Parallel processing and selection of the responses to serotonin during reinnervation of an identified leech neuron

Intracellular recording, sensory field mapping, and culturing identified neurons in the leech, Hirudo medicinalis

The freshwater leech, Hirudo medicinalis, is a versatile model organism that has been used to address scientific questions in the fields of neurophysiology, neuroethology, and developmental biology. The goal of this report is to consolidate experimental techniques from the leech system into a single article that will be of use to physiologists with expertise in other nervous system preparations, or to biology students with little or no electrophysiology experience. We demonstrate how to dissect the leech for recording intracellularly from identified neural circuits in the ganglion. Next we show how individual cells of known function can be removed from the ganglion to be cultured in a Petri dish, and how to record from those neurons in culture. Then we demonstrate how to prepare a patch of innervated skin to be used for mapping sensory or motor fields. These leech preparations are still widely used to address basic electrical properties of neural networks, behavior, synaptogenesis, and development. They are also an appropriate training module for neuroscience or physiology teaching laboratories.

Ionotropic and metabotropic activation of a neuronal chloride channel by serotonin and dopamine in the leech Hirudo medicinalis

1. Cl- channels on the pressure-sensitive (P) neuron in the leech are directly activated by synaptic release of serotonin (5-HT) and are indirectly stimulated by the cAMP second messenger pathway, suggesting an unusual dual regulation of the channels. We have investigated the mode of action of 5-HT and dopamine (DA) on a Cl- channel in adult P cells in culture by recording from cell-attached patches. 2. 5-HT increased Cl- channel activity only when included in the recording pipette and not when applied in the bath. 3. Pipette or, more effectively, bath application of DA led to an increase in Cl- channel activity. This effect was blocked by the potent and specific dopaminergic (DA1) receptor blocker, SCH-23390. 4. The stimulation by DA, but not by 5-HT, was also blocked by the cAMP-dependent protein kinase A (PKA) inhibitor Rp-cAMP and was mimicked by the membrane-permeant cAMP analogue dibutyryl cAMP (db-cAMP). 5. Our results show that 5-HT directly gates a Cl- channel that is also activated by DA via the cAMP pathway. This study demonstrates that a ligand-gated channel can be independently operated by another transmitter acting via a second messenger pathway.

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.

Cell surface contact mediates neuronal recognition and synapse formation between two identified leech neurons

An early event in the formation of the serotonergic synapse by the Retzius (R) onto the pressure-sensitive (P) neurons of the leech is the elimination of an extrasynaptic response to transmitter from sites of contact on the postsynaptic cell. This event during synapse formation is cell-specific in that it is elicited in vitro by contact with the presynaptic R cell but not with other neurons. In the study reported here, we investigated the nature of this interaction between R and P neurons. The loss of the extrasynaptic response of the P cell was elicited by contact with R cells fixed in a mild paraformaldehyde solution, but not by R cells treated with the proteolytic enzyme trypsin prior to fixation. As well, a variety of lectins were assayed for their ability to interfere with synapse formation. The transmitter responses of P cells plated on lectin-coated substrates were unaffected. However, exposure of the R cell to the lectin wheat germ agglutinin (WGA), but not to other lectins, prior to pairing prevented the loss of the extrasynaptic response in contacted P cells and blocked the formation of the R-P synapse in culture. We conclude that recognition by the P cell of the R cell during synapse formation may be mediated by an R cell-specific surface protein which binds wheat germ agglutinin.

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.

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.

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

When serotonergic Retzius (R) neurons of the leech contact pressure-sensitive (P) neurons in culture, P cells selectively lose a protein kinase C-dependent cationic response to serotonin and the R cell reforms the inhibitory, chloride-dependent synapse seen in vivo. In P cells not contacted by R cells, cell-attached patches contained single cation channels sensitive to serotonin and phorbol ester with characteristic properties and high incidence (present in about one-half of the patches). P cells paired with R cells had a cation channel with similar biophysical properties and incidence, but channel activity was not stimulated by serotonin and phorbol ester. These results suggest that the early clearing of the non-synaptic (excitatory) response to serotonin is due to the loss of activation by protein kinase C (and not the number) of cation channels as a prelude to inhibitory synapse formation.

Rapid introduction of long-lasting synaptic changes at crustacean neuromuscular junctions

In this review we present recent evidence implicating second-messenger systems in two forms of long-lasting synaptic change seen at crustacean neuromuscular junctions. Crustacean motor axons are endowed with numerous terminals, each possessing many individual synapses. Some synapses appear to be quiescent or impotent, but can be recruited in response to imposed functional demands. Supernormal impulse activity leads to long-term facilitation (LTF) which persists for many hours. During the persistent phase, additional synapses are physiologically effective, and morphological changes in synapses are seen at the ultrastructural level. Pulsatile application of serotonin, a neuromodulator, also enhances synaptic transmission, but this enhancement declines more rapidly than LTF. Elevation of intraterminal Ca2+ is neither necessary nor sufficient for long-lasting enhancement of transmission, but activation of A-kinase is necessary. LTF is set in motion by an unknown depolarization-dependent mechanism leading to A-kinase activation, whereas serotonin facilitation depends for its initiation on the phosphatidylinositol system. The initial phase of serotonin facilitation may be accounted for by production of inositol triphosphate, whereas the secondary long-lasting phase appears to require participation of both C kinase and A kinase. Neither LTF nor serotonin facilitation requires an intact neuron; both are presynaptic phenomena expressed by the nerve terminals. Brief comparison is made with long-lasting synaptic changes in other systems.