Electrophysiological properties and cholinergic responses of rat ventral oral pontine reticular neurons in vitro.

In order to characterize the electrophysiological properties of morphologically identified neurons of the ventral part of the oral pontine reticular (vRPO) nucleus and the effects of cholinergic agonists on them, intracellular recordings were obtained from 45 cells in a rat brain-slice preparation. Intracellular staining was performed with 2% biocytin in potassium acetate (1 M)-filled micropipettes. Results demonstrated the presence of two types of vRPO neurons. Type I cells (n = 12, 24%) were characterized by a break with a decrease of the depolarizing slope following hyperpolarizing pulses which delayed the return to the resting Vm and subsequent spike-firing. The delay was antagonized by 4-AP (200-500 microM) which specifically blocks the transient outward K+-mediated current I(A). Type II neurons (n = 38, 76%) displayed a typical depolarizing sag during hyperpolarizing current pulses which was blocked by Cs+. This behavior is characteristic of the hyperpolarization-activated current I(Q). These two neuronal types displayed different morphological features. Most type I and II cells (100 and 73.7%, respectively) were depolarized by acetylcholine (1-15 microM), carbachol (0.5-1 microM) and muscarine (1-10 microM) through the activation of post-synaptic muscarinic receptors. The remaining type II cells (26.3%) were hyperpolarized (1-10 min, 3-15 mV) through the activation of post-synaptic muscarinic receptors. Results are consistent with the hypothesis that the vRPO could be a neuronal target of Cch in eliciting paradoxical sleep because most of its neurons are activated by muscarinic agonists.