Ahd. Watson et al., GABA-IMMUNOREACTIVITY IN PROCESSES PRESYNAPTIC TO THE TERMINALS OF AFFERENTS FROM A LOCUST LEG PROPRIOCEPTOR, Journal of neurocytology, 22(7), 1993, pp. 547-557
Individually labelled sensory neurons from the femoral chordotonal org
an, a proprioceptor at the femoro-tibial joint of a locust hindleg, we
re analysed by intracellular recording, and by electron microscopical
immunocytochemistry to reveal the arrangement of their input and outpu
t synapses and to determine whether the input synapses were GABAergic.
Intracellular recordings from these sensory neurons show spikes super
imposed on a barrage of synaptic potentials during movements of the fe
moro-tibial joint. These synaptic inputs can be mimicked by GABA. Inpu
t synapses are made onto the vesicle-containing terminals of afferents
and are often closely associated with the output synapses. By contras
t, the axons of the afferents in the neuropil have no vesicles and nei
ther make nor receive synapses. The input synapses to the afferent ter
minals are made from processes typically a few microns in diameter, wh
ereas the output synapses are made onto much smaller processes of only
0.1-0.2 mum. Input synapses at which an afferent terminal is the only
postsynaptic element are common. Where the synapse is dyadic the seco
nd postsynaptic element does not usually appear to be a chordotonal af
ferent. The output synapses from the afferent terminals are usually dy
adic. At 78% of the input synapses, the presynaptic neurite showed imm
unoreactivity to a GABA antibody, supporting the physiological evidenc
e that the presynaptic effects can be mediated by the release of GABA.
The remaining (22%) immunonegative synapses are intermingled with tho
se showing GABA immunoreactivity, but their putative transmitter is un
known. These morphological observations suggest that the presynaptic c
ontrol of the chordotonal afferents. is largely mediated by GABAergic
neurons, but because other types of neuron also appear to be involved,
presynaptic modulation may be more complex than has yet been revealed
by the physiology.