High-affinity inhibition of glutamate release from corticostriatal synapses by omega-agatoxin TK

Differential calcium channel-mediated dopaminergic modulation in the subthalamonigral synapse

Dopamine (DA) modulates basal ganglia (BG) activity for initiation and execution of goal-directed movements and habits. While most studies are aimed to striatal function, the cellular and molecular mechanisms underlying dopaminergic regulation in other nuclei of the BG are not well understood. Therefore, we set to analyze the dopaminergic modulation occurring in subthalamo-nigral synapse, in both pars compacta (SNc) and pars reticulata (SNr) neurons, because these synapses are important for the integration of information previously processed in striatum and globus pallidus. In this study, electrophysiological and pharmacological evidence of dopaminergic modulation on glutamate release through calcium channels is presented. Using paired pulse ratio (PPR) measurements and selective blockers of these ionic channels, together with agonists and antagonists of DA D₂ -like receptors, we found that blockade of the Ca_V 3 family occludes the presynaptic inhibition produced by the activation of DA receptors pharmacologically profiled as D₃ -type in the STh-SNc synapses. On the contrast, the blockade of Ca_V 2 channels, but not Ca_V 3, occlude with the effect of the D₃ agonist, PD 128907, in the STh-SNr synapse. The functional role of this differential distribution of calcium channels that modulate the release of glutamate in the SN implies a fine adjustment of firing for both classes of neurons. Dopaminergic neurons of the SNc establish a DA tone within the SN based on the excitatory/inhibitory inputs; such tone may contribute to processing information from subthalamic nucleus and could also be involved in pathological DA depletion that drives hyperexcitation of SNr neurons.

The role of Ca2+ -dependent K+ - channels at the rat corticostriatal synapses revealed by paired pulse stimulation

Potassium channels play an important role in modulating synaptic activity both at presynaptic and postsynaptic levels. We have shown before that presynaptically located K_V and K_IR channels modulate the strength of corticostriatal synapses in rat brain, but the role of other types of potassium channels at these synapses remains largely unknown. Here, we show that calcium-dependent potassium channels BK-type but not SK-type channels are located presynaptically in corticostriatal synapses. We stimulated cortical neurons in rat brain slices and recorded postsynaptic excitatory potentials (EPSP) in medium spiny neurons (MSN) in dorsal neostriatum. By using a paired pulse protocol, we induced synaptic facilitation before applying either BK- or SK-specific toxins. Thus, we found that blockage of BK_Ca with iberiotoxin (10 nM) reduces synaptic facilitation and increases the amplitude of the EPSP, while exposure to SK-blocker apamin (100 nM) has no effect. Additionally, we induced train action potentials on striatal MSN by current injection before and after the exposure to K_Ca toxins. We found that the action potential becomes broader when the MSN is exposed to iberiotoxin, although it has no impact on frequency. In contrast, exposure to apamin results in loss of afterhyperpolarization phase and an increase of spike frequency. Therefore, we concluded that postsynaptic SK channels are involved in afterhyperpolarization and modulation of spike frequency while the BK channels are involved on the late repolarization phase of the action potential. Altogether, our results show that calcium-dependent potassium channels modulate both input towards and output from the striatum.

KV1 and KV3 Potassium Channels Identified at Presynaptic Terminals of the Corticostriatal Synapses in Rat

In the last years it has been increasingly clear that K_V-channel activity modulates neurotransmitter release. The subcellular localization and composition of potassium channels are crucial to understanding its influence on neurotransmitter release. To investigate the role of K_V in corticostriatal synapses modulation, we combined extracellular recording of population-spike and pharmacological blockage with specific and nonspecific blockers to identify several families of K_V channels. We induced paired-pulse facilitation (PPF) and studied the changes in paired-pulse ratio (PPR) before and after the addition of specific K_V blockers to determine whether particular K_V subtypes were located pre- or postsynaptically. Initially, the presence of K_V channels was tested by exposing brain slices to tetraethylammonium or 4-aminopyridine; in both cases we observed a decrease in PPR that was dose dependent. Further experiments with tityustoxin, margatoxin, hongotoxin, agitoxin, dendrotoxin, and BDS-I toxins all rendered a reduction in PPR. In contrast heteropodatoxin and phrixotoxin had no effect. Our results reveal that corticostriatal presynaptic K_V channels have a complex stoichiometry, including heterologous combinations K_V1.1, K_V1.2, K_V1.3, and K_V1.6 isoforms, as well as K_V3.4, but not K_V4 channels. The variety of K_V channels offers a wide spectrum of possibilities to regulate neurotransmitter release, providing fine-tuning mechanisms to modulate synaptic strength.

Different levels of cortical excitability reflect clinical fluctuations in migraine

BACKGROUND

In a previous study we demonstrated that high-frequency oscillations (HFOs) elicited by median nerve stimulation are significantly correlated to clinical fluctuations of migraine. We aimed at verifying whether clinical fluctuations and HFO changes are correlated to N20 somatosensory evoked potential (SEP) recovery cycle, which is likely to reflect the functional refractoriness of primary somatosensory cortex neurons.

METHODS

We analysed both HFOs and N20 SEP recovery cycle to paired stimulation in 21 migraine patients and 18 healthy volunteers.

RESULTS

Shortened recovery cycle correlated with low-amplitude HFOs as well as with clinical worsening. By contrast, prolonged recovery cycle correlated with enhanced HFOs, as well as with spontaneous clinical improvement.

CONCLUSIONS

In our migraine patients the strict relationship between presynaptic HFO amplitude and N20 recovery function suggests that changes of both parameters might be caused by modifications of the thalamo-cortical drive. Our findings suggest that the thalamo-cortical drive during interictal stages could fluctuate from abnormally high to abnormally low levels, depending on mechanisms which reduce cortical excitability in spontaneously improving patients, and increase cortical excitability in spontaneously worsening ones.

Identification of estradiol/ERα-regulated genes in the mouse pituitary

Estrogen acts to prime the pituitary prior to the GnRH-induced LH surge by undiscovered mechanisms. This study aimed to identify the key components that mediate estrogen action in priming the pituitary. RNA extracted from the pituitaries of metestrous (low estrogen) and proestrus (high estrogen) stage mice, as well as from ovariectomized wild-type and estrogen receptor α (ERα) knockout mice treated with 17β-estradiol (E(2)) or vehicle, was used for gene expression microarray. Microarray data were then aggregated, built into a functional electronic database, and used for further characterization of E(2)/ERα-regulated genes. These data were used to compile a list of genes representing diverse biological pathways that are regulated by E(2) via an ERα-mediated pathway in the pituitary. This approach substantiates ERα regulation of membrane potential regulators and intracellular vesicle transporters, among others, but not the basic components of secretory machinery. Subsequent characterization of six selected genes (Cacna1a, Cacna1g, Cited1, Abep1, Opn3, and Kcne2) confirmed not only ERα dependency for their pituitary expression but also the significance of their expression in regulating GnRH-induced LH secretion. In conclusion, findings from this study suggest that estrogen primes the pituitary via ERα by equipping pituitary cells with critical cellular components that potentiate LH release on subsequent GnRH stimulations.

N-type calcium channels mediate a GABA(B) presynaptic modulation in the corticostriatal synapse in turtle's paleostriatum augmentatum

Spikes population evoked by a paired pulse protocol were used to assess the influence of GABA(A) and GABA(B) receptors agonists and antagonists on the synaptic potentials and in the S2/S1 ratio in a paired pulse (PP) protocol in the cortico-paleostriatum augmentatum synapses of the turtle. GABA(A) agonist, muscimol, decreased the amplitude of synaptic responses whereas the facilitation produced with the PP protocol did not change, suggesting a postsynaptic action for GABA(A) receptors. GABA(B) agonist, baclofen, enhanced paired pulse ratio indicating a presynaptic modulation through the GABA(B) receptor. Selective antagonists for N- and P/Q-type Ca(2+)-channels also enhanced paired pulse ratio, suggesting that any of these channel types may be involved in neurotransmitter release. However, the strong paired pulse facilitation produced by baclofen was occluded by blocking the N-type Ca2+ channels with omega-conotoxin GVIA (1 microM), but not by the blockage of P/Q-type Ca2+ channels with omega-agatoxin TK (400 nM). These data suggest that N and P/Q channels participate in the neurotransmitter release, whereas only N-type Ca2+ channels are involved in the presynaptic modulation of GABA(B) in the corticostriatal synapse of the turtle.

Delta opioids reduce the neurotransmitter release probability by enhancing transient (KV4) K+-currents in corticostriatal synapses as evaluated by the paired pulse protocol

Field recordings were used to determine the influence of delta-opioid receptor activation on corticostriatal synaptic transmission. Application of the selective delta-opioid receptor agonist, [Tyr-D-Pen-Gly-Phe-D-Pen]-enkephalin (DPDPE, 1 microM), decreased the amplitude of the field-excitatory synaptic potential and at the same time increased the paired pulse ratio (PPR) suggesting a presynaptic site of action. This response reversed rapidly when DPDPE was washed and blocked by 1 nM of the selective delta-receptor antagonist naltrindole. Neither omega-conotoxin GVIA (1 microM) nor omega-agatoxin TK (400 nM), blockers of N- and P/Q-type Ca2+-channels, respectively, nor TEA (1 mM), blocker of some classes of K+-channels, occluded the effects of DPDPE. Instead, 1 mM 4-AP or 400 microM Ba2+ occluded completely the effects of DPDPE. Therefore, the results suggest that the modulation by delta opioids at corticostriatal terminals is mediated by transient (KV4) K+-conductances.

5-HT1A, 5-HT2, and GABAB receptors interact to modulate neurotransmitter release probability in layer 2/3 somatosensory rat cortex as evaluated by the paired pulse protocol

Activation of gamma-aminobutyric acid B (GABA(B)) and 5-hydroxytryptamine (5-HT) receptors produces presynaptic inhibition at glutamatergic terminals in the rat neocortex. To evaluate interactions between these metabotropic receptors, field potentials were recorded in layer 2/3 of somatosensory cortex. In addition, the paired pulse (PP) protocol was used to measure changes in the ratio of the second/first extracellular synaptic potentials (S(2)/S(1) ratio) as an index of glutamate release probability in the area. Lowering extracellular [Ca(2+)](o) to 0.5 mM, increased the S(2)/S(1) ratio by 318 +/- 134%. 5-HT (1 microM) increased the S(2)/S(1) ratio by 61 +/- 15%. In presence of the GABA(A) antagonist bicuculline (10 microM), 5-HT increased the S(2)/S(1) ratio by 98 +/- 15%. This effect did not desensitize after two consecutive applications of the amine, and was dose dependent in the concentration range between 0.03-1 microM (EC(50) = 2.36 x 10(-7) mol/L). The increase of S(2)/S(1) ratio induced by 5-HT (1 microM) was blocked reversibly by the 5-HT(1A) antagonist NAN-190 (10-30 microM), but was unaffected by the selective GABA(B) antagonist CGP 52432 (1 microM). The action of 5-HT was mimicked by the 5-HT(1A/7) agonist 8OH-DPAT (10 microM), increasing the S(2)/S(1) ratio by 84 +/- 2%, a response that was unaffected by the 5-HT(2/7) antagonist ritanserin (2 microM). The 5-HT(1B) agonist CP93129 (10 microM) had no effect. The GABA(B) agonist baclofen (1 microM) increased the S(2)/S(1) ratio up to 308 +/- 33%, which is similar to that produced by low [Ca(2+)](o). When the effect of baclofen was maximal, application of 5-HT (1 microM) reversed the S(2)/S(1) ratio back to 78 +/- 27%, a result that was blocked by the 5-HT(2/7) antagonist ritanserin (100 nM). Notably, the interaction between the GABA(B) agonist and 5-HT was order dependent, because enhancement of the S(2)/S(1) ratio elicited by baclofen was not inhibited if 5-HT was applied first. These results suggest a complex interaction between GABA(B), 5-HT(1A), and 5-HT(2) receptors in layer 2/3 of rat somatosensory cortex. Activation of GABA(B) receptors induces PP facilitation (inhibits glutamate release) more efficiently than does activation of 5-HT(1A) receptors. When the effect of GABA(B) receptor activation is maximal, however, the influence of 5-HT changes to the opposite direction, inhibiting PP facilitation (increasing glutamate release) through activation of 5-HT(2) receptors.

Dopaminergic modulation of axon collaterals interconnecting spiny neurons of the rat striatum

Dopamine is a critical modulator of striatal function; its absence produces Parkinson's disease. Most cellular actions of dopamine are still unknown. This work describes the presynaptic actions of dopaminergic receptor agonists on GABAergic transmission between neostriatal projection neurons. Axon collaterals interconnect projection neurons, the main axons of which project to other basal ganglia nuclei. Most if not all of these projecting axons pass through the globus pallidus. Thus, we lesioned the intrinsic neurons of the globus pallidus and stimulated neostriatal efferent axons antidromically with a bipolar electrode located in this nucleus. This maneuver revealed a bicuculline-sensitive synaptic current while recording in spiny cells. D1 receptor agonists facilitated whereas D2 receptor agonists depressed this synaptic current. In contrast, a bicuculline-sensitive synaptic current evoked by field stimulation inside the neostriatum was not consistently modulated, in agreement with previous studies. The data are discussed in light of the most recent experimental and modeling results. The conclusion was that inhibition of spiny cells by axon collaterals of other spiny cells is quantitatively important; however, to be functionally important, this inhibition might be conditioned to the synchronized firing of spiny neurons. Finally, dopamine exerts a potentially important role regulating the extent of lateral inhibition.

The presynaptic modulation of corticostriatal afferents by mu-opioids is mediated by K+ conductances

Population spikes associated with the paired pulse ratio protocol were used to measure the presynaptic inhibition of corticostriatal transmission caused by mu-opioid receptor activation. A 1 microM of [D-Ala(2), N-MePhe(4), Gly-ol(5)]-enkephalin (DAMGO), a selective mu-opioid receptor agonist, enhanced paired pulse facilitation by 44+/-8%. This effect was completely blocked by 2 nM of the selective mu-receptor antagonist D-Phe-Cys-Tyr-D-Trp-Orn-Thr-NH (CTOP). Antagonists of N- and P/Q-type Ca(2+) channels inhibited, whereas antagonists of potassium channels enhanced, synaptic transmission. A 1 microM of omega-conotoxin GVIA, a blocker of N-type Ca(2+) channels, had no effect on the action of DAMGO, but 400 nM omega-agatoxin TK, a blocker of P/Q-type Ca(2+)-channels, partially blocked the action of this opioid. However, 5 mM Cs(2+) and 400 microM Ba(2+), unselective antagonists of potassium conductances, completely prevented the action of DAMGO on corticostriatal transmission. These data suggest that presynaptic inhibition of corticostriatal afferents by mu-opioids is mediated by the modulation of K(+) conductances in corticostriatal afferents.

Blockade of voltage-gated calcium channels in rat inhibits repetitive cortical spreading depression

Blockers of L-, N-, and P/Q-type voltage-gated calcium channels (VGCCs) were topically applied to the cortical surface of anaesthetized adult rats to study their role in cortical spreading depression (CSD), a correlate of the migraine aura. By pricking the brain, single CSD could still be elicited after blockade of the three different types of VGCCs as in the untreated brain. Topical KCl application to the untreated cortex resulted in repetitive CSD. However, after application of blockers at either L-, or N-, or P/Q-type VGCCs to the cortical surface, application of KCl elicited only one or very few CSD, and their repetition rate was dramatically reduced. The results suggest that cortical excitability resulting in repetitive CSD is markedly influenced by N- and P/Q-type VGCCs and less by L-type VGCCs.

Paired pulse facilitation is turned into paired pulse depression in hippocampal slices after epilepsy induced by 4-aminopyridine in vivo

Modifications in synaptic plasticity seem to play a key role in the origin and persistence of epilepsy. 4-Aminopyridine (4-AP) induces intense and long lasting epileptic seizures and neurodegeneration when applied into the hippocampus in vivo, effects that seem to be mediated by overactivation of glutamate receptors due to the enhancement of glutamate release from nerve endings. We have studied presynaptic modifications of CA1 responses, using the paired pulse paradigm, in hippocampal slices obtained from 4-AP-treated rats killed during epileptic activity (ex vivo). The paired pulse facilitation (PPF) observed in control slices with interstimulus intervals of 10-30 ms was changed into paired pulse depression (PPD) after 100 microM 4-AP added in vitro. A strikingly similar change was observed in the ex vivo slices even though 4-AP was no longer present in the tissue. We conclude that the facilitation of glutamate release induced by 4-AP becomes chronic after a transient exposure to the drug. This suggests that the facilitated neurotransmitter release induced by 4-AP triggers a more permanent plastic change that may be responsible for the persistence of epilepsy.