Neurochemical and anatomical identification of fast- and slow-firing neurones in the rat dorsal raphe nucleus using juxtacellular labelling methods in vivo.
GABA neurones in the dorsal raphe nucleus (DRN) influence ascending 5-hydroxytryptamine (5-HT) neurones but are not physiologically or anatomically characterised. Here, in vivo juxtacellular labelling methods in urethane-anaesthetised rats were used to establish the neurochemical and morphological identity of a fast-firing population of DRN neurones, which recent data suggest may be GABAergic. Slow-firing, putative 5-HT DRN neurones were also identified for the first time using this approach. Fast-firing, DRN neurones were successfully labelled with neurobiotin (n=10) and the majority (n=8/10) were immunoreactive for the GABA synthetic enzyme glutamic acid decarboxylase. These neurones were located in the DRN (mainly lateral regions), and consistently fired spikes with short width (1.1+/-0.1 ms) and high frequency (12.1+/-2.0 Hz). In most cases spike trains were regular but displayed low frequency oscillations (1-2 Hz). These neurones were morphologically heterogeneous but commonly had branching axons with varicosities and dendrites that extended across DRN subregions and the midline. Slow-firing DRN neurones were also successfully labelled with neurobiotin (n=24). These neurones comprised a population of neurones immunopositive for 5-HT and/or tryptophan hydroxylase (n=12) that fired broad spikes (2.2+/-0.2 ms) with high regularity and low frequency (1.7+/-0.2 Hz). However, a slow-firing, less regular population of neurones immunonegative for 5-HT/tryptophan hydroxylase (n=12) was also apparent. In summary, this study chemically identifies fast- and slow-firing neurones in the DRN and establishes for the first time that fast-firing DRN neurones are GABAergic. The electrophysiological and morphological properties of these neurones suggest a novel function involving co-ordination between GABA and 5-HT neurones dispersed across DRN subregions.
University Department of Pharmacology, Mansfield Road, Oxford, UK OX1 3QT. kelly.allers@pharm.ox.ac.uk
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D000200:Action Potentials; D000818:Animals; D001710:Biotin / Q000031:analogs & derivatives*; D004594:Electrophysiology; D005968:Glutamate Decarboxylase / Q000032:analysis; D007150:Immunohistochemistry; D009474:Neurons / Q000737:chemistry* / Q000502:physiology*; D011903:Raphe Nuclei / Q000737:chemistry* / Q000166:cytology / Q000502:physiology*; D051381:Rats; D017207:Rats, Sprague-Dawley; D012701:Serotonin / Q000032:analysis*; D014365:Tryptophan Hydroxylase / Q000032:analysis; D014446:Tyrosine 3-Monooxygenase / Q000032:analysis; D005680:gamma-Aminobutyric Acid
C070614:neurobiotin; D012701:Serotonin; D005680:gamma-Aminobutyric Acid; D001710:Biotin; D014446:Tyrosine 3-Monooxygenase; D014365:Tryptophan Hydroxylase; D005968:Glutamate Decarboxylase