A24 INVESTIGATING THE IMPACT OF PARKINSON’S DISEASE-ASSOCIATED GENES ON INTESTINAL HOMEOSTASIS

Background

Intestinal epithelial cells (IECs) provide an essential physical barrier between harsh luminal contents and underlying host tissue. The maintenance of intestinal homeostasis in this rapidly renewing tissue must be intricately regulated through the proliferation and differentiation of intestinal stem cells (ISCs). Dysregulation of this system results in the loss of barrier function, causing pathologies in both intestinal and extra-intestinal diseases. While Parkinson’s Disease (PD) is primarily a neurodegenerative disorder, there is increasing evidence linking PD progression and gastrointestinal dysfunction. For instance, constipation and increased bowel permeability are frequently observed years prior to development of motor dysfunction in PD, people with inflammatory bowel disease are more likely to develop PD, and a positive correlation exists between gastrointestinal infections and PD incidence. Our group recently developed a model to investigate the role of the gut in PD, demonstrating that mice with genetic ablation of the PD-associated gene Pink1 exhibited motor phenotypes only when previously infected with Gram-negative Citrobacter rodentium intestinal bacteria. As Pink1 and other PD-associated genes are expressed in IECs, we hypothesize that PD-associated gene mutations directly affect the epithelium and impact early PD pathophysiology.

Purpose

Investigate the impact of Pink1 and other PD-associated genes in IECs under steady state and infection.

Method

Single-cell RNA sequencing was performed on IECs isolated from Pink1 WT and KO mice, at steady state and following in vivo C. rodentium infection. Mice were sacrificed at an early timepoint of infection (day 6) to elucidate transcriptional differences between epithelial lineages of each genotype. Additionally, ex vivo colonoids were derived from primary mouse tissue and treated with lipopolysaccharide (LPS) to determine how PINK1 loss-of-function affects the inflammatory response of the epithelium.

Result(s)

Our data revealed that loss-of-function of PINK1 profoundly affected the ISC compartment and several epithelial lineages. Specifically, ISCs from infected Pink1 KO mice demonstrated differentially regulated proliferative and cell cycle genes, while transit amplifying cells showed dysregulated expression of tight junction genes, and enterocytes displayed differentially expressed oxidative damage and apoptotic genes. Preliminary data from colonoids showed that Pink1 KO mice, when stimulated with LPS, had increased pro-inflammatory cytokine gene expression.

Conclusion(s)

In Pink1 KO intestinal epithelial cells, there is indeed an altered cellular response upon infection in vivo and LPS treatment ex vivo. However, more information is needed to decern the mechanistic role of IECs in PD. By investigating the role of PD genes in the gastrointestinal tract, these studies carry important implications for understanding the initiation and progression of PD.

Disclosure of Interest

None Declared

GDNF enhances the synaptic efficacy of dopaminergic neurons in culture

Glial cell line-derived neurotrophic factor (GDNF) is known to promote the survival and differentiation of dopaminergic neurons of the midbrain. GDNF also causes an enhancement of dopamine release by a mechanism which is presently unclear. Using isolated dopaminergic neurons of the rat ventral tegmental area in culture, we have tested the hypothesis that GDNF regulates the establishment and functional properties of synaptic terminals. Previous studies have shown that single dopaminergic neurons in culture can co-release glutamate in addition to dopamine, leading to the generation of a fast excitatory autaptic current via glutamate receptors. Using excitatory autaptic currents as an assay for the activity of synapses established by identified dopaminergic neurons, we found that chronically applied GDNF produced a threefold increase in the amplitude of excitatory autaptic currents. This action was specific for dopaminergic neurons because GDNF had no such effect on ventral tegmental area GABAergic neurons. The enhancement of excitatory autaptic current amplitude caused by GDNF was accompanied by an increase in the frequency of spontaneous miniature excitatory autaptic currents. These observations confirmed a presynaptic locus of change. We identified synaptic terminals by using synapsin-1 immunofluorescence. In single tyrosine hydroxylase-positive neurons, the number of synapsin-positive puncta which represent putative synaptic terminals was found to be approximately doubled in GDNF-treated cells at 5, 10 and 15 days in culture. The number of such morphologically identified terminals in isolated GABAergic neurons was unchanged by GDNF. These results suggest that one mechanism through which GDNF may enhance dopamine release is through promoting the establishment of new functional synaptic terminals.

Neurotensin regulates intracellular calcium in ventral tegmental area astrocytes: evidence for the involvement of multiple receptors

Recent evidence suggests that some types of neurotensin receptors may be expressed by astrocytes. In order to explore the function of neurotensin receptors in astrocytes, the effect of a neurotensin receptor agonist, neurotensin(8-13), on intracellular Ca(2+) dynamics in mixed neuronal/glial cultures prepared from rat ventral tegmental area was examined. It was found that neurotensin(8-13) induces a long-lasting rise in intracellular Ca(2+) concentration in a subset of glial fibrilary acidic protein-positive glial cells. This response displays extensive desensitization and appears to implicate both intracellular and extracellular Ca(2+) sources. In the absence of extracellular Ca(2+), neurotensin(8-13) evokes only a short-lasting rise in intracellular Ca(2+). The neurotensin-evoked intracellular Ca(2+) accumulation is blocked by the phospholipase C inhibitor U73122 and by thapsigargin, suggesting that it is initiated by release of Ca(2+) from an inositol triphosphate-dependent store. The Ca(2+)-mobilizing action of neurotensin(8-13) in astrocytes is dependent on at least two receptors, because the response is blocked in part only by SR48692, a type 1 neurotensin receptor antagonist, and is blocked completely by SR142948A, a novel neurotensin receptor antagonist. The finding that the type 2 neurotensin receptor agonist levocabastine fails to mimic or alter the effects of neurotensin(8-13) on intracellular Ca(2+) makes it unlikely that the type 2 neurotensin receptor is involved. In summary, these results show that functional neurotensin receptors are present in cultured ventral tegmental area astrocytes and that their activation induces a highly desensitizing rise in intracellular Ca(2+). The pharmacological profile of this response suggests that a type 1 neurotensin receptor is involved but that another, possibly novel, non-type 2 neurotensin receptor is also implicated. If present in vivo, such signalling could be involved in some of the physiological actions of neurotensin.

Clozapine inhibits synaptic transmission at GABAergic synapses established by ventral tegmental area neurones in culture

Elucidation of the mechanism of action of the atypical antipsychotic clozapine is complicated by the finding that this molecule interacts with multiple targets including dopaminergic and serotonergic receptors. Binding studies have suggested that clozapine also antagonises GABA(A) receptors, but physiological evidence for such a block at functional synapses is lacking. In this study, we explored this antagonism by using electrophysiological techniques on GABAergic neurones of the ventral tegmental area in culture. Inhibitory post-synaptic currents (IPSCs) evoked in isolated GABAergic neurones were found to be dose-dependently inhibited by clozapine. Compatible with a post-synaptic mechanism, we found that membrane currents evoked by exogenous applications of GABA were similarly dose-dependently inhibited by clozapine. An analysis of miniature inhibitory post-synaptic currents (mIPSCs) showed that clozapine reduced the amplitude of quantal events in a way similar to SR-95531, a specific GABA(A) receptor antagonist. Both drugs caused a similar leftward shift of the cumulative probability distribution of mIPSC amplitudes. This suggests that clozapine acts on both synaptic and extrasynaptic GABA(A) receptors. In conclusion, our work demonstrates that clozapine produces a functional antagonism of GABA(A) receptors at synapses. Because this effect occurs at concentrations that could be found in the brain of patients treated with clozapine, a reduction in GABAergic synaptic transmission could be implicated in the therapeutic actions and/or side-effects of clozapine.

Activation of neurotransmitter release in hippocampal nerve terminals during recovery from intracellular acidification

Intracellular pH may be an important variable regulating neurotransmitter release. A number of pathological conditions, such as anoxia and ischemia, are known to influence intracellular pH, causing acidification of brain cells and excitotoxicity. We examined the effect of acidification on quantal glutamate release. Although acidification caused only modest changes in release, recovery from acidification was associated with a very large (60-fold) increase in the frequency of miniature excitatory postsynaptic currents (mEPSCs) in cultured hippocampal neurons. This was accompanied by a block of evoked EPSCs and a rise in intracellular free Ca2+ ([Ca2+]i). The rise in mEPSC frequency required extracellular Ca2+, but influx did not occur through voltage-operated channels. Because acidic pH is known to activate the Na+/H+ antiporter, we hypothesized that a resulting Na+ load could drive Ca2+ influx through the Na+/Ca2+ exchanger during recovery from acidification. This hypothesis is supported by three observations. First, intracellular Na+ rises during acidification. Second, the elevation in [Ca2+]i and mEPSC frequency during recovery from acidification is prevented by the Na+/H+ antiporter blocker EIPA applied during the acidification step. Third, the rise in free Ca2+ and mEPSC frequency is blocked by the Na+/Ca2+ exchanger blocker dimethylbenzamil. We thus propose that during recovery from intracellular acidification a massive activation of neurotransmitter release occurs because the successive activation of the Na+/H+ and Na+/Ca2+ exchangers in nerve terminals leads to an elevation of intracellular calcium. Our results suggest that changes in intracellular pH and especially recovery from acidification have extensive consequences for the release process in nerve terminals. Excessive release of glutamate through the proposed mechanism could be implicated in excitotoxic insults after anoxic or ischemic episodes.

Modulation of an early step in the secretory machinery in hippocampal nerve terminals

In hippocampal neurons, neurotransmitter release can be regulated by protein kinase A (PKA) through a direct action on the secretory machinery. To identify the site of PKA modulation, we have taken advantage of the ability of the neurotoxin Botulinum A to cleave the synaptic protein SNAP-25. Cleavage of this protein decreases the Ca2+ responsiveness of the secretory machinery by partially uncoupling Ca2+-sensing from fusion per se. This is expressed as a shift toward higher Ca2+ levels of the Ca2+ to neurotransmitter release relationship and as a perturbation of synaptic delay under conditions where secretion induced by the Ca2+-independent secretagogue ruthenium red is unimpaired. We find that SNAP-25 cleavage also perturbs PKA-dependent modulation of secretion; facilitation of ruthenium red-evoked neurotransmitter release by the adenylyl cyclase activator forskolin is blocked completely after Botulinum toxin A action. Together with our observation that forskolin modifies the Ca2+ to neurotransmitter release relationship, our results suggest that SNAP-25 acts as a functional linker between Ca2+ detection and fusion and that PKA modulates an early step in the secretory machinery related to calcium sensing to facilitate synaptic transmission.

Contact-dependent regulation of N-type calcium channel subunits during synaptogenesis

The developmental regulation of the N-type calcium channel during synaptogenesis was studied using cultured rat hippocampal neurons to elucidate the roles of extrinsic versus intrinsic cues in the expression and distribution of this channel. Prior to synapse formation, alpha1B and beta3 subunits of the N-type calcium channel were distributed diffusely throughout neurites, growth cones, and somata. As synaptogenesis proceeded, the subunit distributions became punctate and colocalized with the synaptic vesicle protein synaptotagmin. Isolated neurons were also examined to test for the requirement of extrinsic cues that control N-type calcium channel expression and distribution. These neurons expressed N-type calcium channel subunits, but their distributions remained diffuse. Functional omega-conotoxin GVIA-sensitive channels were expressed in isolated neurons, although the distribution of alpha1B subunits was diffuse. The distribution of the alpha1B subunit and synaptotagmin only became punctate when neuron-neuron contact was allowed. Thus, the expression of functional N-type calcium channels is the result of an intrinsic program while extrinsic regulatory cues mediated by neuron-neuron contact are required to control their distribution during synaptogenesis.

Direct modulation of the secretory machinery underlies PKA-dependent synaptic facilitation in hippocampal neurons

Activation of protein kinase A (PKA) is known to facilitate synaptic transmission. Using synapses established by hippocampal neurons in culture, we show that dialysis of PKA inhibitors in the presynaptic neuron blocks synaptic facilitation produced by the adenylyl cyclase activator forskolin, demonstrating a presynaptic locus of action. Using ruthenium red, a tool that is known to stimulate exocytosis independently of Ca2+ influx, but in a manner sensitive to tetanus toxin, we find that the secretory process is directly up-regulated under conditions where the number of functional terminals remains unchanged, as revealed by imaging of FM1-43, a vital indicator of synaptic vesicle endocytosis. Taken together with our ultrastructural analysis that suggests no enhancement of docking, our data indicate that PKA causes synaptic facilitation by directly elevating the probability of exocytosis of individual vesicles in response to an invariant Ca2+ signal.

Calcium-independent activation of the secretory apparatus by ruthenium red in hippocampal neurons: a new tool to assess modulation of presynaptic function

The functional plasticity of the nervous system may result in part from the direct modulation of the effectiveness of the release machinery of synaptic terminals. To date, direct modulation of secretion in neurons has proven difficult to study because of the lack of a suitable tool to probe the release machinery independently of calcium influx. We report that the polyvalent cation ruthenium red (RR) directly evokes rapid and reversible calcium-independent quantal secretion in hippocampal neurons by binding to external sites on the presynaptic terminal membrane. This binding can be displaced by heparin and is not associated with ultrastructural damage to the synaptic terminals. The use of RR-evoked release as a tool has allowed us to detect a direct modulation of the secretory apparatus after activation of A1 adenosine receptors on hippocampal neurons.

Postsynaptic modifications in long-term facilitation in Aplysia: upregulation of excitatory amino acid receptors

Long-term sensitization of the gill and siphon withdrawal in Aplysia is accompanied by facilitation of sensorimotor synaptic connections that depends on new protein synthesis. This phenomenon has been previously shown to involve presynaptic growth. At the postsynaptic level, a reorganization should occur to parallel the formation of new synaptic contacts. We show here that 24 hr following an application of 5-HT, which produces long-term synaptic facilitation (LTF), the response of the motoneuron to an excitatory amino acid agonist of the synaptic receptors is increased. General inhibition of protein synthesis with anisomycin blocks this enhancement. Inhibiton of protein synthesis limited to the postsynaptic neuron by intracellular injection of gelonin, a ribosome-inactivating toxin, also blocks the increase in the response to the agonist but fails to block 24 hr LTF. These results are compatible with a model of LTF that involves coordinate pre- and postsynaptic changes. The latter may include an upregulation of functional postsynaptic receptors. These may not be initially required for LTF measured at a 24 hr time point, but could become necessary for later stages of LTF. An increase in the number of functional postsynaptic receptors in a reserve pool may also prime the postsynaptic neuron for subsequent learning-associated plasticity.

Presynaptic receptors for FMRFamide, histamine and buccalin regulate acetylcholine release at a neuro-neuronal synapse of Aplysia by modulating N-type Ca2+ channels

At an identified neuro-neuronal synapse of the buccal ganglion of Aplysia, quantal release of acetylcholine (ACh) is increased by FMRFamide and decreased by histamine or buccalin. Activation of presynaptic receptors for these neuromodulators modifies a presynaptic Ca2+ current which is nifedipine-resistant and omega-conotoxin-sensitive. The voltage-sensitivity of these N-type Ca2+ channels is increased by FMRFamide and decreased by histamine through the intermediate of G proteins. Buccalin does not implicate G proteins and reduces the Ca2+ current without affecting the voltage-sensitivity of N-type Ca2+ channels. The possibility of relating the shifts in voltage-dependence of the Ca2+ current induced by FMRFamide and histamine to the phosphorylation state of the N-type Ca2+ channels is discussed. A scheme for the complex regulation of ACh release by presynaptic auto- and heteroreceptors is proposed.

Excitatory amino acid neurotransmission at sensory-motor and interneuronal synapses of Aplysia californica

1. Although the gill and siphon withdrawal reflex of Aplysia has been used as a model system to study learning-associated changes in synaptic transmission, the identity of the neurotransmitter released by the sensory neurons and excitatory interneurons of the network mediating this behavior is still unknown. The identification of the putative neurotransmitter of these neurons should facilitate further studies of synaptic plasticity in Aplysia. 2. We report that sensory-motor transmission within this circuit is mediated through the activation of an excitatory amino acid receptor that is blocked by the non-N-methyl-D-aspartate excitatory amino acid receptor antagonists 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX) and 1-(4-chlorobenzoyl)-piperazine-2,3-dicarboxylic acid (CBPD). Compound postsynaptic potentials evoked in motor neurons by electrical stimulation of the siphon nerve were blocked by 92% with CNQX (75 microM) and 89% with CBPD (75 microM). 3. Simultaneous intracellular recordings were obtained from sensory neurons, excitatory interneurons, and motor neurons. Monosynaptic excitatory postsynaptic potentials (EPSPs) evoked in motor neurons by an action potential in a sensory neuron were blocked by 86% with CNQX (75 microM) and 71% with CBPD (75 microM). The two antagonists also blocked monosynaptic interneuronal EPSPs onto motor neurons by 65% and 67%, respectively. 4. Potential agonists of the synaptic receptors were puff-applied in the intact abdominal ganglion. Homocysteic acid (HCA) was found to mimic the action of the synaptically released transmitter because it strongly excites motor neurons. This effect was blocked by CNQX. Kainate and domoic acid were also effective agonists. 5. The actions of L- and D-glutamate as well as quisqualate were found to be mainly hyperpolarizing, whereas aspartate and (+/-)-amino-3-hydroxy-5-methylisoxazole-4-propionic acid had no effect. 6. Several reasons may be proposed to explain the inability of puff-applied glutamate to excite effectively the postsynaptic neurons in the intact ganglion. It is possible nonetheless that other endogenous amino acids such as HCA act as neurotransmitters at these synapses.

Sensitization of the gill and siphon withdrawal reflex of Aplysia: multiple sites of change in the neuronal network

1. Recent studies have emphasized the major contribution of interneuronal transmission to the mediation and learning-associated modulation of the gill and siphon withdrawal (GSW) reflex of Aplysia. We wish to provide more direct support for the hypothesis that inhibitory junctions are crucial sites of plasticity. 2. In parallel experiments we investigated modulation at five major sites of synaptic transmission in the GSW network: 1) from sensory neurons to motor neurons, 2) from sensory neurons to excitatory interneurons (INTs+) 3) from INTs+ to motor neurons (MNs), 4) from inhibitory interneurons (INTs-) to INTs+, and 5) from INTs+ to INTs-. 3. While recording simultaneously from a single sensory neuron of the LE cluster, an INT+, and a MN, we found that both LE-MN and LE-INTs+ synapses were facilitated by the activation of modulator neurons by stimulation of the left pleuroabdominal connective (185 and 93%, respectively) as well as by serotonin (5-HT) (191 and 84%). Junctions of the second type were therefore less facilitated. The difference in the magnitude of facilitation at these two sites is an indication of a branch-specific, differential efficacy in the modulation of different central synapses made by a single neuron. 4. Although INT(+)-MN junctions have the capacity to display marked posttetanic potentiation, they are not significantly potentiated after connective stimulation. Sensitization of the GSW reflex is therefore not necessarily accompanied by a modification of transmission at these synapses. 5. Inhibitory transmission to INTs+ is significantly reduced by connective stimulation (36%) and by 5-HT (71%). This supports the hypothesis that a reduction of feedback inhibition into INTs+ is a major mechanism of reflex sensitization and may account for the increased evoked firing of INTs+ that is observed after connective stimulation. 6. The excitatory input to INTs- is selectively decreased by 5-HT (50%) and by the molluscan neuropeptide small cardioactive peptide B (38%). This latter effect, which could produce disinhibition of INTs+, may explain the previous observation that this peptide is able to potentiate the evoked input to MNs of the reflex at a concentration (1 microM) that fails to modify monosynaptic sensory-motor transmission. 7. These results indicate that transmission through a small neuronal network that mediates a withdrawal reflex in Aplysia may be modulated at multiple sites and by different mechanisms. These mechanisms include: 1) branch-specific facilitation of sensory neuron outputs and 2) inhibition of INT(-)-INT+ inhibitory postsynaptic potentials by endogenous modulatory neurons and by 5-HT.(ABSTRACT TRUNCATED AT 400 WORDS)

Functional uncoupling of inhibitory interneurons plays an important role in short-term sensitization of Aplysia gill and siphon withdrawal reflex

Attempts to explain learning-associated potentiation of synaptic transmission in model systems such as withdrawal reflexes in the mollusk Aplysia or the hippocampus of vertebrates have focused on the mechanisms by which transmitter release is increased in the principal elements of the circuit. Increased transmission in neuronal networks such as the gill and siphon withdrawal reflex (GSWR) of Aplysia may, however, also be caused by a decrease of transmitter release by inhibitory interneurons. The importance and function of cholinergic inhibitory transmission in the GSWR network were investigated. Central application of the nicotinic cholinergic antagonist d-tubocurarine (d-TC) considerably potentiated gill contractions, evoked either by nerve stimulation or by tactile stimulation of the siphon. Compound EPSPs evoked in motoneurons upon siphon nerve stimulation were also significantly prolonged following application of d-TC, but were unaffected by hexamethonium, a blocker of excitatory ACh receptors in Aplysia. Recordings from excitatory interneurons showed that they received excitation followed by powerful inhibitory input upon stimulation of the siphon nerve. Application of d-TC completely blocked this rapid inhibition, thus prolonging the compound EPSPs evoked in the interneurons. These effects were obtained at a concentration of d-TC (100 microM) that almost totally blocked fast inhibitory cholinergic transmission, but was without effect on monosynaptic connections between sensory neurons and motoneurons of the reflex. Facilitation of (1) compound EPSCs in motoneurons and (2) evoked excitatory interneuronal firing was reduced in preparations already disinhibited by pretreatment with d-TC. Facilitation of sensory-motor synapses, however, was not reduced in the presence of d-TC, indicating that facilitatory interneurons are still activated under cholinergic blockade. These data show that transmission through the GSWR neuronal network is gated by a feedback inhibitory mechanism. They also suggest that a reduction of cholinergic inhibition onto excitatory interneurons may be a mechanism through which transmission within the GSWR network is increased during various forms of learning, such as sensitization. These data place new emphasis on the important role of inhibitory interneurons in determining the plastic properties of neuronal networks, in both invertebrates and vertebrates.

Transmitter release and calcium currents at an Aplysia buccal ganglion synapse--I. Characterization

The Ca2+ current recorded in the presynaptic neuron (B4/B5) of an identified Aplysia synapse was characterized in terms of its activation, voltage sensitivity, Ca2+ dependence of inactivation and pharmacology. It was compared to that recorded in left upper quadrant abdominal ganglion neurons which, unlike B4/B5, display Ca2+ action potentials. The two Ca2+ currents could not be distinguished in terms of their activation threshold or voltage sensitivity. The Ca2+ current recorded in left upper quadrant neurons, however, displayed more important Ca(2+)-dependent inactivation. The peak Ca2+ current in B4/B5 neurons was significantly reduced (30-40%) by the dihydropyridine Ca2+ channel antagonist, nifedipine, while it was increased (15-20%) by the dihydropyridine Ca2+ channel agonist, BAY K8644, although none of these agents had any effect on transmitter release from B4/B5. omega-Conotoxin similarly reduced the Ca2+ current by 30-40%, but unlike nifedipine, it also caused a 50-60% reduction in B4/B5 transmitter release. The pharmacological properties of the Ca2+ current present in left upper quadrant neurons were somewhat different, as this current was unaffected by either BAY K8644 or omega-conotoxin and moderately suppressed (20%) by nifedipine.

Transmitter release and calcium currents at an Aplysia buccal ganglion synapse--II. Modulation by presynaptic receptors

Changes in evoked acetylcholine quantal release induced by histamine, FLRFamide and buccalin were investigated at an identified neuro-neuronal synapse in the buccal ganglion of Aplysia californica. Regulation of acetylcholine release by these neuromodulators was correlated with their actions on the presynaptic Ca2+ current. We have previously reported that FLRFamide and histamine, respectively, increase and decrease acetylcholine release from buccal neurons B4/B5. Buccalin, a peptide specific to the buccal ganglion, lowered the number of acetylcholine quanta released. Consistent with the synaptic effects, the presynaptic nifedipine-resistant Ca2+ current that triggers the release of acetylcholine in B4/B5 neurons [Trudeau L.-E. et al. (1993) Neuroscience 53, 571-580] was lowered by buccalin or by histamine and enhanced by FLRFamide. The analysis of tail currents showed that histamine shifts the voltage dependence of the nifedipine-resistant Ca2+ channels towards more positive voltages, whereas FLRFamide has an opposite action. Buccalin did not affect the voltage dependence of the channels but depressed the amplitude of the Ca2+ current, an effect which could be due either to a reduction of the number of available Ca2+ channels, to a decrease of their unitary conductance or to a modification of their gating. Inactivation of presynaptic G proteins prevented the modulatory actions of FLRFamide and histamine on quantal acetylcholine release and also on the voltage dependence of the nifedipine-resistant Ca2+ channels. This procedure, however, failed to prevent the suppressive effects of buccalin. The possibility of relating the voltage dependence shifts of the Ca2+ current induced by FLRFamide and histamine to the phosphorylation state of the Ca2+ channels is discussed. It is concluded that three independent presynaptic pathways initiated by histamine, FLRFamide and buccalin control presynaptic Ca2+ influx, these modulations being apparent within the physiological range of voltages required to activate Ca2+ channels.

Pre- and postsynaptic actions of nifedipine at an identified cholinergic central synapse of Aplysia

The effects of the dihydropyridine (DHP) Ca2+ channel antagonist, nifedipine, were studied on the cholinergic synapse between the presynaptic neurones B4/B5 and the postsynaptic neurones B3/B6 located in the buccal ganglion of Aplysia californica. Nifedipine (10 microM) decreased the presynaptic Ca2+ current by 30%-40%. Blockade of DHP-sensitive Ca2+ channels, however, did not affect quantal transmitter release from the presynaptic neurones. Thus, at this synapse, DHP-sensitive Ca2+ channels appear not to be involved in acetylcholine (ACh) release. The postsynaptic response to an ionophoretic application of ACh was decreased by nifedipine, pointing to a blocking action of the drug on the postsynaptic receptor/channel complex. Nifedipine was also found to activate protein kinase C, which in turn induces an increase in the nifedipine-resistant presynaptic Ca2+ influx and in the number of released ACh quanta. These effects of nifedipine could be prevented by a previous application of 1,5-(isoquinolinylsulfonyl)-2-methyl-piperazine (H-7), a protein kinase blocker.

Contribution of polysynaptic pathways in the mediation and plasticity of Aplysia gill and siphon withdrawal reflex: evidence for differential modulation

The gill and siphon withdrawal (GSW) reflex of Aplysia is centrally mediated by a monosynaptic and a polysynaptic pathway between sensory and motor neurons. The first objective of this article was to evaluate quantitatively the relative importance of these two components in the mediation of the GSW reflex. We have used an artificial sea water (ASW) solution containing a high concentration of divalent cations to raise the action potential threshold of the interneurons without affecting the monosynaptic component of the reflex (2:1 ASW). Compound EPSPs induced in gill or siphon motor neurons by direct stimulation of the siphon nerve or by tactile stimulation of the siphon skin were reduced by more than 75% in 2:1 ASW. These results indicate that interneurons intercalated between sensory and motor neurons are responsible for a considerable proportion of the afferent input to the motor neurons of the reflex. The second objective of this article was to compare the modulation of the monosynaptic and polysynaptic pathways. We have evaluated their respective contribution in sensitization of the GSW reflex by testing the effects of two neuromodulators of the reflex, 5-HT and small cardioactive peptide B (SCPB). We found that these two neuromodulators have a differential action on the two components of the GSW neuronal network. The polysynaptic pathway was more facilitated than the monosynaptic pathway by the neuropeptide SCPB. By contrast, 5-HT displayed an opposite selectivity. These results suggest that the polysynaptic component of the neuronal network underlying the GSW reflex is very important for its mediation. The data also indicate that the monosynaptic and polysynaptic components of the reflex can be differentially modulated. The diversity of modulatory actions at various sites of the GSW network should be relevant for learning-associated modifications in the intact animal.

Involvement of Ca2+ uptake by a reticulum-like store in the control of transmitter release

At an identified neuro-neuronal synapse of Aplysia, 2,5-diterbutyl 1,4-benzohydroquinone, a selective blocker of the reticulum Ca2+ pump, was found to potentiate evoked quantal release of acetylcholine through an increased accumulation of Ca2+ in the presynaptic neuron during depolarization without any accompanying changes in the presynaptic Ca2+ current. We conclude that a rapid Ca2+ buffering system, similar to that associated with the endoplasmic reticulum, must be present in the nerve terminal and play a role in the control of Ca2+ which reaches the release system.

Xanthine derivatives IBMX and S-9977-2 potentiate transmission at an Aplysia central cholinergic synapse

In an attempt to investigate the role of cAMP-dependent phosphorylations on synaptic transmission at an Aplysia cholinergic buccal ganglion synapse, the effects of xanthine derivatives such as 3-isobutyl-1-methylxanthine (IBMX), which is well known to inhibit phosphodiesterase activity thereby promoting cAMP accumulation, and a novel xanthine derivative, S-9977-2 were evaluated. They were found to potentiate cholinergic transmission by significantly increasing the time constant of decay (Tc) of inhibitory postsynaptic currents (IPSCs). The postsynaptic origin of the phenomenon was supported by the observation that responses to the ionophoretic application of acetylcholine (ACh) were also potentiated in duration as well as in amplitude. No effects of S-9977-2 on the ACh-gated Cl- channel conductance or mean open time were observed. The finding that responses to the hydrolysis-resistant cholinergic analogue carbachol were unaffected by the two xanthines suggested that the observed effects were at least partly caused by an inhibition of acetylcholinesterase (AChE) activity. That these substances inhibit AChE activity was confirmed in vitro. Phosphorylation processes nonetheless appear to be partly involved in the synaptic effect of the xanthines as the kinase blocker H-8 blocked part of the IPSC Tc lengthening. Possible mechanisms are discussed.

Effect of taurine on ethanol-induced changes in open-field locomotor activity

In the present investigation we questioned whether taurine antagonized the effects of ethanol on motor activity measured in the open field. Ten minutes following simultaneous administration (IP) of ethanol (1.0, 1.5, 2.0 and 2.5 g/kg) or saline and taurine (30, 45 and 60 mg/kg) or saline, mice were placed in open field chambers and locomotor activity was measured during a 10 min testing period. A significant interaction was found between taurine and ethanol. Taurine-treated mice displayed lower motor excitation with the 1.0 g/kg dose of ethanol than the saline group treated with the same dose of ethanol. However at the 2.0 g/kg ethanol dose, taurine-treated mice demonstrated higher motor activity than the saline treated mice, once again, treated with the same dose of ethanol. No differences in blood ethanol levels were observed between the two groups. In a second study, taurine administration (30, 45 and 60 mg/kg) did not show any effect on d-amphetamine-induced enhancement of locomotor activity (1, 2, and 3 mg/kg). Data from this study demonstrated an interaction between taurine and ethanol in their effect on locomotor activity in the open field.

Involvement of endogenous opioid mechanisms in the interaction between stress and ethanol

The involvement of endogenous opioid mechanisms in the interaction between stress and ethanol was investigated in the rat. Animals were pretreated with naltrexone (10 mg/kg) or saline 3 h before a second injection consisting of ethanol (1.0 g/kg) or saline. They were then restrained for 15 or 60 min or left in home cages for an equivalent amount of time. After restraint, animals were either subjected to an open-field test or decapitated to collect blood for corticosterone determinations. Locomotor depression was found to be induced by 15 but not 60 min restraint. In naltrexone-treated animals, however, 60 min restraint was also found to induce locomotor depression. Ethanol pretreatment was found to block the locomotor depression induced by 15 min restraint. Such an interaction was in turn antagonized by naltrexone. In the 15 min condition, stress and ethanol were also found to interact in their effects on plasma levels of corticosterone. Naltrexone did not alter any effects of the stressors on corticosterone levels. These results provide support for the involvement of endogenous opioid mechanisms in the interaction of stress and ethanol at a behavioural level.

Stress-ethanol interaction: involvement of endogenous opioid mechanisms

Stress effects vary with different environmental situations or stress intensities. The effects of restraint stress on locomotion and or corticosterone were examined. Rats were restrained for 0, 15, 30, 60, 90 or 120 min, subsequent locomotion was measured for 10 min. Rats were also sacrificed for corticosterone determinations. Restraint stress affects both variables. Locomotion was recorded in rats pretreated with naltrexone or vehicle prior to restraint of 15 or 60 min. Naltrexone influenced the effects of stress differentially. It did not affect the results following 15 min of restraint but suppressed locomotion after 60-min restraint to a level comparable to that found after 15 min. Treatment with ethanol (1.0, 1.5, 2.0 g/kg) prior to 15 or 60 min of restraint resulted in the prevention of hypomotility induced by 15-min stress. It also interacted nonadditively with 15-min stress on corticosterone release. No such interaction occurred with 60 min stress. Also, naltrexone made it possible to block the effect of ethanol on restraint-induced hypomotility. Results describe stress as a nonunitary concept. Its effects tend to vary with its duration. The differential interaction of stress with naltrexone and ethanol depending on its duration supports the above notion. Results further suggest recruitment of opioid systems in long duration stress (60 min).

Effects of ethanol on locomotor depression and corticosterone release induced by restraint-stress: support for a stress-ethanol interaction

The interaction between restraint-stress and ethanol was investigated in the rat. The effects of ethanol pretreatment (0.0, 1.0, 1.5, 2.0 g/kg, 20% v/v) on locomotor depression and corticosterone release induced by restraint-stress (15, 60 min) were measured. Restraint durations of 15, 30, 90 and 120 min were found to decrease locomotor activity while animals restrained for 60 min did not differ from home cage controls. All restraint durations induced a significant increase in plasma levels of corticosterone. Locomotor activity counts of ethanol-pretreated (1.0, 1.5, 2.0 g/kg; 20% v/v) animals restrained for 15 min were not found to be lower than those of ethanol-pretreated animals remaining in home cages. Ethanol pretreatment did not differentially affect the locomotor activity of restrained or home cage animals in the 60-min condition. Plasma corticosterone levels of ethanol-pretreated animals restrained for 15 min were identical to those of ethanol-pretreated home cage controls. However, ethanol-pretreated animals restrained for 60 min demonstrated plasma corticosterone levels higher than those obtained by ethanol pretreatment or 60-min restraint alone. Blood ethanol levels were not found to be different between ethanol-control and ethanol-stress animals. These results provide support for a stress-ethanol interaction. They also suggest a differential interaction of ethanol with different intensities of stress.