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Pre- and Post-synaptic Actions of 5-Hydroxytryptamine in the Rat Lumbar Dorsal Horn In Vitro: Implications for Somatosensory Transmission

Authors

  • J. A. Lopez-Garcia,

    Corresponding author
    1. Department of Physiology, The Worsley Medical and Dental Building, University of Leeds, Leeds LS2 9NQ, UK
      Dr J. A. Lopez-Garcia, Departamento de Fisiologia y Farmacologia, Facultad de Medicina, Universidad de Alcala de Henares, E-28871 Madrid, Spain.
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      Departamento de Fisiologia y Farmacologia, Facultad de Medicina, Universidad de Alcala de Henares, E-28871 Madrid, Spain

  • A. E. King

    1. Department of Physiology, The Worsley Medical and Dental Building, University of Leeds, Leeds LS2 9NQ, UK
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Dr J. A. Lopez-Garcia, Departamento de Fisiologia y Farmacologia, Facultad de Medicina, Universidad de Alcala de Henares, E-28871 Madrid, Spain.

Abstract

Since the relative contribution of pre- versus post-synaptic actions of 5-hydroxytryptamine (5-HT) to modulation of somatosensory processing in the dorsal horn is not known, recordings fro m primary afferents and dorsal horn neurons from in vitro rat spinal cord were used to address this issue. 5-HT produced a depression of spontaneous dorsal root potentials and a slow primary afferent depolarization (PAD): the PAD versus 5-HT concentration-response curve was bell shaped (maximum at 5 μM; 250±C 41.5 μV). In 28/40 dorsal horn neurons, 5-HT elicited a slow depolarization not clearly associated with a specific input resistance change. Excitatory synaptic transmission from primary afferents to dorsal horn neurons was depressed by 5-HT in 40/45 neurons. 5-HT ≥ 5 μM significantly (P≤ 0.05) decreased the amplitude, shortened the total duration and half-decay time of the excitatory post-synaptic potential (EPSP). A dominant effect of 5-HT on longer latency EPSP components was evident. There was no direct relationship between the magnitude of PAD and the reduction of the EPSP by 5-HT. 5-Carboxamidotryptamine, an agonist for 5-HT1 receptors, mimicked the depression of neurotransmission in the dorsal horn without producing PAD. A sample of dorsal horn neurons (n= 8) was injected with biocytin and their morphology described. All had somata within laminae III-VI. In five of these neurons 5-HT depressed the EPSP but in one interneuron-like and one unclassed neuron the EPSP was potentiated. These data suggest that whilst depression of synaptic transmission is the predominant effect of 5-HT in the deep dorsal horn, this is not easily related to PAD or cellular actions of 5-HT on dorsal horn neurons.

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