Neurons with histaminelike immunoreactivity in the segmental and stomatogastric nervous systems of the crayfish Pacifastacus leniusculus and the lobster Homarus americanus.

We used a polyclonal antiserum against histamine to map histaminelike immunoreactivity (HLI) in whole mounts of the segmental ganglia and stomatogastric ganglion of crayfish and lobster. Carbodiimide fixation permitted both HRP-conjugated and FITC-conjugated secondary antibodies to be used effectively to visualize HLI in these whole mounts. Two interneurons that send axons through the inferior ventricular nerve (ivn) and the stomatogastric nerve to the stomatogastric ganglion had strong HLI, both in crayfish and in lobster. These ivn interneurons were known from other evidence to be histaminergic. The neuropil of the stomatogastric ganglion in both crayfish and lobster contained brightly labeled terminals of axons that entered the ganglion from the stomatogastric nerve. No neuronal cell bodies in this ganglion had HLI. Each segmental ganglion contained at least one pair of neurons with HLI. Some neurons in the subesophageal ganglion and in each thoracic ganglion labeled very brightly. Axons of projection interneurons with strong HLI occurred in the dorsal lateral tracts of each segmental ganglion, and sent branches to the lateral neurophils and tract neurophils of each ganglion. All the labeled neurons were interneurons; no HLI was observed in peripheral nerves.

Compartmental models of electrotonic structure and synaptic integration in an identified neurone.

A three-compartment model of the electrotonic structure of an identified motoneurone, the median gastric (m.g.) neurone of the stomatogastric ganglion of the spiny lobster (Panulirus interruptus) was constructed, based on the passive response of the cell to a step of injected current. While its structure is only remotely related to that of the cell, the model is able to predict the passive response of the cell to any wave form of injected current. The shape of the m.g. neurone provided the basis for the development of a multicompartment model of the cell from the simple compartment model. Unlike the three-compartment model, the multicompartment model has a structure that corresponds closely to that of the cell while it retains the ability to predict the passive response of the cell to any wave form of injected current. The multicompartment model was used to analyse the electrotonic structure and synaptic integration of the cell. The axon acts as a current sink, causing steady-state voltage attenuation between the tips of different dendrites and the integrating segment to range between 26 and 89%. Steady-state voltage attenuation in the distal direction is 2% or less. Synaptic inhibition of m.g. by Interneurone 1 was simulated with simultaneously activated conductance-increase synapses located on all dendritic end-compartments of the model. Inhibitory post-synaptic potential (i.p.s.p.) wave forms recorded in the cell soma were duplicated in the soma compartment when the synaptic conductance change in each of the twenty-eight end-compartments was set equal to 5 nS for 8 ms. I.p.s.p. wave forms in dendritic end-compartments were 30% larger than the soma compartment i.p.s.p., while i.p.s.p.s in the integrating segment compartment were intermediate in size. Charge from a 92 mV, 1 ms action potential in the model axon was passively conducted from axonal compartments to the soma compartment of the model, where it reproduced the attenuated, broadened voltage wave forms of action potentials recorded in the cell soma. Passive spread of charge from an axonal action potentials to terminal dendritic compartments evoked potentials there that were 30% larger and faster than the corresponding soma compartment potential.

Sensory alteration of motor patterns in the stomatogastric nervous system of the spiny lobster Panulirus interruptus.

1. Stretching the pyloric region of the lobster's stomach in a manner that resembles pyloric dilation triggers a prolonged burst of impulses in two interneurones with axons in the inferior ventricular nerve (IVN). The burst is activated in the oesophageal ganglion by sensory axons that traverse the lateral ventricular nerves, the dorsal ventricular nerve and the stomatogastric nerve. These sensory axons do not appear to make synaptic contacts in the stomatogastric ganglion. 2. Electrical stimulation of sensory branches of the pyloric nerve triggers similar bursts in the IVN interneurones. 3. The burst of impulses in the IVN interneurones lasts from 2 to 30 s and the impulse frequency ranges from 10 to 80 Hz in different parts of the burst. Once triggered, burst structure and burst duration are independent of the intensity or duration of stimuli applied to the sensory nerves. 4. These bursts alter both the gastric and pyloric motor patterns. The IVN interneurones make a complex pattern of synapses with stomatogastric neurones. These are: pyloric dilators (PD) - excitation and slow inhibition; ventricular dilator (VD) - excitation; gastric mill (GM) neurones - inhibition; lateral posterior gastric neurones (LPGN) - inhibition; and Interneurone I (Int I) - excitation and slow inhibition. The size of the p.s.p.s at each of these synapses depends on the duration and impulse-frequency of the burst in the presynaptic neurones, which in turn alters the firing patterns of the stomatogastric neurones in various ways.

Properties of synapses made by IVN command-interneurones in the stomatogastric ganglion of the spiny lobster Panulirus interruptus.

1. The IVN command interneurones synapse directly onto 11 identified neurones in the stomatogastric ganglion: the two pyloric dilators (PDs), the anterior burster (AB), ventricular dilator (VD), the four gastric mill neurones (GMs), the two lateral posterior gastric neurones (LPGNs), and Interneurone I (Int 1). 2. The IVN p.s.p.s in PD and AB are biphasic, and consist of a fast depolarizing component followed by a slower hyperpolarizing component. 3. The hyperpolarizing component of this biphasic postsynaptic potential is inhibitory, and appears to be the result of a conductance increase to K+ and Cl-. 4. The IVN p.s.p. in VD is excitatory and can drive VD one-for-one. 5. The IVN p.s.p.s in GM and LPGN are inhibitory. The amplitude of a single p.s.p. is small but, at high frequency, summation of p.s.p.s holds the postsynaptic membrane potential below threshold. 6. The facilitation characteristics of the p.s.p.s in each neurone are described. 7. The functional significance of these synaptic characteristics is discussed in terms of the modification of motor output caused by a burst of the IVN interneurones.

Motor-pattern production: interaction of chemical and electrical synapses.

A pair of neurons exhibiting postinhibitory rebound, if connected through reciprocally inhibitory chemical synapses, will exhibit a stable pattern of alternating bursts. If two such oscillating pairs, of similar but not identical properties are connected by means of an electrical synapse and an inhibitory chemical synapse between two neurons, one in each pair, the burst patterns may drift, may lock in synchrony, may entrain in antiphase, may entrain at an intermediate phase, or may be suppressed in the inhibited pair. The behavior depends on the strengths of the chemical and electrical coupling as well as on the degree of depression at the chemical synapse. There relationships of the motor patterns are illustrated quantitatively through theoretical calculations.

Antidromic action potentials fail to demonstrate known interactions between neurons.

An identified motor neuron in the stomatogastric ganglion of Panulirus interruptus inhibits four other motor neurons when it fires spontaneously or in response to depolarization of its soma. It does not inhibit these neurons when it is fired antidromically, although the attenuated antidromic spike is visible at its soma. These findings point out the difficulty of interpreting negative results from antidromic stimulation experiments and the importance of neuronal structure to the integrative activities of nervous systems.