Wind-activated thoracic interneurons of the cockroach: II. Patterns of connection from ventral giant interneurons

A number of thoracic interneurons (TIs) have been found to receive inputs from ventral giant interneurons (vGIs). Each of these cells responds to wind with short latency depolarizations. The previous paper described response properties of several TIs to wind stimuli, including those excited by vGIs. The data showed a correlation between the shape of the TI's wind fields and its morphology. The presence of ventral branches located near the midline of the ganglion predicts a strong response to wind on that side. These ventral median (VM) branches are in the proper location to permit overlap with processes from vGIs. Here we describe the patterns of connections between individual vGIs and 13 of the thoracic interneurons located in the meso- and metathoracic ganglia. A correlation was found between the presence of VM branches and excitation by vGIs. TIs were only excited by vGIs on the side(s) on which VM branches exist. However, presence of a VM branch does not imply that all vGIs on that side make connections with the TI. Summation was found between various vGIs that excited each individual thoracic interneuron. In unilateral thoracic interneurons, no sign of inhibition was found from vGIs on the sides opposite that which contained excitatory vGI axons. Neither was there any evidence of inhibition from dorsal giant interneurons. In addition preliminary evidence indicated that left-right homologues do not inhibit one another. Thus, the data suggest that directional wind fields are primarily the result of selective connection from specific vGIs.     

Response of thoracic interneurons to tactile stimulation in the cockroach,Periplaneta americana

Receåfindings indicate that cockroaches escape in response to tactile stimulation as well as they do in response to the classic wind puff stimulus. The thoracic interneurons that receive inputs from ventral giant interneurons also respond to tactile stimulation and therefore, represent a potential site of convergence between wind and tactile stimulation as well as other sensory modalities. In this article, we characterize the tactile response of these interneurons, which are referred to as type-A thoracic interneurons (TI_As). In response to tactile stimulation of the body cuticle, TI_As typically respond with a short latency biphasic depolarization which often passes threshold for action potentials. The biphasic response is not typical of responses to wind stimulation nor of tactile stimulation of the antennae. It is also not seen in tactile responses of thoracic interneurons that are not part of the TI_A group. The responses of individual TI_As to stimulation of various body locations were mapped. The left-right directional properties of TI_As are consistent with their responses to wind puffs from various different directions. Cells that respond equally well to wind from the left and right side also respond equally well to tactile stimuli on the left and right side of the animal's body. In contrast, cells that are biased to wind on one side are also biased to tactile stimulation on the same side. In general, tactile responses directed at body cuticle are phasic rather than tonic, occurring both when the tactile stimulator is depressed and released. The response reflects stimulus strength and follows repeated stimulation quite well. However, the first phase of the biphasic response is more robust during high-frequency stimulation than the second phase. TI_As also respond to antennal stimulation. However, here the response characteristics are complicated by the fact that movement of either antenna evokes descending activity in both left and right thoracic connectives. The data suggest that the TI_As make up a multimodal site of sensory convergence that is capable of generating an oriental escape turn in response to any one of several sensory cues. 1994 John Wiley & Sons, Inc.     

Responses to acoustic stimulation of thoracic interneurons in noctuid moths

In noctuid moths several types of thoracic interneurons process information from the A1 acoustic sense cell. The most frequently encountered types are repeaters and pulse markers. The present study shows that these interneurons belong to one of two physiological categories: ‘stable followers’ which are capable of following pulse repetition rates greater than 10 Hz, and ‘labile followers’ which respond erratically at a pulse repetition rate of 3 Hz and fail entirely when the rate reaches 5 Hz. A rôle for the labile followers in the avoidance flight response is discussed.     

Responses of descending interneurons to stimulation of wind head receptors in locust

Responses of descending interneurons of the thoracic nerve chain ofLocusta migratoria L. to wind stimulation of the head receptors concerned with the activation of the insect's flight were investigated by means of extracellular microelectrode recordings from their axons. Altogether 11 types of descending effects transmitted by these interneurons to neurons of the segmental ganglia were detected. Ipsilateral and contralateral wind stimulation of the receptors can not only activate hitherto silent interneurons but also effectively modify the character of discharges from spontaneously active, discharging neurons. This influence may be either excitatory (increased spontaneous firing rate) or inhibitory (decreased rate or total abolition of spontaneous discharges). Some descending interneurons give rise not only to on-responses but also to typical off-responses to stimulation. The possible functional role of some of the observed effects in the mechanisms triggering and maintaining the insect's flight is discussed.I. M. Sechenov Institute of Evolutionary Physiology and Biochemistry, Academy of Sciences of the USSR, Leningrad. Biological Research Institute, Hungarian Academy of Sciences, Tihany. Translated from Neirofiziologiya, Vol. 5, No. 6, pp. 602–610, November–December, 1973.     

Dual pathways for tactile sensory information to thoracic interneurons in the cockroach

The escape system of the American cockroach is both fast and directional. In response to wind stimulation both of these characteristics are largely due to the properties of the ventral giant interneurons (vGIs), which conduct sensory information from the cerci on the rear of the animal to type A thoracic interneurons (TI_As) in the thoracic ganglia. The cockroach also escapes from tactile stimuli, and although vGIs are not involved in tactile-mediated escapes, the same thoracic interneurons process tactile sensory information. The response of TI_As to tactile information is typically biphasic. A rapid initial depolarization is followed by a longer latency depolarization that encodes most if not all of the directional information in the tactile stimulus. We report here that the biphasic response of TI_As to tactile stimulation is caused by two separate conducting pathways from the point of stimulation to the thoracic ganglia. Phase 1 is generated by mechanical conduction along the animal's body cuticle or other physical structures. It cannot be eliminated by complete lesion of the nerve cord, and it is not evoked in response to electrical stimulation of abdominal nerves that contain the axons of sensory receptors in abdominal segments. However, it can be eliminated by lesioning the abdominal nerve cord and nerve 7 of the metathoracic ganglion together, suggesting that the relevant sensory structures send axons in nerve 7 and abdominal nerves of anterior abdominal ganglia. Phase 2 of the TI_As tactile response is generated by a typical neural pathway that includes mechanoreceptors in each abdominal segment, which project to interneurons with axons in either abdominal connective. Those interneurons with inputs from receptors that are ipsilateral to their axon have a greater influence on TI_As than those that receive inputs from the contralateral side. The phase 1 response has an important role in reducing initiation time for the escape response. Animals in which the phase 2 pathway has been eliminated by lesion of the abdominal nerve cord are still capable of generating a partial startle response with a typically short latency even when stimulated posterior to the lesion. © 1995 John Wiley & Sons, Inc.     

Evidence that synaptic transmission between giant interneurons and identified thoracic interneurons in the cockroach is cholinergic.

In the cockroach, a population of thoracic interneurons (TIs) receives direct inputs from a population of ventral giant interneurons (vGIs). Synaptic potentials in type-A TIs (TIAs) follow vGI action potentials with constant, short latencies at frequencies up to 200 Hz. These connections are important in the integration of directional wind information involved in determining an oriented escape response. The physiological and biochemical properties of these connections that underlie this decision-making process were examined. Injection of hyperpolarizing or depolarizing current into the postsynaptic TIAs resulted in alterations in the amplitude of the post-synaptic potential (PSP) appropriate for a chemical connection. In addition, bathing cells in zero-calcium, high-magnesium saline resulted in a gradual decrement of the PSP, and ultimately blocked synaptic transmission, reversibly. Single-cell choline acetyltransferase (ChAT) assays of vGI somata were performed. These assays indicated that the vGIs can synthesize acetylcholine. Furthermore, the pharmacological specificity of transmission at the vGI to TIA connections was similar to that previously reported for nicotinic, cholinergic synapses in insects, suggesting that the transmitter released by vGIs at these synapses is acetylcholine.     

Biogenic amines modulate synaptic transmission between identified giant interneurons and thoracic interneurons in the escape system of the cockroach.

In the escape system of the cockroach, Periplaneta americana, a population of uniquely identifiable thoracic interneurons (type A or TIAs) receive information about wind via chemical synapses from a population of ventral giant interneurons (vGIs). The TIAs are involved in the integration of sensory information necessary for orienting the animal during escape. It is likely that there are times in an animal's life when it is advantageous to modify the effectiveness of synaptic transmission between the vGIs and the TIAs. Given the central position of the TIAs in the escape system, this would greatly alter associated motor outputs. We tested the ability of octopamine, serotonin, and dopamine to modulate synaptic transmission between vGIs and TIAs. Both octopamine and dopamine significantly increased the amplitude of vGI-evoked excitatory postsynaptic potentials (EPSPs) in TIAs at 10(-4)-10(-2) M, and 10(-3) M, respectively. On the other hand, serotonin significantly decreased the vGI-evoked EPSPs in TIAs at 10(-4)-10(-3) M. These results indicate that octopamine, serotonin, and dopamine are capable of modulating the efficacy of transmission of important neural connections within this circuit.     

Identification of thoracic interneurons that mediate giant interneuron-to-motor pathways in the cockroach

Paired intracellular recordings were made to identify thoracic interneurons that receive stable short latency excitation from giant interneurons (GIs). Eight metathoracic interneurons were identified in which EPSPs were correlated with GI activity which was evoked either by wind or intracellular electrical stimulation or occurred spontaneously. In all cases EPSPs in the thoracic interneurons followed GI action potentials faithfully at short latencies. EPSPs associated with GI action potentials consistently represented the upper range of amplitudes of a large sample of EPSPs recorded in the thoracic interneurons. Seven of the interneurons were correlated with activity in ventral GIs but were not correlated with activity in dorsal GIs. Four of these interneurons were part of a discrete population of interneurons whose somata are located in the dorsal posterior region of the ganglion. The eighth interneuron (designated the T cell) was positively correlated with activity in dorsal GIs. The four dorsal posterior group interneurons and the T cell were depolarized intracellularly to establish their potential for generating motor activity. In all cases evoked activity was stronger in leg motor neurons (primarily Ds and the common inhibitor) located on the side contralateral to the interneuron's soma. The results indicate that significant polysynaptic pathways exist by which GI activity can evoke motor activity. The implications of this conclusion to investigations on the cockroach escape system are discussed.     

Evidence that synaptic transmission between giant interneurons and identified thoracic interneurons in the cockroach is cholinergic

In the cockroach, a population of thoracic interneurons (TIs) receives direct inputs from a population of ventral giant interneuons (vGIs). Synaptic potentials in type-A TIs (TI_As) follow vGI action potentials with constant, short latencies at frequencies up to 200 Hz. These connections are important in the integration of directional wind information involved in determining an oriented escape response. The physiological and biochemical properties of these connections that underlie this decision-making process were examined. Injection of hyperpolarizing or depolarizing current into the postsynaptic TI_As resulted in alterations in the amplitude of the postsynaptic potential (PSP) appropriate for a chemical connection. In addition, bathing cells in zero-calcium, high magnesium saline resulted in a gradual decrement of the PSP, and ultimately blocked synaptic transmission, reversibly. Single-cell choline acetyltransferase (ChAT) assays of vGI somata were performed. These assays indicated that the vGIs can synthesize acetylcholine. Further more, the pharmacological specificity of transmission at the vGI to TI_A connections was similar to that previously reported for nicotinic, cholinergic synapses in insects, suggesting that the transmitter released by vGIs at these sypapses is acetylcholine. © 1992 John Wiley & Sons, Inc.     

Parallel motor pathways from thoracic interneurons of the ventral giant interneurons system of the cockroach,Periplaneta americana

The data described here complete the principal components of the cockroach wind-mediated escape circuit form cercal afferents to leg motor neurons. It was previously known that the cercal afferents excite ventral giant interneurons which then conduct information on wind stimuli to thoracic ganglia. The ventral giant interneurons connect to a large population of interneurons in the thoracic ganglia which, in turn, are capable of exciting motor neurons that control leg movements. Thoracic interneurons that receive constant short latency inputs from ventral giant interneurons have been referred to as type A thoracic interneurons (TI_As). In this paper, we demonstrate that the motor response of TI_As occurs in adjacent ganglia as well as in the ganglion of origin for the TI_A. We then describe the pathway from TI_As to motor neurons in both ganglia. Our observations reveal complex interactions between thoracic interneurons and leg motor neurons. Two parallel pathways exist. TI_As excite leg motor neurons directly and via local interneurons. Latency and amplitude of post-synaptic potentials (PSPs) in motor neurons and local interneurons either in the ganglion of origin or in adjacent ganglia are all similar. However, the sign of the responses recorded in local interneurons (LI) and motor neurons varies according to the TI_A subpopulation based on the location of their cell bodies. One group, the dorsal posterior group, (DPGs) has dorsal cell bodies, whereas the other group, the ventral median cells, (VMC) has ventral cell bodies. All DPG interneurons either excited postsynaptic cells or failed to show any connection at all. In contrast, all VMC interneurons either inhibited postsynaptic cells or failed to show any connection. It appears that the TI_As utilize directional wind information from the ventral giant interneurons to make a decision on the optimal direction of escape. The output connections, which project not only to cells within the ganglion of origin but also to adjacent ganglia and perhaps beyond, could allow this decision to be made throughout the thoracic ganglia as a single unit. However, nothing in these connections indicates a mechanism for making appropriate coordinated leg movements. Because each pair of legs plays a unique role in the turn, this coordination should be controlled by circuits didicated to each leg. We suggest that this is accomplished by local interneurons between TI_As and leg motor neurons.     

Localization of ventral giant interneuron connections to the ventral median branch of thoracic interneurons in the cockroach

A detailed morphological study was performed to localize the probable sites of connections between two identified populations of interneurons (ventral giant interneurons and type-A thoracic interneurons) in the cockroach. Type-A thoracic interneurons (TIAS) appear to play an important role in orienting the cockroach during wind-mediated escape. However, their large number, approximately 100 neurons, precludes analyzing each cell's role electrophysiologically. The TIAS are characterized by a prominent branch located on one or both sides of the ventral median (VM) region of the thoracic ganglion in which their soma resides. The presence of this ventral median branch can be used to predict connectivity with left or right ventral giant interneurons (vGIs) (Ritzmann and Pollack, 1988) and is correlated with the TIA's directional response to wind (Westin, Ritzmann, and Goddard, 1988), suggesting that this is the locus of synaptic connection. Two approaches were employed to address this hypothesis. Morphological overlap of differentially labelled cells (ethidium bromide, Lucifer Yellow) was examined at the light microscopic level to locate areas of possible synaptic contact. Experiments were also performed in which one-half of the vGI input to the TIAs was surgically removed early in postembryonic development. Although no changes in the overall branching pattern were observed, the VM branches on the operated side were significantly shorter than were those on the unoperated side. Thoracic interneurons that do not receive inputs from vGIs were unaffected by this surgery. The data reported here thereby confirm previous observations by localizing the vGI inputs specifically to the VM branch, and provide a morphological cue for predicting connectivity and function.     

Biogenic amines modulate synaptic transmission between identified giant interneurons and thoracic interneurons in the escape system of the cockroach

In the escape system of the cockroach, Periplaneta americana, a population of uniquely identifiable throacic interneurons (type A or TI_As) receive information about wind via chemical synapses from a population of ventral giant interneurons (vGIs). The TI_As are involved in the integration of sensory information necessary for orienting the animal during escape. It is likely that there are times in an animal's life when it is advantageous to modify the effectiveness of synaptic transmission between the vGIs and the TI_As. Given the central position of the TI_As inthe escape system, this would greatly alter associated motor outputs. We tested the ability of octopamine, serotonin, and dopamine to modulate synaptic transmission between vGIs and TI_As. Both octopamine and dopamine significantly increased the amplitude of vGI-evoked excitatory postsynaptic potentials (EPSPs) in TI_As at 10^?4?10^?2 M, and 10^?3 M, respectively. On the other hand, serotonin significantly decreased the vGI-evoked EPSPs in TI_As at 10^?4?10^?3 M. These results indicate that octopamine, serotonin, and dopamine are capable of modulating the efficacy of transmission of important neural connections within this circuit. © 1992 John Wiley & Sons, Inc.     

Responses of the nematodeCaenorhabditis elegans to controlled chemical stimulation

Summary A new method is described for studying the behavioral responses of nematodes to controlled chemical stimulation. The worm is held by the tail with a suction pipet. Behavior is recorded by an array of light sensors connected to a multichannel recorder. Several types of behavior can be detected in addition to the normal backward propagating waves of about 2 Hz that propel untethered worms forward. The most dramatic of these is the reversal bout, consisting of forward propagating waves of about 0.7 Hz, that propel untethered worms backward. The latter waves are easily distinguished from the former by the large amplitude motion caused by the fact that they contain a sharper bend at the tail. This technique was used to demonstrate that a purely temporal change in chemical stimulation can cause a large change in the probability of occurrrence of a reversal bout. These altered probabilities adapt back to the basal level in about one minute. Increased probabilities adapt faster than decreased. Stronger stimulation causes slower adaptation. Since the reversal bout is associated with changes in direction of locomotion, these observations suggest that klinokinesis with adaptation plays a role in the movement of nematodes in chemical gradients.I would like to thank Deborah Higgins for technical assistance. This work was supported by Grant RR 07024q3 awarded by the Biomedical Research Support Grant Program, Division of Research Resources, National Institutes of Health and by the National Science Foundation under Grant GB-43561.     

Analysis of proprioceptive inputs to DPG interneurons in the cockroach

In this study we report on morphological and physiological analysis of proprioceptive sensory input to thoracic interneurons. Sensory neurons from leg proprioceptors were filled using cobalt chloride. The morphological location of these sensory neurons was compared with that of the DPG interneurons. The interneurons investigated were found to have morphological overlap with the sensory neurons of the specific proprioceptors, suggesting that they have the potential to receive direct input from these proprioceptors. Individual interneurons were recorded intracellularly and identified by intracellular injection of Lucifer Yellow, and the responses of these cells to mechanical stimulation of specific proprioceptors were analyzed. All of the DPG interneurons tested as well as other interneurons receive input from one or more of these proprioceptors. In addition, DPG interneurons have ipsilateral/contralateral biases in their responses to proprioceptors. Paired stimulation of proprioceptors resulted in enhancement or decrement of the response in the interneurons, depending upon which sensory structures were stimulated together. The results of this study show that proprioceptive information is processed by DPG interneurons.     

Parallel motor pathways from thoracic interneurons of the ventral giant interneuron system of the cockroach, Periplaneta americana

The data described here complete the principal components of the cockroach wind-mediated escape circuit from cercal afferents to leg motor neurons. It was previously known that the cercal afferents excite ventral giant interneurons which then conduct information on wind stimuli to thoracic ganglia. The ventral giant interneurons connect to a large population of interneurons in the thoracic ganglia which, in turn, are capable of exciting motor neurons that control leg movements. Thoracic interneurons that receive constant short latency inputs from ventral giant interneurons have been referred to as type A thoracic interneurons (TIAs). In this paper, we demonstrate that the motor response of TIAs occurs in adjacent ganglia as well as in the ganglion of origin for the TIA. We then describe the pathway from TIAs to motor neurons in both ganglia. Our observations reveal complex interactions between thoracic interneurons and leg motor neurons. Two parallel pathways exist. TIAs excite leg motor neurons directly and via local interneurons. Latency and amplitude of post-synaptic potentials (PSPs) in motor neurons and local interneurons either in the ganglion of origin or in adjacent ganglia are all similar. However, the sign of the responses recorded in local interneurons (LI) and motor neurons varies according to the TIA subpopulation based on the location of their cell bodies. One group, the dorsal posterior group, (DPGs) has dorsal cell bodies, whereas the other group, the ventral median cells, (VMC) has ventral cell bodies. All DPG interneurons either excited postsynaptic cells or failed to show any connection at all. In contrast, all VMC interneurons either inhibited postsynaptic cells or failed to show any connection. It appears that the TIAs utilize directional wind information from the ventral giant interneurons to make a decision on the optimal direction of escape. The output connections, which project not only to cells within the ganglion of origin but also to adjacent ganglia and perhaps beyond, could allow this decision to be made throughout the thoracic ganglia as a single unit. However, nothing in these connections indicates a mechanism for making appropriate coordinated leg movements. Because each pair of legs plays a unique role in the turn, this coordination should be controlled by circuits dedicated to each leg. We suggest that this is accomplished by local interneurons between TIAs and leg motor neurons.     

Sex pheromone blend of Manduca sexta: Responses of central olfactory interneurons to antennal stimulation in male moths

Recently, chemical analysis of solvent rinses of the external surfaces of pheromone glands from female Manduca sexta revealed a blend of 12 aldehydes, including the previously identified sex pheromone component, (E,Z)-10,12-hexadecadienal (bombykal). Previous electrophysiological studies showed that olfactory (deutocerebral) interneurons in the antennal lobes of males exhibited a wide range of responsiveness to pheromonal stimulation of the ipsilateral antenna. These experiments were performed with crude extracts of pheromone glands as well as two synthetic compounds: the major pheromone component, bombykal, and (E,Z)-11,13-pentadecadienal, a mimic of a second component of the female's pheromone blend. Using intracellular methods, we have now reexamined similar olfactory interneurons, using each of the 12 chemically identified components as well as synthetic blends of various combinations of them. Eight of the 12 components isolated from female glands elicited some form of response in olfactory interneurons in males. In accordance with biochemical and behavioral data, the most potent are bombykal and two trienals, (E,E,E)- and (E,E,Z)-10,12,14-hexadecatrienal. We also conclude that the C₁₅ dienal is selective for one of the trienal receptors on the antenna, but is much less potent than the natural trienal stimulant.     

Localization of the enhanced input to cockroach giant interneurons after partial deafferentation

The ventral giant interneurons (GIs) in the cockroach have two distinct dendritic fields: a small one ipsilateral to the soma, and a larger, contralateral field from which the axon arises. The major input to these GIs is from the cercus on the axon side; when this cercus is ablated in the last instar before the adult stage, input from the other cercus becomes more effective within 30 days (Vardi and Camhi, 1982b). I wished to determine if the input from the intact, soma-ipsilateral cercus contacted the GIs purely ipsilaterally and if EPSPs at this site were larger in deafferented animals. Consistent with earlier anatomical findings, intracellular recordings from the GI somata showed that the majority of cercal inputs synapse on their own side of the ganglion in normal animals. This was evidenced by differences in the size and shape of the synaptic potentials evoked from the two cerci and by the presence of large EPSPs after a ganglion had been split along the midline. Unitary EPSPs produced by stimulation of single, identified cercal afferents, ipsilateral to the soma, were compared between normal and deafferented animals. Column "h" afferents were chosen because they make a large contribution to the receptive fields of GIs 1 and 2 after ablation of the contralateral cercus. In addition, the arbors of these afferents, when stained with cobalt, did not cross the ganglionic midline in normal animals. Unitary EPSPs recorded in GI 2 were significantly larger in the deafferented animals. There was, however, no significant change in the size of EPSPs in GI 1. Nevertheless, the results from GI 2 suggest that partial deafferentation in the central nervous system can increase the efficacy of synapses distant from the locus of denervation.     

Interaction between descending input and thoracic reflexes for joint coordination in cockroach: I. Descending influence on thoracic sensory reflexes

Tethered cockroaches turn from unilateral antennal contact using asymmetrical movements of mesothoracic (T2) legs (Mu and Ritzmann in J Comp Physiol A 191:1037–1054, 2005). During the turn, the leg on the inside of the turn (the inside T2 leg) has distinctly different motor patterns from those in straight walking. One possible neural mechanism for the transformation from walking to inside leg turning could be that the descending commands alter a few critical reflexes that start a cascade of physical changes in leg movement or posture, leading to further alterations. This hypothesis has two implications: first, the descending activities must be able to influence thoracic reflexes. Second, one should be able to initiate the turning motor pattern without descending signals by mimicking a point farther down in the reflex cascade. We addressed the first implication in this paper by experiments on chordotonal organ reflexes. The activity of depressor muscle (Ds) and slow extensor tibia muscle (SETi) was excited and inhibited by stretching and relaxing the femoral chordotonal organ. However, the Ds responses were altered after eliminating the descending activity, while the SETi responses remain similar. The inhibition to Ds activity by stretching the coxal chordotonal organ was also altered after eliminating the descending activity.