Increased endogenous dopamine prevents myopia in mice.

Experimental evidence suggests that dopamine (DA) modulates refractive eye growth. We evaluated whether increasing endogenous DA activity using pharmacological or genetic approaches decreased myopia susceptibility in mice. First, we assessed the effects of systemic L-3,4-dihydroxyphenylalanine (L-DOPA) injections on form deprivation myopia (FDM) in C57BL/6 J (WTC57) mice. WTC57 mice received daily systemic injections of L-DOPA (n = 11), L-DOPA + ascorbic acid (AA, n = 22), AA (n = 20), or Saline (n = 16). Second, we tested transgenic mice with increased or decreased expression of vesicular monoamine transporter 2 (VMAT2HI, n = 22; WTHI, n = 18; VMAT2LO, n = 18; or WTLO, n = 9) under normal and form deprivation conditions. VMAT2 packages DA into vesicles, affecting DA release. At post-natal day 28 (P28), monocular FD was induced in each genotype. Weekly measurements of refractive error, corneal curvature, and ocular biometry were performed until P42 or P49. WTC57 mice exposed to FD developed a significant myopic shift (treated-contralateral eye) in AA (-3.27 ± 0.73D) or saline (-3.71 ± 0.80D) treated groups that was significantly attenuated by L-DOPA (-0.73 ± 0.90D, p = 0.0002) or L-DOPA + AA (-0.11 ± 0.46D, p = 0.0103). The VMAT2LO mice, with under-expression of VMAT2, were most susceptible to FDM. VMAT2LO mice developed significant myopic shifts to FD after one week compared to VMAT2HI and WT mice (VMAT2LO: -5.48 ± 0.54D; VMAT2HI: -0.52 ± 0.92D, p < 0.05; WT: -2.13 ± 0.78D, p < 0.05; ungoggled control: -0.22 ± 0.24D, p < 0.001). These results indicate that endogenously increasing DA synthesis and release by genetic and pharmacological methods prevents FDM in mice.

Visual conditions affecting eye growth alter diurnal levels of vitreous DOPAC.

In chicks, the diurnal patterns of retinal dopamine synthesis and release are associated with refractive development. To assess the within-day patterns of dopamine release, we assayed vitreal levels of DOPAC (3,4-dihydroxyphenylacetic acid) using high performance liquid chromatography with electrochemical detection, at 4-h intervals over 24 h in eyes with experimental manipulations that change ocular growth rates. Chicks were reared under a 12 h light/12 h dark cycle; experiments began at 12 days of age. Output was assessed by modelling using the robust variance structure of Generalized Estimating Equations. Continuous spectacle lensdefocus or form deprivation: One group experienced non-restricted visual input to both eyes and served as untreated "normal" controls. Three experimental cohorts underwent monocular visual alterations known to alter eye growth and refraction: wearing a diffuser, a negative lens or a positive lens. After one full day of device-wear, chicks were euthanized at 4-h intervals over 24 h (8 birds per time/condition). Brief hyperopic defocus: Chicks wore negative lenses for only 2 daily hours either in the morning (starting at ZT 0; n = 16) or mid-day (starting at ZT 4; n = 8) for 3 days. Vitreal DOPAC was assayed. In chicks with bilateral non-restricted vision, or with continuous defocus or form-deprivation, there was a diurnal variation in vitreal DOPAC levels for all eyes (p < 0.001 for each). In normal controls, DOPAC was highest during the daytime, lowest at night, and equivalent for both eyes. In experimental groups, regardless of whether experiencing a growth stimulatory input (diffuser; negative lens) or growth inhibitory input (positive lens), DOPAC levels were reduced compared both to fellow eyes and to those of normal controls (p < 0.001 for each). These diurnal variations in vitreous DOPAC levels under different visual conditions indicate a complexity for dopaminergic mechanisms in refractive development that requires further study.

Circadian clock genes of goldfish, Carassius auratus: cDNA cloning and rhythmic expression of period and cryptochrome transcripts in retina, liver, and gut.

Clock genes are known to be the molecular core of biological clocks of vertebrates. They are expressed not only in those tissues considered central pacemakers, but also in peripheral tissues. In the present study, partial cDNAs for 6 of the principal clock genes (Period 1-3 and Cryptochrome 1-3) were cloned from a teleost fish, the goldfish (Carassius auratus ). These genes showed high homology (approximately 90%) with the respective cDNAs of zebrafish (Danio rerio), the only other teleost from which clock genes have been cloned. The daily expression pattern of each gene in retina, gut, and liver of goldfish was investigated using quantitative RT-PCR and cosinor analysis. All clock genes analyzed in the retina showed circadian rhythmicity; however, only Per 2-3 and Cry 2-3 were rhythmic in goldfish liver and gut. The amplitude and phase of the expression in liver and gut were different from those found in goldfish retina. Such differences suggest that other cues, such as feeding time, may contribute to the entrainment of oscillators in goldfish liver and gut. Our results support the use of goldfish as a teleost model to investigate the location and functioning of the circadian oscillators.

Light regulation of retinal dopamine that is independent of melanopsin phototransduction.

Light-dependent release of dopamine (DA) in the retina is an important component of light-adaptation mechanisms. Melanopsin-containing inner retinal photoreceptors have been shown to make physical contacts with DA amacrine cells, and have been implicated in the regulation of the local retinal environment in both physiological and anatomical studies. Here we determined whether they contribute to photic regulation of DA in the retina as assayed by the ratio of DA with its primary metabolite, 3,4-dihydroxyphenylacetic acid (DOPAC), and by c-fos induction in tyrosine hydroxylase (TH)-labelled DA amacrine cells. Light treatment (approximately 0.7 log W/m(2) for 90 min) resulted in a substantial increase in DA release (as revealed by an increase in the DOPAC : DA ratio), as well as widespread induction of nuclear c-fos in DA amacrine cells in wild-type mice and in mice lacking melanopsin (Opn4(-/-)). Light-induced DA release was also retained in mice lacking rod phototransduction (Gnat1(-/-)), although the magnitude of this response was substantially reduced compared with wild-types, as was the incidence of light-dependent nuclear c-fos in DAergic amacrines. By contrast, the DAergic system of mice lacking both rods and cones (rd/rd cl) showed no detectable light response. Our data suggest that light regulation of DA, a pivotal retinal neuromodulator, originates primarily with rods and cones, and that melanopsin is neither necessary nor sufficient for this photoresponse.

Light stimulates tyrosine hydroxylase activity and dopamine synthesis in retinal amacrine neurons.

Retinal dopamine-containing amacrine neurons are rapidly activated by light, as shown by an increase in the rate of dopamine formation in vivo and a concomitant increase in the activity of tyrosine hydroxylase, measured in vitro with a subsaturating concentration of pteridine cofactor. Activation of tyrosine hydroxylase also occurs when isolated eyes from rats killed in the dark are exposed to a strobe light. Studies of amacrine neurons should provide basic data about the biochemical processing of visual information, as well as the physiological presynaptic regulatory mechanisms of dopamine-containing neurons.

Dopamine mediates circadian rhythms of rod-cone dominance in the Japanese quail retina.

A circadian clock modulates the functional organization of the Japanese quail retina. Under conditions of constant darkness, rods dominate electroretinogram (ERG) b-wave responses at night, and cones dominate them during the day, yielding a circadian rhythm in retinal sensitivity and rod-cone dominance. The activity of tyrosine hydroxylase, the rate-limiting enzyme in dopamine synthesis, also exhibits a circadian rhythm in the retina with approximately threefold higher levels during the day than at night. The rhythm of tyrosine hydroxylase activity is opposite in phase to the circadian activity of tryptophan hydroxylase, the first enzyme in the melatonin biosynthetic pathway. We tested whether dopamine may be related to the physiological rhythms of the retina by examining the actions of pharmacological agents that effect dopamine receptors. We found that blocking dopamine D2 receptors in the retina during the day mimics the nighttime state by increasing the amplitude of the b-wave and shifting the retina to rod dominance. Conversely, activating D2 receptors at night mimics the daytime state by decreasing the amplitude of the b-wave and shifting the retina to cone dominance. A selective antagonist for D1 dopamine receptors has no effect on retinal sensitivity or rod-cone dominance. Reducing retinal dopamine partially abolishes rhythms in sensitivity and yields a rod-dominated retina regardless of the time of day. These results suggest that dopamine, under the control of a circadian oscillator, has a key role in modulating sensitivity and rod-cone dominance in the Japanese quail retina.

Depletion of catecholamines in the brain of rats differentially affects stimulation of locomotor activity by caffeine, D-amphetamine, and methylphenidate.

The purpose of this study was to assess the role of catecholamines in brain, in the stimulation of locomotor activity, induced by caffeine, as compared to the psychomotor stimulants D-amphetamine and methylphenidate. Adult male rats were pretreated with either (1) 2.5 mg/kg (i.p.) reserpine, 24 hr prior to testing of locomotor activity, (2) 50 mg/kg (i.p.) alpha-methyl-para-tyrosine (AMPT) 6 hr and 2 hr prior to testing of locomotor activity, (3) 200 micrograms/rat (i.c.v.) 6-hydroxydopamine (6-OHDA), or 25 mg/kg (i.p.) desmethylimipramine (DMI) and 200 micrograms/rat 6-OHDA (i.c.v.), 6-8 weeks prior to testing. Each treatment group had a matched control group. Levels of catecholamines in the forebrain were determined in each of the treatment and corresponding control groups. All rats were tested with doses of caffeine, D-amphetamine and methylphenidate (excluding the 6-OHDA-treated animals), administered in random order intraperitoneally 35 min before locomotor activity was measured for 30 min. Pretreatment with either reserpine or AMPT attenuated the stimulation of locomotor activity induced by caffeine and D-amphetamine but not that induced by methylphenidate. The dose-response curve for amphetamine was shifted downward and to the right by reserpine but was flattened by AMPT. The dose-response curve for caffeine was displaced downward in a similar manner by both reserpine and AMPT. Treatment with 6-OHDA or DMI + 6-OHDA produced the expected changes in the content of catecholamines in brain, but failed to modify dose-response curves for caffeine or amphetamine.(ABSTRACT TRUNCATED AT 250 WORDS)

The melatonin antagonist luzindole protects retinal photoreceptors from light damage in the rat.

PURPOSE: Systemic administration of melatonin can increase retinal light damage in the rat. The role of retinal melatonin receptors in modulating light-damage susceptibility was investigated by intravitreally injecting the melatonin receptor antagonist luzindole into rats. METHODS: Nine Sprague-Dawley albino rats 8 to 9 weeks of age were kept in 50 lux cyclic light for at least 7 days before receiving an intravitreal injection of 1 microl 1 mM luzindole in one eye and 1 microl vehicle in the other eye. The injection was given just before the beginning of the normal 12-hour dark phase. At the end of this dark period, animals were exposed to constant light of 2500 lux for 48 hours. Animals were returned to dim cyclic light for 7 days, and dark-adapted electroretinograms (ERGs) were then recorded from the two eyes simultaneously. The eyes were processed for retinal morphology. Photoreceptor nuclei were counted in the outer nuclear layer (ONL), and the thickness of the ONL and that of the rod outer-segment plus inner-segment layer were measured at several points along sections through the vertical meridian. Two age-matched control rats were maintained in dim cyclic light but received no injections. RESULTS: Luzindole-treated eyes had ERG b-wave thresholds of 2.7 +/- 0.5 (mean +/- SEM) log candela (cd)/m2 lower than the fellow eyes injected with vehicle (P < 0.001), and the maximum b-wave amplitude was 1.0 +/- 0.2 log microV greater in luzindole-treated eyes (P < 0.001). Thresholds of the scotopic threshold response were 0.5 +/- 0.1 log cd/m2 lower than those in vehicle-injected eyes (P < 0.05). Luzindole-treated eyes on average had twice as many photoreceptor cells remaining (P < 0.005). In some areas, several rows of photoreceptor nuclei and outer segments remained in the luzindole-treated eye, whereas the fellow control eye showed cells only occasionally and no outer segments. CONCLUSIONS: Eyes pretreated with the melatonin receptor competitive antagonist luzindole before the dark phase preceding constant light exposure were substantially protected from light damage to the retinal photoreceptors. These results implicate the intraocular melatonin-dopamine system in the regulation of light-damage susceptibility.

Effects of apomorphine, a dopamine receptor agonist, on ocular refraction and axial elongation in a primate model of myopia.

The authors examined the effect of local administration of a dopamine receptor agonist on visual deprivation-induced excessive ocular growth and myopia. Eight rhesus monkeys were monocularly deprived of vision from birth with opaque contact lenses. Four of the monkeys received drops of 1% apomorphine HCl 2-3 times/day in the occluded eye; the four control monkeys received vehicle only. Axial lengths were determined by A-scan ultrasonography at birth and at 5-7 months of age. The authors assessed the axial elongation by comparing the postnatal growth in the axial dimension of the occluded eyes with the postnatal growth in nonoccluded eyes. In three of the four control monkeys, occlusion increased axial growth by an average of 1.3 mm. In contrast, they found that growth of the occluded and nonoccluded eyes of the apomorphine-treated monkeys was equivalent, except in one monkey whose nonoccluded eye did not develop normally and was anomalously small. At 6.5-9.5 months of age, three of four controls had myopic refractive errors (-3 to -7 diopters) in the occluded eyes; three of four of the apomorphine-treated monkeys had hyperopic refractive errors (+1-(+)3 diopters) in their occluded eyes. The occluded eye of the fourth monkey was only -0.5 diopters myopic. The findings suggest that apomorphine administration retards excessive axial elongation and the concomitant development of myopia associated with visual deprivation in primates.

Intracellular Ca2+ concentrations in cultured chicken photoreceptor cells: sustained elevation in depolarized cells and the role of dihydropyridine-sensitive Ca2+ channels.

PURPOSE: Retinal photoreceptor cells are tonically depolarized in darkness. Ca2+ influx in darkness plays a critical role in the regulation of neurotransmitter release and melatonin synthesis in these sensory cells. The purpose of the present study was to examine the dynamic changes of intracellular Ca2+ concentrations ([Ca2+]in ) in response to a tonic depolarizing stimulus and to determine the role of dihydropyridine-sensitive calcium channels in the response. METHODS: Photoreceptor cells were prepared from embryonic chick retina and cultured for 6-12 days. Cells were depolarized by exposure to 35 mM extracellular K+. [Ca2+]in of individual photoreceptor cell bodies/synaptic terminals was determined by ratiometric fura-2 image analysis. RESULTS: Chemical depolarization with 35 mM [K+]out greatly increased [Ca2+]in of inner segment/synaptic terminal regions of photoreceptors. The increase usually reached a plateau after the first few minutes of stimulation and was sustained for prolonged periods (>2 h) in the presence of high K+. When the extracellular K+ concentration was reduced, the [Ca2+]in rapidly returned to the basal level. Substitution of 1 mM CoCl2 for CaCl2 in the superfusion medium rapidly and reversibly reduced the [Ca2+]in of depolarized photoreceptor cells. Antagonists of L-type Ca2+ channels, nitrendipine and nifedipine, inhibited the K+-evoked increase of [Ca2+]in. Bay K 8644, a dihydropyridine Ca2+ channel agonist, potentiated the increase of [Ca2+]in elicited by high K+. In some cells, Bay K 8644 alone increased [Ca2+]in under basal conditions. CONCLUSIONS: The increase of [Ca2+]in elicited by depolarization with 35 mM extracellular K+ is due to influx of calcium through the dihydropyridine-sensitive voltage-gated channels. Intracellular [Ca2+] remains elevated for extended periods of time during tonic depolarization. This sustained response requires continuous Ca2+ channel activity.

Circadian expression of tryptophan hydroxylase mRNA in the chicken retina.

Many aspects of retinal physiology are controlled by a circadian clock located within the eye. This clock controls the rhythmic synthesis of melatonin, which results in elevated levels during the night and low levels during the day. The rate-limiting enzyme in melatonin biosynthesis in retina appears to be tryptophan hydroxylase (TPH)[G.M. Cahill and J.C. Besharse, Circadian regulation of melatonin in the retina of Xenopus laevis: Limitation by serotonin availability, J. Neurochem. 54 (1990) 716-719]. In this report, we found that TPH mRNA is strongly expressed in the photoreceptor layer and the vitread portion of the inner nuclear layer; the message is also expressed, but to a lesser extent, in the ganglion cell layer. The abundance of retinal TPH mRNA exhibits a circadian rhythm which persists in constant light or constant darkness. The phase of the rhythm can be reversed by reversing the light:dark cycle. In parallel experiments we found a similar pattern of expression in the chicken pineal gland. However, whereas a pulse of light at midnight suppressed retinal TPH mRNA by 25%, it did not alter pineal TPH mRNA, suggesting that there are tissue-specific differences in photic regulation of TPH mRNA. In retinas treated with kainic acid to destroy serotonin-containing amacrine and bipolar cells, a high amplitude rhythm of TPH mRNA was observed indicating that melatonin-synthesizing photoreceptors are the primary source of the rhythmic message. These observations provide the first evidence that chick retinal TPH mRNA is under control of a circadian clock.

GABA and the regulation of serotonin N-acetyltransferase activity in amphibian retina-I. Effects of GABA agonists and antagonists.

Retinal melatonin biosynthesis is regulated in part by changes in the activity of serotonin N-acetyltransferase (NAT), which increases at night in dark-adapted retinas, but not in light-exposed retinas. Using an in vitro preparation of Xenopus laevis (African clawed frog) eye cups, we have obtained evidence supporting the involvement of gamma-aminobutyric acid (GABA) in the regulation of NAT activity. GABA, the GABA-A receptor agonists muscimol and isoguvacine, and the GABA-B receptor agonist (?)baclofen, in the presence of 3-isobutyl-1-methylxanthine, mimicked dark adaptation by increasing the activity of NAT in light-exposed retinas. The response to GABA agonists was not additive to that observed in darkness. Diazepam increased NAT activity of light-exposed retinas when added in the presence of muscimol, but had no significant effect when added alone. Picrotoxin, an antagonist of the GABA-A receptor-linked Cl(?) channel, blocked both the stimulation caused by dark adaptation and that caused by GABA-A agonists. The increase of NAT activity elicited by muscimol, but not that by baclofen, was blocked by bicuculline methobromide and picrotoxin. The results implicate GABA, acting through GABA-A and possibly GABA-B receptors, in the regulation of NAT activity in retina.

GABA and the regulation of serotonin N-acetyltransferase activity in amphibian retina-II. The role of dopamine.

Retinal melatonin biosynthesis is regulated in part by the activity of serotonin N-acetyltransferase (NAT), which increases in dark-adapted, but not light-exposed, retinas at night. Using an in vitro eye cup preparation from the African clawed frog (Xenopus laevis), we have obtained evidence indicating that dopamine and gamma-aminobutyric acid (GABA) interact in the regulation of the nocturnal rise in NAT activity. Increases of NAT activity induced by the GABA agonist muscimol were suppressed by dopamine. Spiperone, a D2 dopamine receptor antagonist, and muscimol separately increased NAT activity, but were not additive in their effects. Inhibition of NAT activity by the GABA antagonist picrotoxin was blocked by spiperone. Additionally, muscimol decreased concentrations of dopamine and its principle metabolite, 3,4-dihydroxyphenylacetic acid (DOPAC), in light exposed retinas, while picrotoxin increased retinal DOPAC levels in darkness. These data suggest that in darkness, activation of GABA receptors inhibits dopamine secretion, consequently releasing NAT-synthesizing cells from a tonic inhibitory influence.

Melatonin biosynthesis in photoreceptor-enriched chick retinal cell cultures: role of cyclic AMP in the K(+)-evoked, Ca(2+)-dependent induction of serotonin N-acetyltransferase activity.

The roles of cyclic AMP and calcium in the regulation of serotonin N-acetyltransferase (NAT) activity were studied in low density monolayer cultures of chick retinal photoreceptors and neurons. Photoreceptor-enriched retinal cell cultures were prepared from embryonic day 6 retinas and cultured for 6 days. NAT activity in these cultures could be induced by treatment with cyclic AMP protagonists, 8Br-cyclic AMP, forskolin, and 3-isobutyl-1-methylxanthine (IBMX), or by treatment with depolarizing concentrations of extracellular K+. The stimulatory effect of K+, which involves Ca2+ influx through dihydropyridine-sensitive channels, was mediated at least in part by cyclic AMP, as indicated by the following observations. Depolarizing concentrations of K+ stimulated the formation of cyclic AMP, and the stimulatory effects of K+ on both cyclic AMP formation and on NAT activity were synergistically potentiated by the cyclic nucleotide phosphodiesterase inhibitor 3-isobutyl-1-methylxanthine (IBMX). MDL 12,330A, a putative adenylate cyclase inhibitor, inhibited K(+)-evoked cyclic AMP accumulation and induction of NAT activity over the identical concentration range. In contrast, MDL 12,300A failed to inhibit the induction of NAT elicited by 8Br-cyclic AMP. H-89, an inhibitor of cyclic AMP-dependent protein kinase, antagonized the induction of NAT activity by either forskolin or K+ with equal potency for both stimuli. These results suggest that cyclic AMP plays an essential role in the induction of NAT activity that occurs as a consequence of membrane depolarization. Cyclic AMP and Ca2+ may also interact at a step distal to adenylate cyclase.(ABSTRACT TRUNCATED AT 250 WORDS)

Diurnal rhythms of tryptophan hydroxylase activity in Xenopus laevis retina: opposing phases in photoreceptors and inner retinal neurons.

Tryptophan hydroxylase (TPH) is the first enzyme in the biosynthetic pathways of melatonin in photoreceptor cells and of serotonin in amacrine cells. To assess the regulation of TPH activity in photoreceptor cells, we pretreated retinas with kainic acid. The neurotoxin selectively killed inner retinal neurons while sparing photoreceptors. TPH activity in both control and kainate-treated retinas undergoes a day-night rhythm. The rhythms in both preparations fit sinusoidal functions. However, the rhythm in intact retinas peaks at midday while that in kainate-lesioned retinas does so at midnight. The daily rhythm of tryptophan hydroxylase activity in photoreceptors parallels that of melatonin release. Comparing the mean level of activity in rhythms of intact and lesioned retinas, we calculate that the TPH activity in photoreceptors represents 24% of the total activity. Therefore, the TPH activity measured in intact retinas reflects mainly the enzymatic activity in serotonergic neurons, masking that from photoreceptors. In contrast, the levels and diurnal variation of TPH mRNA did not differ in intact and kainate-lesioned retinas indicating that measurements of TPH mRNA content reflect primarily that in photoreceptor cells. Thus, TPH mRNA levels and enzyme activity are differentially regulated in amacrine neurons and photoreceptor cells. This differential regulation markedly impacts the patterns of daily rhythms observed in the intact retina.

Elevation of melatonin in chicken retina by 5-hydroxytryptophan: differential light/dark responses.

Melatonin is synthesized in the chicken retina under the influence of a circadian clock, which also regulates the expression of tryptophan hydroxylase (TPH) and serotonin N-acetyltransferase (AA-NAT). In order to examine the role of substrate supply in the rhythmic synthesis of melatonin in chicken retina, tryptophan and 5-hydroxytryptophan were administered day and night in light or darkness. When administered systemically at night in darkness, 5-hydroxytryptophan, but not tryptophan, dramatically stimulates melatonin levels in the chick retina in a dose-dependent manner. Intraocular administration of 5-hydroxytryptophan also increases melatonin levels locally, indicating a retinal site of action of the serotonin precursor. The effect of 5-hydroxytryptophan is much greater at night, when TPH and AA-NAT activities are high, than during the day, when the enzyme activities are low. Similarly, unexpected light exposure at night, which inactivates AA-NAT, significantly reduces the ability of 5-hydroxytryptophan to increase retinal melatonin levels. The results suggest that TPH, but not AA-NAT or other enzymes in the melatonin biosynthetic pathway, is saturated with substrate in situ. The rate of melatonin production appears to be a function of the concentration of serotonin, which is regulated by TPH, and by the level of activity of AA-NAT.

Behavioral evaluation of hemiparkinsonian MPTP monkeys following dopamine pharmacological manipulation and adrenal co-graft transplantation.

Bradykinesia and rigidity are the symptoms that most directly correlate with loss of striatal dopamine in Parkinson's disease. In the hemiparkinsonian (HP) monkey, this is represented by paucity of movement as measured by coli puterized movement analysis, diminished manual dexterity on clinical examination, and diminished performance on operant behavioral tasks. The present study used an MPTP-induced HP model in rhesus monkeys to evaluate the effectiveness of adrenal medullary and peripheral nerve co-grafts in diminishing parkinsonian symptoms. Unoperated controls (N = 4), surgical controls with caudate lesioning (N = 4), and caudate co-grafted (N = 4) HP monkeys demonstrated diminished movement in the home cage following MPTP. This behavior persisted in unoperated controls, but improved in both surgical control and co-grafted monkeys. Functional hand dexterity evaluations demonstrated similar impairment in all three groups but only surgical controls and co-grafted monkeys demonstrated improvement. In general, rotational behavior in response to apomorphine was consistent with recovery of function in surgical controls and co grafted monkeys, but marked between-subject variability precluded group statistical analyses. None of the monkeys could perform the operant task using the affected limb following MPTP. However, the performance of two co-grafted animals demonstrated partial recovery. L-dopa improved operant performance, demonstrating a dopaminergic component to the task. The results demonstrate recovery of behavioral function after surgical treatment, with adrenal co-grafted monkeys showing the greatest degree of improvement.

Midbrain lesions, dopamine and male sexual behavior.

Destruction of the medial preoptic area (MPOA) eliminates mating in male rats and this region is believed to play a major role in the control of male sexual behavior. Efferents from the MPOA pass through and/or terminate in 4 midbrain regions: the dorsolateral tegmentum (DLT), the central gray, and the A9 and A10 areas. The present study reports the effects of bilateral destruction of each of these midbrain regions on brain catecholamines and sexual behavior in male rats. DLT lesions eliminated mating, reproducing the effect of bilateral preoptic lesions. The sexual activity of males with central gray lesions was accelerated in the sense that the mounting rate for these males was significantly faster than for controls and lesioned males ejaculated more frequently and with shorter latencies than did controls. A9 lesions impaired mating--lesioned males mounted at a slower rate and ejaculated less frequently than controls. Males with A10 lesions took longer to re-initiate mating after an ejaculation than controls, but copulation per se (as reflected in mount rate, ejaculation frequency and latency to ejaculate) was not affected by A10 damage. Brain catecholamine levels were not affected by either DLT or central gray lesions. A9 lesions produced a significant depletion in neostriatal dopamine which was highly correlated with mount latency, mount rate, ejaculation latency and ejaculation frequency. A10 lesions produced a significant depletion of dopamine in the nucleus accumbens and cingulate cortex, but these effects were not significantly correlated with any measure of sexual behavior.

Alterations in light-evoked dopamine metabolism in dystrophic retinas of mutant rds mice.

In dystrophic retinas of rds mice, which are devoid of photoreceptor outer segments, high steady state levels of dopamine were found in dark and light periods. These levels were similar to those observed in normal, BALB/c mouse retinas. Major differences were determined, however, between dopamine turnover in normal and dystrophic retinas. While substantial light-evoked elevation of dopamine synthesis and utilization was observed in normal retinas, dopamine synthesis and metabolism in rds retinas was very low and response to light was depressed. Retinal dopamine metabolism was already depressed in 2 week old rds mice, prior to the onset of photoreceptor cell death, relative to that in age-matched BALB/c mice. At 1 month of age, robust light/dark differences in retinal dopamine metabolism were observed in BALB/c mice, while no significant effect of light was seen in rds mice. The limited ability of the dopaminergic system in rds retinas to respond to light may be due to the absence of normal outer segments. Interestingly, in old rds retinas, although most photoreceptor cells had degenerated, a small but significant light-evoked increase in dopamine metabolism was measured. The presence of relatively high steady state levels of dopamine in rds retinas, despite the reduced dopamine synthetic activity, is maintained by a compensatory reduction in dopamine utilization. Thus, although a considerable amount of dopamine is present in the rds retina, it might not be available to exert its biological functions.

Dopamine receptor-mediated inhibition of serotonin N-acetyltransferase activity in retina.

The possible involvement of catecholamines in the regulation of serotonin N-acetyltransferase (NAT) activity in retina of the African clawed frog was investigated using an in vitro eye cup preparation. Dopamine (10 microM) and norepinephrine (50 microM) had no significant effect on NAT activity of eye cups incubated in the light. However, dopamine inhibited the increase of retinal NAT activity that occurs in eye cups incubated in darkness; the ED50 for dopamine was 0.3 microM. The effect of dopamine on NAT activity was mimicked by the dopamine receptor agonists apomorphine and bromocriptine, but not by agonists of alpha 1-, alpha 2- or beta-adrenergic receptors. Dopamine-mediated inhibition of NAT activity was antagonized by spiroperidol and by alpha-flupenthixol, but not by beta-flupenthixol, phentolamine or timolol. Benztropine, an inhibitor of dopamine reuptake, also decreased NAT activity in eye cups incubated in the dark. The inhibitory effect of benztropine was antagonized by spiroperidol, suggesting that it was mediated by an increase in the extracellular concentration of endogenous dopamine. These studies indicate that the regulation of NAT activity in the retina is subject to modulation by a dopamine receptor-mediated mechanism and suggest that dopamine may play a role in the inhibition of NAT activity by light.