Lateralized hippocampal effects of vasoactive intestinal peptide on learning and memory in rats in a model of depression

Biosynthesis and Function of VIP and Oxytocin: Mechanisms of C-terminal Amidation, Oxytocin Secretion and Transport

In this review, we provide the status of research on vasoactive intestinal peptide (VIP) and oxytocin, typical C-terminal α-amidated peptide hormones, including their precursor protein structures, processing and C-terminal α-amidation, and the recently identified mechanisms of regulation of oxytocin secretion and its transportation through the blood brain barrier. More than half of neural and endocrine peptides, such as VIP and oxytocin, have the α-amide structure at their C-terminus, which is essential for biological activities. We have studied the synthesis and function of C-terminal α-amidated peptides, including VIP and oxytocin, since the 1980s. Human VIP mRNA encoded not only VIP but also another related C-terminal α-amidated peptide, PHM-27 (peptide having amino-terminal histidine, carboxy-terminal methionine amide, and 27 amino acid residues). The human VIP/PHM-27 gene is composed of 7 exons and regulated synergistically by cyclic AMP and protein kinase C pathways. VIP has an essential role in glycemic control using transgenic mouse technology. The peptide C-terminal α-amidation proceeded through a 2-step mechanism catalyzed by 2 different enzymes encoded in a single mRNA. In the oxytocin secretion from the hypothalamus/the posterior pituitary, the CD38-cyclic ADP-ribose signal system, which was first established in the insulin secretion from pancreatic β cells of the islets of Langerhans, was found to be essential. A possible mechanism involving RAGE (receptor for advanced glycation end-products) of the oxytocin transportation from the blood stream into the brain through the blood-brain barrier has also been suggested.

Spatial Distribution of Inhibitory Innervations of Excitatory Pyramidal Cells by Major Interneuron Subtypes in the Auditory Cortex

Mental disorders, characterized by the National Institute of Mental Health as disruptions in neural circuitry, currently account for 13% of the global incidence of such disorders. An increasing number of studies suggest that imbalances between excitatory and inhibitory neurons in neural networks may be a crucial mechanism underlying mental disorders. However, the spatial distribution of inhibitory interneurons in the auditory cortex (ACx) and their relationship with excitatory pyramidal cells (PCs) remain elusive. In this study, we employed a combination of optogenetics, transgenic mice, and patch-clamp recording on brain slices to investigate the microcircuit characteristics of different interneurons (PV, SOM, and VIP) and the spatial pattern of inhibitory inhibition across layers 2/3 to 6 in the ACx. Our findings revealed that PV interneurons provide the strongest and most localized inhibition with no cross-layer innervation or layer specificity. Conversely, SOM and VIP interneurons weakly regulate PC activity over a broader range, exhibiting distinct spatial inhibitory preferences. Specifically, SOM inhibitions are preferentially found in deep infragranular layers, while VIP inhibitions predominantly occur in upper supragranular layers. PV inhibitions are evenly distributed across all layers. These results suggest that the input from inhibitory interneurons to PCs manifests in unique ways, ensuring that both strong and weak inhibitory inputs are evenly dispersed throughout the ACx, thereby maintaining a dynamic excitation-inhibition balance. Our findings contribute to understanding the spatial inhibitory characteristics of PCs and inhibitory interneurons in the ACx at the circuit level, which holds significant clinical implications for identifying and targeting abnormal circuits in auditory system diseases.

VIP-Expressing GABAergic Neurons: Disinhibitory vs. Inhibitory Motif and Its Role in Communication Across Neocortical Areas

GABAergic neurons play a crucial role in shaping cortical activity. Even though GABAergic neurons constitute a small fraction of cortical neurons, their peculiar morphology and functional properties make them an intriguing and challenging task to study. Here, we review the basic anatomical features, the circuit properties, and the possible role in the relevant behavioral task of a subclass of GABAergic neurons that express vasoactive intestinal polypeptide (VIP). These studies were performed using transgenic mice in which the VIP-expressing neurons can be recognized using fluorescent proteins and optogenetic manipulation to control (or regulate) their electrical activity. Cortical VIP-expressing neurons are more abundant in superficial cortical layers than other cortical layers, where they are mainly studied. Optogenetic and paired recordings performed in ex vivo cortical preparations show that VIP-expressing neurons mainly exert their inhibitory effect onto somatostatin-expressing (SOM) inhibitory neurons, leading to a disinhibitory effect onto excitatory pyramidal neurons. However, this subclass of GABAergic neurons also releases neurotransmitters onto other GABAergic and non-GABAergic neurons, suggesting other possible circuit roles than a disinhibitory effect. The heterogeneity of VIP-expressing neurons also suggests their involvement and recruitment during different functions via the inhibition/disinhibition of GABAergic and non-GABAergic neurons locally and distally, depending on the specific local circuit in which they are embedded, with potential effects on the behavioral states of the animal. Although VIP-expressing neurons represent only a tiny fraction of GABAergic inhibitory neurons in the cortex, these neurons’ selective activation/inactivation could produce a relevant behavioral effect in the animal. Regardless of the increasing finding and discoveries on this subclass of GABAergic neurons, there is still a lot of missing information, and more studies should be done to unveil their role at the circuit and behavior level in different cortical layers and across different neocortical areas.

Caffeine-related effects on cognitive performance: Roles of apoptosis in rat hippocampus following sleep deprivation

Caffeine is a common stimulant widely existed in food and has stimulatory effects on the central nervous system, shift-work individuals often rely on caffeine to maintain attention and keep awake. Although sleep deprivation (SD) is widely considered as an independent risk factor for cognition retardations, however, little is well understood about the synergistic role of caffeine dosage and SD for cognitive performance. This research intended to investigate the underlying molecular mechanism of varying caffeine doses on cognitive function after sleep deprivation. The results revealed that SD attenuated the cognitive dysfunction, associated with ultrastructure damage and pyramidal neuron loss in the hippocampus, decreased in the level of VIP and AVP. SD also significantly accelerated the neuropeptide-associated apoptosis in the hippocampus, which may modulate via the cAMP-PKA-CREB signal path axis and activation of the downstream apoptosis genes. Additionally, the data indicated that low-dose caffeine (LC) contributed to cognitive enhancement, and high-dose caffeine (HC) aggravated cognitive impairment by modulating hippocampal neuronal apoptosis. Our studies suggest that caffeine, particularly in high dosage, may be a potential factor to influence the neurocognitive outcome caused by sleep loss, and the appropriate amount of caffeine ingested after sleep deprivation deserves serious consideration.

Hippocampal GABAergic interneurons and their co-localized neuropeptides in stress vulnerability and resilience

Studies in humans and rodents suggest a critical role for the hippocampal formation in cognition and emotion, but also in the adaptation to stressful events. Successful stress adaptation promotes resilience, while its failure may lead to stress-induced psychopathologies such as depression and anxiety disorders. Hippocampal architecture and physiology is shaped by its strong control of activity via diverse classes of inhibitory interneurons that express typical calcium binding proteins and neuropeptides. Celltype-specific opto- and chemogenetic intervention strategies that take advantage of these biochemical markers have bolstered our understanding of the distinct role of different interneurons in anxiety, fear and stress adaptation. Moreover, some of the signature proteins of GABAergic interneurons have a potent impact on emotion and cognition on their own, making them attractive targets for interventions. In particular, neuropeptide Y is a promising endogenous agent for mediating resilience against severe stress. In this review, we evaluate the role of the major types of interneurons across hippocampal subregions in the adaptation to chronic and acute stress and to emotional memory formation.

VIP Modulation of Hippocampal Synaptic Plasticity: A Role for VIP Receptors as Therapeutic Targets in Cognitive Decline and Mesial Temporal Lobe Epilepsy

Vasoactive intestinal peptide (VIP) is an important modulatory peptide throughout the CNS acting as a neurotransmitter, neurotrophic or neuroprotective factor. In the hippocampus, a brain area implicated in learning and memory processes, VIP has a crucial role in the control of GABAergic transmission and pyramidal cell activity in response to specific network activity by either VIP-containing basket cells or interneuron-selective (IS) interneurons and this appears to have a differential impact in hippocampal-dependent cognition. At the cellular level, VIP regulates synaptic transmission by either promoting disinhibition, through activation of VPAC₁ receptors, or enhancing pyramidal cell excitability, through activation of VPAC₂ receptors. These actions also control several important synaptic plasticity phenomena such as long-term potentiation (LTP) and long-term depression (LTD). This paper reviews the current knowledge on the activation and multiple functions of VIP expressing cells in the hippocampus and their role in controlling synaptic transmission, synaptic plasticity and learning and memory processes, discussing also the role of VPAC₁ and VPAC₂ VIP receptors in the regulation of these different processes. Furthermore, we address the current knowledge regarding changes in VIP mediated neurotransmission in epileptogenesis and mesial temporal lobe epilepsy with hippocampal sclerosis (MTLE-HS), and discuss the therapeutic opportunities of using selective VIP receptor ligands to prevent epileptogenesis and cognitive decline in MTLE-HS.

Characterization of GABAergic marker expression in the chronic unpredictable stress model of depression

Evidence continues to build suggesting that the GABAergic neurotransmitter system is altered in brains of patients with major depressive disorder. However, there is little information available related to the extent of these changes or the potential mechanisms associated with these alterations. As stress is a well-established precipitant to depressive episodes, we sought to explore the impact of chronic stress on GABAergic interneurons. Using western blot analyses and quantitative real-time PCR (qPCR) we assessed the effects of five-weeks of chronic unpredictable stress (CUS) exposure on the expression of GABA-synthesizing enzymes (GAD₆₅ and GAD₆₇), calcium-binding proteins (calbindin (CB), parvalbumin (PV) and calretinin (CR)), and neuropeptides co-expressed in GABAergic neurons (somatostatin (SST), neuropeptide Y (NPY), vasoactive intestinal peptide (VIP) and cholecystokinin (CCK)) in the prefrontal cortex (PFC) and hippocampus (HPC) of rats. We also investigated the effects of corticosterone (CORT) and dexamethasone (DEX) exposure on these markers in vitro in primary cortical and hippocampal cultures. We found that CUS induced significant reductions of GAD₆₇ protein levels in both the PFC and HPC of CUS-exposed rats, but did not detect changes in GAD₆₅ protein expression. Similar protein expression changes were found in vitro in cortical neurons. In addition, our results provide clear evidence of reduced markers of interneuron population(s), namely SST and NPY, in the PFC, suggesting these cell types may be selectively vulnerable to chronic stress. Together, this work highlights that chronic stress induces regional and cell type-selective effects on GABAergic interneurons in rats. These findings provide additional supporting evidence that stress-induced GABA neuron dysfunction and cell vulnerability play critical roles in the pathophysiology of stress-related illnesses, including major depressive disorder.

Behavior and the cholinergic parameters in olfactory bulbectomized female rodents: Difference between rats and mice

Olfactory bulbectomy (OBX) in rodents induces a wide spectrum of functional disturbances, including behavioral, neurochemical, and neuromorphological alterations. We have examined the effects of OBX on behavior and the parameters of the cholinergic system in female rats and mice. In rats, OBX resulted in the appearance of some depressive-like behavioral marks, such as the decreased sucrose consumption, hyperactivity, impaired short-term memory and anxiety-like behavioral features, such as shortened presence in the center of the open field arena or open arms of the elevated plus-maze and an enhancement of avoidance behavior. These behavioral abnormalities could be associated with disturbances in hippocampal function, this suggestion being supported by the presence of cellular changes in this brain structure. No effect of OBX on the number of cholinergic neurons in the medial septum-diagonal band as well as on the acetylcholine content and acetylcholinesterase activity in the septum, hippocampus, and neocortex could be detected. In contrast, in mice, OBX impaired spontaneous alternation behavior and decreased the number of cholinergic neurons in the medial septum-diagonal band. These data demonstrate that rats and mice differently respond to OBX, in particular, OBX does not significantly affect the cholinergic system in rats.

Novel peptide VIP-TAT with higher affinity for PAC1 inhibited scopolamine induced amnesia

A novel peptide VIP-TAT with a cell penetrating peptide TAT at the C-terminus of VIP was constructed and prepared using intein mediated purification with an affinity chitin-binding tag (IMPACT) system to enhance the brain uptake efficiency for the medical application in central nervous system. It was found by labeling VIP-TAT and VIP with fluorescein isothiocyanate (FITC) that the extension with TAT increased the brain uptake efficiency of VIP-TAT significantly. Then short-term and long-term treatment with scopolamine (Scop) was used to evaluate the effect of VIP-TAT or VIP on Scop induced amnesia. Both short-term and long-term administration of VIP-TAT inhibited the latent time reduction in step-through test induced by Scop significantly, but long-term administration of VIP aggravated the Scop induced amnesia. Long-term i.p. injection of VIP-TAT was shown to have positive effect by inhibiting the oxidative damage, apoptosis and the cholinergic system activity reduction that induced by Scop, while VIP exerted negative effect in brain opposite to that in periphery system. The in vitro data showed that VIP-TAT had not only protective but also proliferative effect on Neuro2a cells which was inhibited by PAC1 antagonist PACAP(6-38). Competition binding assay and cAMP assay confirmed that VIP-TAT had higher affinity and activation for PAC1 than VIP. So it was concluded that the significantly stronger protective effect of VIP-TAT against Scop induced amnesia than VIP was due to (1) the enhanced brain uptake efficiency of VIP-TAT and (2) the increased affinity and activation of VIP-TAT for receptor PAC1.

Neuropeptides in learning and memory

Dementia conditions and memory deficits of different origins (vascular, metabolic and primary neurodegenerative such as Alzheimer's and Parkinson's diseases) are getting more common and greater clinical problems recently in the aging population. Since the presently available cognitive enhancers have very limited therapeutical applications, there is an emerging need to elucidate the complex pathophysiological mechanisms, identify key mediators and novel targets for future drug development. Neuropeptides are widely distributed in brain regions responsible for learning and memory processes with special emphasis on the hippocampus, amygdala and the basal forebrain. They form networks with each other, and also have complex interactions with the cholinergic, glutamatergic, dopaminergic and GABA-ergic pathways. This review summarizes the extensive experimental data in the well-established rat and mouse models, as well as the few clinical results regarding the expression and the roles of the tachykinin system, somatostatin and the closely related cortistatin, vasoactive intestinal polypeptide (VIP) and pituitary adenylate-cyclase activating polypeptide (PACAP), calcitonin gene-related peptide (CGRP), neuropeptide Y (NPY), opioid peptides and galanin. Furthermore, the main receptorial targets, mechanisms and interactions are described in order to highlight the possible therapeutical potentials. Agents not only symptomatically improving the functional impairments, but also inhibiting the progression of the neurodegenerative processes would be breakthroughs in this area. The most promising mechanisms determined at the level of exploratory investigations in animal models of cognitive disfunctions are somatostatin sst4, NPY Y2, PACAP-VIP VPAC1, tachykinin NK3 and galanin GALR2 receptor agonisms, as well as delta opioid receptor antagonism. Potent and selective non-peptide ligands with good CNS penetration are needed for further characterization of these molecular pathways to complete the preclinical studies and decide if any of the above described targets could be appropriate for clinical investigations.