Involvement Of Presynaptic Mechanisms Research Articles

Functional effects of somatostatin receptor 1 activation on synaptic transmission in the mouse hippocampus

Somatostatin-14 (SRIF) co-localizes with GABA in the hippocampus and regulates neuronal excitability. A role of SRIF in the control of hippocampal activity has been proposed, although the exact contribution of each SRIF receptor (sst(1)-sst(5)) in mediating SRIF action requires some clarification. We used hippocampal slices of wild-type and sst(1) knockout (KO) mice and selective pharmacological tools to provide conclusive evidence for a role of sst(1) in mediating SRIF inhibition of synaptic transmission. With single- and double-label immunohistochemistry, we determined the distribution of sst(1) in hippocampal slices and we quantified sst(1) colocalization with SRIF. With electrophysiology, we found that sst(1) activation with CH-275 inhibited both the NMDA- and the alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA)-mediated responses. Results from sst(1) KO slices confirmed the specificity of CH-275 effects; sst(1) activation did not affect the inhibitory transmission which was in contrast increased by sst(4) activation with L-803,087 in both wild-type and sst(1) KO slices. The AMPA-mediated responses were increased by L-803,087. Functional interaction between sst(1) and sst(4) is suggested by the finding that their combined activation prevented the CH-275-induced inhibition of AMPA transmission. The involvement of pre-synaptic mechanisms in mediating inhibitory effects of sst(1) on excitatory transmission was demonstrated by the finding that CH-275 (i) increased the paired-pulse facilitation ratio, (ii) did not influence the AMPA depolarization in the presence of tetrodotoxin, and (iii) inhibited glutamate release induced by epileptiform treatment. We conclude that SRIF control of excitatory transmission through an action at sst(1) may represent an important contribution to the regulation of hippocampal activity.

Possible role of mitochondria in posttetanic potentiation of GABAergic synaptic transmission in rat neocortical cell cultures

It has been previously demonstrated that mitochondria are of crucial importance for posttetanic potentiation (PTP) at neuromuscular junction. The aim of our study was to examine whether this may also be the case at a central synapse. To address this question, we studied possible mitochondrial involvement in PTP of GABAergic synaptic transmission in rat neocortical cultures, a preparation in which PTP has not been previously documented. Synaptic responses were evoked by local extracellular stimulation. Whole-cell patch-clamp technique was employed to record inhibitory postsynaptic currents (IPSCs) from postsynaptic neurons. Tetanic stimulation (30 Hz, 4 s) of the presynaptic neuron evoked an increase of IPSC amplitude, lasting for about 1 min. PTP was accompanied by a decrease of coefficient of variation of the IPSC and a decrease of paired-pulse (IPSC(2)/IPSC(1)) ratio, indicating involvement of presynaptic mechanism(s) in PTP. Possible role of mitochondria in PTP was addressed using drugs affecting Ca(2+) uptake and subsequent Ca(2+) efflux: carbonyl cyanide 3-chlorophenylhydrazone (CCCP) and tetraphenylphosphonium ions (TPP(+)). It was found that both CCCP (1-2 microM) and TPP(+) (10 microM) either substantially decreased or eliminated PTP. These results further confirm presynaptic origin of PTP in neocortical neurons and suggest an important role of mitochondrial Ca(2+) turnover in this form of synaptic plasticity at the central synapse.

Stimulation of the Lateral Hypothalamus Provokes the Initiation of Robust Long-Term Potentiation of the Thalamo-Cortical Input to the Barrel Field of the Adult, Freely Moving Rat

Long-term potentiation in the thalamo-cortical input to the somatosensory cortex barrel field has been reported to be inducible in vitro only during a narrow critical period of the first postnatal week. Here we explored whether this is due to inability of adult synapses to express LTP or lack of appropriate conditions for LTP induction in slice preparations. We recorded thalamo-cortical field potentials (FPs) from the barrel field of chronically prepared adult rats. In the first series, several parameters of conditioning tetanization of thalamus (T) have been tried. Statistically significant LTP of 135-150% relative to the baseline was observed only in rare cases (3/18) so that the mean changes were not statistically significant. In the second series, five trains of 100 Hz stimulation of T were paired with a "reinforcing" stimulation of the lateral hypothalamus (LH). In most cases (9/13), thalamo-cortical FPs were potentiated. The mean post-tetanic amplitude was 238 +/- 42% (+/- SEM) relative to the baseline (n = 13). The potentiation persisted for >1 h and typically even further increased when tested 24-48 h later. LTP magnitude strongly correlated with the initial paired-pulse ratio (PPR, coefficient of correlation r = 0.98) so that the LTP magnitude was larger (333 +/- 107, n = 6) in cases with PPR > 1.3. The mean PPR tended to decrease after LTP (from 2.05 to 1.65). Altogether the results suggest that LTP is inducible in the thalamo-cortical input to the barrel field of normal adult rats. The dependence of the LTP magnitude upon the initial PPR suggests that inputs with low initial release probability undergo larger LTP. Together with the tendency to a decrease in the PPR this suggests an involvement of presynaptic mechanisms in the maintenance of neocortical LTP.

Impaired long-term potentiation in obese zucker rats: possible involvement of presynaptic mechanism

Electrophysiological investigation of basal synaptic transmission and synaptic plasticity in the CA1 region of the hippocampus was carried out in anesthetized obese Zucker rats (OZR). Comparison of the input/output curves of basal field excitatory postsynaptic potential indicates that these are similar in both the OZR and its lean counterpart suggesting that basal synaptic transmission is intact in the OZR. However, high frequency stimulation evokes long-term potentiation (LTP) in the lean rat but not in the OZR. Since post-tetanic potentiation and paired pulse facilitation, forms of short-term potentiation of presynaptic origin, are also severely impaired in the OZR, the results imply that impairment of CA1 hippocampal LTP in these obese rats may be due, in part, to impaired presynaptic function. The results emphasize the potential deleterious effect of obesity on learning and memory functions of the CNS.

Presynaptic current changes at the mossy fiber-granule cell synapse of cerebellum during LTP.

The involvement of presynaptic mechanisms in the expression of long-term potentiation (LTP), an enhancement of synaptic transmission suggested to take part in learning and memory in the mammalian brain, has not been fully clarified. Although evidence for enhanced vesicle cycling has been reported, it is unknown whether presynaptic terminal excitability could change as has been observed in invertebrate synapses. To address this question, we performed extracellular focal recordings in cerebellar slices. The extracellular current consisted of a pre- (P(1)/N(1)) and postsynaptic (N(2)/SN) component. In ~50% of cases, N(1) could be subdivided into N(1a) and N(1b). Whereas N(1a) was part of the fiber volley (P(1)/N(1a)), N(1b) corresponded to a Ca(2+)-dependent component accounting for 40-50% of N(1), which could be isolated from individual mossy fiber terminals visualized with fast tetramethylindocarbocyanine perchlorate (DiI). The postsynaptic response, given its timing and sensitivity to glutamate receptor antagonists [N(2) was blocked by 10 microM [1,2,3,4-tetrahydro-6-nitro-2,3-dioxo-benzo[f]quinoxaline-7-sulfonamide disodium (NBQX) and SN by 100 microM APV +50 microM 7-Cl-kyn], corresponded to granule cell excitation. N(2) and SN could be reduced by 1) Ca(2+) channel blockers, 2) decreasing the Ca(2+) to Mg(2+) ratio, 3) paired-pulse stimulation, and 4) adenosine receptor activation. However, only the first two manipulations, which modify Ca(2+) influx, were associated with N(1) (or N(1b)) reduction. LTP was induced by theta-burst mossy fiber stimulation (8 trains of 10 impulses at 100 Hz separated by 150-ms pauses). Interestingly, during LTP, both N(1) (or N(1b)) and N(2)/SN persistently increased, whereas P(1) (or P(1)/N(1a)) did not change. Average changes were N(1) = 38.1 +/- 31.9, N(2) = 49.6 +/- 48.8, and SN = 59.1 +/- 35.5%. The presynaptic changes were not observed when LTP was prevented by synaptic inhibition, by N-methyl-D-aspartate and metabotropic glutamate receptor blockage, or by protein kinase C blockage. Moreover, the presynaptic changes were sensitive to Ca(2+) channel blockers (1 mM Ni(2+) and 5 microM omega-CTx-MVIIC) and occluded by K(+) channel blockers (1 mM tetraethylammmonium). Thus regulation of presynaptic terminal excitability may take part in LTP expression at a central mammalian synapse.

AMPA and NMDA currents show different short-term depression in the dorsal lateral geniculate nucleus of the rat.

Paired-pulse depression was studied at the glutamatergic synapse between retinal afferents and thalamocortical cells in the rat dorsal lateral geniculate nucleus. The main objective of this study was to examine the contributions of the pre- and postsynaptic sites to this depression by comparing AMPA- and NMDA-receptor-mediated responses. Equal depression of the two receptor components would indicate involvement of presynaptic mechanisms, while differences in depression would indicate involvement of postsynaptic mechanisms. Pharmacologically isolated AMPA- and NMDA-receptor-mediated currents were recorded using the whole-cell patch-clamp technique in acute thalamic slices. Both the AMPA and the NMDA components showed pronounced depression when retinal afferents were activated by paired pulses. The depression decayed within 5 s. The AMPA component was more strongly depressed than the NMDA component at paired-pulse intervals ranging from 20 to 200 ms, suggesting the involvement of postsynaptic mechanisms. For intervals of 500 ms and longer, the depression of the two components was identical, suggesting the involvement of purely presynaptic mechanisms. The degree of depression measured without the use of pharmacological tools produced similar results, thus excluding the involvement of presynaptic ionotropic glutamate receptors. Cyclothiazide, a blocker of AMPA-receptor desensitisation, reduced the difference in depression between the two components, suggesting that desensitisation of the AMPA receptors is a postsynaptic mechanism that contributes to the difference in depression between the AMPA and the NMDA components.

Changes in Paired Pulse Depression as an Indicator for the Involvement of Presynaptic Mechanism(s) in Modulation of GABA-ergic Synaptic Transmission in Rat Hippocampal Cell Cultures

Paired pulse depression (PPD) of GABA-ergic IPSC was studied in rat hippocampal cell cultures. Synaptic responses were evoked by local extracellular stimulation of the presynaptic units; a whole-cell patch-clamp technique was employed to record responses (IPSC) from the postsynaptic neurons. Paired stimulation (100-msec-long interstimulus interval) resulted in depression of the second IPSC amplitude (IPSC2). This was usually accompanied by an increase in the coefficient of variation (CV) of IPSC2 comparing with that of IPSC1. These results support the involvement of presynaptic mechanism(s) in PPD and suggest a possibility to use PPD alteration as an indicator for the presynaptic origin of modulatory effects. To check this suggestion, we tested whether alteration of the quantum content would affect PPD. We found that reduction of the transmitter release by application of Cd2+ decreased PPD. Post-tetanic potentiation, a well-established presynaptic phenomenon, increased PPD. We conclude that PPD changes may be used as a test for the involvement of presynaptic changes.

Modulation of adrenal catecholamine release by PACAP in vivo.

The aim of the present study was to investigate whether pituitary adenylate cyclase-activating polypeptide-(1-27) (PACAP27) can modulate the adrenal catecholamine (CA) secretion induced by splanchnic nerve stimulation (SNS) and by exogenous acetylcholine (ACh) in anesthetized dogs. Plasma CA concentrations in adrenal venous and aortic blood were quantified by a high-performance liquid chromatography coupled with electrochemical detection. Adrenal venous blood flow was measured by gravimetry. Local infusion of PACAP27 (0.5, 5, and 50 ng) to the left adrenal gland via the adrenolumbar artery resulted in an increase in CA output, reaching a significant level at the highest dose tested. Either direct SNS (2 Hz) or local infusion of ACh (0.5 microgram) to the left adrenal gland produced significant increases in CA output to an extent similar to that obtained with 50 ng of PACAP27 alone. In the presence of PACAP27 (50 ng), CA responses to either SNS or exogenous ACh were significantly potentiated by approximately four- and sixfold, respectively, compared with those obtained in response to each stimulus alone. However, the enhanced CA responses to ACh were not significantly different from those to SNS. The results indicate that the increase in adrenal CA secretion, induced by either direct SNS or exogenous ACh, is synergistically enhanced by PACAP27. The study suggests that the enhanced CA secretion may result from the activation of a PACAP-mediated facilitatory mechanism(s) localized presumably at the postsynaptic level in the canine adrenal medulla in vivo, although the possible involvement of presynaptic mechanisms cannot completely be ruled out in the present study.

Effects of an inhibitor for calcium/calmodulin-dependent protein phosphatase, calcineurin, on induction of long-term potentiation in rat visual cortex

A role of Ca 2+/calmodulin-dependent protein phosphatase (calcineurin) in induction of long-term potentiation (LTP) was investigated using its selective inhibitor, FK506, in visual cortical slices of young rats. Field potentials or excitatory postsynaptic potentials (EPSPs) to test stimulation of white matter were recorded extra- or intracellularly from layer 2 3 , and tetanic stimulation (tetanus) was applied to the white matter at 5 Hz. During the application of FK506 (1 μM), short tetanus (6 s) which had rarely induced LTP in the normal medium, became effective in inducing LTP. Tetanus for 1 min in the presence of FK506 induced LTP with higher probability than in the normal medium. To test possible involvement of presynaptic mechanisms, paired pulses at 50 ms intervals were given to the white matter. The facilitation ratio of the second to first EPSPs was not significantly changed by FK506 and after the induction of LTP, suggesting that the action of FK506 may not be presynaptic. To confirm this, FK506 was injected directly into neurons through recording electrodes. In cases in which stable EPSPs were recorded, the probability of LTP induction became higher than that obtained with normal electrodes. These results suggest that calcineurin plays a role in processes antagonizing the induction of LTP in visual cortex.

Glutamate induces long-term increase in the frequency of single N-methyl- d-aspartate channel openings in hippocampal CA1 neurons examined in situ

Long-term potentiation is currently a leading candidate for a physiological memory mechanism in CNS. The interpretation of this phenomenon is contradictory in many respects. 2,8,10,11 However, there is clear evidence that long-term potentiation is critically dependent on activation of N-methyl- d-aspartate receptors. 3 Recently it has been shown that extracellularly applied glutamate also induces long-lasting changes in the properties of synaptic transmission in the hippocampus that can be attributed to long-term potentiation. 7 The involvement of presynaptic mechanisms has been reported. 6,9 Here we demonstrate a definite increase in both the open time and open-state probability of N-methyl- d-aspartate-operated channels, induced by prolonged application of glutamate to hippocampal slices.

Glutamate iontophoresis induces long-term potentiation in the absence of evoked presynaptic activity

Protocols that induce long-term potentiation (LTP) typically involve afferent stimulation. We tested the hypothesis that LTP induction does not require presynaptic activity. The significance of this hypothesis is underscored by results suggesting that LTP expression may involve activity-dependent presynaptic changes. An induction protocol using glutamate iontophoresis was developed that reliably induced LTP in hippocampal slices without afferent stimulation. Iontophoresis LTP was Ca2+ dependent, was blocked by MK-801, and occluded tetanus-induced LTP. Iontophoresis LTP was induced when excitatory postsynaptic potentials were completely blocked by adenosine plus tetrodotoxin. Our results suggest constraints on the involvement of presynaptic mechanisms and putative retrograde messengers in LTP induction and expression; namely, these processes must function without many forms of activity-dependent presynaptic processes.

Effects of the inhibition of noradrenaline uptake and synthesis on the maintenance of the response to continuous nerve stimulation in the central artery of the rabbit ear.

1. The central artery of the rabbit ear was perfused through its lumen in vitro, with a constant pressure technique, and stimulated continuously via its periarterial sympathetic nerves at the physiological frequency of 5 Hz.2. The vasoconstrictor response, which led initially to an almost complete cessation of intraluminal flow, deteriorated steadily over a period of hours. The involvement of presynaptic mechanisms in this effect was indicated by the finding that noradrenaline, administered extraluminally, produced a similar response before the onset of continuous stimulation and at a late stage when the constriction had decreased markedly. In addition, the noradrenaline precursor DOPA, restored the depressed responses towards their original values, indicating that failure involved depletion of mediator for release.3. Responses to continuous stimulation declined significantly faster after inhibition of tyrosine hydroxylase with alpha-methyl-p-tyrosine. However, inhibition of the uptake of noradrenaline with cocaine did not enhance the decline of the response, even when the sensitization produced by the compound was taken into account.4. It is concluded that synthesis, along with the mobilization of stored mediator, rather than uptake and re-use of noradrenaline maintain the effector response in the central artery of the rabbit ear stimulated continuously at a frequency within the physiological range.