Potassium currents in submucous neurones of guinea-pig caecum and their synaptic modification

Glutamate Regulates the Frequency of Spontaneous Synchronized Ca2+ Spikes Through Group II Metabotropic Glutamate Receptor in Cultured Mouse Cortical Networks

1. Synchronized spontaneous intracellular Ca2+ spikes in networked neurons are believed to play a major role in the development and plasticity of neural circuits. Glutamate-induced signals through the ionotropic glutamate receptors (iGluRs) are profoundly involved in the generation of synchronized Ca2+ spikes. 2. In this study, we examined the involvement of metabotropic glutamate receptors (mGluRs) in cultured mouse cortical neurons. We pharmacologically revealed that glutamate-induced signals through inclusive mGluRs decreased the frequency of Ca2+ spikes. Further experiments indicated that this suppressive effect on the spike frequency was mainly due to the signal through group II mGluR, inactivation of adenylate cyclase-cAMP-PKA signaling pathway. Group I mGluR had little involvement in the spike frequency. 3. Taken together, glutamate generates the synchronized Ca2+ spikes through iGluRs and modulates simultaneously their frequency through group II mGluR-adenylate cyclase-cAMP-PKA signaling pathway in the present in vitro neural network. These results provide the evidence of the profound role of group II mGluR in the spontaneous and synchronous neural activities.

Inhibitory cotransmission or after-hyperpolarizing potentials can regulate firing in recurrent networks with excitatory metabotropic transmission

Recurrent networks of neurons communicating via excitatory connections are common in the nervous system. In the absence of mechanisms to control firing (collectively termed negative feedback), these networks are likely to be bistable and unable to meaningfully encode input signals. In most recurrent circuits, negative feedback is provided by a specialized subpopulation of interneurons, but such neurons are absent from some systems, which therefore require other forms of negative feedback. One such circuit is found within the enteric nervous system of the intestine, where AH/Dogiel type II neurons are interconnected via excitatory synapses acting through metabotropic receptors to produce slow excitatory postsynaptic potentials (slow EPSPs). Negative feedback in this recurrent network may come from either inhibitory postsynaptic potentials arising from the terminals that produce slow EPSPs or from the after hyperpolarizing potentials (AHPs) characteristic of these neurons. We have examined these possibilities using mathematical analysis, based on the Wilson-Cowan model, and computer simulations. Analysis of steady states showed that, under appropriate conditions, both types of negative feedback can provide robust regulation of firing allowing the networks to encode input signals. Numerical simulations were performed using large, anatomically realistic networks with realistic models for metabotropic transmission and suppression of the AHP. In the presence of constant exogenous input, parameters controlling aspects of synaptic events were varied, confirming the analytical results for static stimuli. The simulated networks also responded to time varying inputs in a manner consistent with known physiology. In addition, simulation revealed that neurons in networks with inhibitory contransmission fired in erratic bursts, a phenomenon observed in neurons in unparalysed tissue. Thus, either inhibitory contransmission or AHPs, or both, can allow recurrent networks of AH/Dogiel type II neurons to encode ongoing inputs in a biologically useful way. These neurons appear to be intrinsic primary afferent neurons (IPANs), which implies that the IPANs in a region act in a coordinated fashion.

A simple mathematical model of second-messenger mediated slow excitatory postsynaptic potentials

We have developed a novel and simple mathematical model of a slow excitatory postsynaptic potential (EPSP) based on an abstraction of the processes of activation, inactivation, and summation of a cAMP, protein kinase A (PKA)-dependent second-messenger cascade. The model describes the activation of receptors, G-proteins, and production of cAMP as the first stage and uses first-order, non-rate-limited kinetics. The second stage corresponds to the release of active, PKA catalytic subunit and can use first- or higher-order kinetics. The third stage represents simple phosphorylation of ion channels and is limited by the number of channels available. The decay of each stage is based on first-order, mass-action kinetics. These equations and some variations were solved numerically and values of the parameters were determined by fitting to a variety of experimental data from myenteric neurons of the guinea-pig ileum. The model produced a slow EPSP with a nonlinear stimulus-response relationship that resulted from the underlying kinetics of the signaling cascade. This system of equations is suitable for incorporation into a large-scale computer simulation, and the methodology should be generalizable to other pathways.

Tachykinins in the gut. Part II. Roles in neural excitation, secretion and inflammation

The preprotachykinin-A gene-derived peptides substance (substance P; SP) and neurokinin (NK) A are expressed in intrinsic enteric neurons, which supply all layers of the gut, and extrinsic primary afferent nerve fibers, which innervate primarily the arterial vascular system. The actions of tachykinins on the digestive effector systems are mediated by three different types of tachykinin receptor, termed NK1, NK2 and NK3 receptors. Within the enteric nervous system, SP and NKA are likely to mediate, or comediate, slow synaptic transmission and to modulate neuronal excitability via stimulation of NK3 and NK1 receptors. In the intestinal mucosa, tachykinins cause net secretion of fluid and electrolytes, and it appears as if SP and NKA play a messenger role in intramural secretory reflex pathways. Secretory processes in the salivary glands and pancreas are likewise influenced by tachykinins. The gastrointestinal arterial system may be dilated or constricted by tachykinins, whereas constriction and an increase in the vascular permeability are the only effects seen in the venous system. Various gastrointestinal disorders are associated with distinct changes in the tachykinin system, and there is increasing evidence that tachykinins participate in the hypersecretory, vascular and immunological disturbances associated with infection and inflammatory bowel disease. In a therapeutic perspective, it would seem conceivable that tachykinin antagonists could be exploited as antidiarrheal, antiinflammatory and antinociceptive drugs.

Tachykinins cause inward current through NK1 receptors in bullfrog sensory neurons

The effects of tachykinins on primary afferent neurons of bullfrog dorsal root ganglia (DRG) were examined by using whole-cell patch-clamp methods. Neurokinin A (NKA) caused inward current (INKA) in a concentration-dependent manner. Concentration-response curve showed that the EC50 for NKA was 6 nM. The INKA showed strong tachyphylaxis, when NKA was continuously applied for more than 1 min. Substance P (SP) also produced inward current with potency similar to that of NKA. Neurokinin B (NKB) was less effective in producing the inward current. The order of agonist potency was NKA = SP >> NKB. Spantide ([D-Arg1, D-Trp7.9, Leu11]SP), a non-selective peptide antagonist at tachykinin receptors, reduced the tachykinin-induced current. CP-99,994, a selective non-peptide antagonist for neurokinin-1 (NK1) receptor, inhibited the inward currents produced by NKA and SP. The INKA was associated with decrease in K+ conductance. NKA suppressed both a voltage-dependent K+ current, the M-current (IM), and a voltage-independent background K+ current, IK(B). Intracellular dialysis with GTP gamma S (100 nM) or GDP beta S (100 microM) depressed the INKA. Pre-treatment of DRG neurons with pertussis toxin (PTX) did not prevent the INKA. Depletion of intracellular ATP depressed the INKA. These results suggest that the tachykinin-induced inward current is mediated through the NK1 receptor which mainly couples to PTX-insensitive G-protein in bullfrog primary afferent neurons.

The role of substance P in myocardial dysfunction during ischemia and reperfusion

Impairment of myocardial contraction ("myocardial stunning") occurs during reperfusion after short ischemic periods. Substance P (SP) is widely distributed in heart and can be released by various stimuli including myocardial hypoxia. Our previous study shows SP has a negative inotropic effect in guinea pig heart. The objective of this study was to investigate whether SP contributes to the myocardial stunning after brief global ischemia. Guinea pig hearts in a Langendorff preparation were subjected to 15 min of global ischemia followed by 60 min reperfusion. Experiments were performed without and with pretreatment with neurokinin-1 (NK1) receptor antagonists, spantide (10(-6)M) or CP-99,994-01 (10(-6)M) in order to study the role of SP. Experiments were also performed in hearts which were perfused with atropine, phentolamine, and nadolol (10(-6)M each) to examine the role of neurotransmitters and autonomic receptors. A group of hearts obtained from capsaicin-pretreated guinea pigs was also investigated. Left ventricular developed pressure (LVDP), left ventricular end-diastolic pressure (LVEDP), heart rate, and perfusion pressure were monitored. At the end of reperfusion, the LVDP of control hearts recovered to only 55 +/- 6% (+/- SEM) of preischemic baseline and the LVEDP increased significantly (P > 0.05). With pretreatment with spantide or CP-99,994-01, LVDP recovered to 88 +/- 2% or 78 +/- 2% of the preischemic baseline, respectively. The LVEDP of these hearts was not different from preischemic baseline and much smaller than in control hearts. There were no differences in heart rate and perfusion pressure compared to baseline among all groups. Similar results were obtained in hearts perfused with autonomic blockers. However, recoveries of LVDP and LVEDP were faster in hearts perfused with autonomic blockers during the first 10 min of reperfusion. Pretreatment with capsaicin also significantly improved recovery of LVDP and LVEDP. In conclusion, substance P is involved in postischemic myocardial dysfunction and neurokinin-1 receptors mediate this action. The NK1 receptor antagonists may be useful in prevention of "myocardial stunning".

Potassium currents in adult rat intracardiac neurones

1. Properties of K+ currents were studied in isolated adult rat parasympathetic intracardiac neurones with the use of single-electrode voltage-clamp techniques. 2. A hyperpolarization-activated inward rectifier current was revealed when the membrane was clamped close to the resting level (-60 mV). The slowly developing inward relaxation had a mean amplitude of 450 pA at -150 mV, an activation threshold of -60 to -70 mV and a relaxation time constant of 41 ms at -120 mV. The current was reversibly blocked by Cs+ (1 mM) and became smaller with reduced [K+]o and [Na+]o, indicating that this inward rectifier current probably is a time- and voltage-dependent Na(+)-K+ current. 3. Step depolarizations from the holding potential of -80 mV evoked a transient (< 100 ms at -40 mV) outward K+ current (IA) which was blocked by 4-aminopyridine (4-AP, 1 mM). The time constants for IA inactivation were 20 ms at -50 mV and 16 ms at -20 mV. The steady-state activation and (removal of) inactivation curve showed a small overlap between -70 and -40 mV; the reversal potential of IA was close to EK. 4. Step hyperpolarizations from the depolarized potentials, i.e. -30 mV, revealed a slow inward relaxation associated with the deactivation of a time- and voltage-dependent current. The inward relaxation became faster at more hyperpolarized potentials and reversed at -85 and -53 mV in 4.7 and 15 mM [K+]o. This current was blocked by muscarine (20 microM) and Ba2+ (1 mM) but not affected by Cs+ (1 mM); this current may correspond to the M-current (IM). 5. Depolarization-activated outward K+ currents were evoked by holding the membrane close to the resting potential in the presence of tetrodotoxin (TTX, 3 microM), 4-AP (1 mM) and Ba2+ (1 mM). The amplitude of the outward relaxation and the tail current became smaller as the [K+]o was elevated. The outward tail current was reduced in a Ca(2+)-free solution and the residual current was eliminated by the addition of tetraethylammonium (TEA, 10 mM); the reversal potential was shifted in a direction predicted by the Nernst equation. These findings suggest the presence of delayed rectifier K+ current and Ca(2+)-activated K+ current. 6. Superfusion of TEA, Ba2+ and 4-AP, but not Cs+, induced rhythmic discharges in some of the otherwise quiescent intracardiac neurones.(ABSTRACT TRUNCATED AT 400 WORDS)

Tachykinins as mediators of slow EPSPs in guinea-pig gall-bladder ganglia: involvement of neurokinin-3 receptors

1. The effects of endogenous tachykinins and related peptides on intact guinea-pig gall-bladder neurones were investigated with single-electrode voltage- and current-clamp recording techniques. 2. Pressure ejection of substance P (100 microM) caused a long lasting membrane depolarization that was associated with a decrease in input resistance. In cells that were voltage-clamped to their resting membrane potential, substance P activated an inward current. 3. The reversal potentials of the substance P-induced depolarization and inward current were congruent to 0 mV. In a low-Na+ solution, the substance P-induced depolarization and inward current were reduced in amplitude. 4. Substance P increased the excitability of neurones, as evidenced by a greater anodal break activity and an increase in the number of action potentials generated during a depolarizing current pulse. 5. Substance P, neurokinin A (NKA) and neurokinin B (NKB) were applied by superfusion to determine the relative potencies of these tachykinins. NKB was the most potent, with an EC50 of 24 nM. The EC50 values for NKA and substance P were 47.8 and 281 nM, respectively. 6. The neurokinin-3 (NK-3) receptor agonist senktide depolarized neurones with an EC50 of 6.3 nM. Neither the NK-1 receptor agonist [Sar9,Met(O2)11]-substance P nor the NK-2 receptor agonist [beta-Ala8]-NKA(4-10) caused a measurable depolarization. 7. The NK-3 antagonist [Trp7,beta-Ala8]-NKA (4-10) inhibited the responsiveness of gall-bladder neurones to substance P with a KB (dissociation constant of receptor antagonist) of 49 nM, and depressed both capsaicin-induced depolarizations and stimulus-evoked slow EPSPs. 8. These data indicate that tachykinins mediate slow EPSPs in guinea-pig gall-bladder ganglia by activating NK-3 receptors on gall-bladder neurones. It is proposed that in response to inflammation or high intraluminal pressure, tachykinins may be released within ganglia by sensory fibres and act directly on intrinsic neurones to facilitate ganglionic transmission.

Tachykinin receptors in gastrointestinal motility

For a long time research on the action of TKs on gastrointestinal tissue has been demonstrating the importance of the TKs as non-cholinergic stimulators of motility in most parts of the mammalian gastrointestinal tract. The past years witnessed the development of TK agonists and antagonists selective for the various receptor types, which prompted a wealth of new insight into the pharmacology and molecular biology of the TK receptors. This knowledge now allows a more specific elucidation of the role of TKs and their receptors in the various aspects of gastrointestinal motility, not only in normal tissue but also under pathological conditions.

Interactions between inhibitory and excitatory modulatory signals in single submucosal neurons

Intracellular recordings were made in submucosal neurons from the guinea pig ileum to study the actions of norepinephrine and somatostatin on slow depolarizations induced by 2-chloroadenosine (CADO) and substance P. Local application (by pressure) of CADO and substance P induced a slow depolarization that occurred concomitantly with an increase in input membrane resistance. Norepinephrine, UK-14304 (alpha 2-adrenoceptor agonist), and somatostatin blocked the excitatory responses induced by CADO in a concentration-dependent manner. The alpha 2-adrenoceptor antagonists idazoxan and yohimbine antagonized these inhibitory effects of UK-14304 and norepinephrine. UK-14304 also decreased depolarizations induced by forskolin, but not those induced by the adenosine 3',5'-cyclic monophosphate analogue 8-(4-chlorophenylthio)adenosine 3',5'-cyclic monophosphate. Slow depolarizations induced by substance P were blocked neither by UK-14304 nor by somatostatin. It was previously shown that staurosporine (an inhibitor of various protein kinases) and KT-5720 (an inhibitor of protein kinase A) inhibited slow depolarizations induced by CADO. Here, substance P depolarizations were inhibited by staurosporine and calphostin C (a blocker of protein kinase C) but not by KT-5720. In conclusion, activation of alpha 2-adrenoceptors and somatostatin receptors selectively blocks excitatory responses induced by CADO, most likely by inhibition of adenylyl cyclase and via pertussis toxin-sensitive G proteins. Slow depolarizations induced by substance P are independent of adenylyl cyclase activation and involve activation of protein kinase C.

Neurokinin A mimics the slow excitatory postsynaptic current in submucous plexus neurons of the guinea-pig caecum

Single microelectrode voltage-clamp recordings were made from submucous neurons of the guinea-pig caecum. The slow excitatory postsynaptic current was compared with the currents induced by neurokinin A and substance P. The current induced by neurokinin A (100-300 nM) was associated with a decreased membrane conductance and reversed in polarity between -90 and -100 mV. The neurokinin A current was reduced by Co2+ (1-2 mM), but was not affected by Cs+ (1-2 mM), Ba2+ (10-100 microM) or low Cl- (20-40 mM) solutions. In about 80% of the neurons, the current induced by substance P (100-300 nM) was associated with a decreased membrane conductance and did not reverse with hyperpolarization of the membrane potential up to -130 mV. The current was reduced by Co2+ (1-2 mM) and augmented by low Cl- (20-40 mM) solutions, but was not affected by Cs+ (1-2 mM) or Ba2+ (10-100 microM)-containing solutions. In about 20% of the neurons, the substance P current reversed in polarity between -100 and -120 mV. The slow excitatory postsynaptic current elicited by repetitive nerve stimulation (10-40 Hz, three to five pulses) was accompanied by a decreased membrane conductance, and reversed in polarity between -90 and -100 mV. The slow excitatory postsynaptic current was abolished by Co2+ (1-2 mM) or low Na+ (12 mM) solutions, but was not affected by Cs+ (1-2 mM), Ba2+ (10-100 microM) or low Cl- (20-40 mM) solutions. In such neurons, the neurokinin A current was reversed at approximately the same potential at which the slow excitatory postsynaptic current was reversed, while the substance P current was not reversed even by much stronger hyperpolarizations. It was concluded that the neurokinin A current was mainly due to depression of potassium conductances, while the substance P current resulted from both increased anion conductance and decreased potassium conductances. The conductance change underlying the slow excitatory postsynaptic current is similar to that caused by neurokinin A.

Morphological properties and projections of electrophysiologically characterized neurons in the guinea-pig submucosal plexus

Intracellular recordings were made from 73 guinea-pig submucosal neurons using neurobiotin-filled microelectrodes; subsequently, neuropeptide immunoreactivity, morphology and nerve fibre projections were determined. Five distinct groups of cells could be distinguished: S cells with inhibitory input (61%), S cells without inhibitory input (19%), AH cells (8%), S-AH cells (5%), and glial networks. S cells with inhibitory input were immunoreactive for vasoactive intestinal polypeptide and showed Dogiel Type III morphology with the axon branching and coursing through two to 12 ganglia; varicosities and tufts of varicosities were observed surrounding other cell bodies as well as over blood vessels. S cells without inhibitory input primarily were immunoreactive for neuropeptide Y; they also showed Dogiel Type III morphology and similar, though shorter, axonal projections and varicose features surrounding other neurons. AH cells, which most likely contained substance P, lacked synaptic input and exhibited Dogiel Type II morphology; they branched more extensively than S cells and also formed varicose tufts within other ganglia. S-AH cells combined electrophysiological properties of S cells with inhibitory input and AH cells and did not show consistent morphological or histochemical characteristics. Typical glial networks were observed; in addition, on two occasions unusual networks of dye and electrical coupling between S cells without inhibitory input and a glial complex were observed. These results suggest that vasoactive intestinal polypeptide-containing S cells may act as interneurons which mediate a slow excitatory synaptic potential; that neuropeptide Y-containing S cells, which are known to be cholinergic, may play a role as cholinergic interneurons mediating the nicotinic fast excitatory synaptic potential; and that AH neurons also may provide cholinergic innervation to other submucosal neurons in addition to their previously described dual projections into mucosa and myenteric plexus.

Substance P produces an inward current by suppressing voltage-dependent and -independent K+ currents in bullfrog primary afferent neurons

A whole-cell patch-clamp study was carried out to examine the effect of substance P (SP) on the excitability of neurons in bullfrog dorsal root ganglia (DRG). SP (3 nM to 1 microM) produced an inward current associated with decreased membrane conductance at voltage range between -10 and -130 mV. Neurokinin A (NKA) and neurokinin B (NKB) also produced the inward current in DRG cells; the rank order of agonist potency was NKA = SP much greater than NKB. An antagonist for SP receptors, [D-Arg1, D-Trp7,9, Leu11]SP, did not prevent the response to SP. SP (3 nM to 1 microM) suppressed the voltage-dependent non-inactivating K+ current, the M-current (IM) by reducing the maximum M-conductance. A voltage-independent background K+ current, IK(B), could be recorded at a hyperpolarizing voltage (< or = -60 mV) from DRG neurons. SP (3 nM to 1 microM) produced the inward current associated with decreased IK(B) at a holding potential more negative than -60 mV. The SP-induced inward current reversed its polarity at the equilibrium potential for K ions. Intracellular dialysis with Cs+ blocked the SP-induced responses. Depletion of intracellular ATP reduced SP-induced inward current. These results suggest that the SP-induced inward current was due to suppression of both the IM and IK(B) that are regulated by intracellular activity of ATP in bullfrog DRG neurons.

Adenosine reduces the potassium conductance of guinea pig submucosal plexus neurons by activating protein kinase A

Intracellular recordings were made from S neurons of the submucosal plexus isolated from the guinea pig ileum. Adenosine or its analog 2-chloroadenosine (CADO) depolarized about 80% of neurons; previous work has shown that this results from activation of an A2 receptor. The depolarization was associated with an increase in membrane input resistance, became smaller with membrane hyperpolarization, reversed polarity at the potassium equilibrium potential and was mimicked and occluded by calcium-free solutions or by cadmium, suggesting that it is due to a reduction in a calcium-dependent potassium conductance. Both forskolin (though not 1,9-dideoxyforskolin) and phorbol 12,13-dibutyrate (PDBu) mimicked and occluded the action of CADO. Staurosporine (a nonspecific inhibitor of protein kinases) blocked the depolarization induced by the phorbol ester within 5 min, and blocked the effects of forskolin and CADO in 15-35 min. The depolarization caused by CADO was inhibited by the specific inhibitor of protein kinase A KT5720 [(8R*,9S*,11S*)-(-)-9-hydroxy-9-n-hexylester-8-methyl-2,3,9,10-tet rahydro-8,11-epoxy-1H,8H,11H-2,7b,11a-triazadibenzo[a,g]c ycloocta[cd e]-trin-den-1-one], whereas this inhibitor did not affect the depolarization induced by PDBu. The results are consistent with the control of this potassium conductance by protein kinase C, protein kinase A and intracellular calcium, and they indicate that adenosine reduces the conductance by activating protein kinase A.

Common ionic mechanisms of excitation by substance P and other transmitters in guinea-pig submucosal neurones

1. Intracellular recordings were made from submucosal neurones and single-electrode voltage-clamp methods were used to record membrane currents. The actions of substance P (SP), 5-hydroxytryptamine (5-HT), muscarine, vasoactive intestinal polypeptide (VIP), forskolin and nerve stimulation were studied. 2. Substance P, 5-HT (in the presence of 5-HT3 receptor antagonists), muscarine, VIP, forskolin and slow excitatory synaptic transmission all produced identical responses: an inward current associated with a membrane conductance decrease at the resting potential. The actions of any one occluded the actions of any other and all responses were pertussis-toxin insensitive. 3. These agonists produced a voltage-independent decrease in a 'leak' potassium conductance between -40 and -120 mV in 14% of neurones. 4. These agonists decreased a voltage-dependent, calcium-activated potassium conductance between -40 and -80 mV in all other (86%) neurones. The agonists still evoked an inward current without apparent conductance change at potentials between -90 and -130 mV. 5. In a low calcium solution containing cobalt or cadmium, the agonists produced an inward current associated with a conductance increase from -40 to -120 mV. Ion replacement studies indicated this current was due to an increase in a cation-selective (mainly sodium) conductance. 6. The agonists also reduced the inwardly rectifying potassium current that is activated by somatostatin and alpha 2-adrenoceptor agonists in these neurones. The agonists did not alter the inwardly rectifying potassium current that is present in these neurones in the absence of somatostatin or alpha 2-agonists. 7. Thus, SP, 5-HT, muscarine, VIP and the release of slow excitatory transmitters all appear to act through a common intracellular transduction pathway, an increase in adenylate cyclase. This results in an activation of a sodium-selective cation current and an inhibition of three distinct potassium conductances: the background potassium conductance, the calcium-activated potassium conductance and the inwardly rectifying potassium conductance activated by somatostatin and alpha 2-adrenoceptor agonists.

Fast and slow depolarizations produced by substance P and other tachykinins in sympathetic neurons of rat prevertebral ganglia

Using intracellular recording, we examined the effects of three mammalian tachykinins, substance P (SP), neurokinin A (NKA), and neurokinin B (NKB), on sympathetic neurons of isolated rat coeliac-superior mesenteric ganglia (C-SMG). The 3 tachykinins elicited two distinct depolarizing responses in ganglion cells: fast depolarization with time-to-peak of 1-2 sec and duration of 5-10 sec, and slow depolarization with time-to-peak of about 20 sec and duration of 120-140 sec. Both fast and slow responses persisted in a solution containing low Ca2+ and high Mg2+ or tetrodotoxin, which indicates that the tachykinins directly act on ganglion cells to produce fast and slow depolarizations. The two types of tachykinin-induced responses exhibited clearly distinguishable properties. The membrane conductance was increased during the fast response, but not significantly changed, slightly decreased or sometimes increased during the slow response. Within certain range of membrane potential, the amplitude of fast response increased upon membrane hyperpolarization and decreased upon depolarization of ganglion cells. In contrast, the amplitude of slow response associated with membrane conductance decrease was increased with membrane depolarization and decreased with hyperpolarization. The fast response was markedly suppressed in a Na(+)-deficient solution, a solution containing nominally zero Ca2+ (plus 0.1 mM EGTA in some cases), and in a solution containing Cd2+ or Mn2+, whereas the slow response was not affected in these solutions and was augmented in some cells in K(+)-free solution. Thus it seems that the increase in Ca(2+)-dependent cationic conductance underlies the fast response and that the slow response is produced at least in part by suppression of certain K+ channels. The fast response progressively decreased in amplitude upon repeated application of the peptides with short intervals, whereas the slow response was rather augmented by repeated application. Lowering the temperature markedly depressed the slow response, while the fast response remained almost unaffected. It is therefore likely that the fast and slow depolarizations are mediated by two different subtypes of tachykinin receptors or a single class of receptors linked with two different intracellular mechanisms. Measurement of tachykinins in several sympathetic ganglia by combined use of HPLC and radioimmunoassay revealed that the highest amount of SP occurs in the C-SMG where the content of SP (136.0 pmol/g protein) was higher than those of NKA (44.3) and NKB (18.7). SP thus appears to function as a major tachykinin in rat C-SMG.

Substance P inhibits activation of calcium-dependent potassium conductances in guinea-pig myenteric neurones

1. Intracellular recordings were made from myenteric AH neurones of the guinea-pig ileum in vitro. Some experiments were done with a single-electrode voltage clamp to measure membrane currents. 2. Substance P (SP) applied by superfusion (10 nM-300 nM), pressure ejection (100 nM-10 microM, 760 mmHg, for 10-20 ms) or ionophoresis (1 mM, 100 nA, for 0.2 s) caused a membrane depolarization and an inward current, associated with a decrease in potassium conductance. 3. The SP-induced depolarization was abolished within 15 min by superfusion with calcium-free/high-magnesium (10 mM) solution or solutions containing cobalt, manganese or nickel at 1-3 mM. The response persisted even after 40-60 min of superfusion with calcium-free/normal-magnesium (1.2 mM) solution. In all these solutions, synaptic potentials were abolished within 5 min. 4. SP inhibited a slowly developing outward current and an outward tail current during and after a long depolarizing command pulse (2-10 s), and an outward after-current following single or multiple brief depolarizing command pulses (10-50 ms). These outward currents were suppressed in calcium-free/high-magnesium solution. 5. SP depressed both a calcium-dependent slow after-hyperpolarization following the action potential and an outward after-current preceded by a brief depolarizing command. Both the SP-induced depolarization and the SP-induced inward current were augmented when the peptide was pressure-ejected during the recovery phase of the slow after-hyperpolarization and during that of the slow outward after-current, but both of them were inhibited or almost abolished when SP was applied immediately after spike initiation or a brief depolarizing command. 6. The SP-induced response was depressed by barium (1-2 mM). The SP response was not inhibited by tetraethylammonium at low concentrations (5-10 mM), but was depressed at high concentration (20 mM). 7. Superfusion (1-10 nM) or pressure application of a calcium ionophore, A23187, inhibited or even reversed the SP depolarization and the SP-induced inward current. 8. These results indicate that SP inhibits activation of a calcium-dependent potassium conductance which contributes to both the slow after-hyperpolarization and the resting membrane potential. SP may affect the process by which calcium activates this potassium conductance.

Mechanisms underlying intracellular signal transduction of the slow IPSP in submucous neurones of the guinea-pig caecum

1. Intracellular recordings were obtained from submucous plexus neurones of the guinea-pig caecum. 2. The resting membrane conductance displayed two types of inward rectification: one which developed at potentials more negative than -70 mV, and another that occurred at potentials more negative than the potassium equilibrium potential. The former inward rectification was blocked by extracellular caesium (Cs+; 1-2 mM) and the latter was blocked by Cs+ (1-2 mM) or barium (Ba2+; 30-100 microM). 3. The noradrenaline-induced current measured by subtraction of the current-voltage (I-V) relation before and after adding the agonist also showed an inward rectification around the resting potential. Ba2+ (30-100 microM) blocked both the outward and inward current induced by noradrenaline. The noradrenaline current was not affected by Cs+ (1-2 mM). Both the slow IPSP and the slow IPSC (inhibitory postsynaptic current) were reduced by Ba2+, but not by Cs+. 4. During the intracellular injection of guanosine 5'-O-(3-thiotriphosphate) (GTP-gamma-S), multiple repetitive stimulation or repeated applications of noradrenaline produced irreversible membrane hyperpolarizations with a decreased membrane input resistance, until the membrane had approached the potassium equilibrium potential. 5. Pertussis toxin (1-40 micrograms/ml) abolished both the slow IPSP and the noradrenaline hyperpolarization without affecting the nicotinic fast EPSP or the slow EPSP. 6. Superfusion with a Ca(2+)-free, high-Mg2+ (12 mM) solution caused a membrane depolarization associated with an increased input resistance. It eliminated the Ca2+ spikes, the slow after-hyperpolarizations following the spikes, and the synaptic potentials within 3 min. Prolonged exposure (longer than 20 min) to this solution resulted in a progressive decline of the noradrenaline hyperpolarization. 7. Intracellular injection of ethylene glycol-bis(beta-aminoethylether)N,N,N',N'-tetraacetic acid (EGTA) reduced the slow IPSP and the noradrenaline hyperpolarization. Superfusion with a membrane-permeable Ca2+ chelator, 1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid, tetraacetoxymethyl ester (BAPTA/AM; 10-200 microM) reduced the noradrenaline hyperpolarization. 8. Procaine reversibly reduced the slow IPSP and noradrenaline hyperpolarization without affecting the fast EPSP or slow EPSP at concentrations up to 300 microM.(ABSTRACT TRUNCATED AT 400 WORDS)

Properties of transient K+ currents and underlying single K+ channels in rat olfactory receptor neurons

The transient potassium current, IK(t), of enzymatically dissociated rat olfactory receptor neurons was studied using patch-clamp techniques. Upon depolarization from negative holding potentials, IK(t) activated rapidly and then inactivated with a time course described by the sum of two exponential components with time constants of 22.4 and 143 ms. Single-channel analysis revealed a further small component with a time constant of several seconds. Steady-state inactivation was complete at -20 mV and completely removed at -80 mV (midpoint -45 mV). Activation was significant at -40 mV and appeared to reach a maximum conductance at +40 mV (midpoint -13 mV). Deactivation was described by the sum of two voltage-dependent exponential components. Recovery from inactivation was extraordinarily slow (50 s at -100 mV) and the underlying processes appeared complex. IK(t) was reduced by 4-aminopyridine and tetraethylammonium applied externally. Increasing the external K+ concentration ([K+]o) from 5 to 25 mM partially removed IK(t) inactivation, usually without affecting activation kinetics. The elevated [K+]o also hyperpolarized the steady-state inactivation curve by 9 mV and significantly depolarized the voltage dependence of activation. Single transient K+ channels, with conductances of 17 and 26 pS, were observed in excised patches and often appeared to be localized into large clusters. These channels were similar to IK(t) in their kinetic, pharmacological, and voltage-dependent properties and their inactivation was also subject to modulation by [K+]o. The properties of IK(t) imply a role in action potential repolarization and suggest it may also be important in modulating spike parameters during neuronal burst firing. A simple method is also presented to correct for errors in the measurement of whole-cell resistance (Ro) that can result when patch-clamping very small cells. The analysis revealed a mean corrected Ro of 26 G omega for these cells.

Mechanisms underlying presynaptic inhibition through alpha 2-adrenoceptors in guinea-pig submucosal neurones

1. Intracellular recordings were made from submucosal neurones of the guinea-pig ileum. The actions of noradrenaline, somatostatin and [Met5]enkephalin on nicotinic synaptic potentials (EPSPs) were studied. 2. In one series of experiments, agonists were applied by superfusion; noradrenaline (0.1-20 microM) decreased EPSP amplitude by 95-100% in all neurones. Similar application of somatostatin (1-100 nM) inhibited EPSPs in about half the neurones by a maximum of 40%. [Met5]enkephalin (0.1-10 microM) did not alter EPSPs. Idazoxan and yohimbine competitively antagonized the action of noradrenaline with dissociation equilibrium constants of 20 and 30 nM respectively. 3. In another series of experiments, noradrenaline and somatostatin were applied locally from a pipette so that they reached presynaptic terminals but not the cell bodies or axons of the presynaptic cell: noradrenaline inhibited EPSPs by 90% in all neurones but somatostatin had no effect. When applied locally to the cell bodies giving rise to the presynaptic fibres, both agonists inhibited EPSPs in half the neurones by 40%. 4. When noradrenaline was applied locally to presynaptic terminals, the latency to onset of noradrenaline to inhibit EPSPs was 45-160 ms; cadmium applied similarly depressed EPSPs in 5-50 ms. 5. Pertussis toxin pre-treatment only partially blocked presynaptic inhibition caused by noradrenaline but abolished the reduction of EPSP amplitude by somatostatin. 6. It is concluded that noradrenaline and somatostatin reduce the amplitude of the fast EPSP because they hyperpolarize cell bodies and prevent action potential initiation. Noradrenaline, but not somatostatin, has an additional action to inhibit acetylcholine release by acting at nerve terminal receptors. 7. The presynaptic inhibitory action of noradrenaline results from activation of alpha 2-adrenoceptors at nerve terminals but the mechanism(s) by which these presynaptic receptors act cannot be explained adequately by either activation of a potassium conductance and/or inhibition of a calcium conductance.

Inhibition of calcium currents by noradrenaline, somatostatin and opioids in guinea-pig submucosal neurones

1. Whole-cell recordings were made from submucosal neurones acutely dissociated from guinea-pigs. The actions of noradrenaline, somatostatin and [Met5]enkephalin on currents carried by calcium ions were studied. 2. On depolarization from a holding potential of -70 mV, an inward current activated at -40 mV, reached its peak amplitude at 10 mV and reversed to outward at 72 mV (with external calcium of 5 mM and internal caesium of 160 mM). 3. Cadmium, nickel and cobalt reversibly blocked the calcium current; concentrations causing 50% block were 2.5, 500 and 2000 microM respectively. The calcium current (holding at -70 or -30 mV) was reversibly blocked by omega-conotoxin (100 nM), and unaffected by Bay K 8644 (0.1-10 microM) and nifedipine (1 microM). Cadmium caused an outward shift in holding current at -30 mV, implying that there was a persistent inward calcium current at this potential. 4. Noradrenaline, somatostatin and [Met5]enkephalin decreased the calcium current. The maximal inhibition observed with any one agonist, or with a combination of two agonists, did not exceed 50%; concentrations giving half-maximal inhibition were 5.5 microM for noradrenaline, 4 nM for somatostatin and 1 microM for [Met5]enkephalin. The inhibition was independent of membrane potential. All three agonists also reduced the persistent calcium current at -30 mV. 5. Inhibition of the calcium current by noradrenaline occurred with a latency of not less than 175 ms; cadmium applied by the same method depressed the current within 5-45 ms. 6. Experiments with selective agonists and antagonists indicated that the receptor types involved in calcium current inhibition were alpha 2-adrenoceptors and delta-opioid receptors. Somatostatin acted at a distinct receptor. 7. Calcium currents were also inhibited by intracellular dialysis with guanosine 5'-O-(3-thiotriphosphate) (GTP-gamma-S). Agonists were ineffective in cells pre-treated with pertussis toxin, but their action was restored when purified GTP-binding proteins (Go or Gi) were included in the intracellular recording solution. 8. It is concluded that noradrenaline, somatostatin and [Met5]enkephalin act at their respective receptors on guinea-pig submucosal neurones to inhibit a voltage-dependent calcium current. Activation of the same receptors also increases a potassium conductance in these cells: in both cases a pertussis-sensitive G protein is involved.

Reduction of the N-type calcium current by noradrenaline in neurones of rabbit vesical parasympathetic ganglia

1. Intracellular and single-electrode voltage-clamp recordings were made from neurones of vesical parasympathetic ganglia (VPG) isolated from the rabbit urinary bladder. 2. Noradrenaline (NA, 0.5-5 microM) shortened the duration of the action potentials and depressed the amplitudes of both spike after-hyperpolarization and after-current. 3. Voltage-dependent calcium currents (ICa) were recorded by using microelectrodes filled with 2 M-caesium chloride in a superfusing solution containing tetraethylammonium (TEA, 50 mM) and tetrodotoxin (TTX, 500 nM). Noradrenaline (0.5-5 microM) depressed both the ICa and the tail current evoked by depolarizing voltage jumps from -100 to -50 mV to -30 to +20 mV. 4. Substitution of barium for calcium also produced an inward current (IBa) with no obvious tail current. Noradrenaline (1 microM) reduced the magnitude of the IBa without affecting the voltage dependence of the current-voltage relationship for IBa. 5. Yohimbine (1 microM), but not prazosin (1 microM) or propranolol (1 microM), antagonized the NA-induced inhibition of the IBa. UK 14304, a potent alpha 2-adrenoceptor agonist, mimicked NA in depressing the IBa. 6. The transient low-threshold (T), the transient high-threshold (N) and the slowly inactivating high-threshold (L) calcium currents co-existed in VPG neurones. 7. Noradrenaline reduced the IBa evoked at clamp potentials more positive than -20 mV from holding potentials near the resting membrane potential (-70 to -50 mV). Under these conditions, the IBa consisted primarily of N- and L-current components. In contrast, NA had no effect on the isolated T- and L-currents. It is concluded that NA selectively inhibits the N-type calcium channels by an action at alpha 2-adrenoceptors in the rabbit VPG neurones.

Effects of 5-HT3 receptor antagonists on 5-HT and nicotinic depolarizations in guinea-pig submucosal neurones

1. Intracellular recordings were made from neurones of the guinea-pig submucosal plexus. The effects of several 5-hydroxytryptamine3 (5-HT3) receptor antagonists on depolarizations produced by ionophoretic application of 5-HT and acetylcholine, as well as on fast excitatory postsynaptic potentials (fast e.p.s.ps) produced by nerve stimulation were examined. 2. ICS 205-930, GR 38032F, MDL 72222, cocaine and curare all inhibited the fast e.p.s.p. as well as the depolarizations in response to 5-HT and acetylcholine (ACh) ionophoresis in a dose-dependent fashion. 3. IC50 values for ICS 205-930, GR 38032F, MDL 72222, cocaine and curare in inhibiting the 5-HT mediated depolarizations were 12 nM, 100 nM, 3 microM, 3 microM and 20 microM, respectively. 4. IC50 values for ICS 205-930, GR 38032F, MDL 72222, cocaine and curare in inhibiting the nicotinic depolarizations were 4 microM, 12 microM, 11 microM, 6 microM and 17 microM, respectively. Similar IC50 values were obtained for inhibition of the fast e.p.s.ps by these antagonists. 5. The nicotinic receptor blocker, hexamethonium, inhibited the nicotinic depolarization and the fast e.p.s.p. with IC50 values of 10 microM. Hexamethonium (10 microM-5 mM) did not alter the depolarization induced by 5-HT. 6. These results demonstrate that the pharmacological profile of 5-HT3 receptors present on submucosal neurones is identical to that of 5-HT3 receptors on myenteric neurones and, thus, provide evidence that the enteric neuronal 5-HT3 receptor forms a receptor subtype distinct from that characterized in other parts of the autonomic nervous system.

Extracellular calcium ions are required for muscarine-sensitive potassium current in bullfrog sympathetic neurons

Cultured bullfrog sympathetic neurons were voltage-clamped in the whole-cell configuration. The extracellular medium contained tetrodotoxin (3 microM) and cesium (1 mM) to block and inward sodium current and a hyperpolarization-activated cation current Attempts were made to separate the M-current from four other potassium currents. Tetraethylammonium (30 mM) was used to block a classical delayed rectifier current (IK) and a fast calcium-activated current (IC). Apamin (30 nM) was used to block a slow calcium-activated current (IAHP). 4-Aminopyridine (1 mM) was used to reduce the amplitude of a transient current (IA). In these conditions, the maximum M-conductance near 0 mV was reduced by as much as 90% when divalent cations such as cobalt (1 mM) were added to the superfusate. The maximum M-conductance was also reduced by as much as 60% when calcium ions were removed from the superfusate. The half-activation voltage in the steady-state activation curve and the reversal potential of the M-current were not significantly changed in the calcium-free solution. It is suggested that the presence of calcium ions in the extracellular space is required for the M-current activation.