[Abstract Title]. - Society for Neuroscience

Authors: N. ROTEM 1 , E. SESTIERI 3 , Y. YAROM 2 , *J. HOUNSGAARD 4 ;
1 Dept. of Neurobiology/ The Otto Loewi Ctr., 2 Neurobio., Hebrew Univ., Jerusalem & Eilat,
Israel; 3 Otto Loewi Minerva Ctr., Inter-University Inst., Eilat, Israel; 4 MFI, Panum Inst., 2200N
Copenhagen, Denmark
Abstract: Hair cell driven sensory systems track rapid changes in the environment. However,
early processing of vestibular and auditory information is complicated by a diversity of CNS
projections with specialized primary afferent terminals and target neurons. In elasmobranches the
near-body electric field is monitored by hair cells in Ampullae Lorenzini. This information is
transmitted exclusively to ascending efferent neurons (AENs) in the ipsilateral dorsal
octavolateral brain stem nucleus. Thus, the precise goal-directed behaviors elicited by particular
changes in the near-body electric field crucially depend on the fidelity of mapping primary
afferent activity onto the AENs. Here we present the first study of transmission at this first
synapse in early processing of electrosensory information in the CNS.
Intracellular recordings were obtained from AENs in the isolated brainstem from Iago omanensis
sharks. Stimulating the anterior lateral line nerve selectively activated primary afferents (AFFs).
Only monosynaptic excitatory responses were evoked by the stimulus. The range in stimulus
intensity from just threshold to supramaximal intensity was narrow. In this range the synaptic
response typically increased in amplitude from 0.5 to 4.5 mV. We noted that this increase was
non-continuous. Indeed, amplitude histogram revealed clustering around 2-5 amplitude means.
In most AENs the synaptic response evoked by an AFF stimulus readily elicited an action
potential. Furthermore, the voltage threshold for action potentials was distinctly lower for AFF
evoked response than for depolarization by intracellularly injected current and the synaptic
response evoked by a parallel fiber stimulus. This suggests that each AEN is innervated by only
few afferents that form a highly efficient synaptic transmission.
The response to AFF stimulation was depressed by paired stimulation. Paired-pulse depression
reached a maximum with a delay of 30 ms and was observed at all stimulus intensities. The
depression was not associated with a detectable postsynaptic conductance increase and inhibitory
synaptic potential could not be discerned. Paired-pulse depression was blocked by bicculline and
therefore most likely mediated by GABAA receptors. These observations suggest that pairedpulse
depression is caused by presynaptic inhibition.
In conclusion, electrosensory information in the shark is mapped from primary afferents to AENs
with minimal convergence. Each AEN is dominated by 2-5 afferents that form robustly reliable
excitatory synaptic connections. Temporal integration of electrosensory information is regulated
by feed-forward presynaptic inhibition.
Disclosures: N. Rotem, None; E. Sestieri, None; Y. yarom, None; J. Hounsgaard , None.
Poster
237. Synaptic Integration II
Time: Sunday, November 16, 2008, 1:00 pm - 5:00 pm