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Biomedical subjects

J Z Nowak

Publications and source records attributed to J Z Nowak.

At least 37 records · Page 2Linked to original sources
PubMed1998

A synergistic interaction between histamine and vasoactive intestinal peptide (VIP) on cyclic AMP production in the chick pineal gland.

The effects of vasoactive intestinal peptide (VIP) and histamine, alone and in combination, on cyclic AMP formation have been studied in selected tissues of the central nervous system (CNS) of chick. VIP strongly stimulated cyclic AMP production in the pineal gland, moderately in the retina (maintained in "eye-cup" preparations), and had no effect in cerebral cortical slices. Combination of VIP with forskolin produced a synergistic response in the pineal gland; in the cerebral cortex VIP did not influence the elevation of cyclic AMP production evoked by forskolin, histamine and isoprenaline. Histamine stimulated cyclic AMP synthesis in the all tested CNS tissues, showing the following order of sensitivity to the amine: pineal gland > cerebral cortex > retina. The effects of histamine were stronger in the presence of forskolin. A combination of VIP and histamine produced the cyclic AMP response in the pineal gland clearly more than additive; such a synergistic interaction was antagonized by aminopotentidine, an accepted in mammals H2-histamine receptor blocker. It is concluded that both VIP and histamine can be considered as functionally important neuromodulators in avians (similar to mammalian species). These two substances may play in concert to regulate the pineal physiology in avian species.

Amino Acid Sequence↗
PubMed1997

Histamine-stimulated cyclic AMP formation in the chick pineal gland: role of protein kinase C.

The role of protein kinase C (PKC) in histamine (HA)-stimulated cyclic AMP formation in intact chick pineal glands was investigated. In the pineal gland of chick HA, 2-methylHA, 4-methylHA, and N alpha, N alpha-dimethylHA potently increased cyclic AMP accumulation in a concentration-dependent manner. Treatment of intact glands with PKC inhibitors, i.e. chelerythrine and stautosporine, reduced the stimulatory effect of the HA-ergic compounds on cyclic AMP formation. HA, 2-methylHA, 4-methylHA, and N alpha, N alpha-dimethylHA significantly increased inositol-1,4,5-trisphosphate (IP3) levels in intact chick pineal glands, indicating their activities on phospholipase C and 1,2-diacylglycerol formation. The stimulatory effect of HA on IP3 levels was antagonized by aminopotentidine, a potent blocker of H2-like HA receptors in avian pineal gland. Preincubation of chick pineal glands with a PKC activator, 4 beta-phorbol 12, 13-dibutyrate (4 beta-PDB), enhanced the accumulation of cyclic AMP elicited by HA, 2-methylHA, 4-methylHA, and N alpha, N alpha-dimethylHA. On the other hand, 4 beta-phorbol, inactive on the PKC, was ineffective. Our results point to the possibility that PKC is involved in the regulation by HA of cyclic AMP synthesis in the pineal gland of chick. Furthermore, the cyclic AMP response to pineal HA receptor stimulation can be positively modulated by a concomitant activation of the PKC pathway.

Animals↗
PubMed1997

Histamine in the chick pineal gland: origin, metabolism, and effects on the pineal function.

The chick pineal gland contains histamine and tele-methylhistamine. The levels of both substances are elevated after treatment of chicks with the amino acid precursor of histamine, L-histidine (1 g/kg, ip). In control and L-histidine-loaded animals the pineal levels of histamine and tele-methylhistamine are higher in light-exposed than in dark-adapted animals (measured at the end of the light phase and in the middle of the dark phase of 12 hr light, 12 hr dark illumination cycle, respectively). The chick pineal gland contains histamine-immunofluorescent cells displaying mast cell morphology; they are seen in the vicinity of the capsule and in the parenchyma. Enzymatic studies showed the presence of the activity of histamine synthesizing and inactivating enzyme, i.e., L-histidine decarboxylase (HDC) and histamine-methyltransferase (HMT). The detected enzyme activities were sensitive to specific inhibitors of HDC (alpha-fluoromethylhistidine and alpha-hydrazinohistidine) and HMT (quinacrine and metoprine); inhibitors of aromatic amino acid decarboxylase alpha-methyl-DOPA and NSD-1015 were inactive on HDC. Exogenous histamine added to organ-cultured chick pineals strongly stimulated endogenous cyclic AMP accumulation and moderately increased melatonin secretion. The data, considered collectively, suggest that in avians histamine, probably originating from the pineal mast cell compartment, may function as a regulator of pineal gland activity.

Animals↗
PubMed1997

Prolonged exposure of chicks to light or darkness differentially affects the quinpirole-evoked suppression of serotonin N-acetyltransferase activity in chick retina: an impact on dopamine D4-like receptor.

Dopamine plays an important role in regulation of melatonin biosynthesis in retinas of several vertebrate species. In the avian retina, the dopamine receptor that controls melatonin production represents a D4-like subtype. Stimulation of this receptor by quinpirole (QNP) results in a dose-dependent decline of the nighttime activity of serotonin N-acetyltransferase (NAT, a key regulatory enzyme in melatonin biosynthesis) and melatonin level of the retina. The present study was undertaken to determine whether the ability of QNP to suppress nocturnal NAT activity of chick retina was affected by prolonged adaptation of animals to light and darkness. In the retina of chicks kept under a light:dark (LD) illumination cycle, dopamine and 3,4-dihydroxyphenylacetic acid (DOPAC) levels measured at the end of the light phase were significantly higher than those found in the middle of the dark phase. In animals maintained under continuous light (LL) or darkness (DD) dopamine and DOPAC contents of the retina measured at these two time points were similar and resembled levels found during, respectively, the light and dark phase in the retina of chicks kept under LD illumination cycle. Adaptation of chicks to LL and DD resulted in an attenuated and enhanced, respectively, response of the retinal NAT activity to the suppressive action of QNP. When compared to the LD group, a parallel shift to the right (LL group) or left (DD group) of the dose-response curve for QNP was observed, and the ED50 values for this dopamine receptor agonist were 3.4-times higher (LL) or 2.8-lower (DD) than those calculated for the control LD animals. It is suggested that prolonged exposure to light or darkness, by altering the level of the retinal dopaminergic neurotransmission, may modify the reactivity of the D4-like dopamine receptors regulating NAT activity of the chick retina.

3,4-Dihydroxyphenylacetic Acid↗
PubMed1997

Dopamine D4-like receptors in vertebrate retina: does the retina offer a model for the D4-receptor analysis?

Dopamine is a major catecholamine in vertebrate retina. The amine is localized, depending on the species, in a subset of amacrine and/or interplexiform cells. The dopamine-containing cells in retina are activated upon light exposure, with resulting increase in the amine synthesis and release. Dopamine, acting as a light-adaptive signal, controls many aspects of retinal physiology; among its diverse effects is modulation of the night-driven melatonin biosynthesis, which occurs in photoreceptors. Activation of dopamine receptors belonging to D2-family, localized on photoreceptors, rapidly suppresses the nocturnal cyclic AMP-dependent activity of serotonin N-acetyltransferase (NAT), a key regulatory enzyme in melatonin biosynthesis. Convincing evidence indicates that these NAT activity-modulating receptors represent the D4-subtype dopamine receptors, which--most probably indirectly--control in a negative manner the activity of intraphotoreceptor Ca2+/calmodulin-dependent adenylyl cyclase. This article reviews current knowledge on dopamine D4-subtype receptors, with a special emphasis to their function in vertebrate retina. In addition, some findings resulting from our recent experiments with newly synthesized D4-receptor-selective ligands are presented and discussed. It is proposed that the avian retina, with its ability to synthesize melatonin in a dopamine-sensitive manner (via D4-like dopamine receptor), may offer a suitable specific in vivo model which allows to study potential ligands (both agonists and antagonists) of the D4-subtype dopamine receptor.

Amino Acid Sequence↗
PubMed1996

Characterization of melatonin receptors in the brain of four avian species: duck, goose, pigeon, and turkey.

Melatonin receptor sites were characterized in the brains of four avian species (duck, goose, pigeon, and turkey) by an in vitro binding technique, using 2-[125I]iodomelatonin as a labeled ligand. The specific binding of 2-[125I]iodomelatonin to avian brain membranes was found to be stable, saturable, reversible, and of high affinity. Scatchard analysis revealed that 2-[125I]iodomelatonin binds to a single class of sites in brain membrane preparations of these species with high affinity (affinity constants Kd between 153 and 289 pM) and low capacity (total number of binding sites B(max) between 5.07 and 10.15 fmol/mg protein). Competition studies showed the following relative order of potency of indolic compounds to inhibit the specific 2-[125I]iodomelatonin binding: melatonin >> N-acetyltryptamine > N-acetyl-5-hydroxytryptamine > 5-hydroxytryptamine >>> 5-methoxytryptamine. We conclude that the 2-[125I]iodomelatonin-binding sites in brains of duck, goose, pigeon, and turkey have similar binding and pharmacological characteristics, and may represent ML-1 melatonin receptor sites.

Animals↗
PubMed1996

Pharmacological modifications in dopaminergic neurotransmission affect the quinpirole-evoked suppression of serotonin N-acetyltransferase activity in chick retina: an impact on dopamine D4-like receptors.

Dopamine (DA) plays an important role in the regulation of melatonin biosynthesis in retinas of several vertebrate species. In the retina of chick, the DA receptor controlling melatonin production represents a D4-like subtype. Stimulation of this receptor by quinpirole (QNP) results in a dose-dependent decline of the nighttime activity of serotonin N-acetyltransferase (NAT; a key regulatory enzyme in melatonin biosynthesis) and melatonin level of chick retina. The present study was undertaken to determine whether long-term treatment with antipsychotic drugs (clozapine-30 mg/kg, i.m.; sulpiride-100 mg/kg, i.m.; and raclopride-10 mg/kg, i.p., once daily for 21 days) and L-DOPA (80 mg/kg, i.p., once daily for 7 days) affects the response of the melatonin generating system of chick retina to the suppressive effect of QNP. Chronic administration to chicks of clozapine and sulpiride, but not raclopride, resulted in a markedly increased response of retinal NAT activity to the action of QNP. ED50 values for QNP were 3-times (clozapine) and 4-times (sulpiride) lower than those in the respective vehicle-treated control groups. On the other hand, QNP was significantly less potent in retinas of birds treated with L-DOPA than in control animals; the ED50 value for QNP was 3-times higher in birds injected with L-DOPA than in the vehicle-treated group. These results indicate that long-term treatment with clozapine, sulpiride and L-DOPA may modify the reactivity of D4-like DA receptors regulating NAT activity of chick retina. A possibility of modifications of circadian and electrophysiological processes within the eye following prolonged administration of DA-ergic drugs is discussed.

Animals↗
PubMed1996

The presence of histamine and tele-methylhistamine in the pineal gland of chick.

The levels of histamine (HA) and tele-methylhistamine (t-MeHA) in chick pineal gland were measured by RIA at two time points, i.e. at the end of the light (L) phase and in the middle of the dark (D) phase of 12 h: 12 h L:D cycle. The chick pineal gland showed high HA levels (12.19 and 13.09 ng/pineal at L and D, respectively). The t-MeHA content of chick pineal gland was about 20-times lower than HA level (650.2 and 536.4 pg/pineal at L and D, respectively). An aminoacid precursor of HA, L-histidine (1 g/kg, ip), given to chicks during L or D significantly increased both HA and t-MeHA content of the pineal gland. The L-histidine-evoked elevations in HA level were 2-4-times higher than changes in t-MeHA content. Enzymatic study showed the presence in chick pineal of a moderate activity of L-histidine decarboxylase (HDC), and well expressed activity of HA-methyltransferase (HMT), HA synthesizing and inactivating enzyme, respectively, which suggests that both HA and t-MeHA may be produced within the gland. It is suggested that the metabolic dynamics of the pineal HA may be higher during lighthours than darkhours of the daily light:dark illumination cycle.

Animals↗
PubMed1996

[The effect of light of various wave lengths on suppression of nocturnal serotonin N-acetyltransferase activity in chick retina].

PURPOSE: Effects of white and monochromatic light of various wave length on the night-time retinal serotonin N-acetyltransferase (NAT) activity were examined in chicks. MATERIAL AND METHODS: Male chicks (white leghorn; 3-4 weeks old) were used. Chicks were purchased on the day of hatching. All animals were offered ad libitum access to standard food and water, maintained under an ambient temperature of 27 +/- 2 degrees C, 60 +/- 5% humidity, and exposed to 12 hr light: dark illumination cycle for a minimum of 8-10 days before experiments. The daytime light intensity at the surface of the animals' cages was about 150 luxes. Each experiment was performed at least twice. At the beginning of the fourth hour of the dark phase of the light-dark illumination cycle groups of chicks (four animals per group) were exposed to either white or monochromatic light for 5, 10, 30 or 60 min, and then killed by decapitation. In another set of experiments birds were illuminated for 5 min, returned to darkness for additional 5, 15, 60 or 120 min, and then decapitated. Decapitation was done quickly under dim red light (2 luxes). Retinas were isolated and frozen on dry ice. Exposure of animal to light took place in 25 x 21 cm white plastic chamber. Light produced by 5 W 14 bulb (Osram) was passed through a cotton filter or filtered with glass, narrow band interference filters, 7 nm half-peak band-width. The spectral wave length analysis for each interference filter was performed with the aid of Diode-Spectrophotometer and the light irradiance the three wave lengths used was measured with YSI Radiometer. To measure NAT activity, retinas were sonicated in 10 vols (w/v) in the proportion of 100 microliters of ice-cold 0.05 M Sodium phosphate buffer (pH 6.8). The enzyme activity was determined in supernatants of tissue homogenates with the radioisotopic method of Steinlechner with modifications of Nowak. Data were expressed as mean +/- standard error of mean (SEM) and analysed for statistical significance by analysis of variance and Student-Newman-Keuls test. RESULTS: The potency of the tested lights to suppress NAT activity of the retina had the following rank order. white > or = green (548 nm) > > blue (434 nm) > red (614 nm). CONCLUSION: In chicks, the suppression of the nocturnal NAT activity produced by a short 5-min pulse of monochromatic light was partially reversible in the retina. The studied chick tissues were far less sensitive to pulses of monochromatic light than of white light.

Animals↗
PubMed1996

[The effect of light with various wavelengths and impulse times on nocturnal suppression of N'acetyltransferase activation by serotonin in the pineal gland of the chick].

PURPOSE: Chick pineal gland synthesise melatonin in circadian rhythm, with peak values in the dark phase of an imposed light-dark illumination cycle. Light is the most important environmental factor regulating the melatonin-generating system in this gland. Exposure to light causes a dramatic decline of the night-time levels of the melatoninergic parameters. Effect of white and monochromatic lights of various wavelength on the night-time pineal gland serotonin N-acetyltransferase (NAT) activity were examined in chicks. MATERIAL AND METHODS: Male chicks (white leghorn: 3-4-week old) were used. They were purchased on the day of hatching. All animals were offered ad libitum access to standard food and water, maintained under an ambient temperature of 27 +/- 2 degrees C (chick), 60 +/- 5% humidity, and exposed to 12 hr light: dark illumination cycle for a minimum of 8-10 days before experiments. The daytime light intensity at the surface of the animals' cages was about 150 luxes. Each experiment was done at least twice. At the beginning of the fourth hour of the dark phase of the light-dark illumination cycle groups of chicks (four animals/group) were exposed to either white or monochromatic light for 5, 10, 30 or 60 min, and then killed by decapitation. In another set of experiment birds were illuminated for 5 min, returned to darkness for additional 5, 15, 60 or 120 min, and then decapitated. Decapitation was done quickly under dim red light (2 lux). Pineal glands were isolated and frozen on dry ice. The exposure of an animal to light took place in 25 x 21 cm white plastic chamber. Light produced by 5 W 14 bulbs (Osram) was passed through a cotton filter or filtered with glass, narrow band interference filters, 7 nm half-peak band-width. The spectral wavelength analysis for each interference filter was performed with the aid of Diode-Spectrophotometer, and the irradiance of the light of the three used wavelengths was measured with YSI Radiometer. The estimated peak wavelengths (Imax) of the filters were: 434 nm (blue), 548 nm (green) and 614 nm (red). The NAT activity was determined in supernatants of tissue homogenates with the radioisotopic method of Steinlechner with modifications by Nowak. RESULTS: Significant decrease of the night-time NAT activity of the chick pineal gland was observed after a 5-min pulse of either white or green light. In birds illuminated by blue light a marked reduction (by about 22%) in the pineal NAT activity was found after 10-min exposure, whereas in the group of animals that were exposed to red light a significant decline of the enzyme activity in pineal gland was produced by a 30-min pulse. The enzyme activity started to increase after 15 min in the dark and reached the control values by 1 hr (red light) or 2 hrs (blue and green lights). Two hrs and 5 min after the exposure to white light (5 min light and 120 min dark) pineal NAT activity was still significantly lower (by 16%) than the activity found in the dark control group. CONCLUSION: Suppressive effect of monochromatic (but not white) light on NAT activity was completely reversible in the pineal gland of chick.

Animals↗
PubMed1995

A bizarre receptor mediating stimulatory effect of histamine on cyclic AMP formation in duck pineal gland.

In agreement with our previous findings in chick pineal, histamine also appeared to be a potent stimulator of cAMP formation in intact duck pineal gland (about ten-fold increase above control at 100 microM), showing an EC50 value of 3-5 microM. The effect of histamine in the duck pineal was mimicked by several histaminergic drugs, with the following order of potency: histamine = 4-methylhistamine > 2-methylhistamine > R-alpha-methylhistamine > amthamine > 2-thiazolylethylamine = immepip > imetit > dimaprit. The effect of 100 microM histamine was unaffected by H1-, H2- and H3-receptor selective antagonists, mepyramine, ranitidine and thioperamide, respectively; yet, two other H2-antagonists, aminopotentidine and tiotidine, reduced the histamine effect by only 40%. Thus, these results, being in line with our previous chick data, give support to a suggestion that histamine may be considered a modulator of the pineal activity, and that the avian pineal may contain a novel or avian-specific histamine receptor modulating cAMP formation, whose pharmacology is different from that typical for H1-, H2-, or H3-type receptor.

Animals↗
PubMed1995

Arylamine and arylalkylamine N-acetyltransferases in retina, pineal gland, brain and liver of chicks: a comparative study.

Regulation of arylamine N-acetyltransferase (A-NAT) and arylalkylamine N-acetyltransferase (AA-NAT) was examined in retina, pineal gland, brain and liver of chicks. Enzyme activities were determined using as substrates p-phenetidine and procainamide for A-NAT, tryptamine and phenethylamine for AA-NAT. The activity of A-NAT in all tissues studied does not appear to be regulated by a light-dark cycle. On the other hand, AA-NAT showed distinct light-dark dependent changes (with high values at night) in the retina and pineal gland, but not in brain and liver. The nocturnal increase of retinal and pineal AA-NAT activity was prevented by cycloheximide; the drug did not affect A-NAT activity in these tissues. Treatment of light-adapted chicks with aminophylline significantly increased AA-NAT activity of the retina and pineal gland, without altering the enzyme activity in brain and liver. In these animals, the activity of A-NAT (procainamide) did not change in any tissue studied, whereas the enzyme activity measured using p-phenetidine as a substrate did decrease but only in the retina. A similar pattern of changes in retinal A-NAT and AA-NAT activities was observed after intraocular injection of d,b-cAMP. The rate of inactivation at 4 degrees C was significantly slower for AA-NAT than A-NAT. NATs from brain and liver displayed the highest and lowest, respectively, liability in the cold. The results indicate that the chick retina contains both A-NAT and AA-NAT. The two enzymes have distinct characteristics and the regulation of their activities is different. The retinal A-NAT is similar to A-NAT present in other tested tissues; however, AA-NAT can be induced at night only in the retina and pineal gland. It is suggested that there are two forms of retinal A-NAT, and that, under specified conditions, the activity of one form (A-NAT; p-phenetidine) may be regulated in an opposite manner to AA-NAT activity.

Analysis of Variance↗
PubMed1995

Dopamine-dependent cyclic AMP generating system in chick retina and its relation to melatonin biosynthesis.

Stimulation of a D4-like dopamine (DA) receptors inhibits a cAMP-dependent increase in serotonin N-acetyltransferase activity and melatonin biosynthesis in the chick retina. In order to gain more insight into the molecular mechanisms underlying this suppressive action of DA, the effects of selective stimulation of the D2-family of DA receptors (including the D4-subtype) on cAMP formation were examined in chick retina using two experimental approaches: measurements of adenylyl cyclase activity in retinal homogenates, and cAMP accumulation in eye cup preparation prelabeled with [3H]adenine. The DA-sensitive adenylyl cyclase system is well expressed in chick retina. DA increased both basal and forskolin-stimulated adenylyl cyclase activity. This effect of DA was antagonized by SCH 23390 (a blocker of D1-family of DA receptors) and not affected by sulpiride (a D20-family blocker). Incubation of retinal homogenates with quinpirole (a predominant agonist of D3/D4 DA receptor subtypes) did not produce any major changes in adenylyl cyclase activity. On the other hand, activation of D4-like DA receptor subtype by quinpirole decreased forskolin-stimulated cAMP formation in intact chick retina maintained in "eye-cup" preparations. It is suggested that D4-like DA receptors regulating melatonin biosynthesis in chick retina may be indirectly linked to the cAMP generating system.

Acetyltransferases↗
PubMed1995

Does histamine stimulate cyclic AMP formation in the avian pineal gland via a novel (non-H1, non-H2, non-H3) histamine receptor subtype.

The effects of histamine (HA) and selective HA H1-, H2 and H3-receptor agonists on cyclic AMP formation were investigated in intact chick and duck pineal glands HA potently stimulated the pineal cycle AMP formation. The effect of HA was mimicked fully by N alpha-methylated histamines, and partially by several histaminergic drugs: 2-thiazolylethylamine (H1) amthamine (H2) and R alpha-methyl-histamine (H3). Dimaprit, another selective H2-agonist showed marginal activity. Forskolin highly potentiated the action of HA, and only weakly affected the effects of 2-thiazolylethylamine and amthamine. In the chick pineal, the stimulatory effects of HA and the tested histaminergic drugs were not blocked by mepyramine and thioperamide (H1- and H3-blockers, respectively), but they were antagonized by H2-receptor selective compounds ranitidine and aminopotentidine, which, however, acted in a noncompetitive manner. Another H2-selective blocker zolantidine antagonized the HA effect only when used at very high (30-100 microM) concentrations. In the duck pineal, the stimulatory effect of HA on cyclic AMP production was unaffected by mepyramine (H1), thioperamide (H3), and ranitidine (H2), and only partially inhibited by the H2-blocker aminopotentidine. Electrophysiological experiments revealed that HA is capable of evoking inward currents in most of the tested cells acutely isolated from chick pineal gland. The present findings further indicate that the pharmacological profile of the avian pineal HA receptor, whose stimulation leads to activation of cyclic AMP production, is different from any known HA receptor type (H1, H2, H3), and suggest the existence of either an avian-specific HA receptor, or a novel HA receptor subtype. Electrophysiological data suggest that the pineal HA receptor may be somehow linked to activation of an ionic channel.

Animals↗
PubMed1995

Light-induced suppression of nocturnal serotonin N-acetyltransferase activity in chick pineal gland and retina: a wavelength comparison.

Effects of white and monochromatic (blue-434 nm, green-548 nm, and red-614 nm) lights on the nighttime retinal and pineal NAT activity were examined in chicks. The potency of the tested lights to suppress NAT activity was similar for the retina and pineal gland, with a following rank order: white > green > blue > or = red. The studied tissues of chick were far less sensitive to pulses of monochromatic light than the rat pineal gland. The potency of light to decrease pineal NAT activity of rat was: white > green >> blue > red. In chicks, the suppression of the nocturnal NAT activity produced by a short 5-min pulse of monochromatic light was completely reversible in the pineal gland, and partially reversible in the retina. Our data suggest the existence of some differences between birds and mammals in terms of sensitivity and mechanisms involved in the light-induced suppression of melatonin biosynthesis.

Animals↗
PubMed1994

Does D4 dopamine receptor mediate the inhibitory effect of light on melatonin biosynthesis in chick retina?

The dopamine (DA) receptor mediating the inhibitory effect of light on melatonin formation in the chick retina was characterized pharmacologically. Nighttime serotonin N-acetyltransferase (NAT) activity was significantly decreased by either light exposure or by intraocular (i.oc.) administration of DA and quinpirole (a predominant D3/D4 DA receptor agonist). Several D2-like DA receptor antagonists, i.e. clozapine, haloperidol, spiroperidol, sulpiride, (+)-UH-232 and YM-09151-2, given i.oc. to light-adapted chicks markedly elevated retinal NAT activity. In contrast, raclopride (a D2/D3 DA receptor antagonist) and remoxipride (a D2-selective DA receptor antagonist) were ineffective. Spiroperidol, clozapine, haloperidol and sulpiride significantly increased melatonin content in the light-exposed retina, but had no effect on the activity of hydroxyindole-O-methyltransferase. None of D2-like DA receptor blockers tested modified the nighttime NAT activity in the chick retina. Our results indicate that the light-evoked inhibition of the nocturnal increase in melatonin biosynthesis in chick retina may involve stimulation of the D4 subtype DA receptor.

Acetyl Coenzyme A↗