Burst discharge in mammalian neuroendocrine cells involves an intrinsic regenerative mechanism

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.

Activity dependence and functional role of the apamin-sensitive K+ current in rat supraoptic neurones in vitro

1. Intracellular recordings were obtained from seventy-two magnocellular neurosecretory cells (MNCs) in superfused explants of rat hypothalamus. The current underlying the after-hyperpolarization (IAHP) following spike-evoked trains of action potentials was characterized using the hybrid-clamp technique. The activity-dependent requirements for the genesis of the AHP were determined. The functional role of the conductance was investigated using saturating concentrations (50-300 nM) of apamin, a selective blocker of the AHP in MNCs. 2. IAHP was reversibly abolished by the removal of extracellular Ca2+. The amplitude of IAHP varied linearly as a function of voltage and reversed at -100 +/- 3 mV in 3 mM external K+. Changes in the concentration of extracellular K+ resulted in shifts of the reversal potential consistent with Nernst equation predictions for a K+-selective conductance. 3. Action potentials triggered by brief depolarizing pulses elicited an AHP during trains evoked at frequencies > 1 Hz. Onset of the AHP progressed exponentially, reaching a maximum after the first fifteen to twenty impulses. The steady-state amplitude of the AHP increased logarithmically between 1 and 20 Hz. 4. Switching to voltage clamp during periods of continuous cell activity (firing rate > 4 Hz) confirmed the presence of an apamin-sensitive Ca2(+)-dependent K+ current. 5. Application of apamin produced a threefold increase in the mean firing rate of spontaneously active cells, but was without effect when applied to silent cells (firing rate < 0.5 Hz). 6. Apamin did not affect the ability of MNCs to fire in a phasic manner but caused a dramatic increase in the mean intraburst firing rate. Moreover, inhibition of IAHP by apamin strongly attenuated spike accommodation normally seen at the onset of phasic bursts. 7. While apamin did not enhance the amplitude of depolarizing after-potentials following single spikes, post-train plateau potentials and associated after-discharges were enhanced. 8. The possible consequences of IAHP modulation are discussed in the context of the regulation of firing rate and pattern in MNCs.

Calbindin-D28k: role in determining intrinsically generated firing patterns in rat supraoptic neurones

1. Physiological activation of rat supraoptic nucleus (SON) neurones leads to phasic firing in vasopressin neurones and fast, continuous firing in oxytocin neurones. Using whole-cell patch clamp methods in brain slices, we investigated the role of endogenous calbindin-D28k (calbindin) in determining these intrinsically generated patterns of firing. 2. Direct introduction of calbindin (0.1-0.2 mM) into twelve of twelve phasically firing neurones suppressed Ca(2+)-dependent depolarizing after-potentials (DAPs) and changed activity from phasic to continuous firing. Bovine calcium binding protein (0.3 mM), an analogue of calbindin, had similar effects on both DAPs and firing patterns in five of five cells tested. 3. Introduction of anti-calbindin antiserum (1:2000-5000) into thirteen of thirteen continuously firing neurones unmasked DAPs and converted continuous into phasic firing. Such effects could not be mimicked either by diffusion of normal rabbit serum or antibodies directed against glial fibrillary acidic protein or against neurophysin. 4. Immunocytochemical staining with antisera directed against calbindin revealed more intense staining in the dorsal, oxytocin-rich and less intense staining in the ventral, vasopressin-rich areas of the SON. 5. Elevated intracellular Ca2+ concentration ([Ca2+]i; 0.1 mM) induced DAPs and phasic firing in all twenty-nine SON cells recorded. During chelation of intracellular Ca2+ with (1.1-11 mM) BAPTA, fifty-eight of fifty-eight neurones recorded displayed regular continuous activity and had no DAPs. 6. These data suggest that firing activities in SON cells are dependent on [Ca2+]i and that calbindin, acting as an endogenous Ca2+ buffer, is involved in regulation of intrinsic firing patterns. It is likely that calcium binding proteins have a similar influence on the firing patterns of many neuronal types throughout the nervous system.

Electrophysiological differences between oxytocin and vasopressin neurones recorded from female rats in vitro

1. Intracellular recordings in vitro from immunochemically identified oxytocin (OT) and vasopressin (VP) neurones in the supraoptic nucleus (SON) of virgin or lactating female rats revealed no differences between neurone types in membrane potential (Vm), input resistance and current-voltage relationships (I-V), when taken at resting membrane potentials. 2. When OT (94%), but not VP, neurones (93%) were current clamped at depolarized voltages (above -50 mV), small hyperpolarizing pulses revealed a time- and voltage-dependent outward rectification that was present above -75 mV and that decreased in amplitude as Vm approached the equilibrium potential for potassium (EK). The rectification was more pronounced when the neurones were held at a more depolarized membrane potential, and was larger the longer the neurone was held depolarized, reaching a maximum at 0.6-0.9 s. 3. A rebound depolarization followed the offset of hyperpolarizing pulses that were associated with the rectification. The peak amplitude of the rebound showed a time and a voltage dependence. It followed a bell-shaped curve as the hyperpolarizing commands were made larger, attaining a peak at -65 +/- 1.5 mV. The rebound amplitude increased with pulse duration, achieving a half-maximal amplitude at 0.5 +/- 0.1 s. 4. The expression of the sustained outward rectification and the rebound in OT neurones was similar in virgin and lactating female rats. 5. These results indicate the presence of significant differences in the intrinsic membrane properties, probably K+ currents, between OT and VP neurones in both lactating and virgin female rats.

Voltage-gated calcium currents in the magnocellular neurosecretory cells of the rat supraoptic nucleus

1. Whole-cell patch-clamp techniques were used to analyse voltage-dependent calcium currents in acutely isolated somata of magnocellular neurosecretory cells (MNCs) from the supraoptic nucleus of the hypothalamus of adult rats. Currents were characterized on the basis of their rates of inactivation and their sensitivity to a series of calcium channel blocking agents. 2. Curve fitting analysis of series of long lasting depolarizing voltage steps from a holding potential of -80 mV revealed three current components with different voltage dependences and rates of inactivation (n = 36). These include a low threshold (-60 mV), rapidly inactivating (tau = 42 +/- 3 ms at -10 mV) component, a high threshold (-30 mV), slowly inactivating (tau = 1790 +/- 70 ms) component and a component with an intermediate threshold (-50 mV) and rate of inactivation (tau = 187 +/- 15 ms). There is also a non-inactivating portion of evoked calcium current with a threshold of -50 mV. 3. Based on its voltage dependence, rate of inactivation, greater sensitivity to the divalent cation nickel than to cadmium and insensitivity to omega-conotoxin GVIA (omega-CgTX), the low threshold current appears to be a T-type calcium current. The rate of inactivation, voltage dependence, and sensitivity to omega-CgTX of the slowly inactivating component suggests that it is an N-type current. The characteristics of the intermediate component do not correspond to any identified calcium current type. 4. Portions of the non-inactivating calcium current are sensitive to nifedipine (23 +/- 2% of the total non-inactivating current at -10 mV; n = 10), suggesting the presence of L-type currents, omega-agatoxin-IVA (omega-Aga-IVA; 20 +/- 6% of total; n = 11), suggesting the presence of P-type channels, and omega-CgTX (39 +/- 3% of total; n = 19), suggesting the presence of a non-inactivating N-type current. The non-inactivating component at low potentials (> or = -50 mV) was selectively blocked by nifedipine, suggesting the presence of a novel, low threshold L-type current. 5. We conclude that MNC soma express T-, N-, L-, and P-type calcium currents, as well as a novel low threshold nifedipine-sensitive current and an unidentified inactivating component. This complement of currents is different from that seen in the terminals of these cells, suggesting a spatial and functional segregation of calcium current types in MNCs.

Regulation of spontaneous phasic firing of rat supraoptic vasopressin neurones in vivo by glutamate receptors

1. Vasopressin-secreting neurones in the rat hypothalamic supraoptic nucleus display patterned spontaneous phasic activity, which is apparently maintained in vivo through yet unidentified neurotransmitter system(s). The present investigation used extracellular recording techniques in anaesthetized Long-Evans rats to evaluate whether the neurotransmitter mechanism underlying phasic firing is provided via a family of ionotropic glutamate receptors. 2. N-Methyl-D-aspartate (NMDA) reliably evoked bursts of activity in twenty-seven of twenty-eight phasic neurones. Amino-3-hydroxy-5-methyl-isoxazole-4-propionic acid (AMPA) and kainate also elicited pronounced excitations in twenty-one of twenty-one and and fourteen of fifteen phasic cells, respectively. 3. A rapid blockade of on-going phasic activity was consistently induced following brief applications of both NMDA and non-NMDA receptor antagonists; extended application of antagonists resulted in prolonged silent periods, during which phasic activity failed to recur for minutes. Neither saline nor a cholecystokinin receptor antagonist influenced cell firing. 4. In contrast to putative vasopressin cells, application of NMDA receptor ligands did not affect the spontaneous activity in most putative oxytocin-secreting neurones, whereas kainate and AMPA potently excited seven of nine and four of five putative oxytocin cells, respectively. 5. These results imply that the maintenance of spontaneous phasic discharges in vivo in supraoptic vasopressin-secreting neurones requires tonic synaptic activation involving both NMDA and non-NMDA glutamate receptors. In putative oxytocin-secreting neurones, spontaneous firing appears to be predominantly regulated by non-NMDA receptors. Glutamatergic innervations may be in a unique position to influence the genesis of patterned electrical activity in supraoptic vasopressin neurones.

Control of luteinizing hormone-releasing hormone pulse generation in nonhuman primates

1. The pulsatile release of luteinizing hormone-releasing hormone (LHRH) is critical for reproductive function. However, the exact mechanism of LHRH pulse generation is unclear. The purpose of this article is to review the current knowledge on LHRH pulse generation and to discuss a series of studies in our laboratory. 2. Using push-pull perfusion in the stalk-median eminence of the rhesus monkey several important facts have been revealed. There is evidence indicating that LHRH neurons themselves have endogenous pulse-generating mechanisms but that the pulsatility of LHRH release is also modulated by input from neuropeptide Y (NPY) and norepinephrine (NE) neurons. The release of NPY and NE is pulsatile, with their pulses preceding or occurring simultaneously with LHRH pulses, and the neuroligands NPY and NE and their agonists stimulate LHRH pulses, while the antagonists of the ligands suppress LHRH pulses. 3. The pulsatile release of LHRH increases during the estrogen-induced LH surge as well as the progesterone-induced LH surge. These increases are partly due to the stimulatory effects of estrogen and progesterone on NPY neurons. 4. An increase in pulsatile LHRH release occurs at the onset of puberty. This pubertal increase in LHRH release appears to be due to the removal of tonic inhibition from gamma aminobutyric acid (GABA) neurons and a subsequent increase in the inputs of NPY and NE neurons to LHRH neurons. 5. There are indications that additional neuromodulators are involved in the control of the LHRH pulse generation and that glia may play a role in coordinating pulses of the release of LHRH and neuromodulators. 6. It is concluded that the mechanism generating LHRH pulses appears to comprise highly complex cellular elements in the hypothalamus. The study of neuronal and nonneuronal elements of LHRH pulse generation may serve as a model to study the oscillatory behavior of neurosecretion.

Effects of neurotensin on rat supraoptic nucleus neurones in vitro

1. The electrophysiological actions of neurotensin on magnocellular neurosecretory cells (MNCs) were examined during intracellular recording from seventy-three supraoptic nucleus neurones in superfused explants of rat hypothalamus. 2. Application of neurotensin tridecapeptide (NT(1-13); 1 nM to 3 microM) caused a membrane depolarization and reversibly attenuated the after-hyperpolarization (AHP) which followed current-evoked spike trains. This effect was accompanied by increased firing frequency during depolarizing current pulses evoked from a fixed potential. 3. The effects of neurotensin could be mimicked by the C-terminal fragment, NT(8-13), but not by the N-terminal fragment, NT(1-8). 4. Depolarizing responses to NT(1-13) or NT(8-13), retained during K+ channel blockade with internal Cs+, were accompanied by increased membrane conductance. Current- and voltage-clamp analyses revealed that neurotensin-evoked depolarizations result partly from the activation of a non-selective cationic conductance reversing near -34 mV. 5. Depolarizing responses to neurotensin were retained in the presence of TTX or in Ca(2+)-free solutions, indicating the involvement of receptors located on the plasma membrane of MNCs themselves. 6. Through these effects endogenously released neurotensin may modulate excitability, activity patterns and secretion from the hypothalamo-neurohypophysial axis.

Homogeneity of intracellular electrophysiological properties in different neuronal subtypes in medial preoptic slices containing the sexually dimorphic nucleus of the rat

The sexually dimorphic nucleus of the preoptic area (SDN-POA) is larger in male than in female rats, the male phenotype requiring the presence of circulating androgens perinatally. These experiments investigated the intracellular electrophysiology and morphology of SDN-POA neurons and compared these properties with those of other medial preoptic area (MPOA) neurons. Biocytin-injected cells in the SDN-POA either had one or two primary dendrites, or they had multipolar dendritic arrays; dendrites were aspiny or sparsely spiny and displayed limited branching. Neurons in other parts of the MPOA were similar morphologically. Regardless of morphology, neurons situated in either the SDN-POA or surrounding MPOA had low-threshold potentials and linear or nearly linear current-voltage relations. In most (73%) cells, stimulation of the dorsal preoptic region evoked a fast excitatory postsynaptic potential followed by a fast inhibitory postsynaptic potential (IPSP). Bicuculline blocked the fast IPSPs, which reversed near the Cl2 equilibrium potential (-71 +/- 5 mV), indicating their mediation by gamma-aminobutyric acid (GABA)A receptors. Neurons in the SDN-POA have electrophysiological properties similar to those of other medial preoptic cells. When compared with the hypothalamic paraventricular nucleus, the MPOA appears relatively homogeneous electrophysiologically. This is despite the morphological variability within this population of neurons and heterogeneities that are also apparent at other levels of analysis. Finally, GABA-mediated, inhibitory synaptic contacts are widespread among medial preoptic neurons, consistent with indications from earlier reports that GABA provides a link in the feedback actions of gonadal steroids on the release of gonadotropic hormones.

N-methyl-D-aspartate receptor antagonist ketamine selectively attenuates spontaneous phasic activity of supraoptic vasopressin neurons in vivo

Supraoptic neurosecretory neurons express a prominent N-methyl-D-aspartate receptor system. Recent in vitro evidence reveals that N-methyl-D-aspartate receptor activation dramatically alters the spontaneous discharge patterns of supraoptic neurons. In this study we evaluate whether N-methyl-D-aspartate receptors in vivo contribute to the development of characteristic phasic discharge patterns displayed by vasopressin-secreting neurons. Intravenous administration of ketamine hydrochloride, a non-competitive N-methyl-D-aspartate receptor antagonist, was used to examine whether N-methyl-D-aspartate receptor blockade influences patterned spontaneous discharge observed in supraoptic neurosecretory neurons. Extracellular recordings were obtained from identified hypothalamic supraoptic neurons in pentobarbital-anaesthetized Long-Evans rats. Systemic administration of ketamine (< or = 1.5 mg/kg) potently suppressed spontaneous phasic discharge in 16/19 putative vasopressin-secreting cells. The ketamine-induced blockade was dose dependent, fully reversible and was associated with the complete blockade of activity evoked by local pressure application of N-methyl-D-aspartate, but not the activity evoked by alpha-amino-3-hydroxy-5-methyl-isoxazole-4-propionate receptor agonists (6/6 cells). Ketamine had no detectable effect on threshold or shape of antidromic action potentials. By comparison, the activity in 9/10 continuously active neurons (putative oxytocin-secreting) was unaffected by administration of identical doses of ketamine. These data suggest that N-methyl-D-aspartate receptors play an important role in regulating the onset and maintenance of spontaneous phasic activity patterns displayed by rat supraoptic vasopressin neurons in vivo.

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)

Response of neurons in the dorsal motor nucleus of the vagus to thyrotropin-releasing hormone

Autonomic motoneurons in the dorsal motor nucleus of the vagus (DMX) were recorded intracellularly in an in vitro slice preparation of the guinea pig brainstem. Bath-applied thyrotropin releasing hormone (TRH) (1-10 microM) induced a reversible depolarization of neurons that was typically accompanied by an increase in the spontaneous firing of the cells. In some cells, TRH induced rhythmic bursting activity. The TRH-induced depolarization occurred also in the presence of reduced Ca2+ and TTX. The response was dose-dependent over TRH concentrations of 0.1-10 microM. The TRH-induced depolarization was accompanied by an increase in input resistance. The reversal potential of this effect corresponded to that of K+. Our results indicate that TRH increases the excitability of DMX neurons by reducing a resting K+ conductance.

Properties of supraoptic magnocellular neurones isolated from the adult rat

1. Magnocellular neurosecretory cells (MNCs) were isolated from the supraoptic nucleus of adult Long-Evans rats using an enzymatic procedure. Immunocytochemical staining with antibodies against vasopressin and oxytocin revealed that MNCs can be identified by size. The membrane properties of these cells were examined at 32-34 degrees C using intracellular recording methods. 2. Isolated MNCs displayed a mean (+/- S.E.M.; n = 109) resting membrane potential of -64.1 +/- 1.0 mV, an input resistance of 571 +/- 34 M omega, and a time constant of 8.7 +/- 0.4 ms. Measurements of specific resistivity and input capacitance revealed that the soma of these cells accounts for a mere 20% of their total somato-dendritic membrane in situ. 3. Voltage-current relations measured near -60 mV were linear negative to spike threshold. From more hyperpolarized membrane potentials, voltage responses to depolarizing current steps displayed transient outward rectification and delayed impulse discharge. 4. Action potentials (76.6 +/- 0.9 mV) triggered from an apparent threshold of -59.3 +/- 0.1 mV broadened progressively at the onset of spontaneous or current-evoked spike trains. Steady-state spike duration increased as a logarithmic function of firing frequency with a maximum near 25 Hz. These effects were abolished in Ca(2+)-free solutions. 5. In all cells, evoked spike trains were followed by a prolonged Ca(2+)-sensitive after-hyperpolarization. In contrast, only a small proportion (16%) of MNCs displayed spontaneous bursting activity or depolarizing after-potentials following brief current-evoked bursts. 6. Isolated MNCs responded to amino acids (glutamate and GABA) and to the neuropeptide cholecystokinin, indicating that receptors for these neurotransmitters are expressed postsynaptically by MNCs and are retained following dissociation. 7. Increasing the osmolality of the superfusing solution by 5-30 mosmol kg-1 caused a membrane depolarization associated with a decrease of input resistance and accelerated spontaneous spike discharge in each of thirty-six MNCs tested. Current-clamp analysis suggested that these responses resulted from the activation of a cationic conductance. Excitatory effects of hyperosmolality were not observed in non-magnocellular neurones (n = 6).

Electrophysiology of guinea-pig supraoptic neurones: role of a hyperpolarization-activated cation current in phasic firing

1. Immunocytochemically identified magnocellular neurosecretory cells (MNCs) in the guinea-pig supraoptic nucleus (SON) were studied using the in vitro intracellular recording technique. Cells were identified as containing arginine vasopressin (AVP) or oxytocin (OT) following recordings made with biocytin-filled electrodes. Both AVP and OT MNCs demonstrated a fusiform or pyramidal shape (15-20 microns by 26-39 microns), with two to three processes. There were no significant differences in the proportion of AVP and OT cells in the retrochiasmatic (caudal) versus the rostral slices. 2. No significant differences in passive membrane properties were observed between AVP and OT cells, except that AVP cells exhibited a significantly broader action potential width (1.51 +/- 0.1 ms, n = 11) than did OT cells (1.01 +/- 0.08 ms, n = 7). 3. Firing patterns were recorded for 100 MNCs, 41% of which fired in a phasic manner (repeated clustering of action potentials into bursts). Of the seventy-seven cells which were immunocytochemically identified, only AVP-containing MNCs displayed phasic firing. Phasic firing occurred only in MNCs demonstrating a depolarizing potential which followed hyperpolarizing after-potentials (HAPs). The presence of the depolarizing potential was not always associated with phasic firing, however, as both OT cells and non-phasic AVP cells sometimes exhibited a depolarizing potential. 4. In 160 MNCs examined for the presence of the time-dependent inward rectification (TDR in current clamp, or Ih in voltage clamp), a significant difference in the proportion of cells expressing the Ih was observed in the two cell types. The Ih was expressed in forty-five of fifty-four AVP MNCs (83%) and in six of fifteen OT MNCs (40%). No significant association was found with firing pattern. 5. The Ih exhibited properties similar to those found in other CNS and peripheral tissues. It appeared on steps to potentials more hyperpolarized than -65 mV. It was augmented by raising the extracellular potassium concentration, blocked by 2 mM CsCl, and insensitive to 100-500 microM BaCl2. Activation followed a single exponential, and the time constant of activation was voltage dependent. 6. The adenylate cyclase activator forskolin increased the Ih and shifted its activation curve to more depolarized levels. In cells recorded for several hours, the Ih varied in amplitude, suggesting intrinsic modulation, possibly by intracellular second messenger systems. The Ih in guinea-pig SON MNCs appears to serve an excitatory role, bringing cells closer to firing threshold.

Excitatory effects of thyrotropin-releasing hormone on neurons within the nucleus ambiguus of adult guinea pigs

The electrophysiological effects of thyrotropin-releasing hormone (TRH) on neurons within the nucleus ambiguus (NA) of adult guinea pigs were studied using an in vitro brain stem slice preparation. In 0.01-1.0 micron TRH, NA neurons depolarized (25/39), expressed enhanced postinhibitory rebound (8/8 tested), or exhibited oscillations of the membrane potential (17/39). Because the amplitude of postinhibitory rebound in tetrodotoxin (TTX) at various membrane potentials was not altered by TRH, it suggests that TRH enhanced postinhibitory rebound indirectly by depolarizing the cell membrane. The membrane potential oscillations in NA neurons were persistent in TTX and their frequency was dependent on the membrane potential, suggesting that these oscillations were due to intrinsic membrane properties and not to synaptic inputs. The excitation of NA neurons in vitro by TRH suggests that endogenous TRH may modulate the activity of neurons involved in the regulation of respiratory and autonomic function.

Spontaneous and stimulated firing in cultured rat suprachiasmatic neurons

Neurons from the suprachiasmatic nucleus (SCN) of the hypothalamus, the site of a circadian pacemaker in mammals, were isolated from embryonic rat. After mechanical dissociation neurons were brought into culture for 1-2 weeks, using a chemically defined medium. Recordings were made from 74 bipolar neurons using two different configurations of the patch-clamp technique. During cell attached patch recordings, 45% of neurons fired spontaneously. The mean firing rate was 0.7 +/- 0.6 Hz and the firing pattern was irregular. In whole cell recordings 73% of the investigated neurons showed spontaneous activity with an irregular firing pattern. The mean spontaneous firing rate with an intracellular Cl- concentration of 145 mM was 1.0 +/- 0.6 Hz. The resting membrane potential of the bipolar neurons was estimated to be -62 +/- 24 mV. An intracellular Cl- concentration of 145 mM depolarised the membrane potential. It also increased the probability of spontaneous firing. A depolarising current stimulus produced an action potential with a threshold voltage of -46 +/- 9 mV. Suprathreshold stimuli resulted in repetitive firing with a mean frequency of 12 +/- 4 Hz. The minimum interspike interval was 52 +/- 14 ms. All action potentials either occurring spontaneously or elicited by current stimuli were abolished by the Na(+)-channel blocker TTX. These results indicate that our cultured neurons have some electrophysiological properties in common with SCN neurons in brain slices and in vivo.

Immunohistochemical differentiation of electrophysiologically defined neuronal populations in the region of the rat hypothalamic paraventricular nucleus

Intracellular recording and labeling were combined with neurophysin immunohistochemistry to study neurons in the paraventricular nucleus region of the rat hypothalamus. Neuronal membrane properties were examined in hypothalamic slices, and cells were labeled by injecting biocytin or Lucifer yellow. Slices were then embedded, sectioned, and immunohistochemically processed for neurophysin. Immunoreactivity patterns, and in some cases counterstaining, enabled determinations of the cytoarchitectonic positions of recorded cells to be made. Recorded cells were divided into three types according to their electrophysiological characteristics. The first type lacked low-threshold Ca2+ spikes and displayed linear current-voltage relations, a short time constant, and evidence for an A current. These were relatively large cells that were typically immunoreactive for neurophysin and were situated near other neurophysin-positive neurons. The second type had relatively small low-threshold potentials that did not generate bursts of Na+ spikes. These cells had heterogeneous current-voltage relations and intermediate time constants. They did not label for neurophysin, and most were located in the parvicellular subregion of the paraventricular nucleus. The third type had large low-threshold Ca2- spikes that generated bursts of Na+ spikes, and these cells had nonlinear current-voltage relations and long time constants. These neurons were dorsal or dorsolateral to the paraventricular nucleus and were not immunoreactive for neurophysin. These results indicate that paraventricular magnocellular neurons lack low-threshold potentials, whereas paraventricular parvicellular neurons display low-threshold potentials that generate one or two action potentials. Neurons that fire spike bursts from low-threshold potentials are adjacent to the paraventricular nucleus, confirming earlier reports.

Intracellular study of calcium-related events in cat magnocellular neuroendocrine cells

1. Magnocellular neuroendocrine cells (MNCs) in the supraoptic nucleus (SON) of mammals synthesize vasopressin or oxytocin and release these hormones systemically from their neurohypophysial axon terminals. In the rat, release is facilitated by bursts of action potentials generated by the MNC. However MNC units in the intact cat discharge more slowly and do not display the repetitive bursts (phasic firing) that promote vasopressin secretion. The reasons why these cat endocrine neurones differ so dramatically in their firing behaviour from the rat model were examined using intracellular recording. 2. Cat and rat MNCs displayed similar mean resting potentials approximating -60 mV, and were usually linear in their voltage-current relationship in the hyperpolarizing direction. However cat MNCs displayed a higher mean cell input resistance (301 M omega; n = 56) than those of rat (150 M omega; n = 105). 3. Calcium influx to cat MNCs during firing appeared comparable to rat based on (a) the similar range of action potential broadening observed during a spike train, (b) the shoulder on the action potential's falling phase which was blocked in low-Ca2+ saline, and (c) the ability to evoke tetrodotoxin (TTX)-insensitive spiking and non-synaptic depolarizing potentials, both calcium-mediated events observed in the rat. 4. In cat MNCs, a depolarizing current pulse (100-500 ms; 0.1-0.3 nA) elicited a train of action potentials followed by a prominent after-hyperpolarization (AHP) several times the duration of its counterpart in the rat. The AHP reversed near the equilibrium potential for K+, was not voltage dependent and represented an increased membrane conductance. It was suppressed in low-Ca2+ saline and completely eliminated by the calcium-activated potassium current (IK(Ca)) blockers apamin (100 nM) or d-tubocurarine (50-200 microM). Both blockers decreased spike frequency adaptation but did not induce bursting. Therefore the cat AHP probably represents a Ca(2+)-activated K+ conductance with a similar blocker sensitivity to its briefer counterpart in the rat MNC. 5. The spike hyperpolarizing after-potentials (HAPs) in cat were more than twice the mean amplitude and several times the duration of HAPs in rat. Cat HAPs were qualitatively similar to their rat counterparts, remaining unaffected by apamin or tubocurarine. The intrinsic currents responsible for the AHP and HAP appear to generate the stronger activity-dependent inhibition displayed by cat MNCs. 6. Twenty-one of fifty-two cat MNCs displayed an inward rectification at membrane potentials more negative than -70 mV ([K+]o = 6.24 mM), causing a depolarizing 'sag' in the voltage trajectory lasting 100-200 ms which was TTX resistant.(ABSTRACT TRUNCATED AT 400 WORDS)

Electrophysiological properties of neurones in the region of the paraventricular nucleus in slices of rat hypothalamus

1. Neurones in the region of the hypothalamic paraventricular nucleus (PVN) of the rat were studied with intracellular recording in the coronal slice preparation. Three types of hypothalamic neurones were distinguished according to their membrane properties and anatomical positions. Lucifer Yellow or ethidium bromide was injected intracellularly to determine the morphology of some recorded cells. 2. The most distinctive electrophysiological characteristic was the low-threshold depolarizing potentials which were totally absent in type I neurones, present but variable in type II neurones and very conspicuous in type III neurones. Type II neurones generally showed relatively small low-threshold depolarizations (26.5 +/- 2.2 mV) which generated at most one to two action potentials. Type III neurones, on the other hand, generated large low-threshold potentials (40.3 +/- 2.8 mV) which gave rise to bursts of three to six fast action potentials. Deinactivation of the low-threshold conductance in both type II and type III neurones was voltage- and time-dependent. Low-threshold potentials persisted in TTX (1-3 microM) but were blocked by solutions containing low Ca2+ (0.2 mM) and Cd2+ (0.5 mM), suggesting they were Ca(2+)-dependent. 3. Type I neurones had a significantly shorter membrane time constant (14.5 +/- 1.7 ms) than those of type II (21.6 +/- 1.7 ms) and type III neurones (33.8 +/- 4.4 ms). Input resistance and resting membrane potential did not differ significantly among the cell groups. 4. Current-voltage (I-V) relations were significantly different among the three cell types. Type I neurones had linear I-V relations to -120 mV, while type III neurones all showed marked anomalous rectification. I-V relations among type II neurones were more heterogeneous, although most (75%) had linear I-V curves to about -90 to -100 mV, inward rectification appearing at more negative potentials. 5. Type I neurones generated fast action potentials of relatively large amplitude (64.2 +/- 1.1 mV, threshold to peak) and long duration (1.1 +/- 0.1 ms, measured at half-amplitude); the longer duration was due to a shoulder on the falling phase of the spike. Type II neurones had large spikes (66.5 +/- 1.6 mV) of shorter duration (0.9 +/- 0.1 ms) with no shoulder. Type III neurones had relatively small spikes (56.1 +/- 2.2 mV) of short duration (0.8 +/- 0.1 ms) with no shoulder. 6. The three cell populations showed different patterns of repetitive firing in response to depolarizing current pulses. Type I neurones often generated spike trains with a delayed onset and little spike-frequency adaptation.(ABSTRACT TRUNCATED AT 400 WORDS)

Neuroendocrine rhythms

Hormones are secreted with circhoral, circadian and seasonal periodicities. Circhoral pulsatility is a temporal code, many chronic and acute changes in neuroendocrine status being mediated by changes in the frequency of circhoral release. The identity of the neuronal circuits controlling circhoral release is not known. Circadian release of hormones occurs with a precise temporal order entrained to the light-dark cycle, synchronized to the activity/rest rhythm and generated by circadian oscillators, of which the suprachiasmatic nuclei are the most important. Seasonal rhythms are driven either by an endogenous circannual clock mechanism or by a process of photoperiodic time measurement which is dependent upon the duration of the nocturnal peak of the pineal hormone melatonin.

A technique for controlling the membrane potential of neurons during unit recording

While extracellular unit recordings are technically easier to perform than intracellular recordings, the latter permits control of neuron excitability by changing the membrane potential with steady current passed across the recording micropipette. Utilizing an intracellular amplifier with bridge circuit for the passage of low current (1-10 nA), we show that an extracellular recording micropipette of high resistance (5-20 M omega) can be used in a similar fashion while monitoring single unit activity in the rat brain. This procedure allows fine control of two neuronal mechanisms which are voltage-sensitive: action potential discharge and regenerative bursting. Control of a neuron's membrane potential expands the capabilities of unit recording in the study of evoked synaptic input and bursting behaviour. It can also facilitate classification of neurons based on their firing characteristics.

Voltage-clamp recordings from identified dissociated neuroendocrine cells of the adult rat supraoptic nucleus

In vitro intracellular recordings of membrane potential obtained from the oxytocin and vasopressin neurons of the mammalian hypothalamo-neurohypophysial system in slices (1-3) and expiants (4, 5) have demonstrated many of the intrinsic properties of these magnocellular neuroendocrine cells (MNCs). Voltage-clamp techniques, which are required to study directly the currents underlying intrinsic or transmitter-evoked potential changes, have been applied to cultured embryonic (6) or neonatal supraoptic neurons (7-9) and have been successfully applied to adult supraoptic neurons in situ in only one laboratory (10, 11). We have modified a technique for dissociation of viable adult guineapig hippocampal neurons (12) to dissociate supraoptic MNCs from adult rats for voltage-clamp studies. MNCs were selectively labelled with a fluorescent dye in vivo so that they could be identified after dissociation and prior to making recordings. These data have been published in abstract form elsewhere (13, 14).

Discharge induction in molluscan peptidergic cells requires a specific set of autoexcitatory neuropeptides

The peptidergic caudodorsal cells of the pond snail Lymnaea stagnalis generate long lasting discharges of synchronous spiking activity to release their products. During caudodorsal cell discharges a peptide factor is released which induces similar discharges in silent caudodorsal cells [Ter Maat A. et al. (1988) Brain Res. 438, 77-82]. To identify this factor, the electrophysiological effects of putative caudodorsal cell gene products, calfluxin, caudodorsal cell hormone, four alpha caudodorsal cell peptides and three beta caudodorsal cell peptides, were tested individually and in various combinations. Calfluxin, alpha caudodorsal cell peptide and beta 1 caudodorsal cell peptide each had no effect on membrane potential or excitability of the caudodorsal cells. All other caudodorsal cell peptides caused excitatory responses, but did not induce discharges. Instead, only a specific combination of four caudodorsal cell peptides, caudodorsal cell hormone and alpha caudodorsal cell peptide (1-11, 3-11 and 3-10), evoked caudodorsal cell discharges with similar characteristics to electrically evoked discharges. Incomplete versions of this combination failed to cause a discharge. In addition, antibodies to caudodorsal cell hormone or alpha caudodorsal cell peptide reduced caudodorsal cell excitability and prevented the generation of discharges by electrical stimulation. These results suggest that excitatory autotransmission caused by four caudodorsal cell peptides provides a means to amplify excitatory inputs, thus leading to the generation of the all-or-nothing caudodorsal cell discharge.

Ionic basis for the intrinsic activation of rat supraoptic neurones by hyperosmotic stimuli

1. Magnocellular neurosecretory cells (MNCs) were impaled in the supraoptic nucleus of rat hypothalamic explants maintained in vitro. Current- and voltage-clamp analysis of the osmotically induced response was performed at 34 degrees C. 2. Addition of mannitol or NaCl to cause a rise in fluid osmolarity (greater than +6 mosM) caused a membrane depolarization whose amplitude increased when elicited from more hyperpolarized levels. Changes in temperature (34-28 degrees C), addition of TTX, or superfusion with Na(+)-free or Ca2(+)-free solutions did not block the osmotically induced depolarization. In control solutions the response was consistently accompanied by an increase in the frequency of spontaneous postsynaptic potentials. Thus, osmotic stimuli have a direct effect on MNCs, and they also apparently activate other neurones which are presynaptic to these cells. 3. Under voltage-clamp, hyperosmotic stimuli induced an inward current (Io) accompanied by an increase in membrane conductance. The current was unaffected or slightly enhanced by doubling the external K+ concentration. Io was also characterized by a linear I-V relation (between -100 and -50mV) and an extrapolated reversal potential near -10 mV. Io presumably results from the activation of a voltage-independent and non-selective cationic conductance. 4. Hyperosmotic stimuli did not affect the depolarizing after-current (IDAP) responsible for the production of phasic bursts. However, the inward shift of the post-spike I-V curve caused by Io could reduce or eliminate the region of net outward current which lies negative to spike threshold in silent neurones. Thus in MNCs displaying IDAP, activation of Io by a rise in osmotic pressure can induce or enhance phasic bursting activity. 5. Application of hyperosmotic stimuli sufficient to excite most MNCs (+20 to +80 mosM) did not elicit a response from any of seventeen neurones impaled in areas lateral and caudal to the supraoptic nucleus. Recordings obtained from three CA1 neurones in slices of rat hippocampus revealed that stimuli in excess of +100 mosM are required to evoke appreciable non-specific depolarizations. 6. These studies indicate that the specific endogenous osmosensitivity of MNCs results from the activation of the intrinsic current Io. Furthermore, interactions between Io and IDAP explain how osmotic stimuli can lead to the induction of phasic bursting activity, a response which is known to potentiate the secretion of vasopressin from the neural lobe.

Vasopressin-sensitive neurons in the lateral paraventricular nucleus area in a guinea pig slice preparation

The effects of vasopressin (VP) on hypothalamic neurons located in the region of the paraventricular nucleus (PVN) were analyzed using intracellular techniques in slices of guinea pig brains. Two different classes of neurons were electrophysiologically identified in the magnocellular lateral part of the PVN and in the adjacent area. In the former area, vasopressinergic neurons were identified according to their phasic activity and their endogenous properties. These neurons were not responsive to VP, applied through the perfusion medium or locally by pressure. On the other hand, nonmagnocellular neurons exhibiting low-threshold Ca2+ spikes (LTS) were recorded in the area adjacent to the lateral part of the PVN. LTS were deinactivated at hyperpolarized membrane levels and induced short bursts of action potentials. On these neurons, VP evoked depolarizations accompanied by increases in firing, without modification of membrane resistance. VP effects were not blocked by TTX, suggesting a postsynaptic action of the peptide. These data indicate that VP controls the firing pattern of LTS neurons and suggest that this action may involve collaterals of axons originating from neighbouring vasopressinergic neurons.

Quantitative Comparisons Between the Electrical Activity of Supraoptic Neurons and Vasopressin Release in vitro

Abstract The firing rate of antidromically identified neurons in the tuberal portion of the rat supraoptic nucleus and vasopressin release were compared in an in vitro preparation of the hypothalamo-neurohypophysical system. Extracellular potentials were isolated and held while the perifusing medium was collected for radioimmunoassay of vasopressin. Neurons in the tuberal portion of the supraoptic nucleus were induced to first bursts of action potentials by injecting supra-threshold concentrations (0.01, 0.1 and 1 mM) of the alpha(1)-agonist phenylephrine into the perifusing line. Phenylephrine caused a dose-dependent release of vasopressin into the perifusate and a dose-dependent increase in the peak firing rate, initial burst duration and the total number of action potentials (compared to a 10 min control period) of the recorded neurons. Peak firing rate and peak vasopressin release both increased linearly with the log of the phenylephrine dose, but the efficiency ratio of the increase in vasopressin release to the increase in firing rate was greater from the middle to the highest dose than it was for the lowest to the middle dose, indicating a possible facilitation of hormone release with higher firing frequencies. The total amount of vasopressin released in 10 min post-injection and the total number of evoked action potentials in the same period also increased linearly, but in this case there was no change in the efficiency ratio between the low to middle and the middle to high doses of phenylephrine. Release of vasopressin to phenylephrine was dependent on an intact neural stalk, and in a separate group of isolated neural lobes, 1 mM phenylephrine did not significantly alter the peak or total amount of vasopressin released to an electrical stimulus given in the pattern of an evoked burst. In another group of explants the temporal relationship between firing rate and vasopressin release was examined after a single dose of 1 mM phenylephrine. In these explants the true time-course of release was estimated using a deconvolution procedure which corrected for diffusion of the hormone. Mean vasopressin release and spike activity were still elevated above baseline 4 to 5 min after the first elicited burst, suggesting that the latter parts of the initial burst and/or the after-discharges contribute to the prolonged vasopressin release. However, there was a dramatic drop in the efficiency ratio (vasopressin release/peak firing rate) from the first to the second minute following stimulation. Thus, changes in the frequency of action potentials generated by supraoptic neurons can be directly related to simultaneous changes in the rate of vasopressin release in the same preparation. The results suggest that elevations in firing rate are accompanied by an increased rate of vasopressin release, but as has been demonstrated in isolated neural lobes, this relationship is probably not constant across different stimulation strengths or within a single burst.

Intrinsic and synaptic mechanisms of hypothalamic neurons studied with slice and explant preparations

The use of slice and explant preparations has allowed major advances in our understanding of the membrane physiology of mammalian hypothalamic neurons. This article will review intracellular electrophysiological studies of neurons in or immediately surrounding the supraoptic and paraventricular nuclei. Considerable information is now available on the intrinsic membrane mechanisms that control action potential generation and burst firing in magnocellular neuroendocrine cells (MNCs) within these nuclei. Neurons surrounding the paraventricular nucleus have different electrical properties than the MNCs, including low-threshold Ca2+ spikes and pronounced anomalous rectification. Bicuculline and kynurenic acid strongly depress fast IPSPs and EPSPs in MNCs, thus suggesting that inhibitory and excitatory amino acids mediate fast synaptic transmission in the hypothalamus. The effects of neuromodulators, such as noradrenaline and opioid peptides, have also been examined. Noradrenaline excites supraoptic neurons and leads to phasic firing through an alpha-1 mechanism and decreased K+-conductance. Opioid peptides act directly on mu-receptors to hyperpolarize about half of the neurons through an increased K+-conductance. In conclusion, using the magnocellular neuroendocrine system as a model, in vitro slice and explant preparations have allowed the characterization of electrophysiological properties, the identification of neurotransmitters for synaptic events, and studies on the mechanism of action of neuromodulators.

Repetitive burst-firing neurons in the deep layers of mouse somatosensory cortex

Intracellular recordings were made from neurons of the mouse somatosensory cortex isolated in vitro. Two physiologically distinct classes of pyramidal cells were observed: regular-spiking cells were the majority, and generated accommodating trains of single spikes; bursting cells generated clusters of 2-5 action potentials, and clusters were separated by prolonged afterhyperpolarizations. The bursting cells of the mouse neocortex were unusual in producing repetitive bursting during sustained current pulses, and in being localized to a laminar zone straddling layers V and VI.

Quisqualate receptor-mediated rhythmic bursting of rat hypothalamic neurons in dissociated cell culture

Dissociated hypothalamic neurons from embryonic rat brain exhibit a level of spontaneous synaptic activity after 21 days in culture. When GABA-mediated responses are blocked by picrotoxin or bicuculline (20 microM), the neurons burst rhythmically. Rhythmic burst activity is generated in most cells by postsynaptic excitatory currents (EPSCs) through non-specific cationic channels rather than by intrinsic pacemaker currents. We present evidence that EPSCs are mediated by an excitatory amino acid and a quisqualate receptor type.

Ionic currents in cultured supraoptic neurons: actions of peptides and transmitters

The hypothalamo-neurohypophysial system has proved an excellent model for peptidergic neurons in the central nervous system. Electrophysiological studies using in vivo and in vitro preparations with extracellular and intracellular recording techniques have determined some of the intrinsic and extrinsic mechanisms that generate the striking firing patterns that the neurons exhibit. We have developed a dissociated cell preparation of these neurons and used patch clamp recording techniques to enable detailed studies of membrane properties underlying such activities. Cultured neonatal supraoptic neurons fired spontaneous action potentials which in some cells were distinctively patterned. Under voltage clamp, voltage-activated Na+, K+, and Ca2+ currents were recorded. K+ and Ca2+ currents were modulated by application of alpha-adrenergic agonists, and Ca2+ currents were also modulated by kappa-opioid agonists. The neurons were also sensitive to gamma-aminobutyric acid which acted directly on Cl- channels. Spontaneous, patterned activity, the presence of functional receptors for neurotransmitters and the ability to study the neurons under voltage clamp suggest that this is an excellent model system for studying these peptidergic neurons.

Dynorphin (1-8) inhibits stimulated release of oxytocin but not vasopressin from isolated neurosecretory endings of the rat neurohypophysis

The effects of the opioid peptide dynorphin (1-8) on oxytocin and vasopressin release at the level of isolated neurosecretory endings were investigated. Neurosecretory endings prepared by homogenization and centrifugation were placed on a filter and constantly superfused. Stimulated hormone release was evoked by potassium depolarization (30 mM) and simultaneous increase of the osmolarity (20 mosmol/1). Stimulation resulted in two peaks of hormone release--a short first peak and a longer second one. Addition of dynorphin (1-8) (10(-7) M) to the superfusion buffer significantly diminished the first peak of oxytocin release and totally abolished the second. There was no effect of dynorphin (1-8) on vasopressin release.

Transient calcium-dependent potassium current in magnocellular neurosecretory cells of the rat supraoptic nucleus

1. Magnocellular neurosecretory neurones were impaled in the supraoptic nucleus of perfused explants of rat hypothalamus. Membrane currents were studied at 35 degrees C using the single-microelectrode voltage-clamp technique. 2. Depolarizing voltage steps applied from -100 mV evoked a transient outward current (TOC) from a threshold of -75 mV. From this potential, the amplitude of the current increased non-linearly with voltage. 3. Following its activation TOC reached a peak within 7 ms and subsequently decayed monotonically with a time constant of 30 ms. This time constant did not vary significantly with voltage between -75 and -55 mV. 4. The TOC showed complete steady-state inactivation at potentials positive to -55 mV. Inactivation was removed by hyperpolarization, with a mid-point near -80 mV. The removal of inactivation followed a complex time course with distinct fast (tens of milliseconds) and slow (hundreds of milliseconds) components. 5. Tail current measurements revealed that the TOC equilibrium potential (ETOC) lies near -97 mV in the presence of 3 mM [K+]o. Increasing [K+]o caused a decrease of TOC amplitude and a shift in ETOC of 57 mV/log [K+]o. The TOC is therefore predominantly a K+ current. 6. The TOC was unaffected by tetraethylammonium (up to 12 mM) but was reversibly reduced by 4-aminopyridine (ca. 50% block at 1.0 mM) and dendrotoxin (ca. 50% block at 4 nM). 7. The TOC was strongly inhibited (greater than 70%) by adding Co2+ or Mn2+ (1-3 mM) or Cd2+ (50-400 microM) to Ca-containing solutions, or by removal of Ca2+ from the perfusate. These effects were not accompanied by detectable changes in threshold voltage. The amplitude of TOC was also depressed by the organic Ca2+ channel blocker methoxyverapamil (D600). Finally replacement of Ca2+ by Ba2+ in the perfusate completely and reversibly abolished the TOC. 8. These findings suggest that neurosecretory neurones of the rat supraoptic nucleus display a transient K+ current which is strongly dependent on the presence of external Ca2+. The possible role of this current is briefly discussed.

Chemically mediated positive feedback generates long-lasting afterdischarge in a molluscan neuroendocrine system

The peptidergic neuroendocrine caudodorsal cells (CDCs) of Lymnaea stagnalis control egg laying. The CDC network consists of 100 electrotonically coupled neurons that form two clusters in the cerebral ganglia. Upon prolonged, repeated, intracellular stimulation of one CDC, excitation spreads over the network and leads to a 30-min period of spiking activity: the afterdischarge. During the afterdischarge a number of peptides, including the ovulation hormone, are released. When two ganglia rings from different animals were pinned down next to each other, an afterdischarge initiated in the CDCs of one CNS activated the CDCs of the other CNS, indicating that excitation spreads in the absence of physical contact between the CDCs. A single isolated intercerebral commissure (COM), the neurohaemal area of the CDCs, displayed the same discharge-inducing capability when brought in the vicinity of a second, intact, CNS. Other parts of the CNS did not possess this property. CDC afterdischarges could also induce repetitive spiking in adjacent isolated CDC somata showing that the effect can be directly on the CDCs themselves. The discharge-inducing factor was well separated from the ovulation hormone on a Bio-Gel P-6 column. The factor was pronase-degradable and inhibitors of proteolytic enzymes increased the factor's longevity. It is concluded that, contingent upon the CDC-discharge, a small (less than or equal to 1500 Da) excitatory peptide is released that acts directly on the CDCs. Its function is argued to be: (1) the spread of excitation from a subset of CDCs, receiving external input, over the entire CDC network; and (2) to provide a positive feedback to generate a maximum (all-or-none) response.

Apamin and d-tubocurarine block the afterhyperpolarization of rat supraoptic neurosecretory neurons

Magnocellular neurosecretory cells (MNCs) were impaled in perfused explants of rat hypothalamus. Evoked bursts of spikes were followed by an afterhyperpolarization (AHP) lasting 1-2 s. In each of 22 cells this AHP was selectively suppressed by low nanomolar concentrations of apamin (IC50 = 1.3 nM) or micromolar concentrations of d-tubocurarine (IC50 approximately 40 microM). Blockade of the AHP was accompanied by a decrease in spike accommodation and an unmasking of the late depolarizing afterpotential which induces burst firing in MNCs. Modulation of the Ca2+-dependent K+ (AHP) conductance by an endogenous apamin-like ligand could play an important role in the control of firing rate and pattern in MNCs.

Low-threshold calcium spikes in hypothalamic neurons recorded near the paraventricular nucleus in vitro

From guinea-pig hypothalamic slices, intracellular studies demonstrate the existence of neurons responding to depolarizing current pulses by bursts of fast spikes riding on slow depolarizing potentials, when activated at the resting potential or from hyperpolarized levels (44 cells). Slow depolarizing potentials have a mean amplitude of 17.6 mV and a mean duration of 65.2 msec. They are also produced at the termination of hyperpolarizing current pulses. The ionic basis for these slow potentials have been investigated. Fast spikes constituting the burst discharge are blocked by TTX but the slow component is unaffected, being blocked by Co++ and enhanced by Ba++. Taken together, these results show that the slow depolarizing potentials are generated by a low-threshold Ca++ conductance which is de-inactivated by membrane hyperpolarization. When the neurons are spontaneously active, they exhibit bursts arising from slow depolarizing potentials reminiscent of those evoked by direct stimulation. They also show longer episodes of repetitive firing. Twelve neurons were intracellularly stained and were found in the periphery of the paraventricular nucleus (PVN), in close proximity to the groups of neurophysin-positive neuroendocrine neurons present in the lateral part of this nucleus. Injected neurons have the morphology of reticular cells, judging by their few multipolar, rectilinear and sparsely branched dendrites. Some of their processes are directed towards PVN. Because of their intrinsic electrophysiological properties and their possible relationships with PVN, the population of cells described in the present study may play a role in functions relating to the PVN.

Actions of gamma-aminobutyric acid on rat supraoptic nucleus neurosecretory neurones in vitro

1. Intracellular recordings were obtained from thirty-eight rat supraoptic nucleus (s.o.n.) neurosecretory neurones in perfused hypothalamic explants. Changes in membrane potential and conductance were monitored following application of gamma-aminobutyric acid (GABA), and related agonists and antagonists. 2. GABA depressed action potential discharge of all of thirty-five s.o.n. neurones tested and induced either membrane hyperpolarization or depolarization. Neurones that displayed membrane hyperpolarization in response to lower GABA concentrations (30-300 microM) demonstrated a biphasic membrane voltage change with a later depolarizing phase as a response to higher concentrations (up to 3000 microM). 3. GABA (10-3000 microM) induced a prominent concentration-dependent increase in membrane conductance in all neurones. The critical slope for the log-log plot of [GABA] vs. GABA-induced membrane conductance was 1.7, indicating co-operativity in the GABA receptor-induced conductance change. 4. Muscimol (0.3-30 microM) potently mimicked all the effects of GABA. Bicuculline (1-100 microM) antagonized the effects of GABA and muscimol in a competitive manner. 5. Glycine and taurine (1-10 mM) had weak effects, although comparatively similar to those of GABA. These actions were blocked both by bicuculline (100 microM) and by strychnine (1 microM). At higher concentrations (greater than 10 microM), strychnine also antagonized the actions of GABA. 6. In recordings with potassium-acetate-filled micropipettes, the reversal potential of hyperpolarizing membrane voltage responses to GABA was -72.5 +/- 1.5 mV in close agreement (+/- 5 mV) with the reversal potential of inhibitory post-synaptic potentials (i.p.s.p.s) recorded in the same neurones. Depolarizing responses to GABA reversed polarity at -50 +/- 1.6 mV. In recordings with KCl-filled micropipettes, voltage responses to GABA were always depolarizing and reversed near -40.0 +/- 4.3 mV. Similarly, reduction of the concentration of chloride ions in the perfusion medium from 134 to 10.4 mM induced a positive shift of the GABA reversal potential by 40-50 mV. 7. From measurements of input resistance (Rin) and cell time constant (tau O), input capacitance (Cin; representing total membrane capacitance) was calculated as 78.9 +/- 2.1 pF. During responses to GABA or muscimol, decreased Rin was accompanied by a linearly related decrease in tau o indicating that these substances had no effect on the membrane capacitance of s.o.n. neurones.(ABSTRACT TRUNCATED AT 400 WORDS)

Whole cell voltage clamp recordings from cultured neurons of the supraoptic area of neonatal rat hypothalamus

Whole-cell voltage and current clamp recordings were made from cultured neurons from the hypothalamic supraoptic area of neonatal rats. These neurons fired spontaneous Na+-action potentials, appearing as mixed Na+/K+-currents in voltage clamp. Isolated Na+-currents (less than 3 nA) were rapidly activated and inactivated during positive potential pulses from -80 mV. Two voltage-activated Ba2+-currents (less than 1 nA) were also recorded. These techniques offer a promising new approach for studying the striking electrical behavior of cultured hypothalamic neurons.

Endogenous bursting by rat supraoptic neuroendocrine cells is calcium dependent

1. Phasic bursting by magnocellular neuroendocrine cells (m.n.c.s) in vivo causes increased vasopressin release from axon terminals in the neurohypophysis. In the supraoptic nucleus of the coronal hypothalamic slice thirty-two of sixty-five m.n.c.s recorded intracellularly displayed repetitive bursting, either spontaneously or during a low level of tonic current injection. 2. Of the thirty-two repetitive bursters, twenty-four received no apparent patterned synaptic input and the phasic burst behaviour was voltage dependent. The evidence for these cells being bursting pace-makers and the underlying mechanism driving bursting were further investigated. 3. Phasic bursting by m.n.c.s is usually contingent upon two depolarizing events: a slow depolarization (s.d.) between bursts that brings the membrane potential to burst threshold, and the spike depolarizing after-potential (d.a.p.). One or several d.a.p.s can initiate a burst by summing to form a plateau potential which sustains firing. 4. Of eight phasic cells exposed to tetrodotoxin (TTX) and tonically depolarized with current injection, two cells retained the phasic burst pattern and underlying plateau potentials. Of the remaining six cells in TTX, three of four cells tested regained phasic firing with plateau potentials following the addition of Sr2+, a Ca2+ agonist. Evoked post-synaptic potentials were demonstrably blocked throughout TTX exposure, firmly establishing that some m.n.c.s are bursting pace-makers. 5. The s.d., d.a.p. and plateau potential were retained in TTX or low-Na+ saline, augmented in Sr2+ and blocked in low-Ca2+ saline. All three events were activated at membrane potentials depolarized from -70 mV but steadily inactivated with increasing hyperpolarization to -90 mV. The s.d. and d.a.p. apparently represented partial activation of the same process that drives a burst, the plateau potential. 6. Hyperpolarizing pulses of constant current revealed an apparent decrease in cell conductance underlying the s.d., d.a.p. and plateau potential which was not due to membrane rectification. The plateau potential was reduced in low Na+ and eliminated in low Ca2+. However, it remained relatively unaffected by altering the external K+ concentration and it did not reverse below -90 mV, suggesting a less important role for K+ movement relative to Ca2+ or Na+. A hyperpolarizing pulse during the s.d., d.a.p. or plateau potential probably momentarily inactivated inward Ca2+ current, causing the apparent conductance decrease.(ABSTRACT TRUNCATED AT 400 WORDS)

Isoperiodic bursting by magnocellular neuroendocrine cells in the rat hypothalamic slice

1. Recruitment of magnocellular neuroendocrine cells (m.n.c.s) to a repetitive burst pattern (phasic firing) is associated with increased vasopressin secretion from neurohypophysial terminals in the intact animal. Based on invertebrate studies, bursts of action potentials can arise in two distinct ways: as an intrinsic property of the recorded cell or as an emergent property of synaptic interactions. 2. The majority of phasic m.n.c.s in the hypothalamic slice preparation display an endogenous pace-maker mechanism underlying bursting. It is voltage dependent and varies considerably in periodicity and time course as described in the accompanying paper (Andrew, 1987). 3. In contrast to this intrinsic mechanism, the present study examined if cells might be driven by periodic synaptic input. Intracellular recordings from six of thirty-two phasic m.n.c.s in the supraoptic nucleus revealed an isoperiodic oscillation of the membrane potential, where each depolarizing phase could support a burst. 4. The oscillation had a smooth trajectory and fixed period (range, 5-17 s). The oscillatory frequency was not voltage dependent, i.e. periodicity was unaffected by steady current injection through the recording electrode. 5. The frequency and amplitude of the oscillation remained unaltered by action potential firing. The isoperiodic oscillation could abate spontaneously, leaving intact the endogenous ability to fire a triggered burst driven by an underlying plateau potential. 6. Perfusion with either 10 mM-Mg2+-0.05 mM-Ca2+ or 0.5-2.0 microM-tetrodotoxin blocked both the oscillation and evoked post-synaptic potentials, indicating that the oscillation was synaptically generated. Given that both treatments could also block the intrinsic burst process and that the oscillation could spontaneously abate, the synaptic nature of the oscillation remains a tentative but reasonable conclusion. 7. In total, the evidence suggests that the isoperiodic oscillation has a synaptic origin independent of intrinsic mechanisms. It probably results from synaptic input generated within the slice but the source is not yet identified. This input could support phasic bursting in those m.n.c.s lacking a pace-maker ability and so promote the release of vasopressin in the intact animal.

Calcium-dependent spike after-current induces burst firing in magnocellular neurosecretory cells

Magnocellular neurosecretory cells (MNCs) were impaled in perfused explants of rat hypothalamus. Application of a voltage clamp after 1-5 current-evoked spikes revealed a tetrodotoxin-resistant, but Cd2+-sensitive inward current. From a threshold near -85 mV, the amplitude of this current increased as post-spike commands were made to more positive potentials. Following its activation, the current-voltage relation of the cell displayed a region of negative resistance which crossed the spike threshold. This Ca2+-dependent spike after-current can therefore induce and sustain burst firing in MNCs.

Non-synaptic depolarizing potentials in rat supraoptic neurones recorded in vitro

Intracellular recordings obtained from eighty-two supraoptic nucleus neurones in perfused explants of rat hypothalamus revealed a mean resting membrane potential of -66 +/- 5 mV (S.D.) and spike amplitudes of 70-106 mV. When recorded with K acetate-filled micropipettes, non-spike membrane voltage fluctuations included spontaneous depolarizing and hyperpolarizing potentials. Spontaneous hyperpolarizing potentials peaked in 3-5 ms and decayed exponentially with a mean time constant of 20.2 +/- 0.1 ms, 1.6 times the membrane time constant of 13.8 +/- 0.1 ms. These potentials were identified as spontaneous inhibitory post-synaptic potentials, and all demonstrated a common reversal potential near -80 mV, a depolarizing shift of this reversal potential during intracellular Cl- accumulation, and reversible blockade by raising [Mg2+] to 15 mM in the perfusate. Depolarizing potentials with features typical of spontaneous excitatory post-synaptic potentials i.e. brief (8-20 ms) depolarizing transients, were rarely recorded with K acetate-filled micropipettes. Instead, most neurones demonstrated what are termed non-synaptic depolarizing potentials (n.s.d.p.s) lasting 20-125 ms (mean 86.4 +/- 8.6 ms (S.E. of mean)) with a rise time 21.1 +/- 2.8 ms and a decay time of 16.3 +/- 2.8 ms (n = 28 measured). Unlike typical spontaneous post-synaptic potentials, these events could sustain a constant peak amplitude for most of their duration. These n.s.d.p.s displayed a strong voltage-dependent behaviour and were detected only at membrane potentials within 5-7 mV of the threshold for spike initiation. Spontaneous slow depolarizing membrane shifts preceding or following phasic bursts, or any manipulation (e.g. current step, sinusoid, depolarizing after-potential) causing the membrane potential to enter this range of activation, prompted their appearance. N.s.d.p.s were completely insensitive to the presence of 15 mM-Mg2+ but they were reduced in size and frequency when Ca2+ were replaced with Co2+ or Mn2+. They were detected at a more positive membrane potential when Na+-dependent action potentials were blocked with tetrodotoxin. The size, voltage-dependent and non-synaptic nature of these depolarizing potentials raises the possibility that they reflect the activity of individual (or small clusters of) ionic channels carrying inward current. Their ability to serve as prepotentials to trigger spikes is deemed to be particularly important for promoting the onset of phasic bursts in supraoptic neurosecretory neurones.

Barium ions induce prolonged plateau depolarizations in neurosecretory neurones of the adult rat supraoptic nucleus

The occurrence and ionic basis of prolonged plateau depolarizations were studied during intracellular recordings obtained from thirty-nine supraoptic nucleus (s.o.n.) neurosecretory neurones in perfused explants of rat hypothalamus. Replacement of Ca2+ by Ba2+ in the perfusion media enhanced the shoulder on the repolarization phase of the action potentials in all of eleven cells tested. In Ba2+, spike durations increased as the holding membrane potential was made more positive, resulting in plateaux lasting up to 100 s. These plateaux were characterized by a sustained but slowly decaying absolute potential near 0 mV from which there appeared frequent spontaneous hyperpolarizing transients. A membrane resistance decrease of more than 50% was observed at the onset of a plateau, with gradual restoration during the plateau. Injection of Cs+ into twenty-one cells abolished the spike frequency adaptation and after-hyperpolarization associated with current-evoked bursts of action potentials. Mean spike duration after Cs+ injection increased from 2.5 +/- 0.3 ms to 68 +/- 9 ms (S.E. of mean). Addition of 4-5 mM-tetraethylammonium (TEA) to the perfusion media further increased the spike duration of nine Cs+-injected cells to 320 +/- 70 ms. No further increase could be obtained by doubling the concentration of TEA and/or by the addition of 0.2-0.5 mM-4-aminopyridine to the media. Although spike duration was greatly prolonged during such extensive blockade of K+ channels, plateau potentials lasting for longer than 1 s were observed only when Ba2+ was eventually added to the perfusion media. The special property of Ba2+ that leads to the formation of plateau potentials in s.o.n. neurones is therefore not restricted to its ability to reduce K+ conductances but may reside in its reduced effectiveness as a mediator of Ca2+-dependent inactivation of Ca2+ channels. Injection of Cs+ into s.o.n. neurones increased the slope of their current-voltage relationship below -60 mV from 148 +/- 15 to 257 +/- 41 M omega (S.E. of mean) and eliminated the outward rectification present at potentials above -60 mV. K+ currents are presumably active near the resting potential of these cells. Addition of Ba2+ to the perfusion media revealed a Cd2+-sensitive inward rectification above but not below ca. -55 mV. A slowly inactivating Ba2+ current is therefore carried through Ca2+ channels at potentials above -55 mV.(ABSTRACT TRUNCATED AT 400 WORDS)

Thyrotropin-releasing hormone induces rhythmic bursting in neurons of the nucleus tractus solitarius

The nucleus tractus solitarius (NTS) contains neurons that are part of the central neuronal network controlling rhythmic breathing movements in mammals. Nerve terminals within the NTS show immunoreactivity to thyrotropin-releasing hormone (TRH), a neuropeptide that has potent stimulatory effects on respiration. By means of a brainstem slice preparation in vitro, TRH induced rhythmic bursting in neurons in the respiratory division of the NTS. The frequency of bursting was voltage-dependent and could be reset by short depolarizing current pulses. In the presence of tetrodotoxin, TRH produced rhythmic oscillations in membrane potential whose frequency was also voltage-dependent. These observations suggest that TRH modulates the membrane excitability of NTS neurons and allows them to express endogenous bursting activity.

Calcium-dependent action potentials in rat supraoptic neurosecretory neurones recorded in vitro

Intracellular recordings obtained from thirty-nine supraoptic nucleus neurones in perfused hypothalamic explants displayed a mean resting membrane potential of -69 mV and spike amplitude of 79 mV. Voltage-current plots were linear in the hyperpolarizing direction and revealed a mean slope resistance of 197 M omega. After Na+ channel blockade with tetrodotoxin (TTX; 0.3-16 microM), the voltage-current relationship did not change significantly for hyperpolarizing pulses. An increase in spike threshold in TTX permitted visualization of a reduction in slope resistance to depolarizing current pulses. This rectification was reduced by the addition of the Ca2+ channel blockers Cd2+, Co2+ or Mn2+. High threshold TTX-resistant spikes with amplitudes ranging between 25 and 64 mV were evoked in an all-or-none manner by brief intracellular current pulses applied above 1.0 Hz. Current pulses presented at lower frequencies (less than or equal to 0.5 Hz) evoked graded responses. In seventeen of nineteen cells, prolonged depolarizing current pulses elicited repetitive firing of TTX-resistant spikes with a progressive increase in their amplitude, rise and fall times and after-hyperpolarizations. TTX-resistant spikes were reversibly abolished when CaCl2 was replaced by equimolar amounts of EGTA or by the addition of either Cd2+, Co2+ or Mn2+ to the perfusion medium. In artificial medium containing EGTA, both the shoulder on the repolarization phase of the spike and the subsequent after-hyperpolarization were reversibly abolished. Tetraethylammonium (TEA; 2-5 mM) induced prolongation of mean action potential durations from 1.9 to 12.3 ms (nineteen cells); in TTX, TEA also prolonged the duration and increased the over-all peak amplitude of the TTX-resistant (Ca2+) spike. While TEA also enhanced the amplitude of the Na+ spike, no comparable prolongation in spike duration was observed. These data indicate that somatic action potentials of supraoptic nucleus cells arise from the co-activation of a low threshold Na+ conductance and a higher threshold Ca2+ conductance; the latter is expressed as a shoulder on the repolarization phase of the action potential.

Spike broadening in magnocellular neuroendocrine cells of rat hypothalamic slices

Bursts of action potentials recorded from rat magnocellular neuroendocrine cells (MNCs) are known to be associated with enhanced release of oxytocin or vasopressin from their axon terminals in the neurohypophysis. Intracellular recordings from MNC somata in hypothalamic slices showed that spike broadening was characteristic of such bursts. Transitions from slow to fast firing caused spike broadening in all cells, whether they were silent, slow firing, phasic or fast-continuous. During phasic firing, broadening increased with the intraburst spike frequency. However, the parameters of maximal spike broadening recorded at the soma did not directly coincide with the previously described firing patterns evoking maximal hormone release from neurohypophysial terminals.