1Fachbereich Biologie, Universität Kaiserslautern, D-67653 Kaiserslautern, Germany; and 2Vergleichende Neurobiologie, Universität Ulm, D-89069 Ulm, Germany
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ABSTRACT |
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Büschges, Ansgar and
Harald Wolf.
Phase-dependent presynaptic modulation of mechanosensory signals in the
locust flight system. In the locust flight system, afferents
of a wing hinge mechanoreceptor, the hindwing tegula, make monosynaptic
excitatory connections with motoneurons of the elevator muscles. During
flight motor activity, the excitatory postsynaptic potentials (EPSPs)
produced by these connections changed in amplitude with the phase of
the wingbeat cycle. The largest changes occurred around the phase where
elevator motoneurons passed through their minimum membrane potential.
This phase-dependent modulation was neither due to flight-related
oscillations in motoneuron membrane potential nor to changes in
motoneuron input resistance. This indicates that modulation of EPSP
amplitude is mediated by presynaptic mechanisms that affect the
efficacy of afferent synaptic input. Primary afferent depolarizations
(PADs) were recorded in the terminal arborizations of tegula afferents,
presynaptic to elevator motoneurons in the same hemiganglion. PADs were
attributed to presynaptic inhibitory input because they reduced the
input resistance of the afferents and were sensitive to the
-aminobutyric acid antagonist picrotoxin. PADs occurred either
spontaneously or were elicited by spike activity in the tegula
afferents. In summary, afferent signaling in the locust flight system
appears to be under presynaptic control, a candidate mechanism of which is presynaptic inhibition.
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INTRODUCTION |
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In the control of rhythmic locomotor behavior,
central nervous rhythm generators and sensory signals often interact
closely to produce a functional motor command (e.g., Bässler and
Büschges 1998; Clarac 1991
; Grillner 1985
; Pearson 1993
). Sensory
feedback may sculpture a centrally generated pattern, for instance, by resetting the movement cycle. For the locust flight system it has been
shown that signals from wing proprioceptors modify centrally generated
activity to produce the functional flight motor output. The most
detailed understanding of these interactions concerns the role of the
hindwing tegula in initiating the elevation phase of the wingbeat cycle
(Ramirez and Pearson 1993
; Wolf 1993
;
Wolf and Pearson 1988
). Conversely, central commands may
subject the transmission and processing of sensory signals to
phase-dependent control (Sillar and Skorupski 1986
;
Skorupski and Sillar 1986
). This was demonstrated for
the control of walking (El Manira et al. 1991
;
Gossard et al. 1990
; Wolf and Burrows
1995
). Currently there is just one example (Reichert and
Rowell 1985
) of cyclic modulation of afferent signal processing
in the locust flight system. A number of mechanisms exist for such
phase-dependent modulation, namely, rhythmic changes in membrane
resistance of intercalated neurons, their cyclic de- and
hyperpolarization, or presynaptic inhibition of afferent terminals
(Burrows and Matheson 1994
; see reviews by Clarac
and Cattaert 1996
; Rudomin et al. 1998
). Here we
report that presynaptic gating mechanisms function in the locust flight
system, modulating monosynaptic input from the hindwing tegula to wing
elevator motoneurons in a phase-dependent manner. Presynaptic
inhibition is a candidate mechanism because primary afferent
depolarizations (PADs), which are sensitive to the
-aminobutyric
acid (GABA) antagonist picrotoxin (PCT), can be recorded in the central
terminals of tegula afferents.
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METHODS |
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Fully mature Locusta migratoria from a breeding
colony in Kaiserslautern were used for all experiments. Animals were
dissected from the dorsal side and deafferented according to standard
procedures (Robertson and Pearson 1982). Wind
stimulation of the head elicited flight motor activity ("fictive"
flight). Intracellular recordings were made with an NPI SE10l amplifier
(Polder), in either bridge or discontinuous current clamp mode, from
the neuropil regions of elevator motoneurons or from tegula axons close
to their entrance into the metathoracic ganglion (Fig. 3A
outlines the experimental situation). Extracellular hook electrodes for
stimulation or en passant recording were used in bipolar configuration.
Axons of hindwing tegula afferents were stimulated electrically
according to established procedures, at voltages of 1.0-1.2 T (times
threshold value) (Pearson and Wolf 1988
). PCT was bath
applied to the metathoracic ganglion at a final concentration of 1 × 10
5 M.
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RESULTS AND DISCUSSION |
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Efficacy of synaptic transmission from tegula afferents to elevator motoneurons is modulated in the phase of the wingbeat cycle
The hindwing tegulae provide strong and, in the ipsilateral
hemiganglion monosynaptic, excitatory input to wing elevator
motoneurons (Pearson and Wolf 1988). When recording from
metathoracic elevator motoneurons during flight motor activity, we
observed phase-dependent modulation of the compound excitatory
postsynaptic potentials (EPSPs) elicited by electrical stimulation of
tegula afferents (Fig. 1, A
and B). EPSPs had the largest amplitudes, reaching up to 15 mV in the recording shown, at phases of ~0.4 with regard to depressor
muscle activity (Fig. 1, C and D). Smallest EPSP amplitudes were recorded at phase of ~0.8-1.0 (Fig. 1, C
and D). These observations were consistent in 12 recordings,
from different metathoracic elevator motoneurons in different animals.
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Neither cyclic changes in the membrane potential nor in the input resistance of motoneurons account for modulation of EPSP amplitude
Possible mechanisms of this phase-dependent modulation of EPSP
amplitude (see INTRODUCTION) were examined. First, we
tested if flight-related oscillations in the motoneurons' membrane
potential underlie these changes. At more positive potentials, EPSPs
will decrease in amplitude because of decreasing ionic driving forces [a phenomenon observed during depolarizing current injection into motoneurons of quiescent locusts (e.g., Ramirez and Pearson 1991); this
holds only if no voltage-dependent ion channels are present in the
dendritic region]. Examination of the correlation between EPSP
amplitude and membrane potential (Fig.
2A) yielded data that were
contrary to this expectation. Maximum EPSP amplitudes occurred close to
phase 0.4, at a time when the elevator motoneurons already started to
depolarize, after the interval of least depolarized membrane potential
between phase 0.1 and 0.3 (approximately
56 mV). It was in fact
during this interval of relatively constant membrane potential that the
most dramatic changes in EPSP amplitude occurred. During flight motor
activity, EPSP amplitude was consistently and inversely related to
membrane potential only at more positive potentials (greater than
51
mV). Minimum EPSP amplitudes were observed near phase 0.9, when the
elevator motoneurons already started to repolarize. In addition,
maximal EPSP size was significantly larger during fictive flight than
at rest, despite the more depolarized membrane potentials recorded
during flight motor activity (Fig. 2A). Similar results were
obtained in four other animals.
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Second, cyclic changes in membrane resistance, brought about by flight-related synaptic input to the motoneurons, might cause the observed modulation in EPSP amplitude. Possible changes in membrane resistance were examined by injection of constant-current pulses during fictive flight (Fig. 2B; n = 5). We observed only minor cyclic changes, not remotely sufficient to explain the observed variation in EPSP amplitude (Fig. 2C).
Central arborizations of tegula afferents receive presynaptic inhibitory input
Presynaptic inhibition of tegula afferent terminals is the third
candidate mechanism, which may be responsible for the cyclic modulation
of EPSP amplitude. Indeed, PADs were recorded in the afferent axons of
the hindwing tegula, near their terminal arborizations (Fig.
3A). In quiescent locusts,
PADs occurred spontaneously, but they were also elicited by spike
activity in the tegula afferents (Fig. 3B). A number of
observations suggested that these PADs represent presynaptic inhibitory
input (cf. Clarac and Cattaert 1996). 1) The input
resistance of afferents, determined by the injection of current pulses
(Fig. 3C), decreased by 30% during the occurrence of PADs
(Fig. 3D; n = 3). 2) Bath
application of PCT, an antagonist of the inhibitory transmitter GABA,
abolished PADs (Fig. 3E; n = 3). 3) The
reversal potential of PADs was approximately
60 mV (n = 4) (i.e., close to the equilibrium potential of chloride ions, not
shown). 4) Finally, recordings from the terminal
arborizations of tegula afferents in the metathoracic ganglion revealed
cyclic changes in membrane potential during ongoing flight motor
activity (Fig. 3F). We did not yet determine if these
fluctuations in membrane potential indeed reflect cyclic changes in
presynaptic input.
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In summary, these results provide evidence that the central terminals of hindwing tegula afferents receive presynaptic input modulating their synaptic efficacy in the flight cycle. Presynaptic inhibition is a candidate mechanism for this effect. Further experiments are needed to assess the actual role of presynaptic mechanisms in the phase-dependent modulation of synaptic transmission between tegula afferents and elevator motoneurons and the functional implications for locust flight.
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ACKNOWLEDGMENTS |
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We gratefully acknowledge support of our work by U. Bässler, W. Rathmayer, and R. Wehner. T. Heller and C. Dittrich provided skillful technical assistance. A. Büschges and H. Wolf were Heisenberg fellows of the Deutsche Forschungsgemeinschaft (Bu857 and Wo466) during the experimental work.
Present address of A. Büschges: Zoologisches Institut, Universität Köln, D-50923 Köln, Germany.
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FOOTNOTES |
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Address reprint requests to H. Wolf.
The costs of publication of this article were defrayed in part by the payment of page charges. The article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.
Received 28 May 1998; accepted in final form 1 October 1998.
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REFERENCES |
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