Magnocellular neurons of the rat supraoptic nucleus are endowed with functional nicotinic acetylcholine receptors

Vasopressin: a possible link between hypoxia and hypertension

Cardiovascular and respiratory diseases are frequently associated with transient and prolonged hypoxia, whereas hypoxia exerts pro-hypertensive effects, through stimulation of the sympathetic system and release of pressor endocrine factors. This review is focused on the role of arginine vasopressin (AVP) in dysregulation of the cardiovascular system during hypoxia associated with cardiovascular disorders. AVP is synthesized mainly in the neuroendocrine neurons of the hypothalamic paraventricular nucleus (PVN) and supraoptic nucleus (SON), which send axons to the posterior pituitary and various regions of the central nervous system (CNS). Vasopressinergic neurons are innervated by multiple neuronal projections releasing several neurotransmitters and other regulatory molecules. AVP interacts with V1a, V1b and V2 receptors that are present in the brain and peripheral organs, including the heart, vessels, lungs, and kidneys. Release of vasopressin is intensified during hypernatremia, hypovolemia, inflammation, stress, pain, and hypoxia which frequently occur in cardiovascular patients, and blood AVP concentration is markedly elevated in cardiovascular diseases associated with hypoxemia. There is evidence that hypoxia stimulates AVP release through stimulation of chemoreceptors. It is suggested that acting in the carotid bodies, AVP may fine-tune respiratory and hemodynamic responses to hypoxia and that this effect is intensified in hypertension. There is also evidence that during hypoxia, augmentation of pro-hypertensive effects of vasopressin may result from inappropriate interaction of this hormone with other compounds regulating the cardiovascular system (catecholamines, angiotensins, natriuretic peptides, steroids, nitric oxide). In conclusion, current literature indicates that abnormal mutual interactions between hypoxia and vasopressin may significantly contribute to pathogenesis of hypertension.

Anatomical Markers of Activity in Hypothalamic Neurons

The scientific community has searched for years for ways of examining neuronal tissue to track neural activity with reliable anatomical markers for stimulated neuronal activity. Existing studies that focused on hypothalamic systems offer a few options but do not always compare approaches or validate them for dependence on cell firing, leaving the reader uncertain of the benefits and limitations of each method. Thus, in this article, potential markers will be presented and, where possible, placed into perspective in terms of when and how these methods pertain to hypothalamic function. An example of each approach is included. In reviewing the approaches, one is guided through how neurons work, the consequences of their stimulation, and then the potential markers that could be applied to hypothalamic systems are discussed. Approaches will use features of neuronal glucose utilization, water/oxygen movement, changes in neuron-glial interactions, receptor translocation, cytoskeletal changes, stimulus-synthesis coupling that includes expression of the heteronuclear or mature mRNA for transmitters or the enzymes that make them, and changes in transcription factors (immediate early gene products, precursor buildup, use of promoter-driven surrogate proteins, and induced expression of added transmitters. This article includes discussion of methodological limitations and the power of combining approaches to understand neuronal function. © 2020 American Physiological Society. Compr Physiol 10:549-575, 2020.

Physiological regulation of magnocellular neurosecretory cell activity: integration of intrinsic, local and afferent mechanisms

The hypothalamic supraoptic and paraventricular nuclei contain magnocellular neurosecretory cells (MNCs) that project to the posterior pituitary gland where they secrete either oxytocin or vasopressin (the antidiuretic hormone) into the circulation. Oxytocin is important for delivery at birth and is essential for milk ejection during suckling. Vasopressin primarily promotes water reabsorption in the kidney to maintain body fluid balance, but also increases vasoconstriction. The profile of oxytocin and vasopressin secretion is principally determined by the pattern of action potentials initiated at the cell bodies. Although it has long been known that the activity of MNCs depends upon afferent inputs that relay information on reproductive, osmotic and cardiovascular status, it has recently become clear that activity depends critically on local regulation by glial cells, as well as intrinsic regulation by the MNCs themselves. Here, we provide an overview of recent advances in our understanding of how intrinsic and local extrinsic mechanisms integrate with afferent inputs to generate appropriate physiological regulation of oxytocin and vasopressin MNC activity.

Effect of central nicotinic activation on drinking behavior

Intracerebroventricular injections of angiotensin II (ANG) and nicotine activate the subfornical organ (SFO), an essential central nucleus for ANG-induced drinking. Nicotine has been, however, reported to induce little drinking behavior. To clarify this paradox, we investigated effects of nicotine and ANG on activity of SFO neurons and drinking behavior. In extracellular recordings many SFO neurons (57%) were excited by the both drugs. The nicotine-induced excitation was transient, whereas the ANG-induced was long-lasting. After intracerebroventricular injection of nicotine, the latency to drinking was dose-dependently shortened, but the drinking volumes were much smaller than those by ANG. These suggest that central nicotinic activation contributes to induction of drinking behavior while drinking volume is small because effects of nicotine on neurons are short-lasting.

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.

Muscarinic receptor modulation of slow afterhyperpolarization and phasic firing in rat supraoptic nucleus neurons

A slow posttrain afterhyperpolarization (sAHP) was studied in rat magnocellular neurosecretory cells (MNCs) in vitro. The sAHP was isolated from other afterpotentials by blocking the depolarizing afterpotential (DAP) with Cs(+) and the medium afterhyperpolarization (mAHP) with apamin. The sAHP amplitude increased logarithmically with activity ( approximately 3 mV per e-fold increase in number of impulses) and, when firing stopped, decayed exponentially with a time constant of 2 sec. The sAHP was associated with increased membrane conductance, and its amplitude varied linearly with voltage, reversing at the K(+) equilibrium potential. The sAHP was blocked by Cd(2+) but not by charybdotoxin or iberiotoxin, blockers of intermediate- and big-conductance-type Ca(2+)-dependent K(+) (K(Ca)) channels. The sAHP was reversibly inhibited by muscarine, an effect antagonized by atropine, indicating involvement of muscarinic cholinergic receptors. Muscarine did not affect Ca(2+)-dependent features of action potentials, DAPs, or the mAHP in MNCs, indicating selective modulation of K(Ca) channels causing the sAHP. Muscarinic inhibition of the sAHP enhanced plateau potentials and increased the mean firing rate and duration of afterdischarges that followed spike trains evoked from voltages near threshold. Similarly, the frequency and duration of the spontaneous phasic bursts that characterize physiologically activated vasopressin-releasing MNCs were enhanced by muscarine. MNCs thus express apamin- and voltage-insensitive K(Ca) channels that mediate an sAHP. The activity dependence and kinetics of the sAHP cause it to mask DAPs in a manner that attenuates the amplitude of plateau potentials. Muscarinic inhibition of the sAHP provides an effective mechanism for promoting phasic firing in MNCs.

Nicotinic receptor alpha subunits in magnocellular neurons of rat hypothalamus

Mab35 is a monoclonal antibody against one specific immunogenic region in alpha1, alpha3, alpha5 subunits of nicotinic acetylcholine receptors (N-AChR) of a variety of species. It has previously been claimed that N-AChR-like immunoreactivity (-LI) identified by mab35 is present in vasopressin-containing magnocellular neurons. However, we show here by double immunofluorescence labelling that mab35 immunoreactivity is predominantly localized to oxytocinergic rather than vasopressinergic magnocellular neurons. We further infer that mab35 predominantly stained the alpha3 and/or alpha5 subunits in rat oxytocinergic neurons, and suggest that the unbalanced distribution of these subunits may contribute to some specific physiological properties of oxytocinergic neurons.

Ganglion cells from chick retina display multiple functional nAChR subtypes

We have examined the properties of nicotinic acetylcholine receptors in embryonic chick retinal ganglion cells. Ganglion cells, identified according to morphological and physiological criteria, displayed spontaneous or induced action potentials. In 94/99 cells acetylcholine pulses evoked responses. In current clamp mode, acetylcholine provoked membrane depolarization and triggered action potentials. Under voltage clamp conditions, acetylcholine evoked inward currents that were readily blocked by d-tubocurarine. Antagonists specific for homomeric (alpha-bungarotoxin) and heteromeric (dihydro-beta-erythroidine) receptors revealed that ganglion cells express multiple functional receptor subtypes. These findings demonstrate that ACh modulates the electrical activity of these cells and is likely to mediate synaptic transmission. The presence of multiple receptor subtypes may contribute to processing and transmission of information in the retina.

Nicotinic acetylcholine receptors: from structure to brain function

Nicotinic acetylcholine receptors (nAChRs) are ligand-gated ion channels and can be divided into two groups: muscle receptors, which are found at the skeletal neuromuscular junction where they mediate neuromuscular transmission, and neuronal receptors, which are found throughout the peripheral and central nervous system where they are involved in fast synaptic transmission. nAChRs are pentameric structures that are made up of combinations of individual subunits. Twelve neuronal nAChR subunits have been described, alpha2-alpha10 and beta2-beta4; these are differentially expressed throughout the nervous system and combine to form nAChRs with a wide range of physiological and pharmacological profiles. The nAChR has been proposed as a model of an allosteric protein in which effects arising from the binding of a ligand to a site on the protein can lead to changes in another part of the molecule. A great deal is known about the structure of the pentameric receptor. The extracellular domain contains binding sites for numerous ligands, which alter receptor behavior through allosteric mechanisms. Functional studies have revealed that nAChRs contribute to the control of resting membrane potential, modulation of synaptic transmission and mediation of fast excitatory transmission. To date, ten genes have been identified in the human genome coding for the nAChRs. nAChRs have been demonstrated to be involved in cognitive processes such as learning and memory and control of movement in normal subjects. Recent data from knockout animals has extended the understanding of nAChR function. Dysfunction of nAChR has been linked to a number of human diseases such as schizophrenia, Alzheimer's and Parkinson's diseases. nAChRs also play a significant role in nicotine addiction, which is a major public health concern. A genetically transmissible epilepsy, ADNFLE, has been associated with specific mutations in the gene coding for the alpha4 or beta2 subunits, which leads to altered receptor properties.

Electrical activity of caudal neurosecretory neurons in seawater- and freshwater-adapted flounder: responses to cholinergic agonists

The caudal neurosecretory system (CNSS) of the euryhaline flounder is involved in osmoregulatory responses underlying adaptation to seawater and freshwater. This study compared electrophysiological activity and responses to cholinergic agonists in the neuroendocrine Dahlgren cells in an in vitro preparation taken from fully seawater- (SWA) or freshwater-adapted(FWA) fish. Resting membrane and action potential parameters showed few differences between SWA and FWA cells. The hyperpolarisation-activated sag potential and depolarising afterpotential were present under both conditions;however, amplitude of the latter was significantly greater in SWA cells. The proportions of cells within the population exhibiting different firing patterns were similar in both adaptation states. However, bursting parameters were more variable in FWA cells, suggesting that bursting activity was less robust. The muscarinic agonist, oxotremorine, was largely inhibitory in Dahlgren cells, but increased activity in a non-Dahlgren cell population,α neurons. Nicotine promoted bursting activity in SWA Dahlgren cells,whereas it inhibited over half of FWA cells.

Direct catecholaminergic-cholinergic interactions in the basal forebrain. III. Adrenergic innervation of choline acetyltransferase-containing neurons in the rat

The central adrenergic neurons have been suggested to play a role in the regulation of arousal and in the neuronal control of the cardiovascular system. To provide morphological evidence that these functions could be mediated via the basal forebrain, we performed correlated light and electron microscopic double-immunolabeling experiments using antibodies against phenylethanolamine N-methyltransferase (PNMT) and choline acetyltransferase, the synthesizing enzymes for adrenaline and acetylcholine, respectively. Most adrenergic/cholinergic appositions were located in the horizontal limb of diagonal band of Broca, within the substantia innominata, and in a narrow band bordering the substantia innominata and the globus pallidus. Quantitative analysis indicated that cholinergic neurons of the substantia innominata receive significantly higher numbers of adrenergic appositions than cholinergic cells in the rest of the basal forebrain. In the majority of cases, the ultrastructural analysis revealed axodendritic asymmetric synapses. By comparing the number and distribution of dopamine beta-hydroxylase (DBH)/cholinergic appositions, described earlier, with those of PNMT/cholinergic interactions in the basal forebrain, it can be concluded that a significant proportion of putative DBH/cholinergic contacts may represent adrenergic input. Our results support the hypothesis that the adrenergic/cholinergic link in the basal forebrain may represent a critical component of a central network coordinating autonomic regulation with cortical activation.

Nicotinic cholinergic activation of magnocellular neurons of the hypothalamic paraventricular nucleus

The aim of the present work was to determine whether paraventricular neurons possess functional acetylcholine nicotinic receptors. Using infrared videomicroscopy and differential interference contrast optics, we performed whole-cell recordings in hypothalamic slices containing the paraventricular nucleus. Acetylcholine, locally applied by pressure microejection in the presence of the muscarinic antagonist atropine, evoked a rapidly rising inward current in paraventricular magnocellular endocrine neurons. This current persisted in the presence of blockers of synaptic transmission. It could be reversibly suppressed by nanomolar concentrations of methyllycaconitine, a selective antagonist of alpha 7-containing nicotinic receptors, but was insensitive to micromolar concentrations of dihydro-beta-erythroidine, an antagonist acting preferentially on non-alpha 7 nicotinic receptors. In addition, the effect of acetylcholine could be mimicked by exo-2-(2-pyridyl)-7-azabicyclo[2.2.1]heptane, a recently synthesized nicotinic agonist specific for alpha 7 receptors. Acetylcholine also desensitized paraventricular nicotinic receptors. Desensitization was pronounced and recovery from desensitization was rapid, consistent with the notion that paraventricular nicotinic receptors contain the alpha 7 subunit. Nicotinic currents could not be evoked in paraventricular parvocellular neurons, suggesting that these neurons are devoid of functional nicotinic receptors. The electrophysiological data were corroborated by light microscopic autoradiography, showing that [(125)I]alpha-bungarotoxin binding sites are present in all the magnocellular divisions of the paraventricular nucleus but are undetectable in other areas of this nucleus. Immunohistochemistry, performed using antibodies directed against vasopressin and oxytocin, indicated that responsiveness to nicotinic agonists was a property of vasopressin as well as of oxytocin magnocellular endocrine neurons, in both the paraventricular and the supraoptic nucleus. We conclude that nicotinic agonists can influence the magnocellular neurosecretory system by directly increasing the excitability of magnocellular neurons. By contrast, they are probably without direct effects on paraventricular parvocellular neurons.

Pharmacological properties of rat alpha 7 nicotinic receptors expressed in native and recombinant cell systems

The pharmacological properties of the rat alpha7 nicotinic acetylcholine receptor endogenously expressed in PC12 cells and recombinantly expressed in GH4C1 cells (alpha7-GH4C1 cells) were characterized and compared. Patch-clamp recordings demonstrated that activation by choline and block by methyllycaconitine and dihydro-beta-erythroidine were similar, but block by mecamylamine was different. Whereas in alpha7-GH4C1 cells the inhibition curve for mecamylamine was monophasic (IC(50) of 1.6 microM), it was biphasic in PC12 cells (IC(50) values of 341 nM and 9.6 microM). The same rank order of potency was obtained for various nicotinic agonists, while acetylcholine was 3.7-fold less potent and 1.5-fold more effective in PC12 cells. Dihydro-beta-erythroidine differentially blocked acetylcholine-evoked currents in both systems. Since reverse transcriptase polymerase chain reaction (RT-PCR) experiments revealed expression of alpha3, alpha4, alpha5, alpha7 and beta4 subunits in PC12 cells, whereas GH4C1 cells express only the beta4 subunit, our results suggest that more than one form of alpha7 containing heteromeric nicotinic receptors might be functionally expressed in PC12 cells.

Synaptic potentials mediated by alpha 7 nicotinic acetylcholine receptors in supraoptic nucleus

Brain slice preparations preserving projections from nearby forebrain cholinergic neurons to the supraoptic nucleus (SON) were used to study synaptic potentials mediated by nicotinic acetylcholine receptors (nAChRs) in the hypothalamus. Paired-pulse electrical stimulation in an area anterior to the SON that was rich in cholinergic cells confirmed the monosynaptic nature of the connections to putative oxytocin and vasopressin SON neurons. With ionotropic glutamate and GABA(A) transmission blocked, this stimulation evoked fast, atropine-insensitive EPSPs that were sensitive to nAChR antagonists. Evoked EPSPs were blocked by methyllycaconitine and alpha-bungarotoxin, antagonists that are selective for nAChRs containing the alpha7 subunit, but not by dihydro-beta-erythroidine at concentrations known to antagonize alpha4beta2 nAChRs. Although anatomical evidence exists for postsynaptic alpha4beta2 nAChRs in the SON, these results indicate that postsynaptic alpha7 nAChRs are primarily responsible for the cholinergically mediated EPSPs. Repetitive stimulation suggested partial desensitization of the receptors. With ionotropic glutamate transmission blocked, inhibition of AChE increased spontaneous EPSP frequency and amplitude, suggesting spontaneous ACh release. ACh, nicotine, and choline (a selective alpha7 nAChR agonist) were effective in evoking action potentials and repetitive firing with synaptic transmission blocked by low Ca2+, high Mg2+ medium. These agonists were also effective in evoking the type of phasic bursts characteristic of vasopressin neurons, long thought to be completely dependent on activation of NMDA receptors (NMDARs). Because phasic bursting is Ca2+-dependent, the functional equivalence of alpha7 nAChR and NMDAR activation in this regard is likely attributable to their large Ca2+ fluxing capacities. This is the first demonstration that synaptically released ACh results in fast, alpha7 nAChR-mediated EPSPs in hypothalamic neurons.

Role of neuronal nicotinic receptors in the transmission and processing of information in neurons of the central nervous system

The properties of nicotinic acetylcholine receptors (nAChRs) were studied following exogenous expression in a host system or using whole-cell recordings in brain slices, autoradiography and immunohistochemistry. When expressed in HEK-293 cells, alpha 4 beta 2 nAChRs displayed both a high and a low affinity component. The ratio of these two states was modified by chronic nicotine exposure, resulting in an enhanced sensitivity and a marked reduction in desensitization. Mutations in the gene coding for the alpha 4 subunit are responsible for a particular form of nocturnal epilepsy. When expressed in Xenopus oocytes, alpha 4 beta 2 nAChRs containing these mutations displayed distinct alterations in agonist affinity, desensitization and calcium permeability. Magnocellular endocrine neurons in the supraoptic (SO) nucleus of the hypothalamus were found to express functional alpha 7-containing nAChRs, which could play a role in regulating neurohypophysial peptide secretion. Facial (VII), hypoglossal (XII) and vagal (X) motoneurons of young rats responded to ACh by a fast inward current. The nAChRs present in VII and XII nuclei were of the non-alpha 7-containing type, whereas those present in the X nucleus contained the alpha 7 subunit. In Bcl-2 transgenic mice, facial nerve axotomy caused nAChRs downregulation by interfering negatively with the expression of the alpha 4 subunit. Binding sites corresponding to alpha 7-containing nAChRs were also detected in spinal motor nuclei and axotomy provoked a reduction of the binding. Together, these data indicate that long-term exposure to nicotine can promote neuroadaptive changes in nAChRs and that genetic alterations of neuronal nAChRs can result in transmissible neurological diseases. They also suggest that these receptors probably play a role in the central regulation of autonomic functions, as well as in motor control.

Acetylcholine attenuates synaptic GABA release to supraoptic neurons through presynaptic nicotinic receptors

Both inhibitory GABAergic and excitatory glutamatergic inputs to supraoptic nucleus (SON) neurons can influence the release of vasopressin and oxytocin. Acetylcholine is known to excite SON neurons and to increase vasopressin release. The functional significance of cholinergic receptors, located at the presynaptic nerve terminals, in the regulation of the excitability of SON neurons is not fully known. In this study, we determined the role of presynaptic cholinergic receptors in regulation of the inhibitory GABAergic inputs to the SON neurons. The magnocellular neurons in the rat hypothalamic slice were identified microscopically, and the spontaneous miniature inhibitory postsynaptic currents (mIPSCs) were recorded using the whole-cell voltage-clamp technique. The mIPSCs were abolished by the GABA(A) receptor antagonist, bicuculline (10 microM). Acetylcholine (100 microM) significantly reduced the frequency of mIPSCs of SON neurons from 3.59+/-0.36 to 1.62+/-0.20 Hz (n=37), but did not alter the amplitude and the decay time constant of mIPSCs. Furthermore, the nicotinic receptor antagonist, mecamylamine (10 microM, n=13), eliminated the inhibitory effect of acetylcholine on mIPSCs of SON neurons. The muscarinic receptor antagonist, atropine (100 microM), did not alter significantly the effect of acetylcholine on mIPSCs in most of the 17 SON neurons studied. These results suggest that the excitatory effect of acetylcholine on the SON neurons is mediated, at least in part, by inhibition of presynaptic GABA release. Activation of presynaptic nicotinic receptors located in the GABAergic terminals plays a major role in the cholinergic regulation of the inhibitory GABAergic input to SON neurons.

Ion channel variation causes epilepsies

The discovery of genetically transmissible form of epilepsy associated with a mutation in a gene that codes for a subunit of a ligand-gated channel shined a new light in this field of neurological diseases. Because this gene (CHRNA4) codes for a neuronal nicotinic acetylcholine receptor subunit, functional studies could be designed to evaluate the alterations caused by this mutation. Since this initial observation, five mutations were identified and determination of their functional properties initiated. These experiments were extended to pairwise expression of the control and mutated allele to mimic the heterozygote human genotype. The first common functional trait identified so far, in four of these mutants, is an increased sensitivity to the acetylcholine, suggesting that these mutations may cause a gain of function. An alternative possibility that cannot be excluded is that conditions in the brain are such that these higher responding receptors may be more prone to desensitization. The importance of ionic channels as cause of epilepsies was further demonstrated with the identification of the association between the benign neonatal epilepsy and mutations in genes coding for potassium channel subunits (KCNQ2, KCNQ3). Additional evidences were brought by the identification of mutations in voltage-dependent sodium channels (SCN1A, SCN1B) in a form of generalized epilepsy with febrile seizures.

Neurotransmitter/neuropeptide interactions in the regulation of neurohypophyseal hormone release

Regulation of neurohypophyseal hormone release reflects the convergence of a large number of afferent pathways on the vasopressin (VP)- and oxytocin-producing neurons. These pathways utilize a broad range of neurotransmitters and neuropeptides. In this review, the mechanisms by which this information is coordinated into appropriate physiological responses is discussed with a focus on the responses to agents that are coreleased from A1 catecholamine nerve terminals in the supraoptic nucleus. The A1 pathway transmits hemodynamic information to the vasopressin neurons by releasing several neuroactive agents including ATP, norepinephrine, neuropeptide Y, and substance P. These substances stimulate VP release from explants of the hypothalamo-neurohypophyseal system and certain combinations of these agents elicit potent but selective synergism. Evaluation of the signal cascades elicited by these agents provides insights into mechanisms underlying these synergistic interactions and suggests mechanisms responsible for coordinated responses of the VP neurons to activation of a range of ion-gated ion channel and G-protein-coupled receptors.

Potentiation of glutamatergic synaptic input to supraoptic neurons by presynaptic nicotinic receptors

The release of vasopressin and oxytocin from the supraoptic nucleus (SON) neurons is tonically regulated by excitatory glutamatergic and inhibitory GABAergic synaptic inputs. Acetylcholine is known to excite SON neurons and to elicit vasopressin release. Cholinergic receptors are located pre- and postsynaptically in the SON, but their functional significance in the regulation of SON neurons is not fully understood. In this study, we determined the role of presynaptic cholinergic receptors in regulation of the excitatory glutamatergic inputs to the SON neurons. The magnocellular neurons in the rat hypothalamic slices were identified microscopically, and the spontaneous miniature excitatory postsynaptic currents (mEPSCs) were recorded using the whole cell voltage-clamp technique. The mEPSCs were abolished by the non-NMDA receptor antagonist 6-cyano-7-nitroquinoxaline-2,3-dione (20 microM). Acetylcholine (100 microM) significantly increased the frequency of mEPSCs of 38 SON neurons from 1.87 +/- 0.36 to 3.42 +/- 0.54 Hz but did not alter the amplitude (from 19.61 +/- 0.90 to 19.34 +/- 0.84 pA) and the decay time constant of mEPSCs. Furthermore, the nicotinic receptor antagonist mecamylamine (10 microM, n = 16), but not the muscarinic receptor antagonist atropine (100 microM, n = 12), abolished the excitatory effect of acetylcholine on the frequency of mEPSCs. These data provide new information that the excitatory effect of acetylcholine on the SON neurons is mediated, at least in part, by its effect on presynaptic glutamate release. Activation of presynaptic nicotinic, but not muscarinic, receptors located in the glutamatergic terminals increases the excitatory synaptic input to the SON neurons of the hypothalamus.

Induction of Fos-immunostaining by nicotine and nicotinic receptor antagonists in rat brain

Using Fos protein immunohistochemistry, we have studied the effects of acute nicotine (0.5 mg/kg s.c.) and nicotinic acetylcholine receptor (nAChR) antagonists in eleven rat brain areas. Acute nicotine elevated Fos-like immunostaining (Fos IS) significantly in all studied areas except the medial prefrontal cortex. Nicotine increased the Fos IS in cortical, limbic and hypothalamic areas by 2-10-fold, and in the interpeduncular nucleus as well as in the visual areas the increases were 15-150-fold. When given alone, the nAChR antagonists mecamylamine (1.0 or 5.0 mg/kg i.p.) and dihydro-beta-erythroidine (DHE; 1.4 or 2.8 mg/kg i.p.) increased Fos IS in most brain areas maximally by 2-10-fold, but methyllycaconitine (MLA; 4.0 mg/kg i.p.) only in three areas and maximally by 4-fold. The efficacy of nAChR antagonists in blocking nicotine's effects on Fos IS varied noticeably with respect to region and antagonist, and the combined effect of nicotine+antagonist did not exceed that of either treatment alone. Mecamylamine and DHE significantly reduced nicotine-induced Fos IS in most of the studied areas, and MLA only in two areas. Thus, nAChRs seem to mediate the effects of nicotine on Fos IS, and the differences in the effects of the antagonists studied suggest that more than one subtype of nAChRs are involved. The present experiments also provide evidence that nAChR blockade itself may result in increased Fos protein expression in the brain. This could be due to blockade of presynaptic nAChRs modulating transmitter release or interruption of complex polysynaptic feedback pathways.

Nicotine facilitates noradrenaline release in the rat supraoptic nucleus

Nicotine injected in the supraoptic nucleus facilitates vasopressin release from the neurohypophysis. Nicotinic acetylcholine receptors have been found not only on vasopressin-producing cell bodies but also on presynaptic nerve terminals in the nucleus. Vasopressin cells receive excitatory synaptic inputs from noradrenergic neurones. To test whether nicotine facilitates noradrenaline release in the supraoptic nucleus, we perfused the supraoptic nucleus with nicotine through a microdialysis probe. Nicotine increased the extracellular noradrenaline concentrations in the nucleus. A noradrenaline uptake inhibitor, desipramine, increased the extracellular noradrenaline concentrations in the nucleus and did not block the noradrenaline increase after nicotine. The results suggest that nicotine acts within the supraoptic nucleus to facilitate noradrenaline release pre-synaptically. This presynaptic action may contribute, in part, to vasopressin release after nicotine.

The unusual nature of epibatidine responses at the alpha4beta2 nicotinic acetylcholine receptor

The identification of an equatorial frog toxin, epibatidine, as a potent non-morphinic analgesic, selective for neuronal nicotinic acetylcholine receptors, provoked a marked renewal in our understanding of pain and its mechanisms. In this work we have examined the effects of epibatidine at the major brain rat alpha4beta2 nicotinic acetylcholine receptor expressed in a cell line. Fast drug applications obtained with a modified liquid filament system were used for the analyses of the currents evoked by acetylcholine, nicotine and epibatidine. Characterized by a slow onset and offset, epibatidine responses were of smaller amplitude to those evoked by acetylcholine or nicotine. About a thousand times more sensitive to epibatidine than acetylcholine, the alpha4beta2 receptor also displayed a more pronounced apparent desensitization to this compound. Finally, overnight exposure to 1 nM epibatidine failed to produce the functional upregulation observed with nicotine. These data indicate that, at the rat alpha4beta2 receptor, epibatidine acts as a partial agonist causing a pronounced inhibition of agonist evoked currents at concentrations that do not activate the receptors.

Regulation of feeding-associated peptides and receptors by nicotine

Although numerous epidemiological studies have provided convincing evidence for the inverse association between tobacco smoking and body weight, the molecular mechanisms underlying this relationship are not well-understood. Nicotine, as a potent secretagogue, could be expected to influence the levels and expression of many classes of neurotransmitters, as well as of cell-membrane constituents linked to neurotransmission, including signal transducers and related effectors. A potentially major group of candidate molecules that could be involved in feeding-related actions of nicotine are the numerous neuropeptides and peptide hormones shown in the past two decades to regulate food intake and energy expenditure. These could include neuropeptide Y (NPY), orexins, leptins, and uncoupling proteins (UCPs). Some of these peptides were already shown to respond to nicotine treatment in terms of regulation of levels and of activity at the level of cell-membrane receptors. The primary objective of this review is to summarize our current understanding of the regulatory effects of nicotine on the food intake and energy expenditure as related to the expression levels of leptin, NPY, orexin, uncoupling proteins, and of NPY and orexin receptors.

Effect of acute nicotine on Fos protein expression in rat brain during chronic nicotine and its withdrawal

To study the cholinergic regulation of hypothalamic paraventricular (PVN) and supraoptic (SON) nuclei and interpeduncular nucleus (IPN) we investigated the effects of acute nicotine (0.5 mg/kg, SC, 60 min) on Fos-like immunostaining (IS) during chronic nicotine and its withdrawal in rats. Nicotine or saline was infused to rats via osmotic minipumps (4 mg/kg/day) for 7 days; on the seventh day, the minipumps were removed surgically. In control rats, acute nicotine increased Fos IS significantly in all three brain areas studied. On the seventh day of nicotine infusion this effect partially persisted in IPN but was abolished in PVN and SON. After 72-h withdrawal nicotine-induced elevation of Fos IS was similar to that of control rats in all three areas. The observed attenuation of the response to acute nicotine during constant nicotine infusion in PVN and SON may be attributable to the desensitization of nicotinic acetylcholine receptors (nAChRs) mediating the effects of nicotine in these areas or in their input areas. IPN is connected to midbrain limbic system, so in agreement with our earlier observations, it seems that limbic nicotinic receptors do not very readily desensitize during chronic nicotine infusion. These findings support the suggestions that there are differences in the level of desensitization of nAChRs.