Tyrosine administration does not affect desipramine-induced dopamine levels as measured in vivo in prefrontal cortex

Tyrosine depletion lowers in vivo DOPA synthesis in ventral hippocampus

In vivo dopamine synthesis in the medial prefrontal cortex of the rat is sensitive to the availability of tyrosine. Whether other limbic cortical dopamine terminal regions are similarly tyrosine-dependent is not known. In this study we examined the effects of tyrosine depletion on dopamine synthesis and catecholamine levels in the ventral hippocampus. A tyrosine- and phenylalanine-free neutral amino acid mixture was used to lower brain tyrosine levels in rats undergoing in vivo microdialysis. In one group, NSD-1015 was included in perfusate to permit measurement of DOPA levels. In a second group, NSD-1015 was not included in perfusate so that catecholamine levels could be assayed. Tyrosine depletion significantly lowered DOPA levels in the NSD-1015 treated group and lowered DOPAC but not dopamine or noradrenaline levels in the group not exposed to NSD-1015. We conclude that while catecholamine synthesis in the ventral hippocampus declines when tyrosine availability is lowered, under basal conditions, compensatory mechanisms are able to maintain stable extracellular catecholamine levels.

Acute lithium administration selectively lowers tyrosine levels in serum and brain

Lithium exerts anti-dopaminergic behavioral effects. We examined whether some of these might be mediated by changes in brain levels of tyrosine (TYR), the precursor to dopamine. Lithium chloride (LiCl(2)) 3.0mEq/kg IP acutely lowered serum TYR and the ratio of serum TYR to other large neutral amino acids (LNAAs); it also selectively lowered striatum TYR levels as measured in tissue or in vivo. While LiCl(2) 3.0mEq/kg IP also augmented haloperidol (0.19mg/kg SC)-induced catalepsy, this lithium effect was not attenuated by administration of TYR 100mg/kg IP. We conclude that lithium acutely and selectively lowers brain TYR by lowering serum levels of tyrosine relative to the LNAAs that compete with it for transport across the blood-brain barrier. However, the lowering of TYR does not appear to significantly contribute to the ability of lithium to potentiate haloperidol-mediated catalepsy.

A new strategy for antidepressant prescription

From our research and literature search we propose an understanding of the mechanism of action of antidepressants treatments (ADTs) that should lead to increase efficacy and tolerance. We understand that ADTs promote synaptic plasticity and neurogenesis. This promotion is linked with stimulation of dopaminergic receptors. Previous evidence shows that all ADTs (chemical, electroconvulsive therapy, repetitive transcranial magnetic stimulation, sleep deprivation) increase at least one monoamine neurotransmitter serotonin (5-HT), noradrenaline (NA) or dopamine (DA); this article focuses on DA release or turn-over in the frontal cortex. DA increased dopaminergic activation promotes synaptic plasticity with an inverted U shape dose–response curve. Specific interaction between DA and glutamate is mediated by D1 receptor subtypes and Glutamate (NMDA) receptors with neurotrophic factors likely to play a modulatory role. With the understanding that all ADTs have a common, final, DA-ergic stimulation that promotes synaptic plasticity we can predict that (1) AD efficiency is related to the compound strength for inducing DA-ergic stimulation. (2) ADT efficiency presents a therapeutic window that coincides with the inverted U shape DA response curve. (3) ADT delay of action is related to a “synaptogenesis and neurogenesis delay of action.” (4) The minimum efficient dose can be found by starting at a low dosage and increasing up to the patient response. (5) An increased tolerance requires a concomitant prescription of a few ADTs, with different or opposite adverse effects, at a very low dose. (6) ADTs could improve all diseases with cognitive impairments and synaptic depression by increasing synaptic plasticity and neurogenesis.

Homeostatic mechanisms in dopamine synthesis and release: a mathematical model

BACKGROUND

Dopamine is a catecholamine that is used as a neurotransmitter both in the periphery and in the central nervous system. Dysfunction in various dopaminergic systems is known to be associated with various disorders, including schizophrenia, Parkinson's disease, and Tourette's syndrome. Furthermore, microdialysis studies have shown that addictive drugs increase extracellular dopamine and brain imaging has shown a correlation between euphoria and psycho-stimulant-induced increases in extracellular dopamine 1. These consequences of dopamine dysfunction indicate the importance of maintaining dopamine functionality through homeostatic mechanisms that have been attributed to the delicate balance between synthesis, storage, release, metabolism, and reuptake.

METHODS

We construct a mathematical model of dopamine synthesis, release, and reuptake and use it to study homeostasis in single dopaminergic neuron terminals. We investigate the substrate inhibition of tyrosine hydroxylase by tyrosine, the consequences of the rapid uptake of extracellular dopamine by the dopamine transporters, and the effects of the autoreceoptors on dopaminergic function. The main focus is to understand the regulation and control of synthesis and release and to explicate and interpret experimental findings.

RESULTS

We show that the substrate inhibition of tyrosine hydroxylase by tyrosine stabilizes cytosolic and vesicular dopamine against changes in tyrosine availability due to meals. We find that the autoreceptors dampen the fluctuations in extracellular dopamine caused by changes in tyrosine hydroxylase expression and changes in the rate of firing. We show that short bursts of action potentials create significant dopamine signals against the background of tonic firing. We explain the observed time courses of extracellular dopamine responses to stimulation in wild type mice and mice that have genetically altered dopamine transporter densities and the observed half-lives of extracellular dopamine under various treatment protocols.

CONCLUSION

Dopaminergic systems must respond robustly to important biological signals such as bursts, while at the same time maintaining homeostasis in the face of normal biological fluctuations in inputs, expression levels, and firing rates. This is accomplished through the cooperative effect of many different homeostatic mechanisms including special properties of tyrosine hydroxylase, the dopamine transporters, and the dopamine autoreceptors.

Gamma-butyrolactone-induced dopamine accumulation in prefrontal cortex is affected by tyrosine availability

Gamma-butyrolactone (GBL) elevates striatal and prefrontal cortex dopamine levels; only the striatal dopamine levels are elevated by increased dopamine synthesis. If increased dopamine synthesis is necessary in order for dopamine levels to be affected by tyrosine availability, then GBL-induced prefrontal cortex dopamine levels should be tyrosine insensitive. Rats received either vehicle, tyrosine (50 or 200 mg/kg i.p.) or a tyrosine-depleting mixture prior to GBL 750 mg/kg i.p.. GBL-induced dopamine levels in prefrontal cortex were lowered by tyrosine depletion. GBL-induced striatal dopamine levels were not affected. Hence, increased dopamine synthesis may not be necessary in order for tyrosine availability to affect pharmacologically elevated prefrontal cortex dopamine levels.

Tyrosine depletion lowers dopamine synthesis and desipramine-induced prefrontal cortex catecholamine levels

The relationship between limited tyrosine availability, DA (dopamine) synthesis and DA levels in the medial prefrontal cortex (MPFC) of the rat was examined by in vivo microdialysis. We administered a tyrosine- and phenylalanine-free mixture of large neutral amino acids (LNAA-) IP to lower brain tyrosine, and the norepinephrine transporter inhibitor desipramine (DMI) 10 mg/kg IP to raise MPFC DA levels without affecting DA synthesis. For examination of DOPA levels, NSD-1015 20 microM was included in perfusate. Neither NSD-1015 nor DMI affected tyrosine levels. LNAA- lowered tyrosine levels by 45%, and lowered DOPA levels as well; this was not additionally affected by concurrent DMI 10 mg/kg IP. In parallel studies DMI markedly increased extracellular levels of DA (420% baseline) and norepinephrine (NE) (864% baseline). LNAA- had no effect on baseline levels of DA or NE but robustly lowered DMI-induced DA (176% baseline) as well as NE (237% baseline) levels. Even when DMI (20 microM) was administered in perfusate, LNAA- still lowered DMI-induced DA and NE levels. We conclude that while baseline mesocortical DA synthesis is indeed dependent on tyrosine availability, the MPFC maintains normal extracellular DA and NA levels in the face of moderately lower DA synthesis. During other than baseline conditions, however, tyrosine depletion can lower ECF DA and NE levels in MPFC. These data offer a potential mechanism linking dysregulation of tyrosine transport and cognitive deficits in schizophrenia.

Increased striatal dopamine synthesis is associated with decreased tissue levels of tyrosine

Tyrosine levels do not generally affect indices of dopamine (DA) synthesis or efflux under basal conditions, but can do so when DA synthesis is increased. One possibility is that a high rate of DA synthesis depletes the normally adequate pool of endogenous tyrosine. To study this, we administered drugs known to preferentially increase striatal DA synthesis and examined DOPA levels in striatal microdialysate during perfusion with NSD-1015. In additional groups, we also measured DA, tyrosine and large neutral amino acids in striatal microdialysate, as well as in tissue from striatum and medial prefrontal cortex (MPFC). gamma-butyrolactone (GBL) (750 mg/kg i.p.) increased DOPA levels in striatal microdialysate, increased tissue DA levels in the MPFC and striatum, but lowered tissue tyrosine levels only in striatum. In striatal microdialysate, GBL markedly lowered DA levels; tyrosine levels were only marginally lower. Haloperidol (HAL) (1.0 mg/kg s.c.)+/-amfonelic acid (AFA) (5 mg/kg i.p.) increased striatal DOPA accumulation, increased striatal DA efflux, lowered striatal tissue tyrosine levels, but did not affect microdialysate tyrosine levels. There were no consistent changes in levels of other large neutral amino acids. We conclude that increased tyrosine hydroxylation can significantly deplete the endogenous pool of tyrosine. Under such conditions, near normal extracellular tyrosine levels are maintained despite lower tissue levels. The data are consistent with a net transfer of tyrosine from non-DAergic cells to DA terminals in support of DA synthesis.

In rats chronically treated with clozapine, tyrosine depletion attenuates the clozapine-induced in vivo increase in prefrontal cortex dopamine and norepinephrine levels

We previously reported that depletion of brain tyrosine attenuated the acute clozapine (CLZ)-induced increase in medial prefrontal cortex (MPFC) dopamine (DA) levels. This effect was now examined after chronic CLZ treatment. Male rats received CLZ (10 mg kg(-1) day(-1)) in drinking water for 21 days. On day 18, a cannula was stereotaxically implanted over the MPFC. A microdialysis probe was inserted on day 20. On day 21 after a stable baseline was reached, rats received an acute injection of vehicle (VEH) or a tyrosine- and phenylalanine-free mixture of neutral amino acid [NAA(-)] (total 1 g kg(-1), i.p., two injections, 1 h apart) followed by CLZ (10 mg kg(-1), i.p.) or VEH. Basal tyrosine or norepinephrine (NE) levels were not different between the groups, but basal DA was higher in the group treated chronically with CLZ (p<0.05). Acute CLZ (10 mg kg(-1), i.p.) increased MPFC DA and NE levels to 370% and 510% of baseline, respectively, and similarly in rats chronically pretreated with CLZ or VEH. NAA(-) did not affect basal MPFC DA or NE levels but significantly attenuated acute CLZ-induced DA (220% of baseline) and NE (330% of baseline) levels (p<0.01) in rats pretreated chronically with CLZ or with VEH. These data demonstrate that even after chronic CLZ administration, the acute CLZ-induced increases in MPFC DA and NE levels depend on the availability of brain tyrosine. Judicious manipulation of brain tyrosine levels may provide a useful probe as well as a mechanism for enhancing psychotropic drug actions.