Mark J. Zoran

Formation and modulation of chemical connections: Evoked acetylcholine release from growth cones and neurites of specific identified neurons

The ability to release acetylcholine (ACh) from cultured neurons of Helisoma was assessed by micromanipulating ACh-sensitive somata into contact with presynaptic neurons. ACh release was reliably detected from neurites and growth cones of cholinergic neuron B5, but not neuron B19, as early as 3 s after contact with novel target neurons. The rapid onset of transmission correlates with the ability of neuron B5, but not neuron B19, to indiscriminately form chemical connections and may be related to the specificity of synaptogenesis. The neuropeptide FMRFamide reduces ACh release at early chemical connections. The rapid onset of functional transmission and the ability of FMRFamide to modulate chemical transmission at this early chemical connection suggest that neuron B5 acquires its presynaptic apparatus through an intrinsic program independently of target contact.

Rapid escape reflexes in aquatic oligochaetes: variations in design and function of evolutionarily conserved giant fiber systems

SummaryThis study provides neuroanatomical and electrophysiological evidence that an arrangement of three dorsal giant fibers, functioning as two distinct and dichotomous conduction pathways, has been evolutionarily conserved within the three major orders of aquatic and terrestrial oligochaetes. The medial giant fiber (MGF), activated by afferents of anterior segments, initiates anterior shortening; whereas, the two lateral giant fibers (LGFs), activated in synchrony by afferents of posterior segments, initiate a different response (usually tail withdrawal). Notwithstanding these common features, the design and function of LGF systems differ considerably in aquatic and terrestrial groups. In posterior segments of aquatic species, LGFs are disproportionately larger and conduct faster than MGFs. This contrasts with posterior segments of earthworms in which LGFs are smaller and conduct slower than MGFs.In addition, in aquatic tubificids, a single LGF spike is sufficient to evoke rapid and complete tail withdrawal, whereas a pair of closely-spaced LGF spikes are needed to elicit posterior shortening in earthworms. The graded nature of earthworm escape seems appropriate for worms that burrow in relatively hard substrates and may frequently encounter inanimate stimuli that evoke meaningless giant fiber spiking. On the other hand, the all-or-none nature of the tubificid escape appears advantageous for relatively sedentary worms that are vulnerable to intense predation but reside in aqueous sediments where triggering of giant fiber spikes by non-threatening stimuli is infrequent.Our studies suggest that anatomical and physiological modifications of giant fiber pathways in aquatic and terrestrial worms have occurred during the evolution of oligochaete nervous systems. We hypothesize that differential predation pressures, together with fundamental differences in lifestyle and habitat, have led to this divergence in the structure and function of evolutionarily conserved sets of homologous giant interneurons.

Target-dependent induction of secretory capabilities in an identified motoneuron during synaptogenesis.

Cholinergic neurons isolated from the buccal ganglia of Helisoma were plated into cell culture with a variety of defined target cells to study the specificity of synaptogenesis. Motoneuron B19 selectively formed chemical connections with single dissociated muscle fibers derived from its appropriate target, the supralateral radular tensor (SLT) muscle. B19 did not form such connections with novel neuronal targets. In contrast to neuron B19, cholinergic neuron B5 nonselectively formed chemical connections with novel muscle and neuronal targets. Target cells were micromanipulated into contact with presynaptic neurons to examine the latent period until the onset of functional synaptic transmission. Neuron B5 formed chemical connections within the first minutes of contact with ACh-sensitive neurons and muscle while B19 required sustained periods of muscle-specific contact to induce the acquisition of a functional excitation-secretion coupling mechanism. These different latent periods from the onset of target contact suggest that neuron B5 acquires presynaptic secretory function before target contact, while B19 must receive a specific signal(s) from its appropriate target to induce the transformation of its terminal into a secretory state.

Target contact regulates the calcium responsiveness of the secretory machinery during synaptogenesis

Neuron B19 of Helisoma is selective in synaptogenesis. Presynaptic mechanisms underlying this selectivity were tested. Acetylcholine-sensitive assay cells were micromanipulated into contact with B19 somata to assess its secretory state. Prior to appropriate muscle target contact, spontaneous synaptic currents were detected; however, action potential-evoked release of neurotransmitter was detected only following hours of muscle contact. Photolysis of a calcium cage, DM-nitrophen, accelerated the frequency of synaptic currents in muscle-contacted, but not novel neuron-contacted, B19 somata. These studies demonstrate that contact with appropriate target muscle enhances the responsiveness of this neurons secretory machinery to internal calcium levels, thereby imparting the presynaptic cell with the ability to couple action potentials with neurotransmitter release.

Neurobehavioral specializations for respiratory movements and rapid escape from predators in posterior segments of the tubificid Branchiura sowerbyi

The posterior end of the aquatic oligochaete, Branchiura sowerbyi (Tubificidae) protrudes above the sediments and is specialized to carry out several rhythmic respiratory movements. These include 1) waves of flexion by paired gill filaments on each posterior segment, 2) body undulations, and 3) rectal water pumping. Since execution of these behaviors renders the worms posterior end vulnerable to predation, appropriate neurobehavioral mechanisms have evolved that permit extremely rapid escape of tail segments into the sediments. Some of these mechanisms include 1) highly sensitive sensory apparatus for detecting substrate vibrations, water displacements, or touch, 2) large diameter and rapidly conducting lateral giant nerve fibers, and 3) adequacy of a single lateral giant fiber impulse for evoking an all-or-none longitudinal muscle contraction. The significance of these posterior respiratory and escape reflex specializations are discussed in relation to possible predator foraging strategies.