Olfactory chamber

A further advantage in using fish is anatomical. In air-breathing vertebrates the olfactory chamber extends from the respiratory airway in most fish, however, it is a separate organ divorced from respiratory functions. This feature, and the presence of an aqueous medium, allows us to place a conductivity electrode at the inlet and one at the outlet of the nasal chamber. If electrolytes are used as odorants, their arrival and departure from the chamber can then be measured by conductivity changes. Since conductivity is proportional to concentration we can specify odorant concentration, within known limits, close to the receptors - something which cannot be done with the Intact nasal chamber in air-breathing vertebrates. It is also possible to deliver the odorant in a way that closely imitates that in which it normally arrives ( ). 

Figure l Auburn University olfactory test chamber. 

Examine the head, upper and lower jaws and lips, snout, naris, diagrams and relevant descriptions correspond. Nasolabial sul-cus/cleft, nasal cavity and septum, oral cavity, palate, palatine ridges, incisors, cranium, pinna, eyelid, eye/lens, retina, cornea, vitreous and aqueous chambers, nasopharynx, olfactory lobe, cerebral hemispheres, lateral ventricles, cranial nerves, third ventricle, pituitary, pineal gland, thalamus, perimeningeal space, and internal ear. 

The sacculus is an invagination of the antennal epithelium that consists of three chambers (Itoh et al., 1991 Shanbhag et al., 1995). It contains a few more types of sensilla, some of which have no pores and are likely to contain thermohygroreceptive neurons. The sensilla that contain olfactory neurons are all of the dw category, C sensilla similar to those on the surface. Due to the relative inaccessibility of these sensilla nothing is known about their physiology. 

Of significant interest are the most recent studies in rats that evaluate the thymus-reproductive and thymus-stress axes. Both in vitro administration of thymosin P4 into the medial basal hypothalamus and pituitary in chambers and in vivo administration into the cerebrovascular system have induced elevations of LH in media and serum, respectively (Rebar et al., 1981b Hall et al., 1983). Localization studies have demonstrated that the rat olfactory bulb has the highest concentration of thymosin 4, although it is also present in several other distinct sites (Hannappel et al., 1982). On the other hand, Toj did not influence levels of LH. Nevertheless, Ta is also present in the brain (Hall et al., 1982 Palaszynski et al., 1983), with its highest concentrations in the subcortical nuclei involved with both the autonomic and neuroendocrine system. These sites include both the pituitary gland and the hypothalamus. When injected intracranially, thymosin otj stimulated a rise in serum corticosterone in mice (Hall et al., 1982). The increase was rapid (<3 hours) and did not occur in experiments where the peptide was incubated with cultured adrenal fasciculata cells (Vahouny et al., 1983). 

All these data indicate that, at least in laboratory conditions, adult females are consistently attracted by chemical cues excreted in urine by adult males. There is some evidence, however, that male mouse urine is attractive only for sexually mature females. In one study it was shown that adult females were more attracted to male urine than to female urine, while prepubertal females were more attracted to female urine (Coppola O Connell, 1988). This finding is confirmed by other assays, using preference chambers and stick-chewing tests prepubertal females avoided male signals, but at puberty they began to prefer adult male odors. The opposite pattern of responses was elicited by female olfactory signals, that is, female urine is preferred by females during the prepubertal period and are avoided by adults (Drickamer, 1989). 

If at any time during the test, the subject detects the banana-like odor of lAA, the test is failed. The subject shall quickly exit from the test chamber and leave the test area to avoid olfactory fatigue. 

The whole procedure, from the time the animals brain is removed until the time the slices are placed in ACSF, usually requires 2.5—5 min. However, the timing does not appear to be crucial since viable slices can also be obtained from the hippocampus in the second half of the brain, some 5-7 min after the first set of slices are prepared. When judged by the quality of intracellular and extracellular responses obtained from pyramidal cells in the CAl region, it makes no obvious difference whether the ACSF temperature during preparation is 4, 20, or 36°C. The environment in the recording chamber is usually kept at 33-37°C. At this temperature slices can be maintained for some 10-15 hr. Others, using thicker brain slices of the olfactory cortex, have worked at a chamber temperature of 25°C (Harvey et ai, 1974 Scholfield, 1978a,6 Nicoll et ai, 1980). 

In an early examination of the role of olfaction in toad orientation, Martof (1962) conditioned toads Bufo fowleri) to move onto a block by repeatedly shocking them. Five of the toads that learned to move onto the block were chosen for conditioning in an olfactometer. A current of air passed through an experimental chamber where toads were placed and were subjected to an odorant. Ten seconds after this exposure toads were shocked and conditioned to move onto a block to avoid the shock. The main odorant used was 2,2,4-trimethylpen-tane. Prior to conditioning some toads were rendered anosmic by severing the olfactory tracts. 

Several important control procedures were used in these experiments. First, the experimenters severed the olfactory nerves in some of their subjects to assess olfactory contributions to responding. Second, responses to presentations of filtered air were recorded in addition to responses to presentations of odors. This permitted investigators to distinguish responses mediated by changes in airflow (reflecting the operation of the apparatus) from responses elicited by odor stimuli. Third, at the end of each session, the darkened test chamber was suddenly illuminated to measure the responsiveness of birds to a stimulus with known arousal value. 

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