Reticulo-olivary circuits: An intracellular HRP study in the rat

Abstract

Intracellular recording were obtained from medullary reticular neurons subsequent to electrical stimulation of the ipsilateral or contralateral inferior cerebellar peduncle (ICP) and/or the midbrain. After recording physiological data, the neurons were intracellularly injected with horseradish peroxidase (HRP). Thirty-four HRP filled neurons were subjected to light microscopic analysis. They could be divided into two general groups: (1) those which extend dendritic processes into the neuropil of the inferior olivary complex (n= 19); and (2) those that have no anatomical relationship to the inferior olive (n= 15).. These two populations of reticular neurons differ in their distribution, morphological characteristics and physiological responses. Neurons which extend dendritic processes into the inferior olive are located within 200 μm of the dorsal border of this nuclear complex, between the exiting fibers of the XIIth nerve and the raphe. The cell bodies are located in the nucleus reticularis gigantocellularis and are fusiform or multipolar in shape. Their dendrites extend for long distances in the mediolateral direction; are thin and relatively spine-free except at their distal tips where spines and varicose appendages are evident. Physiologically, midbrain stimulation elicits a fast rising hyperpolarization which is identified as an inhibitory postsynaptic potential. However, only rarely is a response observed subsequent to stimulation of either the ipsilateral or contralateral ICP. Dendrites from 4 neurons from the first group of reticular cells were analyzed at the ultrastructural level. Based on random and serial thin sections, the following features were noted: (1) they contain numerous mitochondria when compared to olivary dendrites; (2) they contribute to the postsynaptic elements within olivary synaptic clusters (glomeruli); and (3) they exhibit focal clusters of synaptic vesicles although conventional synaptic complexes have not been observed. Reticular neurons of the second group, those that do not extend dendritic processes into the inferior olive, are located either lateral to the XIIth nerve or at distances greater than 200 μm from the dorsal border of the inferior olivary complex. Their cell bodies and dendrites are comparable morphologically to the reticular neurons whose dendrites do arborize in the inferior olive. However, rarely are the distal tips of their dendrites characterized by spines or varicose appendages. Physiologically, this population of reticular neurons respond to midbrain stimulation with a low amplitude, short latency depolarizing potential which is interrupted by a hyperpolarizing potential. Responses were shown to be excitatory and inhibitory postsynaptic, amplitude, graded depolarizing potential. No hyperpolarization was observed following this initial response. The present data suggest that there is a distinct population of medullary reticular neurons involved in the circuitry of the inferior olivary complex.