A plexus of multidendritic sensory neurons, the dendritic arborization (da) neurons, innervates the epidermis of soft-bodied insects. Previous studies have indicated that the plexus may comprise distinct subtypes of da neurons, which utilize diverse cyclic 3',5'-guanosine monophosphate signaling pathways and could serve several functions. Here, we identify three distinct classes of da neurons in Manduca, which we term the alpha, beta, and gamma classes. These three classes differ in their sensory responses, branch complexity, peripheral dendritic fields, and axonal projections. The two identified alpha neurons branch over defined regions of the body wall, which in some cases correspond to specific natural folds of the cuticle. These cells project to an intermediate region of the neuropil and appear to function as proprioceptors. Three beta neurons are characterized by long, sinuous dendritic branches and axons that terminate in the ventral neuropil. The function of this group of neurons is unknown. Four neurons belonging to the gamma class have the most complex peripheral dendrites. A representative gamma neuron responds to forceful touch of the cuticle. Although the dendrites of da neurons of different classes may overlap extensively, cells belonging to the same class show minimal dendritic overlap. As a result, the body wall is independently tiled by the beta and gamma da neurons and partially innervated by the alpha neurons. These properties of the da system likely allow insects to discriminate the quality and location of several types of stimuli acting on the cuticle.

Graded synaptic transmission occurs between spiking neurons of the lobster stomatogastric ganglion. In addition to eliciting spike-evoked inhibitory potentials in postsynaptic cells, these neurons also release functionally significant amounts of transmitter below the threshold for action potentials. The spikeless postsynaptic potentials grade in amplitude with presynaptic voltage and can be maintained for long periods. Graded synaptic transmission can be modulated by synaptic input to the presynaptic neuron.This study demonstrates that graded synaptic transmission occurs between spiking neurons of the lobster stomatogastric ganglion. These neurons appear to use two modes of synaptic transmission within the ganglion: presynaptic action potentials (spikes) evoke monosynaptic postsynaptic potentials and, in addition, transmitter is released as a continuously graded function of presynaptic voltage. This study describes the soma-to-soma input-output properties of graded synaptic transmission between a group of spiking motoneurons in the stomatogastric ganglion.It has been known for some time that nonspiking neurons exist (1-13; reviewed in ref. 14) and that such neurons release transmitter as a continuously graded function of presynaptic voltage (5,12,13). In previous studies, spiking neurons have been forced to release transmitter as a graded function of presynaptic voltage by manipulating the amplitude of the action potential (15)(16)(17) or by blocking action potential activity and voltage clamping the presynaptic terminal (18,19). Until now, there has only been suggestive evidence for functionally significant graded synaptic transmission by spiking neurons (3,11,(20)(21)(22). We now report that many stomatogastric neurons appear to use both spike-evoked and graded modes of chemical synaptic transmission as a part of normal function, that their input-output curves are similar to those of other neurons, but show a low threshold for transmitter release, and that synaptic inputs can modulate the graded synaptic transmission. [A brief report has been previously published (23); see also Raper (24), who has shown that graded interactions are sufficient to maintain much of the normal pyloric patterned activity when spikes are blocked but the oscillator is functioning. ]The stomatogastric ganglion of the spiny lobster Panulirus interruptus consists of the cell bodies and neuropil processes of about 30 neurons, most of which are motoneurons innervating the striated muscle of the gut (25). In addition to their excitatory connections onto muscle fibers, these neurons also synaptically inhibit each other (25,26). A diagram of the relevant inhibitory synaptic connections is shown in Fig. 1A.Within the stomatogastric ganglion, synaptic connections are made between fine cellular processes in the neuropil; many individual synaptic contacts distributed over several pre-and Fig. 2). Intracellular recordings were made from cell bodies of identified pre-and postsynaptic neurons (Fig. 1C). Current was injected into the cell ...

The stomatogastric ganglion (STG) of the crab Cancer productus contains approximately 30 neurons arrayed into two different networks (gastric mill and pyloric), each of which produces a distinct motor pattern in vitro. Here we show that the functional division of the STG into these two networks requires intact NO-cGMP signaling. Multiple nitric oxide synthase (NOS)-like proteins are expressed in the stomatogastric nervous system, and NO appears to be released as an orthograde transmitter from descending inputs to the STG. The receptor of NO, a soluble guanylate cyclase (sGC), is expressed in a subset of neurons in both motor networks. When NO diffusion or sGC activation are blocked within the ganglion, the two networks combine into a single conjoint circuit. The gastric mill motor rhythm breaks down, and several gastric neurons pattern switch and begin firing in pyloric time. The functional reorganization of the STG is both rapid and reversible, and the gastric mill motor rhythm is restored when the ganglion is returned to normal saline. Finally, pharmacological manipulations of the NO-cGMP pathway are ineffective when descending modulatory inputs to the STG are blocked. This suggests that the NO-cGMP pathway may interact with other biochemical cascades to partition rhythmic motor output from the ganglion.

The nitric oxide/cyclic 3′,5′‐guanosine monophosphate (NO/cGMP) signaling pathway has been implicated in certain forms of developmental and adult neuronal plasticity. Here we use whole‐mount immunocytochemistry to identify components of this pathway in the nervous system of postembryonic lobsters as they develop through metamorphosis. We find that the synthetic enzyme for NO (nitric oxide synthase, or NOS) and the receptor for this transmitter (NO‐sensitive soluble guanylate cyclase) are broadly distributed in the central nervous system (CNS) at hatching. In the brain, NOS immunoreactivity is intensified during glomerular development in the olfactory and accessory lobes. Whereas only a few neurons express NOS in the CNS, many more neurons synthesize cGMP in the presence of NO. NO‐sensitive guanylate cyclase activity is a stable feature of some cells, while in others it is regulated during development. In the stomatogastric nervous system, a subset of neurons become responsive to NO at metamorphosis, a time when larval networks are reorganized into adult motor circuits. cGMP accumulation was occasionally detected in the nucleus of many cells in the CNS, which suggests that cGMP may have a role in transcription. Based on these findings, we conclude that the NO/cGMP signaling pathway may participate in the development of the lobster nervous system. Furthermore, NO may serve as a modulatory neurotransmitter for diverse neurons throughout the CNS. © 1998 John Wiley & Sons, Inc. J Neurobiol 34: 208–226, 1998

1. Input-output properties of the inhibitory synaptic connection between non-spiking neurons (EX1) and gastric mill (GM) neurons were examined in the stomatogastric ganglion of the spiny lobster, Panulirus interruptus. Current was injected into and the voltage was recorded during current injection, two independent microelectrodes were used. 2. The EX1-GM synaptic connection is a conductance-increase inhibitory type, with an input-output curve that resembles the curve for the squid giant synapse. There is a threshold level of depolarization for transmitter release from the presynaptic cell. Beyond that threshold, increasing presynaptic depolarization causes increasing postsynaptic hyperpolarization (and inhibition). 3. A long presynaptic current step always causes a postsynaptic response with an initial peak of hyperpolarization followed by a decay to a less hyperpolarized plateau level. The plateau level is maintained, in most cells, for the duration of the presynaptic depolarization even over long periods (30 s). 4. The peak, but not the plateau, part of the postsynaptic response is sensitive to the past history of the synaptic connection. If a large conditioning pulse is applied to the presynaptic cell causing a large postsynaptic hyperpolarization, then the postsynaptic response to a later presynaptic test depolarization will have a reduced peak, leaving the plateau component unchanged.