Effects of midbrain stimulation and iontophoretic application of serotonin, noradrenaline, morphine and gaba on electrical thresholds of afferent C- and A-fibre terminals in cat spinal cord

Earl Carstens, H. Gilly, H. Schreiber, M. Zimmermann

Research output: Contribution to journalArticle

35 Scopus citations

Abstract

We have used the single-fibre excitability testing method to investigate whether electrical stimulation in midbrain periaqueductal gray or lateral reticular formation, as well as intraspinal iontophoretic application of the suspected inhibitory neurotransmitters serotonin (5-hydroxytryptamine), noradrenaline, the opiate morphine, or γ-maminobutyric acid (GABA), exert presynaptic actions at the central terminals of cutaneous afferent unnmyelinated or myelinated fibres. Thresholds to antidromically excited 42 single unmyelinated and 18 myelinated fibres in the sural nerve by intraspinal microstimulation were determined before and during periaqueductal gray or lateral reticular formation stimulation (3 100 ms trains s at 100 Hz; 100-900 μA) or intraspinal iontophoretic application (40-300 nA) of 5-hydroxytryptamine, noradrenaline, morphine or GABA from a multibarrel micropipette. Periaqueductal gray or lateral reticular formation stimulation had mixed effects on unmyelinated and myelinated fibre thresholds, with most threshold measurements within ± 10% of control. There was a tendency for thresholds to increase more during periaqueductal gray than lateral reticular formation stimulation. Thresholds of unmyelinated fibres were predominantly raised during iontophoretic application of 5-hydroxytryptamine (20/29 fibres), noradrenaline (10/13) and morphine (15/21), while GABA had mixed effects; thresholds of nearly all myelinated fibres were raised by each drug. Both periaqueductal gray or lateral reticular formation stimulation and application of 5-hydroxytryptamine, noradrenaline or morphine tended to raise thresholds in the majority of the 53 unmyelinated and myelinated fibres tested. Methodological problems in interpreting the physiological significance of these results for presynaptic modulation are discussed.

Original languageEnglish (US)
Pages (from-to)395-406
Number of pages12
JournalNeuroscience
Volume21
Issue number2
DOIs
StatePublished - 1987
Externally publishedYes

ASJC Scopus subject areas

  • Neuroscience(all)

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