Modulation of [3H]-dopamine released by different frequencies of stimulation from rabbit retina

Dopamine transporter immunoreactive terminals in the bovine pineal gland

The dopaminergic system has been proposed to be one of the innervations in controlling the mammalian pineal gland function. The dopamine receptors have been characterized in the pineal and the biphasic effects of dopamine on melatonin production have been demonstrated. Recently, the site of dopamine transporter (DAT), a plasma membrane transport protein of dopaminergic neuron, also has been characterized in the bovine pineal gland. The aim of the present study was to identify the dopaminergic innervation in the bovine pineal gland. The localization and expression of DAT have been performed by using an immunohistochemical method and a reverse transcriptase polymerase chain reaction (RT-PCR) technique. DAT-immunoreactivity was found in the nerve terminals throughout the gland, but not in pinealocytes or neuronal-like cells. Some DAT-immunoreactive nerve fibers were observed along the pineal stalk. DAT mRNA product from RT-PCR was found in the bovine substantia nigra, but not in the pineal gland. The colocalization of DAT with tyrosine hydroxylase (TH) or dopamine beta hydroxylase (DBH) immunoreactivities was observed in nerve terminals. However, no colocalization of DAT with DBH was found in some terminals/fibers. The present results showed the central dopaminergic innervation in the bovine pineal gland distinctively from noradrenergic nerve fibers, and their perikarya origin was located possibly outside of the gland.

Identification of dopamine transporter in bovine pineal gland using [3H]GBR 12935

The mammalian pineal gland contains several neurotransmitters and receptors for amino acids, biogenic amines, and peptides. Some of these, such as D1 and D2 dopamine receptors, have been previously identified and characterized in the bovine pineal gland by our group. As a matter of fact, the density of D1 dopamine receptors in the pineal gland is higher than that of corpus striatum, suggesting that this organ must possess a high affinity dopamine transporter, which has been identified in this study by using [3H]GBR 12935 as a radiological ligand and nomifensine to determine non-specific binding. The association rate of [3H]GBR 12935 binding to the pineal membrane was examined as a function of time. The binding reached equilibrium within 45 min of incubation at 25 degrees C. The specific binding was reversible and saturable. The dissociation time course of the specific [3H]GBR 12935 binding from the bovine pineal membrane was also studied. A half-life (t1/2) of 14-min was obtained. The saturation analysis of the [3H]GBR 12935 binding revealed a dissociation equilibrium constant (Kd) of 6.0 +/- 0.9 nm and a receptor density (Bmax) of 6.9 +/- 0.3 pmol/mg protein, which were comparable with those values obtained from bovine striatum and frontal cortex. In competitive experiments, the concentrations of drugs required to inhibit 50% of the binding (IC50) were in descending order GBR 12909 > GBR 12935 > trans-flupenthixol > nomifensine > cis-flupenthixol > amitriptyline > imipramine > desipramine > dopamine > fluoxetine > fuvoxamine > d-amphetamine. However, nisoxetine, SCH 23390, norepinephrine, and serotonin were unable to displace [3H]GBR binding. These results show that drugs capable of blocking dopamine transporters were effective in displacing [3H]GBR binding; whereas specific norepinephrine and serotonin transporter inhibitors were less effective or ineffective. In addition, the dopamine transporter is ion-dependent as sodium increased [3H]GBR binding in a concentration related manner. These results indicate that a high affinity dopamine transporter exists in the bovine pineal, which may exhibit circadian periodicity, and whose physiological functions need to be delineated and characterized in future investigations.

The role of photoreceptors in the control of refractive state

The current state of research into experimentally induced refractive errors is reviewed. The area is analysed in three components-the transduction of defocus or deprivation, the vector for transmitting the error message from the retina to the outer tunics of the eye, and the identity of the effector for causing growth modulation in the sclera. Anatomical, pharmacological, electrophysiological and optical factors are considered in terms of which elements of the retina are necessary to support a refractive response to deprivation or defocus. Two of the current models are discussed-one emphasizing the role of the choroid in effecting ocular and refractive change, while the second model approaches the problem from the aspect of scleral changes that are associated with growth adaptation without emphasis on the error detection mechanism. A third model is proposed in which the error signal for deprivation or defocus is detected in the outer retina and where error is translated through separate signals for stimulus brightening and darkening into a net signal for fluid flow across and under the active control of the retinal pigment epithelium with the fluid communication between the vitreous chamber and the choroidal lymphatics. The directions of research both fundamental and clinical which are needed to create pharmaceutical or environmental solutions to refractive control are discussed.

Treatment with AC pulsed electromagnetic fields improves olfactory function in Parkinson's disease

Olfactory dysfunction is a common symptom of Parkinson's disease (PD). It may manifest in the early stages of the disease and infrequently may even antedate the onset of motor symptoms. The cause of olfactory dysfunction in PD remains unknown. Pathological changes characteristic of PD (i.e., Lewy bodies) have been demonstrated in the olfactory bulb which contains a large population of dopaminergic neurons involved in olfactory information processing. Since dopaminergic drugs do not affect olfactory threshold in PD patients, it has been suggested that olfactory dysfunction in these patients is not dependent on dopamine deficiency. I present two fully medicated Parkinsonian patients with long standing history of olfactory dysfunction in whom recovery of smell occurred during therapeutic transcranial application of AC pulsed electromagnetic fields (EMFs) in the picotesla flux density. In both patients improvement of smell during administration of EMFs occurred in conjunction with recurrent episodes of yawning. The temporal association between recovery of smell and yawning behavior is remarkable since yawning is mediated by activation of a subpopulation of striatal and limbic postsynaptic dopamine D2 receptors induced by increased synaptic dopamine release. A high density of dopamine D2 receptors is present in the olfactory bulb and tract. Degeneration of olfactory dopaminergic neurons may lead to upregulation (i.e., supersensitivity) of postsynaptic dopamine D2 receptors. Presumably, small amounts of dopamine released into the synapses of the olfactory bulb during magnetic stimulation may cause activation of these supersensitive receptors resulting in enhanced sense of smell. Interestingly, in both patients enhancement of smell perception occurred only during administration of EMFs of 7 Hz frequency implying that the release of dopamine and activation of dopamine D2 receptors in the olfactory bulb was partly frequency dependent. In fact, weak magnetic fields have been found to cause interaction with biological systems only within narrow frequency ranges (i.e., frequency windows) and the existence of such frequency ranges has been explained on the basis of the cyclotron resonance model.

Cellular mechanisms of melatonin action

The pineal hormone melatonin is involved in photic regulations of various kinds, including adaptation to light intensity, daily changes of light and darkness, and seasonal changes of photoperiod lengths. The melatonin effects are mediated by the specific high-affinity receptors localized on plasma membrane and coupled to GTP-binding protein. Two different G proteins coupled to the melatonin receptors have been described, one sensitive to pertussis toxin and the other sensitive to cholera toxin. On the basis of the molecular structure, three subtypes of the melatonin receptors have been described: Mel1A, Mel1B, and Mel1C. The first two subtypes are found in mammals and may be distinguished pharmacologically using selective antagonists. Melatonin receptor regulates several second messengers: cAMP, cGMP, diacylglycerol, inositol trisphosphate, arachidonic acid, and intracellular Ca2+ concentration ([Ca2+]i). In many cases, its effect is inhibitory and requires previous activation of the cell by a stimulatory agent. Melatonin inhibits cAMP accumulation in most of the cells examined, but the indole effects on other messengers have been often observed only in one type of the cells or tissue, until now. Melatonin also regulates the transcription factors, namely, phosphorylation of cAMP-responsive element binding protein and expression of c-Fos. Molecular mechanisms of the melatonin effects are not clear but may involve at least two parallel transduction pathways, one inhibiting adenylyl cyclase and the other regulating phospholipide metabolism and [Ca2+]i.

Flicker parameters are different for suppression of myopia and hyperopia

Axial eye growth rates in the chicken are controlled by local retinal image-processing circuits. These circuits quantify the loss of contrast for different spatial frequencies and promote axial eye growth rates in correlation with the amount of retinal image degradation ("deprivation myopia"). They also distinguish whether the plane of focus lies in front of or behind the retina. How the sign of defocus is detected still remains unclear. Cues from chromatic aberration are not important. In an attempt to isolate retinal circuits controlling the development of myopia or hyperopia, young chickens were raised in flickering light of different frequencies (12 and 6 Hz) and duty cycles (4-75%) produced by rotating chopper disks. The effects of flickering light on refractive errors and change in axial growth rates induced by translucent occluders or defocusing lenses were measured by infrared retinoscopy and A-scan ultrasound, respectively. Retinal electrical activity was evaluated by flicker ERG after matching flicker parameters and stimulation brightness at retinal surface. Changes in retinal and vitreal dopamine content caused by flicker in occluded and normal eyes were determined by HPLC-ECD. Strikingly, suppression of myopia occurred for similar flicker parameters, whether induced by translucent occluders ("deprivation") or negative lenses ("defocus"). The degree to which myopia was suppressed was correlated with the duration of flicker dark phase and with the ERG amplitude. In contrast, suppression of hyperopia did not correlate with these parameters. We conclude that two different retinal circuits with different temporal characteristics are involved in the processing of hyperopic defocus/deprivation and of myopic defocus, the first one dependent on flicker ERG amplitude. However, we did not find any correlation between the rate of dopamine release and the degree of inhibition of deprivation myopia in flickering light.

Autoinhibition of histamine release by H3 receptors in rat brain cortex depends on stimulation frequency

Rat cortical slices preloaded with [3H]histidine released [3H]histamine upon electrical stimulation or after depolarization with elevated K+ levels. The release was dependent on the presence of Ca2+, suggesting a neurosecretory process. Histamine has been shown to inhibit its own release mediated by an autoreceptor belonging to the H3-receptor subclass. In this study we have investigated the autoinhibition using different electrical field stimulation conditions (1, 10, 20 and 33.3 Hz). Applying electrical stimulation, the inhibition of [3H]histamine release by histamine is decreased when the stimulation frequency is elevated. When stimulated with 1 Hz histamine is able to block [3H]histamine release completely, with a p(EC50) of 8.1 +/- 0.1. At higher frequencies histamine still blocks [3H]histamine release completely, but with a lower p(EC50).

Histamine receptors: subclasses and specific ligands

In this review the three main types of histamine receptors are discussed together with their specific ligands. For the classical H1-receptors much emphasis is put on the mechanism by which the receptor is stimulated. For the H1- and H2-receptor the review includes information on the several models available for establishing agonistic or antagonistic activity. In the section on the H3-receptor the ligands are discussed as well as the possible physiological role of this receptor. In the final paragraphs some less well defined activities are presented.

Dopamine and its agonists reduce a light-sensitive pool of cyclic AMP in mouse photoreceptors

The exposure to bright light of dark-adapted (DKA) mouse retinas incubated in the dark (DI) in IBMX-containing medium causes a marked loss of cyclic AMP. This light response also occurs if the medium contains 10 mM aspartate or cobaltous ion, agents believed to confine the effects of light to photoreceptors. Thus, the action of light in the presence of either of these agents defines a light-sensitive pool of cyclic AMP in photoreceptors. This pool could also be reduced or eliminated in DKA-DI retinas by nanomolar to micromolar levels of dopamine (if the medium contained SCH23390, a potent antagonist of D1 receptors), thus indicating an agonistic action of dopamine at D2 receptors. The D2 agonists LY171555 (EC50 10 nM) or (+)-3-PPP also reduced the cyclic AMP level in the dark. Of the D2 antagonists tested, the butyrophenone spiperone (in the presence of the 5HT-2 blocker ketanserin) countered the action of the D2 agonists but substituted benzamides were ineffective. Consistently, the D2 agonists had no effect on cyclic AMP levels of mutant retinas lacking photoreceptors (rd/rd), but reduced cyclic AMP in DKA-DI glutamate-modified retinas which exhibit a major loss of inner retinal neurons without apparent loss of photoreceptors. The D1 antagonist SCH23390 only reduced cyclic AMP levels of DKA-DI retinas when cyclic AMP levels had been elevated by adding dopamine to the incubation medium.

Transmitter release patterns of noradrenergic, dopaminergic and cholinergic axons in rabbit brain slices during short pulse trains, and the operation of presynaptic autoreceptors

Slices of rabbit brain were field-stimulated either by single electrical pulses or by trains of 4 or 8 pulses at 1 or 100 Hz in order to study transmitter release patterns and the autoinhibition of transmitter release. The slices were preincubated with 3H-noradrenaline (cortex), 3H-dopamine (caudate nucleus) or 3H-choline (caudate nucleus). Slices preincubated with 3H-noradrenaline were superfused with medium containing desipramine 1 mumol/l. The overflow of tritium elicited by single pulses amounted to 0.19% of the tritium content of the tissue. The overflow elicited by 4 pulses/1 Hz was similar, whereas that elicited by 4 pulses/100 Hz was 5.1-fold higher. Yohimbine 10-1000 nmol/l increased up to 2.5-fold the overflow evoked by 4 pulses/1 Hz but did not change the overflow evoked by single pulses or 4 pulses/100 Hz. - Slices preincubated with 3H-dopamine were superfused with medium containing nomifensine 1 mumol/l. The overflow of tritium elicited by single pulses was 0.39% of the tritium content of the tissue. The overflow elicited by 4 pulses/1 Hz was 1.3-fold and the overflow elicited by 4 pulses/100 Hz 1.4-fold higher. Domperidone 1-100 nmol/l and sulpiride 10-1000 nmol/l increased up to 2.4-fold the overflow evoked by 4 pulses/1 Hz but increased only slightly the overflow evoked by single pulses or 4 pulses/100 Hz. - Slices preincubated with 3H-choline were superfused either with physostigmine-free medium or with medium containing physostigmine 1 mumol/l. In physostigmine-free medium, atropine did not increase the evoked overflow of tritium at any stimulation condition. In physostigmine-containing medium, the overflow elicited by single pulses was 0.18% of the tritium content of the tissue. The overflow elicited by 8 pulses/1 Hz was 2.0-fold and the overflow elicited by 8 pulses/100 Hz 2.2-fold higher. Atropine 2-200 nmol/l increased up to 2.4-fold the overflow evoked by 8 pulses/1 Hz but increased only slightly the overflow evoked by single pulses or 8 pulses/100 Hz. In physostigmine-free medium, sulpiride 10-1000 nmol/l did not change the single-pulse-evoked overflow of tritium in the absence but increased it in the presence of nomifensine 1 mumol/l. Single pulses elicit a large release of 3H-noradrenaline, 3H-dopamine and 3H-acetylcholine under the conditions of these experiments. Release elicited by single pulses is not subject to autoinhibition except for a small inhibition by spontaneously released transmitter in the case of dopaminergic and cholinergic axons.(ABSTRACT TRUNCATED AT 400 WORDS)

Frequency-dependent autoinhibition of histamine release from rat cortical slices: a possible role for H3 receptor reserve

The inhibition of histamine release after depolarization of rat cerebral cortex slices by electrical stimulation and mediated by the postulated presynaptic autoreceptor (H3) depends strongly on the conditions of stimulation. Using electrically stimulated slices of the cortex a rightwards shift of the concentration-response curve of histamine (an H3 agonist) was observed on increasing the frequency of stimulation. The pA2 value of the H3 antagonist impromidine was, however, not altered at different stimulation frequencies; for a partial agonist only the maximal effect was influenced. These results indicate the existence of a receptor reserve at the H3 autoreceptor.

Dopamine acts on D2 receptors to increase potassium conductance in neurones of the rat substantia nigra zona compacta

1. Intracellular recordings were made from neurones in the substantia nigra zona compacta in slices of rat mesencephalon in vitro. The majority of neurones fired action potentials spontaneously at 0.2-5.6 Hz. Dopamine, applied either by superfusion or from the tip of a pressurized pipette, prevented spontaneous action potential firing and hyperpolarized the membrane. 2. When the membrane potential was held negative to the threshold for action potential firing, the hyperpolarization evoked by dopamine was accompanied by a fall in input resistance. Under voltage clamp, dopamine produced an outward membrane current associated with an increase in membrane conductance. The effects of superfused dopamine on firing rate, membrane potential and membrane current were concentration dependent in the range 1-100 microM. 3. The reversal potential for the hyperpolarizations and the outward currents produced by dopamine was -109.7 +/- 1.7 mV (n = 12) when the potassium concentration was 2.5 mM and -74.0 +/- 5.0 mV (n = 4) when the potassium concentration was 10.5 mM. The change in reversal potentials in these and intermediate potassium concentrations was described by the Nernst equation. 4. The outward current induced by dopamine was reversibly reduced by barium (100-300 microM) and by high concentrations of tetraethylammonium (greater than or equal to 10 mM). Calcium-free solutions with cobalt (0.5-2 mM) did not reduce the current in response to dopamine during the first 5 min of their application. Currents and hyperpolarizations caused by dopamine were unaffected by tetrodotoxin (1 microM). 5. The hyperpolarization produced by dopamine was mimicked by the D2 receptor agonist quinpirole (LY 171555, 0.1-3 microM) and was blocked by the D2 receptor agonists domperidone and (-)-sulpiride. Agonists and antagonists at D1 receptors had no effect. 6. (-)-Sulpiride (30 nM-30 microM) produced a progressive shift to the right in the concentration-response curve to either dopamine or quinpirole. Schild analysis of the antagonism between (-)-sulpiride and quinpirole suggested competitive antagonism with a dissociation equilibrium constant for (-)-sulpiride of about 13 nM. 7. It is concluded that dopamine acts on D2 receptors on neurones of the rat substantia nigra pars compacta to increase the membrane potassium conductance.