The partial dopamine receptor agonists SDZ 208-911 (N-[(8-alpha)-2,6-dimethylergoline-8-yl]-2,2-dimethylpropanamid e), SDZ 208-912 (N-[8-alpha)-2-chloro-6-methylergoline-8-yl]-2,2- dimethylpropanamide) and terguride (transdihydrolisuride; TDHL) were tested in biochemical, behavioral (locomotor activity) and electrophysiological assays in male rats. In reserpine-pretreated rats, SDZ 208-911 and terguride dose-dependently reduced striatal DOPA formation (NSD 1015 treatment) with similar efficacy (-80%) and potency as the selective D2 receptor agonist quinpirole (LY 171555). SDZ 208-912 only produced a partial reduction (-32%) at the highest dose tested. SDZ 208-911 and terguride partially reversed (by approximately 50%) the gamma-butyrolactone (GBL)-induced increase in striatal DOPA accumulation. Quinpirole produced a 100% reversal while SDZ208-912, per se, was inactive. While quinpirole decreased DOPA accumulation, all three partial agonists elevated striatal DOPA accumulation in non-pretreated rats with SDZ 208-912 being as potent and efficacious as haloperidol. The three partial agonists displayed comparatively high affinities in vitro for the dopamine D2 (3H-spiperone) receptor site and somewhat lower affinity for the 5-HT1A (3H-8-OH-DPAT) receptor site. SDZ 208-911 and SDZ 208-912 also showed high affinities for central alpha 2 (3H-idazoxane) receptors. In line with these findings, the partial ergoline agonists dose-dependently elevated the DOPA accumulation in the noradrenaline-rich cortical brain region and decreased the 5-HT synthesis rate (5-HTP accumulation) in the limbic brain region. Furthermore, high doses of SDZ 208-911 and terguride produced weak signs of the 5-HT behavioral syndrome (flat body posture) in reserpinized rats. In the locomotor activity studies in non-pretreated rats, SDZ 208-911, SDZ 208-912 and terguride reduced the activity to 10-20% of controls with SDZ 208-912 being approximately ten times less potent than the other two compounds. Weak postsynaptic dopamine receptor agonist effects of the partial agonists were demonstrated only in reserpine-pretreated rats; all three partial agonists tested produced occasional forward locomotion and the so-called "jerking" behavior. Extracellular single unit recordings were carried out in chloral hydrate-anesthetized rats to investigate the effects on firing rates of dopamine neurons located in the substantia nigra pars compacta. Intravenous administration of SDZ 208-911 and terguride depressed the firing rate by 42 and 53%, respectively, while apomorphine completely inhibited the cells. SDZ 208-912 only reduced the firing by 16% and some cells displayed a biphasic response with a weak depression at low doses that disappeared at high doses.(ABSTRACT TRUNCATED AT 400 WORDS)

Visual masking effects on test flash thresholds were measured under real and simulated eye movement conditions to determine whether visual masking is primarily responsible for elevations in threshold that are sometimes associated with saccadic eye movements. Brief luminous flashes presented to the central retina before, during, and after saccades were masked by stimuli presented either pre-or postsaccadically. The amount and time course of masking were quantitatively dependent on stimulus parameters of intensity and temporal separation and were unaffected by eye movement parameters (amplitude, velocity, direction) as long as retinal stimulus conditions were constant. The duration of forward masking was longer than that of backward masking. When retinal conditions during saccades were mimicked while the eyes were held steady, masking interactions were identical to those obtained during real saccades. These results indicate that masking effects during saccades in ordinary environments are determined solely by the stimulus situation at the retina. Putative nonvisual, centrally originating saccadic suppression suggested by other authors is evidently not additive with visually determined masking during saccades.

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Detection thresholds were measured for a brief test flash projected on a uniform background before, during, and after saccadic eye movement. The amount and duration of threshold elevation during saccades was directly dependent on back-ground illumination; no significant elevations occurred at backgrounds of 2.0 log fl or less. Similar results were obtained during fixation when the backgrounds were "saccadically" displaced. An occipital evoked potential was recorded in association with both eye movements and background displacements at higher background luminances (no test flash). These results may indicate an activation of a selected population of neural elements - probably the "Y" channels - which occurs during saccades in illuminated environments and which renders the channels less responsive to additional, simultaneous, and appropriately structured stimuli.