1 Department of Anatomy and Cell Biology, Queen's University, Kingston, Ontario K7L 3N6, Canada; and 2 Instituto Nacional de Enfermedades Respiratorias and Departamento de Farmacología, Facultad de Medicina, Universidad Nacional Autónoma de Mexico, México D.F, México
![]() |
ABSTRACT |
---|
![]() ![]() ![]() ![]() ![]() ![]() ![]() |
---|
Inhibitory interactions between 5-HT subtype
3 (5-HT3) and P2X receptors were characterized using whole
cell recording techniques. Currents induced by 5-HT
(I5-HT) and ATP (IATP)
were blocked by tropisetron (or ondansetron) and
pyridoxalphosphate-6-azophenyl-2',4'-disulfonic acid, respectively.
Currents induced by 5-HT + ATP (I5-HT+ATP) were only as large as the current induced by the most effective transmitter, revealing current occlusion. Occlusion was observed at
membrane potentials of 60 and 0 mV (for inward currents), but
it was not present at +40 mV (for outward currents). Kinetic and
pharmacological properties of I5-HT+ATP indicate
that they are carried through 5-HT3 and P2X channels.
Current occlusion occurred as fast as activation of
I5-HT and IATP, was still
present in the absence of Ca2+ or Mg2+, after
adding staurosporine, genistein, K-252a, or N-ethylmaleimide to the pipette solution, after substituting ATP with
,
-methylene ATP or GTP with GTP-
-S in the pipette, and was observed at 35°C, 23°C, and 8°C. These results are in agreement with a model that considers that 5-HT3 and P2X channels are in functional
clusters and that these channels might directly inhibit each other.
autonomic neurons; enteric neurons; ligand-gated channels; ion channels; ATP; ATP receptors; P2X receptors; serotonin; 5-hydroxytryptamine; 5-hydroxytryptamine 3 channels; 5-hydroxytryptamine 3 receptors; fast neurotransmission
![]() |
INTRODUCTION |
---|
![]() ![]() ![]() ![]() ![]() ![]() ![]() |
---|
5-HT
AND ATP are known to play a role as neurotransmitters (17,
35, 37, 38) by directly activating cationic channels in the
postsynaptic membrane, which are named 5-HT3 and P2X
receptors, respectively. 5-HT3 receptors are part
of the nicotinic ligand-gated channel superfamily, and two different
subunits have been cloned. Each of these subunits have four
transmembrane domains (14, 29). Of the seven different P2X
subunits that have been cloned, each subunit appears to cross the
membrane only twice (10, 16, 24). Some of these subunits
are able to form homomeric functional cation channels, e.g.,
P2X2 and 5-HT3A. Other ones appear to
participate only in heteromeric channels, e.g., P2X6 and
5-HT3B (14, 27). Current models of
5-HT3 and P2X channels propose that these channels are
formed by a combination of five and three subunits, respectively (14, 32). Recent experimental evidence indicates that the nicotinic acetylcholine (nACh) channels and P2X are not independent and
that they can inhibit each other when they are simultaneously activated
(6, 39). This inhibitory interaction is very fast and
might be mediated by an allosteric interaction between nACh and P2X
channels. A similar cross-inhibition between the P2X2 and
nicotinic 3
4-receptor subtypes was observed when
these were coexpressed in Xenopus oocytes (25).
Similar interactions have been shown between P2X and the
GABAA receptors in dorsal root ganglia (36),
supporting the hypothesis that other members of the nicotinic receptor
superfamily could be interacting with P2X receptors. In myenteric
neurons, however, P2X channels appear to interact with nACh and not
with other members of the nicotinic receptor superfamily present in
these cells, such as the 5-HT3 and GABAA
receptors (39), thus indicating that cross-interactions between these receptors could be tissue specific. To add to this complexity, other types of pharmacological interactions appear to exist
between the serotoninergic and cholinergic systems, and it has been
reported that 5-HT molecules can themselves block nACh channels in
various cell types including submucosal neurons (6, 18).
Our aim in the present study was to investigate and functionally characterize the putative inhibitory interactions between 5-HT3 and P2X native receptors present in submucosal neurons (6, 15, 39). Our findings indicate that activation of 5-HT3 and P2X receptors opens two different channel populations. These two channels, however, negatively modulate each other when they are simultaneously activated. This inhibitory interaction occurs simultaneously with current activation and does not require Ca2+, Mg2+, G proteins, or protein phosphorylation, implying that they are mediated by direct interaction between these receptors. A preliminary report of some of these results has been described previously (3).
![]() |
MATERIALS AND METHODS |
---|
![]() ![]() ![]() ![]() ![]() ![]() ![]() |
---|
Young guinea pigs (150-200 g), either male or female, were killed by decapitation, and 5 cm of proximal jejunum were removed, placed in modified Krebs solution (in mM: 126 NaCl, 1.2 NaH2PO4, 1.2 MgCl2, 2.5 CaCl2, 5 KCl, 25 NaHCO3, and 11 glucose; gassed with 95% O2-5% CO2), and opened longitudinally. The mucosa of this intestinal segment was removed, and the thin submucosal layer (submucosal preparation) was dissected from the underlying layers of smooth muscle.
The submucosal preparation was dissociated using a sequential treatment with two enzymatic solutions, as described elsewhere (9); the first contained papain (0.01 ml/ml; activated with 0.4 mg/ml of L-cysteine), and the second contained collagenase (1 mg/ml) and dispase (4 mg/ml). After these solutions were washed out, the neurons were plated on rounded coverslips coated with sterile rat tail collagen. Culture medium was minimum essential medium 97.5%, containing 2.5% guinea pig serum, 2 mM L-glutamine, 10 U/ml penicillin, 10 µg/ml streptomycin, and 15 mM glucose.
ATP and 5-HT are known to modulate the potassium membrane conductance
of enteric neurons via G protein-linked receptors (4, 12,
19). To isolate the ionic membrane currents mediated by activation of ligand-gated channels, the experiments were carried out
in the presence of Cs+ (a potassium channel blocker) and
currents were measured by the whole cell patch-clamp configuration.
This recording technique prevents effects mediated by second
messengers. ATP- and 5-HT-induced currents were recorded using
short-term (2-40 h) primary cultures of submucosal neurons and the
Axopatch 1D amplifier. Patch pipettes were made as previously described
(9) and had resistances between 1 and 3 M. Sixty to
seventy percent of the series resistance were compensated in about one
third of the experiments reported here. Series resistance compensation,
however, did not affect the lack of additivity of the whole cell
currents activated by (IATP) and 5-HT
(I5-HT; see RESULTS), so, in most
cases, no compensation was made for this factor. Except when otherwise
mentioned, the holding potential was
60 mV. The standard solutions
used, unless otherwise mentioned, had the following composition (in
mM): inside the pipette: 160 Cs-gluconate, 10 EGTA, 5 HEPES, 10 NaCl, 3 ATPMg, and 0.1 GTP; external solution: 160 NaCl, 2 CaCl2,
11 glucose, 5 HEPES, and CsCl 3. The pH of all solutions was adjusted
to 7.3-7.4 with either CsOH (pipette solutions) or NaOH (external
solutions). With these standard solutions, the usual input resistance
of the neurons ranged from 1 to 10 G
. Whole cell currents were
recorded on a PC using Axotape software (Axon Instruments) and analyzed on a Macintosh computer using Axograph software (Axon Instruments). Membrane potentials were corrected for the liquid junction potential (pipette
11 mV). The recording chamber was continuously superfused with external solution at ~2 ml/min. Rapid changes in the external solution were made by using an eight-barrelled device (5). The external application of experimental substances was achieved by
abruptly exchanging the tube delivering control external solution in
front of the cell being recorded for a tube delivering the same
solution plus the drug(s). Experimental substances were removed by
returning back to the tube containing the control external solution.
External solutions were delivered by gravity. Except where otherwise
mentioned, experiments were performed at room temperature (~23°C).
Genistein, ACh, 5-HT, tropisetron (before called ICS-205,930), and pyridoxalphosphate-6-azophenyl-2',4'-disulfonic acid (PPADS) were purchased from Research Biomedical (Natick, MA). Staurosporine and K-252a were supplied by Kamiya (Thousand Oaks, CA). Ondansetron was purchased from Glaxo Wellcome (Mississauga, ON, Canada). All other substances were purchased from Sigma (St. Louis, MO). The pH of the external solution containing ATP, used to induce the IATP, was always readjusted with NaOH. The addition of the other substances to the external solution did not alter its pH.
Results were expressed as means ± SE [number of cells (n)]. The paired Student's t-test was used to evaluate differences between mean values obtained from the same cells, and the unpaired Student's t-test was used for data obtained from different groups of cells. Two-tailed P values of 0.05 or less were considered statistically significant.
![]() |
RESULTS |
---|
![]() ![]() ![]() ![]() ![]() ![]() ![]() |
---|
Pharmacological and electrophysiological properties of the whole cell currents induced by 5-HT and ATP. Electrophysiological and pharmacological properties of I5-HT and IATP in enteric neurons have been previously characterized (5, 7, 15, 38). Both currents are mediated by activation of nonspecific cationic channels, and their slope single-channel conductance have slightly different magnitudes: 9-15 and 15-22 pS for 5-HT3 (15) and P2X (5, 7, 38) receptors, respectively.
5-HT induced an inward current in all neurons tested (n = 140), whereas ATP caused an inward current in only ~88% (n = 123) of a total of 140 neurons. Concentration-response curves (Fig. 1A) were obtained for these transmitters in submucosal neurons and analyzed as previously reported (7). The EC50 values for 5-HT and ATP were 55 and 53 µM, whereas the Hill coefficient values were 1.0 ± 0.1 and 1.1 ± 0.1 (not significantly different than unity), respectively. The currents induced by maximal concentrations of 5-HT and ATP (1 mM) had an average amplitude of
|
|
|
|
Currents induced by 5-HT and ATP are not additive.
The experiments described above demonstrated that
I5-HT and IATP have
completely different kinetics and that they are mediated by activation
of pharmacologically distinct receptors. In the following experiments,
we investigated whether these currents are mediated by independent
populations of channels. If IATP and I5-HT are mediated by independent populations of
channels, then the currents induced by a maximal concentration of these
transmitters (when receptor occupancy is expected to be close to 100%)
should be additive. We therefore measured the peak whole cell currents induced by maximal concentrations (1 mM) of 5-HT, ATP, or simultaneous application of both agonists (I5-HT+ATP) in
the same neuron. We found that the addition of individual currents
[IATP + I5-HT = expected current (Iexpected)] was
significantly larger (P < 0.001) than
I5-HT+ATP, revealing an occlusion between
I5-HT and IATP (Figs.
2-4). In these experiments, I5-HT+ATP
(3,561 ± 369 pA) was not different than the current induced by
the most effective of these transmitters (
3,294 ± 733 pA). For
instance, if IATP was larger than
I5-HT (which was the most common finding), then
I5-HT+ATP had about the same magnitude as
IATP (e.g., Fig. 2, A and
C; see below). Indeed, in 10 analyzed experiments in which
IATP (
3,090 ± 614 pA) was significantly
(P < 0.01) larger than I5-HT
(
1,166 ± 161 pA), I5-HT+ATP
(
3,151 ± 587 pA) was significantly (P < 0.01)
larger than I5-HT but was not different from
IATP. Similarly, in five analyzed neurons in
which I5-HT (
2,742 ± 636 pA) was
significantly (P < 0.05) larger than
IATP (
1,324 ± 270 pA),
I5-HT+ATP (
2,796 ± 620 pA) was also
significantly (P < 0.05) larger than
IATP but was not different from
I5-HT.
Kinetics of the whole cell currents induced by simultaneous application of both agonists. Figure 2C shows the onset of I5-HT, IATP, I5-HT+ATP, and Iexpected of a typical experiment out of 13 analyzed neurons. As it is shown in Fig. 2, C and D, the average time required to reach the half-maximal amplitude of the currents induced by simultaneous application of both agonists was significantly different than that for I5-HT or IATP (P < 0.05). Figure 2C shows that I5-HT+ATP was smaller than Iexpected as soon as currents were detected. Because the sampling frequency during our recordings was 0.3-1 kHz, this current occlusion must be occurring in <5 ms.
Visual inspection of the recordings revealed that I5-HT+ATP desensitized faster than IATP but slower than I5-HT, as seen in Figs. 2A and 3A. To quantify this, in eight neurons, agonists were applied during relatively long periods (1-3 min), and exponential fits were performed using the data from the current peak to the "steady state" (end of the agonist application; Fig. 3A). As with I5-HT, desensitization of I5-HT+ATP was better fitted by the sum of three exponential functions. ThePrereceptor mechanisms and technical artefacts as the origin for
the current occlusion.
The following experiments rule out that current occlusion is due to
prereceptor mechanisms. In six analyzed cells with very low response or
no initial response to ATP (22 ± 16 pA; Fig. 5A),
I5-HT+ATP (
2,147 ± 128 pA) had the same
amplitude and similar kinetics as I5-HT alone
(
2,274 ± 179 pA). Similarly, in another five cells, in which
IATP (
44 ± 27 pA) had been previously blocked with PPADS (Fig. 5B),
I5-HT+ATP (
1,253 ± 288 pA) had the same
amplitude and similar kinetics as I5-HT
(
1,103 ± 273 pA). In three cells with relatively low response
to 5-HT (
304 ± 49 pA; Fig. 5C)
I5-HT+ATP (
1,980 ± 298 pA) had the same amplitude and very similar desensitization kinetics as
IATP (
1,875 ± 260 pA). Similarly, in
another four cells in which I5-HT (
21 ± 18 pA) was blocked with tropisetron (0.3 µM; Fig. 5D),
I5-HT+ATP (
1,900 ± 510 pA) had the same
amplitude and similar kinetics as IATP
(
2,045 ± 509 pA). Similar results were obtain in two neurons in
which ondansetron [0.3 µM, a specific 5-HT3 receptor antagonist (20)] was used to blocked
I5-HT.
|
|
Application of both agonists desensitized both 5-HT3
and P2X receptors.
In the following experiments, we measured the amplitude of both
I5-HT and IATP before and
immediately after (~5 s) a long application of 5-HT, ATP, or
5-HT + ATP. This long application lasted for at least 25 s or
until the induced current had desensitized ~80% (usually within 1 min). Some typical recordings and the average data from such
experiments are shown in Fig. 7,
A-G. We observed that P2X receptor desensitization with ATP
decreased IATP significantly, whereas it did not
affect I5-HT (Fig. 7, A and
B). Similarly, 5-HT3 receptor desensitization
with 5-HT decreased I5-HT significantly, whereas
it did not affect IATP (Fig. 7, C and
D). In other words, no cross-desensitization was observed
between 5-HT3 and P2X receptors. When receptors were
desensitized by the simultaneous application of ATP + 5-HT,
both I5-HT and IATP
were significantly decreased (Fig. 7, E-G). These
observations coupled with the fact
that I5-HT+ATP kinetics are different than the
kinetics of I5-HT or IATP
alone (Figs. 2C, 3, and 4) support the hypothesis that
I5-HT+ATP is carried through both
5-HT3 and P2X channels.
|
Current occlusion was not observed for outward currents.
Current occlusion was not observed at +40 mV (Fig.
8). Thus, at this potential,
I5-HT+ATP was very similar to
Iexpected in six experiments, and the
averages of these currents were not significantly different. At
this membrane potential, I5-HT+ATP (1,111 ± 298 pA) was significantly (P < 0.05) larger than
the current induced by the most effective transmitter (896 ± 250 pA).
|
Role of Ca2+,
Mg2+, and
Cs+ in the current occlusion.
P2X receptors are permeable to Ca2+, suggesting that the
inward current occlusion might be mediated by a raise in the
intracellular concentration of this ion. Against this hypothesis,
however, the current occlusion was still observed in total absence of
Ca2+ in the extracellular and intracellular media (Fig.
9). These experiments were carried out
with the standard extracellular solution but containing no
Ca2+ and 50 µM EGTA and with the standard intracellular
solution + 5 mM BAPTA. The presence of Mg2+ inside the
pipette was also not required for current occlusion, because it was
still observed after this ion was replaced by lithium (n = 7). In these experiments,
I5-HT+ATP (2,315 ± 345 pA) was
significantly lower (P < 0.01) than
Iexpected (
3,433 ± 376 pA). The presence
of Cs+ was also not required for current occlusion, because
it was still observed after this ion was replaced with an equimolar
concentration of Na+ (n = 3) in both the
external and the pipette solutions. In these experiments
I5-HT+ATP (
3,338 ± 628 pA) was
significantly lower (P < 0.05) than
Iexpected (
4,767 ± 933 pA).
|
Role of protein phosphorylation and G proteins in the current occlusion. ATP and 5-HT are also known to activate metabotropic receptors in enteric neurons (4, 12, 19), and their activation might lead to activation of G proteins, changes in second messengers, and protein phosphorylation, which could be responsible for the current occlusion observed here. The following observations, however, do not support this hypothesis.
Current occlusion was still observed after inhibiting protein phosphorylation by either lowering the temperature to 8°C, by replacing ATP of the internal solution with its less hydrolysable analog ![]() |
DISCUSSION |
---|
![]() ![]() ![]() ![]() ![]() ![]() ![]() |
---|
This is the first demonstration that in guinea pig submucosal neurons, activation of 5-HT3 and P2X receptors/channels is not independent and ionic currents carried through them are occluded when they are simultaneously activated. This current occlusion is observed as soon as currents are activated, is voltage dependent, and does not require Ca2+, Mg2+, G protein activation, or protein phosphorylation. Currents induced by the simultaneous application of both agonists are carried through both 5-HT3 and P2X channels. Altogether, these observations suggest that this current occlusion is mediated by cross-inhibition and by a direct interaction between 5-HT3 and P2X receptors/channels. Similar interactions to the ones proposed here have been shown between nACh and P2X receptors (6, 25, 39), between GABAA and P2X receptors (22, 36), between dopamine (D2) and somatostatin (SSTR5) receptors (33), and between dopamine (D5) GABAA channels (28), suggesting that interactions between receptor proteins play and important role in neuronal signaling.
Activation of 5-HT3 and P2X receptors/channels is not independent. The whole cell currents induced by either 5-HT (1 mM) or ATP (1 mM), despite these relatively high concentrations, are mediated by pharmacologically distinct receptors. This is demonstrated by the specific inhibitory effect of tropisetron [or ondansetron; 5-HT3 receptor antagonists (20)] and PPADS [a P2X receptor antagonist (7)] on I5-HT and IATP, respectively. The fact that the amplitudes of these two currents are independent of each other in the recorded neurons indicates that these ligand-gated channels can be expressed independently in these neurons.
Activation of these channels is, however, not independent, as shown by the fact that inward currents carried through 5-HT3 and P2X receptors were not additive when maximally activated. A simple explanation of this current occlusion would be the existence of one single channel gated by both neurotransmitters. Our results obtained at a holding potential of +40 mV rule out this hypothesis. At this potential, the currents induced by simultaneous application of 5-HT + ATP are similar to the sum of the currents induced by individual applications of 5-HT or ATP. Therefore, IATP and I5-HT must be carried through different populations of channels; otherwise, they would not add at any potential. These results are in agreement with the fact that 5-HT3 and P2X currents have different reversal potentials, rectification properties, kinetics (present study), single-channel conductances (5, 7, 15, 38), and different molecular structures (10, 14, 16, 24, 29). The unlikely possibility that the current occlusion was due to interactions between ATP and 5-HT molecules is ruled out by the following observations. First, in neurons in which IATP had been previously blocked with PPADS or with no initial response to ATP, I5-HT+ATP had virtually the same amplitude and kinetics as the current induced by 5-HT alone. Second, 5-HT3 receptor desensitization prevented any 5-HT effect on IATP and similarly, desensitization of P2X receptors also prevented any ATP effect on I5-HT. Third, in cells with no initial response to 5-HT or in which I5-HT had been blocked, I5-HT+ATP had virtually the same amplitude and kinetics as IATP. These observations also rule out the possibility that 5-HT blocks P2X channels or that ATP blocks 5-HT3 channels. An effect that was demonstrated before for 5-HT on nACh channels of submucosal neurons (8).Currents induced by the simultaneous application of both agonists are carried through both 5-HT3 and P2X channels. At least four different observations indicate that I5-HT+ATP is carried through both 5-HT3 and P2X channels and not only through one population of these channels. 1) I5-HT+ATP kinetics is clearly different than the kinetics of I5-HT or IATP alone. Thus the onset of I5-HT+ATP is faster than the onset of I5-HT but slower than IATP. I5-HT+ATP desensitizes slower than I5-HT but more rapidly than IATP. Furthermore, the decay of I5-HT+ATP resembles the decay of both I5-HT and IATP. 2) I5-HT+ATP desensitization kinetics resemble I5-HT when P2X receptors are inhibited with PPADS and resemble IATP when 5-HT3 receptors were inhibited. 3) When ATP + 5-HT are applied simultaneously, both 5-HT3 and P2X receptors are desensitized, whereas no cross-desensitization is observed when 5-HT3 and P2X receptors are desensitized individually. 4) In experiments in which one of the individual currents is significantly larger than the current induced by the other transmitter, I5-HT+ATP amplitude is the same as that of the current induced by the most effective neurotransmitter and is significantly larger than the weaker transmitter. These observations imply that I5-HT+ATP is carried through both 5-HT3 and P2X channels and that inhibition between these channels is reciprocal.
Cross-inhibition between 5-HT3 and P2X channels is
mediated by a direct interaction between these receptors.
Enteric neurons also express metabotropic ATP and 5-HT receptors, which
are known to be linked to G proteins (4, 12, 19), second
messengers, and protein phosphorylation. Several of our observations,
however, indicate that activation of these receptors is not required
for the current occlusion observed here. First, current occlusion
occurs as soon as 5-HT3 and P2X channels are activated,
indicating that this occlusion is as fast as the activation of these
ligand-gated channels. Second, PPADS prevents the effects of ATP on the
5-HT3 channels (present study), but it does not block the
slow depolarization mediated by P2Y receptors (4).
Finally, tropisetron and ondansetron (5-HT3 receptor
antagonists) prevented the effect of 5-HT on the P2X channels. Third,
inhibition of G proteins (with N-ethylmaleimide),
functional modification of G proteins (by replacing GTP with GTP--S
in the pipette solution), and inhibition of protein phosphorylation
(with staurosporine, K-252, or genistein) do not modify the occlusion
observed between I5-HT and
IATP. Fourth, this occlusion is still present
during experiments carried out at 8°C or when ATP was substituted by its less hydrolysable analog
,
-methylene ATP.
Functional implications for these channel interactions. At least some of the current experimental information suggests that inhibitory interactions between ligand-gated channels might be a widely used mechanism to limit the ionic currents through the cellular membrane. Thus a functional interaction such as the one demonstrated here has been shown to exist between P2X and nicotinic channels in enteric neurons (6, 39) and between P2X and other ligand-gated channels (GABAA) in enteric (22) and in root ganglion neurons (36). This does not appear to be always the case, because according to Zhou and Galligan (39), P2X receptors do not interact with GABAA or 5-HT3 receptors in myenteric neurons; however, see Ref. 22.
The role of P2X and the fact that ATP, the endogenous agonist for most P2X receptors, has been shown to be coreleased with various neurotransmitters, including GABA (21), ACh, and noradrenaline (11), suggests the hypothesis that the inhibitory interactions between ligand-gated channels might play an important modulator role for the synaptic transmission. In the submucosal plexus, fast synaptic potentials mediated by 5-HT and ATP have not yet been reported, and, therefore, the role of these interactions in the synaptic transmission of this plexus remains speculative. In other cells, e.g., mast cells (13, 30), ATP and 5-HT can be coreleased, suggesting that the interactions between P2X receptors and 5-HT could play a role in some paracrine effects of these substances. These interactions will limit the excitatory effect when these substances are simultaneously release; particularly, when they reach maximal concentrations. A more general functional significance of the inhibitory interaction between ligand-gated channels would be the saving of cellular energy by limiting ion movements through the cellular membrane. In conclusion, our results indicate that there is a very fast inhibitory interaction between 5-HT3 and P2X channels. These interactions occur as fast as the activation of 5-HT3 and P2X channels, supporting the hypothesis that these receptors are located very close to each other in the neuronal membrane, perhaps forming functional units constituted by at least one channel of each type. A general functional significance of these interactions would be to limit the expenditure of cellular energy by limiting ion movements through the cellular membrane, which might decrease the likelihood of neurotransmitter excitotoxicity. The putative role in synaptic transmission of these interactions would require direct demonstration. ![]() |
ACKNOWLEDGEMENTS |
---|
This work was supported by the Canadian Institutes of Health Research (MOP 36438). C. Barajas-López was supported by the Ontario Ministry of Health (Career Scientist Award #04500).
![]() |
FOOTNOTES |
---|
Address for reprint requests and other correspondence: C. Barajas-López, Botterell Hall, Ninth Floor, Queen's Univ., Kingston, Ontario, Canada K7L 3N6 (E-mail: barajasc{at}meds.queensu.ca).
The costs of publication of this article were defrayed in part by the payment of page charges. The article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.
June 5, 2002;10.1152/ajpgi.00054.2002
Received 11 February 2002; accepted in final form 8 May 2002.
![]() |
REFERENCES |
---|
![]() ![]() ![]() ![]() ![]() ![]() ![]() |
---|
1.
Allende, G,
Franco R,
Mallol J,
Lluis C,
and
Canela EI.
N-ethylmaleimide affects agonist binding to A1 adenosine receptors differently in the presence than in the absence of ligand.
Biochem Biophys Res Commun
181:
213-218,
1991[ISI][Medline].
2.
Barajas-López, C.
Adenosine reduces the potassium conductance of guinea pig submucosal plexus neurons by activating protein kinase A.
Pflügers Arch
424:
410-415,
1993[ISI][Medline].
3.
Barajas-López, C.
Inhibitory interactions between P2X and 5-HT3 channels.
Proc Soc Neurosci
28:
811,
1998.
4.
Barajas-López, C,
Espinosa-Luna R,
and
Christofi FL.
Changes in intracellular Ca2+ by activation of P2 receptors in submucosal neurons in short-term cultures.
Eur J Pharmacol
409:
243-257,
2000[ISI][Medline].
5.
Barajas-López, C,
Espinosa-Luna R,
and
Gerzanich V.
ATP closes a potassium and opens a cationic conductance through different receptors in neurons of guinea pig submucous plexus.
J Pharmacol Exp Ther
268:
1397-1402,
1994[Abstract].
6.
Barajas-López, C,
Espinosa-Luna R,
and
Zhu Y.
Functional interactions between nicotinic and P2X channels in short-term cultures of guinea-pig submucosal neurons.
J Physiol
513:
671-683,
1998
7.
Barajas-López, C,
Huizinga JD,
Collins SM,
Gerzanich V,
Espinosa-Luna R,
and
Peres AL.
P2x-purinoceptors of myenteric neurones from the guinea-pig ileum and their unusual pharmacological properties.
Br J Pharmacol
119:
1541-1548,
1996[Abstract].
8.
Barajas-López, C,
Karanjia R,
and
Espinosa-Luna R.
5-Hydroxytryptamine and atropine inhibit nicotinic receptors in submucosal neurons.
Eur J Pharmacol
414:
113-123,
2001[ISI][Medline].
9.
Barajas-López, C,
Peres AL,
and
Espinosa-Luna R.
Cellular mechanisms underlying adenosine actions on cholinergic transmission in enteric neurons.
Am J Physiol Cell Physiol
271:
C264-C275,
1996
10.
Brake, AJ,
Wagenbach MJ,
and
Julius D.
New structural motif for ligand-gated ion channels defined by an ionotropic ATP receptor.
Nature
371:
519-523,
1994[ISI][Medline].
11.
Burnstock, G.
Purines and cotransmitters in adrenergic and cholinergic neurones.
Prog Brain Res
68:
193-203,
1986[ISI][Medline].
12.
Christofi, FL,
Guan Z,
Wood JD,
Baidan LV,
and
Stokes BT.
Purinergic Ca2+ signaling in myenteric neurons via P2 purinoceptors.
Am J Physiol Gastrointest Liver Physiol
272:
G463-G473,
1997
13.
Church, MK,
Hughes PJ,
and
Vardey CJ.
Studies on the receptor mediating cyclic AMP-independent enhancement by adenosine of IgE-dependent mediator release from rat mast cells.
Br J Pharmacol
87:
233-242,
1986[Abstract].
14.
Davies, PA,
Pistis M,
Hanna MC,
Peters JA,
Lambert JJ,
Hales TG,
and
Kirkness EF.
The 5-HT3B subunit is a major determinant of serotonin-receptor function.
Nature
397:
359-363,
1999[ISI][Medline].
15.
Derkach, V,
Surprenant A,
and
North RA.
5-HT3 receptors are membrane ion channels.
Nature
339:
706-709,
1989[ISI][Medline].
16.
Dunn, PM,
Zhong Y,
and
Burnstock G.
P2X receptors in peripheral neurons.
Prog Neurobiol
65:
107-134,
2001[ISI][Medline].
17.
Evans, RJ,
Derkach V,
and
Surprenant A.
ATP mediates fast synaptic transmission in mammalian neurons.
Nature
357:
503-505,
1992[ISI][Medline].
18.
Garcia-Colunga, J,
and
Miledi R.
Effects of serotonergic agents on neuronal nicotinic acetylcholine receptors.
Proc Natl Acad Sci USA
92:
2919-2923,
1995[Abstract].
19.
Gershon, MD.
Review article: roles played by 5-hydroxytryptamine in the physiology of the bowel.
Aliment Pharmacol Ther
13, Suppl2:
15-30,
1999[ISI][Medline].
20.
Hoyer, D,
Clarke DE,
Fozard JR,
Hartig PR,
Martin GR,
Mylecharane EJ,
Saxena PR,
and
Humphrey PP.
International Union of Pharmacology classification of receptors for 5-hydroxytryptamine (serotonin).
Pharmacol Rev
46:
157-203,
1994[Abstract].
21.
Jo, YH,
and
Schlichter R.
Synaptic corelease of ATP and GABA in cultured spinal neurons.
Nat Neurosci
2:
241-245,
1999[ISI][Medline].
22.
Karanjia, R,
Zhu Y,
Espinosa-Luna R,
and
Barajas-López C.
Inhibitory interactions between P2X and GABAA channels.
Proc Soc Neurosci
31:
602,
2001.
23.
Kase, H,
Iwahashi K,
Nakanishi S,
Matsuda Y,
Yamada K,
Takahashi M,
Sato A,
and
Kaneko M.
K-252 compounds, novel and potent inhibitors of protein kinase C and cyclic nucleotide-dependent protein kinases.
Biochem Biophys Res Commun
142:
436-440,
1987[ISI][Medline].
24.
Khakh, BS,
Burnstock G,
Kennedy C,
King BF,
North RA,
Seguela P,
Voigt M,
and
Humphrey PP.
International union of pharmacology. XXIV. Current status of the nomenclature and properties of P2X receptors and their subunits.
Pharmacol Rev
53:
107-118,
2001
25.
Khakh, BS,
Zhou X,
Sydes J,
Galligan JJ,
and
Lester HA.
State-dependent cross-inhibition between transmitter-gated cation channels.
Nature
406:
405-410,
2000[ISI][Medline].
26.
Kimball, BC,
and
Mulholland MW.
Neuroligands evoke calcium signaling in cultured myenteric neurons.
Surgery
118:
162-169,
1995[ISI][Medline].
27.
Le, KT,
Babinski K,
and
Seguela P.
Central P2X4 and P2X6 channel subunits coassemble into a novel heteromeric ATP receptor.
J Neurosci
18:
7152-7159,
1998[Abstract].
28.
Liu, F,
Wan Q,
Pristupa ZB,
Yu XM,
Wang YT,
and
Niznik HB.
Direct protein-protein coupling enables cross-talk between dopamine D5 and gamma-aminobutyric acid A receptors.
Nature
403:
274-280,
2000[ISI][Medline].
29.
Maricq, AV,
Peterson AS,
Brake AJ,
Myers RM,
and
Julius D.
Primary structure and functional expression of the 5-HT3 receptor, a serotonin-gated ion channel.
Science
254:
432-437,
1991[ISI][Medline].
30.
Marquardt, DL,
Gruber HE,
and
Wasserman SI.
Adenosine release from stimulated mast cells.
Proc Natl Acad Sci USA
81:
6192-6196,
1984[Abstract].
31.
O'Dell, TJ,
Kandel ER,
and
Grant SG.
Long-term potentiation in the hippocampus is blocked by tyrosine kinase inhibitors.
Nature
353:
558-560,
1991[ISI][Medline].
32.
Robertson, SJ,
Ennion SJ,
Evans RJ,
and
Edwards FA.
Synaptic P2X receptors.
Curr Opin Neurobiol
11:
378-386,
2001[ISI][Medline].
33.
Rocheville, M,
Lange DC,
Kumar U,
Patel SC,
Patel RC,
and
Patel YC.
Receptors for dopamine and somatostatin: formation of hetero-oligomers with enhanced functional activity.
Science
288:
154-157,
2000
34.
Shapiro, MS,
Wollmuth LP,
and
Hille B.
Modulation of Ca2+ channels by PTX-sensitive G-proteins is blocked by N-ethylmaleimide in rat sympathetic neurons.
J Neurosci
14:
7109-7116,
1994[Abstract].
35.
Silinsky, EM,
and
Gerzanich V.
On the excitatory effects of ATP and its role as a neurotransmitter in coeliac neurons of the guinea-pig.
J Physiol
464:
197-212,
1993[Abstract].
36.
Sokolova, E,
Nistri A,
and
Giniatullin R.
Negative cross talk between anionic GABAA and cationic P2X ionotropic receptors of rat dorsal root ganglion neurons.
J Neurosci
21:
4958-4968,
2001
37.
Sugita, S,
Shen KZ,
and
North RA.
5-Hydroxytryptamine is a fast excitatory transmitter at 5-HT3 receptors rat amygdala.
Neuron
8:
199-203,
1992[ISI][Medline].
38.
Zhou, X,
and
Galligan JJ.
P2X purinoceptors in cultured myenteric neurons of guinea-pig small intestine.
J Physiol
496:
719-729,
1996[Abstract].
39.
Zhou, X,
and
Galligan JJ.
Non-additive interaction between nicotinic cholinergic and P2X purine receptors in guinea-pig enteric neurons in culture [see comments].
J Physiol
513:
685-697,
1998