T.R. Hall

Hypothalamic monoamine oxidase, a component in the serotonergic control of pituitary prolactin content in Carassius auratus L.

Abstract The role of the enzyme monoamine oxidase (MAO) was examined with regard to the control of pituitary prolactin content in the goldfish. Inhibition of MAO by means of intraperitoneal injection of pargyline resulted in significantly elevated brain serotonin content. Concomitantly, pituitary prolactin stores were also significantly elevated. Time-course and dose-response experiments revealed that pargyline at doses ≥25 mg/kg completely inhibited MAO and significantly increased brain serotonin and pituitary prolactin content within 12 hr. It is concluded that the enzyme MAO may be a significant component of the monoaminergic system of the goldfish, serving to modulate the serotonergic inputs to centers involved in the control of pituitary prolactin secretion.

Monoamine oxidase activity in several tissues of the goldfish. Carassius auratus.

1. Monoamine oxidase (MAO) was determined fluorometrically in male goldfish tissues, and the effects of specific inhibitors determined. 2. MAO activity in kidney greater than intestine greater than pancreas greater than brain approximately liver greater than testis. Apparent Michaelis constant (Km) was higher in the first three and lower in the last three tissues. 3. Specific MAO type A inhibitors (harmaline, clorgyline) were much more effective than a type B inhibitor (deprenyl) in reducing MAO activity. 4. Apparently goldfish tissues contain only a single type A-like MAO.

Photoperiod effects on hypothalamic serotonergic activity in the goldfish, Carassius auratus

Abstract 1. 1. The nature of hypothalamic serotonergic activity in goldfish exposed to various photo-period regimes was examined for day -night differences. 2. 2. Fish maintained throughout the course of these experiments on a short photoperiod (8L/16D, 20°C) showed no day-night differences in hypothalamic serotonin content or in serotonergic activity. 3. 3. Fish transferred from the short photoperiod to a long photoperiod (16L/8D, 20°C) developed day-night differences in hypothalamic serotonin content within 72 h, while day night differences in hypothalamic serotonergic activity were observed after 2 weeks. 4. 4. Data presented suggest that hypothalamic serotonergic mechanisms play a role in mediating the effects of photoperiod on physiological processes in goldfish.

Pinealectomy and melatonin effects on daily variations of the hypothalamic serotonergic system in the goldfish

Abstract 1. 1. Day-night variations of hypothalamic serotonergic activity were noted in goldfish maintained on a 16L/8D, 20°C regime but were not found in fish kept on an 8L/16D, 20°C regime. 2. 2. Pinealectomy had a significant effect on hypothalamic serotonin content and serotonergic activity in fish exposed to the long, but not short, photoperiod. 3. 3. Melatonin administration to control animals (100 μg per day for 10 days) modified the day-night variations of hypothalamic serotonin content and serotonergic activity regardless of photoperiod. 4. 4. The effects of melatonin on hypothalamic serotonin content in pinealectomized animals were dependent upon the photoperiod regime, whereas effects on serotonergic activity were not. 5. 5. It is concluded that the goldfish pineal organ and melatonin may play a role in hypothalamic adjustments to changing photoperiods.

Monoamine oxidase types A and B in the vertebrate brain.

1. Monoamine oxidase (MAO) activity was measured in brains of teleosts, amphibians a reptile, birds and mammals. Selective inhibitors were used to determine the relative proportion of MAO type A and MAO type B. 2. Brain homogenates of all tetrapods showed double sigmoid inhibition curves, indicating the presence of both forms of MAO. 3. Brain homogenates of all teleosts showed single sigmoid inhibition curves, with greater sensitivity to type A inhibitors. 4. These results indicate that tetrapod brains contain both type A and type B MAO activity, whereas teleost brains contain only a type A-like enzyme. 5. It is possible that the presence of two forms of MAO in brain may confer an advantage to a terrestrial lifestyle.

In vivo and in vitro effects of temperature on monoamine oxidase activity in brain and other tissues of the goldfish, carassius auratus L.

1. The maximum velocity (Vmax) and apparent Michaelis constant (Km) of brain and liver monoamine oxidase (MAO) in goldfish were different in fish acclimated to 22 degrees C and to 7 degrees C ambient temperature. 2. In brain, Vmax and Km were dependent upon incubation temperature, but both parameters were lower in 7 degrees C, adapted fish over most of the incubation temperature range. 3. The values obtained for Km showed a plateau at incubation temperatures at and below 25 degrees C for warm water fish, and at and below 20 degrees C for cold water fish. The activation energy of brain MAO was lower in fish adapted to the colder water. 4. These results show that goldfish MAO displays changes in functional activity in response to a change in environmental temperature. Apparently the purpose of this adaptation is to compensate for a reduction in enzyme concentration.

Effects of inhibitors on monoamine oxidase activity in perch (Perca flavescens) brain in vitro

1. Perch brain homogenates were incubated in vitro and monoamine oxidase (MAO) activity was determined fluorometrically, using a kynuramine substrate. 2. Clorgyline, harmaline and deprenyl inhibited MAO activity in a concentration-related manner, with single sigmoid inhibition curves, and the type A inhibitors harmaline and clorgyline were more effective than the type B inhibitor deprenyl. 3. Two types of inhibition were recognized in vitro; a fast-onsetting inhibition, similar to that produced by a reversible inhibitor, and a slow-onsetting inhibition, which is time- and concentration-dependent and presumably represents inactivation of the enzyme.

Characteristics of monoamine oxidase activity in brain and other organs of the adult bullfrog, Rana catesbeiana.

1. Monoamine oxidase (MAO) activity was measured in brain, liver, kidney and intestine of the adult bullfrog by a fluorometric method. 2. All tissues contained both type A and type B MAO, on the basis of responses to specific inhibitors, but with different ratios in each tissue. 3. MAO activity was optimum at 30 degrees C. However, MAO type B showed greater activity changes related to incubation temperature than did type A. 4. The Michaelis constant (Km) of MAO also varied with temperature, with a nadir around 20 degrees C. The functional significance of this is not clear. 5. Arrhenius plots showed that the activation energy for MAO B was higher than for MAO A.

Serotoninergic drugs affect prolactin and growth hormone secretion in the domestic fowl

Adult fowl of both sexes injected with the monoamine oxidase inhibitor pargyline showed elevated circulating prolactin concentrations and reduced growth hormone concentrations. Young cockerels injected with the serotonin agonist quipazine and the antagonist methysergide showed responses consistent with a serotoninergic stimulatory control of prolactin. Injection of the serotonin precursor tryptophan and the serotonin re-uptake blocker imipramine resulted in elevated prolactin and reduced growth hormone levels. The similarities and differences in the control of prolactin and growth hormone in birds and mammals were discussed.

Effects of para-chlorophenylalanine, a brain serotonin depletor, on pituitary cyclic AMP levels in the rainbow trout, Salmo gairdneri.

Abstract The tryptophan hydroxylase inhibitor para -chlorophenylalanine (pCPA) was employed in an investigation of the role of brain serotonin in the regulation of pituitary cyclic AMP levels in Salmo gairdneri . Hypothalamic serotonin content was measured with a fluorometric assay, while pituitary cAMP was determined by means of a double-antibody radioimmunoassay. A time-course study revealed that 250 mg pCPA/kg significantly reduced hypothalamic serotonin content within 1 hr after injection. Lower doses were effective in reducing serotonin content after 48 hr. Pituitary cAMP levels declined significantly 1 hr after injection of 250 mg pCPA/kg. Administration of pCPA at 50 and 250 mg/kg doses reduced cAMP levels within 24 hr. These data indicate that a serotonergic mechanism is involved with the regulation of pituitary cyclic AMP levels in the rainbow trout.