Arousal changes in the locust optomotor system

J. Insect Physiol., 1976, Vol. 22, pp. 393 to 396. Pergamon Press. Printed in Great Britain.

AROUSAL

CHANGES

IN THE LOCUST SYSTEM

OPTOMOTOR

J. KIEN* Fachbereich

Biologie-Zoologie

der Technischen

Hochschule,

61 Darmstadt,

W. Germany

(Receiwed 18 August 1975) Abstract-The activity of the optomotor interneurons in the locust optic lobe varies with arousal states. Two behaviourally different arousal situations can be readily reproduced in the locust, a spontaneous and an evoked arousal change. These produce different neuronal correlates in the optomotor interneurons, suggesting the presence of different arousal pathways possibly acting at different sites in the motion detection chain. Furthermore, an additional arousal effect can be demonstrated at the motoneurons showing that a reflex system is sensitive to arousal modulation at several levels. INTRODUCTION

IN ARTHROPODS, periods of increased muscle tone and motor activity can be described as periods of increased arousal exactly as in mammals (ROWELL, 1971). Also, like mammals, arthropods show an improvement in the performance of reflex responses during arousal. For example, the optomotor following eye or head movements of a crab placed inside a striped drum can be considerably increased by sudden flicks of the drum (HORRIDGE,1966) or by weak electrical stimulation to the head (BURROWS and HORRIDGE,1968). Also the firing rates of the motoneurons activated during the optomotor reflexes are more than doubled a~ the animal is aroused (WIERSMA and OBERJAT, 1968 ; WIER~MA and FIORE, 1971). However, unlike mammals neither the source of these arousal changes nor their site of action is known. For example, in a reflex system do the arousal changes occur at the motoneurons or is the change in motor response due to previous effects on interneurons ? One such reflex system which undergoes arousal effects is the locust optomotor system which consists of a relatively simple chain of interneurons and motoneurons many of whose responses have been characterized (KEN, 1974, 1975, and in preparation). Recordings can be made at three levels: (1) the optic lobe output neurons, the two types of directionally selective monocular neurons, labelled Ml and M2; (2) the neurons which are the output from the brain and are of the same types as the optic lobe neurons but are binocular and therefore are labelled Bl and B2; and (3) the responses of some of the motoneurons responsible for yawing head movements. The responses of these interneurons and motoneurons were examined during * Present address: Max Planck Institut fiir Verhaltensphysiologic, Abteilung Huber, 8131 Seewiesen, W. Germany.

two different arousal situations, firstly during the steady decline in arousal such as occurs during the standard dectrophysiological experiment, and, secondly, during the immediate arousal which occurs after sudden application of a strong and novel stimulus. MATERIALS

AND

METHODS

The animals used were Locusta migratoria obtained from laboratory culture. The single-unit recording methods and stimulus apparatus have been described in detail elsewhere (KIEN, 1974, 1975). In order to monitor both the degree of muscular activity and the motor output of the optomotor system, myograms were recorded from muscles 50 and 51 (SHEPHEARD,1973), two of the horizontal head-turning muscles. Both muscles receive innervation from one small and two large motoneurons in the suboesophageal ganglion (Kien and Altman, in preparation). The myograms were recorded with fine insulated copper wires (0.1 mm dia.) whose position on the muscles was confirmed by dissection after each experiment. Data were stored on tape and then passed to an integrator which gave a linear voltage output dependent on the number of spikes within a set interval. Stripe patterns produced by a tungsten lamp in the centre of a slotted metal drum were projected onto a diffuser which stimulated the whole of one eye. The light level at the eye was ca. 600 cd/m2. The non-stimulated eye faced a blank black background. Air pufFs were provided by a fine Pasteur pipette held near the head, thorax, or abdomen. RESULTS Two

experimentally

AND

DISCUSSION

different

arousal

situations

can be easily reproduced. The first is the spontaneous and gradual change in the arousal state in 393

394

J. KEN

an animal either unstimulated or given repetitive stimuli for long periods. Such spontaneous changes in the arousal state are extremely conspicuous during standard electrophysiological experiments; there is usually a gradual decrease in muscle tone and motor activity indicating a continually decreasing arousal level. Concomitantly with this decline there is a diminuition of the optomotor responses. Fig. l(a, b) shows an example of the decrease which 0

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the animal as quiet as before. However, as spontaneous occurrence of this activity pattern is very rare, though it can be easily evoked by appropriate stimuli, this pattern will be called the evoked arousal change. The two arousal situations produce different correlates in the optomotor inter-neurons. The spontaneous arousal change can be monitored by myograms during an ordinary electrophysiological experiment. By the time the interneuron recording can start (usually about 45 min after beginning a preparation), the arousal level of the animal, as indicated by the myograms, is quite low and remains so, providing a stable situation. However, occasionally a recording can be obtained quickly and then the neuron’s response shows great variability. Fig. 2 shows one such case where in the l

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spikedsec

Zsec Fig. l(a, b). The decrease in muscle activity and optomotor responses during a recording from a restrained animal. Upper trace, the integrator output shows a myogram recorded from muscle 50 on the left side. In this and all the following figures the integrator counted spikes over a 250 msec interval. The lower trace indicates the occurrence of a stimulus movement at a velocity of Pl”/sec in an anticlockwise direction. (a) Response at the start of the experiment. (b) Response to the same stimulus less than 1 hr later. Note both the decrease in ongoing muscle activity and the loss of clear optomotor responses.

can occur after the animal has been in the experimental situation for less than 1 hr. Similarly, spontaneous increases in the arousal level (increased muscle tone) are reflected by increases in the optomotor responses of the motoneurons where the spike frequency may rise more than threefold for one or two stimuli before dropping back to the previous level. This pattern of arousal change with alterations of the visual response magnitude will henceforth be called the spontaneous arousal change. The second arousal response is most easily elicited by the sudden application of a strong stimulus, for example air puffs to the head, thorax, or abdomen. Such stimuli, given to a quiescent animal, arouse it to extremely active body and leg movements. In all cases these movements appear undirected and non-specific, and no optomotor movements can be elicited until this random activity has ceased. Very rarely, this same pattern of activity will occur spontaneously; a previously quiescent animal suddenly begins a series of vigorous movements which quickly subside leaving

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Fig. 2. The variation of an M2 interneuron response with time. Response values were obtained by subtracting the spikes in the preceding 0.5 set from those during the 0.5 set movement. Each point is averaged from five movements. The first velocity run (0) was made in the sequence indicated by the numbers. Note the low values obtained for the tirst two parts of the run and the change in the third and fourth parts. The velocity run was repeated immediately (e) and the response level remained high. After 30 min in which there was no stimulation the response level (A) fell to an intermediate level. first part of the velocity run a response frequency of no more than 25 spikes/set could be obtained. Ten min after the start of the run, coinciding with increased ongoing activity in the myograms, there was a sudden increase in the visual response which was maintained during the completion of this run

and for the duration of the next (20 min). HOWever, 30 min later, during which the animal was not stimulated or disturbed, the response fell again. These changes in responsiveness occur only in the visual response and there are no changes in the ongoing activity. There is no clear relationship between the ongoing firing frequency before a movement and the magnitude of a visual response. The evoked arousal change produces a quite different response in the optic lobe interneurons. Fig. 3(A) shows the responses of an Ml neuron to a series of air puffs given after the animal had been in the recording

situation

for over 1 hr.

The neuron

Arousal changes in the locust optomotor system

-I 2sec

395

50 spikes /set

Fig. 3. Effect of sudden arousal by air puffs on the responses of an Ml interneuron (A) and a B2 interneuron (B). (a) Motion stimulation, (b) air puffs to the body were applied continuously during the time indicated by the bar, (c) motion stimuli immediately afterwards, and then (d) motion stimuli and continuous blowing applied simultaneously. The visual stimulus speed in (A) is 8,6”/sec and in (B) 1.3”/sec. does not respond to each air puff but the spontaneous firing rate rises to a level previously obtained only by high-speed movement stimulation. Although the air puffs continue, this high firing rate declines within 0.3 set and settles to a rate approximately the same as before the air puffs were given. Motion stimuli immediately after the air puffs also elicit the same response as before, thus emphasizing that the evoked arousal produces different changes from the spontaneous arousal. If the motion stimuli and air puffs are given simultaneously the ongoing activity rate rises again. The visual response does not appear to add to this rise because no clear optomotor response is superimposed on the raised spontaneous frequency although the neuron has not reached its maximal firing rate. The binocular interneurons show no different responses to air puffs (Fig. 3B) indicating that the arousal effects arise predominantly in the optic lobe. In much the same way as the spontaneous eruptions of body movements can be occasionally observed in the quiescent animal, on rare occasions the ongoing activity of an interneuron suddenly spontaneoudy increases, e.g. from 5 to 20 spikes/set or from 20 to 40 spikes/set. These increases in ongoing activity resemble the evoked arousal effects of high ongoing frequency (e.g. 4O/sec) and the visual responses can no longer be clearly seen. The situation is as in Fig. 3(d). As the increase in the ongoing firing rate of the interneuron is not coupled with an increased visual response it would appear that the ongoing activity

of the neuron is generated independently of the visual input. This was also suggested by the results from the spontaneous arousal changes. However, whereas the spontaneous arousal changes affect the visual response of the interneuron, the evoked arousal changes the ongoing activity. As the spontaneous arousal change alters the transfer of visual information it must act on the visual inputs to the optomotor interneuron or on the interneuron, or on both. The evoked arousal effect, on the other hand, alters the ongoing activity and therefore most probably acts directly on the intemeuron. Thus comparisons of the neuronal correlates of the two arousal situations raise the possibility of two different arousal inputs which may even act at different sites on the optomotor interneurons or their inputs. Such an arrangement would not be surprising as other movement detectors in the locust optic lobe are known to receive several inputs arriving at different sites (O’SHEA and ROWELL, 1975). Although the interneuron responses to air puffs cIarify why no optomotor head movements can be elicited during the evoked arousal, the question remains whether or not these intemeuron effects are solely responsible for the changes in the behavioural optomotor responses. But clearly there must be a separate input to the motoneurons as, during spontaneous changes in arousal state, there is a change in the ongoing activity in the muscle but no change in the optomotor interneurons’ ongoing activity. That this input does affect the

396

J. KIEN

-_I

50 spikeslsec

2 set

Fig. 4. Simultaneous recording of an M2 interneuron (a) and both motor units of muscles 50 and 51 (b and c) on the left side. Positive on the lowest trace marks movement in an anticlockwise direction at a velocity of 3l”/sec. In this experiment the legs, wings, antennae, and ocelli were removed. Wing bases and many hair fields on the head, pronotum, and prothorax were covered with wax. Note the variation in the activity of the first motor unit (b) which does not follow the variations in interneuron activity.

ability of the motoneurons to respond to visual movement can be seen in simultaneous recordings of the interneuron and the motor units innervating muscles 50 and 51. Such a recording (Fig. 4) shows a variability in the motoneurons that is not echoed by the interneuron. The sudden apparently spontaneous increase in motoneuron firing rate during an experiment where the animal was clamped and deprived of much sensory input requires the existence of an input independent from the optomotor interneuron Therefore the recorded. behaviour of the motor output reflects an effect in the motoneurons as well as effects at the precedent interneurons. The experiments here have shown that the state of excitement or arousal level of an insect has a profound influence at many sites within a reflex system. In the locust optomotor system, changes in arousal produce separate effects at three points ; (1) on the visual responses of the optic lobe interneurons (site of action: the motion inputs or the interneuron, or both); (2) a separate effect on the interneuron ongoing activity (site of action most probably the interneuron itself); and (3) directly on the motor system. Furthermore, the different effects are produced by the different experimental situations, described here as the spontaneous arousal change or the evoked arousal change. The cause of the difference between the two arousal situations remains obscure. One possible explanation can be derived from the rare and sudden jumps in interneuron ongoing activity which follow the pattern of evoked arousal. In bees, these sudden changes in ongoing activity are thought to be correlated with body movements (Kaiser, personal communication). Therefore it is possible that the evoked arousal effect is specifically related to the production or occurrence of actual movements rather than just increases in muscle tone. Certainly

the possible correlation of evoked arousal with body movement deserves further study as do the origin of arousal effects, their transmission to sensory interneurons, and their sites of action in reflex systems. Acknowledgements-This work was supported by Deutsche Forschungsgemeinschaft grant no. Me 365/4 to Professor R. MENZEL while the author was supported by an Alexander von Humboldt stipendium.

REFERENCES BURROWSM. and HORRIDGEG. A. (1968) Eyecup withdrawal in the crab Cur&us and the interaction with the optokinetic responses. J. exp. Biol. 49, 285-297. HORRIDGE G. A. (1966) Adaptation and other phenomena in the optokinetic response of the crab Carcinrrs. J. exp. Biol. 44, 285-295. KIEN J. (1974) Sensory integration in the locust optomotor system-II. Direction selective neurons in the circumoesophageal connectives and the optic lobe. Vision Res. 14, 1255-1268. KIEN J. (1975) Neuronal mechanisms subserving directional selectivity in the locust optomotor system. J. con@. Physiol. 102, 337-355. O’SHEA M. and ROWELL C. H. F. (1975) Protection from habituation by lateral inhibition. Nature, Land. 254, 53-55. ROWELL C. H. F. (1971) Antenna1 cleaning, arousal and visual interneuron responsiveness in a locust. g. exp. Biol. 55, 749-761. SHEPHEARDP. (1973) Musculature and innervation of the neck muscles of the desert locust Schistocerca gregaria (ForskLl). J. Morph. 139, 439-464. WIER~MAC. A. G. and OBERJATT. (1968) The selective responsiveness of various crayfish occulomotor fibres to sensory stimuli. Comp. Biochem. Physiol. 26, 1-16. WIER~MAC. A. G. and FIORE L. (1971) Factors regulating discharge frequency in optomotor fibres of Carcinus maenas. J. exp. Biol. 54, 497-505.