Monoaminergic Synapses Research Articles

The residence of synaptically released dopamine on D2 autoreceptors.

SUMMARYNeuromodulation mediated by synaptically released endogenous transmitters acting in G-protein-coupled receptors (GPCRs) is slow primarily because of multistep downstream signaling. What is less well understood is the spatial and temporal kinetics of transmitter and receptor interaction. The present work uses the combination of the dopamine sensor, dLight, to detect the spatial release and diffusion of dopamine and a caged form of a D2-dopamine receptor antagonist, CyHQ-sulpiride, to rapidly block the D2 autoreceptors. Photoactivation of the CyHQ-sulpiride blocks receptors in milliseconds such that the time course of dopamine/receptor interaction is mapped onto the downstream signaling. The results show that highly localized release, but not dopamine diffusion, defines the time course of the functional interaction between dopamine and D2 autoreceptors, which determines downstream inhibition.

Neuropathology of the Brainstem to Mechanistically Understand and to Treat Alzheimer's Disease.

Alzheimer’s disease (AD) is a devastating neurodegenerative disorder as yet without effective therapy. Symptoms of this disorder typically reflect cortical malfunction with local neurohistopathology, which biased investigators to search for focal triggers and molecular mechanisms. Cortex, however, receives massive afferents from caudal brain structures, which do not only convey specific information but powerfully tune ensemble activity. Moreover, there is evidence that the start of AD is subcortical. The brainstem harbors monoamine systems, which establish a dense innervation in both allo- and neocortex. Monoaminergic synapses can co-release neuropeptides either by precisely terminating on cortical neurons or, when being “en passant”, can instigate local volume transmission. Especially due to its early damage, malfunction of the ascending monoaminergic system emerges as an early sign and possible trigger of AD. This review summarizes the involvement and cascaded impairment of brainstem monoaminergic neurons in AD and discusses cellular mechanisms that lead to their dysfunction. We highlight the significance and therapeutic challenges of transmitter co-release in ascending activating system, describe the role and changes of local connections and distant afferents of brainstem nuclei in AD, and summon the rapidly increasing diagnostic window during the last few years.

Diurnal variation of cerebrospinal fluid hypocretin-1 (Orexin-A) levels in control and depressed subjects

Background Hypocretins, excitatory neuropeptides at monoaminergic synapses, appear to regulate human sleep-wake cycles. Undetectable cerebrospinal fluid hypocretin-1 levels are seen in narcolepsy, which is frequently associated with secondary depression. Shortened rapid eye movement latency is observed in both narcolepsy and depression. Cerebrospinal fluid hypocretin-1 levels have not been reported in mood disorders. Methods We examined hypocretin-1 levels in 14 control and 15 depressed subjects. Cerebrospinal fluid was drawn continuously in supine subjects for 24 hours with an indwelling intrathecal catheter under entrained light-dark conditions. Depressed subjects were studied before and after 5 weeks of sertraline (n = 10, three nonresponders) or bupropion (n = 5, two nonresponders). Results Hypocretin-1 levels varied slightly (amplitude 10%) but significantly across the diurnal cycle in control subjects, with amplitude significantly reduced in depression (3%). Levels were lowest at midday, surprising for a hypothetically wake-promoting peptide. Mean hypocretin levels trended higher in depressive than in control subjects. Hypocretin-1 levels decreased modestly but significantly after sertraline (−14%) but not bupropion. Conclusions Our results are consistent with previous physiologic findings in depression indicating dampened diurnal variations in hypocretin-1. The finding that sertraline but not bupropion slightly decreased cerebrospinal fluid hypocretin-1 indicates a serotoninergic influence on hypocretin tone.

Some cholinergic themes related to Alzheimer's disease: Synaptology of the nucleus basalis, location of m2 receptors, interactions with amyloid metabolism, and perturbations of cortical plasticity

Cholinergic neurons in the nucleus basalis of Meynert (nbM) receive cholinergic, GABAergic and monoaminergic synapses. Only few of these neurons display the sort of intense m2 immunoreactivity that would be expected if they were expressing m2 as their presynaptic autoreceptor. The depletion of cortical m2 in Alzheimer's disease (AD) appears to reflect the loss of presynaptic autoreceptors located on incoming axons from the nucleus basalis of Meynert (nbM) and also the loss of postsynaptic receptors located on a novel group of nitric oxide producing interstitial neurons in the cerebral cortex. The defect of cholinergic transmission in AD may enhance the neurotoxicity of amyloid β, leading to a vicious cycle which can potentially accelerate the pathological process. Because acetylcholine plays a critical role in regulating axonal growth and synaptic remodeling, the cholinergic loss in AD can perturb cortical plasticity so as to undermine the already fragile compensatory reserve of the aging cerebral cortex.

3H-dopamine uptake and 3H-haloperidol binding in striatum after administration of methyl mercury to rats.

Methyl mercury inhibits dopamine (DA) and serotonin (5-HT) uptake by brain synaptosomes and decreases antagonist binding to striatal dopaminergic D2 receptors in vitro. To assess the effects in vivo, adult rats were given methyl mercury, either as a single dose (10 mg/kg by gavage) or a cumulative total dose of 50 mg/kg in 2 weeks. The repeated dosing decreased body weight and caused neuromuscular dysfunction. In spite of this overt toxicity, neither 3H-DA uptake nor 3H-haloperidol binding changed in striatal synaptosomes. There were no significant alterations in 3H-5-HT uptake by hypothalamic synaptosomes or 3H-flunitrazepam binding in cerebellar synaptosomes. The results suggest that monoaminergic synapses and the benzodiazepine binding sites, associated with cerebellar GABA receptors, remain functionally normal at doses of methyl mercury that are otherwise toxic. The results also emphasize the importance of due care when extrapolating cellular or biochemical data to the level of the whole organism.

A neurochemical study of developmental impairment of the brain caused by the administration of cytosine arabinoside during the fetal or neonatal period of rats.

Injection of pregnant rats with cytosine arabinoside (ara-C) (280 mg/kg) on day 15 of gestation caused a significant rise (about two times the control value) in monoamine concentrations (norepinephrine, dopamine, and serotonin) accompanied by a decrease (about 60% of the control) in the brain weight and DNA content in the cerebrum of the offspring at 60 days of age. When neonatal rats were injected with ara-C (30 mg/kg/day) for four consecutive days from the fourth to seventh days after birth, a decrease of DNA content per cerebellum and an elevation of monoamine concentrations in the cerebellum were found. However, the total content of each monoamine per cerebrum or cerebellum showed no difference from the control. These results suggest that monoaminergic neurons may remain intact, with normal monoaminergic synapses compressed into a small brain volume. The neonatal administration of ara-C caused an elevation of 2',3'-cyclic nucleotide 3'-phosphodiesterase (CNPase) (EC 3.1.4.37) activity and myelin protein content in the cerebellum, suggesting a relative increase in myelin concentration as a result of hypoplasia of granule cells.

Intermittent Hypoendorphinaemia in Migraine Attack

Beta-endorphin (RIA method, previous chromatographic extraction) was evaluated in plasma of migraine sufferers in free periods and during attacks. Decreased levels of the endogenous opioid peptide were found in plasma sampled during the attacks but not in free periods. Even chronic headache sufferers exhibited significantly lowered levels of beta-endorphin, when compared with control subjects with a negative personal and family history of head pains. The mechanism of the hypoendorphinaemia is unknown: lowered levels of the neuropeptide, which controls nociception, vegetative functions and hedonia, could be important in a syndrome such as migraine, characterized by pain, dysautonomia and anhedonia. The impairment of monoaminergic synapses ("empty neuron" condition) constantly present in sufferers from serious headaches, could be due to the fact that opioid neuropeptides, because of a receptoral or metabolic impairment, poorly modulate the respective monoaminergic neurons, resulting in imbalance of synaptic neurotransmission.

Two types of dopaminergic nerve terminals in the rat neostriatum. An ultrastructural study

In the neostriatum 7.7% of all boutons absorbed α-methylnoradrenaline, thus staining their synaptic vesicles profiles granular. These presumably monoaminergic synapses were divided into two types according to ultrastructural features. Type A was composed of a relatively large axon terminal and a dendritic spine with postsynaptic membrane thickening. The axon terminal of type B, that is a bouton en passant, was smaller than the postsynaptic profile, while the synaptic contact was symmetrical. Following nigral coagulation, both types of synapse degenerated. It is concluded that both types of synapse are dopaminergic.

Monoaminergic synapses in infant mouse neocortex: Comparison of cortical fields in seizure-prone and resistant mice

In a quantitative electron microscopic study, 5-hydroxydopamine was utilized to identify putative monoamine-containing synaptic boutons in the temporal and in the somatosensory cortex of 6-day-old mice. In Swiss mice, a stratiform distribution of a synapses is seen in both fields. Although the overall synaptic density is slightly lower in the temporal cortex, the relative density of small granular vesicles synapses is equivalent in both fields. In the temporal cortex of seizure-prone DBA mice, only 4% of the synapses are labeled by 5-hydroxydopamine compared with 20% in the temporal cortex of normal Swiss mice; the degree of labeling in the somatosensory cortex is comparable in both strains. Morphometric analysis of the structure of the labeled synapses suggests that the typical monoamine-containing synapse in DBA mice differs from that in Swiss mice. Assuming that 5-hydroxydopamine is a reliable marker for monoamine-containing synapses, we conclude that the temporal cortex differs in several key ways from other fields in the cortex of DBA mice and from the same region in non-seizure-prone Swiss mice.

Development of neocortical circuitry: Quantitative ultrastructural analysis of putative monoaminergic synapses

The distribution, numbers and morphology of presumed monoaminergic (MA) synapses were examined in somatosensory cortex of neonatal rats and mice (newborn to 16 days of age). MA synapses were identified using an ultrastructural cytochemical marker, 5-hydroxydopamine (5-OHDA), which results in the appearance of small granular vesicles (SGV) in their presynaptic terminals. From birth to 7 days of age, 20–30% of all synapses sampled in somatosensory cortex contain SGVs. However, few SGV synapses are seen in 8-day-old cortex and by 12 days of age, SGVs are no longer detectable in cortex. A specific laminar distribution for these SGV synapses—which is distinct from the overall synaptic distribution—is first seen at 3 days of age and is essentially unchanged until 7 days postnatally. During this entire period, the SGV synapses predominate in the primordium of layer IV, where they account for 50–70% of all synapses. Morphometric analysis of SGV synapses indicates that there are differences in junctional symmetry, vesicle shape and configuration of the contact zone between SGV and non-SGV synapses, as well as between SGV synapses themselves in the various cortical layers. The laminar distribution and morphological characteristics of SGV synapses suggest that the MA projection to neocortex exhibits a high degree of spatial specificity during its ingrowth. Also, the relatively high proportion of SGV synapses in the first postnatal week may reflect a potent influence exerted by the MA inputs on immature neocortex. The decreased numerical density of SGV synapses after 7 days of age is probably due to the development of the blood-brain barrier to 5-OHDA.

Monoaminergic synapses, including dendro-dendritic synapses in the rat substantia nigra.

Intraventricular administration of 1 or 2 mg of the osmiophilic "false transmitter" 5-hydroxydopamine (5-OHDA) was used to label monoamine storage and release sites in the rat substantia nigra. Vesicles containing unusually dense cores indicative of the presence of the marker were seen forming from the Golgi apparatus in the cell bodies of medium-sized neurons of the substantia nigra, pars compacta, and from smooth endoplasmic reticulum in the dendrites of those neurons and in small unmyelinated axons of unknown origin. In serial sections, both axons and dendrites containing synaptic vesicles marked with 5-OHDA were seen to form synapses "en passage" in pars compacta, and some presynaptic dendrites containing vesicles filled by the marker were also observed to form contacts with dendrites in pars reticulata. The only identified postsynaptic elements engaging in monoaminergic synapses in the substantia nigra were dendrites of medium-sized pars compacta neurons.

Problems of Understanding the Substructure of Synapses

This chapter discusses the morphological aspects of synapses that are thought to have a chemical mode of transmission. Using special prefixation and fixation techniques on the synapses of the rat central nervous system (cerebral and cerebellar cortices) and posterior pituitary endings, and frog forebrain, retina and motor endplate, microtubules can be observed focused on the terminal membrane. Synaptic vesicles can usually be observed associated with the microtubules except in the rat pituitary and frog retina. The present results give strong support to Schmitt's original hypothesis that synaptic vesicles slide along the microtubules and so arrive at the active zone of the presynaptic membrane. Therefore, now there is a morphological explanation of the clustering of synaptic vesicles at the appropriate zone of the presynaptic membrane for transmitter release. Monoaminergic synapses are, without doubt, the best characterized synapses at currently; the detection of the amine by the means of fluorescence microscopy, the presence of a dense core in a percentage of relatively small synaptic vesicles, the depletion or prevention of the uptake of the amine by a variety of drugs, the uptake of false transmitters—5- and 6-hydroxydopamine (the latter causing specific degeneration, and the uptake of tritiated noradrenaline, which can be detected by EM autoradiography, all provide important clues for identification.

The action of para-methoxyphenylethylamine (PMPEA) on monosynaptic reflex transmission in the cat.

1. The intravenous injection of para-methoxyphenylethylamine (PMPEA) into cats produces an increase in the size of the spinal cord monosynaptic reflex. The reflex elevation occurs within 30 s of drug administration, reaches a peak within 2 min, and lasts about 20 min.2. The action of PMPEA is similar for extensor (gastrocnemius-soleus) and flexor (posterior biceps-semitendinosus) monosynaptic reflexes.3. Repeated doses of PMPEA give comparable effects. The degree of monosynaptic reflex elevation is dose related.4. The action of PMPEA is antagonized by phenoxybenzamine and by methysergide or cyproheptadine. The combination of phenoxybenzamine with either of the latter is particularly effective in preventing the reflex facilitation by PMPEA.5. It is concluded that PMPEA has a central action on the spinal cord. It seems likely that monoaminergic synapses are involved.