synaptic current could be detected following repetitive preganglionic stimulation: a net inward current resulting from inhibition of the voltage-dependent outward K+ current IM; a net outward current associated with a fall in membrane conductance when IM was deactivated by membrane hyperpolarization or inhibited with external Ba2+ or internal Cs+; and an occasional late inward current associated with an increased membrane conductance. As a result, synaptic current amplitudes showed complex changes with changes in membrane potential. Both the inward current associated with IM inhibition and the outward current were enhanced by neostigmine and blocked by atropine or pirenzepine, and therefore resulted from activation of muscarinic receptors. In unclamped neurones, equivalent stimulation produced a membrane depolarization and induced or facilitated repetitive spike discharges. It is concluded that the principal synaptic response to muscarinic receptor activation is IM inhibition, leading to a net inward current and increased excitability, but that this response may be modified under certain circumstances by other synaptic currents.

Hexamethonium increases the excitability of sympathetic neurons by the blockade of the Ca2+-activated K+ channels

Effects of hexamethonium (C6) on the excitability of sympathetic ganglion cells were examined by means of intracellular recording. When DC currents were injected, high concentrations of C6 significantly augmented the repetitive firing of the cells without any change in threshold voltage for initiation of the spike. The Ca2+-sensitive component of the after-hyperpolarization following a spike was reduced by C6 in a dose-dependent fashion (0.3 to 10 mM). C6 slightly affected parameters for the spike but neither the resting membrane potential nor the input membrane resistance. The amplitude of the Ca2+ spike (in the presence of 1 microM tetrodotoxin and 10 mM tetraethylammonium) was not affected even by 30 mM C6. Bee venom (0.3 micrograms/ml) which contains apamin showed similar effects. These results suggest that C6 blocks the Ca2+-activated K+ channels, resulting in an increase in excitability of the cells.

Outward currents in voltage-clamped rat sympathetic neurones

Outward membrane currents were studied in neurones of the isolated rat superior cervical ganglion by using a two-micro-electrode or single-micro-electrode voltage-clamp technique. Under current clamp, depolarization elicited electrotonic potentials that displayed marked outward rectification. From negative resting potentials (-70 mV) a short latency, short duration outward rectification was observed. From more positive potentials (-40 mV) a longer latency persistent outward rectification could be demonstrated. Under voltage clamp, four distinct outward currents were observed: a delayed rectifier (IK); a transient outward current (IA); a Ca2+-activated current (IC) and the M-current (IM). The maximum amplitude of IK, IA and IC was 1-2 orders of magnitude greater than IM. Depolarizing from -40 mV to potentials more positive than -20 mV co-activated IK and IC, producing a characteristic N-shaped current voltage curve with a minimum at about +80 mV. Superfusion with Mn2+-containing solutions reduced outward current at all voltages and abolished the N-characteristic; the remaining current (IK) slowly inactivated (tau greater than 1 s). Raising [K+]o from 6 to 36 mmol/l reversed outward tail currents observed in normal solution. Addition of tetraethylammonium ions (1-3 mmol/l) strongly reduced the amplitude of IK and IC. IA was characterized by very rapid activation at potentials more positive than -60 mV and by fast and complete inactivation at potentials in the activation range. The amplitude of IA was dependent on [K+]o and was reduced by external 4-aminopyridine (1-3 mmol/l). The activation appeared to depend on the nature and concentration of divalent cations present in the superfusate. It is concluded that the soma membrane of rat sympathetic neurones, like many other vertebrate and invertebrate neurones, contains multiple populations of K+ channels. The possible functions of these in the control of ganglion cell excitability are discussed.

Muscarinic agonists depress calcium-dependent gK in bullfrog sympathetic neurons

Intracellular recordings were made from 'fast B' [9] neurons in bullfrog sympathetic ganglia. A single soma action potential was followed by a prolonged after-hyperpolarization lasting for several hundred milliseconds up to 2 s. The spike afterhyperpolarization, which is generated by calcium-dependent potassium conductance increase (gKCa) [3,20-24], was shortened by the muscarinic action of acetylcholine and oxotremorine (30-300 nM). These concentrations of muscarinic agonists were too low to cause any detectable changes in resting membrane potential, input resistance or action potential wave form. ACh released from presynaptic terminal under a physiological condition also caused the shortening of the calcium-dependent hyperpolarization. The results suggested that the shortening of calcium-dependent spike afterhyperpolarization may permit the neuron to pass the high frequency of discharge during the muscarinic excitation.

Ca-activated potassium current in vertebrate sympathetic neurons

Ca-activated K-currents (IC) in sympathetic neurones have been triggered by intracellular Ca-injection or by activating ICa. IC is strongly voltage-dependent, with a peak slope of 11 mV/e-fold depolarization above -50 mV. Relaxation, fluctuation and single channel analysis suggests this to result from voltage-dependent opening and closing rates. Time-constants for channel opening and closing are about 15 msec near zero mV. Single channel conductance is about 100 pS. Currents can be blocked by TEA. IC is activated very rapidly (less than or equal to 5 msec) and sometimes transiently by a depolarizing voltage-step. It is suggested that IC contributes to both spike repolarization and spike after-hyperpolarization. Spontaneous miniature ICs have also been recorded, probably activated by the release of packets of intracellular Ca.

Calcium-activated outward current in voltage-clamped hippocampal neurones of the guinea-pig

Slow clamp currents were recorded from CA1 and CA3 pyramidal neurones in slices of guinea-pig hippocampus maintained in vitro, using a single micro-electrode sample-and-hold technique. Depolarizing voltage commands evoked a time- and voltage-dependent outward current which was suppressed by removing external Ca or by adding Cd (0.5 mM) or Mn (5 mM). This Ca-dependent current (Ic) was not reduced by muscarinic agonists (unlike IM) but was greatly reduced by 5-20 mM-tetraethylammonium (TEA). Repolarizing IC tail currents reversed at -73 +/- 5 mV in 3 mM-K solution. The reversal potential became about 30 mV more positive on raising [K]o to 15 mM. No clear change in current amplitude or tail-current reversal potential occurred on adding Cs (2 mM), reducing [Cl]o from 128 to 10 mM, or replacing external Na with Tris. The underlying conductance GC was activated at membrane potentials positive to -45 mV. At -32 mV GC showed an approximately exponential increase with time, with a time constant of approximately 0.6 sec at 26 degrees C. Repolarizing tail currents declined exponentially with time, the time constant becoming shorter with increasing negative post-pulse potentials. When the clamp was switched off at the end of a depolarizing command of sufficient amplitude and duration to activate IC, a membrane hyperpolarization to -73 mV ensued, of similar amplitude and decay time to that following spontaneous action potentials. It is concluded that the clamp current observed in these experiments is probably the Ca-activated K current thought to contribute to the post-activation after-hyperpolarization in hippocampal neurones.