Dihydroxyphenylalanine and dopamine are released from portal vein together with noradrenaline and dihydroxyphenylglycol during nerve stimulation

L-DOPA-induced Neurogenesis in the Hippocampus is Mediated through GPR143, a Distinct Mechanism of Dopamine

Neurogenesis occurs in the hippocampus through life and is implicated in various physiological brain functions such as memory encoding and mood regulation. L-3,4-dihydroxyphenylalanine (L-DOPA) has long been believed to be an inert precursor of dopamine. Here, we show that L-DOPA and its receptor, GPR143, the gene product of ocular albinism 1, regulate neurogenesis in the dentate gyrus in a dopamine-independent manner. L-DOPA at concentrations far lower than that of dopamine promoted proliferation of neural stem and progenitor cells in wild-type mice under the inhibition of its conversion to dopamine; this effect was abolished in GPR143-gene-deficient (Gpr143  -/y) mice. Hippocampal neurogenesis decreased during development and adulthood, and exacerbated depression-like behavior was observed in adult Gpr143  -/y mice. Replenishment of GPR143 in the dentate gyrus attenuated the impaired neurogenesis and depression-like behavior. Our findings suggest that L-DOPA through GPR143 modulates hippocampal neurogenesis, thereby playing a role in mood regulation in the hippocampus.

L-DOPA sensitizes vasomotor tone by modulating the vascular alpha1-adrenergic receptor

Blood pressure is regulated by extrinsic factors including noradrenaline, the sympathetic neurotransmitter that controls cardiovascular functions through adrenergic receptors. However, the fine-tuning system of noradrenaline signaling is relatively unknown. We here show that l-3,4-dihydroxyphenylalanine (L-DOPA), a precursor of catecholamines, sensitizes the vascular adrenergic receptor alpha1 (ADRA1) through activation of L-DOPA receptor GPR143. In WT mice, intravenous infusion of the ADRA1 agonist phenylephrine induced a transient elevation of blood pressure. This response was attenuated in Gpr143 gene-deficient (Gpr143-/y) mice. Specific knockout of Gpr143 in vascular smooth muscle cells (VSMCs) also showed a similar phenotype, indicating that L-DOPA directly modulates ADRA1 signaling in the VSMCs. L-DOPA at nanomolar concentrations alone produced no effect on the VSMCs, but it enhanced phenylephrine-induced vasoconstriction and intracellular Ca2+ responses. Phenylephrine also augmented the phosphorylation of extracellular signal-regulated kinases in cultured VSMCs from WT but not Gpr143-/y mice. In WT mice, blood pressure increased during the transition from light-rest to dark-active phases. This elevation was not observed in Gpr143-/y mice. Taken together, our findings provide evidence for L-DOPA/GPR143 signaling that exerts precursor control of sympathetic neurotransmission through sensitizing vascular ADRA1.

Vascular effects of singlet oxygen (1O2) generated by photo-excitation on adrenergic neurotransmission in isolated rabbit mesenteric vein

It has been suggested that reactive oxygen species (ROS) may be involved in the regulation of vascular tone. However, the nature of ROS effects on vascular sensitivity remains to be elucidated. The present study was designed to investigate the effects of ROS, especially 1O2, on neurotransmission at the sympathetic neurovascular junction. Basal noradrenaline (NA) release, release of NA induced by electrical stimulation (ES), and resting NA release at the sympathetic nerve terminals were determined using a superfusion technique. The amount of NA was determined by HPLC; isometric tension changes evoked by ES were also recorded simultaneously. 1O2 was generated from Rose Bengal by photo-activation. The generation of 1O2 in the superfusate was monitored by electron spin resonance (ESR) using the spin trap 2,2,6,6-tetramethyl-4-piperidinol throughout the experimental time course. The ESR results confirmed that 1O2 was generated by photo-activation of Rose Bengal via the formation of 2,2,6,6-tetramethyl-4-hydroxyl-piperidinyloxy. Exposure of helical strips of rabbit mesenteric vein to 1O2 induced a significant increase in tension and NA release during the basal period, but had no effect on ES-induced release. L-histidine, an 1O2 scavenger, significantly inhibited the observed effects on vascular tension and NA release in response to 1O2. These results suggest that 1O2 may induce NA-mediated vasoconstriction at the postjunctional site, and may be associated with Ca(2+)-independent NA release from the prejunctional site of adrenergic neurotransmission.

L-3,4-Dihydroxyphenylalanine as a neurotransmitter candidate in the central nervous system

Historically, 3,4-dihydroxyphenylalanine (DOPA) has been believed to be an inert amino acid that alleviates the symptoms of Parkinson's disease by its conversion to dopamine via the enzyme aromatic L-amino acid decarboxylase. In contrast to this generally accepted idea, we propose that DOPA itself is a neurotransmitter and/or neuromodulator, in addition to being a precursor of dopamine. Several criteria, such as synthesis, metabolism, active transport, existence, physiological release, competitive antagonism, and physiological or pharmacological responses, must be satisfied before a compound is accepted as a neurotransmitter. Recent evidence suggests that DOPA fulfills these criteria in its involvement mainly in baroreflex neurotransmission in the lower brainstem and in delayed neuronal death by transient ischemia in the striatum and the hippocampal CA1 region of rats.

Clonidine-induced nitric oxide-dependent vasorelaxation mediated by endothelial alpha(2)-adrenoceptor activation

1. To assess the involvement of endothelial alpha(2)-adrenoceptors in the clonidine-induced vasodilatation, the mesenteric artery of Sprague Dawley rats was cannulated and perfused with Tyrode solution (2 ml min(-1)). We measured perfusion pressure, nitric oxide (NO) in the perfusate using chemiluminescence, and tissue cyclic GMP by RIA. 2. In phenylephrine-precontracted mesenteries, clonidine elicited concentration-dependent vasodilatations associated to a rise in luminal NO. One hundred nM rauwolscine or 100 microM L(omega)-nitro-L-arginine antagonized the clonidine-induced vasodilatation. Guanabenz, guanfacine, and oxymetazoline mimicked the clonidine-induced vasorelaxation. 3. In non-contracted mesenteries, 100 nM clonidine elicited a maximal rise of NO (123+/-13 pmol); associated to a peak in tissue cyclic GMP. Endothelium removal, L(omega)-nitro-L-arginine, or rauwolscine ablated the rise in NO. One hundred nM aminoclonidine, guanfacine, guanabenz, UK14,304 and oxymetazoline mimicked the clonidine-induced surge of NO. Ten microM ODQ obliterated the clonidine-induced vasorelaxation and the associated tissue cyclic GMP accumulation; 10 - 100 nM sildenafil increased tissue cyclic GMP accumulation without altering the clonidine-induced NO release. 4. alpha(2)-Adrenergic blockers antagonized the clonidine-induced rise in NO. Consistent with a preferential alpha(2D)-adrenoceptor activation, the K(B)s for yohimbine, rauwolscine, phentolamine, WB-4101, and prazosin were: 6.8, 24, 19, 165, and 1489 nM, respectively. 5. Rat pretreatment with 100 mg kg(-1) 6-hydroxydopamine reduced 95% tissue noradrenaline and 60% neuropeptide Y. In these preparations, 100 nM clonidine elicited a rise of 91.9+/-15.5 pmol NO. Perfusion with 1 microM guanethidine or 1 microM guanethidine plus 1 microM atropine did not modify the NO surge evoked by 100 nM clonidine. 6. Clonidine and congeners activate endothelial alpha(2D)-adrenoceptors coupled to the L-arginine pathway, suggesting that the antihypertensive action of clonidine involves an endothelial vasorelaxation mediated by NO release, in addition to presynaptic mechanisms.

Differential sensitivity to hydroxyl radicals of pre- and postjunctional neurovascular transmission in the isolated canine mesenteric vein

In some pathophysiological conditions, the first target of reactive oxygen intermediates is the vascular system. Superoxide anions, when generated in the vascular circulation, may then escape into the extracellular space via an anion channel and, following dismutation to hydrogen peroxide (H(2)O(2)), form hydroxyl radicals (HO(*)). In an attempt to understand the role of HO(*) in the regulation of transmission at the sympathetic neurovascular junction, the effect of HO(*) at nerve terminals was examined by measuring the amount of noradrenaline (NA) released from isolated, spirally cut, superfused canine mesenteric vein during basal and electrical stimulation (ES; 5Hz, 2ms, 9V); tension development evoked by ES was also recorded simultaneously. HO(*) was generated from Fenton's reagent (1. 5x10(-4)M H(2)O(2) plus 10(-4)M FeSO(4)); generation of HO(*) from H(2)O(2)/FeSO(4) in the superfusate was monitored by electron spin resonance spectroscopy using the spin-trap 5, 5-dimethyl-1-pyrroline-N-oxide throughout the experimental time course. Exposure to HO(*) of the helical strips produced an irreversible decrease in tension development evoked by ES with no effect on NA release, suggesting that the observed effect is elicited postjunctionally. The susceptibility of the processes of NA-mediated contraction to HO(*) may differ greatly from that of the NA release mechanism at the prejunctional site. Exposure of the strip preparation to HO(*) leads to a substantial stimulation of basal release of NA without affecting ES-evoked NA release, possibly due to enhanced non-exocytotic Ca(2+)-independent release elicited by HO(*). A direct demonstration of this concept was obtained by showing a significant increase in the basal response of NA release in Ca(2+)-free solution. The major conclusion of the present study is that HO(*) can damage NA-mediated contraction of the vascular preparations at the postjunctional site, and may selectively induce a non-exocytotic release of NA from the prejunctional site of sympathetic neurotransmission.

Inhibition by midazolam of the adrenergic function in the isolated canine mesenteric vein

BACKGROUND

Midazolam has been reported to cause hypotension or to depress sympathetic activity following intravenous injection. However, little information is available concerning the mechanism of these effects. The aim of the present study was to determine the effects of midazolam on release of noradrenaline (NA) at nerve terminals and on receptors in the venous smooth muscle.

METHODS

The effect of midazolam at nerve terminals was examined by measuring the amount of NA release from superfused canine mesenteric vein helical strips during electrical stimulation (ES; 5 Hz, 2 ms, 9 V). The NA was quantified by high-performance liquid chromatography with electrochemical detection; tension development evoked by ES was also recorded simultaneously. In a separate series of experiments, ring preparations from the isolated vein were mounted in Krebs-Ringer solution for isometric tension recording to assess the effect of midazolam on alpha-adrenoceptors.

RESULTS

Application of tetrodotoxin (10(-6) M) or replacement of superfusate with Ca(2+)-free solution decreased both the release of NA and the tension development evoked by ES. Yohimbine (5 x 10(-8) M) increased the ES-evoked release of NA, whereas it decreased tension development in the vein strips. Midazolam (10(-4) M) did not affect either the basal release of NA or the basal tension, but inhibited both the NA release (P < 0.01) and the tension development (P < 0.01) during ES; midazolam at 10(-5) M inhibited the tension development (P < 0.05) but had no effect on NA release. In the ring preparations, midazolam (10(-5) and 10(-4) M) attenuated responses to NA (a mixed alpha 1- and alpha 2-adrenoceptor agonist, 10(-8) to 10(-3) M), phenylephrine (the alpha 1-adrenoceptor agonist, 10(-8) to 10(-3) M) and 5-bromo-6-[2-imidazolin-2yl-amino]-quinoxaline (UK14304; the alpha 2-adrenoceptor agonist, 10(-7) to 10(-3) M) in a dose-dependent manner.

CONCLUSION

The data obtained in the present study suggest that midazolam at 10(-4) M may reduce venous tone by inhibiting the release of NA from sympathetic nerve endings and both alpha 1- and alpha 2-adrenoceptor mediated smooth muscle contractions. It is also postulated that a stage of the post-receptor transduction mechanism linked to the venous smooth muscle contraction may be more sensitive to midazolam than the NA release mechanism at nerve terminals since midazolam at the low concentration tested inhibited ES-evoked tension development with no effect on the release of NA.

Nitric oxide reduces basal efflux of catecholamines from perfused dog adrenal glands

Norepinephrine, epinephrine, dopamine, and both the free and extended forms of [met]enkephalin spontaneously efflux from adrenal glands under basal conditions. The present study was done to determine whether nitric oxide has a regulatory role in these effluxes. Isolated adrenal glands (n = 63) from mongrel dogs were perfused retrogradely with Krebs-Ringer solution. In some experiments NG-monomethyl-L-arginine (3 x 10(-4) M), an inhibitor of nitric oxide synthesis, was added to the perfusate. In other experiments one of the nitric oxide donors, 3-morpholinosydnonimine (10(-7) M or 10(-5) M) or sodium nitroprusside (10(-6) M or 10(-4) M) was added. Norepinephrine, epinephrine, dopamine and their metabolites 3,4-dihydroxyphenylglycol and 3,4-dihydroxyphenylacetic acid in perfusates were quantitated by high performance liquid chromatography with electrochemical detection and in some experiments the [met]enkephalins were determined by radioimmunoassay. In the presence of NG-monomethyl-L-arginine, the basal effluxes of norepinephrine, epinephrine, and dopamine were significantly increased from control, but the effluxes of the free and extended forms of the [met]enkephalins were not changed. The effects of NG-monomethyl-L-arginine on catecholamine efflux were reversed in the presence of L-arginine (10(-3) M). Sodium nitroprusside (10(-6) M) inhibited effluxes of norepinephrine and epinephrine and 3-morpholinosydnonimine had no effect on these effluxes. Dopamine efflux appeared to be under different controls from those of norepinephrine and epinephrine since dopamine efflux was unaffected by sodium nitroprusside and was decreased over time by 3-morpholinosydnonimine (10(-7) M). It is concluded that endogenously produced nitric oxide inhibits the basal efflux of norepinephrine, epinephrine, and dopamine from isolated dog adrenal glands; this inhibition appears to be near maximal for norepinephrine and epinephrine but not for dopamine.

Neuropeptide Y release and contractile properties: differences between canine veins and arteries

During intense sympathetic activation, as occurs during hemorrhage, veins constrict to a greater degree than do arteries. This study determined if differences in the amounts or actions of the sympathetic cotransmitter neuropeptide Y released from perivascular nerves could contribute to these differences. Strips of canine mesenteric and popliteal arteries and of saphenous and portal veins were superfused, and the releases of noradrenaline and neuropeptide Y evoked by transmural stimulation were assessed. Both compounds were released in greater amounts in the veins than in the arteries. In other experiments rings of each vessel were mounted in organ chambers for isometric-tension recording. Neuropeptide Y (up to 10(-4) M) did not contract any vessel; however, at 3 x 10(-7) M it shifted the frequency-response and concentration-response curves to noradrenaline in the arteries only. In the veins neuropeptide Y had no postsynaptic effect on strong contractions. These results suggest that neuropeptide Y functions locally to affect vasoconstriction of the arteries studied, and may have a different role in the veins. Further, processes involving neuropeptide Y do not appear to account for the differences in responsiveness of these arteries as compared to the veins during intense sympathetic stimulation.

Neurobiology of L-DOPAergic systems

L-DOPA is proposed to be a neurotransmitter and/or neuromodulator in CNS. It is released probably from neurons, which may contain L-DOPA as an end-product, and/or from some compartment other than catecholamine-containing vesicles. The L-DOPA itself produces presynaptic and postsynaptic responses. All are stereoselective and most are antagonized by competitive antagonist. In striatum, L-DOPA is neuromodulator, mother of catecholamines, not only a precursor for dopamine but also a potentiator of children for presynaptic beta-adrenoceptors to facilitate dopamine release and postsynaptic D2 receptors, and ACh release inhibitor. All may cooperate for Parkinson's disease. Meanwhile, supersensitization of increase in L-glutamate release to nanomolar levodopa was seen in Parkinson's model rats, which may relate to dyskinesia or "on-off" during chronic therapy. In lower brainstem, L-DOPA tonically activates postsynaptic depressor sites of NTS and CVLM and pressor sites of RVLM. L-DOPA is probably a neurotransmitter of primary baroreceptor afferents terminating in NTS. GABA, the inhibitory neuromodulator for baroreflex in NTS, tonically functions to inhibit, via GABAA receptors, L-DOPA release and depressor responses to levodopa. Levodopa inversely releases GABA. L-DOPAergic monosynaptic relay from NTS to CVLM and from PHN to RVLM is suggested. Tonic L-DOPAergic baroreceptor-aortic nerve-NTS-CVLM relay seems to carry baroreflex information. Disturbance of neuronal activity to release L-DOPA in NTS, loss of the activity in CVLM, enhancement of the activity with decreased decarboxylation and increase in sensitivity to levodopa in RVLM may be involved in maintenance of hypertension in SHR. This is a story of "L-DOPAergic receptors" with extremely high affinity and low density.

Norepinephrine release during vasoconstriction induced by cross-linked hemoglobin

The pressor effect of hemoglobin-based blood substitutes is due partly to their capacity to scavenge nitric oxide (NO), a potent vasodilator. NO also appears to modulate the release of norepinephrine (NE) from sympathetic nerve endings in some blood vessels. Thus studies were designed to determine if contraction occurring in response to alpha alpha-cross-linked hemoglobin (XL-Hb) is due in part to increased exit of NE from vascular nerve endings. Helical strips of canine femoral artery were superfused in vitro with Krebs-Ringer solution and, for each strip, the overflow of NE into the superfusate as well as contractile responses were measured concurrently during basal conditions, during nerve stimulation and during tyramine-evoked release of NE. XL-Hb (10 microM) contracted unstimulated strips without affecting NE overflow. NE overflow also was unchanged by NG-monomethyl-L-arginine (L-NMMA; 300 microM), an inhibitor of NO synthesis; by sodium nitroprusside (SNP; 1 microM) an NO donor; by a combination of XL-Hb and L-NMMA; or of XL-Hb and SNP. These treatments contracted the strips to the same degree as did XL-Hb alone, except for SNP, which induced relaxation. Transmural stimulation of the strips at 2 and 10 Hz induced NE overflow and contraction, neither of which was affected by any treatment except SNP which significantly (P < 0.05) increased NE overflow while inhibiting contraction. In other experiments, XL-Hb augmented contractions induced by tyramine (10 microM) although the resulting NE release was unaffected. These results suggest that, in the femoral artery, contractions induced by XL-Hb are not due to increased efflux of NE from vascular nerve endings but are consistent with inhibition of the the actions of NO.

Sympathectomy does not modify the levels of dopa or dopamine in the rat dorsal root ganglion

In the present study we report on the levels of dihydroxyphenylalanine (DOPA), dopamine and their metabolites, 3-O-methyl-DOPA, homovanillic acid and dihydroxyphenylacetic acid, in the dorsal root ganglion (DRG) where dopaminergic sensory neurons have recently been identified. HPLC with electrochemical detection was used. Chemical sympathectomy, induced by daily injection of 30 mg/kg/sc of guanethidine over 4 days, did not modify these levels. DOPA, dopamine and their metabolites were also detected in the dorsal root in both intact and sympathectomized rats. The present results show that in the DRG, DOPA and dopamine do not derive from sympathetic nerves, and agree with previous reports suggesting that there is a peripheral dopaminergic sensory innervation. In addition the identification of 3-O-methylDOPA, the extraneuronal metabolite of DOPA is not only the precursor of dopamine in the DRG but may be released from DRG cells. The high 3-O-methyl-DOPA/DOPA ratio found in the DRG and the dorsal root suggests that DOPA could play a role as neurotransmitter or modulator in the peripheral sensory innervation, as it has been proposed for the central nervous system and the sympathetic system.

Catecholamine release and excretion in rats with immunologically induced preganglionic sympathectomy

Plasma and urinary catecholamines were quantified to assess global sympathoadrenal function in rats with preganglionic lesions caused by antibodies to acetylcholinesterase (AChE). Rats were given intravenous injections of normal mouse IgG or murine monoclonal anti-acetylcholinesterase IgG (1.5 mg). Five or 16 days afterward, basal blood samples were taken through indwelling arterial cannulate. A few hours later, the rats were immobilized for 10 min in padded restrainers, and another blood sample was drawn. HPLC determinations showed low basal levels of norepinephrine and epinephrine (< 0.2 ng/ml in all rat plasma samples). In control rats, immobilization stress increased levels of plasma catecholamines up to 35-fold. In rats tested 5 days after injection of antibody, the norepinephrine response was much smaller (15% of control), and the epinephrine response was nearly abolished (5% of control). There was some recovery at 16 days after antibody treatment, but stress-induced catecholamine release was still markedly impaired. Reduced stress-induced release was not accompanied by major changes in tissue epinephrine or norepinephrine (heart, spleen, adrenal glands, and brain), although adrenal dopamine content dropped by 60%. Urinary excretion was studied in parallel experiments to gain insight into the effects of AChE antibodies on basal sympathoadrenal activity. Epinephrine, norepinephrine, dopamine, and selected metabolites were quantified in 24-h urine samples collected at frequent intervals for 30 days after antibody injection. No statistically significant changes were detected in the urinary output of dopamine, 3-methoxytyramine, normetanephrine, or 3-methoxy-4-hydroxyphenylglycol. On the other hand, epinephrine and norepinephrine output increased sharply at the time of antibody injection and then fell significantly below control levels. Norepinephrine output returned to normal after 2 weeks, but epinephrine output remained depressed. These results are consistent with previous evidence of widespread and persistent antibody-mediated damage to the preganglionic sympathetic system.

Increase in plasma 3,4-dihydroxyphenylalanine (DOPA) appearance rate after inhibition of DOPA decarboxylase in humans

Concentrations of DOPA in plasma are relatively high as compared to norepinephrine. The significance of plasma DOPA has not been elucidated. One would expect that substantial amounts of DOPA are derived from sympathetic nerves. There appears, however, neither to be a depot of DOPA in nerves nor is there a close correlation between plasma DOPA and sympathetic activity. The aim of the present study was to obtain further information about plasma DOPA by studying DOPA kinetics in healthy humans both with and without inhibition of DOPA decarboxylase by benserazide. Plasma DOPA and other catecholamines were measured by reverse-phase HPLC with electrochemical detection and DOPA clearance and appearance rate were studied using infusion of 3H-DOPA. The plasma clearance of DOPA was 1.02 1 min-1. Approximately 20% of this value could be explained by DOPA being decarboxylated in the kidneys and excreted as dopamine. The DOPA appearance rate was 1.13 micrograms min-1 and the extremities accounted for approximately 1/5 of this value. After inhibition of DOPA decarboxylase by benserazide the DOPA appearance rate increased 7-fold, whereas the DOPA clearance only decreased slightly and insignificantly. These findings are probably explained by two factors: (1) There is normally a large production of DOPA in some tissues from which DOPA spillover into plasma only occurs to a minor extent and tracer DOPA only mixes with this compartments to a small degree; (2) These compartments are permeable to benserazide, which blocks the decarboxylation of DOPA, which then leaves the tissues and spillover to plasma.(ABSTRACT TRUNCATED AT 250 WORDS)

Inhibition of aromatic L-amino acid decarboxylase under physiological conditions: optimization of 3-hydroxybenzylhydrazine concentration to prevent concurrent inhibition of monoamine oxidase

The activity of the enzyme tyrosine hydroxylase (TH; EC 1.14.16.2) is commonly studied indirectly by quantifying the formation of the product, 3,4-dihydroxyphenylalanine (DOPA), after inhibition of aromatic L-amino acid decarboxylase (AAAD; EC 4.1.1.28), the enzyme which metabolizes DOPA. This study was done to determine if the concentration of the hydrazine derivative 3-hydroxybenzylhydrazine (NSD-1015), a drug frequently used in vitro to inhibit AAAD, could be adjusted such that it would inhibit that enzyme, but would not simultaneously inhibit a second, potentially important enzyme, monoamine oxidase (MAO; EC 1.4.3.4). MAO catalyzes the formation of 3,4-dihydroxyphenylacetic acid (DOPAC) and 3,4-dihydroxyphenylglycol (DOPEG) from dopamine (DA) and norepinephrine (NE), respectively. Five concentrations of NSD-1015 in superfusate (0.01 to 20 microM) were tested in strips of canine portal vein superfused and stimulated in vitro. DOPA, DA, NE, and DOPEG in superfusate and in the veins after superfusion were quantified by HPLC with electrochemical detection. The efficacy of NSD-1015 in inhibiting AAAD and MAO was determined by examining the levels of DOPA and DOPEG, respectively. NSD-1015, only when applied at 0.1 microM, resulted in the marked augmentation of total DOPA levels, but did not affect levels of DOPEG, which suggests that this concentration of the drug inhibits AAAD, but does not inhibit MAO. Therefore, it is concluded that, of the concentrations of NSD-1015 tested, 0.1 microM is the optimum concentration to use in this preparation for studies designed to examine TH activity by measuring DOPA after the inhibition of AAAD.

Two PC12 pheochromocytoma lines sealed in hollow fiber-based capsules tonically release L-dopa in vitro

Two PC12 cell-derived lines have been studied following encapsulation into polymer-based hollow fibers with respect to secreted catecholamines and their metabolites. Cellular encapsulation provides a chronic microperfusion environment within which basally secreted PC12 products can be readily measured. Encapsulated PC12 cells grown and held under the conditions specified in this report basally release amounts exceeding their total cellular stores of the dopamine precursor L-DOPA and the electrochemically active dopamine metabolites DOPAC and HVA during 45-min static incubations. Under these same conditions, these cells release less than 0.1% of their total cellular store of dopamine. Depolarizing incubations enhance dopamine secretion eightyfold and enhance secretion of L-DOPA, HVA, and DOPAC about twofold. The relative composition of products basally secreted differs between PC12-derived cell lines, and an inverse relationship exists between basal release of L-DOPA and total cellular store of dopamine. These results further indicate that selected PC12 cell lines are potentially a source of both dopamine and L-DOPA in therapeutic cellular replacement applications.