Localization of the GABAergic and non‐GABAergic neurons projecting to the sublaterodorsal nucleus and potentially gating paradoxical sleep onset

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Abstract
We recently determined in rats that iontophoretic application of bicuculline or gabazine [two GABAa antagonists] and kainic acid (a glutamate agonist) in the sublaterodorsal nucleus (SLD) induces with a very short latency a paradoxical sleep‐like state. From these results, we proposed that GABAergic and glutamatergic inputs to the SLD paradoxical sleep (PS)‐executive neurons gate the onset of PS [R. Boissard et al. (2002) Eur. J. Neurosci., 16, 1959–1973]. We therefore decided to determine the origin of the GABAergic and non‐GABAergic inputs to the SLD combining ejection of a retrograde tracer [cholera‐toxin B subunit (CTb)] with glutamate decarboxylase (GAD) immunohistochemistry. The presence of GAD‐immunoreactive neurons in the SLD was confirmed. Then, following CTb ejections centred on the SLD, combined with GAD and CTb immunohistochemistry, double‐labelled cells were observed in the mesencephalic and pontine reticular nuclei and to a lesser extent the parvicellular reticular nucleus. A large number of GAD‐negative retrogradely labelled cells was also seen in these structures as well as in the primary motor area of the frontal cortex, the central nucleus of the amygdala, the ventral and lateral bed nucleus of the stria terminalis, the lateral hypothalamic area, the lateral and ventrolateral periaqueductal grey and the lateral paragigantocellular reticular nucleus. From these results, we propose that the activation of PS‐executive neurons from the SLD is due to the removal of a tonic inhibition from GABAergic neurons localized in the SLD, and the mesencephalic and pontine reticular nuclei. Strong non‐GABAergic inputs to the SLD could be excitatory and responsible for the tonic glutamatergic input on the PS‐on neurons we have previously described. They could also terminate on SLD GABAergic interneurons and be indirectly responsible for the inhibition of the PS‐on neurons during waking and slow‐wave sleep.