Non-oscillatory discharges of an F-prostaglandin responsive neuron population in the olfactory bulb-telencephalon transition area in lake whitefish.

Our previous studies on olfactory bulbar responses in salmonid fishes suggest that pheromone signals might be processed by a mechanism distinct from that of other odorants. Using in vivo single-unit and electroencephalographic recordings, we investigated response characteristics of olfactory neurons in lake whitefish, Coregonus clupeaformis, a species characterized by high electrophysiological and behavioral sensitivities to the reproductive pheromone candidates F-prostaglandins. We found a neuron population responsive to F-prostaglandins in the ventromedial brain tissue strip connecting the olfactory bulb to the telencephalon. Of the 64 neurons examined in this area, 33% showed excitatory and 11% inhibitory responses to F-prostaglandins, while 52% were non-responsive to all the stimuli tested. Both phasic and tonic F-prostaglandin neuron response patterns were observed during the 10-s stimulus period; some responses were delayed from the onset of stimulation, and some persisted for a long time following stimulus cessation. This neuron population did not induce synchronized oscillatory waves upon stimulation with F-prostaglandins, despite massive discharges. We demonstrate for the first time that the olfactory bulb-telencephalon area of the brain is a distinct neural structure through which putative reproductive pheromone signals are integrated. Amino acid and F-prostaglandin neuron population discharges have different temporal characteristics, suggesting different processing mechanisms exist for odorant and pheromone signals. The observed sustained neuron discharges may play a role in amplifying pheromone signals required for triggering stereotyped neuroendocrine and/or behavior changes.

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.

Biochemical and physiological evidence that bile acids produced and released by lake char (Salvelinus namaycush) function as chemical signals.

It has been hypothesized that faeces of juvenile lake char (Salvelinus namaycush) may contain chemical cues that mediate behaviour of conspecifics. However, our knowledge of bile acids naturally produced and released by fish is limited. Using HPLC, we fractionated bile acids produced and released by lake char and examined their stimulatory effectiveness using electro-olfactogram recordings. Taurocholic acid, taurochenodeoxycholic acid, taurooxocholanic acid, taurooxodeoxycholic acid 3alpha-sulphate, trace amounts of taurolithocholic acid and an unidentified sulphated bile steroid were found in bile and faeces. Bile acids were either taurine amidated or sulphated, or both. Lake char released an average of 4 nmol min(-1) bile acids per kilogram of body weight into their tank water. Urinary bile acids accounted for only a small portion of total bile acids released into water. Water and faeces contained higher proportion of taurochenodeoxycholic acid and sulphated bile acids (relative to taurocholic acid) than bile. The electro-olfactogram recordings demonstrated that bile acids released by lake char were detectable by their olfactory system at nanomolar concentrations, which is well below the levels of bile acids released into water. The exquisite olfactory sensitivity of lake char to water-borne bile acids released by their conspecifics is consistent with a role for these compounds as important chemical signals.

Olfactory sensitivity and specificity of Arctic char, Salvelinus alpinus, to a putative male pheromone, prostaglandin f(2)alpha.

Actively spawning male Arctic char, Salvelinus alpinus, release immunoreactive F-prostaglandins into the water that attract ovulated females and elicit spawning behaviour. To investigate a possible role played by the olfactory system, we determined the sensitivity and specificity of the char to prostaglandins by the electro-olfactogram (EOG) using a newly devised stimulatory apparatus. Of 18 prostaglandins tested, PGF(2)alpha and two synthetic analogues, 16,16-dimethyl-PGF(2)alpha and U-46619, were detected at threshold concentrations of 0.5-1.0 x 10(-11) M, and at a concentration of 10(-8) M evoked an EOG response equivalent to that of 10(-5) M L-serine. Olfactory nerve twig recordings further demonstrated that the EOG responses were transduced into nerve impulses and transmitted to the brain. No difference was found in the responses between sexes and maturational stages. We conclude that PGF(2)alpha is a candidate of the male pheromone of Arctic char, supporting our previous behavioural studies.

Topographic bulbar projections and dual neural pathways of the primary olfactory neurons in salmonid fishes.

A growing body of evidence indicates spatial patterning of molecular expression and physiological activities in the olfactory epithelium and primary afferent circuits of the vertebrate olfactory bulb. Because our previous studies indicate that olfactory receptors specific for amino acids and a bile acid, taurocholic acid, project to segregated coding centres in the olfactory bulb, we further examined the afferent projections and pathways of the primary neuronal responses to putative pheromones by recording the electroencephalogram from various regions of the olfactory bulb. First, using the electro-olfactogram, we determined olfactory sensitivities of six salmonid species to these odorants. Prostaglandin F2 alpha and 15-keto-prostaglandin F2 alpha were potent olfactory stimulants for all tested salmonids, except rainbow trout (Oncorhynchus mykiss). None of the salmonids responded to 17 alpha,20 beta-dihydroxy-4-pregnen-3-one. However, they were sensitive to etiocholan-3 alpha-ol-17-one glucuronide. In all salmonids examined, electroencephalograms to amino acids and taurocholic acid, applied singly or in combination, projected to two segregated regions, the lateroposterior and mid-olfactory bulb, respectively. Neither prostaglandin F2 alpha, 15-keto-prostaglandin F2 alpha nor etiocholan-3 alpha-ol-17-one glucuronide elicited electroencephalograms. These data indicate that, in salmonids, olfactory neurons responsive to amino acids and taurocholic acid project to spatially segregated regions, and thereby generated signals are encoded spatially and temporarily. The results also suggest that olfactory signals due to hormonal pheromones are processed in a manner distinct from those for amino acids and bile acids, and may possibly be mediated by extrabulbar primary olfactory fibres bypassing the bulb.

Effects of L-thyroxine on brain monoamines during parr-smolt transformation of Atlantic salmon (Salmo salar L.).

During spring, seaward migrating juvenile Atlantic salmon (Salmo salar) undergo parr-smolt transformation (PST) which involves changes in physiology, including one or two peaks in plasma thyroxine (T4). To investigate if changes in plasma T4 influence neural function, we measured levels of dopamine (DA) and its metabolites, 3,4-dihydroxyphenylacetic acid (DOPAC) and homovanillic acid (HVA), and also measured serotonin (5-hydroxytryptamine, 5-HT) and its metabolite, 5-hydroxyindoleacetic acid (5-HIAA) in brain regions of two groups of Atlantic salmon parr on an 8:16 h light/dark photoperiod. One group was treated with ambient T4 to simulate the natural smolt peak in plasma T4. T4 treatment depressed DOPAC levels as well as DOPAC/DA and 5-HIAA/5-HT ratios in the olfactory system but with no changes in the optic tectum. We conclude that during PST monoaminergic functions in specific brain regions of juvenile Atlantic salmon are affected by T4 treatment.

Spatial projections to the olfactory bulb of functionally distinct and randomly distributed primary neurons in salmonid fishes.

In fish, olfactory sensory neurons expressing specific odorant receptors are randomly distributed throughout the olfactory epithelium, and, yet, these subsets of olfactory neurons segregate as they enter the olfactory bulb and project to restricted regions. We investigated the functional significance of this projection pattern by recording electroencephalographic responses (EEGs) from the olfactory bulb, while simultaneously monitoring electro-olfactograms (EOGs) in response to two distinct odorant groups, amino acids (AA) and a bile acid, taurocholic acid (TCA), in Atlantic salmon (Salmo salar) and rainbow trout (Oncorhynchus mykiss). EEGs to AA and TCA distributed in two segregated regions, lateroposterior and mid olfactory bulb. When olfactory rosettes were subjected to partial lamellectomy (removal of the anterior, posterior, medial, or lateral half), both EOGs and EEGs to these odorants were uniformly reduced, and the degree of reduction was not dependent on the rosette region removed. These results indicate that the entire rosette contributes to the generation of EOGs, leading to transmission to the bulb. We conclude that in salmonid fishes olfactory neurons responsive to AA and TCA are randomly distributed throughout the olfactory epithelium, and yet project to spatially segregated regions and thereby generated signals are encoded independently in the bulb.

Nitric oxide synthase in the olfactory mucosa of the larval sea lamprey (Petromyzon marinus).

The use of nitric oxide, a product of enzymatic reduction of L-arginine by nitric oxide synthase, as a modulator of processes within the olfactory mucosa was investigated in larval sea lampreys, extant fish of ancient vertebrate origin. In the present study, we demonstrated that the sea lamprey olfactory mucosa is specifically sensitive to L-arginine, that the L-arginine responses are inhibited by an inhibitor of nitric oxide synthase, N omega-nitro-L-arginine, and that nitric oxide synthase is present in olfactory receptor cells, sustentacular cells, and basal cells. Electron microscopic examination using NADPH-diaphorase histochemistry revealed intense labeling within secretory vesicles of sustentacular cells and in proximity to mitochondria within olfactory receptor cell dendrites and sustentacular cells. At the base of the olfactory epithelium, NADPH-diaphorase staining was intense in the perinuclear cytoplasm of a subpopulation of basal cells, moderate in sustentacular cell foot processes, and scattered in olfactory receptor cell axons. Throughout axons in the olfactory epithelium and the lamina propria, labeling predominated in axonal profiles with mitochondria. These physiological and ultrastructural studies imply that in sea lamprey larvae, nitric oxide modulates peri-receptor events of L-arginine chemostimulation, olfactory receptor cell axonal activity, and developmental processes.

T4 depresses olfactory responses to L-alanine and plasma T3 and T3 production in smoltifying Atlantic salmon.

During natural or induced smoltification, Atlantic salmon exhibit spring elevations in olfactory activity and plasma L-thyroxine (T4). To determine whether T4 influences olfactory activity, we administered T4 to late parr in early April and measured olfactory bulb electroencephalogram and olfactory epithelium electroolfactorogram responses to L-alanine nasal stimulation, plasma T4 and 3,5,3'-triiodo-L-thyronine (T3) levels, and hepatic and brain monodeiodination. T4 treatment raised plasma T4 to 15 ng/ml, simulating the smolt T4 surge, and depressed plasma T3 and electroencephalogram responses, without modifying electroolfactorogram responses. Decreased plasma T3 may be explained by inhibition of hepatic T4 outer-ring deiodination, generating T3, and stimulation of inner-ring deiodination, degrading T4 and T3. Brain T4 outer-ring deiodination was also strongly inhibited. In conclusion, creation of a high plasma T4 concentration simulating the natural smolt peak depressed olfactory bulb responses to L-alanine due to the high T4 concentration or the depressed T3 availability in brain induced by T4. T4 may terminate the period of heightened olfactory responsiveness during smoltification.

Olfaction and gustation in fish: an overview.

Living in an aquatic environment, often devoid of light but rich in dissolved compounds, fish have highly developed chemosensory and chemical signalling systems. The olfactory and gustatory systems comprise the major chemosensory pathways. Despite considerable variations in structural organization of the peripheral olfactory organ throughout fish species, ultrastructural organization of the olfactory sensory epithelium is extremely consistent. The olfactory receptor cell is a bipolar neurone which is directly exposed to the external environment and sends information to the brain by its own axon (cranial nerve I). Four major classes of chemicals have been identified as specific olfactory stimuli and their stimulatory effectiveness characterized: amino acids, sex steroids, bile acids/salts and prostaglandins. Olfactory signals such as those involved in reproduction and feeding may be processed independently through two distinct subsystems: the lateral and medial olfactory systems. The taste buds constitute the structural basis of the gustatory organ. Taste buds may occur not only in the oropharyngeal cavity, but on the whole body surface. Chemical information detected by specialized epithelial cells, gustatory cells, is transmitted to the central nervous system by cranial nerve VII (facial), IX (glossopharyngeal), and X (vagal). Besides diverse sensitivities and specificities for amino acids, fish gustatory receptors detect various organic acids, nucleotides and bile salts. Putative receptors, molecular mechanisms of transduction and the role played by olfaction and gustation in feeding, reproduction, migration and other fish behaviours are discussed.

Thyroid hormone deiodination in brain, liver, gill, heart and muscle of Atlantic salmon (Salmo salar) during photoperiodically-induced parr-smolt transformation. I. Outer- and inner-ring thyroxine deiodination.

Outer-ring deiodinase (ORD) and inner-ring deiodinase (IRD) pathways for L-thyroxine (T4) were examined in the microsomes of tissues of 20-month-old Atlantic salmon induced to undergo parr-smolt transformation (PST) in late February and March by imposing a 16-hr photoperiod. Smolt external characteristics, decline in condition factor, increases in brain-somatic index, cardiac-somatic index, gill-somatic index, gill Na+/K+ ATPase activity, and food consumption developed progressively during the 5-week study. Plasma T4 (4.1 ng/ml, Week 1) rose to 13.5 and 12.5 ng/ml (Weeks 3 and 4) and fell to 7.5 ng/ml (Week 5). Plasma T3 rose from 2.1 ng/ml (Week 1) to 3.8 ng/ml (Week 2) and fell to 2.4 ng/ml (Week 5). T4ORD activity occurred in liver, heart, gill, brain, and skeletal muscle, increasing in liver and heart between Weeks 1 and 2, and in brain between Weeks 4 and 5. Only liver T4ORD activity was correlated (r = +0.946) with plasma T3 concentrations, suggesting that hepatic T4ORD may determine the plasma T3 concentration. For hepatic T4ORD the mean Km was 0.42 nM and the mean Vmax 1.2 pmols T4 converted.hr-1.mg microsomal protein-1. HPLC analysis revealed 3,3',5-T3(rT3) and 3,3'-diiodo-L-thyronine, as evidence of T4IRD activity, but only in liver and brain. The lower bound estimates of T4IRD activities to generate rT3 were 28% (liver) and 50% (brain) of the respective T4ORD activities. Brain T4IRD increased progressively during PST. In post-smolts in later October, plasma T3 concentrations and both hepatic and brain T4ORD and T4IRD activities were low, but some rT3 was produced. We conclude that during induced PST in Atlantic salmon (i) T4ORD activity increases in liver, heart, and brain, but not in gill or skeletal muscles; (ii) the hepatic T4ORD is highly correlated with plasma T3 and may therefore determine plasma T3; and (iii) there are changes in T4ORD and T4IRD in brain, which may reflect regulation of intracellular thyroid hormone metabolism in that tissue. However, it remains to be determined how changes in deiodinase activity relate to the surge in plasma T4 and the role of thyroid hormones during PST.

Thyroid hormone deiodination in brain, liver, gill, heart and muscle of Atlantic salmon (Salmo salar) during photoperiodically-induced parr-smolt transformation. II. Outer- and inner-ring 3,5,3'-triiodo-L-thyronine and 3,3',5'-triiodo-L-thyronine (reverse T3) deiodination.

Outer-ring deiodinase (ORD) and inner-ring deiodinase (IRD) pathways for 3,5,3'-triiodo-L-thyronine (T3) and 3,3',5'-T3 (reverse T3, rT3) were examined in microsomal fractions of liver, heart, gill, brain, and skeletal muscle of 20-month-old Atlantic salmon induced to undergo parr-smolt transformation (PST) in late February and March by imposing a 16-hr photoperiod. All tissues showed negligible T3ORD activity. T3IRD activity was detected in both the liver (Km = 0.65 nM; Vmax = 15.5 pmol T3 deiodinated.hr-1.mg microsomal protein-1) and brain of smolts, but not in gill, heart, or skeletal muscle. rT3ORD was detected in liver, brain, and muscle, and at very low levels in gill and heart. rT3IRD activity occurred to some extent in all tissues except brain. T3IRD activity changed in brain during PST, and was low in brain and liver of post-smolts examined in late October. We conclude that (i) deiodination of T3 proceeds exclusively through an IRD pathway, which may permit regulation of T3 degradation independently of the ORD pathway responsible for T3 formation; (ii) deiodination of rT3 proceeds mainly through an ORD pathway but rT3IRD activity does occur in some tissues; and (iii) the altered brain T3IRD activity during PST suggests regulation of T3 turnover in the brain at this time.

Ciliated and microvillar receptor cells degenerate and then differentiate in the olfactory epithelium of rainbow trout following olfactory nerve section.

We used scanning (SEM) and transmission (TEM) electron microscopy to examine ultrastructural changes in the olfactory epithelium (OE) of rainbow trout following unilateral olfactory nerve section. Both ciliated receptor cells (CRC) and microvillar receptor cells (MRC) degenerated and subsequently differentiated from unidentified precursor cells. The following changes took place in fish that were held at 10 degrees C at the stated period following olfactory nerve section: on day 7, MRC and CRC contained intracellular vacuoles; on day 12, the olfactory knobs appeared disrupted; by day 26, olfactory receptor cells were absent from the OE; on day 42, there were receptor cell bodies and a few CRC with short cilia at the apical surface; and on day 55, a small number of both CRC and MRC had differentiated. By day 76, both CRC and MRC repopulated the OE. Degenerative changes in the cytoplasm of the sustentacular cells (SC) and ciliated nonsensory cells (CNC) were observed in the first 26 days following olfactory nerve section, but these cells remained intact throughout the experiment. The degeneration and subsequent differentiation of CRC and MRC supports and extends previous observations that both cell types are olfactory receptor neurons with axons that extend along the olfactory nerve to the olfactory bulb.

Sex pheromones selectively stimulate the medial olfactory tracts of male goldfish.

The olfactory tracts of teleost fish are comprised of medial and lateral sub-tracts which previous studies suggest convey responses to pheromones and food odors respectively. This study tested this possibility by recording electrical responses from the medial and lateral tracts of male goldfish exposed to sex pheromones and food odors. Only the medial olfactory tract responded to pheromones and both tracts responded to an L-amino acid and crude food odor. These findings verify earlier studies of peripheral olfactory sensitivity to pheromones and confirm that pheromonal information is carried within the medial tracts. They also suggest that the neural processes responsible for food recognition are more complex than previously supposed.

Effects of cortisol on aspects of 3,5,3'-triiodo-L-thyronine metabolism in rainbow trout (Oncorhynchus mykiss).

Aspects of 3,5,3'-triiodo-L-thyronine (T3) metabolism were studied in fed rainbow trout (Oncorhynchus mykiss) held at 11.5-14 degrees and intraperitoneally implanted with hydrogenated corn oil (controls) or oil containing cortisol. Cortisol implants caused dose-related plasma cortisol elevations within the physiological range for 2-3 weeks, loss in body weight, and depression in plasma T3 and free T3 index with no consistent change in plasma thyroxine (T4) or free T4 index. Plasma T3 clearance rate and plasma T3 appearance rate were both increased by cortisol, with no change in hepatic microsomal T4 5'-monodeiodinase activity (Km or Vmax), but with a significant decrease in muscle T3 concentration. It is concluded that chronic physiologic cortisol treatment enhances plasma T3 clearance without change in hepatic T4 to T3 conversion, resulting in a decline in T3 concentration in both plasma and tissue (muscle) compartments.

The olfactory system, not the terminal nerve, functions as the primary chemosensory pathway mediating responses to sex pheromones in male goldfish.

Studies of the neural mechanisms underlying responsiveness to sex pheromones in male goldfish suggest that, contrary to a currently popular hypothesis, the olfactory system (cranial nerve 1), and not the terminal nerve (cranial nerve 0), mediates chemosensory responses to pheromones. When the olfactory epithelium of male goldfish was exposed to two identified sex pheromones, 17 alpha, 20 beta-dihydroxy-4-pregnen-3-one and a mixture of prostaglandin F2 alpha and its metabolite 15-keto-prostaglandin F2 alpha, the spontaneous activity of olfactory neurons located in the medical portion of the olfactory bulb changed, while activity of terminal nerve cell bodies did not. A variety of other synthetic and natural odors also failed to alter the activity of terminal nerve cell bodies as did visual, magnetic, thermal, and auditory cues. Terminal nerve activity was, however, inhibited by tactile stimulation, suggesting that this system may have a modulatory role associated with the physical interactions that characterize goldfish spawning behavior.

F prostaglandins function as potent olfactory stimulants that comprise the postovulatory female sex pheromone in goldfish.

This study establishes that ovulated female goldfish release F type prostaglandins (PGFs) to the water where they stimulate male spawning behavior and comprise the goldfish postovulatory pheromone. We first demonstrated that ovulated and prostaglandin-injected female goldfish release immunoreactive PGFs to the water. Next, using electro-olfactogram recording (EOG), we determined that waterborne prostaglandins function as potent olfactory stimulants for mature male goldfish. Prostaglandin F2 alpha (PGF2 alpha) and its metabolite 15-keto-prostaglandin F2 alpha (15K-PGF2 alpha) were the most potent prostaglandins; the former had a detection threshold of 10(-10) M and the latter a detection threshold of 10(-12) M. Studies of prostaglandin-injected fish indicated that PGF metabolites are an important component of the pheromone. Cross-adaptation experiments using the EOG demonstrated that goldfish have separate olfactory receptor sites for PGF2 alpha and 15K-PGF2 alpha that are independent from those that detect other olfactory stimulants. Finally, we established that male goldfish exposed to low concentrations of waterborne PGFs exhibit reproductive behaviors similar to those elicited by exposure to the odor of ovulated fish. Together with our recent discovery that a steroidal maturational hormone functions as a preovulatory "priming" pheromone for goldfish, these findings suggest that hormones and their metabolites may commonly serve as reproductive pheromones in fish.

Morphological and physiological development of olfactory receptor cells in rainbow trout (Salmo gairdneri) embryos.

The morphological and functional differentiation of the olfactory receptor cells were investigated in developing rainbow trout (Salmo gairdneri) embryos by means of light and electron (transmission and scanning) microscopy and electrophysiology. Ciliated receptor cells first appeared when the olfactory placode was folded to form a groovelike structure rostrad to the eye at stage 24 (day 18; 18 days postfertilization). Ciliated receptor cells predominated until immature microvillar receptor cells developed in stage 28 (day 26) embryos. At stage 29, the day of hatching, the anterior edge of the olfactory epithelium contained only ciliated receptor cells, and the midregion contained both ciliated and microvillar receptor cells. Spontaneous neural firing activity was recorded from the olfactory mucosa as early as stage 25. The neural responses to amino acids were initially recorded from stage 26 embryos, containing sparse ciliated receptor cells with a few short cilia. The D-enantiomers of amino acids were less effective. From these results we concluded that in rainbow trout the olfactory receptor cell has two separate morphological forms, ciliated and microvillar. These are ontogenetically distinct; the ciliated receptor cells preceded the microvillar. The ciliated receptor cells respond to amino acid stimulation.

A protocol for estimation of cortisol plasma clearance in acid-exposed rainbow trout (Salmo gairdneri).

The cortisol metabolic clearance rate (MCR) and degradation rate (DR) were determined in acid (H2SO4)-stressed and control rainbow trout (260 g) cannulated via the dorsal aorta. Recovery from catheterization, as judged by plasma cortisol, glucose, protein, and packed cell volume (PCV), was complete by 6 days. However, serial blood sampling increased plasma cortisol. Furthermore, although no major or consistent diel change in plasma cortisol occurred in terminally sampled free-swimming control trout, fluctuations were observed in serially bled catheterized trout. These findings preclude cortisol MCR estimation by any serial sampling method. Although plasma cortisol was temporarily elevated by constant infusion (70 microliter hr-1) of saline:ethanol vehicle, a satisfactory protocol was established for determining cortisol MCR by infusion of labeled cortisol to constant plasma specific activity. The MCR for control trout in water, pH 7.7, for 7 days was 30.3 +/- 4.2 ml hr-1 100 g-1, which did not differ from that of trout in water, pH 5.0. However, the cortisol DR was greater at pH 5.0 (2.13 +/- 0.46 micrograms hr-1 100 g-1) than at pH 7.7 (0.56 +/- 0.12) due primarily to increased plasma cortisol at pH 5.0.

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.