The High-Precision Liquid Chromatography with Electrochemical Detection (HPLC-ECD) for Monoamines Neurotransmitters and Their Metabolites: A Review

This review highlights the advantages of high-precision liquid chromatography with an electrochemical detector (HPLC-ECD) in detecting and quantifying biological samples obtained through intracerebral microdialysis, specifically the serotonergic and dopaminergic systems: Serotonin (5-HT), 5-hydroxyindolacetic acid (5-HIAA), 3,4-dihydroxyphenylacetic acid (DOPAC), dopamine (DA), 3-metoxytryptamin (3-MT) and homovanillic acid (HVA). Recognized for its speed and selectivity, HPLC enables direct analysis of intracerebral microdialysis samples without complex derivatization. Various chromatographic methods, including reverse phase (RP), are explored for neurotransmitters (NTs) and metabolites separation. Electrochemical detector (ECD), particularly with glassy carbon (GC) electrodes, is emphasized for its simplicity and sensitivity, aimed at enhancing reproducibility through optimization strategies such as modified electrode materials. This paper underscores the determination of limits of detection (LOD) and quantification (LOQ) and the linear range (L.R.) showcasing the potential for real-time monitoring of compounds concentrations. A non-exhaustive compilation of literature values for LOD, LOQ, and L.R. from recent publications is included.

Effects of pollutants and microplastics ingestion on oxidative stress and monoaminergic activity of seabream brains

Nowadays, microplastics (MPs) and adsorbed pollutants are considered a global thread to marine ecosystems. This study describes the effects of pollutants and MPs ingestion on fish brains through the assessment of oxidative stress biomarkers and monoaminergic neurotransmitters using gilthead seabream (Sparus aurata) as fish model. Juveniles were experimentally exposed to three different dietary treatments for 90 days: Control treatment (C) consisted of standard feed; Virgin treatment (V) contained feed enriched with 10% of MPs; and Exposed treatment (E) consisted of feed with 10% of MPs that were exposed to seawater in an anthropogenically impacted area for 2 months in order to enrich the plastic with the pollutants within the water column. Sampling was made at the start of the experiment (T0), at the end of the dietary treatments (T90) and after a posterior detoxification period of 30 days (T120). Results evidenced that a MPs and pollutants enriched diet increases the activity of some of the oxidative stress biomarkers (e.g. CAT and GST), and it was shown for the first time alterations on dopaminergic and serotonergic system activity on seabream brains, indicating potential neurofunctional effects associated to MPs and pollutants ingestion. In addition, results showed a tendency to recover enzymatic and brain monoaminergic neurotransmitter levels after a 30-day detoxification period. In conclusion, MPs and pollutants exposure for 90 days induced oxidative stress and changes on monoaminergic activity in the brain of S. aurata.

Cerebrospinal Fluid Biogenic Monoamine Analysis for Diagnosis of Primary Neurotransmitter Disorders

Biogenic amine neurotransmitters metabolism is a multistep pathway with pterin and pyridoxal phosphate (vitamin B6) as cofactors. A defect in biogenic amine and cofactor metabolism and vesicular transporters result in a primary neurotransmitter disorders. These are a well-recognized groups of inherited disorders and often present with features overlapping with other neurological conditions. Their diagnosis is made by analysis of biogenic amine metabolites in cerebrospinal fluid (CSF) and other body fluids and respective enzyme assays. Many of these disorders are treatable and deficits can be reverted by timely intervention. CSF biogenic amine or cofactor metabolite analysis is one of the primary indicators of a neurotransmitter disorder. In this paper, 3 cases are reported-2 of cofactor deficiency and 1 with enzyme deficiency wherein biogenic amine estimation has assisted in diagnosis.

The Multilayer Connectome of Caenorhabditis elegans

Connectomics has focused primarily on the mapping of synaptic links in the brain; yet it is well established that extrasynaptic volume transmission, especially via monoamines and neuropeptides, is also critical to brain function and occurs primarily outside the synaptic connectome. We have mapped the putative monoamine connections, as well as a subset of neuropeptide connections, in C. elegans based on new and published gene expression data. The monoamine and neuropeptide networks exhibit distinct topological properties, with the monoamine network displaying a highly disassortative star-like structure with a rich-club of interconnected broadcasting hubs, and the neuropeptide network showing a more recurrent, highly clustered topology. Despite the low degree of overlap between the extrasynaptic (or wireless) and synaptic (or wired) connectomes, we find highly significant multilink motifs of interaction, pinpointing locations in the network where aminergic and neuropeptide signalling modulate synaptic activity. Thus, the C. elegans connectome can be mapped as a multiplex network with synaptic, gap junction, and neuromodulator layers representing alternative modes of interaction between neurons. This provides a new topological plan for understanding how aminergic and peptidergic modulation of behaviour is achieved by specific motifs and loci of integration between hard-wired synaptic or junctional circuits and extrasynaptic signals wirelessly broadcast from a small number of modulatory neurons.

Major depressive disorder

Major depressive disorder (MDD) is a debilitating disease that is characterized by depressed mood, diminished interests, impaired cognitive function and vegetative symptoms, such as disturbed sleep or appetite. MDD occurs about twice as often in women than it does in men and affects one in six adults in their lifetime. The aetiology of MDD is multifactorial and its heritability is estimated to be approximately 35%. In addition, environmental factors, such as sexual, physical or emotional abuse during childhood, are strongly associated with the risk of developing MDD. No established mechanism can explain all aspects of the disease. However, MDD is associated with alterations in regional brain volumes, particularly the hippocampus, and with functional changes in brain circuits, such as the cognitive control network and the affective-salience network. Furthermore, disturbances in the main neurobiological stress-responsive systems, including the hypothalamic-pituitary-adrenal axis and the immune system, occur in MDD. Management primarily comprises psychotherapy and pharmacological treatment. For treatment-resistant patients who have not responded to several augmentation or combination treatment attempts, electroconvulsive therapy is the treatment with the best empirical evidence. In this Primer, we provide an overview of the current evidence of MDD, including its epidemiology, aetiology, pathophysiology, diagnosis and treatment.

"Brain sex differentiation" in teleosts: Emerging concepts with potential biomarkers

"Brain sex differentiation" in teleosts is a contentious topic of research as most of the earlier reports tend to suggest that gonadal sex differentiation drives brain sex differentiation. However, identification of sex-specific marker genes in the developing brain of teleosts signifies brain-gonadal interaction during early sexual development in lower vertebrates. In this context, the influence of gonadotropin-releasing hormone (GnRH)-gonadotropin (GTH) axis on gonadal sex differentiation, if any requires in depth analysis. Presence of seabream (sb) GnRH immunoreactivity (ir-) in the brain of XY Nile tilapia was found as early as 5days post hatch (dph) followed by qualitative reduction in the preoptic area-hypothalamus region. In contrast, in the XX female brain a steady ir- of sbGnRH was evident from 15dph. Earlier studies using sea bass already implied the importance of hypothalamic gonadotropic axis completion during sex differentiation period. Such biphasic pattern of localization was also seen in pituitary GTHs using heterologous antisera in tilapia. However, more recent analysis in the same species could not detect any sexually dimorphic pattern using homologous antisera for pituitary GTHs. Detailed studies on the development of hypothalamo-hypophyseal-gonadal axis in teleosts focusing on hypothalamic monoamines (MA) and MA-related enzymes demonstrated sex-specific differential expression of tryptophan hydroxylase (Tph) in the early stages of developing male and female brains of tilapia and catfish. The changes in Tph expression was in agreement with the levels of serotonin (5-HT) and 5-hydroxytryptophan in the preoptic area-hypothalamus. Considering the stimulatory influence of 5-HT on GnRH and GTH release, it is possible to propose a network association between these correlates during early development, which may bring about brain sex dimorphism in males. A recent study from our laboratory during female brain sex development demonstrated high expression of tyrosine hydroxylase in correlation with catecholamine levels, brain aromatase and its related transcription factors such as fushi tarazu factor 1, Ftz-f1 and fork head box protein L2, foxl2. Taken together, gender differences in the levels of various transcripts provide new perspectives on brain sex differentiation in lower vertebrates. Sexually dimorphic or differentially expressing genes may play an essential role at the level of brain in response to gonadal differentiation, which might consequentially or causatively respond to gonadal sex.

[Pathophysiology of depression]

Neurobiological findings of depression are reviewed in this paper. Modern neurobiological methods have revealed pathophysiological mechanism associated with depression. Monoamine hypothesis, which was advocated in the 1950's, emphasizes that the deficiency of monoamine neurotransmitters bring about depressive symptoms. This theory played an important role in promoting the development of new antidepressants, but some inconsistent findings were pointed out concerning this theory. Neuroendocrine studies have revealed the hypothalamic-pituitary-adrenal (HPA) axis dysfunctions in depressive patients, and increased activity of HPA axis are considered as state marker of depression. Morphological changes of hippocampus, polymorphism of serotonin transporter gene, and down regulation of neurotrophin are also discussed in this review.

New approaches to antidepressant drug discovery: beyond monoamines

All available antidepressant medications are based on serendipitous discoveries of the clinical efficacy of two classes of antidepressants more than 50 years ago. These tricyclic and monoamine oxidase inhibitor antidepressants were subsequently found to promote serotonin or noradrenaline function in the brain. Newer agents are more specific but have the same core mechanisms of action in promoting these monoamine neurotransmitters. This is unfortunate, because only approximately 50% of individuals with depression show full remission in response to these mechanisms. This review summarizes the obstacles that have hindered the development of non-monoamine-based antidepressants, and provides a progress report on some of the most promising current strategies.

Effects of estrogen on brain development and neuroprotection--implications for negative symptoms in schizophrenia

Increasing evidence during the last few years suggests that there are gender-specific differences in schizophrenia, influencing the age of onset, treatment outcome and the prevalence of negative symptoms. With respect to the latter in postmortem brain and cerebrospinal fluid of schizophrenic patients with negative symptoms a reduction of dopaminergic activity became evident. Measures of noradrenergic activity, dopamine beta-hydroxylase and the metabolite MHPG, appear to decrease with brain atrophy seen in patients with negative symptoms. Serotonergic activity tends to be low in patients with impaired cognitive function as is seen in negative schizophrenia. In these patients ventricular enlargement is associated with the severity of negative symptoms, low monoamine activity and low cerebral glucose metabolism. On the other hand atypical antipsychotic drugs that modulate also glutamate receptor activity, suggest an additional alternative mechanism of antipsychotic action beyond aminergic neurotransmitters. These drugs improve glutamatergic transmission and decrease negative symptoms; this suggests a glutamatergic deficiency as an extension of the dopamine model. The glutamate-dopamine interaction illustrates the importance of cross-talk between projections to the cortex, striatum, and lower brainstem for the expression of negative symptomatology. On the other hand, estradiol-17beta the most potent female sex hormone influences not only primary and secondary sexual characteristics but also embryonal and fetal growth as well as development of the brain aminergic networks, which are involved in schizophrenia. Estradiol-l7beta possesses neuroprotective properties, which are relevant for the course of schizophrenia and this may explain the pronounced gender differences with respect to progression and therapeutic response of schizophrenia. The present review attempts an update and synthesis of the information about the hormonal influence on neuronal pathways in negative symptoms of schizophrenia. It shows that estradiol-l7beta influences transporters and receptors as well as the morphological appearance of neuronal systems and that it may be an integral part of the neuroprotective system ameliorating schizophrenia.

Chemical neuroanatomy of the vesicular amine transporters

Acetylcholine, catecholamines, serotonin, and histamine are classical neurotransmitters. These small molecules also play important roles in the endocrine and immune/inflammatory systems. Serotonin secreted from enterochromaffin cells of the gut epithelium regulates gut motility; histamine secreted from basophils and mast cells is a major regulator of vascular permeability and skin inflammatory responses; epinephrine is a classical hormone released from the adrenal medulla. Each of these molecules is released from neural, endocrine, or immune/inflammatory cells only in response to specific physiological stimuli. Regulated secretion is possible because amines are stored in secretory vesicles and released via a stimulus-dependent exocytotic event. Amine storage-at concentrations orders of magnitude higher than in the cytoplasm-is accomplished in turn by specific secretory vesicle transporters that recognize the amines and move them from the cytosol into the vesicle. Immunohistochemical visualization of specific vesicular amine transporters (VATs) in neuronal, endocrine, and inflammatory cells provides important new information about how amine-handling cell phenotypes arise during development and how vesicular transport is regulated during homeostatic response events. Comparison of the chemical neuroanatomy of VATs and amine biosynthetic enzymes has also revealed cell groups that express vesicular transporters but not enzymes for monoamine synthesis, and vice versa: their function and regulation is a new topic of investigation in mammalian neurobiology. The chemical neuroanatomy of the vesicular amine transporters is reviewed here. These and similar data emerging from the study of the localization of the recently characterized vesicular inhibitory and excitatory amino acid transporters will contribute to understanding chemically coded synaptic circuitry in the brain, and amine-handling neuroendocrine and immune/inflammatory cell regulation.

In vivo imaging of the vesicular acetylcholine transporter and the vesicular monoamine transporter

Validation of the vesicular acetylcholine transporter (VAChT) and the neuronal vesicular monoamine transporter (VMAT2) as important molecular targets in the cholinergic and dopamine neurons, respectively, has sparked interest in the development of radiotracers for studying these markers in vitro and in vivo. Currently, a number of selective high-affinity radiotracers are available for studying these targets in vivo with positron emission tomography (PET) or single photon emission computed tomography (SPECT). PET studies of VMAT2 in neuropathology reveal changes in the density of this marker that can be verified independently. Similarly, in vivo studies with VAChT ligands suggest that the latter are potentially useful in detecting cholinergic lesions in vivo; however, additional development is required to fully realize the potential of these radioligands.

Neurochemical effects of consumption of Great Lakes salmon by rats

This study, part of a larger project to determine the health consequences of both perinatal and adult exposure to contaminated salmon from the Great Lakes, determined the neurochemical effects of exposure of rats to chow adulterated with lyophilized salmon fillets. Concentrations of biogenic amines, their metabolites, and choline acetyltransferase (ChAT) were determined in the frontal cortex (FC), nucleus accumbens, caudate nucleus (CN), hippocampus (HC), and substantia nigra (SN) of adult rats who had been exposed, both perinatally and as adults, to standard rat chow adulterated with either 5 or 20% (w/w) lyophilized fillets from either Lake Huron (LH) or Lake Ontario (LO) salmon. Dopamine (DA) concentrations in the FC were significantly decreased following exposure to both 20% fish diets. CN DA concentrations were significantly reduced in rats exposed to all diets, while SN DA was decreased only in the LO20-fed animals. SN norepinephrine concentrations were reduced in all groups except for the LO5-fed rats. 3,4-Dihydroxyphenylacetic acid (DOPAC) concentrations in the FC were significantly increased in the LH20 and LO5 groups, while CN DOPAC concentrations were reduced in LH20, LO5, and LO20 animals. 5-Hydroxyindoleacetic acid concentrations were reduced in the FC and CN of all animals exposed to diets adulterated with Great Lakes salmon. ChAT concentrations were unaffected in rats exposed to any of the adulterated diets. The significant reductions in DA, particularly in the FC and CN, suggest that either fish-borne contaminants or consumption of fish, per se, may affect behaviors that require inhibition of normal responding. We conclude that consumption of contaminated fish from the Great Lakes may result in sufficient reductions in biogenic amine function to result in significant deficits in important behavioral functions in the rat and, by inference, in the perinatally exposed human.

[A new way for neural transplantation: the grafted central neurons migration from the subarachnoid space into the spinal cord and cerebral cortex]

In the studies of neural transplantation, Chen and his colleagues observed that the grafted central neurons could migrate from the subarachnoid space into the spinal cord and cerebral cortex. This finding promises an attractive possibility that a lot of neurons might be introduced into a long distance of the spinal cord and the extensive superficial layer of the brain which has suffered neuronal loss and thus might reinnervate a wide range of denervated area. Female rats were used in the study. Neural tissues or cell suspension containing fetal monoaminergic or arginine vasopressin (AVP) neurons were implanted into the subarachnoid space of the transected spinal cord or the normal brain and spinal cord. The rats were treated in different ways: some were grafted and the spinal cord was cut at the same time; grafted one month later or before the spinal cord was cut; grafted but the brain and spinal cord remained intact. Immunohistochemical method was used to monitor the grafted neurons after these rats survived for one or nine months. The results showed that there were a few to hundreds of 5-HT, TH or AVP-immunoreactive neurons and fibers found in the spinal cord or cerebral cortex near the grafted region. These neurons grew well and displayed their capacity of adaptability and long-time survival in the host CNS. No neurons, however, were found in the subarachnoid space of the grafted rats which had survived for another month or longer. On the other side, grafted with tissue block, it was found that transplants left in the grafted region grew as a "nodule" attaching to the surface of the spinal cord. The "nodule" was also found occasionally when the cell suspension was centered heavily at one region of the subarachnoid space. In both cases, the pia mater between them disappeared. Immunoreactive neurons were found in the "nodule" and neighboring spinal cord, some fibers from the neurons in the "nodule" extended into the spinal cord. It is supposed that a part of cells in the suspension died after grafted in the subarachnoid space and released hydrolases which injured the near pia mater by hydrolysis. As a result, some neurons in the suspension had a chance to migrate into the spinal cord and cerebral cortex. After the transplant in the subarachnoid space was gradually cleaned out, the enzymatic hydrolysis to the pia mater became weaker and finally stopped, the lesioned pia mater was gradually repaired. If the transplant was not cleared timely, the pia mater could not withstand the persistent hydrolysis and collapsed finally, leading the transplant to be fused with the host CNS.

Effects of D-cycloserine and (+)-HA-966 on the locomotor stimulation induced by NMDA antagonists and clonidine in monoamine-depleted mice

We have previously observed that an N-methyl-D-aspartate (NMDA) antagonist in combination with the alpha 2-adrenoceptor agonist clonidine produces a marked locomotor stimulation in monoamine-depleted mice. In this paper we report on how the partial glycine agonists D-cycloserine (high intrinsic activity) and (+)-HA-966 [(+)-3-amino-1-hydroxypyrrolid-2-one; low intrinsic activity] affect this response; the interaction with both an uncompetitive and a competitive NMDA antagonist was investigated. (+)-HA-966 was found to counteract the locomotor stimulation produced by clonidine combined with either an uncompetitive (MK-801 = dizocilpine) or a competitive [D-CPPene = 3-(2-carboxypiperazine-4-yl)-1-propenyl-1-phosphonic acid] NMDA antagonist. D-cycloserine potentiated the locomotor stimulation produced by either NMDA antagonist combined with clonidine, although statistical significance was achieved only in the case of MK-801. If the present hyperactivity model has any relevance for psychosis the prediction based on the present results would be that d-cycloserine, contrary to current hopes, might not be so effective in schizophrenia, whereas (+)-HA-966 might be an interesting candidate.