Toshiaki J. Hara

Neurobiology of fish olfaction: a review

The last decade saw important advances in our understanding of the olfactory system function. In some animals, we now have the basic knowledge necessary to investigate coding mechanisms employed in olfaction. So far, studies of the fish olfactory system have focused on odor detection and the early processing of olfactory information in the olfactory bulb. How this information is integrated in the forebrain is unknown. Here, we first describe the anatomy of the fish olfactory system. The problems faced when describing the anatomy of the terminal nerve complex and nucleus olfactoretinalis are highlighted. Olfactory sensory neurons are randomly distributed over the entire olfactory epithelium, a unique feature of the olfactory sense. These primary olfactory neurons converge upon their second-order targets in segregated areas of the olfactory bulb. Exchange of information occurs in the glomeruli and glomerular plexus, where primary neurons synapse on mitral cell dendrites. The spatial distribution of glomerular activity induced by odorants of different classes shows that distinct neuron populations of the olfactory bulb encode information related to different odorant groups. In most cases, these neuron populations synchronize their alternating sequences of firing and silence when stimulated by primary input. Synchronized oscillations of these second-order neurons could contain important coding information, or represent a mechanism by which learning is facilitated. Alternatively, oscillations could be solely used to shape the olfactory bulb response. The nature of the olfactory information that reaches the forebrain and decoding of this information by the central nervous system are discussed.

Olfactory responses to amino acids in rainbow trout, Salmo gairdneri

Abstract 1. 1. The olfactory bulbar responses induced when the nares were infused with solutions of amino acids and related compounds were studied in rainbow trout, Salmo gairdneri . 2. 2. The order of magnitude of the response to the eleven most effective of forty-nine amino acids tested were: l -glutamine > l -methionine > l -leucine > homoserine > l -asparagine > l -alanine > l -cystine > l -cysteine > glycine > l -serine > l -histidine. 3. 3. The threshold concentrations were estimated to range between 10 −7 and 10 −8 M for those most effective amino acids. 4. 4. The l -isomer of an amino acid was always more effective than its d -isomer. 5. 5. The correlation between stimulatory effectiveness of amino acids and their molecular structures is discussed.

Extreme olfactory sensitivity of mature and gonadally-regressed goldfish to a potent steroidal pheromone, 17α,20β-dihydroxy-4-pregnen-3-one

Summary1.Electrical responses (electro-olfactogram; EOG) were obtained from the olfactory epithelium of goldfish exposed to a variety of sex steroids thought to function as pheromones, severall-amino acids, and a bile acid. Responses of male and female goldfish with both mature and regressed gonads were compared.2.17α,20β-dihydroxy-4-pregnen-3-one (17,20P) and progesterone were potent odorants. 17,20P had a detection threshold of 10−13M and at a concentration of 10−8M evoked an EOG response two to three times that of 10−5Ml-serine. Progesterone was less stimulatory than 17,20P. This finding corroborates recent endocrinological and behavioral studies (Stacey and Sorensen 1986; Dulka et al. 1987) which indicate that 17,20P is a potent pheromone in mature male goldfish.3.Preliminary cross-adaptation experiments suggest that 17,20P is detected by receptor site(s) differing from those which detect bile acids andl-amino acids.4.The goldfish olfactory epithelium did not respond to etiocholanolone glucuronide, 17β-estradiol glucuronide, testosterone glucuronide and 17β-estradiol, which have also been hypothesized to function as pheromones in fish.5.The olfactory thresholds of goldfish tol-amino acids and taurocholic acid were similar to other species of fish: approximately 10−13M forl-serine and 10−9M forl-cysteine and taurocholic acid.6.The olfactory sensitivities of mature male, mature female and gonadally-regressed fish were similar to all compounds tested suggesting that differences in olfactory receptor function are not responsible for the fact that only mature males exhibit behavioral and endocrine responses to pheromonal 17,20P.

The characteristics of the electro-olfactogram (EOG): Its loss and recovery following olfactory nerve section in rainbow trout (Salmo gairdneri)

Electro-olfactograms (EOGs) were recorded from both sensory and non-sensory epithelia on the olfactory lamellae and from other areas within the rosette of rainbow trout (Salmo gairdneri). The trout EOG induced by amino acids was a monophasic negative voltage composed of a phasic component which declined to a steady level (tonic component) that was maintained throughout stimulus duration. The time period (2 min) for complete recovery of the EOG was obtained by applying two identical stimuli (10(-5) mol/liter L-serine for 5 s) successively at increasing intervals. EOG response to amino acids increased nearly exponentially with concentration and no saturation was reached. Gradual deterioration of olfactory receptor cells occurred within 14 days after olfactory nerve section (axotomy) as indicated by phospholipid staining. Regeneration of the receptor cells started at 56 days postaxotomy, reached approximately normal density and the elongate form of mature neurons at 84 days. In axotomized fish the magnitude of EOG response decreased relative to the controls at 7 days with minimal sensitivity between 14 and 28 days postaxotomy. EOGs were not evoked by L-serine nor L-leucine in most fish 17-22 days postaxotomy. Restoration of the EOG response coincided with morphological repopulation of receptor neurons after 84 days. EOG recovery was only 50% of the control values at 230 days. The EOG responses evoked by HCl in sensory and non-sensory epithelia were indistinguishable from each other, indicating that receptor neurons are not likely to be their primary origin.

Morpholine, Bile Acids and Skin Mucus as Possible Chemical Cues in Salmonid Homing: Electrophysiological Re-Evaluation

The olfactory-imprinting and pheromone hypotheses of salmon homing recognize the involvement of olfaction in the recognition of the homestream. However, physiological basis for olfactory recognition and the nature of homestream odors have not yet been established. In this paper the state of knowledge and advances in the study of chemical cues relevant to salmonid homing are reviewed, with special emphasis on 1) imprinting to morpholine, 2) skin mucus as a chemical signal, and 3) chemoreceptor responses to bile acids. Although homing of salmonids artificially imprinted to morpholine appears evident from behavioral studies, the olfactory detection of morpholine has not been adequately demonstrated. The skin mucus has been shown to be a potent olfactory stimulus for salmonids. Chemical characterization revealed that free amino acids present in the mucus were primarily responsible for olfactory stimulation; a synthetic mucus, a mixture of amino acids based on the analysis data, induced olfactory response indistinguishable from that induced by the original mucus. Electrophysiological studies showed that bile acids, especially taurine conjugates, were not only potent olfactory stimulants, but also highly-specific taste stimuli for rainbow trout. The threshold concentration for taurolithocholic acid, the most potent bile acid tested, was estimated at 10-12 M, nearly 4 log units lower than that for L-proline, the most potent taste stimulant reported for this species. In the olfactory system bile acids were as stimulatory as amino acids, with the threshold being almost 1,000 times higher than those for trout taste receptors. Because high sensitivity of the salmonid gustatory system to certain chemicals has now been demonstrated, it is no longer appropriate to consider olfaction to be the sensory modality for chemical detection only on the basis of its high sensitivity. In the light of these findings some important issues for future study are discussed.

Gustatory responses of the rainbow trout (Salmo gairdneri) palate to amino acids and derivatives

Summary1.Gustatory responses to amino acids and derivatives obtained from the palatine nerve (VIIth cranial nerve) of rainbow trout (Salmogairdneri) were studied.2.The response to an amino acid was characterized by its fast-adapting, phasic nature that returned to baseline within 5 s during continuous stimulation.3.Complete recovery of the response took place 120 s after stimulation when tested with two identical stimuli given successively (Figs. 2, 3).4.Among common amino acids tested, onlyL-isomers of proline, hydroxyproline, alanine, leucine, and phenylalanine were stimulatory (Table 1), demonstrating that the facial taste system is more narrowly-tuned to amino acid stimuli than the olfactory system. The arginine derivative,L-α-amino-β-guanidinopropionic acid (L-AGPA) andL-argininic acid, and betaine were also effective.5.Concentration-response relations, when plotted semi-logarithmically, were generally sigmoidal, saturating at higher concentrations (>1 mM) (Fig. 5). The threshold concentration forL-proline, the most effective amino acid tested, was estimated to be 50 nM. The thresholds for other stimulatory chemicals ranged between 1 and 100 μM.6.L-AGPA, though having higher threshold (10 μM), could induce responses three times the magnitude of that ofL-proline at 1 mM.7.The stimulatory activity of heterocyclic-imino acids having 4-, 5-, and 6-membered rings and also of thioproline and hydroxyproline suggests that the receptor recognizes primarily the imino acid region (Fig. 6).8.Taste responses to amino acids were independent of pH, except that arginine analogues were only active at basic pHs (Fig. 7 and Table 3). larginine was active only at pHs higher than 8.5. The palatal chemoreceptors were stimulated by waters with pHs below 7.0, suggesting the existence of receptors for pH and/or CO2 (Fig. 7).

Structural and Functional Development of the Olfactory Organ in Teleosts

Abstract Fish detect chemical stimuli through two major chemosensory channels, olfaction and gustation. All current evidence indicates that olfaction is a major mediator of chemical signals and is involved in diverse teleost behavior. This paper describes the morphological components and the basic principles of developmental patterns of the olfactory organ as well as cellular differentiation of receptors in rainbow trout Oncorhynchus mykiss (formerly Salmo gairdneri). The olfactory organ originates from an anlage formed by the ectoderm on the ventrolateral part of the head during stage 19 (11 d postfertilization). Ciliated receptor cells appear at stage 24 (day 18) and dominate until immature microvillar receptor cells develop in stage-28 (day 26) embryos. During this period, the olfactory placode continues a lateral and upward migration. At stage 29, the day of hatching, the olfactory groove is enclosed by flaps of skin, and by stage 33, distinct anterior and posterior nares are formed. Spontaneous elect...

Comparison of the olfactory response to amino acids in rainbow trout, brook trout, and whitefish

Abstract 1. 1. The olfactory apparatus and the bulbar electrical responses induced by nasal infusion of amino acid solutions were compared among three salmonid species, rainbow trout, brook trout and whitefish. 2. 2. Morphology of the olfactory organ was similar in all species examined, except that the peripherial olfactory organ of whitefish was slightly smaller. 3. 3. The threshold concentration for the most effective amino acids ranged from 10 −7 to 10 −8 M in rainbow and brook trout and was higher in whitefish. 4. 4. Olfactory bulbar response spectra to amino acids were similar in all species. However, correlation was slightly higher between rainbow and brook trout ( r = 0·89) than between rainbow trout and whitefish ( r = 0·82).

Mature males of Arctic charr,Salvelinus alpinus, release F-type prostaglandins to attract conspecific mature females and stimulate their spawning behaviour

SynopsisWe examined the behavioural and biological functions of prostaglandins (PG) of F-type as a chemosensory signal during spawning in Arctic charr. Immunoassay revealed F-type PGs in water inhabited by ripe charr of both sexes performing spawning-related acts. High concentrations of immunoreactive PGFs seemed to coincide with male spawning activity. Ripe isolated males were more odorous to other charr (as assayed by EOG) than either females or immature charr. When these males received chemical cues from ovulated females, they increased their release of odorants, including F-type PGs, into the water. PGF2α at concentrations of 0.1 nM attracted ovulated females and ripe males. It also stimulated digging behaviour in mature females. It is concluded that F-type PGs or their derivatives are released by ripe males on the spawning grounds to attract females and elicit their spawning behaviour.

Structure-activity relationships of amino acids in fish olfaction.

Abstract o (1) Investigation of the specificity of olfactory stimulation of amino acids and analogues by recording the electrical responses from the olfactory bulb of rainbow trout (Salmo gairdneri) has led to the establishment of definite structure-activity relationships. (2) α-Amino and α-carboxyl groups are essential; replacement with other functional groups or structural alterations of either or both groups results in markedly decreased responses or in elimination of the response. Amino acids whose amino group is located at a position other than the alpha are far less effective. (3) The α-hydrogen of amino acids must be free. Replacement of the β-hydrogen with a functional group also results in decreased activity. (4) The size of the fourth α-moiety is important. Overall molecular length exceeding six carbon atoms reduces effectiveness. Aromatic amino acids are only slightly stimulatory. (5) The presence of a terminal carboxyl group reduces effectiveness. However, amidation of this group increases activity markedly. (6) The amino acids inducing stimulation are thus characterized by being simple, short and straightchained, with only certain attached groups. A hypothetical receptor site has been proposed which involves two charged centres, one cationic and one anionic, capable of interacting with ionized groups of these stimulatory amino acids.