Edmund A. Arbas

Physiology and morphology of projection neurons in the antennal lobe of the male mothManduca sexta

Summary1.We have used intracellular recording and staining, followed by reconstruction from serial sections, to characterize the responses and structure of projection neurons (PNs) that link the antennal lobe (AL) to other regions of the brain of the male sphinx mothManduca sexta.2.Dendritic arborizations of the AL PNs were usually restricted either to ordinary glomeruli or to the male-specific macroglomerular complex (MGC) within the AL neuropil. Dendritic fields in the MGC appeared to belong to distinct partitions within the MGC (Figs. 2, 3). PNs innervating the ordinary glomeruli had arborizations in a single glomerulus (uniglomerular) (Figs. 6, 7, 9, 11, 12A) or in more than one ordinary glomerulus of one AL (multiglomerular) (Figs. 12B, C, 14, 15), or in one case, in single glomeruli in both ALs (bilateral-uniglomerular) (Fig. 16). One PN innervated the MGC and many or all ordinary glomeruli of the AL (Fig. 13).3.PNs with dendritic arborizations in the ordinary glomeruli and PNs associated with the MGC typically projected both to the calyces of the ipsilateral mushroom body and to the lateral protocerebrum, but some differences in the patterns of termination in those regions have been noted for the two classes of PNs (Figs. 2, 3, 6, 7, 9, 16). One PN conspicuously lacked branches in the calyces but did project to the lateral protocerebrum (Fig. 14). The PN innervating the MGC and many ordinary glomeruli projected to the calyces of the ipsilateral mushroom body and the superior protocerebrum (Fig. 13).4.Crude sex-pheromone extracts excited all neurons with arborizations in the MGC, although some were inhibited by other odors (Figs. 3, 4). One P(MGC) was excited by crude sex-pheromone extract and by a mimic of one component of the pheromone blend but was inhibited by another component of the blend (Fig. 5).5.PNs with dendritic arborizations in ordinary glomeruli were excited (Figs. 7, 8, 10) or inhibited (Figs. 9, 11) by certain non-pheromonal odors. Some of these PNs also responded to mechanosensory stimulation of the antennae (Figs. 10, 11, 15, 16).6.The PN with dendritic arborizations in the MGC and many ordinary glomeruli was excited by crude sex-pheromone extracts and non-pheromonal odors and also responded to mechanosensory stimulation of the antenna (Fig. 13).

Odor-modulated upwind flight of the sphinx moth, Manduca sexta L.

Summary1.Male and female Manduca sexta flew upwind in response to the odor of female sex-pheromone gland extract or fresh tobacco leaf respectively, and generated very similar zigzagging tracks along the odor plume.2.After loss of odor during flight, males and females alike: (1) first flew slower and steered their flight more across the wind, then (2) stopped moving upwind, and finally (3) regressed downwind.3.Males flying upwind in a pheromone plume in wind of different velocities maintained their ground speed near a relatively constant ‘preferred’ value by increasing their air speed as the velocity of the wind increased, and also maintained the average angle of their resultant flight tracks with respect to the wind at a preferred value by steering a course more precisely due upwind.4.The inter-turn duration and turn rate, two measures of the temporal aspects of the flight track, were maintained, on average, with remarkable consistency across all wind velocities and in both sexes. The inter-turn durations also decreased significantly as moths approached the odor source, suggesting modulation of the temporal pattern of turning by some feature of the odor plume. This temporal regularity of turning appears to be one of the most stereotyped features of odor-modulated flight in M. sexta.

Physiology and morphology of descending neurons in pheromone-processing olfactory pathways in the male moth Manduca sexta

Summary1.We have characterized the responses and structure of olfactory descending neurons (DNs) that reside in the protocerebrum (PC) of the brain of male sphinx moths Manduca sexta and project toward thoracic ganglia.2.Excitatory responses of DNs to pheromone blends were of two general types: (a) brief excitation (BE) that recovered to background in <1 s after the stimulus, and (b) long-lasting excitation (LLE) that outlasted the stimulus by ≥1 s and, in many cases, as long as 30 s. Individual pheromone components were ineffective in eliciting LLE.3.Some neurons showing LLE also exhibited state-dependent responses to pheromonal stimuli. When such neurons were in a state of low background firing, stimulation with pheromone blend elicited LLE. When they were in a state of LLE, an identical stimulus reduced firing for 5–10 s after which firing gradually increased to the initial higher level.4.Thirteen stained DNs were reconstructed from serial sections for detailed analysis of their morphology in the brain. DNs exhibiting LLE had neurites concentrated in the lateral accessory lobes (LALs) in the protocerebrum and adjacent neuropil. Most DNs exhibiting only BE to pheromonal stimuli and other DNs showing responses only to visual or mechanosensory stimuli did not have branches in the LALs.

Control of hindlimb posture by wind-sensitive hairs and antennae during locust flight

Summary1.Steering movements of tethered, flying locusts,Schistocerca gregaria, subjected to simulated yaw were examined under open-loop conditions. Lateral movements of hindlimbs or curling of the abdomen were monitored with a capacitive movement transducer and were interpreted as indicating the tendency of the animal to turn.2.Three responses to simulated yaw were noted: 1) Yaw-correcting upwind turning tendencies (Figs. 1, 2, 3). 2) Downwind turning tendencies (Figs. 2, 3, 4, 5), and 3) transient adjustments of hindlimb position consistent with an upwind turning tendency occurred in animals that made either no sustained postural adjustments of hindlimbs, or that exhibited sustained downwind turning tendencies (Figs. 4, 5).3.Ablations of certain mechanoreceptors tested their roles in wind detection and wind angle determination. The expression of upwind turning tendencies, whether sustained or transient, depends on inputs from cephalic mechanosensory hairplates (Figs. 2, 3, 4, 5). With hairplates occluded, all locusts exhibited downwind turning tendencies. All downwind turning tendencies depend on inputs from the antennae (Figs. 2, 3).4.Antennae and hairplates operate in an apparent antagonism in the steering responses they produce, which may provide the control flexibility required for complex flight maneuvering.

Physiology and morphology of protocerebral olfactory neurons in the male moth Manduca sexta

Summary1.We have used intracellular recording and staining with Lucifer Yellow, followed by reconstruction from serial sections, to characterize the responses and structure of olfactory neurons in the protocerebrum (PC) of the brain of the male sphinx moth Manduca sexta.2.Many olfactory protocerebral neurons (PCNs) innervate a particular neuropil region lateral to the central body, the lateral accessory lobe (LAL), which appears to be important for processing olfactory information.3.Each LAL is linked by its constituent neurons to the ipsilateral lateral PC, where projection neurons from the antennal lobe terminate, as well as to other regions of the PC. The LALs are also linked to each other by bilateral neurons with arborizations in each LAL.4.Some PC neurons showed long-lasting excitation (LLE) that outlasted the olfactory stimuli by ≥ 1 s, and as long as 30 s in some preparations. LLE was more frequently elicited by the sex-pheromone blend than by individual pheromone components. All bilateral neurons that showed LLE had arborizations in the LALs. LLE responses were also recorded in a single local neuron innervating the mushroom body.5.In some other PC neurons, pheromonal stimuli elicited brief excitations that recovered to background firing rates <1 s after stimulation.

Leydig neuron activity modulates heartbeat in the medicinal leech

Summary1.Leydig neurons fire spontaneously at low rates ( < 4 Hz), but their activity increases with mechanical stimulation or electrical stimulation of mechanosensory neurons (Figs. 1, 2, 4). These conditions also cause acceleration of bursting in heart motor neurons (Figs. 1, 2).2.The firing rate of Leydig cells was found to regulate heart rate in chains of isolated ganglia (Fig. 5). When Leydig neurons were made to fire action potentials at relatively high frequencies (ca. 5–10 Hz), however, heart motor neurons ceased bursting and were either silenced (Fig. 6), or fired erratically.3.Firing of Leydig neurons at high rates caused bilateral heart interneurons of ganglia 3 or 4 to fire tonically rather than in their normal alternating bursts (Figs. 6, 7). Tonic firing of these heart interneurons accounts for the prolonged barrages of ipsps recorded in heart motor neurons and the disruption of their normal cyclic activity (Fig. 6).4.Preventing spontaneous activity of Leydig neurons with injected currents in isolated ganglia caused deceleration of the heartbeat rhythm but did not halt oscillation.5.Electrical stimulation of peripheral nerve roots with Leydig neuron activity suppressed in isolated ganglia caused acceleration of heart rate (Fig. 8).

Active Behavior and Reflexive Responses: Another Perspective on Odor-Modulated Locomotion

That male moths orient their flight into the wind upon sensing wind-borne pheromone from a conspecific female, and modulate their subsequent flight performance with respect to pheromone and wind stimuli, is generally accepted by most that study this behavior (see Baker and Vickers this volume; Kramer, Carde and MafraNeto this volume; and Witzgall this volume). The behavioral and physiological mechanisms that enable a male moth to accomplish this complex feat are still a matter for considerable (and lively) debate. As we initiated our approach to this behavior from an explicitly neuroethological perspective (see Arbas this volume) using the moth Manduca sexta as our model, we decided that, rather than assume all male moths locate pheromone sources using the same mechanisms, we would start “from scratch.” This approach has forced us to examine our data in new and different ways and, at times, to come to conclusions and propose mechanisms that are at odds with some of the currently accepted views of the control of pheromone-modulated flight in moths (Arbas and Willis 1994, Willis and Arbas 1994). However, careful review of the work of earlier researchers in this field, especially that of J.S. Kennedy, has revealed hints of many of the interpretations that we have come to with regard to our own data. In order to proceed in the development of our own ideas regarding the generation and control of this biologically and agriculturally interesting behavior, it has been critical to clearly understand the existing hypotheses.

Olfaction in Manduca sexta: Cellular Mechanisms of Responses to Sex Pheromone

In insects, olfaction plays a major role in the control of many kinds of behavior. Orientation and movement toward, and interactions with, receptive mating partners, appropriate sites for oviposition, sources of food, and hosts for parasitism usually involve olfactory signals that initiate, sustain, and guide the behaviors. Because of their prominence in the zoosphere, their economic and medical importance, and their usefulness as models for both behavioral and neurobiological research, insects have been extensively studied by investigators interested in mechanisms of olfactory control of behavior. Insects respond to a variety of semiochemicals, including pheromones (chemical messengers within a species, such as sex attractants) and kairomones (chemical messengers between species and adaptively favorable to the recipient, such as attractants and stimulants for oviposition and feeding emitted by a host plant). Studies of insect responses to such biologically significant odors have shown that the quality and quantity of odorants in complex mixtures present in the environment are encoded in patterns of activity in multiple olfactory receptor cells (ORCs) in the antennae. These ‘messages’ are decoded and integrated in the olfactory centers of the central nervous system (CNS), and it is there that olfactorily induced changes in the behavior or physiology of the insect are initiated.

MUSCLE DEGENERATION FOLLOWING REMOTE NERVE INJURY

Muscle depends upon innervation and contraction to maintain a differentiated state. Denervation can therefore induce muscle atrophy. In grasshoppers, muscle degeneration can also be triggered by the severing of a leg during autotomy. In this case, the muscles that degenerate are neither damaged nor denervated. This phenomenon suggests the existence of transneuronal mechanisms that influence muscle survival. To characterize this autotomy-induced process, we studied the degeneration of a thoracic tergotrochanteral depressor muscle (M#133b,c) subsequent to the shedding of a hindlimb in the grasshoppers Barytettix psolus and Barytettix humphreysii. Both histochemical and electrophysiological methods were used to follow muscle degeneration 1, 3, 5, 10, and 15 days postautotomy. Muscle fibers began to show denervation-like electrophysiological changes (i.e., depolarized resting membrane potentials and postinhibitory rebound) as soon as 3 days postautotomy. By 10 days, significant muscle degeneration was evident and electrophysiological changes were found in all animals tested. Muscle anatomical degeneration was not induced by synaptic transmission failure, because neuromuscular transmission was maintained in most fibers. The rate of muscle degeneration was not constant. Between 1 and 10 days, mean fiber cross-sectional area did not change on the autotomized side, although this is normally a time of muscle growth. However after 10 days, cross-sectional area became drastically reduced and the number of muscle fibers within M#133b,c was decreased. The variability in rate of fiber degeneration was not dependent upon fiber type, since M#133b,c only contains fast-type fibers.

Motor Patterns Underlying Pheromone-Modulated Flight in Male Moths, Manduca sexta

Male moths typically find mates by flying a characteristic zigzag path up a plume of sex-pheromones released by a conspecific female. This upwind path results from a combination of self-steered maneuvers and continuous reactions to visual, olfactory, and mechanosensory inputs during flight. To characterize motor patterns underlying these flight maneuvers, we have simultaneously video-recorded the orientation tracks, and recorded electromyograms (EMGs) from flight muscles of male Manduca sexta flying freely in a plume of female pheromones. EMG activity was synchronized with video recordings of zigzag flight tracks in a laboratory wind tunnel. The activity of up to five muscles was recorded: (1) the left dorsal longitudinal muscle (DLM), the principal wing-depressor; (2 and 3) the left and right third axillary muscles (AxM), thought to be “steering” muscles; and (4 and 5) the left and right first basalar muscles (BaM), which generate lift and thrust by altering the angle of attack of the fore-wing. Parameters measured from the EMGs were: burst period (ms), and burst duration (ms) for all five muscles; and the time of onset of AxM and BaM activity relative to DLM burst onset (defined as the phase of onset).