Preliminary studies of the sodium borohydride stabilizable binding of phenylethylamine and tyramine to brain preparations

The borohydride stabilizable binding of 2-phenylethylamine and p-tyramine to mouse brain homogenates was compared to that of tryptamine and of serotonin. The highest binding was found to be that of tryptamine, followed by that of serotonin, tyramine, and phenylethylamine. The stabilizable binding of phenylethylamine to calf midbrain (including corpus striatum) homogenates and synaptic membranes was decreased by dopamine; this amine and D-amphetamine also decreased the stabilizable binding of tyramine to rat brain homogenates. The subsynaptosomal distribution of the binding of phenylethylamine to synaptic calf midbrain fractions was also investigated. The highest binding capacity was found in the 0.8 M fraction, rich in myelin.

Tyramine-binding by synaptosomes from rat brain: effect of centrally active drugs

Incubation of p-tyramine (TRM) with rat midbrain plus corpus striatum (MB+ CS) crude synaptosomal preparations, under conditions which reduce to a minimum amine uptake, results in appreciable binding of this amine by synaptosomes. This process is inhibited by preincubation with a number of drugs active in the CNS, e.g., chlorpromazine, desipramine, d-amphetamine, and diphenhydramine. Morphine, however, does not affect this binding. The Ki for each one of these compounds, as well as the K association constant and concentration of the binding sites of TRM, were determined. These results suggest a role for TRM in synaptic transmission mechanisms occurring in the nigrostriatal system, a function which could be regulated by a number of substances representing some of the major chemical classes of centrally active drugs (CD).

Pathways linking L-phenylalanine and 2-phenylethylamine with p-tyramine in rabbit brain

Concentrations of labeled p-tyramine and 2-phenylethylamine were measured in rabbit brain 10 min after the intraventricular administration of either radioactive amine. In both cases the recoveries of the newly synthesized amine and of the unchanged precursor were significantly increased in animals pretreated with the monoamine oxidase inhibitor pargyline. Significant amounts of both amines were present in rabbit brain 10 min after the intraventricular injection of labeled L-phenylalanine. Pretreatment with pargyline increased their recoveries, whereas alpha-methyldopa (an L-aromatic amino acid decarboxylase enzyme inhibitor, L-AAADI) decreased them considerably, and no significant alteration was found in L-alpha-methyldopa hydrazine (a peripheral L-AAADI) pretreated animals. These results provide evidence for a new biochemical pathway in brain for p-tyramine biosynthesis, with L-phenylalanine (bypassing the formation of L-tyrosine) or 2-phenylethylamine as precursors. The significance and implications of these metabolic routes are discussed, especially considering that p-tyramine itself can be converted to catecholamines.

Biochemical plasticity of synaptic transmission: a critical review of Dale's Principle

"Dale's Principle" states that each neuron releases one and only one synaptic transmitter. Mental disorders and behavioral drug effects are attributed to activation or blockade of one or more of these specific transmitters. A series of biochemical, electrophysiological, and behavioral studies suggests the alternative view that at each monoaminergic synapse the action of the transmitter is modulated by several metabolically related substances: amine analogs (2-phenylethylamine [PEA], p-tyramine, etc.), deaminated products (aldehydes, acids, and alcohols), and possibly also amino acid precursors. In support of this view, the authors present evidence for the presence, synthesis, metabolism, and biological activity (at the cellular level, using microelectrode techniques) of amino acid, amines, and deaminated compounds metabolically related to catecholamines and sorotonin. That neuroamino acids exert direct effects (not mediated via their amine metabolites) is illustrated by the rapid effects of microiontophoretic dopa upon cortical unit activity, and by the observation that neither the lethargic effect of 5-hydroxytryptophan (considered to support Jouvet's serotonergic theory of sleep) nor the behavioral stimulant effects of dopa (considered to support the catecholamine theory of affective behavior) are significantly prevented by L-aromatic amino acid decarboxylase inhibitors. The biological activity of the deaminated metabolites of catecholamines and serotonin is illustrated by the effects of their microiontophoretic administration upon cortical units. Further, probenecid (an inhibitor of acid transport across the blood-brain barrier) is shown to qualitatively alter the effects of intraventricularly administered PEA and of its metabolite phenylacetic acid upon visual evoked potentials. Rabbit brain is shown to synthesize a series of pharmacologically active noncatecholic phenylethylamines as by-products of catecholamine metabolism. Amine modulators such as PEA differ from typical transmitters by their ability to cross biological barriers; inhibition of decarboxylase in peripheral tissues only (using alpha-methyldopa hydrazine) markedly depletes brain PEA (but not catecholamines). Because of the homeostatic control of the rate of transmitter synthesis and disposition, physiological, pharmacological, and pathological changes may be expected to affect more the tissue levels of related modulators. This modulator theory of drug action is illustrated by the effect of several psychotropic drugs upon the brain levels of PEA and of norepinephrine. For instance, amphetamine initially decreases and then increases brain PEA levels, without altering brain norepinephrine levels. The authors propose an expanded "Dale's Principle": each neuron is specific in that it releases at all its endings the same pool of chemical messengers, composed of one transmitter and metabolically related modulators, the relative proportion of which is determined by the physiological state of the cell (biochemical plasticity)...

Penetration of phenylacetic acid across the blood-cerebrospinal fluid barrier

Labeled phenylacetic acid (14C-PAA) penetrates blood-cerebrospinal fluid (CSF) barrier in cats only under a high gradient of concentration; under these conditions the administration of large doses of either PAA or probenecid decreases the plasma:CSF 14C-PAA ratio. 14C-PAA is quickly removed from the CSF and blood and is excreted into urine; this excretion is blocked by probenecid. It appears, therefore, that PAA crosses the blood-CSF barrier with difficulty and that is secreted by the kidney.