The effects of neurotensin, vasoactive intestinal polypeptide and other neuropeptides on the secretion of somatostatin from cerebral cortical cells

Neuropeptide System Regulation of Prefrontal Cortex Circuitry: Implications for Neuropsychiatric Disorders

Neuropeptides, a diverse class of signaling molecules in the nervous system, modulate various biological effects including membrane excitability, synaptic transmission and synaptogenesis, gene expression, and glial cell architecture and function. To date, most of what is known about neuropeptide action is limited to subcortical brain structures and tissue outside of the central nervous system. Thus, there is a knowledge gap in our understanding of neuropeptide function within cortical circuits. In this review, we provide a comprehensive overview of various families of neuropeptides and their cognate receptors that are expressed in the prefrontal cortex (PFC). Specifically, we highlight dynorphin, enkephalin, corticotropin-releasing factor, cholecystokinin, somatostatin, neuropeptide Y, and vasoactive intestinal peptide. Further, we review the implication of neuropeptide signaling in prefrontal cortical circuit function and use as potential therapeutic targets. Together, this review summarizes established knowledge and highlights unknowns of neuropeptide modulation of neural function underlying various biological effects while offering insights for future research. An increased emphasis in this area of study is necessary to elucidate basic principles of the diverse signaling molecules used in cortical circuits beyond fast excitatory and inhibitory transmitters as well as consider components of neuropeptide action in the PFC as a potential therapeutic target for neurological disorders. Therefore, this review not only sheds light on the importance of cortical neuropeptide studies, but also provides a comprehensive overview of neuropeptide action in the PFC to serve as a roadmap for future studies in this field.

Vasoactive intestinal peptide fluctuates in human blood with a circadian rhythm

The vasoactive intestinal peptide (VIP) may be radioimmunoassayed in systemic venous blood. The plasma concentrations of VIP were investigated in human blood according to a chronobiological design. The study documented a circadian rhythmicity in time-qualified concentrations of VIP. Accordingly, VIP may be ascribed to biological variables characterized by periodicity in their physiological attributes. The rhythmic physiology of VIP is, however, highly disturbed in its tonic and phasic properties during senescence.

Endogenous vasoactive intestinal peptide (VIP) regulates somatostatin secretion by cultured fetal rat cerebral cortical and hypothalamic cells

To determine the possible physiological role of endogenous vasoactive intestinal peptide (VIP) in the control of cerebral somatostatin (SS), we studied the effect of endogenous VIP blockade on immunoreactive SS (IR-SS) accumulation by fetal rat cerebral cortical and hypothalamic cells in culture. Cells were cultured in minimum essential medium (MEM) with 10% fetal calf serum and 10% horse serum. After 7-10 days 'in vitro' media were replaced with MEMs without sera containing anti-VIP immunoglobulins G (IgG) for 1, 3, 6, 24 or 48 h. Controls received the same amount of IgG from normal rabbit serum (NRS). In another group of experiments, cells were incubated with VIP (10(-11) M to 10(-7) M) for 1, 3, 6 or 24 h. Exposure to anti-VIP IgG resulted in a decreased accumulation of IR-SS in both cerebral cortical and hypothalamic cells, whereas the addition of VIP caused a dose-dependent increase in total IR-SS, these effects being evident after 3 h incubation. The stimulatory action VIP on IR-SS was up to 129%, this being decreased to 86% by the addition of anti-VIP to plates containing 10(-7) M VIP. Patterns of IR-SS accumulation throughout prolonged incubation periods were qualitatively similar (in both cerebrocortical and hypothalamic cells) in the presence or absence of anti-VIP IgG. However, in plates containing anti-VIP, the total amount of IR-SS was lower than in the control groups (IgG from NRS). These findings demonstrate that, at this time of brain development, somatostatinergic neurons may be under the physiological regulation of locally produced VIP.

Neurotensin innervation of the human cerebral cortex: lack of colocalization with catecholamines

We have localized neurotensin (NT) with immunocytochemical methods in the normal human cerebral cortex. Extensive areas of the frontal cortex, the hippocampal formation, and selected areas of the parietal, temporal and occipital lobes, were examined using post-mortem brain tissue. The peptidergic innervation was characteristically restricted to the limbic belt and to the dorsally contiguous regions. NT-labeled perikarya were found throughout the subiculum, including its dorsal supra-callosal continuation. NT terminal plexuses were particularly abundant in layers I-VI of the anterior cingulate cortex, in layer I of area 32 and of medical areas 9, 8, 6 and in layers II-III of area 29, of the presubiculum and entorhinal cortex. Elsewhere, NT fibers were scarce being more frequent in layer I. This regional and laminar pattern differed significantly from that of tyrosine hydroxylase (TH), which was used to label catecholaminergic axons, and preferentially the dopaminergic ones. Even in zones where TH and NT innervations were abundant, such as the anterior cingulate cortex or area 32, double-labeling procedures disclosed no colocalized fibers. The lack of NT-TH colocalization in human, contrasts with previous findings in the rodent cortex, where a contingent of the DA cortical afferents contains NT. The DA mesocortical neuronal population, labeled by TH antisera, thus seems to change its chemical phenotype, by losing the expression of an associated peptidergic neurotransmitter; this could be related to the predominant extension in the ascent of the phylogenetic scale of the non-colocalized, type of cortical DA innervation which is also found in rodents. The possible origins of the cortical, non-dopaminergic NT innervation in human are discussed: thalamo-cortical, subiculo-cortical or intrinsic. Such cortical NT innervation could be very important in limbic circuitry as a regulatory peptide in affective processes and could be involved in the physiology of pain and memory.

Depolarizing influences regulate somatostatin synthesis and processing in cultured cerebral cortical cells

There is increasing evidence that persistent depolarization plays a critical role not only in excitation-secretion coupling, but also in the mechanisms linking excitation of neuronal cells to long-term adaptative changes in biosynthesis of neuropeptides. Somatostatin (SRIF) release and synthesis are affected by numerous agents, such as high concentrations of potassium that cause depolarization of cellular membrane. In the present work, we tried to determine whether prolonged exposure to veratridine (VTD) regulates SRIF synthesis. We found that exposure to VTD (100 microM) resulted in the stimulation of total (cell content + media) immunoreactive SRIF (IR-SRIF). This effect was calcium- and sodium-dependent, since it was prevented when verapamil (VPM) 20 microM or tetrodotoxin (TTX) 1 microM were added simultaneously with VTD. Cerebral cortical cells were exposed to high potassium concentrations, and the nature of the IR-SRIF was characterized by high-pressure liquid chromatography (HPLC) or gel filtration. It was evident that chronic exposure to high potassium concentrations modified the elution profile of medium IR-SRIF on HPLC and gel filtration, causing an increase in somatostatin-28 (S-28) and a decrease in somatostatin-14 (S-14). The results indicate that chronic exposure to VTD or high potassium concentration increases immunoreactive somatostatin and augments synthesis of its high-molecular-weight forms. This suggests that chronic membrane depolarization activating sodium and calcium channels initiates the entry of calcium ions, which triggers somatostatin release and causes a depletion of its intracellular stores. The stimulation of somatostatin secretion could be coupled to synthesis of the peptide.

VIP: molecular biology and neurobiological function

In the mammalian brain, a major regulatory peptide is vasoactive intestinal peptide (VIP). This 28 amino acid peptide, originally isolated from the porcine duodenum, was later found in the central and peripheral nervous systems and in endocrine cells, where it exhibits neurotransmitter and hormonal roles. Increasing evidence points to VIP's importance as a mediator or a modulator of several basic functions. Thus, VIP is a major factor in brain activity, neuroendocrine functions, cardiac activity, respiration, digestion, and sexual potency. In view of this peptide's importance, the mechanisms controlling its production and the pathways regulating its functions have been reviewed. VIP is a member of a peptide family, including peptides such as glucagon, secretin, and growth hormone releasing hormone. These peptides may have evolved by exon duplication coupled with gene duplication. The human VIP gene contains seven exons, each encoding a distinct functional domain on the protein precursor or the mRNA. VIP gene transcripts are mainly found in neurons or neuron-related cells. VIP gene expression is regulated by neuronal and endocrine signals that contribute to its developmental control. VIP exerts its function via receptor-mediated systems, activating signal transduction pathways, including cAMP. It can act as a neurotransmitter, neuromodulator, and a secretagog. As a growth and developmental regulator, VIP may have a crucial effect as a neuronal survival factor. We shall proceed from the gene to its multiple functions.

Neuroendocrine significance of vasoactive intestinal polypeptide

The new data reported here, and available in the literature, are interpreted to indicate that acute release of PRL in stress is probably mediated by secretion of VIP and PHI arising from a subpopulation of paraventricular cells in the tuberoinfundibular system, and that this secretion is under serotonergic control, presumably by way of the raphe nuclear projection to the hypothalamus. The acute PRL response to suckling response is only partially under VIP/PHI control, and may be regulated by an as yet unidentified neural lobe hormone. In addition to the hypothalamic component of PRL regulation, there is a well-defined population of VIP cells within the pituitary, representing the only known example of VIP expression outside of nerve cells. This population of VIP cells is exquisitely responsive to thyroid status, and in common with the thyrotrope cell, is activated by hypothyroidism. Since VIP secretion is enhanced in the hypothyroid pituitary, and VIP release is stimulated by TRH, it is reasonable to postulate that paracrine VIP secretion may play a role in the reasonable to postulate that paracrine VIP secretion may play a role in the hyperprolactinemia prolactinemia that occurs in the hypothyroid human, although this is clearly not the case for the rat in whom hyperprolactinemia was not demonstrable. The role of VIP/PHI in human pituitary disease is unknown. We have been unable to identify any tumors that contain immunoreactive material using tissues prepared by standard methods. It may be that the demonstration of VIP/PHI is more demanding and will require better techniques for staining. The role of tuberoinfundibular VIP hypersecretion remains to be established but the evidence for stress-induced PRL hypersecretion in man encourages us to believe that at least some cases may be due to excessive hypothalamic activity. Additional potential neuroendocrine actions of VIP are in the secretomotor control of the ovary and thyroid, and in the regulation of somatostatin secretion and synthesis. In dispersed cell cultures (but not in whole hypothalamic slices from adult animals), VIP stimulates somatostatin secretion and independently stimulates the formation of somatostatin mRNA, an effect that can be duplicated in mixed cultures by treatment with forskolin, a postreceptor cAMP stimulator. In work carried out by Montminy and colleagues, the cAMP action has shown to be mediated by formation of a soluble protein that appears to activate the somatostatin gene promotor through interaction with a specific gene sequence.

Adenylate cyclase activation is not sufficient to stimulate somatostatin release from dispersed cerebral cortical and diencephalic cells in glia-free cultures

Under conditions in which vasoactive intestinal peptide (VIP) induces somatostatin release from cortical and diencephalic neuronal cultures, VIP causes large increases in intracellular cyclic AMP. Both the release of somatostatin and the increase in cyclic AMP elicited by VIP require exogenous calcium, can be blocked by cobalt ion, and can be qualitatively mimicked by depolarizating concentrations of exogenous potassium ion. Direct activation of adenylate cyclase by forskolin causes large increases in cyclic AMP content but does not induce somatostatin release. In the absence of VIP, the calcium ionophore, ionomycin, and the phorbol ester, phorbol 12-myristate-13-acetate, also stimulate somatostatin release. These results indicate that VIP-stimulation of cyclic AMP formation and VIP-stimulation of somatostatin release are calcium-dependent and that the two phenomena are dissociatable. Cyclic AMP formation is not a necessary condition for VIP-induced somatostatin release. Nucleotide formation may be a sufficient condition for release or, possibly in association with calcium influx, it may be an event unrelated to the release process.

Distribution of neurotensin-like immunoreactivity in the diencephalon of the Japanese monkey (Macaca fuscata)

The distribution of neurotensin-like immunoreactive (NT-LI) neurons was examined in the thalamus and hypothalamus of the Japanese monkey (Macaca fuscata) by using the peroxidase-antiperoxidase immunocytochemistry technique. In the thalamus, NT-LI neuronal perikarya were distributed mainly in the midline nuclear group and the dorsomedial nucleus, and partially in the intralaminar nucleus: Immunoreactive fibers were mainly distributed in the midline nucleus, particularly in the nucleus rhomboidalis. Numerous immunoreactive fibers were also detected in the regions that contain the pathways to extrathalamic areas such as the stratum zonale and inferior thalamic peduncle. In the hypothalamus, many immunoreactive neuronal perikarya were distributed in the lateral hypothalamic area and in the arcuate nucleus. Immunoreactive fibers were disseminated throughout the hypothalamus, but they were dense in the preoptic area and sparse in the ventromedial nucleus. An accumulation of dense immunoreactive endings was also observed in the external layer of the median eminence. NT-LI fibers in the external layer of the median eminence were considered to represent nerve endings near portal vessels. Functional roles of neurotensin in the thalamus and hypothalamus are discussed from the anatomical point of view.

Neurotransmitters in the cerebral cortex

This article surveys the conventional neurotransmitters and modulatory neuropeptides that are found in the cerebral cortex and attempts to place them into the perspective of both intracortical circuitry and cortical disease. The distribution of these substances is related, where possible, to particular types of cortical neuron or to afferent or efferent fibers. Their physiological actions, where known, on cortical neurons are surveyed, and their potential roles in disease states such as the dementias, epilepsy, and stroke are assessed. Conventional transmitters that occur in afferent fibers to the cortex from brain-stem and basal forebrain sites are: serotonin, noradrenaline, dopamine, and acetylcholine. All of these except dopamine are distributed to all cortical areas: dopamine is distributed to frontal and cingulate areas only. The transmitter in thalamic afferent systems is unknown. Gamma aminobutyric acid (GABA) is the transmitter used by the majority of cortical interneurons and has a profound effect upon the shaping of receptive field properties. The vast majority of the known cortical peptides are found in GABAergic neurons, and the possibility exists that they may act as trophic substances for other neurons. Levels of certain neuropeptides decline in cases of dementia of cortical origin. Acetylcholine is the only other known transmitter of cortical neurons. It, too, is contained in neurons that also contain a neuropeptide. The transmitter(s) used by excitatory cortical interneurons and by the efferent pyramidal cells is unknown, but it may be glutamate or aspartate. It is possible that excitotoxins released in anoxic disease of the cortex may produce damage by acting on receptors for these or related transmitter agents.

Somatostatin release from dissociated cerebral cortical cell cultures

Primary monolayer cultures of dispersed fetal cerebral cortical cells can be used to measure the release of the neuropeptide, somatostatin. Three to five percent of cellular IRS is released basally into KRB in 10 min. Basal release is stable for at least 60 min and stimulated levels of release can be induced by introducing ionophores, neurotransmitters, or peptides. The peptide content of the incubation samples is readily measured by a well-characterized, sensitive RIA. Table II summarizes the major factors that must be taken into consideration when developing this system for measuring peptide release.