Actions of vasoactive intestinal polypeptide (VIP) on neocortical neurons of the rat in vitro

Distribution of vasoactive intestinal peptide (VIP) immunoreactivity in the rat pallial and subpallial amygdala and colocalization with γ-aminobutyric acid (GABA)

The amygdaloid complex, also known as the amygdala, is a heterogeneous group of distinct nuclear and cortical pallial and subpallial structures. The amygdala plays an important role in several complex functions including emotional behavior and learning. The expression of calcium-binding proteins and peptides in GABAergic neurons located in the pallial and subpallial amygdala is not uniform and is sometimes restricted to specific groups of cells. Vasoactive intestinal polypeptide (VIP) is present in specific subpopulations of GABAergic cells in the amygdala. VIP immunoreactivity has been observed in somatodendritic and axonal profiles of the rat basolateral and central amygdala. However, a comprehensive analysis of the distribution of VIP immunoreactivity in the various pallial and subpallial structures is currently lacking. The present study used immunohistochemical and morphometric techniques to analyze the distribution and the neuronal localization of VIP immunoreactivity in the rat pallial and subpallial amygdala. In the pallial amygdala, VIP-IR neurons are local inhibitory interneurons that presumably directly and indirectly regulate the activity of excitatory pyramidal neurons. In the subpallial amygdala, VIP immunoreactivity is expressed in several inhibitory cell types, presumably acting as projection or local interneurons. The distribution of VIP immunoreactivity is non-homogeneous throughout the different areas of the amygdaloid complex, suggesting a distinct influence of this neuropeptide on local neuronal circuits and, consequently, on the cognitive, emotional, behavioral and endocrine activities mediated by the amygdala.

Role of vasoactive intestinal peptide in the light input to the circadian system

The neuropeptide vasoactive intestinal peptide (VIP) is expressed at high levels in a subset of neurons in the ventral region of the suprachiasmatic nucleus (SCN). While VIP is known to be important for the synchronization of the SCN network, the role of VIP in photic regulation of the circadian system has received less attention. In the present study, we found that the light-evoked increase in electrical activity in vivo was unaltered by the loss of VIP. In the absence of VIP, the ventral SCN still exhibited N-methyl-d-aspartate-evoked responses in a brain slice preparation, although the absolute levels of neural activity before and after treatment were significantly reduced. Next, we used calcium imaging techniques to determine if the loss of VIP altered the calcium influx due to retinohypothalamic tract stimulation. The magnitude of the evoked calcium influx was not reduced in the ventral SCN, but did decline in the dorsal SCN regions. We examined the time course of the photic induction of Period1 in the SCN using in situ hybridization in VIP-mutant mice. We found that the initial induction of Period1 was not reduced by the loss of this signaling peptide. However, the sustained increase in Period1 expression (after 30 min) was significantly reduced. Similar results were found by measuring the light induction of cFOS in the SCN. These findings suggest that VIP is critical for longer-term changes within the SCN circuit, but does not play a role in the acute light response.

Vasoactive intestinal peptide produces long-lasting changes in neural activity in the suprachiasmatic nucleus

The neuropeptide vasoactive intestinal peptide (VIP) is expressed at high levels in the neurons of the suprachiasmatic nucleus (SCN). While VIP is known to be important to the input and output pathways from the SCN, the physiological effects of VIP on electrical activity of SCN neurons are not well known. Here the impact of VIP on firing rate of SCN neurons was investigated in mouse slice cultures recorded during the night. The application of VIP produced an increase in electrical activity in SCN slices that lasted several hours after treatment. This is a novel mechanism by which this peptide can produce long-term changes in central nervous system physiology. The increase in action potential frequency was blocked by a VIP receptor antagonist and lost in a VIP receptor knockout mouse. In addition, inhibitors of both the Epac family of cAMP binding proteins and cAMP-dependent protein kinase (PKA) blocked the induction by VIP. The persistent increase in spike rate following VIP application was not seen in SCN neurons from mice deficient in Kv3 channel proteins and was dependent on the clock protein PER1. These findings suggest that VIP regulates the long-term firing rate of SCN neurons through a VIPR2-mediated increase in the cAMP pathway and implicate the fast delayed rectifier (FDR) potassium currents as one of the targets of this regulation.

VIP, CRF, and PACAP act at distinct receptors to elicit different cAMP/PKA dynamics in the neocortex

The functional significance of diverse neuropeptide coexpression and convergence onto common second messenger pathways remains unclear. To address this question, we characterized responses to corticotropin-releasing factor (CRF), pituitary adenylate cyclase-activating peptide (PACAP), and vasoactive intestinal peptide (VIP) in rat neocortical slices using optical recordings of cyclic adenosine monophosphate (cAMP) and protein kinase A (PKA) sensors, patch-clamp, and single-cell reverse transcription-polymerase chain reaction. Responses of pyramidal neurons to the 3 neuropeptides markedly differed in time-course and amplitude. Effects of these neuropeptides on the PKA-sensitive slow afterhyperpolarization current were consistent with those observed with cAMP/PKA sensors. CRF-1 receptors, primarily expressed in pyramidal cells, reportedly mediate the neocortical effects of CRF. PACAP and VIP activated distinct PAC1 and VPAC1 receptors, respectively. Indeed, a selective VPAC1 antagonist prevented VIP responses but had a minor effect on PACAP responses, which were mimicked by a specific PAC1 agonist. While PAC1 and VPAC1 were coexpressed in pyramidal cells, PAC1 expression was also frequently detected in interneurons, suggesting that PACAP has widespread effects on the neuronal network. Our results suggest that VIP and CRF, originating from interneurons, and PACAP, expressed mainly by pyramidal cells, finely tune the excitability and gene expression in the neocortical network via distinct cAMP/PKA-mediated effects.

Vasoactive intestinal polypeptide immunoreactivity in the human cerebellum: qualitative and quantitative analyses

Although autoradiographic, reverse transcription-polymerase chain reaction and immunohistochemical studies have demonstrated receptors for vasoactive intestinal polypeptide (VIP) in the cerebellum of various species, immunohistochemistry has never shown immunoreactivity for VIP within cerebellar neuronal bodies and processes. The present study aimed to ascertain whether VIP immunoreactivity really does exist in the human cerebellum by making a systematic analysis of samples removed post-mortem from all of the cerebellar lobes. The study was carried out using light microscopy immunohistochemical techniques based on a set of four different antibodies (three polyclonal and one monoclonal) against VIP, carefully selected on the basis of control tests performed on human colon. All of the antibodies used showed VIP-immunoreactive neuronal bodies and processes distributed in the cerebellar cortex and subjacent white matter of all of the cerebellum lobes, having similar qualitative patterns of distribution. Immunoreactive neurons included subpopulations of the main neuron types of the cortex. Statistical analysis of the quantitative data on the VIP immunoreactivity revealed by the different antibodies in the different cerebellar lobes did not demonstrate any significant differences. In conclusion, using four different anti-VIP antibodies, the first evidence of VIP immunoreactivity is herein supplied in the human post-mortem cerebellum, with similar qualitative/quantitative patterns of distribution among the different cerebellum lobes. Owing to the function performed by VIP as a neurotransmitter/neuromodulator, it is a candidate for a role in intrinsic and extrinsic (projective) circuits of the cerebellum, in agreement with previous demonstrations of receptors for VIP in the cerebellar cortex and nuclei. As VIP signalling pathways are implicated in the regulation of cognitive and psychic functions, cerebral blood flow and metabolism, processes of histomorphogenesis, differentiation and outgrowth of nervous tissues, the results of this study could be applied to clinical neurology and psychiatry, opening new perspectives for the interpretation of neurodevelopment disorders and development of new therapeutic strategies in cerebellar diseases.

Cholecystokinin action on layer 6b neurons in somatosensory cortex

Layer 6b in neocortex is a distinct sublamina at the ventral portion of layer 6. Corticothalamic projections arise from 6b neurons, but few studies have examined the functional properties of these cells. In the present study we examined the actions of cholecystokinin (CCK) on layer 6b neocortical neurons using whole-cell patch clamp recording techniques. We found that the general CCK receptor agonist CCK8S (sulfated CCK octapeptide) strongly depolarized the neurons, and this action persisted in the presence of tetrodotoxin, suggesting a postsynaptic site of action. The excitatory actions of CCK8S were mimicked by the selective CCK(B) receptor agonist CCK4, and attenuated by the selective CCK(B) receptor antagonist L365260, indicating a role for CCK(B) receptors. Voltage-clamp recordings revealed that CCK8S produced a slow inward current associated with a decreased conductance with a reversal potential near the K(+) equilibrium potential. In addition, intracellular cesium also blocked the inward current, suggesting the involvement of a K(+) conductance, likely K(leak). Our data indicate that CCK, acting via CCK(B) receptors, produces a long-lasting excitation of layer 6b neocortical neurons, and this action may play a critical role in modulation of corticothalamic circuit activity.

Excitatory actions of peptide histidine isoleucine on thalamic relay neurons

Peptide histidine isoleucine (PHI) and vasoactive intestinal peptide (VIP) are neuropeptides synthesized from a common precursor, prepro-VIP, and share structural similarity and biological functions in many systems. Within the central nervous system and peripheral tissues, PHI and VIP have overlapping distribution. PHI-mediated functions are generally via activation of VIP receptors; however, the potency and affinity of PHI for VIP receptors are significantly lower than VIP. In addition, several studies suggest distinct PHI receptors that are independent of VIP receptors. PHI receptors have been cloned and characterized in fish, but their existence in mammals is still unknown. This study focuses on the functional role of PHI in the thalamus because of the localization of both PHI and VIP receptors in this brain region. Using extracellular multiple-unit recording techniques, we found that PHI strongly attenuated the slow intrathalamic rhythmic activity. Using intracellular recording techniques, we found that PHI selectively depolarized thalamic relay neurons via an enhancement of the hyperpolarization-activated mixed cation current, Ih. Further, the actions of PHI were occluded by VIP and dopamine, indicating these modulators converge onto a common mechanism. In contrast to previous work, we found that PHI was more potent than VIP in producing excitatory actions on thalamic neurons. We next used the transgenic mice lacking a specific VIP receptor, VPAC2, to identify its possible role in PHI-mediated actions in the thalamus. PHI depolarized all relay neurons tested from wild-type mice (VPAC2(+/+)); however, in knockout mice (VPAC2(-/-)), PHI produced no change in membrane potential in all neurons tested. Our findings indicate that excitatory actions of PHI are mediated by VPAC2 receptors, not by its own PHI receptors and the excitatory actions of PHI clearly attenuate intrathalamic rhythmic activities, and likely influence information transfer through thalamocortical circuits.

Postsynaptic targets of somatostatin-containing interneurons in the rat basolateral amygdala

The basolateral amygdala contains several subpopulations of inhibitory interneurons that can be distinguished on the basis of their content of calcium-binding proteins or peptides. Although previous studies have shown that interneuronal subpopulations containing parvalbumin (PV) or vasoactive intestinal peptide (VIP) innervate distinct postsynaptic domains of pyramidal cells as well as other interneurons, very little is known about the synaptic outputs of the interneuronal subpopulation that expresses somatostatin (SOM). The present study utilized dual-labeling immunocytochemical techniques at the light and electron microscopic levels to analyze the innervation of pyramidal cells, PV+ interneurons, and VIP+ interneurons in the anterior basolateral amygdalar nucleus (BLa) by SOM+ axon terminals. Pyramidal cell somata and dendrites were selectively labeled with antibodies to calcium/calmodulin-dependent protein kinase II (CaMK); previous studies have shown that the vast majority of dendritic spines, whether CAMK+ or not, arise from pyramidal cells. Almost all SOM+ axon terminals formed symmetrical synapses. The main postsynaptic targets of SOM+ terminals were small-caliber CaMK+ dendrites and dendritic spines, some of which were CaMK+. These SOM+ synapses with dendrites were often in close proximity to asymmetrical (excitatory) synapses to these same structures formed by unlabeled terminals. Few SOM+ terminals formed synapses with CaMK+ pyramidal cell somata or large-caliber (proximal) dendrites. Likewise, only 15% of SOM+ terminals formed synapses with PV+, VIP+, or SOM+ interneurons. These findings suggest that inhibitory inputs from SOM+ interneurons may interact with excitatory inputs to pyramidal cell distal dendrites in the BLa. These interactions might affect synaptic plasticity related to emotional learning.

Vasoactive intestinal peptide selectively depolarizes thalamic relay neurons and attenuates intrathalamic rhythmic activity

The reciprocal synaptic relationship between the relay thalamus and surrounding thalamic reticular nucleus can lead to the generation of various rhythmic activities that are associated with different levels of behavioral states as well as certain pathophysiological conditions. Intrathalamic rhythmic activities may be attenuated by numerous neuromodulators that arise from a variety of brain stem nuclei. This study focuses on the potential role of a particular neuropeptide, vasoactive intestinal peptide (VIP). VIP and its receptors are localized within the thalamic circuit and thus may serve as an endogenous modulator of the rhythmic activity. Using extracellular multiple-unit recording techniques, we found that VIP strongly attenuated the slow, 2- to 4-Hz intrathalamic rhythm. This rhythm is similar to that observed during slow wave sleep and certain pathophysiological conditions such as generalized absence epilepsy. Using intracellular recording techniques, we found that VIP selectively depolarized relay neurons in the ventrobasal nucleus but had negligible actions on neurons in thalamic reticular nucleus. The VIP-mediated depolarization is produced via an enhancement of the nonselective cation conductance, Ih. The antioscillatory actions of VIP likely occur by shifting the membrane potential to decrease the probability of burst discharge by relay neurons, a requirement to maintain the rhythmic activity. Not only does VIP alter the intrathalamic rhythmic activity, this peptide that is endogenous to the thalamic circuit may also play a significant role in the regulation of information transfer through the thalamocortical circuit.

Noradrenergic innervation of the developing and mature visual and motor cortex of the rat brain: a light and electron microscopic immunocytochemical analysis

The noradrenergic (NA) innervation of the developing and adult visual and motor cortex of the rat was examined with light and electron microscopic immunocytochemistry by using antibodies against dopamine-beta-hydroxylase. At birth, NA fibers were present in both cortical areas, appearing as two tangential streams, one above and the other below the cortical plate. During the subsequent weeks, these two streams arborized gradually innervating all cortical layers. The adult pattern of distribution was attained by postnatal day 14, but the density of innervation, which was higher in the motor than in the visual cortex, appeared similar to the adult by the end of the third postnatal week. Electron microscopic analysis revealed that a low proportion of NA varicosities (the highest value was 12% in the adult motor cortex in single sections) were engaged in synaptic contact, throughout development, in both areas examined. The overwhelming majority of these synapses were symmetrical, involving predominantly small or medium dendrites. This evidence suggests that transmission by diffusion is the major mode of NA action in the developing and adult cerebral cortex. Noradrenaline released in the rare synaptic junctions may act mainly to reduce the activity of its cortical targets. The results altogether provide morphologic evidence for an involvement of noradrenaline in the development of the neocortex and, along with earlier data on the serotonergic system, indicate that the monoaminergic systems are endowed with a specific anatomic organization in various areas of the brain.

NMDA and AMPA receptors on neocortical neurons are differentially distributed

The distribution of glutamate receptor subtypes on the surface of neurons is highly relevant for synaptic activation and signal processing in the neocortex. As a novel approach we have used infra-red videomicroscopy in combination with photostimulation or microiontophoresis in brain slices of rat neocortex to map the distribution of N-methyl-D-aspartate (NMDA) and alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptors on pyramidal neurons of layer V. Both modes of application revealed a spatially distinct distribution of glutamate receptor subtypes: the soma and the proximal dendrite of neurons are highly sensitive to NMDA, whereas the more distal parts of the dendrite are more sensitive to AMPA. An implication is that NMDA receptors near the soma might regulate the amplification of synaptic signals resulting from AMPA receptor activation on remote dendritic sites.

Neurochemical features and synaptic connections of large physiologically-identified GABAergic cells in the rat frontal cortex

Physiological and morphological properties of large non-pyramidal cells immunoreactive for cholecystokinin, parvalbumin or somatostatin were investigated in vitro in the frontal cortex of 18-22-day-old rats. These three peptides were expressed in separate populations including large cells. Cholecystokinin cells and parvalbumin cells made boutons apposed to other cell bodies, but differed in their firing patterns in response to depolarizing current pulses. Parvalbumin cells belonged to fast-spiking cells. Parvalbumin fast-spiking cells also included chandelier cells. In contrast, cholecystokinin cells were found to be regular-spiking non-pyramidal cells or burst-spiking non-pyramidal cells with bursting activity from hyperpolarized potentials (two or more spikes on slow depolarizing humps). Large somatostatin cells belonged to the regular-spiking non-pyramidal category and featured wide or ascending axonal arbors (wide arbor cells and Martinotti cells) which did not seem to be apposed to the somata so frequently as large cholecystokinin and parvalbumin cells. For electron microscopic observations, another population of eight immunohistochemically-uncharacterized non-pyramidal cells were selected: (i) five fast spiking cells including one chandelier cell which are supposed to contain parvalbumin, and (ii) three large regular-spiking non-pyramidal cells with terminals apposed to somata, which are not considered to include somatostatin cells, but some of which may belong to cholecystokinin cells. The fast-spiking cells other than a chandelier cell and the large regular-spiking non-pyramidal cells made GABA-positive synapses on somata (4% and 12% of the synapses in two small to medium fast-spiking cells, 22% and 35% of the synapses in two large fast-spiking cells, and 10%, 18% and 37% of the synapses in three large regular-spiking non-pyramidal cells). A few terminals of the fast-spiking and regular-spiking non-pyramidal cells innervated GABAergic cells. About 30% of the fast-spiking cell terminals innervated spines, but few of the regular-spiking non-pyramidal cell terminals did. A fast-spiking chandelier cell made GABA-positive synapses on GABA-negative axon initial segments. These results suggest that large GABAergic cells are heterogeneous in neuroactive substances, firing patterns and synaptic connections, and that cortical cells receive heterogeneous GABAergic somatic inputs.

Ultrastructural evidence for combined action of noradrenaline and vasoactive intestinal polypeptide upon neurons, astrocytes, and blood vessels of the rat cerebral cortex

The intracortical organization of the noradrenaline (NA) and vasoactive intestinal polypeptide (VIP) systems provides ample opportunity for functional convergence, and accumulated evidence indicates that NA and VIP share certain cellular actions upon both neuronal and nonneuronal cortical elements. In the present study, a double immunolabeling method was combined with a silver-gold intensification procedure to examine the ultrastructural relationships of the NA coeruleocortical afferents and the intrinsic VIP neurons with three main constituents of the cortex: neurons, astrocytes, and blood vessels. Electron microscopy of singly or doubly labeled material indicated that NA and VIP boutons are engaged in a variety of anatomical relationships with both neuronal and nonneuronal elements. Dendritic shafts and perikarya of nonpyramidal neurons, some of which are VIP positive, receive combined NA and VIP synapses. A significant number of cortical microvessels are in intimate contact with NA or VIP profiles. NA axons often form perivascular loops, and VIP dendritic shafts of large diameter are frequently observed to bend around the vessel circumference. Serial section examination demonstrates that some NA boutons are directly apposed to the capillary wall at sites of glial end-feet discontinuities, whereas VIP boutons contact astrocytic sleeves of capillaries but never cross the perivascular astroglial barrier. Some VIP dendrites containing coated vesicles make intimate contact with the capillary basal lamina. Astrocytic perikarya, mainly in the supragranular layers, are also directly apposed to NA and/or VIP elements. These complex anatomical relationships provide a structural basis for the known interactions between NA and VIP in the control of cortical metabolism and function.

Interneurons immunoreactive for vasoactive intestinal polypeptide (VIP) are extensively innervated by parvalbumin-containing boutons in rat primary somatosensory cortex

Sensory perception results from the synchronized action of large ensembles of cortical neurons. Receptive field properties of such neurons in sensory areas strongly depend on circuits utilizing the inhibitory amino acid transmitter gamma-aminobutyric acid (GABA). GABAergic neurons often co-localize neuropeptides and/or calcium-binding proteins in a cell-type specific manner. We have taken advantage of this fact to study the synaptic circuitry involving presynaptic parvalbumin-containing boutons (originating from horizontally extensive basket cells) and postsynaptic VIP-immunoreactive GABAergic targets which mostly have a vertically oriented axonal field. Abundant appositions between parvalbumin-immunoreactive boutons and all VIP-stained neurons were observed at the light microscopic level. The numbers of contacts ranged between three and well over 20 for single VIP cells. The higher numbers were especially frequent in the supragranular and granular layers contacting the numerous bipolar, as well as multipolar VIP cells located there; but the VIP-immunoreactive neurons in the infragranular layers were also targeted by parvalbumin-immunoreactive boutons without exception, albeit in more variable, mostly lower numbers. Correlated electron microscopic investigations revealed that virtually all of these light microscopically observed appositions resembled symmetric synaptic specializations. The vast majority were located on the soma or proximal dendrites of the VIP-positive neurons. Since pyramidal cells, in turn, represent a major target for the parvalbumin and VIP synapses, the activation of these synapses may lead to coherent oscillations providing the necessary clock function to synchronize pyramidal cell discharges, both across and within cortical columns.