Overview of cellular electrophysiological actions of vasopressin.

The nonapeptide vasopressin acts both as a hormone and as a neurotransmitter/neuromodulator. As a hormone, its target organs include kidney, blood vessels, liver, platelets and anterior pituitary. As a neurotransmitter/neuromodulator, vasopressin plays a role in autonomic functions, such as cardiovascular regulation and temperature regulation and is involved in complex behavioral and cognitive functions, such as sexual behavior, pair-bond formation and social recognition. At the neuronal level, vasopressin acts by enhancing membrane excitability and by modulating synaptic transmission. The present review will focus on the electrophysiological effects of vasopressin at the cellular level. A large proportion of the experiments summarized here have been performed in in vitro systems, especially in brain and spinal cord slices of the rat. Vasopressin exerts a powerful excitatory action on motoneurons of young rats and mice. It acts by generating a cationic inward current and/or by reducing a potassium conductance. In addition, vasopressin enhances the inhibitory synaptic input to motoneurons. By virtue of these actions, vasopressin may regulate the functioning of neuronal networks involved in motor control. In the amygdala, vasopressin can directly excite a subpopulation of neurons, whereas oxytocin, a related neuropeptide, can indirectly inhibit these same neurons. In the lateral septum, vasopressin exerts a similar dual action: it excites directly a neuronal subpopulation, but causes indirect inhibition of virtually all lateral septal neurons. The actions of vasopressin in the amygdala and lateral septum may represent at least part of the neuronal substrate by which vasopressin influences fear and anxiety-related behavior and social recognition, respectively. Central vasopressin can modulate cardiovascular parameters by causing excitation of spinal sympathetic preganglionic neurons, by increasing the inhibitory input to cardiac parasympathetic neurons in the nucleus ambiguus, by depressing the excitatory input to parabrachial neurons, or by inhibiting glutamate release at solitary tract axon terminals. By acting in or near the hypothalamic supraoptic nucleus, vasopressin can influence magnocellular neuron activity, suggesting that the peptide may exert some control on its own release at neurohypophyseal axon terminals. The central actions of vasopressin are mainly mediated by receptors of the V(1A) type, although recent studies have also reported the presence of vasopressin V(1B) receptors in the brain. Major unsolved problems are: (i) what is the transduction pathway activated following stimulation of central vasopressin V(1A) receptors? (ii) What is the precise nature of the cation channels and/or potassium channels operated by vasopressin? (iii) Does vasopressin, by virtue of its second messenger(s), interfere with other neurotransmitter/neuromodulator systems? In recent years, information concerning the mechanism of action of vasopressin at the neuronal level and its possible role and function at the whole-animal level has been accumulating. Translation of peptide actions at the cellular level into autonomic, behavioral and cognitive effects requires an intermediate level of integration, i.e. the level of neuronal circuitry. Here, detailed information is lacking. Further progress will probably require the introduction of new techniques, such as targeted in vivo whole-cell recording, large-scale recordings from neuronal ensembles or in vivo imaging in small animals.

Identified motoneurons involved in sexual and eliminative functions in the rat are powerfully excited by vasopressin and tachykinins.

The pudendal motor system is constituted by striated muscles of the pelvic floor and the spinal motoneurons that innervate them. It plays a role in eliminative functions of the bladder and intestine and in sexual function. Pudendal motoneurons are located in the ventral horn of the caudal lumbar spinal cord and send their axon into the pudendal nerve. In the rat, binding sites for vasopressin and tachykinin are present in the dorsomedial and dorsolateral pudendal nuclei, suggesting that these neuropeptides may affect pudendal motoneurons. The aim of the present study was to investigate possible effects of vasopressin and tachykinins on these motoneurons. Recordings were performed in spinal cord slices of young male rats using the whole-cell patch-clamp technique. Before recording, motoneurons were identified by 1,1'-dilinoleyl-3,3,3',3'-tetramethylindocarbocyanine, 4-chlorobenzenesulfonate retrograde labeling. The identification was confirmed, a posteriori, by choline acetyltransferase immunocytochemistry. Vasopressin and tachykinins caused a powerful excitation of pudendal motoneurons. The peptide-evoked depolarization, or the peptide-evoked inward current, persisted in the presence of tetrodotoxin, indicating that these effects were mainly postsynaptic. By using selective receptor agonists and antagonist, we determined that vasopressin acted via vasopressin 1a (V1a), but not V1b, V2, or oxytocin receptors, whereas tachykinins acted via neurokinin 1 (NK1), but not NK2 or NK3, receptors. Vasopressin acted by enhancing a nonselective cationic conductance; in some motoneurons, it also probably suppressed a resting K+ conductance. Our data show that vasopressin and tachykinins can excite pudendal motoneurons and thus influence the force of striated perineal muscles involved in eliminative and sexual functions.

Intracellular targeting of truncated secretory peptides in the mammalian heart and brain.

Secretory polypeptides are vital for nervous system function, sleep, reproduction, growth, and metabolism. Ribosomes scanning the 5'-end of mRNA usually detect the first AUG site for initiating translation. The nascent propeptide chain is then directed via a signal-peptide into the endoplasmic reticulum, processed through the Golgi stacks, and packaged into secretory vesicles. By expressing prepropeptide-EGFP fusion proteins, we observed unusual destinations, mitochondria, nucleus, and cytoplasm, of neuropeptide Y (NPY), atrial natriuretic peptide, and growth hormone in living murine cardiac cells and hypothalamic slices. Subcellular expression was modulated by Zn++ or mutations of N-terminal prohormone sequences but was not due to overexpression in the trans-Golgi network. Mitochondrial targeting of NPY also occurred without the EGFP tag, was enhanced by site-directed mutagenesis of the first AUG initiation site, and abolished by mutation of the second AUG. Immunological methods indicated the presence of N-terminal truncated NPY in mitochondria. Imaging studies showed depolarization of NPY-containing mitochondria. P-SORT software correctly predicted the secondary intracellular destinations and suggested such destinations for many neuropeptides and peptide hormones known. Thus, mammalian cells may retarget secretory peptides from extracellular to intracellular sites by skipping the first translation-initiation codon and thereby alter mitochondrial function, gene expression, and secretion.

Alpha7 neuronal nicotinic acetylcholine receptors are negatively regulated by tyrosine phosphorylation and Src-family kinases.

Nicotine, a component of tobacco, is highly addictive but possesses beneficial properties such as cognitive improvements and memory maintenance. Involved in these processes is the neuronal nicotinic acetylcholine receptor (nAChR) alpha7, whose activation triggers depolarization, intracellular signaling cascades, and synaptic plasticity underlying addiction and cognition. It is therefore important to investigate intracellular mechanisms by which a cell regulates alpha7 nAChR activity. We have examined the role of phosphorylation by combining molecular biology, biochemistry, and electrophysiology in SH-SY5Y neuroblastoma cells, Xenopus oocytes, rat hippocampal interneurons, and neurons from the supraoptic nucleus, and we found tyrosine phosphorylation of alpha7 nAChRs. Tyrosine kinase inhibition by genistein decreased alpha7 nAChR phosphorylation but strongly increased acetylcholine-evoked currents, whereas tyrosine phosphatase inhibition by pervanadate produced opposite effects. Src-family kinases (SFKs) directly interacted with the cytoplasmic loop of alpha7 nAChRs and phosphorylated the receptors at the plasma membrane. SFK inhibition by PP2 [4-amino-5-(4-chlorophenyl)-7-(t-butyl)pyrazolo[3,4-d]pyrimidine] or SU6656 (2,3-dihydro-N,N-dimethyl-2-oxo-3-[(4,5,6,7-tetrahydro-1H-indol-2-yl)methylene]-1H-indole-5-sulfonamide) increased alpha7 nAChR-mediated responses, whereas expression of active Src reduced alpha7 nAChR activity. Mutant alpha7 nAChRs lacking cytoplasmic loop tyrosine residues because of alanine replacement of Tyr-386 and Tyr-442 were more active than wild-type receptors and insensitive to kinase or phosphatase inhibition. Because the amount of surface alpha7 receptors was not affected by kinase or phosphatase inhibitors, these data show that functional properties of alpha7 nAChRs depend on the tyrosine phosphorylation status of the receptor and are the result of a balance between SFKs and tyrosine phosphatases. These findings reveal novel regulatory mechanisms that may help to understand nicotinic receptor-dependent plasticity, addiction, and pathology.

A novel positive allosteric modulator of the alpha7 neuronal nicotinic acetylcholine receptor: in vitro and in vivo characterization.

Several lines of evidence suggest a link between the alpha7 neuronal nicotinic acetylcholine receptor (nAChR) and brain disorders including schizophrenia, Alzheimer's disease, and traumatic brain injury. The present work describes a novel molecule, 1-(5-chloro-2,4-dimethoxy-phenyl)-3-(5-methyl-isoxazol-3-yl)-urea (PNU-120596), which acts as a powerful positive allosteric modulator of the alpha7 nAChR. Discovered in a high-throughput screen, PNU-120596 increased agonist-evoked calcium flux mediated by an engineered variant of the human alpha7 nAChR. Electrophysiology studies confirmed that PNU-120596 increased peak agonist-evoked currents mediated by wild-type receptors and also demonstrated a pronounced prolongation of the evoked response in the continued presence of agonist. In contrast, PNU-120596 produced no detectable change in currents mediated by alpha4beta2, alpha3beta4, and alpha9alpha10 nAChRs. PNU-120596 increased the channel mean open time of alpha7 nAChRs but had no effect on ion selectivity and relatively little, if any, effect on unitary conductance. When applied to acute hippocampal slices, PNU-120596 increased the frequency of ACh-evoked GABAergic postsynaptic currents measured in pyramidal neurons; this effect was suppressed by TTX, suggesting that PNU-120596 modulated the function of alpha7 nAChRs located on the somatodendritic membrane of hippocampal interneurons. Accordingly, PNU-120596 greatly enhanced the ACh-evoked inward currents in these interneurons. Systemic administration of PNU-120596 to rats improved the auditory gating deficit caused by amphetamine, a model proposed to reflect a circuit level disturbance associated with schizophrenia. Together, these results suggest that PNU-120596 represents a new class of molecule that enhances alpha7 nAChR function and thus has the potential to treat psychiatric and neurological disorders.

Suppression of potassium channels elicits calcium-dependent plateau potentials in suprachiasmatic neurons of the rat.

By using whole-cell recordings in acute and organotypic hypothalamic slices, we found that following K+ channel blockade, sustained plateau potentials can be elicited by current injection in suprachiasmatic neurons. In an attempt to determine the ionic basis of these potentials, ion-substitution experiments were carried out. It appeared that to generate plateau potentials, calcium influx was required. Plateau potentials were also present when extracellular calcium was replaced by barium, but were independent upon an increase in the intracellular free calcium concentration. Substitution of extracellular sodium by the impermeant cation N-methyl-D-glucamine indicated that sodium influx could also contribute to plateau potentials. To gain some information on the pharmacological profile of the Ca++ channels responsible for plateau potentials, selective blocker of various types of Ca++ channel were tested. Plateau potentials were unaffected by isradipine, an L-type Ca++ channel blocker. However, they were slightly reduced by omega-conotoxin GVIA and omega-agatoxin TK, blockers of N-type and P/Q-type Ca++ channels, respectively. These data suggest that R-type Ca++ channels probably play a major role in the genesis of plateau potentials. We speculate that neurotransmitters/neuromodulators capable of reducing or suppressing potassium conductance(s) may elicit a Ca++-dependent plateau potential in suprachiasmatic neurons, thus promoting sustained firing activity and neuropeptide release.

A three-dimensional multi-electrode array for multi-site stimulation and recording in acute brain slices.

Several multi-electrode array devices integrating planar metal electrodes were designed in the past 30 years for extracellular stimulation and recording from cultured neuronal cells and organotypic brain slices. However, these devices are not well suited for recordings from acute brain slice preparations due to a dead cell layer at the tissue slice border that appears during the cutting procedure. To overcome this problem, we propose the use of protruding 3D electrodes, i.e. tip-shaped electrodes, allowing tissue penetration in order to get closer to living neurons in the tissue slice. In this paper, we describe the design and fabrication of planar and 3D protruding multi-electrode arrays. The electrical differences between planar and 3D protruding electrode configuration were simulated and verified experimentally. Finally, a comparison between the planar and 3D protruding electrode configuration was realized by stimulation and recording from acute rat hippocampus slices. The results show that larger signal amplitudes in the millivolt range can be obtained with the 3D electrode devices. Spikes corresponding to single cell activity could be monitored in the hippocampus CA3 and CA1 region using 3D electrodes.

Nicotinic receptors in circuit excitability and epilepsy.

Neuronal nicotinic acetylcholine receptors belong to the family of excitatory ligand-gated channels and result from the assembly of five subunits. Functional heteromeric nictonic receptors are present in the hippocampus and neocortex, thalamus, mesolimbic dopamine system and brainstem motor nuclei, where they may play a role, respectively, in memory, sensory processing, addiction and motor control. Some forms of autosomal dominant nocturnal frontal lobe epilepsy (ADNFLE) have been found to be associated with mutations in the genes coding for the alpha 4 or beta2 subunits of the nicotinic receptor. Mutant receptors display an increased acetylcholine sensitivity with respect to normal receptors. Since the thalamus and the cortex are strongly innervated by cholinergic neurons projecting from the brainstem and basal forebrain, an unbalance between excitation and inhibition, brought about by the presence of mutant receptors, could generate seizures by facilitating and synchronizing spontaneous oscillations in thalamo-cortical circuits.