Some laboratory observations on the toxicity and acceptability of norbormide to wild Rattus norvegicus and on feeding behaviour associated with sublethal dosing

Studies on the development of a microencapsulated delivery system for norbormide, a species-specific acute rodenticide

Norbormide, a selective rat toxicant, was microencapsulated to both mask the flavour and delay the release until after a lethal dose has been ingested by the rat. To this end, gelatine microspheres containing norbormide were made and over coated with either shellac resin or an equal mixture of shellac and Eudragit RS in a fluid-bed coating machine. The microcapsules absorb water, swell and burst to release their contents. In rats an 8 h window is available to delay the release of encapsulated material. In initial experiments, a shellac coating of 20% w/w was established as suitable for delaying the release. A capsule size range of 200-400 microm was selected, from capsule mastication experiment, for oral gavaging and feeding studies in rat. Oral gavage study has demonstrated for the first time that a substantial delay in release of a lethal dose of an acute poison has been achieved by microencapsulation. Feeding test has demonstrated that there is a fine balance between the size and density of the capsules in the bait to overcome mastication of the capsules by rats. A combination of shellac and Eudragit RS resins is a viable polymeric wall material to control the rate of penetration of water in to microcapsules.

Norbormide: a calcium entry blocker with selective vasoconstrictor activity in rat peripheral arteries

Norbormide is a unique vasoactive substance endowed with species- and tissue-specific, endothelium independent, vasoconstrictor activity that is restricted to the peripheral arteries of rat. In rat aorta and in all tested arteries of other species norbormide exhibits vasorelaxant property presumably due to the blockade of calcium channels. A calcium entry blocker effect of norbormide has also been described in isolated, perfused guinea pig hearts. In these preparations norbormide produced coronary vasodilator, as well as negative inotropic and dromotropic effects. In single ventricular myocytes of guinea pigs norbormide reduces L-type calcium current. The mechanism underlying the selective vasoconstrictor effect of norbormide is unknown. In rat caudal artery, a vessel contracted by norbormide, the drug activates phospholipase C (PLC) signal cascade which is the biochemical pathway involved in the contractile effect triggered by most receptor-activating vasoactive agents. Therefore, norbormide-induced contraction of rat peripheral vessels is likely to be due to the activation of a PLC-coupled receptor abundantly or selectively expressed in vascular smooth muscle cells. The identification of this putative receptor could facilitate the development of tissue-selective pharmacological agents.

Laboratory tests of 5-p-chlorophenyl silatrane as a rodenticide

The properties of 5-p-chlorophenyl silatrane as a rodenticide against Rattus norvegicus and Mus musculus were investigated in the laboratory. The high oral toxicity of the compound was confirmed. When the compound was given to laboratory rats and mice by stomach tube at lethal dosages, signs of poisoning were observed within a minute. When caged wild rats and mice were given a choice between plain and poisoned baits the optimum rodenticidal concentration in the bait was about 0.5% for both species, producing 50% mortality in wild rats and 95% mortality in wild mice. The results are discussed in relation to safety in use and the probable effectiveness of the compound as a rodenticide in field conditions.

A comparative trial of norbormide and zinc phosphide against Rattus norvegicus on farms

Norbormide at 1·0 and 0·5% and zinc phosphide at 5·0 and 2·5% were each tested in four types of cereal bait, after prebaiting and as direct poisons, against infestations of R. norvegicus on forty-eight farms in Montgomeryshire.

The relative success of treatments was measured by the reduction in the number of takes from wheat baits put down for 2 days, 13 or 16 days before, and 5 days after, poisoning.

Treatments with zinc phosphide were significantly more successful than those with norbormide, irrespective of the cereal bait, concentration of poison or method of treatment used, and in spite of conditions on many farms that partially restricted the distribution of the baits containing zinc phosphide.

Differences between these results and results of laboratory tests with the same rodenticides are discussed.

Norbormide is recommended for use in situations where zinc phosphide cannot be used efficiently without risk to livestock.

We are indebted to Messrs Tavolek Laboratories Limited, Slough, for the free supply of norbormide made available early in the trial; also to Messrs. Lloyd and Pritchard, who were responsible for all the treatments; to Miss E. J. Taylor and Miss P. Cullen for assistance with some of the census baiting, and to J. H. Greaves for permission to examine original data and present them in Table 3.