Sensory evidence for olfactory receptors with opposite chiral selectivity.

Human odor responses to alpha-ionone enantiomers were measured by psycho-physical methods. Surprisingly, subjects' relative sensitivities for these enantiomers were found to diverge widely, some subjects being much more sensitive to (+) than to (-) and vice versa. In 63 subjects the (+)/(-) threshold concentration ratio varied over 4 orders of magnitude. Nine subjects with extreme (+)/(-) ratios were further tested several times over a 7-month period and retained stable ratios. These same nine subjects, when tested for threshold sensitivity to carvone enantiomers, showed (+)/(-) ratios that varied only in a narrow range and did not correlate with their ionone ratios. It is suggested that odor discrimination of alpha-ionone enantiomers involves at least two receptor types of opposite chiral selectivity and that their distribution varies independently in the human population.

Odor similarity between stress-inducing odorants in wistar rats.

Odor similarity to the odor of 4-mercapto-4-methyl-2-pentanone (I) was measured on eight rats by generalization of a conditioned avoidance response tocis- andtrans-8-mercapto-p-menthan-3-one (II and III), 3-mercapto-3-methyl-2-pentanone (V), andt-amyl mercaptan (VI). Previously, these odorants had been found to induce stress in rats in an open-field situation. In the present experiment, rats generalized the avoidance response learned with I, for V, VI, and to a lesser extenttrans-isomer III, implying odor similarities;cis isomer II was discriminated. Odor similarity between mercapto ketone I and mercaptan VI is surprising since VI lacks the keto group.

Facilitation of generalization performances in spatial learning problems by posttrial stimulation of the mesencephalic reticular formation.

The effects of low posttrial stimulation of the mesenphalic reticular formation (MRF) on spatial learning problems were studied in two consecutive stages. First, the performances of stimulated (S) and no-stimulated (NS) subjects were assessed in a reconvergent three-arm maze. Secondly, the same animals were tested in a generalization paradigm, since they had to solve a similar task in a radial four-arm maze, each group being divided into newly stimulated (S-S, NS-S) or no-stimulated (S-NS, NS-NS) subjects. In the first experiment, no differences were found between the acquisition processes of groups S and NS. Conversely, in the second experiment, previously stimulated animals (S-S and S-NS) showed better generalization of the experimental rule. The results are discussed in terms of preservation of the neural activity after the early MRF stimulation. From a functional point of view, our data indicate that an enhancement of the neural activity during acquisition facilitates not only a high accessibility of memories concerning the experimental rule in a subsequent similar situation but also the capability to apply a previously elaborated response system in a more complex situation.