Pineal acetylserotonin methyltransferase activity in the teleost fishes, Hesperoleucus symmetricus and Salmo gairdneri, with evidence for lack of effect of constant light and darkness

Probable mechanism of catalysis of pineal gland hydroxyindole-O-methyltransferase (HIOMT) from rainbow trout (Salmo gairdneri)

Activity of trout pineal HIOMT was found to increase with increase in incubation temperature from 5 to 40 degrees C although the activation energy remained constant over this range. From examination of the effects of the products of HIOMT catalysis on the enzyme it was apparent that the catalytic mechanism was ordered Bi-Bi with S-adenosylmethionine as the obligatory first substrate. Trout HIOMT was found to methylate all the common pineal hydroxyindoles with hydroxytryptophol having the greatest affinity for the enzyme. The pH optimum for trout HIOMT was found to be about pH 9.0 although routine use of a pH of 7.9 is recommended to limit potentially deliterious effects caused by degradation of S-adenosylmethionine at elevated pHs.

Pineal gland hydroxyindole-O-methyl transferase (HIOMT) from rainbow trout (Salmo gairdneri) subject to temperature-dependent substrate inhibition by N-acetylserotonin

The effect of different incubation temperatures on the activity of trout hydroxyindole-O-methyltransferase (HIOMT) was determined in order to study the potential effects of changes in environmental temperature on production of melatonin. In the study, N-acetylserotonin (NAS) was found to inhibit trout pineal HIOMT, inhibitory concentration decreasing with a decrease in incubation temperature. An increase in incubation temperature was accompanied by an increase in maximal velocity (Vm) and a decrease in affinity of the substrate NAS for the enzyme, while the affinity of the inhibitor NAS remained more or less constant with increase in temperature up to 30 degrees C. It was interesting to note that at subinhibitory concentrations of NAS, possibly concentrations within the normal physiological range, there was a balance between increased velocity and decreased affinity for substrate, with increase in temperature resulting in a constant rate of melatonin production. On the basis of these results it is suggested that substrate inhibition might be involved in modulation of melatonin production and that trout pineal HIOMT can maintain a constant rate of melatonin production over a wide range of temperatures by a combination of compensatory changes in enzyme kinetic parameters.

Examination of in vitro melatonin secretion from superfused trout (Salmo gairdneri) pineal organs maintained under diel illumination or continuous darkness

Melatonin secretion was measured from rainbow trout (Salmo gairdneri) pineal organs maintained individually under flow-through whole organ culture (superfusion) conditions. Radioimmunoassay of perfusate fractions collected during controlled photic conditions demonstrated that melatonin secretion in vitro remained basal during the photophase and underwent increases in titer during the scotophase. While amounts of melatonin (mel) secreted were characteristic of individual pineal organs, photophase values ranged between 0.25 and 0.75 ng mel/ml and scotophase values ranged from 6 to 10 ng mel/ml of perfusate. Diel melatonin secretion profiles reflected the illumination regimen, with light associated with low melatonin titer in the perfusate and darkness associated with high titer. Light pulses during a normal scotophase resulted in a depression in melatonin secretion regardless of whether it was administered early or late in the dark period. Pulses of darkness given early or late in a normal photophase resulted in increased melatonin secretion. Superfused trout pineal organs did not display endogenous rhythmicity in melatonin secretion when subjected to prolonged exposure to continuous darkness (DD), whether first exposed to entraining light/dark (LD) cycles prior to DD or exposed to DD at the initiation of superfusion. In both studies, elevated melatonin secretion gradually declined over time. But exposure to a 4:4LD cycle after DD resulted in decreased (with light) and increased (with darkness) melatonin secretion. These results demonstrate that the trout pineal organ can be maintained for extended periods of time in superfusion culture, that the trout pineal organ is very responsive to light or dark for regulating melatonin synthesis, and that an endogenous rhythm in melatonin synthesis when organs were maintained in DD was not detectable.

Effect of pinealectomy on thyroid hormone (T4 and T3) levels in plasma during annual reproductive cycle in the freshwater catfish, Clarias batrachus

Thyroxine and triiodothyronine (T3) were measured by radioimmunoassay in serum and thyroid gland of the freshwater teleost Clarias batrachus. Pinealectomy increased the glandular levels of the thyroid hormones but decreased the plasma levels of T4 concomitantly with the increase of T3 levels. Most of the effects of pinealectomy on thyroid hormones were restricted to the gonadal development periods. Pinealectomy has shown no significant impact on thyroid hormones during the other periods of the reproductive cycle. Thus, it appears that the pineal gland is inhibitory to thyroid hormones during gonadal development and maturation.

Uptake and metabolism of indole compounds by the goldfish pineal organ

Indole metabolism was studied in the pineal organ of the goldfish by radioautography and high-performance liquid chromatography. The rate of uptake of tritiated serotonin was rapid in vitro with dense labeling over the photoreceptor cells. Tritiated tryptophan was taken up at a slower rate and the label was distributed evenly over the epithelium. Continual light caused a reduction in the concentration of serotonin (5-HT) and 5-hydroxyindoleacetic acid (5-HIAA) compared to groups exposed to constant darkness both in vivo and in explants, suggesting that these effects are not derived from photoreceptors outside the pineal organ. These data are consistent with the hypothesis that indole metabolism is functionally linked to phototransduction events in the pineal organ of lower vertebrates.

Characteristics and non-rhythmicity of retinal hydroxyindole-O-methyltransferase activity in trout (Salmo gairdneri)

The validation and optimization of an hydroxyindole-O-methyltransferase (HIOMT) activity assay system for trout ocular enzyme was undertaken. High-performance liquid chromatographic analysis, cross-validated with a standard thin-layer chromatographic technique verified that HIOMT activity was present in trout ocular tissue by identifying the 3H-melatonin end product. This activity was observed in the neural retina and was restricted to those fractions containing the cytosolic component. The enzyme displayed bimodal pH optima, one at pH 7.6, the other at 8.4. N-Acetylserotonin was O-methylated preferentially and a Km of 4.5 X 10(-7) M was calculated for this substrate. The S-[3H]adenosylmethionine Km was 4.6 X 10(-6) M. Time of incubation and enzyme concentration studies were conducted. The analysis of retinal HIOMT activity in groups of trout killed over a day--night period did not detect a rhythm in enzyme activity. Results of these investigations are discussed in terms of the evolution of melatonin synthesis and functions.

The presence of two populations of sensory-type cells in the pineal organ of the five-bearded rockling, Ciliata mustela L. (Teleostei)

The pineal organ of the five-bearded rockling, Ciliata mustela L., was examined by means of electron microscopy. Two categories of sensory cells are described: 1) Sensory cells 1 (or photoreceptor cells sensu stricto) showing the characteristic ultrastructure of photoreceptor cells with a well-developed receptor pole (outer segment) and a transmitter pole (ribbon-type synapse in the basal pedicle contacting dendritic processes), and a segmental organization of organelles. 2) Sensory cells 2 (or photoneuroendocrine cells) displaying no particular segmentation. The ultrastructure of the receptor pole (outer segment) is variable in shape (with either long or short disks) and in the number of disks; some outer segments are simple cilia of the 9 + 0 type. This second cell category is rich in smooth endoplasmic reticulum, beta-particles of glycogen, dense inclusions of variable size and content, and dense-core vesicles 130 nm in diameter. These cells have an extended contact area with the perivascular space. The functional significance of both cell categories is discussed in terms of the known physiological responses of the pineal organ. A possible confusion in identification of interstitial cells and neuroendocrine cells in some teleost species is discussed.

The effects of constant light and constant darkness on daily changes in the morphology of the pineal organ in the goldfish, Carassius auratus

The fine structure of photoreceptor cells in the pineal organ of the goldfish was found to vary quantitatively over a 24-hour period. Stereological analysis revealed significant daily changes in the volume of the cell and inner segment, nuclear volume and nucleolar diameter, volume of endoplasmic reticulum and Golgi bodies, area of both rough and smooth endoplasmic reticulum, and number of vesicles associated with each Golgi body. Peak values of these variables occurred either during the dark phase or latter part of the light phase. These findings agree closely with those reported in higher vertebrates, and suggest that metabolic activities, and possible secretory functions, of the pineal organ of fishes are synchronized to the light:dark cycle. Daily changes in these variables generally persisted in fish exposed to constant darkness for seven days, with the peak in these rhythms coinciding closely with those observed in fish exposed to a light:dark cycle. In contrast, the rhythms in all variables were abolished in fish kept in continual light for seven days. Photoreceptor cells from fish exposed to continuous light had larger nucleoli and greater amounts of rough endoplasmic reticulum indicating a further effect of light on pineal metabolism in lower vertebrates.

A light and electron microscopic study of the pineal in the blind goby, Typhlogobius californiensis (Pisces: Gobiidae)

The pineal of the burrow-inhabiting, blind goby, Typhlogobius californiensis, was studied by light and electron microscopy. The two predominant cell types were photoreceptor and supportive cells. Ganglion cells could not be positively identified. The majority of photoreceptor cells had well developed outer segments and made apparent synaptic contact with nerve processes of unknown origin. Similar unmyelinated nerve fibers containing dense-cored vesicles were seen descending the pineal stalk. The cytoplasm of supportive cells contained several Golgi zones, occasional dense aggregations of smooth endoplasmic reticulum, abundant quantities of glycogen, some dense-cored vesicles and unusual rod crystalline inclusions measuring approximately 0.5 micrometer in width and 4-5 micrometer in length. These observations suggest that the pineal in this light-deprived species has retained its photoreceptive function and is a metabolically active organ which may also function in secretion.

The pineal of the troglophilic fish, Chologaster agassizi: an ultrastructural study

The pineal of the facultative, cave-dwelling fish, Chologaster agassizi, was examined electron microscopically. Two cell types, photoreceptor and supportive cells, were identified in the pineal epithelium. The photoreceptor cells had well developed outer segments and contained Golgi bodies which were surrounded by both clear and dense-cored vesicles. Vesicle-crowned rods were frequently seen in various regions of the cell. The supportive cells also contained Golgi bodies from which both clear and dense-cored vesicles appeared to originate. In addition, these cells were characterized by peculiar arrangements of the smooth endoplasmic reticulum and the presence of pigment granules. Large quantities of glycogen were observed in both cell types. Small, unmyelinated nerve fibers were seen coursing throughout the pineal epithelium. Terminals filled with pleomorphic, clear vesicles and dense-cored vesicles were present in the vicinity of these nerve fibers. Similar vesicle-filled terminals were observed in close association with the supportive cells. The results of this study indicate that the pineal in this light-deprived species is a metabolically active organ capable of photoreception. Specializations of the organelles in the pineal cells were similar to those observed in other vertebrates living in environments of low light levels.

Light and electron microscopic study on the pineal complex of the coelacanth, Latimeria chalumnae Smith

The pineal complex of the coelacanth, Latimeria chalumnae was studied light and electron microscopically. It consists of two vesicles representing parapineal and pineal organs. Both occur intracranially and openly communicate with each other and the brain ventricle. The entire complex shows a striking photoreceptor morphology with sensory, ependymal and nerve cells. The last cell type is more abundant in the parapineal vesicle than in the pineal organ. The following ultrastructural details of the parapineal are noteworthy: 1. The sensory cells possess large inner and outer segments protruding freely in the vesicular cavity. The outer segments measure 8-10 mum in length and consist of as many as 275 lamellae. The basal processes of these cells terminate in neuropil-like regions. Occasionally, dense granules (500-1000 A) of uncertain identity occur in the perinuclear and inner segment cytoplasm of the cells. 2. The supporting cells are of the ependymal type. Their cytoplasm contains a filamentous feltwork and pinocytotic vesicles, but lacks secretory granules. Cytosomes are particularly abundant in cell processes in the neuropil-like zones. The basal end-feet of these cells isolate the receptor and nerve cells from the perivascular space. 3. In the neuropil-like regions, terminals of sensory cells make synaptic contacts with neuronal dendrites. Synaptic ribbon-like profiles in the terminals characterize the contact zones. Only unmyelinated nerve fibers could be observed in the small area of the tissue examined. The results are discussed with regard to photoreceptive and secretory functions of the pineal complex and its evolution in lower vertebrates.