Neuropeptide deficits in schizophrenia vs. Alzheimer's disease cerebral cortex

Neuropeptides involved in the pathophysiology of schizophrenia and major depression

The present review summarizes the findings on the role of neuropeptides in the pathophysiology of schizophrenia and major depression. Several neuropeptides as vasopressin and endorphins in particular, beta-endorphin and gamma-type endorphins, cholecystokinin (CCK), neurotensin, somatostatin and Neuropeptide Y have been implicated in schizophrenia. During the last decade, however, few attempts to explore the significance of most of these and other neuropeptides in the pathophysiology of the disease or their therapeutic potential are found in the literature. An exception is neurotensin, which exerts neuroleptic-like effects in animal studies, while CSF, brain and blood studies are inconclusive. Things are different in major depression. Here much attention is paid to the endocrine abnormalities found in this disorder in particular the increased activity of the hypothalamic-pituitary-adrenal (HPA) axis. Neuropeptides as corticotropin-releasing hormone (CRH), vasopressin and corticosteroids are implicated in the symptomatology of this disorder. As a consequence much work is going on investigating the influence of CRH and corticosteroid antagonists or inhibitors of the synthesis of corticosteroids as potential therapeutic agents. This review emphasizes the role of vasopressin in the increased activity of the HPA axis in major depression and suggests exploration of the influence of the now available non-peptidergic vasopressin orally active V1 antagonists.

Early maternal deprivation alters hippocampal levels of neuropeptide Y and calcitonin-gene related peptide in adult rats

Stressful events early in life are reported to be more prevalent among patients with an adult life psychiatric disorder. Early maternal deprivation is considered an animal model of early life stress. Maternally deprived adult rats display long-term alterations in the neuroendocrine system, brain and behavior that are in many ways analogous to depressive and schizophrenic symptomatology. Neuropeptide Y (NPY) and calcitonin-gene related peptide (CGRP) have been implicated in both disorders and also been suggested to play a role in the neuroadaptational response to stress. Consequently, male Wistar rat-pups were subjected to early maternal deprivation or control handling, on postnatal day (pnd) 9. On pnd 21, pups were weaned and split into two groups that were reared either on a saw-dust floor or on a grid-floor, considered to be a mild stressor. On pnd 67, all animals were subjected to the prepulse inhibition test. One week later, the animals were sacrificed, the brains removed and dissected on ice. Levels of NPY-like immunoreactivity (LI) and CGRP-LI were quantified by radioimmunoassay in brain regional extracts. Maternal deprivation led to a significant reduction in basal startle amplitude and disruption of prepulse inhibition. These findings were paralleled by significantly reduced levels of NPY and CGRP in the hippocampus and occipital cortex. It is hypothesised that these changes may be of relevance to aspects of schizophrenic and affective symptomatology. The present study further shows that brain NPY and, in particular, CGRP are sensitive to long-term mild stress and further implicate the involvement of these peptides in the neuroendocrine stress response.

Schizophrenia and viral infection during neurodevelopment: a focus on mechanisms

The task of defining schizophrenia pathogenesis has fascinated and frustrated researchers for nearly a century. In recent years, unprecedented advances from diverse fields of study have given credence to both viral and developmental theories. This review considers possible mechanisms by which viral and developmental processes may interact to engender schizophrenia. Many of the current controversies in schizophrenia pathogenesis are reviewed in light of the viral hypothesis, including: epidemiological findings and the role of a genetic diathesis, phenotype heterogeneity, abnormalities in excitatory and inhibitory neurotransmitter systems, anomalous cerebral latereralization, and static vs progressive disease. The importance of animal models in elucidating the impact of viral infections on developing neurons is illustrated by recent studies in which neonatal rats are infected with lymphocytic choriomeningitis virus in order to examine alterations in hippocampal circuitry. Finally, consideration is given to a new hypothesis that some cases of schizophrenia could be instigated by a viral infection that disrupts developing inhibitory circuits, consequently unleashing glutamatergic neurotransmission leading to selective excitotoxicity, and a degenerative disease course.

Neuropeptide Y, peptide YY and aluminum in Alzheimer's disease: is there an etiological relationship?

Neuropeptide Y (NPY) and peptide YY (PYY) are members of the pancreatic polypeptide family which have a high degree of primary and tertiary structural homology. They function as neurotransmitters and humoral agents in central nervous system and gastrointestinal function. During the last two decades, NPY body fluid concentrations and NPY/PYY brain receptor numbers have been demonstrated to be altered during the course of Alzheimer's disease. Recent research has shown that both NPY and PYY may be involved in aluminum metabolism in animal models. A brief discussion of the structure, biological activity and possible involvement of these peptides in aluminum metabolism and Alzheimer's disease is contained herein.

Neurochemical correlates of cortical GABAergic deficits in schizophrenia: selective losses of calcium binding protein immunoreactivity

Deficits in a variety of different neurochemical species are consistent with a loss of cortical gamma-aminobutyric acid (GABA)ergic interneurons in schizophrenia. As well as neurochemical markers that indicate all neurons using GABA as a transmitter, and which include GABA uptake sites and glutamate decarboxylase, deficits of certain neuropeptides and calcium binding proteins coexisting with GABA have been reported. These abnormalities are indicative of losses specific to certain subtypes of GABAergic neurons. The calcium binding proteins in particular demonstrate selective deficits; we find losses of parvalbumin- and calbindin-, but not calretinin-immunoreactive cells in the prefrontal cortex in schizophrenia. These selective reductions in the density of parvalbumin- and calbindin-containing neurons could reflect functional loss of expression in intact cells or alternatively a deficit in the density of certain GABAergic neuronal subtypes. The latter interpretation is consistent with a neurodevelopmental pathogenesis involving neuronal damage at a time prior to the expression of these protective calcium-binding proteins. In this review we discuss the evidence for altered GABAergic transmission in schizophrenia.

Auditory verbal hallucinations and dysfunction of the neural substrates of speech

OBJECTIVE

to evaluate the neural substrate of auditory verbal hallucinations (AVH), the correlation between AVH and subvocal speech (hereafter SVS), and the relationship between speech and AVH.

METHOD

we reviewed the papers found by an electronic literature search on hallucinations and speech. The review was extended to the papers cited in these publications and to classical works.

RESULTS

there is no conclusive evidence of structural abnormality of the speech perception area in hallucinating schizophrenic patients. However there is evidence of electrophysiological abnormalities of the auditory and speech perception cortices. Functional imaging data are inconsistent, yet point to the left superior temporal gyrus as one of the neural substrates for AVH. There is also evidence that SVS could accompany the experience of AVH.

CONCLUSION

there is evidence that dysfunction of brain areas responsible for speech generation is a fundamental mechanism for generating AVH in schizophrenia. It results in a secondary activation of Wernicke's area (speech perception) and Broca's area (speech expression). The first leading to the experience of hallucinations, and the second, eventually, gives rise to a variable degree of vocal muscle activity detectable by EMG, and/or faint vocalizations detectable by sensitive microphones placed at proximity of the larynx. Direct stimulation or disease of Wernicke's area produces AVH without SVS.

The alpha7-nicotinic acetylcholine receptor and the pathology of hippocampal interneurons in schizophrenia

This paper is a review of a recent findings on the pathology of hippocampal interneurons in schizophrenia, with specific emphasis on a protein expressed by these cells, the alpha7-nicotinic acetylcholine receptor subunit. Convergent information indicates that interneurons in the hippocampus and other forebrain structures are decreased in number and function in subjects with schizophrenia. Among the neurochemical markers that are decreased in the hippocampus are synapsin I, cholecystokinin, somatostatin, glutamic acid decarboxylase, and nitric oxide synthase. GABA uptake sites and the GABA synthetic enzyme glutamic acid decarboxylase are also diminished. Included among these findings is decreased binding of alpha-bungarotoxin, which binds to low-affinity nicotinic acetylcholine receptors, such as the alpha7-nicotinic receptor. Co-labeling experiments in rodents indicate that these markers are expressed on overlapping populations of hippocampal interneurons. Thus, the finding of decreased neurochemical function of hippocampal interneurons is a widely replicated finding, with different groups reporting markedly similar findings using independent post mortem samples and different neurochemical strategies. Decreased alpha-bungarotoxin binding or decreased alpha7-nicotinic receptor immunoreactivity has also been found in the frontal cortex and in the nucleus reticularis thalami of schizophrenic subjects. The alpha7-nicotinic receptor subunit gene on chromosome 15q14 is a site of heritability for schizophrenia and bipolar affective disorder, and in, particular, for a deficit in inhibitory neuronal function associated with these illnesses. Thus, the post mortem data are further supported by psychophysiologic and genetic investigations that indicate a deficit in inhibitory interneuronal function, involving the alpha7-nicotinic receptor. The alpha7-receptor is a ligand-gated ion channel that admits calcium ions into cells, and it has been proposed to have various developmental roles. Its malfunction may be part of the developmental pathogenesis of schizophrenia.

Effects of typical and atypical antipsychotics on neuropeptide Y in rat brain tissue and microdialysates from ventral striatum

The main goal of this study was to investigate effects of typical (haloperidol) and atypical (risperidone) antipsychotic drugs on brain regional neuropeptide Y (NPY)-like immunoreactivity (-LI) tissue concentrations and on release of NPY-LI in freely moving rats. An additional aim was to explore the effect of d-amphetamine on NPY-LI release following pretreatment with typical and atypical antipsychotics. During a 4-week period, male Wistar rats were fed chow to which vehicle, risperidone (1.15 mg/100 g food or 2.3 mg/100 g food), or haloperidol (1.15 mg/100 g food) were added. In one series of experiments, the animals were sacrificed on day 30 with focused microwave irradiation, the brain regions dissected and extracted for radioimmunoassay of NPY-LI. In another experimental series, probes were inserted into the ventral striatum. The perfusates were collected at 60-min intervals; NPY-LI was determined by radioimmunoassay. Haloperidol significantly increased NPY-LI in hypothalamus and the occipital cortex. In contrast, haloperidol decreased tissue levels of NPY-LI in striatum. Moreover, haloperidol and risperidone also significantly decreased extracellular NPY-LI concentrations in the ventral striatum. d-amphetamine (1.5 mg/kg) significantly increased extracellular NPY-LI in the vehicle group. Both haloperidol and risperidone pretreatments abolished the effect of d-amphetamine. The results show that d-amphetamine as well as haloperidol and risperidone selectively and specifically affect NPY-LI concentrations in brain tissue and microdialysates and that the effect of d-amphetamine is abolished by both typical and atypical antipsychotics.

Abnormal expression of brain-derived neurotrophic factor and its receptor in the corticolimbic system of schizophrenic patients

Previous neuropathological studies have revealed that the corticolimbic system of schizophrenic patients expresses abnormal levels of various synaptic molecules, which are known to be influenced by the neuronal differentiation factors, neurotrophins. Therefore, we determined levels of neurotrophins and their receptors in the postmortem brains of schizophrenic patients and control subjects in relation to molecular impairments in schizophrenia. Among the neurotrophins examined, levels of brain-derived neurotrophic factor (BDNF) were elevated specifically in the anterior cingulate cortex and hippocampus of schizophrenic patients, but levels of nerve growth factors and neurotrophin-3 showed no change in any of the regions examined. In parallel, the expressions of TrkB receptor and calbindin-D, which are both influenced by BDNF, were reduced significantly in the hippocampus or the prefrontal cortex. However, neuroleptic treatment did not appear to mimic the neurotrophic change. Neither withdrawal of drug treatment in patients nor chronic administration of haloperidol to rats altered levels of BDNF. These findings suggest that neurotrophic abnormality is associated with the corticolimbic structures of schizophrenic patients and might provide the molecular substrate for pathological manifestations of the illness.

Expression of the cannabinoid receptor CB1 in distinct neuronal subpopulations in the adult mouse forebrain

Cannabinoids can modulate motor behaviour, learning and memory, cognition and pain perception. These effects correlate with the expression of the cannabinoid receptor 1 (CB1) and with the presence of endogenous cannabinoids in the brain. In trying to obtain further insights into the mechanisms underlying the modulatory effects of cannabinoids, CB1-positive neurons were determined in the murine forebrain at a single cell resolution. We performed a double in situ hybridization study to detect mRNA of CB1 in combination with mRNA of glutamic acid decarboxylase 65k, neuropeptide cholecystokinin (CCK), parvalbumin, calretinin and calbindin D28k, respectively. Our results revealed that CB1-expressing cells can be divided into distinct neuronal subpopulations. There is a clear distinction between neurons containing CB1 mRNA either at high levels or low levels. The majority of high CB1-expressing cells are GABAergic (gamma-aminobutyric acid) neurons belonging mainly to the cholecystokinin-positive and parvalbumin-negative type of interneurons (basket cells) and, to a lower extent, to the calbindin D28k-positive mid-proximal dendritic inhibitory interneurons. Only a fraction of low CB1-expressing cells is GABAergic. In the hippocampus, amygdala and entorhinal cortex area, CB1 mRNA is present at low but significant levels in many non-GABAergic cells that can be considered as projecting principal neurons. Thus, a complex mechanism appears to underlie the modulatory effects of cannabinoids. They might act on principal glutamatergic circuits as well as modulate local GABAergic inhibitory circuits. CB1 is very highly coexpressed with CCK. It is known that cannabinoids and CCK often have opposite effects on behaviour and physiology. Therefore, we suggest that a putative cross-talk between cannabinoids and CCK might exist and will be relevant to better understanding of physiology and pharmacology of the cannabinoid system.

Reduction in somatostatin and substance P levels and choline acetyltransferase activity in the cortex and hippocampus of the rat after chronic intracerebroventricular infusion of beta-amyloid (1-40)

The present study investigated the neurochemical and behavioural sequelae following chronic intracerebroventricular infusion of beta-amyloid (1-40) in rats. beta-amyloid was either infused intermittently via implanted cannulae on the day of operation and subsequently on postsurgical days 4, 7, 10, and 13 (Experiment 1), or continuously using osmotic pumps for 14 days (Experiment 2). The same amount of beta-amyloid was delivered under both infusion regimes. In both experiments, beta-amyloid infusion led to severe deficits in the acquisition of a spatial reference memory task conducted on postoperative days 10 to 14. The animals were sacrificed on the postoperative day 15 for neurochemical analyses. These included radioenzymatic and radioimmunoassays, designed to determine choline acetyltransferase activity and the contents of neuropeptides (somatostatin, substance P, and neuropeptide Y), respectively. Experiment 2 also included solution-hybridisation-RNAase protection assay for preprosomatostatin mRNA quantification. There was a significant reduction in choline acetyltransferase activity and in the levels of substance P as well as somatostatin and preprosomatostatin mRNA in the cortical mantle of beta-amyloid-treated rats, compared to controls in both experiments. Appreciable reductions in choline acetyltransferase activity and somatostatin level were also apparent in the hippocampus. In contrast, beta-amyloid infusion did not significantly affect the brain level of neuropeptide Y. The present study demonstrated that chronic infusion of beta-amyloid can lead to a reduction in the levels of selected neuropeptides resembling the pattern seen in Alzheimer's disease patients.

Cerebrospinal fluid somatostatin, mood, and cognition in multiple sclerosis

BACKGROUND

Cerebrospinal fluid (CSF) somatostatin (SS) levels have been shown to be decreased in multiple sclerosis (MS) during relapse as well as in disorders characterized by depression or cognitive impairment. Since MS is often associated with depression and cognitive impairment, we examined both the effect of course of illness on CSF SS as well as the variance in SS attributable to associated features (e.g., depression or cognitive impairment).

METHODS

Fifteen patients with chronic progressive MS participating in a 2-year cyclosporine trial underwent lumbar punctures for CSF SS at baseline and at 12 and 24 months. Additionally, patients were evaluated by neuropsychological testing, and physical disability and mood ratings. Baseline CSF SS levels were also obtained in a group of control subjects (n = 10).

RESULTS

At baseline, CSF SS levels were lower in MS patients than control subjects (p < .001). Decreased CSF SS at 24 months was correlated with decreased cognitive performance on several measures and was best and significantly predicted by cognitive deterioration at 24 months.

CONCLUSIONS

Our data support those from previous studies that found lower levels of CSF SS in MS during relapse and suggest that changes in CSF SS are related to the process responsible for diminished cognitive function in MS.

Early appearance of acetylcholinergic, serotoninergic, and peptidergic neurons and fibers in the developing human central nervous system

Animal experiments have already shown that neurotransmitters and neuropeptides are not only important for normal functioning of the adult central nervous system (CNS) but are also crucial to its development. However, information on the spatio-temporal distribution of these endogenous substances in the developing human CNS is still scarce. With the use of immunocytochemical staining and a constant supply of properly fixed human abortuses from southern China, an early appearance of acetylcholinesterase, enkephalin, and substance P immunoreactivities was detected first in the spinal cord (weeks 5 to 7 of gestation), then in the brainstem nuclei (weeks 11 to 12). Their overlapping localizations in many regions of the CNS suggest possible interactions among neurons containing these substances, which are in turn important for the proper establishment of the neuronal circuitry. Immunoreactivity for neuropeptide Y appeared initially in the lateral region of upper segments of the spinal cord at week 12 of gestation, then spread latero-medially and cranio-caudally to the sacral region. In the hippocampus, neuropeptide Y neurons appeared from week 15 onwards. Serotoninergic neurons were found in the dorsal raphe nucleus at week 10 and then decreased in number as the fetus grew older. Somatostatin releasing inhibitory factor, vasopressin, and oxytocin were detected in the hypothalamus from weeks 12 to 14 onwards, and monoamine oxidase, succinic dehydrogenase, parvalbumin, calbindin D28K, and vasoactive intestinal peptide were found in the visual cortex at midgestation. The early appearance and the abundance of the neurotransmitters and neuropeptides in the developing CNS indicate that they may play a key role in neuronal differentiation.

Marked decrease of neuropeptide Y Y2 receptor binding sites in the hippocampus in murine prion disease

Using autoradiographic binding methodology with monoiodinated peptide YY together with the agonists neuropeptide Y (NPY) and NPY (13-36), as well as in situ hybridization with oligonucleotide probes complementary to the NPY Y2 receptor (Y2-R) mRNA, we have studied whether or not intracerebral prion inoculation affects Y2-Rs in male CD-1 mice. Monoiodinated peptide YY binding, mainly representing Y2-Rs, was down-regulated by 85% in the CA1 strata oriens and radiatum and by 50-65% in the CA3 stratum oriens 110-140 days postinoculation. In the CA3 stratum radiatum, where the mossy fibers from the dentate granule cells project, there was a significant decrease in PYY binding at 110-120 days. Y2-R mRNA, moderately expressed both in the CA1 and CA3 pyramidal cell layers and the granule cell layer in the dentate gyrus, showed a slight, but not significant, decrease in CA3 neurons 130 days postinoculation. The results indicate that the accumulation of the scrapie prion protein in the CA1-3 region strongly inhibits NPY binding at the Y2-Rs, which, however, is only marginally due to reduced Y2-R mRNA expression. The loss of the ability of NPY to bind to inhibitory Y2-Rs may cause dysfunction of hippocampal circuits and may contribute to the clinical symptoms in mouse scrapie.