Intersegmental interneurons serving larval and pupal mechanosensory reflexes in the moth Manduca sexta

1. Intersegmental interneurons (INs) that participate in the larval bending reflex and the pupal gin trap closure reflex were identified in the isolated ventral nerve cord of Manduca sexta. INs 305, 504, and 703 show qualitatively different responses in the pupa than in the larva to electrical stimulation of sensory neurons that are retained during the larval-pupal transition to serve both reflexes. Action potentials produced by current injected into the 3 interneurons excite motor neurons that are directly involved in the larval and pupal reflexes. The excitation of the motor neurons is not associated with EPSPs at a fixed latency following action potentials in the interneurons, and thus there do not seem to be direct synaptic connections between the interneurons and the motor neurons. 2. IN 305 (Fig. 2) has a lateral soma, processes in most of the dorsal neuropil ipsilateral to the soma, and a crossing neurite that gives rise to a single contralateral descending axon. IN 305 is excited by stimulation of the sensory nerve ipsilateral to its soma in the larva and the pupa. Stimulation of the sensory nerve contralateral to its soma produces an inhibitory response in the larva, but a mixed excitatory/inhibitory response to the identical stimulus in the pupa. 3. IN 504 (Fig. 3) has a lateral soma, processes throughout most of the neuropil ipsilateral to the soma, and a crossing neurite that bifurcates to give rise to a process extending to the caudal limit of the neuropil and an ascending axon. IN 504 is excited by stimulation of the sensory nerve ipsilateral to its soma in both larvae and pupae, while the response to stimulation of the sensory nerve contralateral to its soma is inhibitory in the larva but mixed (excitatory/inhibitory) in the pupa. 4. IN 703 has a large antero-lateral soma, a neurite that extends across to the contralateral side giving rise to processes located primarily dorsally in both ipsilateral and contralateral neuropils, and two axons that ascend and descend in the connectives contralateral to the soma (Fig. 4). IN 703 responds to stimulation of the sensory nerves on either side of the ganglion, but the form of the response changes during the larval-pupal transition. In the larva, the response consists of very phasic (0-2 spikes) excitation, but in the pupa there is a prolonged excitation that greatly outlasts the stimulus (Fig. 6).(ABSTRACT TRUNCATED AT 400 WORDS)

Development of the gin trap reflex in Manduca sexta: a comparison of larval and pupal motor responses

1. Responses of motor neurons in larvae and pupae of Manduca sexta to stimulation of tactile sensory neurons were measured in both semi-intact, and isolated nerve cord preparations. These motor neurons innervate abdominal intersegmental muscles which are involved in the production of a general flexion reflex in the larva, and the closure reflex of the pupal gin traps. 2. Larval motor neurons respond to stimulation of sensory neurons innervating abdominal mechanosensory hairs with prolonged, tonic excitation ipsilaterally, and either weak excitation or inhibition contralaterally (Figs. 4A, 6). 3. Pupae respond to tactile stimulation of mechanosensory hairs within the gin traps with a rapid closure reflex. Motor neurons which innervate muscles ipsilateral to the stimulus exhibit a large depolarization, high frequency firing, and abrupt termination (Figs. 2, 4B). Generally, contralateral motor neurons fire antiphasically to the ipsilateral motor neurons, producing a characteristic triphasic firing pattern (Figs. 7, 8) which is not seen in the larva. 4. Pupal motor neurons can also respond to sensory stimulation with other types of patterns, including rotational responses (Fig. 3A), gin trap opening reflexes (Fig. 3B), and 'flip-flop' responses (Fig. 9). 5. Pupal motor neurons, like larval motor neurons, do not show oscillatory responses to tonic current injection, nor do motor neurons of either stage appear to interact synaptically with one another. Most pupal motor neurons also exhibit i-V properties similar to those of larval motor neurons (Table 1; Fig. 10). Some pupal motor neurons, however, show a marked non-linear response to depolarizing current injection (Fig. 11).

The use of hormonally induced mosaics to study alterations in the synaptic connections made by persistent sensory neurons during insect metamorphosis

During the initial phase of metamorphosis in the hawkmoth, Manduca sexta, persistent mechanosensory neurons expand their terminal arborizations within the CNS and evoke a reflex response in the pupa which is different than in the larva. In an effort to determine the contribution of sensory neuron modifications to the difference in reflex responses, manipulations of juvenile hormone and 20-hydroxyecdysone were used to generate mosaic animals in which the sensory neurons were advanced or delayed developmentally with respect to the rest of the animal, including circuit components within the CNS. In the larval stage electrical stimulation of the sensory axons evokes a slow depolarization and a prolonged burst of action potentials in the ipsilateral intersegmental muscle motor neurons. By contrast, in pupal preparations the same motor neurons respond to an identical stimulus with a larger, more rapid depolarization which leads to a relatively brief, high-frequency burst of action potentials. Motor responses on the contralateral side of the body are also altered during pupal development. In mosaic animals where larval-like sensory neurons interact with a pupal CNS, a larval reflex response is generated. In the converse situation, pupal-like sensory neurons interacting with a larval or prepupal CNS evoke a motor response that is typical of larvae or prepupae. We conclude, therefore, that pupal development of the sensory neurons is necessary, but not sufficient, for the production of the pupal reflex.

Physiology and pharmacology of acetylcholinergic responses of interneurons in the antennal lobes of the moth Manduca sexta

1. Neurons in the antennal lobe (AL) of the moth Manduca sexta respond to the application, via pressure injection into the neuropil, of acetylcholine (ACh). When synaptic transmission is not blocked, both excitatory (Fig. 2) and inhibitory (Fig. 3) responses are seen. 2. Responses to ACh appear to be receptor-mediated, as they are associated with an increase in input conductance (Figs. 2B and 3B) and are dose-dependent (Fig. 2 C). 3. All neurons responsive to ACh are also excited by nicotine. Responses to nicotine are stronger and more prolonged than responses to ACh (Fig. 4C). No responses are observed to the muscarinic agonist, oxotremorine (Fig. 4 B). 4. Curare blocks responses of AL neurons to applied ACh, while atropine and dexetimide are only weakly effective at reducing ACh responses (Figs. 5 and 6). 5. Curare is also more effective than atropine or dexetimide at reducing synaptically-mediated responses of AL neurons (Fig. 7). 6. In one AL neuron, bicuculline methiodide (BMI) blocked the IPSP produced by electrical stimulation of the antennal nerve, but it did not reduce the inhibitory response to application of ACh (Fig. 8).

GABA-mediated synaptic inhibition of projection neurons in the antennal lobes of the sphinx moth, Manduca sexta

Responses of neurons in the antennal lobe (AL) of the moth Manduca sexta to stimulation of the ipsilateral antenna by odors consist of excitatory and inhibitory synaptic potentials. Stimulation of primary afferent fibers by electrical shock of the antennal nerve causes a characteristic IPSP-EPSP synaptic response in AL projection neurons. The IPSP in projection neurons reverses below the resting potential, is sensitive to changes in external and internal chloride concentration, and thus is apparently mediated by an increase in chloride conductance. The IPSP is reversibly blocked by 100 microM picrotoxin or bicuculline. Many AL neurons respond to application of GABA with a strong hyperpolarization and an inhibition of spontaneous spiking activity. GABA responses are associated with an increase in neuronal input conductance and a reversal potential below the resting potential. Application of GABA blocks inhibitory synaptic inputs and reduces or blocks excitatory inputs. EPSPs can be protected from depression by application of GABA. Muscimol, a GABA analog that mimics GABA responses at GABAA receptors but not at GABAB receptors in the vertebrate CNS, inhibits many AL neurons in the moth.

Synaptic mechanisms of a tonic EPSP in crustacean visual interneurons: analysis and simulation

Light-evoked synaptic responses of identified visual interneurons, sustaining fibers (SFs), were quantitatively analyzed, and a neuronal cable model was used to calculate voltage attenuations and predict synaptic responses. The cable model is based on morphological measurements of SFs filled with Lucifer yellow and passive membrane properties assessed by current injection in the proximal portion of the dendritic arbor. The morphological and electrophysiological measurements were made in different preparations on homologues of the same identified interneurons. The excitatory postsynaptic potential (EPSP) elicited with high-intensity light consists of a transient phase (mean amplitude 33.1 mV) and a second phase (the plateau) that decays slowly relative to the membrane time constant (mean amplitude 24.4 mV). The mean extrapolated reversal potentials are -19.1 mV for the transient and -22.3 mV during the plateau. The change in input conductance associated with the plateau phase of the response showed a peak of 121% above the resting input conductance and decayed to approximately 50% above resting conductance over several seconds. Compartmental cable models (18, 19) were used to calculate voltage attenuations and local synaptic conductances within the SF dendritic tree. The dendrites are electrotonically compact, and voltage attenuations average 6% for current flowing distally from the recording site (injected) and 45% for current flowing proximally to the recording site. The steady-state EPSP is associated with a calculated 80% decrease in the net dendritic membrane resistivity. The synaptic response, calculated for an EPSP distributed throughout the dentritic tree (using this conductance change and the measured steady-state reversal potential) was 27.0 mV, compared with an observed mean value of 24.4 mV. The calculated relationship between steady-state EPSP amplitude and dendritic membrane resistivity (Rs) is a sigmoidal function that resembles the intensity/response function of the SF. We can therefore correctly predict the transformation from light intensity to compound EPSP amplitude by calculating the intervening synaptic membrane resistivity and voltage values. These functions are affected in a predictable manner by the passive membrane resistivity (Rm) and the EPSP reversal potentials. Tetrodotoxin (TTX) application abolished all SF spiking but left the EPSP essentially unchanged, suggesting that the neuronal pathway from photoreceptors to SFs is mainly or entirely comprised of nonspiking (i.e., TTX-insensitive) elements.

Linear integration of convergent visual inputs in an oculomotor reflex pathway

The functional connectivity between identified visual interneurons [sustaining fibers (SF)] and oculomotor neurons was assessed by simultaneous recording and cross-correlation analysis. A small group of SFs exhibit excitatory functional connections to an identified tonic oculomotor neuron. The excitatory interactions are found exclusively between SFs and oculomotor neurons with similar and/or overlapping excitatory receptive fields. A second group of SFs exhibit inhibitory connections to motor neurons. The excitatory receptive fields of these SFs correspond to the inhibitory receptive fields of the motor neurons. The collective action of the SFs is sufficient to produce all of the steady-state visual behavior of the motor neurons including the increment in firing rate elicited by illumination, unique features of the motor neuron receptive field, and differential sensitivity to blue light and polarized light. Pairs of SFs that converge on the same motor neuron sum their effects linearly. Thus the joint interaction of two SFs on a motor neuron is equal to the sum of the two postsynaptic effects taken separately. Coactivation of excitatory and inhibitory SF inputs to a motor neuron results in a partial cancellation of their postsynaptic effects on the motor neuron's firing rate. The antagonistic interactions protect the system from perturbations by stray light, visual adaptation, and variations in the central excited state. The ensemble information code, at the SF level of the optomotor pathway, is a set of differentially weighted mean firing rates. The weightings reflect differences in the strengths of the several SF-to- motor neuron interactions. One consequence of these differences is a selective weighting of the effects of illumination (in different regions of visual space) on the compensatory eye reflex.

A quantitative correlation of contour sensitivity with dendritic density in an identified visual neuron

Crayfish sustaining fibers are visual interneurons that are tonically excited by increases in illumination within their receptive fields. The receptive fields of sustaining fibers (SFs) are correlated with the two-dimensional position of their dendritic fields within the central neuropil. Dendritic branching patterns and contour sensitivity maps were obtained for SF 019 in several preparations. The contour sensitivity along the dorsal-ventral visual arc (which intersects 019's receptive field) is positively correlated with the variations in the spatial frequency of 019's dendritic arbor along the dorsal ventral midline of the neuropil. We conclude that an SF's receptive field is determined by the position and extent of its dendritic field and that sensitivity variations within the receptive field are principally determined by changes in its dendritic density.