Calcium influx-independent depression of transmitter release by 5-HT at lamprey spinal cord synapses.
1. The mechanisms by which 5-hydroxytryptamine (5-HT) depresses transmitter release from lamprey reticulospinal axons were investigated. These axons make glutamatergic synapses onto spinal ventral horn neurons. 5-HT reduces release at these synapses, yet the mechanisms remain unclear. 2. Excitatory postsynaptic currents (EPSCs) evoked by stimulation of reticulospinal axons were recorded in ventral horn neurons. 5-HT depressed the EPSCs in a dose-dependent manner with an apparent Km of 2.3 microM. 3. To examine the presynaptic effect of 5-HT, electrophysiological and optical recordings were made from presynaptic axons. Action potentials evoked Ca(2+) transients in the axons loaded with a Ca(2+)-sensitive dye. 5-HT slightly reduced the Ca(2+) transient. 4. A third-power relationship between Ca(2+) entry and transmitter release was determined. However, presynaptic Ca(2+) currents were unaffected by 5-HT. 5. Further, in the presence of a K(+) channel blocker, 4-aminopyridine (4-AP), 5-HT left unaltered the presynaptic Ca(2+) transient, ruling out the possibility of its direct action on presynaptic Ca(2+) current. 5-HT activated a 4-AP-sensitive current with a reversal potential of -95 mV in these axons. 6. The basal Ca(2+) concentration did not affect 5-HT-mediated inhibition of release. Although 5-HT caused a subtle reduction in resting axonal [Ca(2+)]i, synaptic responses recorded during enhanced resting [Ca(2+)]i, by giving stimulus trains, were equally depressed by 5-HT. 7. 5-HT reduced the frequency of TTX-insensitive spontaneous EPSCs at these synapses, but had no effect on their amplitude. We propose a mechanism of inhibition for transmitter release by 5-HT that is independent of presynaptic Ca(2+) entry.
Takahashi
M
M
Department of Physiology, Northwestern University Medical School, Chicago, IL 60611, USA.
Freed
R
R
Blackmer
T
T
Alford
S
S
D015761:4-Aminopyridine / Q000494:pharmacology; D000200:Action Potentials / Q000187:drug effects; D000818:Animals; D001369:Axons / Q000502:physiology; D002118:Calcium / Q000378:metabolism*; D004594:Electrophysiology; D018691:Excitatory Amino Acid Antagonists / Q000494:pharmacology; D018698:Glutamic Acid / Q000378:metabolism; D007798:Lampreys / Q000502:physiology*; D009046:Motor Neurons / Q000502:physiology; D018377:Neurotransmitter Agents / Q000378:metabolism*; D017729:Presynaptic Terminals / Q000187:drug effects / Q000378:metabolism; D012701:Serotonin / Q000494:pharmacology / Q000502:physiology*; D013116:Spinal Cord / Q000502:physiology*; D013569:Synapses / Q000187:drug effects / Q000502:physiology*; D009435:Synaptic Transmission / Q000187:drug effects
D018691:Excitatory Amino Acid Antagonists; D018377:Neurotransmitter Agents; D012701:Serotonin; D018698:Glutamic Acid; D015761:4-Aminopyridine; D002118:Calcium