α₂-adrenoceptor-mediated presynaptic modulation of GABAergic transmission in mechanically dissociated rat ventrolateral preoptic neurons

The ventrolateral preoptic nucleus (VLPO) is a key nucleus involved in the homeostatic regulation of sleep-wakefulness. Little is known, however, about the cellular mechanisms underlying its role in sleep regulation and how the neurotransmitters, such as GABA and noradrenaline (NA), are involved. In the present study we investigated GABAergic transmission to acutely dissociated VLPO neurons using an enzyme-free, mechanical dissociation procedure in which functional terminals remained adherent and we investigated how this GABAergic transmission was modulated by NA. As previously reported in slices, NA hyperpolarized multipolar VLPO neurons and depolarized bipolar VLPO neurons. NA also inhibited the release of GABA onto multipolar VLPO neurons but had no effect on GABAergic transmission to bipolar neurons. The inhibition of release was mediated by presynaptic α₂ adrenoceptors coupled to N-ethylmaleimide (NEM)-sensitive G-proteins which appeared to act via inhibition of adenylate cyclase and subsequent decreases in protein kinase A activity. The inhibition of GABA release did not, however, involve an inhibition of external Ca²⁺ influx. The results indicate that all VLPO neurons contain GABAergic inputs and that the different morphological subgroups of VLPO neurons are correlated not only to different postsynaptic responses to NA but also to different presynaptic NA responses. Furthermore our results demonstrate an additional mechanism by which NA can modulate the excitability of multipolar VLPO neurons which may have important implications for its role in regulating sleep/wakefulness.