1. C. Glycine is the major inhibitory amino acid transmitter in the spinal cord, and strychnine is a relatively selective antagonist of glycine. Strychnine has very little if any action at the GABA receptor-chloride channel complex.
3. A. Analeptic stimulants, such as pentylenetetrazol and picrotoxin, act by inhibiting chloride influx at the GABAA receptor-chloride channel complex. This antagonism can occur through interaction with one of several binding sites or allosteric modifiers of receptor-channel function.
4. B. Methylxanthines have been proposed to be inhibitors of phosphodiesterase, which would elevate intracellular levels of cAMP. However, the concentration of cAMP that is required for such action is above the threshold of CNS stimulation. Since the methylxanthines are relatively potent antagonists of adenosine and since adenosine has been shown to be a reasonably potent inhibitor of both central and peripheral neurons, the most likely mechanism by which CNS stimulation occurs is through antagonism of adenosine receptors.
5. B. Psychomotor stimulants such as amphetamine are also indirectly acting sympathomimetics that increase the release of catecholamines from sympathetic nerve terminals. While amphetamine and other congeners possess additional actions on peripheral sympathetic nerves, this is the most likely explanation for the cardiac stimulation observed following the administration of these agents.
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