WHOLE CELL RECORDINGS FROM RESPIRATORY NEURONS IN THE MEDULLA OF BRAIN-STEM-SPINAL CORD PREPARATIONS ISOLATED FROM NEWBORN RATS

In brainstem-spinal cord preparations isolated from newborn rats, a whole cell recording technique was applied to record membrane potentials of inspiratory (Insp) and pre-inspiratory (Pre-I) neurons in the ventrolateral medulla. Labelling of these respiratory neurons with Lucifer Yellow allowed analysis of their locations and morphology. Intracellular membrane potentials from 25 Insp neurons were recorded. Average resting membrane potential was -49 mV (n = 25) and input resistance was 306 M-OMEGA. Insp neurons were classified into three types from the patterns of synaptic potentials. Type I neurons (n = 11) had a high probability of excitatory postsynaptic potentials (EPSPs) in the pre- and post-inspiratory phases. Type II neurons (n = 7) showed abrupt transition to the burst phase from the resting potential level without increased EPSPs in the pre-inspiratory phases. Type III neurons (n = 7) were hyperpolarized by inhibitory postsynaptic potentials (IPSPs) in the pre- and post-inspiratory phases. These Insp neurons, located in the ventrolateral medulla 80-490-mu-m from the ventral surface, were 10-30-mu-m in diameter, and had various soma shapes (pyramidal, spherical or fusiform). Intracellular membrane potentials from 24 Pre-I neurons were recorded. The average resting membrane potential was -45 mV (n = 24), and the input resistance was 320 M-OMEGA. Typical Pre-I neurons showed fairly great depolarization accompanied by action potentials during their burst phase and repolarization during the inspiratory phase. Most Pre-I neurons appeared to have a high level of synaptic activity. These cells were located in the ventrolateral medulla 50-440-mu-m below the ventral surface and had pyramidal or fusiform somas of 10-25-mu-m in diameter. Stimulation of the ipsilateral IXth, Xth roots or the spinal cord (C3 level) induced orthodromic responses in most Insp or Pre-I neurons. An antidromic action potential was induced in only one Pre-I neuron by stimulation at the ipsilateral C3 level. Many Insp or Pre-I neurons had dendrites that terminated close to the ventral surface of the medulla. The present study revealed postsynaptic activity of respiratory neurons in the rostral ventrolateral medulla, which is consistent with the excitatory and inhibitory synaptic connections from Pre-I neurons to Insp neurons, and inhibitory synaptic connections for Insp neurons to Pre-I neurons.