L-Dopa-like Regulatory Actions of l-threo-3,4-Dihydroxyphenylserine on the Release of Endogenous Noradrenaline Via Presynaptic Receptors in Rat Hypothalamic Slices

Effects of l-threo-3,4-dihydroxyphenylserine (l-threo-DOPS) on the spontaneous release and the stimulus(2 Hz)-evoked release of endogenous noradrenaline were studied in rat hypothalamic slices with functioning l-aromatic amino acid decarboxylase (AADC) and with AADC inhibition. In non-inhibited slices, spontaneous release was not modified by l-threo-DOPS at 1 Pm-100Nm, tended to increase at 1–10 μm and increased at 100 μm. Noradrenaline tissue content slightly increased at 100 μm. Stimulated release was concentration-dependently facilitated at 1–1000 Pm and tended to decrease gradually from a maximum at 10 Nm-10 μm. Under AADC inhibition, spontaneous release concentration-dependently increased at 10–100 μm by 60% of the increase seen in slices without AADC inhibition. Increase in noradrenaline tissue content was abolished. l-threo-DOPS produced a triphasic pattern on stimulated release; concentration-dependent facilitation at 1–1000 Pm similar to that seen in slices with functional AADC, no facilitation at 10–1000 Nm, and a concentration-dependent increment at 10–100 μm. The facilitation at 1 Nm was stereoselective and was antagonized by (–)-propranolol 10 Nm, and no facilitation at 100 Nm was restored to the maximum by yohimbine 10 Nm, DG-5128 10 Nm or S-sulpiride 1 Nm. Furthermore, l-threo-DOPS (1–1000 Pm)-induced facilitation was competitively antagonized by l-dopa methyl ester, a competitive antagonist for l-dopa, with a pA2 value of 13·6, whereas it was noncompetitively antagonized by (–)-propranolol.

L-dopaergic components in the caudal ventrolateral medulla in baroreflex neurotransmission

L-3,4-Dihydroxyphenylalanine (L-DOPA) is probably a transmitter of the primary baroreceptor afferents terminating in the nucleus tractus solitarii; L-DOPA functions tonically to activate depressor sites of the caudal ventrolateral medulla, which receives input from the nucleus tractus solitarii [Misu Y. et al. (1996) Prog. Neurobiol. 49, 415-454]. We have attempted to clarify whether or not L-DOPAergic components within the caudal ventrolateral medulla are involved in baroreflex neurotransmission in anesthetized rats. Electrolytic lesions of the right nucleus tractus solitarii (1 mA d.c. for 10 s, 10 days before measurement) selectively decreased by 45% the tissue content of L-DOPA in the dissected ipsilateral caudal ventrolateral medulla. Electrolytic lesions did not decrease dopamine, norepinephrine and epinephrine levels. During microdialysis of the right caudal ventrolateral medulla, extracellular levels of L-DOPA, norepinephrine, epinephrine and 3,4-dihydroxyphenylacetic acid were consistently detectable using high-performance liquid chromatography with electrochemical detection. However, extracellular dopamine levels were lower than the assay limit. Baroreceptor activation by i.v. phenylephrine selectively evoked L-DOPA without increasing the levels of norepinephrine, epinephrine and 3,4-dihydroxyphenylacetic acid. This L-DOPA release was suppressed by acute lesion in the ipsilateral nucleus tractus solitarii. Intermittent stimulation of the right aortic depressor nerve (20 Hz, 3 V, 0.3 ms duration, for 30 min) repetitively and constantly caused L-DOPA release, hypotension and bradycardia, without increases in levels of norepinephrine, epinephrine and 3,4-dihydroxyphenylacetic acid. Local inhibition of L-DOPA synthesis with alpha-methyl-p-tyrosine (30 microM) infused into the ipsilateral caudal ventrolateral medulla gradually decreased basal levels of L-DOPA and 3,4-dihydroxyphenylacetic acid without decreasing norepinephrine and epinephrine. The inhibition of L-DOPA synthesis interrupted L-DOPA release and decreased by 65% depressor responses elicited by aortic nerve stimulation; however, it produced no effect on bradycardic responses. CoCl2 (119 ng), a mainly presynaptic inhibitory transmission marker, and L-DOPA methyl ester (1 microg), a competitive L-DOPA antagonist, when microinjected into depressor sites of the right caudal ventrolateral medulla, reduced by 60% depressor responses to transient ipsilateral stimulation of the aortic nerve (20 Hz, 3 V, 0.1 ms duration, for 10 s). No changes in bradycardic responses were observed. There may exist an L-DOPAergic relay from the nucleus tractus solitarii to the caudal ventrolateral medulla. L-DOPAergic components in the caudal ventrolateral medulla are involved in baroreflex neurotransmission via a baroreceptor-aortic depressor nerve-nucleus tractus solitarii-caudal ventrolateral medulla relay in the rat.

Depressor action of L-threo-dihydroxyphenylserine in the rat nucleus tractus solitarii

Microinjections of L-threo-dihydroxyphenylserine (L-threo-DOPS, 0.1-3 ng), a synthetic precursor amino acid of noradrenaline, into the medial area of the nucleus tractus solitarii produced dose-dependent depressor and bradycardic responses in anesthetized rats treated with or without i.p. 3-hydroxybenzylhydrazine, a central inhibitor of L-aromatic amino acid decarboxylase. D-threo-DOPS (3 ng) produced no effect. L-Dihydroxyphenylalanine (L-DOPA) methyl ester (1 microgram), a competitive antagonist of L-DOPA, microinjected into the nucleus tractus solitarii, blocked the depressor and bradycardic responses to L-threo-DOPS itself produces vasodepressor actions without its conversion to noradrenaline, probably via a recognition site for L-DOPA in the rat nucleus tractus solitarii.

Altered L-DOPA systems for blood pressure regulation in the lower brainstem of spontaneously hypertensive rats

Recent findings have enhanced our understanding of the roles played by the L-DOPA system in the baroreceptor reflex and in blood pressure regulation in the lower brainstem. L-DOPA is probably a neurotransmitter of primary baroreceptor afferents terminating in depressor sites of the nucleus tractus solitarii (NTS). It also seems to be a neurotransmitter in depressor sites of the caudal ventrolateral medulla (CVLM) and in pressor sites of the rostral ventrolateral medulla (RVLM) of normotensive Wistar rats. We have explored whether or not presynaptic and postsynaptic functions of the L-DOPA system in these areas are altered to maintain hypertension in adult spontaneously hypertensive rats, as compared with age-matched Wistar Kyoto rats. In this review article, we survey the roles of the L-DOPA system in the baroreceptor reflex and in blood pressure regulation in the rat lower brainstem.

Altered basal release and pressor effect of L-DOPA in the rostral ventrolateral medulla of spontaneously hypertensive rats

1. Transmitter-like L-DOPA functions as a tonic to produce postsynaptic cardiopressor responses in the rostral ventrolateral medulla (RVLM) of rats. We attempted to clarify whether a transmitter-like L-DOPA system is altered in the RVLM of spontaneously hypertensive rats (SHR) to maintain the hypertension. 2. By microdialysis of left RVLM area, the basal L-DOPA release was higher in SHR than in Wistar-Kyoto (WKY) rats. This release was partially inhibited by tetrodotoxin (TTX, 1 mu mol/L) to a similar degree in both strains. TTX-sensitive L-DOPA release was higher in SHR than in WKY. 3. L-DOPA (10-600 ng) and L-glutamate (10-300 ng) microinjected into left RVLM produced dose-dependent hypertension and tachycardia. Pressor but not tachycardiac responses to L-DOPA at lower doses were slightly greater in SHR than in WKY, whereas no difference to L-glutamate was observed in either strain. 4. In RVLM regions, there was no difference of tyrosine hydroxylase activity in SHR or WKY; however, L-aromatic amino acid decarboxylase activity was lower in SHR than in WKY. 5. Enhanced presynaptic neuronal L-DOPA release, including a decrease in decarboxylation and sensitization of postsynaptic pressor sites to L-DOPA in RVLM, may be involved in the maintenance of hypertension in SHR.

Altered basal release and depressor effect of L-DOPA in the nucleus tractus solitarii of spontaneously hypertensive rats

1. L-DOPA as a probable neurotransmitter of baroreceptor afferents functions as a tonic to mediate cardiodepressor control in the nucleus tractus solitarii (NTS). We attempted to clarify further whether a transmitter-like L-DOPA system is altered in NTS of adult spontaneously hypertensive rats (SHR). 2. By microdialysis of left NTS area, the basal L-DOPA release was lower in SHR than in Wistar-Kyoto (WKY) rats. This release was partially inhibited by tetrodotoxin (TTX, 1 mu mol/L) to a similar degree in both strains. TTX-sensitive L-DOPA release was lower in SHR than in WKY. 3. L-DOPA (10-300 ng) and L-glutamate (3-100 ng) microinjected into left NTS produced dose-dependent hypotension and bradycardia. No difference of responses to L-glutamate was seen in either strain. However, depressor but not bradycardic responses to L-DOPA at higher doses were slightly greater in SHR than in WKY. 4. In caudal dorsomedial medulla including NTS, tyrosine hydroxylase activity was increased in SHR compared to WKY, while there was no difference in either strain of L-aromatic amino acid decarboxylase activity. 5. Impaired tonic neuronal activity to release L-DOPA in NTS may be involved in the maintenance of hypertension in SHR. An increase in sensitivity of a recognition site for L-DOPA seems to occur as a compensatory mechanism for impairment of the neuronal activity.

Loss of tonic neuronal activity to release L-DOPA in the caudal ventrolateral medulla of spontaneously hypertensive rats

Experiments were designed to clarify whether a tonic L-DOPA system is altered in the caudal ventrolateral medulla (CVLM) of adult spontaneously hypertensive rats (SHR), compared to age-matched Wistar-Kyoto rats (WKY). By microdialysis in CVLM, basal L-DOPA release was constantly detectable and was lower in SHR than that in WKY. This release was reduced by tetrodotoxin perfusion (1 microM) in WKY to a basal level in SHR, whereas no modification occurred with tetrodotoxin in SHR. No difference of tyrosine hydroxylase and DOPA decarboxylase activities in the CVLM region was seen between the two strains. By microinjections into depressor sites of CVLM, L-DOPA (10-300 ng) or L-glutamate (3-300 ng) elicited dose-dependent depressor and bradycardic responses and greater depressor responses to both amino acids were seen at high doses in SHR, compared to WKY. Tonic neuronal activity to release L-DOPA is lost in the CVLM of adult SHR and this loss may contribute to maintenance of hypertension in SHR.

L-DOPA systems for blood pressure regulation in the lower brainstem

We have explored probable neurotransmitter roles of L-3,4-dihydroxyphenylalanine (L-DOPA) in baroreceptor reflex and blood pressure regulation in depressor sites of the nucleus tractus solitarii (NTS) and the caudal ventrolateral medulla (CVLM), and in pressor sites of the rostral ventrolateral medulla (RVLM) in anesthetized rats. During microdialysis of these three areas, the basal L-DOPA release is in part tetrodotoxin (TTX)-sensitive and Ca2(+)-dependent, high K+ Ca2(+)-dependently releases dL-DOPA. L-DOPA microinjected (10-300 ng) dose-dependently produces postsynaptic depressor responses in the NTS and CVLM and pressor responses in the RVLM, and a recognition site for L-DOPA functions tonically to activate depressor neurons in the NTS and CVLM and pressor neurons in the RVLM. It is highly probable that L-DOPA is a neurotransmitter of the baroreceptor afferents terminating in the NTS, which is based on further findings such as (1) antagonism by a competitive L-DOPA antagonist against depressor responses to aortic nerve stimulation, (2) TTX-sensitive L-DOPA release by aortic nerve stimulation, (3) abolition of baroreceptor-stimulated L-DOPA release by bilateral sino-aortic denervation and (4) decreases in tyrosine hydroxylase (TH)- and L-DOPA-immunoreactivities without modifications of dopamine- and DBH-immunoreactivities in the left NTS and ganglion nodosum 7 days after ipsilateral aortic nerve denervation peripheral to the ganglion. In the NTS, GABA tonically functions to inhibit via GABAA receptors L-DOPA release and depressor responses to L-DOPA, whereas L-DOPA induces GABA release. Impaired TTX-sensitive neuronal activity to release L-DOPA in the NTS and enhanced TTX-sensitive neuronal activity including a decrease in decarboxylation of L-DOPA to dopamine and an increase in sensitivity of the recognition site to L-DOPA in the RVLM are relevant to the maintenance of hypertension in spontaneously hypertensive rats. Decreases in the contents of L-DOPA in the right CVLM 10 days after electrical lesion of the ipsilateral NTS suggest a 'L-DOPAergic' and monosynaptic relay from the NTS to the CVLM. L-DOPA seems to play major roles as a neurotransmitter for baroreceptor reflex and blood pressure regulation in the lower brainstem of rats.

Altered tonic L-3,4-dihydroxyphenylalanine systems in the nucleus tractus solitarii and the rostral ventrolateral medulla of spontaneously hypertensive rats

We have proposed that L-3,4-dihydroxyphenylalanine (L-DOPA) is a neurotransmitter in the central nervous system [Y. Misu et al. (1995) Adv. Pharmac. 32, 427-459]. L-DOPA as a probable neurotransmitter for the primary baroreceptor afferents tonically functions to mediate cardiodepressor control in the nucleus tractus solitarii and also tonically functions to mediate cardiopressor control in the rostral ventrolateral medulla of rats. We further attempted to clarify whether a transmitter-like L-DOPA system is altered in these areas of adult spontaneously hypertensive rats. By microdialysis in the left nucleus tractus solitarii area, the basal L-DOPA release was lower in spontaneously hypertensive rats than that in Wistar-Kyoto rats. This release was partially reduced by tetrodotoxin (1 microM) to the same absolute levels in the two strains. Tonic neuronal L-DOPA release is impaired in this nucleus of spontaneously hypertensive rats. This impairment is not secondarily due to decrease in formation or increase in decarboxylation of L-DOPA, since tyrosine hydroxylase activity was increased in spontaneously hypertensive rats, compared to Wistar-Kyoto rats, while no difference of L-aromatic amino acid decarboxylase activity was seen in the caudal dorsomedial medulla including the nucleus. L-DOPA (10-300 ng) microinjected into the nucleus produced dose-dependent hypotension and bradycardia. A maximum depressor response of spontaneously hypertensive rats to L-DOPA at higher doses was slightly greater than that of Wistar-Kyoto rats. On the other hand, in the left rostral ventrolateral medulla, the basal L-DOPA release was higher in spontaneously hypertensive rats than that in Wistar-Kyoto rats. This release was also partially reduced by tetrodotoxin to the same absolute levels in the two strains. Tonic neuronal L-DOPA release is enhanced in spontaneously hypertensive rats. This enhancement seems to include partially a decrease in decarboxylation of L-DOPA, since L-aromatic amino acid decarboxylase activity was decreased in spontaneously hypertensive rats compared to Wistar-Kyoto rats, while no difference in tyrosine hydroxylase activity was seen. L-DOPA (10-600 ng) produced dose-dependent hypertension and tachycardia. Importantly, a pressor response of spontaneously hypertensive rats to L-DOPA at lower doses was slightly greater than that of Wistar-Kyoto rats. L-DOPA seems to play a transmitter-like role in blood pressure regulation at levels of the nucleus tractus solitarii and rostral ventrolateral medulla in rats.(ABSTRACT TRUNCATED AT 400 WORDS)

Supersensitization of neurochemical responses by L-DOPA and dopamine receptor agonists in the striatum of experimental Parkinson's disease model rats

We studied the mechanisms of dopamine receptor agonist- and L-DOPA-mediated supersensitization in experimental Parkinson's disease model rats, by measuring in vivo acetylcholine (ACh) release, GTPase activities, and mRNA expression in the striatum of 6-hydroxydopamine-treated rats. D1 agonist (SKF38393) and D2/D3 agonists (bromocriptine and quinpirole) showed more potent stimulation or inhibitions on ACh release in the model rat than in the control. However, quinpirole-evoked stimulation of GTPase activity was enhance in the model rats, compared to the control, while there was no significant enhancement of the bromocriptine-evoked stimulation. On the other hand, L-DOPA at 0.3-10 pM showed a biphasic action including significant inhibition on the GTPase activity in the lesioned striatal membranes, but not in the control. In the RNAase protection assay, neither D1, D2, Gi1 alpha, GoA alpha nor Gs alpha mRNA expression in the model was significantly different from the control. These findings suggest that there is supersensitization of D1 and D2/D3 receptors in the experimental Parkinson's disease model, while the upregulation of their receptors or GTP-binding proteins (G-proteins) to be coupled to their receptors is unlikely involved in major parts of such mechanisms. In addition, the present report provides the first evidence that L-DOPA mediates neurochemical responses in the plasma membranes, possibly through its receptor.

Baroreceptor-aortic nerve-mediated release of endogenous L-3,4-dihydroxyphenylalanine and its tonic depressor function in the nucleus tractus solitarii of rats

We have proposed that L-3,4-dihydroxyphenylalanine (L-DOPA) is a neurotransmitter and/or neuromodulator in the central nervous system [Misu Y. and Goshima Y. (1993) Trends pharmac. Sci. 14, 119-123]. This study aimed to explore whether or not endogenous L-DOPA, as a neurotransmitter candidate of the primary baroreceptor afferents, tonically functions to activate depressor neurons in the nucleus tractus solitarii of anesthetized rats. By parallel microdialysis in bilateral nucleus tractus solitarii areas, the basal L-DOPA release was in part inhibited by tetrodotoxin perfusion (1 microM) or Ca2+ deprivation, and was markedly reduced by alpha-methyl-p-tyrosine (200 mg/kg, i.p.), a tyrosine hydroxylase inhibitor. Forty to 100 mM K+ concentration-dependently released L-DOPA. Fifty millimoles K+ repetitively and constantly released L-DOPA. This release was Ca(2+)-dependent. Stimulation of the left aortic nerve (100 Hz, 8 V) repetitively and constantly released L-DOPA and this release was tetrodotoxin-sensitive. Phenylephrine i.v. infused produced L-DOPA release and reflex bradycardia, temporally associated with a rise and subsequent recovery of blood pressure. This release and bradycardia were abolished by denervation of the bilateral carotid sinus and aortic nerves. In addition, L-DOPA methyl ester, a competitive L-DOPA antagonist, when microinjected into depressor sites of the left nucleus tractus solitarii, antagonized depressor responses to mild stimulation (20 Hz, 3 V) of the ipsilateral aortic nerve. This antagonist alone, microinjected bilaterally, elicited a dose-dependent hypertension, which was abolished by alpha-methyl-p-tyrosine. Furthermore, by immunocytochemical analysis seven days after denervation of the left aortic nerve, tyrosine hydroxylase- and L-DOPA-, but not dopamine- and dopamine-beta-hydroxylase-immunoreactivities decreased in the ipsilateral nucleus tractus solitarii and dorsal motor vagus nucleus complex area. In the left ganglion nodosum, denervation decreased staining and number of L-DOPA-immunoreactive cells and staining of tyrosine hydroxylase-immunoreactive cells, but no modification of dopamine-immunoreactive cells was seen. Taken together with previous findings that L-DOPA itself is stereoselectively responsible for cardiovascular control in this nucleus, it is probable that L-DOPA is a neurotransmitter of the primary baroreceptor afferents terminating directly in depressor neurons and/or indirectly in some neurons within a microcircuit, including depressor neurons of the nucleus tractus solitarii. Endogenously released L-DOPA itself tonically functions to activate depressor neurons for regulation of blood pressure in the rat nucleus tractus solitarii.

Transcutaneous ultrasound of the cervical esophagus in patients with esophageal carcinoma

Transcutaneous ultrasound of the cervical esophagus was performed in 46 patients with esophageal carcinoma and in 35 controls. The former had 24 upper segmental lesions and 22 lower segmental lesions. The level of the sternoclavicular joint was used to divide the esophagus into the upper segmental (USE) and lower segmental esophagus (LSE). The anterior esophageal wall thickness and luminal dimensions were measured before and immediately after phonation. The mean wall thickness in the controls was 1.8 mm before phonation and 2.1 mm after phonation, with a significant difference (t test, P < 0.05). The mean wall thickness in the USE carcinoma group was 4.3 mm and 4.4 mm before and after phonation respectively. There was a significant difference between the controls and USE carcinoma groups (t test, P < 0.05). The cross sectional area, which was calculated as the product of anterio-posterior and lateral diameters, averaged 28 mm2 before phonation in the controls and increased to 44 mm2 after phonation (t test, P < 0.05). The mean cross sectional area in the USE carcinoma group was significantly smaller before phonation than that in the controls (t test, P < 0.05) and showed little change after phonation. No significant difference was found between LSE carcinoma and control groups. Transcutaneous ultrasound appears to be a promising non-invasive method of investigation for cervical esophageal carcinoma.

Non-effective dose of exogenously applied L-dopa itself stereoselectively potentiates postsynaptic D2 receptor-mediated locomotor activities of conscious rats

We attempted to clarify whether or not under inhibition of central dopa decarboxylase non-effective i.p. doses of L-dopa potentiate D2 receptor-mediated locomotor activities without conversion to dopamine in normal and i.v.t. 6-hydroxydopamine-treated rats. In normal rats, only the highest dose of quinpirole, a selective D2 agonist, at ranges used (0.01-1 mg/kg, s.c.) slightly increased the total counts of locomotor activities for 140 min after injection. A simultaneously injected non-effective dose of L-DOPA (30 mg/kg) potentiated locomotor activities by quinpirole (0.1 and 1 mg/kg). L-dopa potentiated quinpirole (1 mg/kg)-induced locomotor activities 90 to 140 min after the injection with marked increase in basal release of L-DOPA without increase in dopamine release simultaneously monitored during striatal microdialysis, compared to quinpirole alone. D-dopa (30 mg/kg) produced no potentiation. In 6-hydroxydopamine-treated rats, a non-effective dose of L-dopa (10 mg/kg) also potentiated quinpirole (0.3 mg/kg)-induced locomotor activities. L-dopa acting on a recognition site for itself stereoselectively potentiates postsynaptic D2 receptor-mediated locomotor activities of rats.

Endogenously released L-dopa itself tonically functions to potentiate postsynaptic D2 receptor-mediated locomotor activities of conscious rats

A non-effective dose of exogenously applied L-dopa itself stereoselectively potentiates postsynaptic D2 receptor-mediated locomotor activities of rats. We further attempted to clarify whether or not endogenously released L-dopa tonically functions to potentiate activities of these receptors, simultaneously monitoring locomotor activities and basal release of L-dopa and dopamine during striatal microdialysis in conscious rats. Quinpirole (1 mg/kg, s.c.) alone, a selective D2 agonist, increased locomotor activities and decreased basal L-dopa and dopamine release 20-140 min after injection. Pretreatment with alpha-methyl-p-tyrosine (3 mg/kg, i.p.), a tyrosine hydroxylase inhibitor, decreased locomotor activities and further decreased L-dopa release without modification of dopamine release, compared to quinpirole alone, whereas 3-hydroxybenzylhydrazine (100 mg/kg, i.p.), a central dopa decarboxylase inhibitor, further increased locomotor activities and markedly increased L-dopa release without modification of dopamine release. Endogenously released L-dopa itself functions tonically to potentiate activities of postsynaptic D2 receptors relevant to locomotor movement of rats.

Evidence for a dopamine receptor subtype sensitive to combination of D1 with D2 antagonist in measurement of G-protein activity using rat striatal membranes

In rat striatal membranes, various kinds of dopamine receptor agonists stimulated low-Km GTPase activity in a concentration-dependent manner. This stimulation by bromocriptine, pergolide and apomorphine was partially inhibited by sulpiride (SUL), a D2-selective antagonist, markedly inhibited by combination of SUL with SCH 23390 (SCH), a D1-selective antagonist, and not modified by SCH alone. The stimulation by BAM-1110 was resistant to SUL or SCH alone but abolished by combination of SUL with SCH. These findings suggest the presence of another subtype of a dopamine receptor in a functional in vitro bioassay system in rat striata.

Evidence for L-dopa relevant to modulation of sympathetic activity in the rostral ventrolateral medulla of rats

By microdialysis in the rostral ventrolateral medulla (RVLM) of anesthetized rats, the spontaneous L-3,4-dihydroxyphenylalanine (DOPA) release was partially Ca(2+)-dependent and tetrodotoxin-sensitive and was markedly reduced by alpha-methyl-p-tyrosine (alpha-MPT; 200 mg/kg, i.p.). K+ (50 mM) Ca(2+)-dependently evoked L-DOPA. By microinjections into unilateral RVLM, L-DOPA (30-300 ng) produced dose-dependent hypertension and tachycardia similarly in rats untreated, treated with i.p. 3-hydroxybenzylhydrazine, a central DOPA decarboxylase inhibitor, or with i.v.t. 6-hydroxydopamine. These responses were antagonized by L-DOPA methyl ester (1.5 micrograms), a competitive L-DOPA antagonist. D-DOPA, dopamine, noradrenaline or adrenaline (300 ng) produced no effect. Furthermore, L-DOPA methyl ester alone microinjected into bilateral RVLM (2 micrograms x 2) produced prolonged hypotension and bradycardia, which were abolished by alpha-MPT. These data suggest that L-DOPA is relevant to modulation of sympathetic activity in the rat RVLM.

Transmitter-like L-3,4-dihydroxyphenylalanine tonically functions to mediate vasodepressor control in the caudal ventrolateral medulla of rats

By microdialysis in the unilateral caudal ventrolateral medulla (CVLM) of anesthetized rats, the spontaneous L-3,4-dihydroxyphenylalanine (L-DOPA) release was in part tetrodotoxin-sensitive or Ca(2+)-dependent and was abolished by i.p. alpha-methyl-p-tyrosine (alpha-MPT), a tyrosine hydroxylase inhibitor. High K+ (50 mM) Ca(2+)-dependently evoked L-DOPA. By unilateral microinjections into the CVLM, L-DOPA (10-100 ng) produced dose-dependent, marked hypotension and bradycardia similarly in rats untreated, treated with i.p. 3-hydroxybenzylhydrazine, a central DOPA decarboxylase inhibitor, or with i.v.t. 6-hydroxydopamine. These responses were antagonized by L-DOPA methyl ester, a competitive L-DOPA antagonist. A depressor response to dopamine or noradrenaline (100 ng) was far smaller and slower in onset than that to L-DOPA (30 ng). D-DOPA (100 ng) produced no effect. Furthermore, L-DOPA methyl ester microinjected into bilateral CVLM produced some hypertension and tachycardia, which were markedly reduced by alpha-MPT. Transmitter-like L-DOPA tonically functions to mediate vasodepressor control in CVLM of rats.

Review on the relationship between nicotinic acetylcholine receptors and dopaminergic neurotransmission in the central nervous system--dopa is an endogenous neuroactive substance

L-3,4-Dihydroxyphenylalanine (DOPA) is believed to be an inert precursor for dopamine (DA). Contrary, transmitter-like endogenous DOPA is released from in vitro and in vivo striata: DOPA is released by neuronal activities under physiological conditions from striata of conscious rats. Furthermore, exogenous nanomolar DOPA itself produces an in vitro presynaptic response to facilitate the catecholamine release. An in vivo postsynaptic depressor response is elicited by DOPA microinjected into the nucleus tractus solitarii. These responses are antagonized by L-DOPA methyl ester, a competitive DOPA antagonist. In striata, DOPA is an endogenous potentiator for presynaptic beta-adrenoceptors to facilitate the DA release and also probably for postsynaptic D2-receptors to increase locomotor activities. Nicotine releases DA and transmitter-like DOPA in vitro and in vivo striata. Nicotine (0.1-1.0 mg/kg, sc) dose-dependently increases locomotor activities. This increase is stereoselective and mecamylamine (1.0 mg/kg, sc)-sensitive but not antagonized by L-DOPA methyl ester (200 micrograms, ivt). Then, a selective low ip dose of alpha-methyl-p-tyrosine (alpha-MPT) to inhibit the basal release of DOPA without decreasing the basal release of DA was explored in vivo striata: it was 3 mg/kg. Pretreatment with this dose did inhibit the nicotine-induced increases in locomotor activities. This result suggests that endogenously released DOPA is in part relevant to nicotine-induced behavior in rats.

Endogenously released DOPA is probably relevant to nicotine-induced increases in locomotor activities of rats

The relevance of endogenously released DOPA to the (+/-)-nicotine-induced increase in locomotor activities was explored in rats. This increase was dose (0.1-1.0 mg/kg, s.c.)-dependent, stereo-selective and mecamylamine (1.0 mg/kg, s.c.)-sensitive, but was not antagonized by L-dopa methyl ester (200 micrograms, i.v.t.), a competitive L-dopa antagonist. Then, by microdialysis, low doses of alpha-methyl-p-tyrosine (alpha-MPT, i.p.) at 1-10 mg/kg, were tested to find a dose that selectively inhibits the basal DOPA release in the striata: the release was consistently inhibited by 3 mg/kg without any tendency to inhibit the basal dopamine release. Pretreatment with this dose did inhibit the nicotine-induced increases in locomotor activities. This suggests that endogeneously released DOPA is in part relevant to the nicotine-induced behavior in rats.

Transmitter-like 3,4-dihydroxyphenylalanine is tonically released by nicotine in striata of conscious rats

Microdialysis and high performance liquid chromatography with an electrochemical detector were applied to compare the characteristics of nicotine-evoked release of endogenous 3,4-dihydroxyphenylalanine (DOPA) from striata of conscious rats and those of the release of dopamine (DA). Dialysates were collected every 20 min 3-8 h after the start of perfusion. Nicotine was perfused for 20 min through a probe. (+/-)-Nicotine (100-300 microM) constantly and repeatedly released DOPA and DA over a similar time course in a dose-dependent manner. The ratio of the DOPA and DA release evoked was approximately 1:3. The (+/-)-nicotine (200 microM)-induced DOPA release was mecamylamine (500 microM)-sensitive, tetrodotoxin (100 nM)-sensitive and Ca2+ (removal plus 12.5 mM Mg2+ addition)-dependent. The (+)-isomer produced no DOPA release. These characteristics of DOPA release were almost the same as those of DA release. Furthermore, mecamylamine alone inhibited the basal release of DOPA but not of DA. Nicotine released stereoselectively endogenous DOPA via nicotinic acetylcholine receptors from striata of freely moving rats in a manner similar to transmitter DA. These acetylcholine receptors function tonically for the release of DOPA. These findings are further support for our proposal that DOPA is an endogenous neuroactive substance.

Evidence for L-dopa systems responsible for cardiovascular control in the nucleus tractus solitarii of the rat

Microinjections of L-DOPA (10-100 ng) into the medial area of the nucleus tractus solitarii (NTS) led to dose-dependent decreases in arterial blood pressure and heart rate in rats treated with i.p. 3-hydroxybenzylhydrazine, a central inhibitor of DOPA decarboxylase, or similarly with intraventricular 6-hydroxydopamine. D-DOPA, dopamine or noradrenaline (100 ng) produced no effect. L-DOPA methyl ester (1 microgram), a competitive antagonist for L-DOPA, microinjected into NTS, blocked the depressor and bradycardic responses to L-DOPA. High K+ (40 mM) released endogenous DOPA in a Ca(2+)-dependent manner from slices of the rat dorsomedial medulla including NTS. These results support the hypothesis that there exist systems of L-DOPA itself responsible for cardiovascular regulation in NTS of rats. This regulatory action of L-DOPA seems to be postsynaptic in nature.

Transmitter-Like Basal and K+-Evoked Release of 3,4-Dihydroxyphenylalanine from the Striatum in Conscious Rats Studied by Microdialysis

Using microdialysis and HPLC, characteristics of the release of endogenous 3,4-dihydroxyphenylalanine (DOPA) from striatum in conscious rats were studied in comparison with those of 3,4-dihydroxyphenylethylamine (dopamine; DA). Purified L-aromatic amino acid decarboxylase (AADC) converted a putative peak of DOPA to DA. The retention time of DOPA differed from that of DA and major metabolites of DA and norepinephrine. The DOPA peak of dialysates comigrated with that of authentic DOPA when the pH of the HPLC buffer was modified. The ratio of the basal release of DOPA:DA was 1:2. 3-Hydroxybenzylhydrazine (NSD-1015; 100 mg/kg, i.p.), an AADC inhibitor, markedly increased the basal release of DOPA but produced no effect on DA. The basal release of DOPA was markedly decreased by alpha-methyl-p-tyrosine (200 mg/kg, i.p.), substantially tetrodotoxin (1 microM) sensitive, and Ca2+ (removal plus 12.5 mM Mg2+ addition) dependent. Fifty millimolar K+ released DOPA and this release was also Ca2+ dependent. These characteristics of the basal and evoked release of DOPA were similar to those of DA. The ratio of the evoked release of DOPA:DA was 1:3. These results indicate that DOPA is released under physiological conditions and by K(+)-induced depolarization in a manner similar to that for transmitter DA from striatum in freely moving rats.