Biomedical subjects

G I Hatton

Publications and source records attributed to G I Hatton.

At least 19 recordsLinked to original sources

PubMed2006

Vesicular glutamate transporter expression in supraoptic neurones suggests a glutamatergic phenotype.

Magnocellular neuroendocrine cells of the supraoptic nucleus (SON) release the peptides oxytocin (OT) and vasopressin (VP) from their dendrites and terminals. In addition to peptide-containing large dense-core vesicles, axon terminals from these cells contain clear microvesicles that have been shown to contain glutamate. Using multilabelling confocal microscopy, we investigated the presence of vesicular glutamate transporters (VGLUTs) in astrocytes as well as VP and OT neurones of the SON. Simultaneous probing of the SON with antibodies against VGLUT isoforms 1-3, OT, VP and glial fibrillary acidic protein (GFAP) revealed the presence of VGLUT-2 in somata and dendrites of SON neurones. Immunoreactivity (-ir) for VGLUT-3 was also detected in both OT and VP neurones as well as in GFAP-ir astrocytes and other cells of the ventral glial lamina. Colocalisation of VGLUT-2 and VGLUT-3 in individual SON neurones was also examined and VGLUT-ir with both antibodies could be detected in both types of SON neurones. Although VGLUT-1-ir was strong lateral to the SON, only sparse labelling was apparent within the nucleus, and no colocalisation with either SON neurones or astrocytes was observed. The SON or the SON plus its surrounding perinuclear zone was probed using the reverse transcriptase-polymerase chain reaction and the presence of mRNA for all three VGLUT isoforms was detected. These results suggest that similar arrangements of transmitters exist in SON neuronal dendrites and their neurohypophysial terminals and that magnocellular neuroendocrine somata and dendrites may be capable of glutamatergic transmission.

Animals↗

PubMed2001

Plasticity of neurohypophysial terminals with increased hormonal release during dehydration: ultrastructural and biochemical analyses.

Arginine vasopressin- (AVP) and oxytocin- (OXT) secreting magnocellular neurons undergo gross structural changes with chronic physiological stimulation. Here, we investigated subcellular aspects of plasticity in rat neurohypophysial terminals during dehydration. Ultrastructural analyses demonstrated that chronic dehydration by 2% NaCl drinking for 7 days significantly decreased the numbers of neurosecretory granules and microvesicles but not the numbers of mitochondria. Moreover, in dehydrated rats, terminals making neurovascular contacts enlarged, whereas terminals in apposition to astrocytes, i.e., neuroglial contacts, became smaller. Western blot analyses demonstrated significant decreases in the levels of F3 and Thy-1 together with those of AVP- and OXT-neurophysin, but the levels of synaptophysin, SNAP-25, and GAP-43 were unchanged. Both F3 and Thy-1 were recovered in the buffer-insoluble pellet, and phosphatidyl inositol-specific phospholipase C treatment released both molecules from the crude membrane fraction, indicating that they are attached to terminal membranes by glycosylphosphatidyl inositol anchors. Confocal microscopic observations demonstrated that F3 colocalized with Thy-1 in the same terminals of magnocellular neurons. In contrast, the level of calretinin, a Ca(2+) binding protein was significantly increased with chronic dehydration. Thus, the present results suggest that enhancement of neurovascular contacts results from rearrangement of terminal-astrocyte and terminal-vessel contacts rather than enlargement or sprouting of magnocellular terminals themselves. The down-regulation of F3 and Thy-1 may contribute to enhancement of neurovascular contacts that accompany increased peptide release during dehydration.

Animals↗

PubMed2001

Ionotropic histamine receptors and H2 receptors modulate supraoptic oxytocin neuronal excitability and dye coupling.

Histaminergic neurons of the tuberomammillary nucleus (TM) project monosynaptically to the supraoptic nucleus (SON). This projection remains intact in our hypothalamic slices and permits investigation of both brief synaptic responses and the effects of repetitively activating this pathway. SON oxytocin (OX) neurons respond to single TM stimuli with fast IPSPs, whose kinetics resemble those of GABA(A) or glycine receptors. IPSPs were blocked by the Cl(-) channel blocker picrotoxin, but not by bicuculline or strychnine, and by histamine H(2), but not by H(1) or H(3) receptor antagonists, suggesting the presence of an ionotropic histamine receptor and the possible nonspecificity of currently used H(2) antagonists. G-protein mediation of the IPSPs was ruled out using guanosine 5'-O-(2-thiodiphosphate) (GDP-betaS), pertussis toxin, and Rp-adenosine 3',5'-cyclic monophosphothioate triethylamine (Rp-cAMPs), none of which blocked evoked IPSPs. We also investigated the effects of synaptically released histamine on dye coupling and neuronal excitability. One hundred seventy-three OX neurons were Lucifer yellow-injected in horizontal slices. Repetitive TM stimulation (10 Hz, 5-10 min) reduced coupling, an effect blocked by H(2), but not by H(1) or H(3), receptor antagonists. Because H(2) receptors are linked to activation of adenylyl cyclase, TM-stimulated reduction in coupling was blocked by GDP-betaS, pertussis toxin, and Rp-cAMPs and was mimicked by 8-bromo-cAMP, 3-isobutyl-1-methylxanthine, and Sp-cAMP. Membrane potentials of OX and vasopressin neurons were hyperpolarized, accompanied by decreased conductances, in response to bath application of 8-bromo-cAMP but not the membrane-impermeable cAMP. These results suggest that synaptically released histamine, in addition to evoking fast IPSPs in OX cells, mediates a prolonged decrease in excitability and uncoupling of the neurons.

Animals↗

PubMed2001

Connexin 26 and basic fibroblast growth factor are expressed primarily in the subpial and subependymal layers in adult brain parenchyma: roles in stem cell proliferation and morphological plasticity?

The gap junction protein connexin 26 (Cx26) has been detected previously in the parenchyma of the developing brain and in the developing and adult meninges, but there is no clear evidence for the presence of this connexin in adult brain parenchyma. Confocal mapping of Cx26 through serial sections of the meningeal-intact rat brain with four antibodies revealed an intense Cx26 immunoreactivity in both parenchyma and extraparenchyma. In the extraparenchyma, a continuum of Cx26-immunoreactive puncta was observed throughout the three meningeal layers, the perineurium of cranial nerves, and meningeal projections into the brain, including sheaths of blood vessels and stroma of the choroid plexus. In the parenchyma, Cx26-immunoreactive puncta were located primarily in subependymal, subpial, and perivascular zones and were associated primarily with glial fibrillary acidic protein-positive (GFAP+) astrocytes, the nuclei of which are strongly immunoreactive for basic fibroblast growth factor (bFGF). Although it was found to a lesser extent than in astrocytes, bFGF immunoreactivity also was intense in the nuclei of meningeal fibroblasts. In addition, we have found a close correlation between the distribution of Cx26 and vimentin immunoreactivities in the meninges and their projections into the brain. We previously showed vimentin and S100beta immunoreactivities through a network of meningeal fibroblasts in the three layers of meninges, perivascular cells, and ependymocytes and in a population of astrocytes. The related topography of this network with GFAP+ astrocytes has also been demonstrated. Considering that connexin immunoreactivity may reflect the presence of functional gap junctions, the present results are consistent with our hypothesis that all of these various cell types may communicate in a cooperative network.

Animals↗

PubMed2000

Immunocytochemical basis for a meningeo-glial network.

Evidence is presented here for a cellular network that courses through all layers of meninges, the vasculature of both the brain and meninges, and extends into the brain parenchyma. Confocal mapping of calcium-binding protein S100beta immunoreactivity (S100beta-ir) and of the intermediate filament vimentin-ir through serial sections of the meningeal-intact adult rat brain revealed this network. In all tissues examined, S100beta-ir and vimentin-ir were primarily colocalized, and were found in cells with elongated processes through which these cells contacted one another to form a network. The location of labeling and the morphology of the cells labeled were consistent with the possibility that this network consists of fibroblasts in the meninges and the walls of large blood vessels, of pericytes at the level of capillaries, and of ependymocytes and a population of astrocytes in the brain parenchyma. At many sites along the borders of the brain parenchyma itself and of the brain blood vessels, it was possible to detect S100beta-ir and vimentin-ir cell processes that cross the basal laminae. This suggested the probable means by which the S100beta-ir cells of the extraparenchymal tissues anatomically contact the cells that express the same markers in the brain. Privileged anatomical relationships of the S100beta/vimentin network with the glial fibrillary acidic protein (GFAP) astrocytes further suggested that, together, they form the structural basis for a general meningeo-glial network. This organization challenges the current model of brain architecture, calls for a reconsideration of the role of meninges and vascular tissues, and appears to reflect the existence of hitherto unsuspected systems of communication.

Animals↗

PubMed2000

Calbindin-D28k and calretinin in the rat posterior pituitary; light and electron microscopic localization and upregulation with dehydration.

Ca(2+) binding proteins (CaBPs), calbindin-D(28k) (calbindin) and calretinin, are thought to contribute to the regulation of intracellular Ca(2+) in many neuronal populations and perhaps more importantly, signal functional modulation in neuronal activity. In the present experiments, light microscopic immunohistochemistry revealed that the immunoreactivity of calbindin and calretinin was contained in varicose axons in the posterior pituitary. The dual labeling study with confocal microscopy demonstrated that calbindin immunoreactivity was present in the terminals of both oxytocin (OXT) and arginine-vasopressin (AVP) neurons. However, calretinin immunoreactivity was exclusively seen in the OXT terminals. Moreover, the dual labeling study showed that most calretinin-positive terminals contained calbindin immunoreactivity, demonstrating the colocalization of calbindin and calretinin in the same OXT nerve terminals. By electron microscopy, calbindin and calretinin immunoreactivities were seen in the neurosecretory axons and nerve terminals. These immunoreactive nerve terminals were seen to contain more clear microvesicles than dense-core neurosecretory granules. This immunoelectron microscopic observation suggests that both calbindin and calretinin localize preferentially in the active zone of the nerve terminals, which usually face the perivascular space around fenestrated capillaries. In spite of similar localization of calbindin and calretinin within the posterior pituitary, Western blot analysis showed some differences between the two CaBPs. Calbindin was present mostly in the soluble fraction with little in the insoluble fraction, but a substantial portion of calretinin was present in both the insoluble and soluble fractions. Moreover, dehydration induced by drinking 2% NaCl solution and deprivation of drinking water increased calretinin levels in the posterior pituitary as compared with control, but the calbindin level was not changed. The present findings demonstrate that calbindin and calretinin colocalize in the active zones of OXT nerve terminals, but only calretinin is upregulated with dehydration, suggesting different physiological role of calbindin and calretinin in the nerve terminals.

Animals↗

PubMed2000

Histamine suppresses non-NMDA excitatory synaptic currents in rat supraoptic nucleus neurons.

Whole cell patch-clamp recordings were obtained from supraoptic neurons to investigate the effects of histamine on excitatory postsynaptic currents evoked by electrical stimulation of areas around the posterior supraoptic nucleus. When cells were voltage-clamped at -70 mV, evoked excitatory postsynaptic currents had amplitudes of 88.4 +/- 9.6 pA and durations of 41.1 +/- 3.0 ms (mean +/- SE; n = 43). With twin stimulus pulses (20 Hz) used, paired-pulse facilitation ratios were 1.93 +/- 0.12. Bath application of 6-cyano-7-nitroquinoxalene-2,3-dione (CNQX) abolished synaptic currents. Histamine at concentrations approximately 0.1-10 microM reversibly suppressed excitatory postsynaptic currents in all supraoptic neurons tested. Within 2 min after application of (10 microM) histamine, current amplitudes and durations decreased by 61. 5 and 31.0%, respectively, with little change in the paired-pulse facilitation ratio. Dimaprit or imetit (H(2) or H(3) receptor agonists) did not reduce synaptic currents, whereas pyrilamine (H(1) receptor antagonist) blocked histamine-induced suppression of synaptic currents. When patch electrodes containing guanosine 5'-O-(2-thiodiphosphate) (GDP-beta-S) were used to record cells, histamine still suppressed current amplitudes by 49.1% and durations by 41.9%. Similarly, intracellular diffusion of bis-(o-aminophenoxy)-N,N,N',N'-tetraacetic acid (BAPTA) and H(7) did not abolish histamine-induced suppression of synaptic currents, either. Bath perifusion of 8-bromo-quanosine 3',5'-cyclic monophosphate reduced current amplitudes by 32.3% and durations by 27.9%. After bath perfusion of slices with N(omega)-nitro-L-arginine methyl ester (L-NAME), histamine injection decreased current amplitudes only by 31.9%, much less than the inhibition rate in control (P < 0.01). In addition, histamine induced little change in current durations and paired-pulse facilitation ratios, representing a partial blockade of histamine effects on synaptic currents by L-NAME. In supraoptic neurons recorded using electrodes containing BAPTA and perifused with L-NAME, the effects of histamine on synaptic currents were completely abolished. Norepinephrine injection reversibly decreased current amplitudes by 39.1% and duration by 64.5%, with a drop in the paired-pulse facilitation ratio of 47.9%. Bath perifusion of L-NAME, as well as intracellular diffusion of GDP-beta-S, 1-(5-isoquinolinylsulfonyl)-2-methyl-piperazine, or BAPTA, failed to block norepinephrine-induced suppression of evoked synaptic currents. The present results suggest that histamine suppresses non-N-methyl-D-aspartate synaptic currents in supraoptic neurons through activation of H(1) receptors. It is possible that histamine first acts at supraoptic cells (perhaps both neuronal and nonneuronal) and induces the production of nitric oxide, which then diffuses to nearby neurons and modulates synaptic transmission by a postsynaptic mechanism.

Action Potentials↗

PubMed1999

Nitric oxide via cGMP-dependent mechanisms increases dye coupling and excitability of rat supraoptic nucleus neurons.

Unlike many neuron populations, supraoptic nucleus (SON) neurons are rich in both nitric oxide synthase (NOS) and the NO receptor-soluble guanylyl cyclase (GC), the activation of which leads to cGMP accumulation. Elevations in cGMP result in increased coupling among SON neurons. We investigated the effect of NO on dye coupling in SONs from male, proestrus virgin female, and lactating rats. In 167 slices 263 SON neurons were recorded; 210 of these neurons were injected intracellularly (one neuron per SON) with Lucifer yellow (LY). The typically minimal coupling seen in virgin females was increased nearly fourfold by the NO precursor, L-arginine, or the NO donor, sodium nitroprusside (SNP). L-Arginine-induced coupling was abolished by a NOS inhibitor. In slices from male and lactating rats who have a higher basal incidence of coupling, SNP increased coupling by approximately twofold over control (p < 0.03). SNP effects were prevented by the NO scavenger hemoglobin (20 microM) and by the selective blocker of NO-activated GC, ODQ (10 microM). These results suggest that NO released from cells within the SON can expand the coupled network of neurons and that this action occurs via cGMP-dependent processes. Because increased coupling is associated with elevated SON neuronal excitability, we also studied the effects of 8-bromo-cGMP on excitability. In both phasically and continuously firing neurons 8-bromo-cGMP (1-2 mM), but not cGMP, produced membrane depolarizations accompanied by membrane conductance increases. Conductance increases remained when depolarizations were eliminated by current-clamping the membrane potential. Thus, NO-induced cGMP increases SON neuronal coupling and excitability.

Animals↗

PubMed1999

Inositol 1,4,5-trisphosphate-sensitive Ca2+ stores in rat supraoptic neurons: involvement in histamine-induced enhancement of depolarizing afterpotentials.

Histamine, a putative neuromodulator and neurotransmitter, can depolarize supraoptic neurons and enhance depolarizing afterpotentials that play a key role in determining the excitability of these neurons. This study investigated intracellular signal transduction involved in histamine-induced enhancement of depolarizing afterpotentials utilizing immunohistochemical and electrophysiological methods. Abundant inositol 1,4,5-trisphosphate receptor-related immunostaining was seen in all parts of the supraoptic nucleus, mainly within somata and proximal processes of the magnocellular neurons, but also in astrocytes of the ventral glial lamina. In supraoptic neurons displaying depolarizing afterpotentials, three brief depolarizations evoked a slow inward current. Bath application of histamine (1-2.5 microM) reversibly enhanced this slow inward current in almost all supraoptic neurons tested. Amplitudes and durations of the slow inward current were increased by 68.1% and 22.8%, respectively. Pretreatment of cells with a histamine receptor (subtype 1) antagonist (pyrilamine) or inhibitors of phospholipase C activation (neomycin or U73122) prevented histamine-induced enhancement of the slow inward current. When electrodes containing heparin, an inositol 1,4,5-trisphosphate receptor blocker, were used for recording, histamine had no effect on the slow inward current. Heparin, however, failed to abolish norepinephrine-induced enhancement of the slow inward current. After H7 [1-(5-isoquinolinylsulfonyl)-2-methylpiperazine], an inhibitor of protein kinase C, was infused into supraoptic neurons via the electrodes, histamine-induced enhancement of the slow inward current was also blocked. These results indicate the presence of, and functional roles for, inositol 1,4,5-trisphosphate receptor-sensitive Ca2+ stores in supraoptic neurons. Following activation of histamine receptors (subtype 1) and phospholipase C, Ca2+ mobilization from internal stores participates in mediating histamine-induced enhancement of depolarizing afterpotentials.

Animals↗

PubMed1999

Astroglial modulation of neurotransmitter/peptide release from the neurohypophysis: present status.

Reviewed in this article are those studies that have contributed heavily to our current conceptualizations of glial participation in the functioning of the magnocellular hypothalamo-neurohypophysial system. This system undergoes remarkable morphological and functional reorganization induced by increased demand for peptide synthesis and release, and this reorganization involves the astrocytic elements in primary roles. Under basal conditions, these glia appear to be vested with the responsibility of controlling the neuronal microenvironment in ways that reduce neuronal excitability, restrict access to neuronal membranes by neuroactive substances and deter neuron neuron interactions within the system. With physiological activation, the glial elements, via receptor-mediated mechanisms, take up new positions. This permissively facilitates neuron neuron interactions such as the exposure of neuronal membranes to released peptides and the formation of gap junctions and new synapses, enhances and prolongs the actions of those excitatory neurotransmitters for which there are glial uptake mechanisms, and facilitates the entry of peptides into the blood. In addition, subpopulations of these glia either newly synthesize or increase synthesis of neuroactive peptides for which their neuronal neighbors have receptors. Release of these peptides by the glia or their functional roles in the system have not yet been demonstrated.

Animals↗

PubMed1998

Colocalization of calretinin and calbindin-D28k with oxytocin and vasopressin in rat supraoptic nucleus neurons: a quantitative study.

Recent electrophysiological experiments, in which purified calbindin-D28k (calbindin) and calretinin antibodies were diffused into these neurons, showed that Ca2+-dependent membrane potentials and firing patterns were profoundly and predictably affected by Ca2+-binding proteins (CaBPs). The present study used quantitative analyses of a dual-labeling immunofluorescence method to investigate the colocalization of the CaBPs, calbindin and calretinin in oxytocin (OT)- and (VP)-containing neurons of the supraoptic nucleus. Analyses of tissue immunostained with two different dilutions of each CaBP antibody used, revealed that 84% and 72% of the OT neurons were positive for calbindin immunoreactivity (-ir) at the higher and lower antibody concentrations, respectively. 52% and 50% of OT neurons were positive for calretinin-ir; thus, many OT neurons express both calbindin and calretinin. In contrast, only 25% and 18% of VP neurons showed calbindin-ir, and they were virtually devoid of calretinin-ir. These results provide evidence that CaBP expression in OT neurons is both greater and more diverse than in VP neurons, and are consistent with the hypothesis that Ca2+ buffering capacity contributes to the control of intrinsic firing patterns.

Analysis of Variance↗

PubMed1998

Synaptic modulation of neuronal coupling.

Electrotonic coupling among neurons in the vertebrate, and more specifically the mammalian, brain has now been demonstrated to exist in all major brain subdivisions and in the spinal cord. For many of these brain areas, recent studies have investigated the possibilities of modulation of that coupling by synaptically released transmitters and/or neuromodulators. Reviewed here is the evidence for coupling, the synaptically related factors that play roles in up- or downregulation of this type of intercellular interaction and, to the extent that they have been investigated, the intracellular mechanisms operative in changing the extent of coupled networks in the brain. The functional significance of coupling and its modulation is discussed for some of these areas.

Animals↗

PubMed1998

Mechanisms of neuroendocrine cell excitability.

Oxytocin (OT) and vasopressin (VP), two neuronally synthesized nonapeptides, are made in the hypothalamic paraventricular and supraoptic nuclei of mammals and released into their blood, eventually to have profound hormonal actions on peripheral tissues. In the rat both OT and VP neurons fire slowly and irregularly under conditions of low demand for peptide release, but natural or artificial depolarizing stimuli result in differential patterns of activity: either regular continuous firing, strongly associated with OT cells, or phasic bursting, characteristic of VP neurons. Recently published findings offer an explanation for the dominant presence of certain Ca(2+)-dependent membrane potentials that typically lead to phasic firing in VP neurons. Mechanisms of excitability involved in the differential activities of the two cell types, as well as of the same cell type under different physiological conditions, include such factors as Ca2+ binding proteins, voltage- and ligand-gated ion channels, release of Ca2+ from internal stores and gap junctional conductances. The evidence for these factors is reviewed here.

Animals↗

PubMed1998

Neurophysiology of magnocellular neuroendocrine cells: recent advances.

Magnocellular neuroendocrine cells of the hypothalamic paraventricular and supraoptic nuclei are responsible for most of the vasopressin and oxytocin in the peripheral blood as well as for central release of these peptides in selected brain areas. As the principal component of the hypothalamo-neurohypophysial system, these neurons have been a subject of continual study for half a century. The wealth of solid information from decades of in vivo studies has provided a firm basis for in vitro, brain slice and explant investigations of neural mechanisms involved in the control and regulation of vasopressin and oxytocin neurons. In vitro methods have revealed the presence and permitted the study of monosynaptic projections to supraoptic neurons from the olfactory bulbs, the tuberomammillary nuclei of the posterior hypothalamus and from the organum vasculosum of the lamina terminalis. Such methods have also facilitated the elucidation of the various ionic currents controlling neurosecretory cell activity as well as the roles of calcium binding proteins and release of calcium from internal stores. This review summarizes recent advances in our understanding of the afferent inputs that impinge upon these two cell types, and the cellular and molecular mechanisms intrinsic to these neurons that determine their activity patterns and, in part, their responses to incoming stimuli.

Action Potentials↗

PubMed1998

Intrinsic controls of intracellular calcium and intercellular communication in the regulation of neuroendocrine cell activity.

1. The magnocellular hypothalamoneurohypophysial system, consisting chiefly of the supraoptic and paraventricular nuclei and their axonal projections to the pituitary neural lobe, has become a model for the study of neuroendocrine cell morphology, function, and plasticity. 2. Decades of research have produced a wealth of knowledge about the physiological conditions that activate this system, the peripheral target tissues affected by its outputs, and its capacity to undergo use-dependent, reversible reorganization. 3. Earlier research on the neural control of this system concentrated largely on the synaptic inputs that influence the activity of these magnocellular neurons and, while that task is still far from completed, methods have now been developed that permit insights to be gained into the control exercised by intrinsic cellular and molecular mechanisms. 4. This article reviews the current state of knowledge of roles played by these intrinsic mechanisms, including influences of intracellular calcium buffering, calcium release from internal stores and intercellular communication through gap junctions, in the control of neuroendocrine cell activity.

Animals↗

PubMed1997

Reduced outward K+ conductances generate depolarizing after-potentials in rat supraoptic nucleus neurones.

1. Whole-cell patch clamp recordings were obtained from sixty-five rat supraoptic nucleus (SON) neurones in brain slices to investigate ionic mechanisms underlying depolarizing after-potentials (DAPs). When cells were voltage clamped around -58 mV, slow inward currents mediating DAPs (IDAP), evoked by three brief depolarizing pulses, had a peak of 17 +/- 1 pA (mean +/- S.E.M.) and lasted for 2.8 +/- 0.1 s. 2. No significant differences in the amplitude and duration were observed when one to three preceding depolarizing pulses were applied, although there was a tendency for twin pulses to evoke larger IDAP than a single pulse. The IDAP was absent when membrane potentials were more negative than -70 mV. In the range -70 to -50 mV, IDAP amplitudes and durations increased as the membrane became more depolarized, with an activation threshold of -65.7 +/- 0.7 mV. 3. IDAP with normal amplitude and duration could be evoked during the decay of a preceding IDAP. As frequencies of depolarizing pulses rose from 2 to 20 Hz, the times to peak IDAP amplitude were reduced but the amplitudes and durations did not change. 4. A consistent reduction in membrane conductance during the IDAP was observed in all SON neurones tested, and averaged 34.6 +/- 3.3%. Small hyperpolarizing pulses used to measure membrane conductances appeared not to disturb major ionic mechanisms underlying IDAP, since the slope and duration of IDAP with and without test pulses were similar. 5. The IDAP had an averaged reversal potential of -87.4 +/- 1.6 mV, which was close to the K+ equilibrium potential. An elevation in [K+]o reduced or abolished the IDAP, and shifted its reversal potential toward more positive levels. Perifusion of slices with 7.5-10 mM TEA, a K+ channel blocker, reversibly suppressed the IDAP. 6. Both Na+ and Ca2+ currents failed to induce an IDAP-like current during perifusion of slices with media containing high [K+]o or TEA. However, the IDAP was abolished by replacing external Ca2+ with Co2+, or replacing 82% of external Na+ with choline or Li+. Perifusion of slices with media containing 1-2 microM TTX also reduced IDAP by 55.5 +/- 9.0%. 7. These results suggest that the generation of DAPs in SON neurones mainly involves a reduction in outward K+ current(s), which probably has little or no inactivation and can be inhibited by [Ca2+]i transients, due to Ca2+ influx during action potentials and Ca2+ release from internal stores. Na+ influx might provide a permissive influence for Ca(2+)-induced reduction of K+ conductances and/or help to raise [Ca2+]i via reverse-mode Ca(2+)-Na+ exchange. Other conductances, making minor contributions to the IDAP, may also be involved.

Animals↗

PubMed1997

Electrophysiology of excitatory and inhibitory afferents to rat histaminergic tuberomammillary nucleus neurons from hypothalamic and forebrain sites.

Anatomical evidence exists for projections to the tuberomammillary nucleus (TM) from the nucleus of the diagonal band of Broca (DBB) and the lateral preoptic area (LPO). The physiological effects of activating these inputs were studied by recording postsynaptic responses intracellularly from TM cells during both electrical stimulation and local nanodrop application of glutamate in horizontally cut brain slices. Electrical stimulation of the DBB, LPO and anterior lateral hypothalamic area (LH) usually evoked fast IPSPs (approximately 75% of responses) which were blocked by bicuculline or picrotoxin, suggesting GABA(A) mediation. The remaining excitatory responses evoked by stimulation of the LPO and LH were blocked by non-NMDA receptor antagonists (CNQX or NBQX) and the NMDA receptor antagonist, AP-5. Glutamate applied to the above areas induced postsynaptic responses in TM cells similar to those seen with electrical stimulation. Spontaneous firing in TM cells was suppressed by glutamate applied in the DBB. Glutamate applied in the LPO or LH evoked both inhibitory and excitatory responses. Changes in PSPs and firing rates were interpreted to result from glutamate activation of the neurons in the DBB, LPO and LH areas with inhibitory or excitatory connections to recorded TM neurons. These results support previous anatomical findings and suggest that inhibitory and excitatory synaptic control of TM activity is exerted by the DBB, LPO and LH areas.

Afferent Pathways↗

PubMed1997

Taurine in rat posterior pituitary: localization in astrocytes and selective release by hypoosmotic stimulation.

Taurine, a gamma-aminobutyric acid (GABA)-like acidic amino acid, has previously been shown to be prominently localized to astrocytes in the supraoptic nucleus, the neurons of which contain only small amounts, and to have inhibitory actions on supraoptic neuronal activity. In the present study, taurine distribution in the neurohypophysis was determined by using a well-characterized monoclonal antibody against taurine itself. Preembedding immunohistochemistry was performed at light and electron microscopic levels by using diaminobenzidine and gold-substituted silver-intensified peroxidase (GSSP) methods. At the light microscopic level, the distribution pattern and cellular localization of taurine immunoreactivity corresponded to that of glial fibrillary acidic protein. Pituicyte cell bodies and processes displayed dense taurine immunoreactivity. Electron microscopic observations revealed strong taurine GSSP reactions in these neural lobe astrocytes, but weak taurine reactivity was seen within only some neurosecretory axons. High-performance liquid chromatography analyses demonstrated that in vitro hypoosmotic stimulation (reduction of 40 mOsm/kg) of isolated posterior pituitaries resulted in preferential increases in taurine release into the bathing medium without increased release of other amino acids. Conversely, tissue concentrations of taurine significantly decreased with hypoosmotic perfusion, while glutamate, glutamine, and GABA concentrations were not reduced. These results indicate that taurine is mainly concentrated in neurohypophysial astrocytes, which are known to engulf the neurosecretory axonal processes and terminals. Taurine released from pituicytes under basal and hypoosmotic conditions may act to suppress axon terminal depolarization and thereby depress release of neurohypophysial peptides.

Animals↗