Changes in reticular formation unit response patterns associated with pentylenetetrazol-induced enhancement of sensory evoked responses

Comparative effects of convulsant drugs on the sensory responses of neurons in the amygdala and brainstem reticular formation

The sensory responses of neurons in the amygdala and mesencephalic reticular formation in the cat were enhanced following the intravenous administration of subconvulsant doses of bicuculline, strychnine, bemegride, pentylenetetrazol and physostigmine. The degree and intensity of the enhancement of the response was considerably greater in the reticular formation than in the amygdala. The latency of the response in simultaneously-recorded pairs of neurons in the amygdala and reticular formation was significantly shorter in the mesencephalic reticular formation. The enhancement induced by convulsants does not appear to be transmitter-specific, since enhancement was produced with sequential administration of convulsant drugs which affect gamma-aminobutyric acid (GABA), glycine or acetylcholine. These findings suggest that the reticular formation is involved, to a larger degree than the amygdala, in the ability of sensory stimuli to initiate generalized convulsive seizures in animals treated with these convulsant drugs. The enhancement of the response in the hippocampus and cortex, which has previously been shown to exhibit a longer latency and a lower degree of enhancement than the reticular formation, coupled with the findings in the amygdala, suggest that the reticular formation may mediate the enhancement of the response of these other regions of the brain. The spread of the enhancement of the response to other structures in the brain via the widely distributed output pathways from the reticular formation may lead to initiation of generalized seizures by a recruitment-like process, which may involve enlargement of the sensory hyperresponsive neural network of the brain until a critical neural mass is reached and initiation of seizures results.

Behavioral functions of the reticular formation

Studies of the behavioral correlates of activity in reticular formation cells, usually performed in restrained animals, have found units whose discharge relates to sensory stimuli, pain and escape behavior, conditioning and habituation, arousal, complex motivational states, REM sleep, eye movements, respiration and locomotion. Units with these different behavioral correlates were found in the same anatomical areas. Most studies report that a large proportion of encountered cells related to the behavior being studied. If one adds up the reported percentages, the total far exceeds 100%. Therefore it appears that many investigators are looking at the same cells and reaching very different conclusions about their behavioral roles. On the basis of observations in unrestrained cats, it is hypothesized that discharge in most RF cells is primarily related to the excitation of small groups of muscles. This hypothesis can parsimoniously explain many previous observations on the behavioral correlates of these cells, and is consistent with anatomical, physiological and phylogenetic studies of the reticular formation. The hypothesized simplicity of reticular formation unit function is contrasted with the complexity of the behavioral functions mediated by the RF, and the implications of this contrast discussed.