[Bombesin-mediated non-cholinergic late slow excitatory postsynaptic potentials in guinea pig inferior mesenteric ganglion in vitro].

The effect of bombesin (BOM) on non-cholinergic excitatory synaptic transmission of the guinea pig inferior mesenteric ganglion (IMG) was investigated by intracellular recording. Repetitive stimulation of the colon nerves (1 ms, 25 Hz, 4 s) elicited a burst of action potentials, which was followed by a long-lasting depolarization in 74.3% (52/70) of the IMG neurons. The depolarization was not blocked by nicotinic (d-tubocurarine, 100 micromol/L) and muscarinic (atropine, 1 micromol/L) antagonists, but was eliminated in a low Ca(2+)/high Mg(2+) Krebs solution, indicating that the depolarization was due to the release of non-cholinergic transmitters. Superfusing the ganglia with BOM (10 micromol/L, 1 min) induced a slow depolarization in 66.5% (109/164) neurons tested. The BOM response was not appreciably changed in low Ca(2+)/high Mg(2+) Krebs solution (n=6, P>0.05), suggesting that BOM depolarized the neurons by acting directly on the postsynaptic membrane rather than via a release of other endogenous depolarizing substances. In a total of 102 cells that exhibited late slow excitatory postsynaptic potential (ls-EPSP), superfusion of the ganglia with BOM produced a membrane depolarization in 82 neurons (80%), while the remaining 20 cells (20%) exhibited no response to BOM. In 18 neurons with ls-EPSP, 4 (22%) neurons were sensitive to both BOM and SP; 6 (33%) and 5 (28%) neurons were only sensitive to BOM and SP, respectively. The remaining 3 (17%) neurons were insensitive to both BOM and SP. Membrane resistance (Rm) had no apparent change in 47.3%, 59.5 % of the neurons tested during the ls-EPSP (n=55) and BOM depolarization (n=84), respectively, but had a marked decrease in 38.2%, 27.4%, and a marked increase in the remaining 14.5%, 13.1% of the neurons. However, when the Rm change accompanying ls-EPSP was compared with that accompanying BOM depolarization (n=20) in the same neuron, the changes in Rm were always parallel. Moreover, ls-EPSP (n=6) and BOM depolarization (n=8) were all augmented by conditioning hyperpolarization. The extrapolated values of the reversal potentials of ls-EPSP and BOM depolarization were 46.0+/-8.0 and 50.0+/-7.0 mV (n=8, P>0.05), respectively. In 14 BOM-sensitive neurons, a ls-EPSP was elicited by repetitive colon nerve stimulation. Superfusion of BOM (10 micromol/L) in these cells initially caused a large depolarization and then membrane potential gradually subsided to resting level in the continuous presence of BOM. Stimulation of the presynaptic nerves at this time failed to elicit a detecable ls-EPSP in 2 neurons and induced a much smaller one in 10 cells, while the ls-EPSP in the remaining 2 neurons was not appreciably affected. On the other hand, prolonged superfusion of BOM had no effect on the amplitude and duration of ls-EPSP in 6 BOM-insensititive neurons studied (P>0.05). The amplitude and duration of SP-induced depolarization were not altered by prolonged superfusion of BOM (n=4, P>0.05) Superfusion of tyr(4) D-phe(12) bombesin (1 micromol/L, 10 15 min), a BOM receptor antagonist, did not cause any noticeable changes in passive membrane properties nor block nicotinic f-EPSPs, but markedly suppressed (n=5) or completely abolished (n=11) BOM depolarization in all 16 neurons tested Similarly, tyr(4) D-phe(12) bombesin partially or completely antagonized the ls-EPSP in 9 out of a total of BOM sensitive neurons (n=11). The ls-EPSP elicited in the remaining two neurons was insignificantly affected by this drug. However, following 10 20 min of wash with Krebs solution the ls-EPSP was reversed. In contrast, superfusion of the ganglia with tyr(4) D-phe(12) bombesin did not change the amplitude and duration (P>0.05) of ls-EPSP in 10 BOM-insensitive cells. Similarly, the amplitude and duration of SP-induced depolarization were not appreciably affected by tyr(4) D-phe(12) bombesin (n=6, P>0.05). In conclusion, our results indicate that BOM may be another transmitter mediating the ls-EPSP in the guinea pig IMG and that there is no cross-desensitization of BOM receptors and SP receptors.