Uptake and release of meta-tyramine, para-tyramine, and dopamine in rat striatal slices

Tyramine-immunoreactive neuronal structures in the rat brain: Abundance in the median eminence of the mediobasal hypothalamus

Immunoreactivity to p-tyramine, one of the natural trace amines, was studied in the rat brain by an anti-p-tyramine antibody. Immunoreactivity to this amine is very weak in the nigrostriatal dopaminergic neurons and terminals, and weak in the locus coeruleus noradrenergic ones. It was intensified in these structures after monoamine oxidase inhibition. On the other hand, this amine was highly concentrated in the median eminence of the mediobasal hypothalamus, in which its physiological function on prolactin release has been demonstrated.

Effects of monoamine oxidase inhibitors on the acid metabolites of some trace amines and of dopamine in the rat striatum

The effects of the administration of selective and non-selective inhibitors of monoamine oxidase (MAO) on the concentrations of three trace acid metabolites [phenylacetic acid (PAA); m-hydroxyphenylacetic acid (mHPAA); and p-hydroxyphenylacetic acid (pHPAA)] and of an acid metabolite of dopamine [3,4-dihydroxyphenylacetic acid (DOPAC)] in the rat striatum were determined. Administration of brofaromine (1-100 mg/kg, s.c.) a type AMAO inhibitor, dose-dependently decreased DOPAC and mHPAA levels. pHPAA levels were decreased by 100 mg/kg brofaromine, but PAA levels were unaffected. Doses of deprenyl of less than 100 mg/kg, i.p., had no effect on any of the acids, while 100 mg/kg decreased DOPAC, mHPAA and pHPAA but not PAA levels. Clorgyline, pargyline and tranylcypromine treatment decreased the levels of DOPAC, mHPAA and pHPAA but not PAA. Administration of alpha-monofluoromethyldopa, an inhibitor of aromatic amino acid decarboxylase, decreased the levels of all four acids. It was concluded that deamination of the respective parent amine by type A MAO is primarily responsible for the synthesis of DOPAC and mHPAA, but that another pathway contributes to pHPAA synthesis. It appears that either PAA arises predominantly independently from the actions of MAO or that is removal via transport or further metabolism regulates its concentration.

Release of some endogenous trace amines from rat striatal slices in the presence and absence of a monoamine oxidase inhibitor

The basal and 50 mM K+-stimulated release of m-tyramine (mTA), p-tyramine (pTA), tryptamine (TR) and phenylethylamine (PE) from striatal slices obtained from rats pretreated with a monoamine oxidase inhibitor (MAOI) was investigated. A K+-stimulated release of mTA and pTA was observed, but K+ did not stimulate either TR or PE release. The latter two amines, therefore, are unlikely to be conventional neurotransmitters in the rat striatum. The release of endogenous striatal pTA from control rats was also investigated. Veratridine stimulated endogenous pTA release, but 50 mM K+ did not. It is possible, therefore, that endogenous pTA can be released in a transmitter-like fashion.

Tryptamine transport in rat brain slices: a comparison with 5-hydroxytryptamine

The uptake of [14C]tryptamine (14C-T) and [3H]serotonin (3H-5HT) into slices of rat hypothalamus (HT), fronto-parietal cortex (CX), and caudate nucleus (Cau) has been investigated. In all three brain areas, the uptake of 3H-5HT at 37 degrees C was much greater than that in an ice-bath at 1.0-1.5 degrees C. In contrast, the uptake of 14C-T at 37 degrees C was not much greater than uptake at 1.0-1.5 degrees C. While markedly different amounts of 3H-5HT were accumulated by each of the brain areas studied, the regional uptake of 14C-T was quantitatively similar. In general the uptake of 14C-T was inhibited less than 3H-5HT by cocaine, DNP, ouabain, and decreased Na+ concentrations. Similarly, 14C-T was less susceptible to serotonin uptake inhibitors except in the caudate. It was concluded that though a common indoleamine uptake system accumulates both T and 5HT, a non-specific low affinity or diffusional process also transports both amines and is predominantly responsible for T, but not 5HT, uptake. The spontaneous release, or wash-out, of 14C-T from the caudate was much faster than that of 3H-5HT. In addition, while depolarizing stimuli caused little or no release of 14C-T, large releases of 3H-5HT were observed. T, therefore, does not behave like a conventional neurotransmitter.

The behavioural effects of phenelzine and phenylethylamine may be due to amine release

Intraperitoneal administration of phenelzine (PLZ) or of beta-phenylethylamine (PE) quickly leads to short periods of behavioural activation, which are primarily dopaminergic and serotonergic in nature, respectively. In order to determine whether these behavioural effects are due to PLZ- or PE-induced displacement of dopamine (DA) or serotonin (5HT), the ability of these compounds to stimulate release of radiolabelled DA and 5HT from slices of rat striatum was assessed. Additionally, at various times following IP administration of behaviourally active doses of PLZ or PE to rats, the striatal levels of the drugs were measured. Both PLZ and PE were able to stimulate a greater release of radiolabelled DA than of 5HT. Lower concentrations of PE (0.8, 8.0 microM) than of PLZ (100 microM, 1 mM) stimulated release. The striatal levels (approximately 200 microM) of PLZ, as well as of PE, attained at times corresponding to behavioural activation seemed sufficient to cause release of both DA and 5HT. It was concluded, therefore, that the mode of action of these compounds may involve displacement, or release, of these biogenic amines, though it is not clear how such release ultimately leads to the behaviours observed.

Cerebral decarboxylation of meta- and para-tyrosine

The decarboxylase inhibitor DL-alpha-monofluoromethyldopa reduces, in a dose dependent manner, the concentration of striatal p-tyramine in the mouse. Homovanillic acid is also significantly reduced. Conversely, this treatment increases the m-tyramine concentration. Administration of m-tyrosine produces large increases in m-tyramine and a slight decrease in p-tyramine; these changes are potentiated in the presence of the decarboxylase inhibitor. Such data along with other recently published results permit the conclusion that m-tyramine arises from phenylalanine via m-tyrosine and that p-tyramine arises by decarboxylation of p-tyrosine. Both these reactions are closely related to the activity of tyrosine hydroxylase and the availability of appropriate substrates.

The in vitro release of endogenous m-tyramine, p-tyramine and dopamine from rat striatum

Administration of phenelzine (100 mg/kg. i.p., 18 hr) increased rat striatal concentrations of pTA, mTA and DA by 30, 6.7 and 1.5 fold, respectively. Lesions of the medial forebrain bundle prevented these increases, permitting the conclusion that the phenelzine-induced amine increases were localized in the synaptic terminals. The release of endogenous pTA, mTA and DA from striatal slices obtained from phenelzine-treated rats was investigated, 50 mM KC1l elicited releases of pTA, mTA and DA which were significantly greater than their respective basal releases. These K+-stimulated releases were antagonized significantly by 15 mM MgCl2, suggesting that they are calcium-dependent in nature. We have concluded, therefore, that mTA and pTA, as well as DA, are released from striatal nerve terminals in vivo. The total amounts of mTA and DA, but not pTA, released in the release experiments were greater than those found in the nonincubated tissue. It appears, therefore, that the biosynthesis of mTA and DA was stimulated during the incubation of the striatal slices.

Release of radiolabeled dopamine, p-tyramine, and m-tyramine from rat striatal slices by some aminotetralins

The effect of several 2-aminotetralins (2ATs) on the uptake and release of [14C] dopamine and [2H]m- or [3H]p-tyramine by rat striatal slices was examined. 6,7-Dihydroxy-2AT (6,7OHAT) and 5,6-dihydroxy-2-methyl-AT (5,6OHMeAT) were the most potent uptake inhibitors as well as the most potent releasers of the three labeled amines. The 5-, 6-, and 7-hydroxy-2-N, N-dipropyl-ATs (5-, 6-, and 7OHdiPrAT) and 5,6-dihydroxy-2-N,N-dipropyl-AT (5,6OHdiPrAT) significantly inhibited the uptakes of the three labeled amines, but they released only the tyramines. The dipropyl substitutions of a 2AT appeared to confer a tyraminergic specificity to its release properties. To verify this supposition, 2AT was compared to 2-N,N-dipropyl-AT (diPrAT). Although 2AT released both [3H]p-tyramine and [14C]dopamine, diPrAT released only [3H]p-tyramine. None of the compounds, however, differentiated between m- and p-tyramine. It was concluded that the release of tyramines could be implicated in the actions of some of the 2ATs and that the tyramines can be transported independently from dopamine.

A comparison of the effects of methylphenidate and amphetamine on the simultaneous release of radiolabelled dopamine and p- or m-tyramine from rat striatal slices

The release of [14C]dopamine (DA) from slices of rat caudate nucleus was studied simultaneously with the release of either [3H]para-tyramine (pTA) or [3H]meta-tyramine (mTA). Amphetamine (10(-5) M) caused a large concurrent release of [14C]DA and [3H]pTA; similar results were obtained when [14C]DA and [3H]mTA release were studied. The release of all three amines by amphetamine was quantitatively similar. In contrast, methylphenidate caused a release of [3H]pTA similar to that seen with amphetamine, but only a very small simultaneous release of [14C]DA. [3H]mTA was also strongly released by methylphenidate concurrent with a minimal release of [14C]DA. The inclusion of reserpine in the incubation medium had no detectable effect on the release of any of the three amines by amphetamine. Methylphenidate-induced release of tritiated mTA and pTA was also unaffected by reserpine. However, the release of [14C]DA by methylphenidate was potentiated in the presence of reserpine. The uptake of radiolabelled pTA, mTA and DA was inhibited by both amphetamine and methylphenidate, although amphetamine was a stronger inhibitor of the uptake of all three amines. It is suggested that release of endogenous tyramines may be involved in mediating some actions of psychomotor stimulant drugs.