GABAergic and non-GABAergic thalamic, hypothalamic and basal forebrain projections to the ventral oral pontine reticular nucleus: Their implication in REM sleep modulation

The ventral part of the oral pontine reticular nucleus (vRPO) is a demonstrated site of brainstem REM-sleep generation and maintenance. The vRPO has reciprocal connections with structures that control other states of the sleep–wakefulness cycle, many situated in the basal forebrain and the diencephalon. Some of these connections utilize the inhibitory neurotransmitter GABA. The aim of the present work is to map the local origin of the basal forebrain and diencephalon projections to the vRPO whether GABAergic or non-GABAergic. A double-labelling technique combining vRPO injections of the neuronal tracer, cholera-toxin (CTB), with GAD-immunohistochemistry, was used for this purpose in adult cats. All of the numerous CTB-positive neurons in the reticular thalamic and dorsocaudal hypothalamic nuclei were double-labelled (CTB/GAD-positive) neurons. Approximately 15%, 14% and 16% of the CTB-positive neurons in the zona incerta and the dorsal and lateral hypothalamic areas are, respectively, CTB/GAD-positive neurons. However, only some double-labelled neurons were found in other hypothalamic nuclei with abundant CTB-positive neurons, such as the paraventricular nucleus, perifornical area and H1 Forel field. In addition, CTB-positive neurons were abundant in the central amygdaline nucleus, terminal stria bed nuclei, median preoptic nucleus, medial and lateral preoptic areas, dorsomedial and ventromedial hypothalamic nuclei, posterior hypothalamic area and periventricular thalamic nucleus. The GABAergic and non-GABAergic connections described here may be the morphological pillar through which these prosencephalic structures modulate, either by inhibiting or by exciting, the vRPO REM-sleep inducing neurons during the different sleep–wakefulness cycle states.