Research reportFast wave activity in the rat rhinencephalon: elicitation by the odors of phytochemicals, organic solvents, and a rodent predator
Introduction
Numerous publications have described large (>1 mV) bursts of rhythmical, 15–85 Hz fast waves in the rhinencephalon of several mammalian species, including the rat 1, 2, 6, 7, 13, 14, 15, 16, 17, 20, 21, 22, 41, 45. Recently, it has been found that bursts of 15–50 Hz fast activity (average frequency ≈20 Hz) are elicited in the pyriform cortex and dentate gyrus of the rat by the odors of several organic solvents (diethyl ether, methyl methacrylate, oil of turpentine, toluene, and xylene) and synthetic components of the odors of the red fox (trimethylthiazoline) and weasel (2-propylthietane) 20, 21, 22, 41, 45. These waves are specifically olfactory since they are elicited by the flow of odorized air through the nasal passages but not by auditory, gustatory, somatosensory, or visual inputs, and have no relation to concurrent motor activity in normal or scopolamine-treated rats 22, 41, 45.
Several of the odorants that are effective in eliciting the 20-Hz fast waves in the pyriform cortex and dentate gyrus have been shown to suppress feeding in various lagomorphs and rodents (i.e., oil of camphor, 2-propylthietane, trimethylthiazoline, and toluene (10, 19, 23, 32, 33, 34, 35, 36, 42)). In contrast, the strong and unpleasant (to humans) odors of butyric acid, cadaverine, and caproic acid fail to elicit 20-Hz fast waves and do not suppress feeding in rats 19, 22, 23, 45. Thus, the easily recorded rhinencephalic fast wave response might provide an assay capable of detecting chemical compounds that would be useful in protecting orchards or reforestation plantations from predation by wild lagomorphs or rodents. In recent years, several other odorous compounds including benzyl alcohol, carvacrol, eucalyptol, isopentenylmethyl sulfide (a component of the anal gland secretion of foxes, [43]), pine oil, pure pinosylvin and pinosylvin methyl ether, and salicylaldehyde (salicaldehyde) have been used in attempts to reduce crop destruction by lagomorphs, mice, and opossums 3, 4, 8, 11, 29, 37, 44. However, the effect of these compounds on the electrical activity of olfactory-related areas in the brain has not been studied. Consequently, the present study has tested the reactions of the pyriform cortex and dentate gyrus to these and other similar compounds when presented as odorants to freely moving rats. Further, a number of additional organic solvents were tested to determine whether all members of this broad group are effective in generating rhinencephalic fast activity.
Finally, there has been a recent suggestion that the bursts of 20-Hz fast waves which occur during the presentation of organic solvents are not related to normal mechanisms involved in olfactory functions but, rather, are kindling-induced seizure-like reactions of the olfactory brain after exposure to toxic chemicals [24]. This possibility was examined in rats during brief or prolonged exposures to some organic solvents.
Section snippets
Animals
Experiments were conducted on 17 male, Long–Evans rats weighing between 364 and 804 g at experimental onset. The rats were housed individually in wire cages (on a 12:12 h light/dark cycle) and were given free access to Agway rat chow and bottled tap water. The housing environment was maintained at ≈20°C. All testing was conducted during the light phase.
Surgery
The rats were anesthetized with sodium pentobarbital (50 mg/kg, i.p.), placed in a stereotaxic device, and using the methods of Zibrowski and
Histology
Olfactory mucosa electrodes were located in the labyrinth of the ethmoid bone in 17 rats. Deep electrodes were located in 10 of the 12 rats (in the remaining two cases adequate histological material was not available) with implants in the pyriform cortex and in the five rats with dentate gyrus implants (see Fig. 1). The deep pyriform electrodes were located in the superficial or deep cell layers but surface electrode tip locations could not be identified since they lay just outside the brain.
Discussion
The major new finding of the present research is that in addition to many organic solvents and components of rodent predator odors 20, 21, 22, 41, 45, several phytochemicals including the odors of carvacrol, eucalyptol, and salicylaldehyde are capable of eliciting bursts of rhythmical 20-Hz fast waves in the rhinencephalic cortex of the rat. Oil of camphor is also moderately effective [45]. The responsitivity shown by the rat rhinencephalon to the odors of eucalyptol and salicylaldehyde, in
Acknowledgements
We thank Richard Cooley for his technical assistance; Duncan Hunter, Department of Chemistry, University of Western Ontario, for his insight and for providing many of the organic solvents used in the experiments; David Wakarchuk of Phero Tech, Delta, BC who provided benzyl alcohol, benzylgentisate, carvacrol, eucalyptol, isopentenylmethyl sulfide, pine stump extract, pinosylvin, resveratrol, and salicylaldehyde; and the Natural Sciences and Engineering Research Council of Canada which provided
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