Decrease In N-acetyltransferase Activity Research Articles

50-Hz SINUSOIDAL MAGNETIC FIELD EFFECT ONIN VITROPINEALN-ACETYLTRANSFERASE ACTIVITY

Some studies have shown a decrease in pineal N-acetyltransferase (NAT) activity and/or blood melatonin concentration in rodents exposed to extremely low-frequency (ELF) and low magnetic flux density electromagnetic fields. The mechanism/s involved in such effects are not known. It has been hypothesized that the magnetic fields (MF) could act on the pineal gland directly and/or indirectly through the retina. The aim of this work was to study whether MFs could modify NAT activity through a direct effect on the gland. Pineal glands obtained from rats sacrificed in the middle of the dark period were exposed during a 1-h incubation to 10-, 100-, or 1,000-μT, 50-Hz, sinusoidal MFs. The results showed that the glands exposed to the highest magnetic flux density responded with a significant decrease in NAT activity. The data obtained from these experiments support the idea that the pineal gland can be directly affected by ELF electromagnetic fields.

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.

The effects of lindane poisoning on N-acetyldopamine and N-acetyl 5-hydroxytryptamine concentrations in the brain of Locusta migratoria L.

High-performance liquid chromatography with electrochemical detection was used to measure N-acetyldopamine and N-acetyl 5-hydroxytryptamine in the cerebral ganglia of the African migratory locust, Locusta migratoria (fifth instar) exposed to lindane, a chlorinated insecticide. Locusts were poisoned by this insecticide in order to assess the effects of this neuroactive compound on these acetylated metabolites levels. Relatively low concentrations of N-acetyldopamine were observed in the cerebral ganglia. Incubation of ganglia resulted in increased concentrations of N-acetyldopamine and N-acetyl 5-hydroxytryptamine. Lindane significantly reduces N-acetyldopamine and N-acetyl 5-hydroxytryptamine levels in the cerebral ganglia. These results can be correlated with a decrease in N-acetyltransferase activity.

Ontogeny of Light-Induced Decrease of N-Acetyltransferase Activity in Explanted Chick Pineal Glands

The ontogeny of chick pineal serotonin N-acetyltransferase (NAT) activity was investigated in explanted chick pineal glands at 4, 10 and 21 d of age. Nocturnal levels of the enzyme and the response of the enzyme to light exposure were determined in pineal glands maintained in short-term culture at each age. The results indicate that nocturnal NAT activity was increased in the glands from older birds. Nocturnal levels of NAT activity at the time of the initiation of the experiment were threefold greater in glands from 21-d-old birds as compared to that in glands from 4-day-old chicks. The response to light was similar in all three ages examined; light induced a significant decrease in NAT activity within 60 min in explanted glands from 4-d-old chicks and within 180 min in the glands from the 10- and 21-d-old chicks. A paradoxical transient increase in enzyme activity occurred immediately (within 5 min) following light exposure which was significant in the glands from the youngest chicks, and present, but more variable, in the older chicks. These data indicate that the nocturnal enzyme activity is greater in glands from older birds, but that light exposure of explanted glands initiates a transient rise followed by a decrease in NAT activity at all three ages.

Androgenic control of N‐acetyltransferase activity in the harderian glands of the syrian hamster is mediated by 5α‐dihydrotestosterone

N-acetyltransferase (NAT) activity in the Harderian glands of intact and gonadectomized male and female Syrian hamsters was evaluated. The exogenous administration of 5 alpha-dihydrotestosterone (DHT) to castrated males and intact females produced an increase in NAT values, which reached the values present in the glands of intact males. The administration of a 5 alpha-reductase inhibitor to intact males led to a decrease in NAT activity, suggesting that testosterone is converted in DHT within the glands. It is concluded that NAT activity in the Syrian hamster Harderian glands is under androgenic control, the active steroid being DHT.

Light-induced decrease of serotonin N-acetyltransferase activity and melatonin in the chicken pineal gland and retina

In the pineal gland and retina of chickens, light exposure at night when serotonin N-acetyltransferase (NAT) activity levels are high causes a 4–5-fold decrease in NAT activity. Thet 1/2 of NAT inactivation is 10 min and the kinetics of inactivation are similar in the pineal gland and retina. A brief pulse of light can initiate the process of NAT inactivation which continues in the dark for 30 min before a partial recovery of NAT activity occurs. In blinded chicks, there is less inactivation of pineal NAT by light than in intact chicks, indicating that the eyes are involved in the pineal's response to light. In chicks that have had their superior cervical ganglia removed inactivation of pineal NAT by light is intermediate between that of intact and blind chicks, indicating that ganglionectomy does not completely mimic the effects of blinding. The pineal gland itself is light-sensitive in culture. Light causes a 4–5-fold decrease in NAT activity in static organ culture, and inhibits melatonin release in flow-through organ culture. Drugs that increase cyclic nucleotide levels in cells (cholera toxin, RO 20-1724, monobutyryl cyclic AMP, monobutyryl cyclic GMP) block the NAT decrease by light, whereas high potassium or EGTA do not block this light-induced NAT inactivation.

Correlation of changes in pineal gland cAMP metabolism with light-mediated decrease in N-acetyltransferase activity

The present study investigated the correlation between the light-mediated decrease in rat pineal N-acetyltransferase (NAT) observed in vivo and changes in cAMP metabolism. While exposing dark-adapted rats to light for short time periods (0-10 min) resulted in a rapid decrease in pineal NAT activity, cAMP exhibited a biphasic response. Following light exposure for 30 s there was a 50% decrease in cAMP levels. However, after 6 min of light exposure the cyclic nucleotide levels had increased 2-3 times above control values. These responses were prevented by phenoxybenzamine pretreatment and the initial decrease was mimicked by i.v. propranolol administration. Examination of cAMP metabolic enzymes, adenylate cyclase and phosphodiesterase revealed an increase in adenylate cyclase activity following 6 min of exposure to light. We discussed how the results observed in vivo compare with those observed using cultured pinealocytes.

Age-associated changes in pineal serotonin N-acetyltransferase activity and melatonin content in the male gerbil.

Pineal serotonin N-acetyltransferase (NAT) activity and melatonin content were measured in 2- and 15-month old male gerbils at various times over a 24h light:dark cycle (LD 14:10). A nocturnal increase over the low daytime values in NAT activity and melatonin content was noted in both 2- and 15-month old gerbils, suggesting the continued existence of a circadian rhythm in each of these biochemical parameters of pineal secretory function at least until 15-months of age. Notable differences in the levels of NAT activity and melatonin content between 2- and 15-month old gerbils included (1) a phase shift in peak NAT activity and melatonin content in 15-month old versus 2-month old gerbils and (2) a latent decrease in NAT activity in 15-month old gerbils after the onset of the light period relative to that in 2-month old gerbils.

Beta adrenergic-blockers decrease adrenergically stimulated N-acetyltransferase activity in pineal glands in organ culture

Treatment with 10 μM l-propranolol caused a rapid decrease in the high level of acetyl coenzyme A: serotonin N-acetyltransferase (EC 2.3.1.5, N-acetyltransferase) activity in pineal glands cultured in the presence of 1 μM norepinephrine. d-Propranolol was ineffective except at high concentration (1 mM). The same stereospecificity was also observed in organ culture when d- or l-propranolol was tested as a blocker of the norepinephrine-induced increase in N-acetyltransferase activity. Practolol (10–300 μM), a relatively specific β 1,-blocker, also initiated a decrease and inhibited the norepinephrineinduced increase in N-acetyltransferase. Butoxamine (10–300 μM), a relatively specific β 2-blocker, had no significant effect in either experimental situation. The percentage decrease in N-acetyltransferase activity after 40 min of treatment with l-propranolol was strongly dependent on temperature. At 40° C enzyme activity decreased by 92% but at 34° C enzyme activity decreased by only 50%. l-Propranolol (20 μM) did not inhibit the increase or initiate a decrease in N-acetyltransferase activity in pineal glands treated with 1 mM N 6,O 2 -dibutyryl adenosine cyclic 3',5'-monophosphate. The fast response observed when l-propranolol was added to the culture medium of norepinephrinetreated glands was not blocked by cycloheximide treatment. Concentrations of ouabain or K +, which are known to completely inhibit the norepinephrine-stimulated increase in N-acetyltransferase activity, caused only a small decrease in induced N-acetyltransferase activity, similar to that due to cycloheximide treatment.