Loss of channel modulation by transmitter and protein kinase C during innervation of an identified leech neuron

Ca2+ influx into identified leech neurons induced by 5-hydroxytryptamine

The role of 5-hydroxytryptamine (5-HT, serotonin) in the control of leech behavior is well established and has been analyzed extensively on the cellular level; however, hitherto little is known about the effect of 5-HT on the cytosolic free calcium concentration ([Ca(2+)](i)) in leech neurons. As [Ca(2+)](i) plays a pivotal role in numerous cellular processes, we investigated the effect of 5-HT on [Ca(2+)](i) (measured by Fura-2) in identified leech neurons under different experimental conditions, such as changed extracellular ion composition and blockade of excitatory synaptic transmission. In pressure (P), lateral nociceptive (N1), and Leydig neurons, 5-HT induced a [Ca(2+)](i) increase which was predominantly due to Ca(2+) influx since it was abolished in Ca(2+)-free solution. The 5-HT-induced Ca(2+) influx occurred only if the cells depolarized sufficiently, indicating that it was mediated by voltage-dependent Ca(2+) channels. In P and N1 neurons, the membrane depolarization was due to Na(+) influx through cation channels coupled to 5-HT receptors, whereby the dose-dependency suggests an involvement in excitatory synaptic transmission. In Leydig neurons, 5-HT receptor-coupled cation channels seem to be absent. In these cells, the membrane depolarization activating the voltage-dependent Ca(2+) channels was evoked by 5-HT-triggered excitatory glutamatergic input. In Retzius, anterior pagoda (AP), annulus erector (AE), and median nociceptive (N2) neurons, 5-HT had no effect on [Ca(2+)](i).

Antidepressants differentially influence the transcriptional activity of the glucocorticoid receptor in vitro

Functional normalization of the hypothalamic-pituitary-adrenal axis in depressive patients by successful treatment with antidepressants is associated with increased efficiency of corticosteroid signal transduction. Accordingly, some antidepressants have been shown to influence the activity of the glucocorticoid receptor (GR) in cultured cells. It is not clear, however, whether this is a common principle for all antidepressants throughout all classes. Therefore, we screened a range of 18 antidepressants of different classes for their effect on GR signaling in a reporter gene assay using the mouse hippocampal cell line HT22. We evaluated GR-mediated gene transcription after short-time incubation (24 h) with different concentrations of each antidepressant (1 or 10 microM) in the presence or absence of the synthetic steroid dexamethasone (0.01 or 1 microM). The majority of antidepressants had a tendency to enhance steroid-induced GR-mediated gene transcription at high concentrations of antidepressant and low concentrations of steroid. Some antidepressants reduced the steroid-independent background activity of GR. This reduction was not due to unspecific inhibition of GR by oxidative stress, since no induction of intracellular peroxides was detectable in the concentration range of antidepressants used in our study. Furthermore, no significant change in GR activity was observed by concomitant treatment of HT22 cells with the antioxidant alpha-tocopherol (vitamin E). In conclusion, we report that many antidepressants enhance GR signaling in an in vitro neuronal system at clinically relevant concentrations. Those not showing an effect in vitro apparently use different mechanisms to influence GR activity that require an in vivo setting.

Calcium influx into dendrites of the leech Retzius neuron evoked by 5-hydroxytryptamine

5-Hydroxytryptamine (5-HT) is a ubiquitous neurotransmitter and neuromodulator that affects neural circuits and behaviours in vertebrates and invertebrates. In the present study, we have investigated 5-HT-induced Ca(2+) transients in subcellular compartments of Retzius neurons in the leech central nervous system using confocal laser scanning microscopy, and studied the effect of 5-HT on the electrical coupling between the Retzius neurons. Bath application of 5-HT (50mM) induced a Ca(2+) transient in axon, dendrites and cell body of the Retzius neuron. This Ca(2+) transient was significantly faster and larger in dendrites than in axon and cell body, and was half-maximal at a 5-HT concentration of 5-12mM. The Ca(2+) transient was suppressed in the absence of extracellular Ca(2+) and by methysergide (100mM), a non-specific antagonist of metabotropic 5-HT receptors, and was strongly reduced by bath application of the Ca(2+) channel blocker Co(2+) (2mM). Injection of the non-hydrolysable GTP analogue GTPgammaS increased and prolonged the dendritic 5-HT-induced Ca(2+) transient. The non-selective protein kinase inhibitor H7 (100mM) and the adenylate cyclase inhibitor SQ22536 (500 mM) did not affect the Ca(2+) transient, and the membrane-permeable cAMP analogue dibutyryl-cAMP (500 mM) did not mimic the effect of 5-HT application. 5-HT reduced the apparent electrical coupling between the two Retzius neurons, whereas suppression of the Ca(2+) influx by removal of external Ca(2+) improved the transmission of action potentials at the electrical synapses which are located between the dendrites of the adjacent Retzius neurons. The results indicate that 5-HT induces a Ca(2+) influx through calcium channels located primarily in the dendrites, and presumably activated by a G protein-coupled 5-HT receptor. The dendritic Ca(2+) increase appears to modulate the excitability of, and the synchronization between, the two Retzius neurons.

Excitatory peptides and osmotic pressure modulate mechanosensitive cation channels in concert

Behavioral and neuroendocrine responses underlying systemic osmoregulation are synergistically controlled by osmoreceptors and neuropeptides released within the hypothalamus. Although mechanisms underlying osmoreception are understood, the cellular basis for the integration of osmotic and peptidergic signals remains unknown. Here we show that the excitatory effects of angiotensin II, cholecystokinin and neurotensin on supraoptic neurosecretory neurons are due to the stimulation of the stretch-inactivated cation channels responsible for osmoreception. This molecular convergence underlies the facilitatory effects of neuropeptides on responses to osmotic stimulation and provides a basis for the gating effects of plasma osmolality on the responsiveness of osmoregulatory neurons to peptidergic stimulation.

Requirement for tyrosine phosphatase during serotonergic neuromodulation by protein kinase C

Tyrosine kinases and phosphatases are abundant in the nervous system, where they signal cellular differentiation, mediate the responses to growth factors, and direct neurite outgrowth during development. Tyrosine phosphorylation can also alter ion channel activity, but its physiological significance remains unclear. In an identified leech mechanosensory neuron, the ubiquitous neuromodulator serotonin increases the activity of a cation channel by activating protein kinase C (PKC), resulting in membrane depolarization and modulation of the receptive field properties. We observed that the effects on isolated neurons and channels were blocked by inhibiting tyrosine phosphatases. Serotonergic stimulation of PKC thus activates a tyrosine phosphatase activity associated with the channels, which reverses their constitutive inhibition by tyrosine phosphorylation, representing a novel form of neuromodulation.

Modulation and selection of neurotransmitter responses during synapse formation between identified leech neurons

1. Serotonin (5-HT) modulates two different responses in the pressure sensitive neurons (P) of the leech: an inhibitory, Cl- dependent synaptic response and a depolarizing extrasynaptic response. 2. Serotonergic Retzius cells (R) in vivo and in culture elicit inhibitory Cl- dependent responses in P neurons. Moreover, at discrete sites of contact between R and P cells, the excitatory response to 5-HT is gradually lost prior to synapse formation. This phenomenon is specifically mediated by R cells. 3. The extrasynaptic response is mediated by cation channels sensitive to protein kinase C (PKC). Cation channels are present at the sites of contact but they become insensitive to PKC. Moreover, cation channels from single P cells are no longer modulated by PKC if they are inserted (by cramming the patch pipette) into the cytoplasm of a P cell in contact with an R cell. 4. Blockers of tyrosine kinases prevent the uncoupling of cation channel modulation and inhibit synapse formation between the R and the P neurons. 5. We suggest that cell contact induces an intracellular, tyrosine kinase-dependent signal as part of the mechanism of neuronal recognition leading to synapse formation.

Synapse formation and function: insights from identified leech neurons in culture

Identified leech neurons in culture are providing novel insights to the signals underlying synapse formation and function. Identified neurons from the central nervous system of the leech can be removed individually and plated in culture, where they retain their characteristic physiological properties, grow neurites, and form specific synapses that are directly accessible by a variety of approaches. Synapses between cultured neurons can be chemical or electrical (either rectifying or not) or may not form, depending on the neuronal identities. Furthermore, the characteristics of these synapses depend on the regions of the cells that come into contact. The formation and physiology of synapses between the Retzius cell and its partners have been well characterized. Retzius cells form purely chemical, inhibitory synapses with pressure-sensitive (P) cells where serotonin (5-HT) is the transmitter. Retzius cells synthesize 5-HT, which is stored in vesicles that recycle after 5-HT is secreted on stimulation. The release of 5-HT is quantal, calcium-dependent, and shows activity-dependent facilitation and depression. Anterograde and retrograde signals during synapse formation modify calcium currents, responses to 5-HT, and neurite outgrowth. The nature of these synaptogenic signals is being elucidated. For example, contact specifically with Retzius cells induces a localized selection of transmitter responses in postsynaptic P cells. This effect is signaled by tyrosine phosphorylation prior to synapse formation.

Tyrosine phosphorylation during synapse formation between identified leech neurons

1. We have examined whether tyrosine phosphorylation is required for synapse formation between identified neurons from the central nervous system of the leech in culture. 2. Within a few hours of contact with the cell body of the serotonergic Retzius neuron (R cell), the soma of the postsynaptic pressure-sensitive neuron (P cell), but not the R cell, could be labelled intracellularly with an antibody against phosphotyrosine residues. The labelling seemed specific for P cells contacted by R cells, as it was greatly reduced in pairs of either R or P cells and in single cells. Genistein (20 microM) and lavendustin A (10 microM), selective inhibitors of tyrosine kinases, blocked the labelling of contacted P cells, whereas their ineffective analogues (genistein and lavendustin B) had no effect on labelling. 3. R cell contact also induced the loss of an extrasynaptic, depolarizing response (due to modulation of cation channels) to serotonin (5-HT) in the P cell within a few days of juxtaposing cell bodies and within an hour of contact with growth cones. Treatment of the neurons with the tyrosine kinase inhibitors (but not the ineffective analogues) prevented the loss of the depolarizing response and of single cation channel modulation by 5-HT. 4. R cells formed inhibitory, Cl(-)-dependent synapses with P cells. Synapse formation was prevented by the tyrosine kinase inhibitors but not by their ineffective analogues. These compounds had no obvious effect on neurite outgrowth or cell adhesion. We conclude that tyrosine phosphorylation is a signal during the formation of this synapse.

Signalling synapse formation between identified neurons

We have investigated the signals between identified leech neurons during the formation of specific synapses in culture. At an inhibitory serotonergic synapse between two well-studied neurons, the postsynaptic cell has an additional (extrasynaptic) excitatory response to 5-HT which may underly a form of activity-dependent modulation. Thus, the presynaptic neuron must select which 5-HT response will be activated and which will be excluded at its synapses. The selection of these responses preceded synapse formation and was specifically induced at sites of contact with the presynaptic neuron, this not being observed for other cell pairings. Aldehyde-fixed presynaptic cells were equally effective, unless pre-treated with trypsin or wheat germ agglutinin, suggesting that contact with a specific cell-surface glycoprotein induced this physiological change in 5-HT sensitivity. The mechanism underlying the selective loss of the extrasynaptic response has been examined by single channel recording. Cation channels in the postsynaptic neuron were modulated by protein kinase C (PKC) upon binding of 5-HT to a 5-HT2 receptor. However, at sites of contact with the presynaptic neuron, the channels were no longer sensitive to PKC. Furthermore, when cation channels from uncontacted neurons were inserted or 'crammed' into contacted neurons, they were rapidly rendered insensitive to PKC, demonstrating a cytoplasmic signal for the uncoupling of channel modulation. Interestingly, the cytoplasm of contacted postsynaptic neurons showed immunoreactivity for tyrosine phosphorylation: exposure of the neurons to specific inhibitors of tyrosine kinases prevented tyrosine phosphorylation, the loss of cation channel modulation and synapse formation.(ABSTRACT TRUNCATED AT 250 WORDS)

Interleukin-1 and interleukin-6 modify protein phosphorylation in the central nervous system of Hirudo medicinalis

Recent data suggest that IL-1-like molecules have been conserved during evolution. The signal transduction mechanism of IL-1 is not known, but kinase activation has been reported. With the aim of studying if human IL-1 has any effect on the leech nervous system, we have added this cytokine to segmental ganglia labeled with [32P]ortophosphoric acid; proteins have been separated by electrophoresis and phosphoproteins detected by autoradiography. In the present paper we show that human IL-1 and IL-6 are able to induce changes on protein phosphorylation in the leech central nervous system and that these changes are similar to those ones induced by the neurotransmitter serotonin.

Membrane properties of microglial cells isolated from the leech central nervous system

Membrane potentials and channel properties of microglial cells isolated from the leech central nervous system and maintained in culture on different substrates were investigated by using the patch-clamp technique. As expected, microglia on concanavalin A (con-A) were round and stationary, whereas those on extract of extracellular matrix (ECM) were spindle shaped and mobile. The mean membrane capacitance was 9 +/- 1 pF (s.e., n = 46), and the input resistance ranged from 0.135 G omega to 21 G omega with a mean of 4.2 +/- 1.6 G omega (n = 19). On-cell patches exhibited no single-channel activity. Voltage-dependent Na+, K+ and Ca2+ currents typical of neurons were absent. Currents evoked in response to voltage ramps from -100 mV to +100 mV or to steps of 4 s duration reversed in sign at or near 0 mV and exhibited single-channel activity of increasing amplitude for incrementally larger positive and negative voltage steps. No differences between the membrane properties of microglial cells on con-A and on ECM were evident. Currents were increased in fluid in which Na+ was substituted with K+, and were decreased when Na+ was substituted with N-methyl-D-glucamine. Varying external [Cl-] was without effect, as was addition to the fluid of 100 microM anthracene-9-carboxylate, a Cl- channel blocker. Together these characteristics indicated a cation channel. Bath application of 100 microM serotonin reversibly increased both inward and outward currents as well as single-channel activity. It is concluded that cultured microglial cells isolated from the adult leech have high membrane resistance and cation channel activity influenced by serotonin.

Cellular and molecular approaches to neural repair in the medicinal leech

Investigation of repair processes and the effects of damage in the leech has the advantage that it is possible to work with single neurons, and that leech neurons can regenerate. The effects of surface topology on neuronal outgrowth in culture were investigated using patterned substrates produced by photolithography. On grooved substrates with a depth, width and separation of 2 microns, the processes of cultured neurons showed significant alignment. In studies on regeneration, different mechanosensory neurons were shown to respond differently to lesions of their axons. High threshold units (N cells) respond by sprouting of additional processes from the axon hillock region within the CNS, whereas low threshold cells respond with a sprouting only at the lesion site in the nerve root and not within the CNS. Sprouted processes are retained for over a year. When a neuron of a particular modality is killed, cells of the same modality (but not the other) expand their receptive fields to cover the denervated skin. Single channel patch clamp recording experiments have been initiated on damaged neurons. Experiments are in progress to clone genes whose expression is regulated after peripheral nerve injury.

Selection of transmitter responses at sites of neurite contact during synapse formation between identified leech neurons

1. Pressure sensitive (P) neurons of the leech Hirudo medicinalis show both an inhibitory, Cl(-)-dependent response and a depolarizing, cationic response to pipette application of serotonin (5-HT). Serotonergic Retzius (R) neurons in culture reform inhibitory, Cl(-)-dependent synapses with P neurons but fail to elicit the extrasynaptic, depolarizing response to 5-HT. We have examined the localization of the selection of 5-HT responses by testing the sensitivity of P cell growth cones and neurites to 5-HT application. 2. As measured by intracellular recording at the P cell soma, synaptic release of 5-HT from R cell processes activated only the Cl(-)-dependent response in P cell neurites. Focal application of 5-HT from a micropipette depolarized uncontacted P cell growth cones and neurites. In contrast, processes from the same P cells that were contacted by R cells were rarely depolarized by 5-HT application unless the application pipette was moved along the neurites away from the sites of contact. 3. The channels underlying the depolarizing response to 5-HT were identified in patch clamp recordings from P cell growth cones. These cation channels showed rare, brief openings in the absence of 5-HT. Application of 5-HT in the bath (outside the patch pipette) increased channel activity in uncontacted P cell growth cones but not in growth cones of the same P cells contacted by R cells. 4. We conclude that the selection of transmitter responses during synapse formation was localized to discrete sites of contact between the synaptic partners.

Tyrosine kinase-dependent selection of transmitter responses induced by neuronal contact

Transmitter receptors are localized to discrete cellular sites such that only those responses appropriate for a particular pattern of inputs are activated. How neurons select between synaptic and extrasynaptic responses during development is not understood. We have investigated how contact during synapse formation between identified leech neurons selectively suppresses the modulation of extrasynaptic channels by protein kinase C. A microelectrode with an isolated membrane patch containing channels from an uninnervated target neuron was 'crammed' into a similar cell contacted by a presynaptic partner. We report here that within a few minutes, the crammed channels were rendered insensitive to activation of protein kinase C, demonstrating the action of a cytoplasmic signal. Treatment of the neurons with selective inhibitors of tyrosine kinases, which are signalling molecules during normal and oncogenic cellular differentiation, prevented the loss of channel modulation. Thus, tyrosine kinases mediate early functional changes during specific synapse formation that are induced by neuronal contact.

Effect of serotonin on protein phosphorylation in the central nervous system of the leech Hirudo medicinalis

1. Phosphoproteins of different regions of the Hirudo medicinalis central nervous system have been analysed by means of two-dimensional electrophoresis. 2. Serotonin, 8-Br-cAMP and phorbol 12,13-dibutyrate stimulate phosphorylation of a number of proteins whose isoelectric points and molecular weights are presented. 3. A group of proteins of 78 kDa and pI = 6-6.5, whose level of phosphorylation increases in the presence of serotonin, 8-Br-cAMP and phorbol ester, is observed only in segmental but not in cephalic or caudal ganglia. 4. The putative roles of these phosphoproteins are discussed.

Loss of extrasynaptic channel modulation by protein kinase C underlies the selection of serotonin responses in an identified leech neuron

Pressure-sensitive (P) neurons contacted by serotonergic Retzius (R) neurons of the leech in culture selectively reduce a protein kinase C (PKC)-dependent cation response to serotonin and are innervated by the inhibitory, Cl(-)-dependent synapse seen in vivo. We have examined whether the reduction of extrasynaptic cation channel modulation is due to changes in sensitivity of the channels to second messenger. In inside-out membrane patches from single, uncontacted P cells in culture, cation channel activity was increased by rat brain PKC and cofactors. In contrast, the activity of cation channels in patches isolated from P cells paired with R cells was unaffected by PKC. These results demonstrate the loss of extrasynaptic channel modulation by PKC during synapse formation.

Effect of phorbol ester on protein phosphorylation in the central nervous system of the leech Hirudo medicinalis: a two-dimensional electrophoretical analysis

1. Proteins of different regions of the Hirudo medicinalis central nervous system have been analyzed by means of two-dimensional electrophoresis. 2. Subcellular distribution of phosphoproteins has been studied in leech segmental ganglia. 3. Phorbol 12,13-dibutyrate, a protein kinase C activator, stimulates the phosphorylation of a number of proteins whose isoelectric points and mol. wts are presented. 4. Putative roles for these phosphoproteins are discussed.