Physiological studies of brainstem reticular connectivity. II. Responses of mPRF neurons to stimulation of mesencephalic and contralateral pontine reticular formation.

The connectivity between medial pontine reticular formation (mPRF) and the contralateral mPRF and between mPRF and the mesencephalic reticular formation (MRF) was studied by intracellular recordings of mPRF neuronal responses to microstimulation of the contralateral gigantocellular field (cFTG) portion of mPRF and ipsilateral MRF in unanesthetized, undrugged cats. There was a very high percentage (75-86%) of monosynaptic latency postsynaptic potentials (PSPs) in mPRF neurons in response to microstimulation of cFTG and MRF, and most PSPs (72-82%) were excitatory ones (EPSPs). The initial EPSPs from cFTG stimulation were characterized by a rapid rise time and a relatively constant latency, while those from MRF had a less rapid rise time and a longer plateau; EPSPs from both sites frequently led to spike potential generation. In contrast, the percentage of initial monosynaptic inhibitory PSPs (EPSPs) was less than 4% from each of these regions, statistically significantly less than that from bulbar FTM and bulbar FTG stimulation (about 12%) reported in the companion paper. Injection of depolarizing current in mPRF neurons unmasked hyperpolarizing PSP responses to stimulation that followed initial depolarizing PSPs. Intracellular HRP labeling indicated that these data were from recordings from neurons with 20-100 microns diameters, with 80% greater than 40 microns. Neurons with a different discharge pattern for this area of the pons, a stereotyped burst pattern, were recorded just ventral to mPRF; this discharge pattern resembled that found in inhibitory interneurons in other central nervous system regions. There were no differences in the density and pattern of orthodromic PSPs between those mPRF neurons that were antidromically activated from cFTG and the general population that was not antidromically activated from cFTG or other stimulated sites; this suggests, when combined with data of the companion paper, an identity of input and output elements in mPRF with respect to synaptic response properties. The high degree of connectivity between reticular regions may furnish a substrate for functional interaction.