Ontogeny of mechanisms which regulate light/dark differences in retinal dopamine synthesis.

The ability of bicuculline, a GABA antagonist, to enhance dopamine (DA) synthesis in retinas of rats 1, 4, 7, 15 and 60 days after eye opening was assessed and compared to the time course of postnatal development of the light-induced increase in DA synthesis. The accumulation of dihydroxyphenylalanine (DOPA) following administration of the L-aromatic amino acid decarboxylase inhibitor, NSD 1015, was used to estimate DA synthesis. In dark-adapted rats, neither bicuculline nor light enhanced DOPA accumulation 1 day after eye opening, but on the remaining days either treatment significantly augmented DA synthesis, and by day 15 the effects were as great as those observed in adult retinas. At each time point, the magnitude of the drug effect on DA synthesis in the dark was similar to that observed following light exposure. These results suggest that an endogenous GABAergic input to the DA neurons appears at the same time as the acquisition of the dopaminergic response to light. The effect of bicuculline treatment on DA synthesis in light-exposed animals was also assessed. At 4 and 7 days the drug significantly enhanced DOPA accumulation over that produced by exposure to light alone, but on later days bicuculline exerted no such additive effect. These data imply that early in the maturation of the light response mechanisms other than removal of an inhibitory GABAergic tone may be partially responsible for excitation of the DA neurons.

Effect of strychnine and of bicuculline on dopamine synthesis in retinas of dark-maintained rats.

The intravitreal administration of strychnine produced a significant elevation of dopamine (DA) synthesis in the retinas of dark-adapted rats. Strychnine had no antagonistic effect on the ability of muscimol to suppress the normal increase in retinal DA synthesis observed after exposure of rats to light. This latter observation suggests that strychnine is not merely acting in a non-selective fashion to suppress the inhibitory GABAergic tone exerted on the DA neurons in darkness. These results suggest that there may be a direct or indirect strychnine-sensitive input to the retinal DA neurons in the dark. This input may not be glycinergic, however, as even high dosages of glycine did not suppress the light-mediated enhancement of DA synthesis in the retina.

Extended exposure to continuous low intensity light abolishes the photosensitivity of retinal dopamine neurons.

Male Sprague-Dawley rats were divided into 2 groups. One group (experimental) was housed for 6 months in continuous low intensity light while the other (control) was exposed to standard 14 hr light: 10 hr dark cyclic lighting conditions for the entire time. For both the control and experimental groups the light intensity was 350-700 Lux. After 6 months, the experimental rats were returned to cyclic lighting. At one week and again at 2 months the light aversion behavior of all rats was tested in a light/dark test box. The experimental rats chose the dark side of the box only 58% of the time while control animals preferred the dark 79% of the time. Since rats normally are nocturnal and avoid light, these results suggest that the experimental rats may have permanently lost a functionally significant portion of the ability to detect light. After the second behavioral test all rats were dark adapted and 15 hr later the effect of short term (30 or 60 min) exposure to light on DA turnover in one retina from each rat was assessed. The other retina from each rat was fixed and examined histologically. Light significantly enhanced the alpha methyl-p-tyrosine induced decline of DA in the retinas of the control rats but exerted no similar effect in the experimental animals. The retinal DA contents of the experimental rats were substantially depleted. Histological examination suggested that the outer nuclear layers of the experimental retinas were more severely damaged than those from rats exposed to continuous light for 4 months but still contained a few pycnotic photoreceptor nuclei and nearly normal looking inner neural layers. These results indicate that extended exposure to light eventually abolishes light aversion behavior and at this time there is also a loss of the photosensitivity of the dopaminergic amacrine neurons.

Effects of excitatory amino acids on dopamine synthesis in the rat retina.

Intraocular administration of quisqualate but not of N-methyl-D-aspartate or kainate augments dopamine synthesis in the retinas of dark-maintained rats. The effect of quisqualate is dosage-dependent but not blocked by the glutamate antagonist, glutamate diethyl ester. These results are the first to show that the quiescent dopamine neurons of the dark-adapted rat retina can be activated by an excitatory amino acid analogue of the putative endogenous neurotransmitter, glutamate.

Postnatal development of the light response of the dopaminergic neurons in the rat retina.

The effect of light on retinal dopamine (DA) synthesis and content in dark-adapted rats was assessed 15 h and 2, 4, 7 and 16 days after eye opening (13 to 14 days after birth). The accumulation of dihydroxyphenylalanine (DOPA) following inhibition of its decarboxylation was used to estimate DA synthesis. At 7 and 16 days, but not earlier, light significantly augmented DOPA formation. These increases were as dramatic as those reported for adult rats. DA in dark-adapted retinas ranged from 0 (undetectable) at 15 h to 83% of adult levels at 16 days, but were only 36% of that of adult retinas at 7 days. Light produced a significant decline in DA at 16 days but not at any other time point. These results indicate that the dopaminergic neurons synthesize transmitter and respond to light at a time when the neuronal pools of DA are not yet mature.

Benzodiazepines suppress the light response of retinal dopaminergic neurons in vivo.

Sedative dosages of the benzodiazepine, diazepam (76 micro mol/kg, i.p.), reduce the light-evoked increase in retinal dopamine turnover, while intraocular applications of the water-soluble benzodiazepine, flurazepam (0.50 or 1.0 micro mol/eyeball), produce a dose-dependent suppression of light-enhanced dopamine synthesis. These results provide the first evidence that the benzodiazepines alter a physiologically important retinal response in vivo. They also suggest that some the visual effects produced by the benzodiazepines may have an intraretinal locus of action.

Use of liquid chromatography with electrochemistry to measure effects of varying intensities of white light on DOPA accumulation in rat retinas.

Exposure of dark-adapted rats to light enhances the activity of the retinal dopamine (DA) neurons. The purpose of this study was to determine if the response of these neurons to light varies with different intensities of light. The accumulation of dihydroxyphenylalanine (DOPA) after inhibition of L-aromatic amino acid decarboxylase with NSD-1015 was used as a measure of the in vivo activity of these DA neurons. Retinal DOPA accumulation was significantly increased in dark-adapted rats that had been exposed to light for only 5 min. The activation of the retinal DA neurons by cool white fluorescent lighting was dependent upon the light intensity. Light intensities of 0.1 and 0.5 lux did not stimulate the retinal DA neurons. There was a significant, but submaximal, activation of the neurons by 5.0 lux, and intensities of 32.2 lux or more maximally stimulated the neurons. The method involving liquid chromatography (LC) with electrochemistry (EC) which was used in these experiments to measure retinal DOPA and DA concentrations is also described in detail.

GABA antagonists enhance dopamine turnover in the rat retina in vivo.

Previous results provided evidence for an inhibitory GABAergic influence on the dopamine neurons of the rat retina, without proving that endogenous GABA physiologically regulates the activity of these cells. We injected picrotoxinin intraocularly to dark-adapted rats and measured retinal dopamine turnover. Dopamine was analyzed radioenzymatically, and the decline in dopamine after alpha-methyl-p-tyrosine was used as an index of dopamine turnover. Picrotoxinin significantly stimulated dopamine turnover (P less than 0.05). In similar experiments with light-exposed rats picrotoxinin slightly enhanced dopamine turnover beyond that produced by light alone. Intraocular bicuculline methiodide produced similar results in dark-adapted and also in light-exposed rats. These data suggest that endogenous GABA tonically inhibits the activity of the retinal dopamine neurons in the dark and that there may be some remaining GABA tone on these cells in the light-exposed rat. Atropine pretreatment did not affect the picrotoxinin-induced activation of retinal dopamine turnover which negates the involvement of a cholinergic interneuron.

Measurement of mouse brain glucose utilization in vivo using [U-14C]glucose.

The rate of [2-14c]glucose uptake has been used as an indication of the status of energy consumption by the rat brain, but the cost of this radiolabel can be prohibitive and the surgical manipulation involved in published methods is extensive. A method for measuring glucose utilization in vivo in mouse brain with [U-14C]glucose is described in this article. Glucose consumption in whole mouse brain obtained with [U-14C]glucose or [2-14C]glucose was 0.650 +/- 0.022 -and 0.716 +/- .036 nmol/mg/min, respectively. In all instances the rate obtained with the uniformly labeled isotope was somewhat lower than that found with [2-14C]glucose. The rate of glucose utilization measured with either isotope was significantly depressed in sodium pentobarbital anesthetized mice. The method described here is advantageous because [U-14C]glucose is substantially less expensive than [2-14C]glucose and surgical intervention is avoided.