Changes in Na(+)-K(+)-Cl(-) cotransporter immunoreactivity in the gerbil hippocampus following spontaneous seizure

The immunoreactivity of Na(+)-K(+)-Cl(-) cotransporter (NKCC) in the gerbil hippocampus associated with various sequelae of spontaneous seizures were investigated in order to identify the roles of NKCC in the epileptogenesis and the recovery mechanisms in these animals. The NKCC immunoreactivities in the CA2-3 regions, the subiculum and the entorhinal cortex, were significantly more intensified in the pre-seizure group of seizure sensitive (SS) gerbils than in the seizure resistant (SR) gerbils. Following the on-set of seizure, the immunoreactivity of NKCC was significantly changed. In the hippocampal complex except the CA1 region, NKCC immunoreactivity in GABAergic neurons was significantly decreased 30 min after seizure on-set, versus the pre-seizure group. On the other hand, NKCC immunoreactivity was dramatically elevated in the CA1 regions, and 3 h postictal NKCC immunoreactivity increased significantly in the dentate gyrus and the dendrites of the pyramidal cells in the CA2-3 regions. These findings suggest that altered NKCC expression may be associated with seizure activity, and have an important role in the postictal recovery by regulating GABA-mediated inhibitory circuit in the hippocampal complex of the gerbil.     

Changes in Na(+)-K(+)-Cl(-) cotransporter immunoreactivity in the gerbil hippocampus following transient ischemia

We examined alterations in Na(+)-K(+)-Cl(-) cotransporter 1 (NKCC1) immunoreactivity following ischemia. Twelve hours after ischemia, NKCC1 immunoreactivity in the CA1 region and in the hilar region was significantly diminished. Twenty-four hours after ischemia, NKCC1 immunoreactivity was intensified in these hippocampal regions as well as CA2-3. Two days after ischemia, NKCC1 immunoreactivity in the CA1 and the hilar neurons had disappeared, although in the CA2-3 and the granule cell layer NKCC1 immunoreactivities had recovered to the sham level. This finding suggests that NKCC1 may play an important role in the ischemic neuronal injury induced by excitotoxicity as well as neuronal edema.     

K(+)-induced changes in the properties of the hyperpolarization-activated cation current I(h) in rat CA1 pyramidal cells

The effects of rises in external K(+) (K(ext)) on I(h) were investigated in CA1 pyramidal cells of rat hippocampal slices using the whole-cell patch clamp technique. At the basal K(ext) level (2.5 mM), hyperpolarization-activated cation current (I(h)) had a maximal amplitude of -350+/-60 pA which was enhanced by approximately 60 and approximately 95% at 5 and 7.5 mM K(ext), respectively. The midpoint activation voltage was significantly shifted from -80 mV in the negative direction to about -87 mV at both 5 and 7.5 mM K(ext), without appreciable alterations of the current kinetics. The maximal conductance was approximately 2.4 nS under control conditions and significantly increased to approximately 3.3 and approximately 5.6 nS at 5 and 7.5 mM K(ext), respectively. The reversal potential was shifted in the positive direction, from a control value of approximately -30 mV by approximately 6 and approximately 14 mV at 5 and 7.5 mM K(ext), respectively. Our data demonstrate that even moderate changes in K(ext) have a substantial effect on the properties of I(h).     

Peptidergic counter-regulation of Ca(2+)- and Na(+)-dependent K(+) currents modulates the shape of action potentials in neurosecretory insect neurons

Influx of Ca(2+) and Na(+) ions during an action potential can strongly affect the repolarization and the fast afterhyperpolarization (fAHP) if a neuron expresses Ca(2+)- and Na(+)-dependent K(+) currents (K(Ca) and K(Na)). This applies to cockroach abdominal dorsal unpaired median neurons (DUMs). Here the rapid activation of K(Ca) depends mainly on the P/Q-type Ca(2+) current. Adipokinetic hormones (AKHs)-insect counterparts to mammalian glucagon-mobilize energy reserves but also modulate neuronal activity and lead to enhanced locomotor activity. Cockroach AKH I accelerates spiking and enhances the fAHP of octopaminergic DUM neurons, and it is generally held that enhanced release of the biogenic amine from these and other neurons may lead to general arousal. AKH I modulates the voltage-gated Na(+) and P/Q-type Ca(2+) current and the background Ca(2+) current. Upregulation of P/Q-type Ca(2+) current increases the K(Ca) current, whereas enhanced inactivation of Na(+) current decreases the K(Na) current. We quantified the hormone-induced changes in ion currents in terms of Hodgkin-Huxley models and simulated the resulting activity of DUM neurons. Upregulation of P/Q-type Ca(2+) and K(Ca) current enhanced the hyperpolarization but had a weak effect on spiking. Downregulation of Na(+) and K(Na) current decreased hyperpolarization and slightly accelerated spiking. Superposition of these modulations produced an increase in fAHP while the spike frequency remained unchanged. Only when the upregulation of the pacemaking Ca(2+) background current was included in the simulated modulation the model reproduced the experimentally observed AKH-I-induced changes. The possible physiological relevance of this dual effect is discussed in respect to transmitter release and synaptic integration.     

Transient K current in the somatic membrane of cultured central neurons of embryonic rat.

1. Somatic K currents of cultured hippocampal, striatal, and spinal cord neurons of embryonic rat were recorded under voltage clamp in membrane spheres ("blebs") excised by means of a tight-seal pipette. 2. The somatic K current in blebs was subject to rapid and near complete inactivation during 300-ms depolarizations, whereas whole-cell K currents included a substantial maintained component. Size and kinetic properties of bleb and whole-cell currents were stable throughout the recording period. 3. The steady-state inactivation of somatic A current was steeply voltage dependent and complete near voltage levels that activated current, whereas peak conductances did not saturate during depolarizations up to +90 mV. Activation started with a delay. Half-times of activation decreased with depolarization, but half-times of inactivation varied little with depolarization. Recovery from inactivation followed a sigmoidal time course with half-times of approximately 50 ms. 4. Half-times of activation and inactivation varied over more than an order of magnitude between individual neurons. Midpoint potentials of inactivation and peak conductance varied over approximately 40 mV. The parameter ranges of hippocampal, striatal, and spinal cord neurons overlapped. 5. Individual soma membranes revealed signs of K channel heterogeneity in their 4-aminopyridine block, current fluctuations, and current kinetics. On the other hand, currents elicited after conditioning pulses that established varied degrees of steady-state inactivation or of recovery from full inactivation had superimposable time courses. 6. The described characteristics of the somatic A channels are compared with those reported for the RCK4, Raw3, and mShal products expressed in Xenopus oocytes. Whereas the ranges of voltage dependencies and of most kinetic characteristics are compatible among native and cloned channels, these three cloned channels recover much more slowly from inactivation. In addition, inactivation in native channels, unlike that in RCK4 and Raw3 channels, was stable after excision in a subcellular fragment.     

Electrogenic pump (Na+/K(+)-ATPase) activity in rat optic nerve

Rat optic nerves were studied in a sucrose gap chamber in order to study the origin of a late afterhyperpolarization that follows repetitive activity. The results provide evidence for electrogenic pump (Na+/K(+)-ATPase) activity in central nervous system myelinated axons and demonstrate an effect on axonal excitability. Repetitive stimulation (25-200 Hz; 200-5000 ms) led to a prolonged, temperature-dependent post-train afterhyperpolarization with duration up to about 40 s. The post-train afterhyperpolarization was blocked by the Na+/K(+)-ATPase blockers strophanthidin and ouabain, and the substitution of Li+ for Na+ in the test solution, which also blocks Na+/K(+)-ATPase. The peak amplitude of the post-train afterhyperpolarization was minimally changed by the potassium-channel blocker tetraethylammonium (10 mM), and the Ca2(+)-channel blocker CoCl2 (4 mM). Hyperpolarizing constant current did not reverse the afterhyperpolarization. The amplitude of the hyperpolarization was increased in the presence of the potassium-channel blocker 4-aminopyridine (1 mM). In the presence of 4-amino-pyridine, the post-train hyperpolarization was much reduced by strophanthidin, except for a residual early component lasting several hundred milliseconds which was blocked by the potassium-channel blocker tetraethylammonium. This finding indicates that after exposure to 4-aminopyridine, repetitive stimulation leads to activation of a tetraethylammonium-sensitive K(+)-channel that contributes during the first several hundred milliseconds to the post-train afterhyperpolarization. The amplitude of the compound action potential elicited by a single submaximal stimulus during the post-train hyperpolarization was smaller than that of the control response.The decrement in amplitude was not present under identical stimulation conditions when the post-train hyperpolarization was blocked by strophanthidin, indicating that the hyperpolarization associated with repetitive stimulation reduced excitability.(ABSTRACT TRUNCATED AT 250 WORDS)     

d(-)3-Hydroxybutyrate cotransport with Na in rat renal brush border membrane vesicles

Which are the driving forces ford(-)3-hydroxybutyrate (HB) transport in rat renal brush border membranes (RBB)? Sodium, even in the absence of gradients, accelerates the unidirectional (1–5 s) flux of HB into rat RBB vesicles. Valinomycin (andK _i=K _o) does not significantly alter the NaCl gradient driven HB influx. Thus, the Na-dependent HB influx is driven by the chemical Na⁺ gradient but it is not driven by changes in the transmembrane electrical potential. Indeed, in valinomycin-treated membranes, vesicle-inside more negative potentials (K-gluconate_in-Na-gluconate_out) sufficient to accelerate Na-glucose cotransport, did not stimulate HB influx, in the presence of inwardly directed Na⁺ gradients, and did not significantly inhibit when in the absence of Na⁺. Thus, cotransport of HB with Na in rat RBB membranes does not involve the net transfer of positive charge and the passive conductance of this membrane for HB^? is not large. However, vesicle inside more negative potentials (induced by inwardly directed NaNO₃ gradients or by outwardly directed K⁺ gradients and valinomycin in the presence of inwardly directed Na⁺ gradients) inhibited HB influx, suggesting that another potential sensitive mechanism, perhaps redistribution of intramembrane charges, may influence HB influx. Acidification (pH_i=pH_o=6.4 vs. 7.4) or inwardly directed H⁺ gradients (pH_o/pH_i=6.4/7.4) did not alter HB influx, in the absence of Na⁺. Thus there is no evidence for a H⁺ driven HB influx. HB influx is significantly inhibited by high (100 mEq/l) trans concentration of Na⁺. Also, influx of 2.25 mM¹⁴C-HB was significantly increased by 5–10 mM intravesicular HB under Na-equilibrated conditions. Thus, the rate of translocation of the free carrier appears to limit HB influx through the cotransport system.     

A Ba(2+)-sensitive K(+) current contributes to the resting membrane potential of neurons in rat suprachiasmatic nucleus

A Ba(2+)-sensitive K(+) current was studied in neurons of the suprachiasmatic nucleus (SCN) using the whole cell patch-clamp technique in acutely prepared brain slices. This Ba(2+)-sensitive K(+) current was found in approximately 90% of the SCN neurons and was uniformly distributed across the SCN. Current-clamp studies revealed that Ba(2+) (500 microM) reversibly depolarized the membrane potential by 6.7 +/- 1.3 mV (n = 22) and concomitantly Ba(2+) induced an increase in the spontaneous firing rate of 0.8 +/- 0.2 Hz (n = 12). The Ba(2+)-evoked depolarizations did not depend on firing activity or spike dependent synaptic transmission. No significant day/night difference in the hyperpolarizing contribution to the resting membrane potential of the present Ba(2+)-sensitive current was observed. Voltage-clamp experiments showed that Ba(2+) (500 microM) reduced a fast-activating, voltage-dependent K(+) current. This current was activated at levels below firing threshold and exhibited outward rectification. The Ba(2+)-sensitive K(+) current was strongly reduced by tetraethylammonium (TEA; 20 and 60 mM) but was insensitive to 4-aminopyridine (4-AP; 5 mM) and quinine (100 microM). A component of Ba(2+)-sensitive K(+) current remaining in the presence of TEA exhibited no clear voltage dependence and is less likely to contribute to the resting membrane potential. The voltage dependence, kinetics and pharmacological properties of the Ba(2+)- and TEA-sensitive K(+) current make it unlikely that this current is a delayed rectifier, Ca(2+)-activated K(+) current, ATP-sensitive K(+) current, M-current or K(+) inward rectifier. Our data are consistent with the Ba(2+)- and TEA-sensitive K(+) current in SCN neurons being an outward rectifying K(+) current of a novel identity or belonging to a known family of K(+) channels with related properties. Regardless of its precise molecular identity, the current appears to exert a significant hyperpolarizing effect on the resting potential of SCN neurons.     

Na+,K+,2Cl- cotransport and intracellular chloride regulation in rat primary sensory neurons: thermodynamic and kinetic aspects

Adult primary afferent neurons are depolarized by GABA throughout their entire surface, including their somata located in dorsal root ganglia (DRG). Primary afferent depolarization (PAD) mediated by GABA released from spinal interneurons determines presynaptic inhibition, a key mechanism in somatosensory processing. The depolarization is due to Cl(-) efflux through GABA(A) channels; the outward Cl(-) gradient is generated by a Na+,K+,2Cl(-) cotransporter (NKCC) as first established in amphibians. Using fluorescence imaging microscopy we measured [Cl(-)]i and cell water volume (CWV) in dissociated rat DRG cells (P0-P21) loaded with N-(ethoxycarbonylmethyl)-6-methoxyquinolinium bromide and calcein, respectively. Basal [Cl(-)]i was 44.2 +/- 1.2 mM (mean +/- SE), Cl(-) equilibrium potential (E Cl) was -27.0 +/- 0.7 mV (n = 75). This [Cl(-)]i is about four times higher than electrochemical equilibrium. On isosmotic removal of external Cl(-), cells lost Cl(-) and shrank. On returning to control solution, cells reaccumulated Cl(-) and recovered CWV. Cl(-) reaccumulation had Na+-dependent (SDC) and Na+-independent (SIC) components. The SIC stabilized at [Cl(-)]i = 13.2 +/- 1.2 mM, suggesting that it was passive (E(Cl) = -60.5 +/- 3 mV). Bumetanide blocked CWV recovery and most (65%) of the SDC (IC50 = 5.7 microM), indicating that both were mediated by NKCC. Active Cl(-) uptake fell with increasing [Cl(-)]i and became negligible when [Cl(-)]i reached basal levels. The kinetics of active Cl(-) uptake suggests a negative feedback system in which intracellular Cl(-)regulates its own influx thereby keeping [Cl(-)]i constant, above electrochemical equilibrium but below the value that would attain if NKCC reached thermodynamic equilibrium.     

Chemical anoxia activates ATP-sensitive and blocks Ca(2+)-dependent K(+) channels in rat dorsal vagal neurons in situ

The contribution of subclasses of K(+) channels to the response of mammalian neurons to anoxia is not yet clear. We investigated the role of ATP-sensitive (K(ATP)) and Ca(2+)-activated K(+) currents (small conductance, SK, big conductance, BK) in mediating the effects of chemical anoxia by cyanide, as determined by electrophysiological analysis and fluorometric Ca(2+) measurements in dorsal vagal neurons of rat brainstem slices. The cyanide-evoked persistent outward current was abolished by the K(ATP) channel blocker tolbutamide, but not changed by the SK and BK channel blockers apamin or tetraethylammonium. The K(+) channel blockers also revealed that ongoing activation of K(ATP) and SK channels counteracts a tonic, spike-related rise in intracellular Ca(2+) ([Ca(2+)](i)) under normoxic conditions, but did not modify the rise of [Ca(2+)](i) associated with the cyanide-induced outward current. Cyanide depressed the SK channel-mediated afterhyperpolarizing current without changing the depolarization-induced [Ca(2+)](i) transient, but did not affect spike duration that is determined by BK channels. The afterhyperpolarizing current and the concomitant [Ca(2+)](i) rise were abolished by Ca(2+)-free superfusate that changed neither the cyanide-induced outward current nor the associated [Ca(2+)](i) increase. Intracellular BAPTA for Ca(2+) chelation blocked the afterhyperpolarizing current and the accompanying [Ca(2+)](i) increase, but had no effect on the cyanide-induced outward current although the associated [Ca(2+)](i) increase was noticeably attenuated. Reproducing the cyanide-evoked [Ca(2+)](i) transient with the Ca(2+) pump blocker cyclopiazonic acid did not evoke an outward current.Our results show that anoxia mediates a persistent hyperpolarization due to activation of K(ATP) channels, blocks SK channels and has no effect on BK channels, and that the anoxic rise of [Ca(2+)](i) does not interfere with the activity of these K(+) channels.     

Differential role of KIR channel and Na(+)/K(+)-pump in the regulation of extracellular K(+) in rat hippocampus.

Little information is available on the specific roles of different cellular mechanisms involved in extracellular K(+) homeostasis during neuronal activity in situ. These studies have been hampered by the lack of an adequate experimental paradigm able to separate K(+)-buffering activity from the superimposed extrusion of K(+) from variably active neurons. We have devised a new protocol that allows for such an analysis. We used paired field- and K(+)-selective microelectrode recordings from CA3 stratum pyramidale during maximal Schaffer collateral stimulation in the presence of excitatory synapse blockade to evoke purely antidromic spikes in CA3. Under these conditions of controlled neuronal firing, we studied the [K(+)]o baseline during 0.05 Hz stimulation, and the accumulation and rate of recovery of extracellular K(+) at higher frequency stimulation (1-3 Hz). In the first set of experiments, we showed that neuronal hyperpolarization by extracellular application of ZD7288 (11 microM), a selective blocker of neuronal I(h) currents, does not affect the dynamics of extracellular K(+). This indicates that the K(+) dynamics evoked by controlled pyramidal cell firing do not depend on neuronal membrane potential, but only on the balance between K(+) extruded by firing neurons and K(+) buffered by neuronal and glial mechanisms. In the second set of experiments, we showed that di-hydro-ouabain (5 microM), a selective blocker of the Na(+)/K(+)-pump, yields an elevation of baseline [K(+)]o and abolishes the K(+) recovery during higher frequency stimulation and its undershoot during the ensuing period. In the third set of experiments, we showed that Ba(2+) (200 microM), a selective blocker of inwardly rectifying K(+) channels (KIR), does not affect the posttetanus rate of recovery of [K(+)]o, nor does it affect the rate of K(+) recovery during high-frequency stimulation. It does, however, cause an elevation of baseline [K(+)]o and an increase in the amplitude of the ensuing undershoot. We show for the first time that it is possible to differentiate the specific roles of Na(+)/K(+)-pump and KIR channels in buffering extracellular K(+). Neuronal and glial Na(+)/K(+)-pumps are involved in setting baseline [K(+)]o levels, determining the rate of its recovery during sustained high-frequency firing, and determining its postactivity undershoot. Conversely, glial KIR channels are involved in the regulation of baseline levels of K(+), and in decreasing the amplitude of the postactivity [K(+)]o undershoot, but do not affect the rate of K(+) clearance during neuronal firing. The results presented provide new insights into the specific physiological role of glial KIR channels in extracellular K(+) homeostasis.     

Involvement of actin cytoskeleton in modulation of Ca(2+)-activated K(+) channels from rat hippocampal CA1 pyramidal neurons

Anterograde tracing techniques combined with postembedding immunocytochemical staining were used to determine the gamma amino butyric acid (GABA) content of pretectogeniculate (PT-LGN) terminals and their postsynaptic targets. The results provide evidence that PT-LGN terminals are GABAergic and that they contact GABAergic interneurons. These results corroborate previous anatomical studies and support the idea that the PT-LGN projection functions to disinhibit thalamocortical cells in the dorsal lateral geniculate nucleus.     

Amygdala kindling induces upregulation of mRNA for NKCC1, a Na(+), K(+)-2Cl(-) cotransporter,in the rat piriform cortex

GABA, the main inhibitory neurotransmitter in the brain, elicits a hyperpolarizing response by activation of the GABA(A)-receptor/chloride-channel complex under conditions of normal Cl(-) homeostasis. Thus the pathogenesis of epilepsy could involve an impairment of GABA(A)-receptor-mediated inhibition due to a collapse of the Cl(-) gradient. We examined the expression patterns of Cl(-) transporters and a Cl(-) channel in a rat amygdala-kindling model. Activity-dependent increases were observed in the mRNA for NKCC1, an inwardly-directed Cl(-) transporter, in the piriform cortex. This suggests that an increase in [Cl(-)](i) and a resultant reduction in GABAergic inhibition may occur in the kindled piriform cortex.     

Dynorphin-immunoreactive neurons in the central nervous system of the rat

An antiserum specific for the C-terminal region of dynorphin_1–17 (DYN) was used to examine the distribution of this endogenous opioid peptide in the rat brain with the indirect immunofluorescence technique. DYN-positive nerve cell bodies and fibers were found in many nuclei in the spinal cord, medulla, mesencephalon, hypothalamus and forebrain. These findings indicate that a widespread system of DYN neurons is present in the brain distinct from the previously described enkephalin and endorphin systems.     

Agonist effects at putative central 5-HT4 receptors in rat hippocampus by R(+)- and S(-)-zacopride; no evidence for stereo-selectivity.

The EEG of halothane anaesthetized rats was recorded from an electrode implanted into the hippocampus. In the present study the effect of R(+)- and S(-)-zacopride, administered intra-cerebroventricularly, on different frequency bands of the EEG was investigated. Both enantiomers induced similar dose-dependent (5-20 micrograms) increases in all frequency bands. The effects of R(+)- and S(-)-zacopride were inhibited by pretreatment with a high dose of ICS 205-930 (1 micrograms i.c.v.), which suggests the involvement of 5-HT4 receptors. The lack of stereo-selectivity of the zacopride enantiomers is in contrast to observations made in in vitro studies.     

Development of Na(+)- and K(+)-currents in the cochlear ganglion of the chick embryo.

The development of Na(+)- and K(+)-currents in the primary afferent neurons of the cochlear ganglion was studied using the patch-clamp technique. Cells were dissociated between days 6 and 17 of development and membrane currents recorded within the following 24 h. Outward currents were the first to appear between days 6 and 7 of embryonic development and their magnitude increased throughout development from 200 pA on day 7 to 900 pA on days 14-16. Threshold for activation decreased by 20 mV between days 8 and 14. Outward currents were absent when Cs+ replaced K+ in the pipette and were partially blocked by external tetraethylammonium. Outward currents contained at least three components: (i) a non-inactivating outward current, similar to the delayed-rectifier, predominating in mature neurons; (ii) a slowly inactivating current (tau about 200 ms), most evident in early and intermediate stages (days 7-10); and (iii) a rapidly inactivating outward current (tau about 20 ms) similar to the A-current (IA) described in other neurons, which was distinctly expressed in mature neurons. Sodium currents were identified as fast transient inward currents, sensitive to tetrodotoxin and extracellular Na(+)-removal. They appeared later than K(+)-currents and increased in size from about 100 pA between days 9-11 to 600 pA by days 13-16. The development of membrane currents in cochlear ganglion neurons corresponded to defined stages of the innervation pattern of the chick cochlea [Whitehead and Morest (1985) Neuroscience 14, 255-276]. These currents could be functionally related to the establishment of synaptic connections between transducing cells and primary afferent neurons.     

Inhibition of large-conductance Ca(2+)-activated K(+) channels in hippocampal neurons by toosendanin

The effect of toosendanin, a selective presynaptic blocker and effective antibotulismic agent, on large-conductance Ca(2+)-activated K(+) channels was studied in inside-out patches of pyramidal neurons freshly isolated from the hippocampal CA1 region of the rat. Toosendanin (1 x 10(-6)g/ml approximately 1 x 10(-4)g/ml) was found to inhibit large-conductance Ca(2+)-activated K(+) channels by reducing its open probability significantly in a concentration-dependent manner, although the effective concentration of toosendanin was lower in a symmetrical K(+) (150 mM) solution than under asymmetrical conditions (changing K(+) concentration in pipette solution to 5mM). The action was partially reversible by washing. By decreasing the slow open time constant, toosendanin shortened the open dwell time of large-conductance Ca(2+)-activated K(+) channels in a dose-dependent manner. A dose-dependent reduction of unitary current amplitude of the channel was detected after toosendanin perfusion. On elevating the intracellular free calcium concentration from 1 to 10 microM, a similar effect on large-conductance Ca(2+)-activated K(+) channels by toosendanin was also observed, but its efficacy was diminished.These results show that toosendanin inhibits large-conductance Ca(2+)-activated K(+) channels in hippocampal neurons by reducing the open probability and unitary current amplitude of the channel, and that Ca(2+) interferes with the effect. These data provide an explanation for toosendanin-induced facilitation of neurotransmitter release and the antibotulismic effect of the drug.     

The effects of calcitonin on central neurons in the rat

The effects of salmon calcitonin on central neurons were studied in anesthetized rats. Calcitonin applied iontophoretically consistently inhibited spontaneous activity in half of the neurons tested in the anterior hypothalamic nucleus and subthalamus but had virtually no effect on cortical and thalamic neurons. Calcitonin also inhibited glutamate-evoked activity in the neurons tested. Calcitonin administered into the brain ventricular system led to a marked decrease in spontaneous discharge of hypothalamic cells in the majority of cells tested. The onset of this response began within 20 and 30 min of calcitonin application.     

GDNF acutely modulates excitability and A-type K(+) channels in midbrain dopaminergic neurons

Glial cell line-derived neurotrophic factor (GDNF) prevents lesion-induced death of midbrain dopaminergic neurons, but its function in normal brain remains uncertain. Here we show that GDNF acutely and reversibly potentiated the excitability of cultured midbrain neurons by inhibiting transient A-type K(+) channels. The effects of GDNF were limited to large, tyrosine hydroxylase (TH)-positive dopaminergic neurons, and were mediated by mitogen associated protein (MAP) kinase. Application of GDNF also elicited a MAP kinase-dependent enhancement of the excitability in dopaminergic neurons in midbrain slice. These results demonstrate an acute regulation of GDNF on ion channels and its underlying signaling mechanism, and reveal an unexpected role of GDNF in normal midbrain dopaminergic neurons.