The effect of haloperidol, spiperone and pimozide on the flexor reflex of the hind limb of the spinal rat

Potential changes of frog afferent terminals in response to dopamine

The actions of dopamine on the membrane potential of afferent fibers of the isolated hemisected frog spinal cord were studied by sucrose gap techniques. The most prominent effect seen after addition of dopamine to the superfusing Ringer's solution was a slow reversible hyperpolarization at concentrations as low as 0.01 microM; its amplitude and duration were dependent upon concentration and length of application. Biphasic responses with an initial dominant hyperpolarization and a much smaller, later depolarization were also noted and were particularly prominent when dopamine was applied at higher concentrations. Exposure of the cord to apomorphine, a non-selective agonist, to SKF 38393A, a D-1 selective agonist, or to LY-14186, a D-2 selective agonist, hyperpolarized the dorsal root in a manner similar to that of dopamine, but only when the former compounds were applied at higher concentrations (100 microM or greater). Apomorphine also elicited a late depolarization. The non-selective dopamine antagonists, fluphenazine and haloperidol, reversibly reduced dopamine's actions. Similar effects were produced by the selective D-2 antagonists, sulpiride and metoclopramide, which had no effect on hyperpolarizations evoked by norepinephrine. Dopamine did not appear to activate adrenergic or serotonergic receptors, for its effects were not affected by yohimbine, corynanthine, propranolol, or methysergide. The effect of dopamine appeared to result from an action of the amine on both afferent fibers and interneurons. This inference was drawn because the potential changes produced by dopamine were substantially reduced, but never eliminated, by superfusion of the cord with solutions containing Mn2+ ions, tetrodotoxin or mephenesin.(ABSTRACT TRUNCATED AT 250 WORDS)

Dopamine receptor-mediated depression of spinal monosynaptic transmission

The effects of the dopamine agonists apomorphine, lisuride and lergotrile were evaluated on cat spinal cord monosynaptic transmission by stimulating the dorsal root and recording the ventral root compound action potential. All 3 agonists decreased the area of the monosynaptic response. This effect was prevented by pretreatment with the dopamine antagonists haloperidol and metoclopramide, but not with the alpha-adrenergic antagonist phentolamine. These results suggest the existence of spinal cord dopamine receptors which can modulate motor output.