Post-tetanic potentiation at the nerve-muscle junction in the longitudinal muscle of the guinea-pig ileum. Possible role of substance P

The effect of substance P on nerve action potential propagation and cholinergic transmission in the myenteric plexus of the guinea-pig ileum

The effect of substance P on nerve terminals in myenteric plexus of the guinea-pig ileum was investigated. Neurogenic twitches of the myenteric plexus longitudinal muscle strip were recorded. Twitches of the strip portion where excitation involved the most distal parts of cholinergic nerve terminals were more increased by local application of substance P (0.1 and 0.4 nmol/l) than twitches of the portion where excitation involved both distal and proximal parts of nerve terminals. Substance P addition to a portion of the strip conducting nerve action potentials to invade the neighbouring strip portion also augmented twitches of the latter portion so that the interference with the propagation process was considered. The effect of substance P was poorly antagonized by the addition of a substance P antagonist, (D-Arg1, D-Pro2, D-Trp7,9, Leu11)-substance P. Compound nerve action potentials were evoked in strands of fibres of the myenteric plexus by low-frequency train stimulation (1 Hz). The addition of substance P prevented a decrease of the amplitude of responses observed under control conditions. Using high-frequency train stimulation (30 Hz) the amplitude of responses to impulses 2-7 was augmented over that to the first impulse; substance P further increased such facilitation regularly. It seems that substance P might promote nerve action potential invasion of the distal parts of nerve terminals.

The topographical basis of cholinergic transmission in guinea-pig ileum myenteric plexus

Myenteric plexus-longitudinal muscle strips were used to study nerve action potential propagation and transmission and their differences between the proximal and the distal regions of cholinergic terminals. Neurogenic twitches of a portion of the strip were evoked by focal electrical stimulation. Twitches mediated by the distal regions of cholinergic nerve terminals were more influenced by drugs affecting Ca2+ "utilization" (Bay K 8644, kappa opiate ligand ethylketocyclazocine, changes in extracellular Ca2+ or Co2+ concentration) in contrast to twitches mediated by proximal regions of these terminals which were more influenced by drugs affecting sodium-potassium spike (tetrodotoxin, dendrotoxin, 4-aminopyridine, tetraethylammonium). Post-tetanic potentiation of twitches was prominent with that portion of the strip where the distal regions of nerve terminals were involved. Drugs interfering with Na+/K+ spikes indiscriminately influenced both the twitch height and post-tetanic potentiation whereas changes in extracellular Ca2+ concentration affected selectively only post-tetanic potentiation. Release of [3H]acetylcholine from pre-labelled strips evoked by 1 Hz continuous stimulation or by train stimulation at 30 Hz was measured selectively from portions containing either proximal and distal or only distal regions of nerve terminals. The release from portions containing the distal regions was relatively higher when evoked by 30 Hz than by 1 Hz. The distal regions of nerve terminals might be thus recruited to participate in transmission by a frequency-dependent process. Nerve impulses were recorded from strands of nerve fibres in the myenteric plexus. At 1 and 5 mm distance from the stimulation focus nerve impulses were completely suppressed by tetrodotoxin. At 5 mm, in some strands the amplitude of nerve impulses was also subject to the effect of drugs affecting Ca2+ "utilization"; facilitation of nerve impulse amplitude during 30 Hz train stimulation was always influenced by drugs affecting Ca2+ "utilization". Propagation of nerve impulses in the distal region of cholinergic nerve terminals was found to be Ca-sensitive and frequency-dependent; this might form the basis for facilitation and post-tetanic potentiation of muscarinic transmission.

The topography of cholinergic transmission in the mechanism of drug action at muscarinic synapses of the guinea-pig ileum

The pharmacology of cholinergic neurogenic responses evoked by the participation of only the endings of axon terminals was compared to that of responses evoked by participation of the more proximal parts of the terminals also. Myenteric plexus-longitudinal muscle strips of the guinea-pig ileum were drawn through narrow orifices in 2 rubber membranes dividing a bath into 3 separate compartments. Oral segments were stimulated electrically by single impulses or by trains and local neurogenic contractions were evoked. The contractions of the aboral segment due to nerve impulses transmitted from the oral segment via the middle segment were also recorded. The opioid ligands ketocyclazocine and [D-Ala2,MePhe4,Met(O)5-ol]enkephalin and noradrenaline inhibited the twitches of the aboral segment evoked by oral segment stimulation more than the local twitches of the oral segment when these agents were applied directly to the respective compartments. The twitches of the aboral segment were also inhibited by the application of these drugs into the middle compartment adjusted to 10 mm width. Verapamil and the alkaline earth metal ions cobalt and lanthanum had similar effects. 4-Aminopyridine increased twitch amplitude more in the aboral segment than in the oral segment when applied directly; similar effects in the aboral segment were seen when the agents were applied to the middle compartment. The action of atropine, papaverine, d-tubocurarine and prostigmine did not discriminate between twitches in the oral and aboral segment when applied directly and all drugs except prostigmine were without effect when applied to the middle compartment.(ABSTRACT TRUNCATED AT 250 WORDS)

Post-tetanic contractile events further support the interaction of multiple neurotransmitters in the neuroeffector junction of the rat vas deferens

Post-tetanic events were recorded in isolated, superfused, epididymal and prostatic halves of the rat vas deferens. Increasing the frequency of nerve stimulation from 0.15 to 15 Hz (1-30 s) and then back to 0.15 Hz produced a post-tetanic potentiation (PTP) of the muscular responses in the epididymal end but a post-tetanic inhibitory response (PTI) in the prostatic half. Both effects were abolished by tetrodotoxin or animal pretreatment with 6-hydroxy dopamine (6-OHDA). PTP was markedly reduced by reserpine treatment or tissue incubation with prazosin. PTI was not altered by adrenergic drugs but partially reduced by tissue application of bicuculline or strychnine, revealing that gamma-aminobutyric acid (GABA) may modulate the motor transmission towards the prostatic half of the rat ductus.

Interactions between the duration of stimulation and noradrenaline on cholinergic transmission in the myenteric plexus-smooth muscle preparation

Output of acetylcholine (ACh), electromyogram (EMG) recordings and contractions of myenteric plexus-longitudinal muscle strip preparations from the guinea-pig ileum were studied during stimulation by single impulses or by trains (30 Hz; 2 to 128 impulses) under control conditions and in the presence of noradrenaline (NA). During supramaximal stimulation NA (2.5 microM) inhibited both contractions of the smooth muscle and the release of ACh evoked by single impulses more effectively than those evoked by train stimulation so that in a train of 4 impulses the output of ACh per impulse after the 2nd to 4th impulses was 69 to 104% higher than the output after the 1st impulse. During submaximal stimulation, contractions and ACh release evoked by single impulses were almost completely inhibited by NA. The neurogenic EMG, a direct consequence of the localized action of released transmitter (ACh), was recorded in the longitudinal muscle 4 and 10 mm aborally from the focal stimulation site. The incidence of the neurogenic response was much higher at the proximal (4 mm) than at the distal (10 mm) site and was proportional to the number of impulses in a train (100 Hz). NA inhibited propagation of the neurogenic response evoked by single impulses whereas its effect during train stimulation was less. It is concluded that in the course of train stimulation, sites of transmission more distant from the stimulation focus was recruited, and consequently the secretion of ACh in succeeding impulses was enhanced. NA could interfere with this process; it might inhibit the invasion by action potentials of cholinergic nerve terminal varicosities, thereby reducing the release of ACh.

How far from the stimulation site in myenteric plexus-longitudinal muscle preparations the neurogenic cholinergic contraction can be evoked?

Neurogenic contractions of a segment of myenteric plexus-longitudinal muscle strip preparations from the guinea-pig ileum were evoked. The site of electric stimulation was separated from the contracting segment by a gap preventing the spread of muscle action potentials set up in other regions. The width of the separating gap (2-20 mm) indicated the length of nerve fibers that could conduct impulses across and trigger cholinergic contractions behind the gap; it was more than 12 and less than 16 mm. Transmission of excitation was more effective in the aboral direction compared to the oral direction and was not apparently affected by noradrenaline nor by substance P.