Neurochemical and anatomical identification of fast- and slow-firing neurones in the rat dorsal raphe nucleus using juxtacellular labelling methods in vivo

Abstract

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.