Determination of kinetic properties of serotonin-N-acetyltransferase in bovine pineal gland using HPLC with fluorimetric detection

Retinal Organ Cultures as Alternative Research Models

Ex vivo organ cultures represent unique research models, as they combine the advantages of cell cultures with those of animal models. Being able to mimic in vivo situations through the use of organ cultures provides an excellent opportunity to investigate cellular processes, molecular pathways and cell–cell interactions, as well as structural and synaptic organisation. Human and animal organ cultures are now well established and comprise sensitive, easy-to-manipulate experimental systems that raise minimal ethical concerns. The eye, in particular, is a very complex organ that is not easy to reproduce in vitro. However, a lot of research has been dedicated to the development of suitable ocular organ cultures. This review covers the various ex vivo retinal organ culture systems available for use in ophthalmology research and compares them with commonly used animal models. In particular, bovine and porcine retinal organ culture systems are described, because the size, anatomy, physiology and vessel morphology of bovine and porcine eyes are similar to the human eye in an undisputed way, thus making them good models. In addition, these animals are widely used by the food industry and the eyes are considered surplus material. A short overview of murine, rat, rabbit, cat, canine and simian retinal organ cultures is also provided.

Kinetic analysis of the catalytic mechanism of serotonin N-acetyltransferase (EC 2.3.1.87)

Serotonin N-acetyltransferase (arylalkylamine N-acetyltransferase, AANAT, EC 2.3.1.87) is the penultimate enzyme in melatonin biosynthesis. This enzyme is of special biological interest because large changes in its activity drive the large night/day rhythm in circulating melatonin in vertebrates. In this study the kinetic mechanism of AANAT action was studied using bacterially expressed glutathione S-transferase (GST)-AANAT fusion protein. The enzymologic behavior of GST-AANAT and cleaved AANAT was essentially identical. Two-substrate kinetic analysis generated an intersecting line pattern characteristic of a ternary complex mechanism. The dead end inhibitor analog desulfo-CoA was competitive versus acetyl-CoA and noncompetitive versus tryptamine. Tryptophol was not an alternative substrate but was a dead end competitive inhibitor versus tryptamine and an uncompetitive inhibitor versus acetyl-CoA, indicative of an ordered binding mechanism requiring binding of acetyl-CoA first. N-Acetyltryptamine, a reaction product, was a noncompetitive inhibitor versus tryptamine and uncompetitive with respect to acetyl-CoA. Taken together these results support an ordered BiBi ternary complex (sequential) kinetic mechanism for AANAT and provide a framework for inhibitor design.

Daily time course of the contents in monoamines and their metabolites in the pineal gland of Syrian hamster

Twenty-four h changes in endogenous tyrosine hydroxylase (TH) activity, monoamines and their metabolites as well as N-acetyltransferase (NAT) activity and melatonin content were investigated in the Syrian hamster pineal gland. Both NAT activity and melatonin content exhibited an expected rise 8 h (0400 h) after darkness onset. Pineal dopamine (DA) and 3,4-dihydroxyphenyl acetic acid (DOPAC) levels were highest during the dark phase reaching a peak at 0200 h, just prior to the rise in melatonin production. However no significant difference were found in both norepinephrine (NE) and TH activity levels during the 24 h period. However, in the present study, the rat pineal gland exhibited a clear day/night difference in its TH activity. These data suggest a possible role of pineal DA in the induction of melatonin synthesis in addition to its function as a NE precursor in the Syrian hamster pineal gland and also indicate that catecholamines may act on the hamster pineal in a different manner than in the rat.

Parasympathetic inhibition of pineal indole metabolism by prejunctional modulation of noradrenaline release

The role of the parasympathetic nervous system in rat pineal indole metabolism was investigated by transpineal in vivo microdialysis. On-line coupling to a high performance liquid chromatography system with fluorescence detection (HPLC-FD) allowed simultaneous analysis of three major indolic compounds from the pineal, i.e. serotonin, N-acetylserotonin and melatonin. Infusion of the muscarinic receptor agonists, carbachol and oxotremorine, during the dark period resulted in a marked decrease of melatonin release. This effect was suggested to be mediated by a decrease in N-acetyltransferase activity, since a similar decrease was seen in N-acetylserotonin release, while serotonin levels increased simultaneously. Nicotine did show a very slight effect on the three indoles under these circumstances. Neostigmine failed to influence pineal indole metabolism, indicating that the endogenous tonus of acetylcholine release is either absent or extremely low in the middle of the dark period. The involvement of sympathetic innervation in the muscarinic effects was investigated by measurement of noradrenaline release from the pineal by sensitive off-line HPLC-FD analysis of noradrenaline in the dialysates. Carbachol markedly decreased the noradrenaline input during the infusion. Noradrenaline release returned to baseline values immediately after infusion with carbachol. These data suggest that the in vivo inhibitory effect of muscarinic receptor agonists on pineal melatonin production is mediated by presynaptic muscarinic receptors, located on the sympathetic nerve endings. This prejunctional inhibition of noradrenaline release causes a reduced induction of N-acetyltransferase activity, resulting in decreased melatonin release.

Tyrosine hydroxylase activity in peripherally denervated rat pineal gland

The presence of tyrosine hydroxylase (TH) in the rat pineal gland was studied using a combination of immunochemical and biochemical methods. In superior cervical ganglionectomized (SCGx) animals and in isolated pineals incubated for 72 h, both TH immunoreactive (TH-IR) fibers and TH biochemical activity were still present but reduced. Conversely, in dispersed pinealocytes incubated for only 24 h we were unable to detect either TH activity or TH-positive cells. Since in the pineal gland of intact rats total 3-methoxy-4-hydroxy phenylglycol (MHPG) was undetectable, and only traces of norepinephrine (NE) were present in the pineal of ganglionectomized animals, the results suggest a central pinealopetal catecholaminergic pathway which could use dopamine as a neurotransmitter.

Progesterone does not mediate the inhibition of pineal melatonin production during the rat proestrous night

N-Acetyltransferase (NAT) activity and melatonin content were determined in pineal glands obtained during the proestrous stage of the rat estrous cycle. In control animals, both pineal NAT activity and melatonin levels were reduced during the first hours of proestrous night, as compared with day-time proestrous levels. The administration of antiprogestagen RU486, in a dose which blocked the preovulatory rise of serum luteinizing hormone and follicle-stimulating hormone levels, failed to prevent the reduction of pineal melatonin synthesis associated with the ovulatory events at proestrous night. These results suggest that estradiol, but not progesterone, might be the ovarian hormone responsible for the inhibition of pineal melatonin synthesis observed in the normal cycling female rat during proestrous night.