Modulation of visual cortex inhibition during reticular evoked arousal

The effects of LSD and some analogues on the responses of single cortical neurons of the cat to optical stimulation

The effects of lysergic acid diethylamide (LSD) and its analogues, 2-bromo-LSD (BOL) and methysergide, have been investigated on the responses to photic stimulation of single neurons in the striate cortex of the paralyzed, anesthetized cat. Systemic LSD (0.1--50 micrograms/kg, i.v.) produced: (a) enhancement or depression of evoked activity, the former being common with low, the latter with high doses; (b) changes in directional selectivity; and (c) changes in unstimulated background discharges. The effectiveness of the drug was reduced by repeated administration. Both BOL (10--75 micrograms/kg) and methysergide (100-700 micrograms/kg) produced effects qualitatively similar to LSD, but were considerably less potent. Microelectrophoretic administrations of LSD to single cortical neurons had actions similar to those caused by intravenous administration. BOL and methysergide required much larger currents to produce any effect and sometimes no effect could be induced by iontophoresis. It was concluded that these drugs influence visually evoked neuronal responses mainly by acting directly on cortical cells or synapses; and that the interference with visual cortical function could account for the distortion of visual perception caused by lysergic acid analogues; but that the hallucinogenic and psychotomimetic actions of LSD probably require additional subcortical effects.

Cortical mechanisms that augment or reduce evoked potentials in cats

The augmenting or reducing of flash-evoked potentials in paralyzed cats was significantly correlated with their previously videotaped behaviors. Augmenting reducing was due to alterations in cortical rather than subcortical responsiveness, and was duplicated by increasing cortical arousal by means of reticular stimulation while the animal was exposed to visual stimuli of constant intensity. Brainstem activation of cortical inhibition may underlie the behavioral results as well as the results obtained with the evoked potentials.

Reticular suppression of flash-evoked IPSPs in visual cortex neurons

In a majority of visual cortex (VC) neurons recorded intracellularly in chronically implanted rabbits, light flashes evoked responses consisting of protracted IPSPs (duration 160-200 msec) followed in some cases by a rebound. Inhibition of spontaneous activity through high frequency reticular stimulation (MRF) or through presentations of non-visual arousing stimuli, was not associated with strong hyperpolarization of the cell membrane. Reticular stimulation evoking both activation and inhibition of spontaneous neuronal activity elicited attenuation or complete elimination elimination (disinhibition) of flash-evoked protracted IPSPs and of the postinhibitory rebounds. Rhythmic neuronal discharges elicited by stimulation of the visual pathway was also reduced during reticular activation, due to attenuation of the IPSP-rebound sequence.