Mechanisms of orexin 2 receptor-mediated depolarization in the rat paraventricular nucleus of the hypothalamus

The paraventricular nucleus of the hypothalamus (PVN) contains dense orexin 2 (OX₂) receptor. We examined the mechanisms of OX₂ receptor-mediated excitation on electrophysiologically identified type I (putative magnocellular), low-threshold spikes (LTS)-expressing type II (putative preautonomic), and non-LTS type II (putative parvocellular neuroendocrine) neurons. In the presence of tetrodotoxin, an OX₂ receptor agonist, ALOXB (30-1000 nM) depolarized 56% of type I, and 73-75% of type II neurons. In type I neurons, ALOXB-induced inward current displayed increased-conductance current-voltage (I-V) relationship and reversed polarity at -27.5 ± 4.8 mV. A Na⁺-Ca²⁺ exchanger (NCX) inhibitor, KBR-7943, attenuated ALOXB responses in the majority of type I neurons, while no attenuation was observed in nearly all type II neurons. Type II neurons exhibited three types of I-V relationships in response to ALOXB, characterized by decreased, increased, and unchanged conductance, respectively. The reversal potential of the decreased-conductance responses was near the equilibrium potential of K⁺ (E_k⁺) and became more positive in a high-K⁺ solution, suggesting that K⁺ conductance blockade is involved. In a low-Na⁺ solution, non-reversed I-V curves of increased-conductance responses became decreased-conductance responses and reversed polarity near E_k⁺, suggesting the involvement of both K⁺ conductance and non-selective cation conductance (NSCC). Approximately 35% of LTS-expressing type II neurons were vasopressin-immunoreactive and 71% of them responded to ALOXB. In conclusion, orexins may activate OX₂ receptor on PVN neurons and cause depolarization by promoting NCX and/or NSCC in magnocellular neurons, and by decreasing K⁺ conductance and/or increasing NSCC in parvocellular neurons. Furthermore, the majority of vasopressinergic preautonomic neurons are under OX₂ receptor regulation.