Intrinsic mechanisms involved in the electrophysiological properties of the vasopressin-releasing neurons of the hypothalamus

Heterogeneity of neuronal firing type and morphology in retrosplenial cortex of male F344 rats

The rodent granular retrosplenial cortex (gRSC) has reciprocal connections to the hippocampus to support fear memories. Although activity-dependent plasticity occurs within the RSC during memory formation, the intrinsic and morphological properties of RSC neurons are poorly understood. The present study used whole-cell recordings to examine intrinsic neuronal firing and morphology of neurons in layer 2/3 (L2/3) and layer 5 (L5) of the gRSC in adult male rats. Five different classifications were observed: regular-spiking (RS), regular-spiking afterdepolarization (RS_ADP), late-spiking (LS), burst-spiking (BS), and fast-spiking (FS) neurons. RS_ADP neurons were the most commonly observed neuronal class, identified by their robust spike frequency adaptation and pronounced afterdepolarization (ADP) following an action potential (AP). They also had the most extensive dendritic branching compared with other cell types. LS neurons were predominantly found in L2/3 and exhibited a long delay before onset of their initial AP. They also had reduced dendritic branching compared with other cell types. BS neurons were limited to L5 and generated an initial burst of two or more APs. FS neurons demonstrated sustained firing and little frequency adaptation and were the only nonpyramidal firing type. Relative to adults, RS neurons from juvenile rats (PND 14-30) lacked an ADP and were less excitable. Bath application of group 1 mGluR blockers attenuated the ADP in adult neurons. In other fear-related brain structures, the ADP has been shown to enhance excitability and synaptic plasticity. Thus, understanding cellular mechanisms of the gRSC will provide insight regarding its precise role in memory-related processes across the lifespan.NEW & NOTEWORTHY This is the first study to demonstrate that granular retrosplenial cortical (gRSC) neurons exhibit five distinctive firing types: regular spiking (RS), regular spiking with an afterdepolarization (RS_ADP), late spiking (LS), burst spiking (BS), and fast spiking (FS). RS_ADP neurons were the most frequently observed cell type in adult gRSC neurons. Interestingly, RS neurons without an ADP were most common in gRSC neurons of juvenile rats (PND 14-30). Thus, the ADP property, which was previously shown to enhance neuronal excitability, emerges during development.

Hormone secretion in transgenic rats and electrophysiological activity in their gonadotropin releasing-hormone neurons

Expression of GFP in GnRH neurons has allowed for studies of individual GnRH neurons. We have demonstrated previously the preservation of physiological function in male GnRH-GFP mice. In the present study, we confirm using biocytin-filled GFP-positive neurons in the hypothalamic slice preparation that GFP-expressing somata, axons, and dendrites in hypothalamic slices from GnRH-GFP rats are GnRH1 peptide positive. Second, we used repetitive sampling to study hormone secretion from GnRH-GFP transgenic rats in the homozygous, heterozygous, and wild-type state and between transgenic and Wistar males after ~4 yr of backcrossing. Parameters of hormone secretion were not different between the three genetic groups or between transgenic males and Wistar controls. Finally, we performed long-term recording in as many GFP-identified GnRH neurons as possible in hypothalamic slices to determine their patterns of discharge. In some cases, we obtained GnRH neuronal recordings from individual males in which blood samples had been collected the previous day. Activity in individual GnRH neurons was expressed as total quiescence, a continuous pattern of firing of either low or relatively high frequencies or an intermittent pattern of firing. In males with both intensive blood sampling (at 6-min intervals) and recordings from their GnRH neurons, we analyzed the activity of GnRH neurons with intermittent activity above 2 Hz using cluster analysis on both data sets. The average number of pulses was 3.9 ± 0.6/h. The average number of episodes of firing was 4.0 ± 0.6/h. Therefore, the GnRH pulse generator may be maintained in the sagittal hypothalamic slice preparation.

Emerging methodologies for the study of hypothalamic gonadotropin-releasing-hormone (GnRH) neurons

Gonadotropin-releasing-hormone (GnRH) neurons form part of a central neural oscillator that controls sexual reproduction through intermittent release of the GnRH peptide. Activity of GnRH neurons, and by extension release of GnRH, has been proposed to reflect intrinsic properties and synaptic input of GnRH neurons. To study GnRH neurons, we used traditional electrophysiology and computational methods. These emerging methodologies enhance the elucidation of processing in GnRH neurons. We used dynamic current-clamping to understand how living GnRH somata process input from glutamate and GABA, two key neurotransmitters in the neuroendocrine hypothalamus. In order to study the impact of synaptic integration in dendrites and neuronal morphology, we have developed full-morphology models of GnRH neurons. Using dynamic clamping, we have demonstrated that small-amplitude glutamatergic currents can drive repetitive firing in GnRH neurons. Furthermore, application of simulated GABAergic synapses with a depolarized reversal potential have revealed two functional subpopulations of GnRH neurons: one population in which GABA chronically depolarizes membrane potential (without inducing action potentials) and a second population in which GABAergic excitation results in slow spiking. Finally, when AMPA-type and GABA-type simulated inputs are applied together, action potentials occur when the AMPA-type conductance occurs during the descending phase of GABAergic excitation and at the nadir of GABAergic inhibition. Compartmental computer models have shown that excitatory synapses at >300 microns from somtata are unable to drive spiking with purely passive dendrites. In models with active dendrites, distal synapses are more efficient at driving spiking than somatic inputs. We then used our models to extend the results from dynamic current clamping at GnRH somata to distribute synaptic inputs along the dendrite. We show that propagation delays for dendritic synapses alter synaptic integration in GnRH neurons by widening the temporal window of interaction for the generation of action potentials. Finally, we have shown that changes in dendrite morphology can modulate the output of GnRH neurons by altering the efficacy of action potential generation in response to after-depolarization potentials (ADPs). Taken together, the methodologies of dynamic current clamping and multi-compartmental modeling can make major contributions to the study of synaptic integration and structure-function relationships in hypothalamic GnRH neurons. Use of these methodological approaches will continue to provide keen insights leading to conceptual advances in our understanding of reproductive hormone secretion in normal and pathological physiology and open the door to understanding whether the mechanisms of pulsatile GnRH release are conserved across species.

Dendrites determine the contribution of after depolarization potentials (ADPs) to generation of repetitive action potentials in hypothalamic gonadotropin releasing-hormone (GnRH) neurons

The impact of structure in modulating synaptic signals originating in dendrites is widely recognized. In this study, we focused on the impact of dendrite morphology on a local spike generating mechanism which has been implicated in hormone secretion, the after depolarization potential (ADP). Using multi-compartmental models of hypothalamic GnRH neurons, we systematically truncated dendrite length and determined the consequence on ADP amplitude and repetitive firing. Decreasing the length of the dendrite significantly increased the amplitude of the ADP and increased repetitive firing. These effects were observed in dendrites both with and without active conductances suggesting they largely reflect passive characteristics of the dendrite. In order to test the findings of the model, we performed whole-cell recordings in GnRH neurons and elicited ADPs using current injection. During recordings, neurons were filled with biocytin so that we could determine dendritic and total projection (dendrite plus axon) length. Neurons exhibited ADPs and increasing ADP amplitude was associated with decreasing dendrite length, in keeping with the predictions of the models. Thus, despite the relatively simple morphology of the GnRH neuron's dendrite, it can still exert a substantial impact on the final neuronal output.

Defining the gonadotrophin-releasing hormone neuronal network: transgenic approaches to understanding neurocircuitry

The gonadotrophin-releasing hormone (GnRH) neurones are the final downstream effector neurones driving the central regulation of reproductive function and fertility in all mammalian species. Although it is abundantly clear that successful fertility relies upon the communication of a variety signals regarding internal and external cues to the GnRH neuronal population, how this is achieved remains poorly understood. A range of technical limitations has posed significant hurdles to defining, with any certainty, the complexities of the synaptic neuronal network regulating GnRH neurones. However, recent advances in transgenic technology have opened up new avenues to reconsider questions aimed at understanding this critical network. This article addresses some of the latest advances that use transgenic mouse models as tools to understand the neuronal circuitry underpinning the regulation of the GnRH neurones. By incorporating standard morphological and viral tract tracing techniques with innovative transgenic tools, recent studies have uncovered previously unappreciated qualities of the GnRH neurone, including extensive dendritic lengths, numerous somal and dendritic spines and plasticity over pubertal development, along with beginning to define the primary and higher-order afferents that make up the GnRH neuronal network.

Dendritic processing of excitatory synaptic input in hypothalamic gonadotropin releasing-hormone neurons

The activity of hypothalamic GnRH neurons results in the intermittent release of GnRH required for reproductive function. This intermittent neurosecretory activity has been proposed to reflect integration of intrinsic properties of and synaptic input to GnRH neurons. Determining the relative impact of synaptic inputs at different locations on the GnRH neuron is difficult, if not impossible, using only experimental approaches. Thus, we used electrophysiological recordings and neuronal reconstructions to generate computer models of GnRH neurons to examine the effects of synaptic inputs at varying distances from the soma along dendrites. The parameters of the models were adjusted to duplicate measured passive and active electrophysiology of cells from mouse brain slices. Our morphological findings reinforce the emerging picture of a complex dendritic structure of GnRH neurons. Furthermore, analysis of reduced morphology models indicated that this population of cells is unlikely to exhibit low-frequency tonic spiking in the absence of synaptic input. Finally, applying realistic patterns of synaptic input to modeled GnRH neurons indicates that synapses located more than about 30% of the average dendrite length from the soma cannot drive firing at frequencies consistent with neuropeptide release. Thus, processing of synaptic input to dendrites of GnRH neurons is probably more complex than simple summation.

Control of firing by small (S)-alpha-amino-3-hydroxy-5-methyl-isoxazolepropionic acid-like inputs in hypothalamic gonadotropin releasing-hormone (GnRH) neurons

Episodic release of gonadotropin releasing hormone (GnRH) is obligatory for mammalian reproduction. The contribution of synaptic input to intermittent firing of GnRH neurons is unclear. GnRH neurons have very few synapses and most post-synaptic currents are small. Therefore, the impact of synaptic currents on firing in GnRH neurons was directly examined using simulated (S)-alpha-amino-3-hydroxy-5-methyl-isoxazolepropionic acid (AMPA)-like inputs applied with the method of dynamic current clamping. Tightly synchronized inputs and 50 ms bursts of excitatory input resulted in action potentials that were coincident with the stimulus. Neither input pattern resulted in sustained firing. When ongoing patterns of simulated inputs were applied over a range of parameters, action potentials were associated with clusters of AMPA-like inputs of 250 pS (approximately 15 pA amplitudes), while single inputs of 500 pS (approximately 30 pA amplitudes) resulted in action potentials. Ongoing inputs of 500 pS drove firing at 4-9 Hz. These findings provide evidence that small, simulated glutamatergic inputs can control firing in GnRH neurons and suggest that despite the small amplitudes, endogenous synaptic input mediated by glutamate may contribute to firing in GnRH neurons.

Baclofen inhibits guinea pig magnocellular neurones via activation of an inwardly rectifying K+ conductance

The GABAB receptors GABAB-R1 and GABAB-R2 have been cloned in several mammalian species, and the functional receptor has been shown to exist as a heterodimeric complex. We have cloned guinea pig GABAB-R1 and GABAB-R2 receptor sequences and, using in situ hybridization and immunocytochemistry for vasopressin (AVP), we found that GABAB-R1 and -R2 receptors are expressed in vasopressin neurones of the supraoptic (SON) and paraventricular nuclei (PVN). Therefore, we used both sharp electrode and whole-cell patch recording techniques to examine the effects of the selective GABAB agonist baclofen on SON and PVN magnocellular neurones and to determine the coupling of the GABAB receptor to effector pathways. Recordings were made in coronal hypothalamic slices from both female (ovariectomized) and male guinea pigs. In the presence of tetrodotoxin (TTX), baclofen hyperpolarized (DeltaVmax = 5.6 mV, EC50 = 2.3 microM) SON magnocellular neurones (n = 27) under current clamp, or induced an outward current that reversed at EK (DeltaImax = 24.2 pA) in PVN magnocellular neurones (n = 33) under voltage clamp. Seventeen of the 24 biocytin-labelled SON magnocellular neurones were identified as AVP neurones, and ten of the 33 biocytin-labelled PVN neurones were identified as AVP or neurophysin-containing neurones, although all of the cells were clustered in the vasopressin-rich core. In the absence of TTX, baclofen activated an outward K+ current that hyperpolarized SON and PVN neurones and significantly reduced their firing rate. The outward current showed inward rectification and was blocked by the K+ channel blocker barium and the GABAB receptor antagonist CGP 35348. Therefore, GABAB receptors are coupled to inwardly rectifying K+ channels in SON and PVN magnocellular neurones and may play a prominent role in modulating phasic bursting activity in guinea pig vasopressin neurones.

Priming of excitatory synapses by alpha1 adrenoceptor-mediated inhibition of group III metabotropic glutamate receptors

Adaptive responses mediated by the hypothalamus require sustained activation until homeostasis is achieved. Increases in excitatory drive to the magnocellular neuroendocrine cells that mediate these responses, however, result in the activation of a presynaptic metabotropic glutamate receptor (mGluR) that curtails synaptic excitability. Recent evidence that group III mGluRs can be inhibited by protein kinase C prompted us to test the hypothesis that activation of PKC by noradrenaline (NA) inhibits group III mGluRs and increases excitatory synaptic input to these cells. To examine the effects of NA on miniature EPSCs (mEPSCs), we obtained whole-cell recordings from magnocellular vasopressin and oxytocin neurons in the paraventricular nucleus of the hypothalamus. All of the neurons tested in the current study displayed an alpha1 adrenoceptor-mediated increase in mEPSC frequency in response to NA (1-200 microm). The excitatory effects of NA were mimicked by the phorbol ester PMA and blocked by the PKC inhibitor calphostin C. The activation of PKC inhibits the efficacy of group III mGluRs, resulting in an increase in mEPSC frequency in response to a subsequent exposure to NA. By removing feedback inhibition, this mechanism effectively primes the synapses such that subsequent activation is more efficacious. The novel form of synaptic rescaling afforded by this cross-talk between distinct metabotropic receptors provides a means by which ascending catecholamine inputs can facilitate the control of homeostasis by hypothalamic networks.

Changes in the active membrane properties of rat supraoptic neurones during pregnancy and lactation

To better understand the plasticity of intrinsic membrane properties of supraoptic magnocellular neuroendocrine cells associated with reproductive function, intracellular recordings were performed in oxytocin (OT) and vasopressin (VP) neurones from virgin, late pregnant (E19-22), and lactating (8-12 days of lactation) rats in vitro, using hypothalamic explants. OT neurones from virgin rats displayed a narrower spike width than neurones from pregnant and lactating rats, characterized by faster rise and decay times. Spike width changes in VP neurones were not as prominent as those observed in OT neurones. In OT neurones, the amplitude and the decay of the afterhyperpolarization following spike trains was significantly larger and faster, respectively, in pregnant and lactating rats compared to virgin rats. These properties did not change during pregnancy and lactation in VP neurones. The incidence of the depolarizing afterpotential following spikes significantly increased from approximately 20% in virgin rats to 40-50% during pregnancy and lactation in OT neurones, but was stable (80-90%) across states in VP neurones. Repetitive firing properties (frequency adaptation, the first interspike interval frequency and frequency-current (F-I) relationship) were altered during pregnancy and lactation in OT neurones, but not VP neurones. The increased incidence of depolarizing afterpotentials in OT neurones enhances excitability, while the increased afterhyperpolarization results in suppression of firing rate. Thus, the changes may favour the short bursting activity seen in OT neurones during lactation. These results confirmed reproductive state-dependent changes in intrinsic membrane properties of OT neurones during lactation, and suggest these changes are in place during late pregnancy. This argues that the plasticity in the electrical properties in OT neurones associated with lactation is not instigated by suckling.

Bursting properties of caudal neurosecretory cells in the flounder Platichthys flesus, in vitro

Bursting activity in type 1 Dahlgren cells was studied using intra- and extracellular recording from an in vitro preparation of the caudal neurosecretory system of the euryhaline flounder. 45% of cells showed spontaneous bursts of approximately 120s duration and 380s cycle period. Similar bursts were triggered by short duration (<5s) depolarising or hyperpolarising pulses. Cells displayed a characteristic depolarising after potential, following either an action potential with associated afterhyperpolarisation, or a hyperpolarising current pulse. This depolarising after potential was related to a ‘sag’ potential, which developed during the hyperpolarising pulse. Both the depolarising after potential and the sag potential occurred only in cells at more depolarised (<60mV) holding potentials. In addition, the amplitude of the depolarising after potential was dependent on the amplitude and the duration of the hyperpolarising pulse. The depolarising after potential following action potentials may provide a mechanism for facilitating repetitive firing during a burst. Extracellular recording revealed similar bursting in individual units which was not, however, synchronised between units. Spontaneous bursting activity recorded both intra- and extracellularly was inhibited by application of a known neuromodulator of the system, 5-hydroxytryptamine. This study provides a basis for investigating the relationship between physiological status, Dahlgren cell activity and neuropeptide secretion.

Synchronized clusters of action potentials can increase or decrease the excitability of the axons of magnocellular hypothalamic neurosecretory cells

Extracellular recordings were made from supraoptic nucleus (SON) cells in urethane anaesthetized male rats in vivo. Two stimulating electrodes were positioned to activate the cells antidromically, one in the mid axon region of the cells and the other at the axon terminals. Trains of 5-20 just-subthreshold stimuli at 5 s intervals decreased the threshold for antidromic activation from both sites. Whereas neither single stimuli, nor the stimuli at the beginning of a train of 20 stimuli evoked antidromic action potentials, later action potentials did so. Paradoxically, trains of 20 just-suprathreshold stimuli increased the threshold for activation of both axons and terminals. In recordings from the same cells, stimuli were applied singly at 5 s intervals at an intensity which almost invariably evoked an antidromic action potential. Identical stimuli were then applied in trains of 20 stimuli at 50 Hz. After the first train, the initial stimulus pulses of the trains frequently fell below threshold. Following a conditioning train of five stimuli applied to one electrode, the period of decreased threshold (increased excitability) at the other electrode lasted less than 100 ms and the period of increased threshold (decreased excitability) after 12 trains of 20 stimuli lasted between 5 and 10 s. Both decreased and increased excitability were seen at axons and terminals of both putative oxytocin and vasopressin cells. Since the excitability changes were shown in vivo at frequencies encountered during recordings, it is likely that they influence the probability of spike propagation and hormone secretion under physiological conditions.

Activity-dependent morphological synaptic plasticity in an adult neurosecretory system: magnocellular oxytocin neurons of the hypothalamus

Oxytocin and vasopressin neurons, located in the supraoptic and paraventricular nuclei of the hypothalamus, send their axons to the neurohypophysis where the neurohormones are released directly into the general circulation. Hormone release depends on the electrical activity of the neurons, which in turn is regulated by different afferent inputs. During conditions that enhance oxytocin secretion (parturition, lactation, and dehydration), these afferents undergo morphological remodelling which results in an increased number of synapses contacting oxytocin neurons. The synaptic changes are reversible with cessation of stimulation. Using quantitative analyses on immunolabelled preparations, we have established that this morphological synaptic plasticity affects both inhibitory and excitatory afferent inputs to oxytocin neurons. This review describes such synaptic modifications, their functional significance, and the cellular mechanisms that may be responsible.Key words: oxytocin, vasopressin, GABA, glutamate, noradrenaline, hypothalamo-neurohypophysial system, lactation.

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.

Phenotypic and state-dependent expression of the electrical and morphological properties of oxytocin and vasopressin neurones

Oxytocin and vasopressin secreting neurones of the hypothalamic supraoptic nucleus share many membrane characteristics and a roughly similar morphology. However, these two neurone types differ in the relative expression of some intrinsic and synaptic currents, and in the extent of their respective dendritic arbors. Spike depolarizing afterpotentials are present in both types, but more frequently give rise to prolonged burst discharges in vasopressin neurones. Oxytocin, but not vasopressin neurones, are characterized by a depolarization-activated, sustained outward rectifier which turns on near spike threshold, and which can produce prolonged spike frequency adaptation. When this sustained current is deactivated by small hyperpolarizing pulses, a rebound depolarization sufficient to evoke short spike trains follows the offset of these pulses. Both oxytocin and vasopressin neurones exhibit a transient outward rectification underlain by an Ia-type current. This transient rectifier delays spiking to depolarizing stimuli from a relatively hyperpolarized baseline, and is more prominent in vasopressin neurones. As a result, oxytocin neurones may be more reactive to depolarizing inputs. Both cell types receive glutamatergic, excitatory synaptic inputs and both possess R,S- alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid (AMPA) and N-methyl-D-aspartate (NMDA) receptor subtypes. The AMPA receptor channel on both cell types is characterized by a relatively high calcium permeability and voltage-dependent rectification, characteristic of a diminished presence of the GluR2 AMPA subunit. However, AMPA-mediated synaptic transients are larger, and decay faster, in oxytocin compared with vasopressin neurones, suggesting a potential difference for synaptic integration. The characteristics of NMDA-mediated synaptic transients are similar in oxytocin and vasopressin neurones, but some data suggest NMDA receptors may be less involved in the glutamatergic activation of oxytocin neurones. In both cell types, synaptic release of glutamate often coactivates AMPA and NMDA receptors. The dendritic morphology of oxytocin and vasopressin neurones in female rats differs from one another and exhibits considerable plasticity as a function of endocrine state. In virgin rats, oxytocin neurones have more dendritic branches and a greater total dendritic length compared with lactation, when the arbor is much less extensive. A complementary change occurs in vasopressin dendrites, which are more extensive during lactation. This reorganization suggests that oxytocin neurones may be more electronically compact during lactation. In addition, such dramatic shifts in overall dendritic length imply that significant gains and losses in either the total number of synapses, or in synaptic density, are incurred by both cell types as a function of reproductive state.

Noradrenergic excitation of magnocellular neurons in the rat hypothalamic paraventricular nucleus via intranuclear glutamatergic circuits

Noradrenergic projections to the hypothalamus play a critical role in the afferent control of oxytocin and vasopressin release. Recent evidence for intrahypothalamic glutamatergic circuits prompted us to test the hypothesis that the excitatory effect of noradrenergic inputs on oxytocin and vasopressin release is mediated in part by local glutamatergic interneurons. The voltage response to norepinephrine (30-300 microM) was tested with whole-cell recordings in putative magnocellular neurons of the paraventricular nucleus (PVN) in hypothalamic slices (400 micrometers). Norepinephrine elicited an alpha1 receptor-mediated direct depolarization in 23% of the magnocellular neurons tested; however, the most prominent response, seen in 42% of the magnocellular neurons, was an alpha1 receptor-mediated increase in the frequency of EPSPs. The norepinephrine-induced increase in EPSPs was blocked by tetrodotoxin and by ionotropic glutamate receptor antagonists, suggesting that norepinephrine excited presynaptic glutamate neurons to cause an increase in spike-mediated transmitter release. The increase in EPSPs also was observed in a surgically isolated PVN preparation (64% of cells) and with microdrop applications of norepinephrine (1 mM, 33% of cells) and glutamate (0.5-1 mM, 28%) in the PVN, indicating that the norepinephrine-sensitive presynaptic glutamate neurons are located within the PVN. Biocytin injection and subsequent immunohistochemical labeling revealed that both oxytocin and vasopressin neurons responded to norepinephrine. Our data indicate that magnocellular neurons of the PVN receive excitatory inputs from intranuclear glutamatergic neurons that express alpha1-adrenoreceptors. These glutamatergic interneurons may serve as an excitatory relay in the afferent noradrenergic control of oxytocin and vasopressin release under certain physiological conditions.

Electrophysiological distinctions between oxytocin and vasopressin neurons in the supraoptic nucleus

Oxytocin and vasopressin neurons can be differentiated from one another, and from neurons in the immediately adjacent perinuclear zone, by their electrophysiological properties. In both sexes, oxytocin and vasopressin neurons are characterized by a prominent transient outward rectification which is conspicuously lacking in most perinuclear neurons. In addition, perinuclear neurons, some of which project to the supraoptic nucleus, exhibit a transient depolarization which underlies short bursts of spikes. Oxytocin neurons are characterized by: 1) the presence of a sustained outward rectifier above -50 mV, active below spike threshold; 2) a rebound depolarization following deactivation of the sustained rectification which can sustain short spike trains; and 3) a smaller transient outward rectification, probably associated with the potassium current, Ia. Vasopressin neurons show little of the sustained outward rectification and rebound depolarization, but have a stronger transient outward rectification. Although both cell types exhibit depolarizing afterpotentials, in vasopressin neurons these lead to plateau potentials underlying prolonged discharges. In oxytocin neurons, the depolarizing potential usually sustains a short spike discharge, but less often leads to prolonged bursts. These data suggest that the intrinsic properties of oxytocin and vasopressin neurons lead to quantitatively different forms of burst discharges, both of which may facilitate hormone release.

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.

Components of after-hyperpolarization in magnocellular neurones of the rat supraoptic nucleus in vitro

1. The pharmacological sensitivity of hyperpolarizing components of spike train after-potentials was examined in sixty-one magnocellular neurones of the rat supraoptic nucleus using intracellular recording techniques in a brain slice preparation. 2. In 26 % of all neurones a slow after-hyperpolarization (AHP) was observed in addition to a fast AHP. In 31 % of all neurones a depolarizing after-potential (DAP) was observed. 3. The fast AHP was blocked by apamin whereas the slow AHP was blocked by charybdotoxin (ChTX). The DAP was enhanced by ChTX or a DAP was unmasked if not present during the control period. 4. Low concentrations of TEA (0.15-1.5 mM) induced effects on the slow AHP and the DAP essentially resembling those of ChTX. The same was true for the effects of CoCl2 (1 mM). 5. Spike train after-potentials were not affected by either iberiotoxin (IbTX), a selective high-conductance potassium (BK) channel antagonist, or margatoxin (MgTX), a Kv1.3 alpha-subunit antagonist. 6. Kv1.3 alpha-subunit immunohistochemistry revealed that these units are not expressed in the somato-dendritic region of supraoptic neurones. 7. The effects of ChTX, IbTX, MgTX, TEA, CoCl2 and CdCl2 on spike train after-potentials are interpreted in terms of an induction of the slow AHP by the activation of calcium-dependent potassium channels of intermediate single channel conductance (IK channels). 8. The results suggest that at least the majority of supraoptic magnocellular neurones share the capability of generating both a slow AHP and a DAP. The slow AHP may act to control the expression of the DAP, thus regulating the excitability of magnocellular neurones. The interaction of the slow AHP and the DAP may be important for the control of phasic discharge.

Characterization of carbachol-induced rhythmic bursting discharges in neurons from guinea pig lateral septum slices

A brain slice preparation from guinea pigs and intracellular recording techniques were used to examine the effects of carbachol application on the three classes (A, B, and C) of neurons (n = 68, 40 of class A, 12 of class B, 16 of class C) within the mediolateral part of the lateral septum (LSml). Bath application of carbachol elicited a sustained depolarization associated with an increase in membrane input resistance, action-potential firing and triggered rhythmic bursting discharges in 59% of recorded neurons. According to the configuration of these bursts, LSml neurons were classified into type I, II, and III neurons with reference to their response to carbachol. The frequency of spontaneous bursts was increased by depolarization caused by applied DC current in the three types of neurons. Bursts in type II and III neurons were voltage and dose dependent. These dependences were responsible for a continuum of variation in carbachol responses in these two types of neurons. As the neuron depolarized in the presence of carbachol, spontaneous action potentials increased in frequency and slow afterdepolarizing potentials (sADPs) appeared and preceded the occurrence of the first burst. These sADPs from adjacent action potentials appeared to progressively increase to initiate a burst. In the presence of carbachol, sADPs and bursts were also observable after action potentials evoked by depolarizing current pulses at the resting membrane potential (RMP) in LSml neurons. Evoked sADPs and bursts were associated with an apparent increase in input conductance. Application of low Na+ medium blocked both the sADP and bursts. Application of zero Ca2+ medium either 1) blocked completely the generation of sADPs and bursts (n = 16), or 2) did not block bursts (n = 14). Evoked sADPs and bursts were blocked by tetraethylammonium but were resistant to external Cs+. The results indicate that the activation of cholinergic receptors does not differentially affect the three classes of LSml neurons. The responses to carbachol in type II and III neurons form a continuum of variation, whereas these of type I neurons constitute a discrete entity. The selective cholinergic induction of a sADP, and the progressive activation of these sADPs in LSml neurons are thought to be responsible for the onset of the three types of rhythmic bursting discharges. We propose that sADPs and bursts induced by carbachol are generated by a cationic conductance largely permeable to Na+. In a subpopulation of LSml neurons (n = 16), the bursts are dependent on the presence of Ca2+ in the medium.

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.

Electrophysiological and morphological characteristics of neurons in perinuclear zone of supraoptic nucleus

Electrophysiological and morphological characteristics of neurons in perinuclear zone of supraoptic nucleus. J. Neurophysiol. 78: 2427-2437, 1997. Neurons in the perinuclear zone (PZ) of the supraoptic nucleus (SON) are thought to serve as interneurons and may mediate changes in neurohypophysial hormone release in response to physiological changes in blood pressure. However, the morphology and electrophysiological characteristics of PZ neurons are unknown. In the present study, PZ neurons from male and female rats were recorded intracellularly to determine some membrane properties, then filled with biocytin or biotinamide for morphological analysis. In general, PZ neurons had faster spikes than magnocellular SON neurons, and the great majority were characterized by a subthreshold depolarizing hump when depolarized from a hyperpolarized (less than -80 mV) membrane potential. In most neurons, this hump was similar to low-threshold spikes described in other CNS regions. Near-threshold, fast action potentials were clustered near the onset of these depolarizations. Conspicuously absent in all PZ neurons was the strong transient and subthreshold outward rectification characteristic of vasopressin and oxytocin neurons of the SON. These results suggest that PZ neurons are electrophysiologically distinct from neurosecretory neurons of the SON. No differences were found between male and female rats in any of the basic properties examined, including input resistance, membrane time constant, spike height, spike width, spike threshold, and the size of the spike afterhyperpolarization. Morphologically, PZ neurons were diverse but were divided into spiny and aspiny groups. Three spiny neurons and one aspiny neuron contributed an axonal projection to the SON characterized by varicosities suggestive of terminals. In the case of the three spiny neurons, the SON projection was clearly a minor collateral projection. The axon arborized in the PZ, but one or more branches were cut at the edge of the explant, indicating a longer projection. In the remaining neurons, no axonal projection to the SON was detected and several had axons leaving the explant. Some portion of the dendritic tree penetrated the SON in several neurons. The morphology of PZ neurons was thus heterogeneous and suggests that, for some cells at least, the projection to the SON may be a minor collateral component of a much wider axonal projection.

Antisense oligonucleotides in neuroendocrinology: enthusiasm and frustration

Antisense oligodeoxynucleotides (ODN) offer the potential advantage to manipulate neuropeptide or neuropeptide receptor expression within the brain transiently and site-specifically, thus providing a tool for neuroendocrinological research into the physiological function of a particular neuropeptide system. In this study, various approaches are introduced which reveal that antisense ODN may exert acute, short-term effects on neuronal responsiveness to afferent stimuli, as well as long-term effects on neuropeptide/receptor protein availability in a given system depending on the duration of treatment. Short-term effects were seen in that oxytocin (OXT) and vasopressin (AVP) antisense ODN affected electrophysiological and secretory parameters of oxytocinergic and vasopressinergic neurons, respectively, as well as their ability to express the Fos protein in response to afferent stimulation a few hours after a single infusion into the hypothalamic supraoptic nucleus. In this study, two methodological approaches to study long-term effects of the antisense ODN are exemplified, in which antisense ODN directed against the mRNA coding for the neuropeptide itself or its receptor were used. The repeated infusion of corticotropin releasing hormone (CRH) antisense ODN into the paraventricular nucleus resulted in reduced immunoreactive CRH, but not AVP, in the external zone of the median eminence. Furthermore, in order to evaluate the receptor-mediated effects of CRH and AVP released locally within the paraventricular nucleus on adrenocorticotropin (ACTH) release from the pituitary, CRH receptor (and also AVP receptor) antisense ODN were repeatedly infused into the hypothalamic nuclei; this treatment resulted in an elevation of stimulated, but not basal, ACTH release into the blood. However, in addition to these obvious antisense effects, results are discussed which demonstrate sequence-unspecific effects of phosphorothioated ODN, suggesting that some of their mechanisms of action are not yet understood.

Outward potassium currents of supraoptic magnocellular neurosecretory cells isolated from the adult guinea-pig

1. Several types of whole-cell outward K+ current recorded from magnocellular neurosecretory cells (MNCs) dissociated from the supraoptic nucleus of the adult guinea-pig were identified on the basis of their voltage dependence, kinetics, pharmacology and Ca2+ dependence. 2. The predominant K+ current evoked from a holding potential of -40 mV was slowly activating, long-lasting, tetraethylammonium (TEA) sensitive and showed little steady-state inactivation. Also, this current was reduced by extracellular Cd2+. These data suggest that in supraoptic MNCs classical Ca(2+)-insensitive, delayed rectifier channels (KV) and Ca(2+)-sensitive, non-inactivating channels (KCa) both contribute to the sustained current. 3. A transient, low-threshold K+ current, which was 4-aminopyridine (4-AP) sensitive and showed significant steady-state inactivation, was evoked along with the sustained current from a holding potential of -90 mV. Based on these characteristics, this current corresponds to the A-current (IK(A)) described in other neurons. 4. IK(A) was activated when Ca2+ influx was blocked or when Ca2+ was absent from the extracellular medium, suggesting that Ca2+ influx is not necessary for activation of the current. 5. In many recordings, a transient 4-AP-insensitive outward current was evoked from a holding potential of -40 mV. This high-threshold transient K+ current was abolished by extracellular Cd2+ or TEA and was absent when extracellular Ca2+ was replaced by Sr2+, suggesting that it is a transient Ca(2+)-dependent K+ current. 6. We conclude that the presence of multiple types of K+ current may, in part, underlie the complex firing patterns of oxytocinergic and vasopressinergic MNCs.

Oscillatory bursting of phasically firing rat supraoptic neurones in low-Ca2+ medium: Na+ influx, cytosolic Ca2+ and gap junctions

1. Whole-cell patch recordings were obtained from supraoptic nucleus (SON) neurones in horizontal brain slices of adult male rats. Low-Ca2+ or Ca(2+)-free perifusion medium induced oscillatory bursting activity in all sixty-nine cells displaying both phasic firing and depolarizing after-potentials (DAPs). In fifteen non-phasic cells without DAPs, Ca(2+)-free medium produced little or no oscillatory bursting. 2. Typical bursts started with rapid membrane depolarization, resulting in a plateau with superimposed action potentials, and ended several hundred milliseconds later in swift repolarization. Prominent bursting was observed at membrane potentials from -50 to -70 mV, with maximum amplitudes of 12.2 +/- 0.7 mV (mean +/- S.E.M.) around -70 mV. Development of oscillatory bursting was dependent on reduction of [Ca2+]o, with a threshold for the bursting < or = 1.2 mM Ca2+. 3. Bursting was abolished by addition of TTX, Co2+, Ni2+ or Mg2+ into the Ca(2+)-free medium, or by replacement of external Na+ with choline or Li+. Low concentrations of TEA or increased [K+]o prolonged burst durations and enlarged oscillation amplitudes. 4. Voltage-clamp techniques were used to examine the persistent Na+ current (INaP), and revealed that low [Ca2+]o shifted the threshold for INaP activation in a negative direction and enhanced the amplitude of this current. These changes in INaP were abolished by adding Co2+ or Mg2+ to Ca(2+)-free medium. 5. Direct diffusion of BAPTA or heparin into neurones or bath application of ryanodine suppressed bursting. Oscillations were also eliminated by the uncoupling agents heptanol, halothane or acidification. 6. CNQX, APV, bicuculline, CGP35348 (GABAB receptor antagonist), promethazine, atropine, d-tubocurarine and suramin had no obvious effects on oscillatory bursting. Blockers of transient Ca2+, or hyperpolarization-activating cation currents also did not alter bursting activity. 7. These results suggest that intrinsic burst activity in SON neurons perifused with low-Ca2+ or Ca(2+)-free medium involves enhanced Na+ influx through persistent Na+ channels, and requires the presence of rapid intracellular Ca2+ mobilization that might also explain the selective existence of oscillatory bursting in phasically firing cells. Intercellular communication through gap junctions appears to be important in determining neuronal activity of the neuroendocrine cells in low-Ca2+ medium.

Electrophysiological characteristics of immunochemically identified rat oxytocin and vasopressin neurones in vitro

1. Intracellular recordings were made from supraoptic neurones in vitro from hypothalamic explants prepared from adult male rats. Neurones were injected with biotinylated markers, and of thirty-nine labelled neurones, nineteen were identified immunocytochemically as containing oxytocin-neurophysin and twenty as containing vasopressin-neurophysin. 2. Vasopressin and oxytocin neurones did not differ in their resting membrane potential, input resistance, membrane time constant, action potential height from threshold, action potential width at half-amplitude, and spike hyperpolarizing after-potential amplitude. Both cell types exhibited spike broadening during brief, evoked spike trains (6-8 spikes), but the degree of broadening was slightly greater for vasopressin neurones. When hyperpolarized below -75 mV, all but one neurone exhibited a transient outward rectification to depolarizing pulses, which delayed the occurrence of the first spike. 3. Both cell types exhibited a long after-hyperpolarizing potential (AHP) following brief spike trains evoked either with a square wave pulse or using 5 ms pulses in a train. There were no significant differences between cell types in the size of the AHP evoked with nine spikes, or in the time constant of its decay. The maximal AHP evoked by a 180 ms pulse was elicited by an average of twelve to thirteen spikes, and neither the size of this maximal AHP nor its time constant of decay were different for the two cell types. 4. In most oxytocin and vasopressin neurones the AHP, and concomitantly spike frequency adaptation, were markedly reduced by the bee venom apamin and by d-tubocurarine, known blockers of a Ca(2+)-mediated K+ conductance. However, a minority of neurones, of both cell types, were relatively resistant to both agents. 5. In untreated neurones, 55% of vasopressin neurones and 32% of oxytocin neurones exhibited a depolarizing after-potential (DAP) after individual spikes or, more commonly, after brief trains of spikes evoked with current pulses. For each neurone with a DAP, bursts of spikes could be evoked if the membrane potential was sufficiently depolarized such that the DAP reached spike threshold. In four out of five vasopressin neurones a DAP became evident only after pharmacological blockade of the AHP, whereas in six oxytocin neurones tested no such masking was found. 6. The firing patterns of neurones were examined at rest and after varying the membrane potential with continuous current injection. No identifying pattern was strictly associated with either cell type, and a substantial number of neurones were silent at rest.(ABSTRACT TRUNCATED AT 400 WORDS)