On the mechanism of imipramine's influence in lowering p-hydroxyphenylglycol concentrations in the brain. The role of tyrosine

Further studies on the mechanism of the cardiovascular effects of L-tyrosine

Tyrosine is the precursor amino acid of catecholamines. Low doses of tyrosine produce tachycardia and hypertension, while higher doses induce bradycardia and hypotension in anaesthetised rats. The mechanism and site of action of L-tyrosine are not fully understood. Eight groups of Wistar rats received different pretreatments in order to study the influence of blockade of various receptor mechanisms on the cardiovascular effects of L-tyrosine. The effects mediated by the autonomic nervous system were inhibited by ganglion blockade (hexamethonium), by alpha 1- and beta 1-adrenoceptor blockade (prazosin and atenolol) and by parasympathetic acetylcholine receptor blockade (atropine). The possible role of histamine receptors was studied by inducing H1 and H2-receptor blockade (diphenhydramine and cimetidine, respectively). The effect of inhibition of prostaglandin synthesis by indomethacin was also studied. The L-tyrosine-induced tachycardia was completely blocked by atenolol. Both atenolol and prazosin partly inhibited the hypertensive effects of low doses of tyrosine. The tyrosine-induced bradycardia was not inhibited, and the hypotension was only partly blocked by the pretreatments. Therefore, adrenergic mechanisms seem to mediate the stimulatory cardiovascular effects of tyrosine. The depressant effects of high doses of tyrosine do not appear to be mediated by cholinergic activation, histamine receptor activation, or prostaglandin synthesis.

Cardiovascular effects of L-tyrosine: influence of blockade of tyrosine metabolism

Tyrosine is the precursor of catecholamines. Small doses of tyrosine produce tachycardia and hypertension while higher doses produce bradycardia and hypotension in anaesthetised rats. The mechanism of these effects has not been established. An increased synthesis and release of catecholamines has been suggested to be the mechanism. Various pretreatments were given to anaesthetised Wistar rats to study the influence of a blockade of L-tyrosine metabolism and thus a blockade of catecholamine synthesis, on these cardiovascular effects: valine, which inhibits tyrosine uptake into brain, alpha-methyl-p-tyrosine, which blocks the rate-limiting enzyme, tyrosine hydroxylase, carbidopa and benserazide, which both inhibit dopa decarboxylase, and desipramine, which blocks catecholamine re-uptake. Benserazide and alpha-methyl-p-tyrosine partially blocked the stimulatory effects of tyrosine. None of the pretreatments were able to block effectively the inhibitory effects of L-tyrosine. Therefore, the metabolism of tyrosine to form catecholamines may be involved in the stimulatory but not in the inhibitory cardiovascular effects of L-tyrosine. Valine pretreatment did not antagonize the depressant effects of tyrosine. Since valine blocks the uptake of L-tyrosine into the brain, the depressant effects of L-tyrosine might be peripheral rather than central in origin.

Persistent sensitization of clonidine-induced hypokinesia following one exposure to a stressor: possible relevance to panic disorder and its treatment

Based on previous findings of this laboratory that a single exposure to a stressful stimulus can induce a very long-lasting, sensitizing influence on the actions of drugs of multiple clinical and structural classes, the hypothesis was tested that a single stressful event might exert such an action on the alpha-2 norepinephrine agonist clonidine. Male rats received a single injection of the highly stressful convulsant stimulant pentylenetetrazole (PTZ; 40 mg/kg, IP) and were tested for locomotion after treatment with clonidine (25 micrograms/kg, IP) 1 h, 1 week or 2 weeks later. As expected, clonidine itself induced the hypokinesia typically associated with low doses of this compound. More importantly, all groups pretreated with PTZ showed a significant enhancement of this effect. The influence of PTZ 1 or 2 weeks prior to clonidine cannot be explained as simply due to a lingering impairment of locomotion by PTZ, since no hypokinesia was observed when activity in these groups was examined immediately prior to clonidine administration. Such impairment appears, however, to have been a factor in the heightened hypokinesia observed in the group receiving PTZ only 1 h before clonidine. Mass spectrometric analysis of norepinephrine and 3-methoxy-4-hydroxyphenylglycol levels in hippocampus and cortical areas failed to reveal any changes which could explain the persistent behavioral sensitization we observed. Plasma corticosterone determinations confirmed the stressful nature of PTZ but similarly failed to provide an explanation for the observed behavioral sensitization. The major finding of a long-term sensitizing influence on clonidine of an acute stressful experience is consistent with what is known of the precipitants and treatment of panic disorder.

Decreases in tyrosine and p-hydroxyphenylglycol caused by various antidepressants

The effects of eleven different antidepressant drugs on brain p-hydroxyphenylglycol (pHPG) and on brain and plasma tyrosine concentrations were investigated in rats. Imipramine, amitriptyline, amoxapine, desmethylimipramine and iprindole (20 mg/kg each) and bupropion (50 mg/kg) decreased brain pHPG levels 4.5 or 6 hr after injection. Each of these drugs also significantly reduced plasma tyrosine levels 1.5 hr after injection. In contrast, zimelidine, amitriptylinoxide, trimipramine and trazodone had no significant effect on either brain pHPG or plasma tyrosine. Mianserin significantly lowered plasma tyrosine but produced a nonsignificant decrease in brain pHPG. The decreases in brain pHPG caused by the various drugs were significantly correlated with 3,4-dihydroxyphenylethyleneglycol. Moreover, decreases in brain pHPG and brain and plasma tyrosine concentrations were correlated with the potencies of these drugs to inhibit in vitro norepinephrine uptake. These results suggest the possibility that noradrenergic (or similar) mechanisms regulate both pHPG and tyrosine levels. However, the decreases in pHPG cannot be explained entirely by a deficiency in tyrosine, since the depletions in pHPG were much larger and longer lasting than those of tyrosine.