Heterosynaptic facilitation in the giant cell of Aplysia

New concepts of molecular communication among neurons

Recently a number of complex electrophysiological responses to neurotransmitters have been observed that cannot be described as simple excitation or inhibition. These responses are often characterized as modulatory, although there is no consensus on what defines modulation. Morphological studies reveal certain neurotransmitters stored in what might be release sites without synaptic contact. There is no direct evidence for nonsynaptic release from CNS sites, although such release does occur in the periphery and in invertebrates. Nonsynaptic release might provide a basis for diffuse one-cell-to-many communication, but it might also simply be a means of sending the transmitter to a broader area of a single neuron than occurs in typical synapses. Several kinds of macromolecules have been found to be transported in a retrograde direction – and in some cases transsynaptically. There have been suggestions that some neurons may release more than one type of transmitter. Particularly intriguing is the possibility of release of substances that modulate actions of a primary transmitter. Taken together this range of evidence suggests that neurons may use a variety of forms of molecular communication in addition to traditionally described synaptic transmission.

Several authors have suggested modes of communication distinct from classical synaptic transmission and have classified released substances using terms such as neurohumor, neurohormone, neuroregulator, and modulator. These suggestions have the heuristic value of drawing together diverse kinds of data, but it remains to be established that the pieces fit together in that fashion – for example, that complex electrophysiological effects are associated with substances released nonsynaptically. In order to reduce confusion, a flexible, generic approach to nomenclature for substances released from neurons and for hypothetical modes of communication is recommended. Some behavioral implications of nonconventional transmission are considered.

Invertebrate preparations and their contribution to neurobiology in the second half of the 20th century

This review summarized the contribution to neurobiology achieved through the use of invertebrate preparations in the second half of the 20th century. This fascinating period was preceded by pioneers who explored a wide variety of invertebrate phyla and developed various preparations appropriate for electrophysiological studies. Their work advanced general knowledge about neuronal properties (dendritic, somatic, and axonal excitability; pre- and postsynaptic mechanisms). The study of invertebrates made it possible to identify cell bodies in different ganglia, and monitor their operation in the course of behavior. In the 1970s, the details of central neural circuits in worms, molluscs, insects, and crustaceans were characterized for the first time and well before equivalent findings were made in vertebrate preparations. The concept and nature of a central pattern generator (CPG) have been studied in detail, and the stomatogastric nervous system (STNS) is a fine example, having led to many major developments since it was first examined. The final part of the review is a discussion of recent neuroethological studies that have addressed simple cognitive functions and confirmed the utility of invertebrate models. After presenting our invertebrate "mice," the worm Caenorhabditis elegans and the fruit fly Drosophila melanogaster, our conclusion, based on arguments very different from those used fifty years ago, is that invertebrate models are still essential for acquiring insight into the complexity of the brain.

Enhanced synaptic transmission at identified synaptic connections in the cerebral ganglion of Aplysia

The identified A-B neuron synaptic connections in the cerebral ganglion of Aplysia exhibited a novel form of enhanced synaptic transmission. A brief high-frequency train of action potentials (2 s, 10-30 Hz) in the presynaptic A neurons produced a long-lasting increase in the amplitude of excitatory postsynaptic potentials (EPSPs) in B neurons. The increase in synaptic efficacy was termed slow developing potentiation (SDP) since the EPSP amplitude increased slowly with the peak occurring 5 min after the tetanizing train. Peak EPSP amplitudes increased relative to the initial EPSP by an average of greater than 250%. SDP decayed as a single exponential with a time constant of tau = 24 min. The enhanced transmission was neuron specific. Only the connections made by the tetanized A neuron were potentiated. However, potentiation apparently occurred at all the synapses made by the tetanized A neuron. Tetanizing the postsynaptic B neurons neither induced, nor when paired with A neuron tetanization, increased SDP. SDP appears to be primarily due to increased transmitter release by the presynaptic neuron.

Effects of c-AMP, caffeine, theophylline, and vinblastine on spontaneous transmitter release at locust nerve-muscle junctions

The effects of c-AMP, phosphodiesterase inhibitors (caffeine and theophylline) and vinblastine on spontaneous transmitter release was investigated at locust neuromuscular junctions. c-AMP, theophylline, caffeine, and vinblastine caused facilitation of transmitter release. None of these drugs had any effect on the amplitude of miniature excitatory postsynaptic potentials (min. E.P.S.P.'s), or on the resting membrane potential, but vinblastine increased the proportion of 'large' min. E.P.S.P.'s. The effect of theophylline (but not c-AMP and caffeine) on min. E.P.S.P. frequency was found to be calcium dependent. The effects of these drugs on the locust glutamatergic synapse are compared with their actions at other synapses.

Two identified interneurons modulate the firing pattern of pacemaker bursting cells in Helix

Two interneurons modulating the firing pattern of two endogeneous bursting neurons have been identified in the visceral ganglion. They are characterized by large depolarizing afterpotentials and doublets due to the delayed firing of a dense layer of terminal processes converging onto the axon branches from the two bursters. Firing of the terminal processes upon soma or axon stimulation needs either subthreshold depolarization of the cell or conditioning nerve stimulation acting as a gating mechanism.

Voltage-dependent actions of endogenous and exogenous serotonin on identified neurones

1. Impulse activity in an identified serotonin-containing neuron (GSN) produces a slow excitatory synaptic response in another identified neuron, the A neuron. An axon process can be traced close to the follower neuron perikaryon after Lucifer Yellow injection of the GSN perikaryon. 2. The synaptic response is markedly voltage-sensitive being increased at depolarized potentials and almost abolished and not inverted at potentials in excess of about -55mV. 3. Serotonin locally applied produces a similar response. 4. The response to serotonin does not involve a change in conductance to either sodium or chloride ions, but calcium ions do appear to be important either because of their influence on potassium ion permeability or in a direct transfer of charge across the membrane. 5. Another follower neuron exhibits a complex GSN-induced synaptic response comprising a slow potential similar to that seen in the A neuron and also a fast, probably sodium dependent, potential. 6. In addition to producing a weak direct excitation of the A neuron, GSN-activation can also partly reverse accommodation and also prolong the duration of the impulse in the A neuron. 7. Exogenously applied serotonin produces a similar voltage-dependent inward current response in the GSN as seen in the A neuron. It is suggested that the receptors mediating the response on the GSNs may normally be involved in feedback regulation. 8. Cyproheptadine (reversibly), methergoline, mianserin and propranolol (all irreversibly) antagonised the response in the GSN. These agents probably all have action on the ionic mechanism underlying the serotonin response.

Cyclic AMP, 5-HT, and the modulation of transmitter release at the crayfish neuromuscular junction

In this study it was found that several agents which elevate cAMP levels in cells also increase dramatically the quantity of transmitter released from crayfish excitatory nerve terminals in response to a stimulus. With respect to time course and magnitude, the increase produced by one of these agents, the cyclic nucleotide phosphodiesterase inhibitor Squibb 20,009 (SQ 20,009), is unlike any reported for such a drug at a synapse. Additionally, SQ 20,009 potentiated the facilitation of transmitter release produced by serotonin (5-HT) at this synapse. These results establish a possible role for cAMP in the control and modulation of transmitter release at the crayfish neuromuscular junction (NMJ). They further suggest that 5-HT functions here by activation of a presynaptically located adenylate cyclase.

An electron microscope study of synaptic contacts in the abdominal ganglion of Aplysia californica

The fine structure of the abdominal ganglion of Aplysia californica has been studied in preparations fixed by immersion in aldehydes, either directly or after a survival of a few hours in artificial sea water. The central core of neuropil is surrounded by a rind of neuronal cell bodies floating in a subcapsular space containing a loose meshwork of neuronal and glial processes, separated by wide extracellular spaces. Large primary processes with deeply infolded membranes leave the neuronal perikarya and enter the neuropil where they branch into smaller processes containing either neurofilaments, neurotubules or both. Some have the appearance of initial segments. The neuropil is not a homogeneous structure. Rather, four types of zones can be distinguished: (1) zones of fibers of passage coursing together in the neuropil and making few synaptic contacts: (2) zones of neurosecretory fibers containing large granules and dense-core vesicles, again making few synaptic contacts: (3) zones with a great variety of synaptic contacts between medium size and small profiles; and (4) glomerular zones. The differentiated membranes of the synapses are characterized by a slight increase in density and by being regularly parallel to each other. Presynaptic densities are sometimes quite prominent but specialized dense cytoplasmic opacities have never been seen bordering the postsynaptic membranes, i.e., all synapses are of the symmetrical type. Interlemmal opacities vary considerably in density. In zone 3, the synaptic vesicles are of several sizes, are round, oval or flat, and are either clear or filled with different types of dense material. The population of vesicles within a single profile may consist either of a homogeneous group of similar vesicles or of various mixtures of two or three kinds of vesicles. In profiles with mixtures of clear and large dense-core vesicles, it is often only the clear vesicles which agglomerate towards the differentiated membranes. In such cases the large dense-core vesicles lie as a peripheral halo around the clear vesicles. Here, and especially in other large neuronal profiles not forming contact in the plane of section, they can be seen to associate specifically with mitochondria and glycogen. It is proposed that they do not contain neurotransmitters but are related to mitochondrial activities such as the storage of ATP or the movement of calcium ions. In profiles with mixtures of clear and small dense-core vesicles, both types of vesicles often touch the presynaptic membrane, suggesting the release of two transmitters or of a modulator or neurohormone with a transmitter, by a single terminal. Serial synapses are present in this zone. The glomerular zones contain small profiles forming many synaptic contacts, some of which are arranged in such a way as to suggest the existence of "reciprocal" serial synapses.