Calcium entry blockade as a mechanism for chlordimeform-induced inhibition of motor activity in the isolated guinea-pig ileum

Cultured neuronal networks as environmental biosensors

Contamination of water by toxins, either intentionally or unintentionally, is a growing concern for both military and civilian agencies and thus there is a need for systems capable of monitoring a wide range of natural and industrial toxicants. The EILATox-Oregon Workshop held in September 2002 provided an opportunity to test the capabilities of a prototype neuronal network-based biosensor with unknown contaminants in water samples. The biosensor is a portable device capable of recording the action potential activity from a network of mammalian neurons grown on glass microelectrode arrays. Changes in the action potential fi ring rate across the network are monitored to determine exposure to toxicants. A series of three neuronal networks derived from mice was used to test seven unknown samples. Two of these unknowns later were revealed to be blanks, to which the neuronal networks did not respond. Of the five remaining unknowns, a significant change in network activity was detected for four of the compounds at concentrations below a lethal level for humans: mercuric chloride, sodium arsenite, phosdrin and chlordimeform. These compounds--two heavy metals, an organophosphate and an insecticide--demonstrate the breadth of detection possible with neuronal networks. The results generated at the workshop show the promise of the neuronal network biosensor as an environmental detector but there is still considerable effort needed to produce a device suitable for routine environmental threat monitoring.

Modulation of peristalsis by cannabinoid CB(1) ligands in the isolated guinea-pig ileum

The effect of cannabinoid drugs on peristalsis in the guinea-pig ileum was studied. Peristalsis was induced by delivering fluid into the oral end of an isolated intestinal segment. Longitudinal muscle reflex contraction, threshold pressure and threshold volume to trigger peristalsis, compliance of the intestinal wall during the preparatory phase (a reflection of the resistance of the wall to distension) and maximal ejection pressure during the emptying phase of peristalsis were measured. The cannabinoid agonists WIN 55,212-2 (0.3 - 300 nM) and CP55,940 (0.3 - 300 nM) significantly decreased longitudinal muscle reflex contraction, compliance and maximal ejection pressure, while increased threshold pressure and volume to elicit peristalsis. These effects were not modified by the opioid antagonist naloxone (1 microM) and by the alpha-adrenoceptor antagonist phentolamine (1 microM). The inhibitory effect of both WIN 55,212-2 and CP55,940 on intestinal peristalsis was antagonized by the cannabinoid CB(1) receptor antagonist SR141716A (0.1 microM), but not by the cannabinoid CB(2) receptor antagonist SR144528 (0.1 microM). In absence of other drugs, the CB(1) receptor antagonists SR141716A (0.01 - 1 microM) and AM281 (0.01 - 1 microM) slightly (approximatively 20%) but significantly increased maximal ejection pressure during the empty phase of peristalsis without modifying longitudinal muscle reflex contraction, threshold pressure, threshold volume to trigger peristalsis and compliance. It is concluded that activation of CB(1) receptors reduces peristalsis efficiency in the isolated guinea-pig, and that the emptying phase of peristalsis could be tonically inhibited by the endogenous cannabinoid system.

Effects of mercuric chloride and methyl mercury on cholinergic neuromuscular transmission in the guinea-pig ileum

The effects of mercuric chloride (HgCl2) and methyl mercury (MeHg) were examined on basal mechanical activity and electrically-induced neurogenic cholinergic contractions (twitch contractions) in longitudinal muscle-myenteric plexus strips from guinea-pig distal ileum. Both compounds at 0.33 microM slightly enhanced the amplitude of twitch contractions in approximately 50% preparations. This effect was probably due to facilitation of acetylcholine (ACh) release since 0.1 and 1 microM mercurials increased electrically-evoked tritium outflow from [3H]choline preloaded muscle layer with attached myenteric plexus. Conversely, higher mercury concentrations inhibited twitch contractions (HgCl2 IC50 = 21.3 +/- 6.4 microM; MeHg IC50 = 45.1 +/- 5.5 microM), as well as contractions to exogenous ACh (0.1 microM) in resting preparations, and concomitantly increased the basal tone. The former effects possibly reflected an antimuscarinic activity of mercury, while the latter was related to alterations of calcium homeostasis in the effector cells. Indeed, the effect of HgCl2 on basal tone was antagonized by the Ca2+ entry blocker nifedipine (3, 10, 30 nM), indicating Hg-induced facilitation of Ca2+ influx through voltage-dependent channels. On the whole, our results suggest that cholinergic neuromuscular transmission and Ca(2+)-dependent mechanisms underlying smooth muscle contractility are targets for mercury toxicity in the intestine.