Concentration and Membrane Fluidity Dependence of Odor Discrimination in the Turtle Olfactory System

In the present study, we examined the concentration dependenceof odor discrimination in turtle olfactory bulbar responsesusing the cross-adaptation technique. In the odorant pairs withdiverse molecular structures, the degree of discrimination wasunchanged or only slightly decreased with an increase in odorantconcentrations, suggesting that odorants are well discriminatedeven at high concentrations. In the odorant pairs with closelyrelated molecular structures, the degree of discrimination wasdecreased with an increase in odorant concentrations. An increasein the temperature of turtle olfactory epithelium also decreasedthe ability to discriminate these odorants. There was a goodcorrelation between changes in the odor discriminating abilityinduced by an increase in odor concentrations and those inducedby a temperature increase. The liposomes were made of lipidsextracted from the turtle olfactory epithelia and changes oftheir membrane fluidity induced by adsorption of odorants weremonitored with DPH. There was a good correlation between a decreasein odor discriminating ability and the membrane fluidity changesinduced by odorants. We suggest that decreases in odor discriminatingability induced either by an increase in odor concentrationor by a temperature increase are ultimately caused by changesin the membrane fluidity. Chem. Senses 22: 553–563, 1997.     

Protease-sensitive urinary pheromones induce region-specific Fos-expression in rat accessory olfactory bulb.

Vomeronasal organs of female Wistar rats were exposed with sprayed urine preparations of male Wistar rats prior to sacrifice. Exposure to crude urine and ultrafiltrated urine preparation (<5000 Da) induced significant Fos expression, which is correlated with cellular activity, in the mitral/tufted cell layer of the accessory olfactory bulb (AOB), while exposure to the remaining substances after ultrafiltration (>5000 Da) and control salt solution did not. Exposure to urine preparation treated with papain induced expression of Fos-immunoreactive cells in the rostral region of the AOB, but did not induce such expression in the caudal region. Exposure to urine preparation treated with pronase induced urine-specific Fos immunoreactivity neither in the rostral nor in the caudal region. These results suggest that at least two different peptides carrying pheromonal activities are contained in male Wistar rat urine.     

Pyrazine Analogues Are Active Components of Wolf Urine That Induce Avoidance and Freezing Behaviours in Mice

Background

The common grey wolf (Canis lupus) is found throughout the entire Northern hemisphere and preys on many kinds of mammals. The urine of the wolf contains a number of volatile constituents that can potentially be used for predator–prey chemosignalling. Although wolf urine is put to practical use to keep rabbits, rodents, deer and so on at bay, we are unaware of any prior behavioural studies or chemical analyses regarding the fear-inducing impact of wolf urine on laboratory mice.

Methodology/Principal Findings

Three wolf urine samples harvested at different times were used in this study. All of them induced stereotypical fear-associated behaviors (i.e., avoidance and freezing) in female mice. The levels of certain urinary volatiles varied widely among the samples. To identify the volatiles that provoked avoidance and freezing, behavioural, chemical, and immunohistochemical analyses were performed. One of the urine samples (sample C) had higher levels of 2,6-dimethylpyrazine (DMP), trimethylpyrazine (TMP), and 3-ethyl-2,5-dimethyl pyrazine (EDMP) compared with the other two urine samples (samples A and B). In addition, sample C induced avoidance and freezing behaviours more effectively than samples A and B. Moreover, only sample C led to pronounced expression of Fos-immunoreactive cells in the accessory olfactory bulb (AOB) of female mice. Freezing behaviour and Fos immunoreactivity were markedly enhanced when the mice were confronted with a mixture of purified DMP, TMP, and EDMP vs. any one pyrazine alone.

Conclusions/Significance

The current results suggest that wolf urinary volatiles can engender aversive and fear-related responses in mice. Pyrazine analogues were identified as the predominant active components among these volatiles to induce avoidance and freezing behaviours via stimulation of the murine AOB.     

Hard-Diet Feeding Recovers Neurogenesis in the Subventricular Zone and Olfactory Functions of Mice Impaired by Soft-Diet Feeding

The subventricular zone (SVZ) generates an immense number of neurons even during adulthood. These neurons migrate to the olfactory bulb (OB) and differentiate into granule cells and periglomerular cells. The information broadcast by general odorants is received by the olfactory sensory neurons and transmitted to the OB. Recent studies have shown that a reduction of mastication impairs both neurogenesis in the hippocampus and brain functions. To examine these effects, we first measured the difference in Fos-immunoreactivity (Fos-ir) at the principal sensory trigeminal nucleus (Pr5), which receives intraoral touch information via the trigeminal nerve, when female adult mice ingested a hard or soft diet to explore whether soft-diet feeding could mimic impaired mastication. Ingestion of a hard diet induced greater expression of Fos-ir cells at the Pr5 than did a soft diet or no diet. Bromodeoxyuridine-immunoreactive (BrdU-ir) structures in sagittal sections of the SVZ and in the OB of mice fed a soft or hard diet were studied to explore the effects of changes in mastication on newly generated neurons. After 1 month, the density of BrdU-ir cells in the SVZ and OB was lower in the soft-diet-fed mice than in the hard-diet-fed mice. The odor preferences of individual female mice to butyric acid were tested in a Y-maze apparatus. Avoidance of butyric acid was reduced by the soft-diet feeding. We then explored the effects of the hard-diet feeding on olfactory functions and neurogenesis in the SVZ of mice impaired by soft-diet feeding. At 3 months of hard-diet feeding, avoidance of butyric acid was reversed and responses to odors and neurogenesis were recovered in the SVZ. The present results suggest that feeding with a hard diet improves neurogenesis in the SVZ, which in turn enhances olfactory function at the OB.     

Responses to putative second messengers and odorants in water nose olfactory neurons of Xenopus laevis

Using the whole-cell mode of the patch-clamp technique, we attempted to record inward currents in response to cAMP, inositol 1,4, 5-trisphosphate (IP(3)) and odorants from sensory neurons in the olfactory epithelium of the Xenopus laevis lateral diverticulum (water nose). Dialysis of 100 microM of IP(3) induced inward currents, while dialysis of 1 mM of cAMP into olfactory neurons did not induce any response under the voltage-clamp conditions. Changes in membrane conductance were examined by applying ramp pulses. The slope of the current-voltage (I-V) curve during the IP(3)-induced response was steeper than that after the response, indicating that IP(3) increased the membrane conductance. The water nose olfactory neurons have been shown to respond to both amino acids and volatile odorants. The slopes of I-V curves during responses to amino acids and a volatile odorant, lilial, were similar to those before the responses, suggesting that the total membrane conductance was not changed during responses to amino acids and the volatile odorant.