Effects of sexual interactions on the in vivo rate of monoamine synthesis in forebrain regions of the male rat.

The effects of heterosexual interactions on the in vivo rate of regional brain monoamine synthesis were examined in the male rat. To this end, the animals were administered an inhibitor of cerebral aromatic L-amino acid decarboxylase, NSD-1015 (100 mg.kg-1 i.p.), and regional brain DOPA and 5-HTP accumulation, over a 15-35 min period of sexual interaction, was compared with the DOPA or 5-HTP accumulation in time-matched home cage controls. Using the DOPA and 5-HTP accumulation as an estimate for the rate of tyrosine and tryptophan hydroxylase activity, respectively, the present results demonstrate: (1) an increased demand on catecholamine synthesis in the neocortex, the amygdala and in the septal area; and (2) an increased dopamine and serotonin synthesis in the ventral striatum (excluding the olfactory tubercle), and in the dorsal striatum.     

Intracranial self-stimulation increases differentially in vivo hydroxylation of tyrosine but similarly in vivo hydroxylation of tryptophan in rat medial prefrontal cortex, nucleus accumbens and striatum

We have examined using microdialysis the effect of intracranial self-stimulation (ICSS) on the in vivo hydroxylation rate of tyrosine and tryptophan in the medial prefrontal cortex (mPFC), nucleus accumbens (NAC) and striatum (STR). A decarboxylase inhibitor NSD-1015 was included in the perfusate, which enabled the simultaneous measurement of 3,4-dihydroxyphenylalanine (DOPA) and 5-hydroxytryptophan (5-HTP) as an index of the in vivo hydroxylation level of tyrosine and tryptophan. When rats were exposed to 1 h of ICSS at the medial forebrain bundle (MFB), their extracellular levels of DOPA significantly increased in the mPFC, NAC and STR, but with a different magnitude and time course. The same stimulation produced a delayed increase in extracellular 5-HTP, compared to DOPA, in these brain regions. The profile of 5-HTP response demonstrated no apparent difference among the regions. These findings indicate that ICSS of the MFB can increase differentially the in vivo hydroxylation of tyrosine but similarly the in vivo hydroxylation of tryptophan in the mPFC, NAC and STR.     

Effects of 5-HT and 8-OH-DPAT on forebrain monoamine synthesis after local application into the median and dorsal raphe nuclei of the rat

Summary 5-HT (10 and 40 g) and 8-OH-DPAT (1 and 5 g) were locally applied into the dorsal or median raphe nuclei in awake, unrestrained, rats. All animals were also treated with the 5-HTP and DOPA decarboxylase inhibitor NSD-1015, 100mg kg^–1 SC, 30 min before decapitation. 5-HT or 8-OH-DPAT were administered 5 min before NSD-1015. The regional brain in vivo rate of tyrosine and tryptophan hydroxylase activity was estimated by measuring the accumulation of DOPA and 5-HTP. The following brain regions were sampled: neocortex, hippocampus, dorso-lateral neostriatum, ventro-medial neostriatum, nucleus accumbens, olfactory tubercle, globus pallidus, septum and the amygdala.Compared to normal controls, there were small and inconsistent effects on forebrain 5-HTP accumulation by saline injections into the dorsal or the median raphe (an increase in 3 out of 36 experiments), whereas strong effects by the injection procedure were noted on forebrain DOPA accumulation (an increase in 15 out of 36 experiments).Injections of 5-HT (same effect by 10 or 40 g) into the dorsal raphe, produced a decrease in 5-HTP accumulation in all forebrain areas except for the hippocampus and the septum, whereas no effects were seen in any area after median raphe injections. In contrast, 8-OH-DPAT preferentially produced a decrease in forebrain 5-HTP accumulation after median raphe injections and less, but statistically significant effects by dorsal raphe injections. The 8-OH-DPAT injection into the median raphe primarily affected limbic forebrain areas (hippocampus, nucleus accumbens, ventro-medial neostriatum, amygdala and the septum).This dissociation of the effects of 5-HT and 8-OH-DPAT on forebrain 5-HT synthesis after local application into the dorsal or the median raphe strongly supports the contention of heterogeniety in the brain 5-HT receptor population in terms of receptor subtypes and/or receptor regulation.     

Suggestive evidence for inhibitory effects of galanin on mesolimbic dopaminergic neurotransmission

The objective was to examine effects of galaninrat on forebrain monoamine synthesis and on spontaneous locomotor activity in the rat. The rate of monoamine synthesis was estimated by measuring the accumulation of l-DOPA and 5-HTP, following inhibition of cerebral aromatic l-amino acid decarboxylase by means of NSD-1015 (100 mg kg-1 i.p.), after i.c.v. or intracerebral administration of galanin in adult male Wistar rats. Spontaneous locomotor activity was observed in an automated open-field arena ( approximately 0.5 m2). The i.c.v. administration of galanin (0.5-5.0 nmol bilaterally) produced a dose-dependent, statistically significant, increase in DOPA accumulation throughout the neostriatum, and in the olfactory bulb, indicating an increase in the rate of DA synthesis. No increase was observed in brain areas where noradrenaline is the predominant catecholamine, such as the neocortex or the ventral hippocampus. In addition, there was a tendency for an increase in 5-HTP accumulation in the dorso-lateral neostriatum and in the accumbens. The same i.c.v. administration of galanin produced a dose-dependent, and statistically significant, decrease in spontaneous locomotor activity. The effect on forebrain DA synthesis could also be produced by local bilateral application of galanin (2x1 nmol) into the ventral tegmental area, but not the nucleus accumbens (2x2 nmol). There were no effects on forebrain DOPA or 5-HTP accumulation by the local application of galanin into the locus coeruleus, or into the dorsal raphe nucleus. It is concluded that the neuropeptide galanin modulates forebrain dopaminergic neurotransmission. The effect appears to be mediated at the somato-dendritic level of the meso-neostriatal pathway, and could perhaps be utilized to normalize perturbations ascribed to dysfunction in this neuronal pathway, such as schizophrenia.     

Tryptophan hydroxylase and 5-HTP-decarboxylase activity in cockroach brain and the effects of p-chlorophenylalanine and 3-hydroxybenzylhydrazine (NSD-1015)

The activities of two enzymes of the serotonin biosynthetic pathway, namely tryptophan hydroxylase (TPH) and 5-hydroxytryptophan-decarboxylase (5-HTP-D), were determined in the cockroach, Periplaneta americana, brain using radiometric techniques, significant levels of both enzymes were found. p-Chlorophenylalanine (PCPA) inhibited brain TPH activity strongly both in vitro and in vivo. In vitro, 10 mM PCPA reduced the TPH activity by 76% and in vivo either 15 micrograms/g or 30 micrograms/g of PCPA inhibited brain TPH activity by 80-85%. The activity of 5-HTP-D enzyme was inhibited significantly in vivo by 3-hydroxybenzylhydrazine, commonly known as NSD-1015. A recovery in the activity of 5-HTP-D enzyme was observed 12 h after 50 micrograms/g of NSD-1015 but 100 micrograms/g caused an inhibition lasting for the 24 h period studied.     

Biosynthesis of dopamine and serotonin in the rat brain after repeated cocaine injections: A microdissection mapping study

The purpose of the present study was to examine the effects of chronic cocaine on dopamine (DA) and serotonin (5-HT) synthesis in several rat brain regions implicated in drug reinforcement. Male rats were treated twice daily with cocaine (15 mg/kg, ip) or saline for 1 week. After 42 hr of abstinence, rats were challenged with either cocaine (15 mg/kg, ip) or saline, followed by the aromatic L-amino acid decarboxylase inhibitor 3-hydroxybenzylhydrazine (NSD-1015; 100 mg/kg, ip). Animals were decapitated 30 min after NSD-1015 and discrete brain regions were microdissected from 300 μm frozen sections. Postmortem tissue levels of 3, 4-dihydroxyphenylalanine (DOPA) and 5-hyroxytryptophan (5-HTP) were quantified by HPLC with electrochemical detection and used to estimate biosynthesis of DA and 5-HT, respectively. In chronic saline-treated rats, cocaine dramatically suppressed DA and 5-HT synthesis in all forebrain regions examined, including: medial prefrontal cortex, nucleus accumbens, caudate nucleus, olfactory tubercle, and basolateral amygdala. The degree of inhibition ranged from 35-65% and was more pronounced in 5-HT neurons compared to DA neurons in the same tissue sample. In general, chronic cocaine did not significantly alter basal levels of DOPA or 5-HTP; a notable exception was lateral hypothalamus, where chronic cocaine reduced basal DA synthesis to 75% of control. After repeated cocaine injections, the synthesisiinhibiting effect of a challenge injection of cocaine was attenuated in many brain areas. These data suggest that whereas acute cocaine decreases DA and 5-HT synthesis in forebrain, chronic cocaine is not neurotoxic to DA and 5-HT neurons. In addition, the mechanism(s) mediating cocaine-induced suppression of monoamine synthesis may become desensitized by chronic exposure to the drug. Published 1993 Wiley-Liss, Inc.     

Dihydalazine inhibits tyrosine hydroxylasein vivo in the rat

Summary The effect of dihydralazine on monoamine metabolism in the rat was investigated. Dihydralazine, 5 mg/kg i.v., reduced noradrenaline (NA) in the heart. After pretreatment with phenoxybenzamine an NA depletion was evident also in the brain. Dihydralazine did not affect the utilization of NA or dopamine in the brain as judged by the disappearance rates of these amines following synthesis inhibition by-methyl-p-tyrosine. However, dihydralazine reduced the synthesis of monoamines as evidenced by a decreased accumulation of the monoamine precursor dihydroxyphenylalanine (and 5-hydroxytryptamine) subsequent to treatment with NSD 1015.It is concluded that dihydralazine inhibits central tyrosine hydroxylase (and tryptophan hydroxylase) in the rat.     

Differential effect of immobilization stress on in vivo synthesis rate of monoamines in medial prefrontal cortex and nucleus accumbens of conscious rats.

We have used microdialysis to measure the in vivo hydroxylation level of tyrosine and tryptophan in the medial prefrontal cortex and nucleus accumbens of conscious rats that were subjected to immobilization. The brain was perfused with an inhibitor of aromatic L-amino acid decarboxylase, 3-hydroxybenzylhydrazine, and the amount of 3,4-dihydroxyphenylalanine (DOPA) and 5-hydroxytryptophan (5-HTP) accumulating in the dialysate was measured as an index of the in vivo hydroxylation rate of tyrosine and tryptophan. One hour of immobilization caused a significant increase in extracellular DOPAin the medial prefrontal cortex but not nucleus accumbens. The same manipulation produced a significant and more prolonged elevation in extracellular 5-HTP in the nucleus accumbens as well as medial prefrontal cortex. The observed profile of stress-induced 5-HTP response was comparable in two brain regions. The results suggest that in vivo catecholamine synthesis is heterogenous, whereas in vivo serotonin synthesis is homogenous, with respect to responsiveness to stress in the medial prefrontal cortex and nucleus accumbens.     

Effects of neonatal bilateral eye enucleation on postnatal development of the monoamines in posterior thalamus of the rat

Summary Levels of dopamine (DA), noradrenaline (NA) and 5-hydroxytryptamine (serotonin, 5-HT) and their metabolites, and the activities of tyrosine hydroxylase (TH), tryptophan hydroxylase (TPH) and monoamine oxidase A and B (MAO-A and MAO-B) have been determined in the rat posterior thalamus after enucleation during postnatal development. DA and 5-HT turnover rate have been measured as 3,4-dihydroxyphenylalanine (DOPA) and 5-hydroxytryptophan (5-HTP) accumulation rates after central decarboxylase inhibition by 3-hydroxybenzylhydrazine (NSD-1015). The major changes were an increase in noradrenergic and serotoninergic metabolism in enucleated animals compared with control animals. A decrease of the MAO-A to MAO-B ratio during postnatal development was found.Abbreviations DA dopamine - DOPAC 3,4-dihydroxyphenylacetic acid - HVA homovanillic acid - DOPA 3,4-dihydroxyphenylalanine - NA noradrenaline - MHPG 3-methoxy-4-hydroxyphenylgly-col - NMN normetanephrine - TRP tryptophan - 5-HTP 5-hydroxytryptophan - 5-HT serotonin - 5-HIAA 5-hydroxy-3-indolacetic acid - TH tyrosine hydroxylase - TPH tryptophan hydroxylase - MAO-A monoamine oxidase-A - MAO-B monoamine oxidase-B Recipients of fellowships from I.I.E. del FISS Reincorporación de doctores y tecnólogos del M.E.C., and Perfeccionamiento de doctores y tecnólogos del MEC, Spain     

The central aromatic amino acid DOPA decarboxylase inhibitor, NSD-1015, does not inhibit L-DOPA-induced circling in unilateral 6-OHDA-lesioned-rats

The centrally acting aromatic amino acid dopa decarboxylase (AADC) inhibitor, 3-hydroxybenzyl hydrazine (NSD-1015), is widely used to study the neurotransmitter-like actions of L-DOPA. However, the effects of NSD-1015 on L-DOPA-induced motor activity are unclear as both increases and decreases have been reported. We now investigate the effects of NSD-1015 on L-DOPA-induced contralateral circling behaviour in 6-OHDA-lesioned rats and on striatal levels of L-DOPA, 3-O-methyl-DOPA (3-OMD), dopamine, dihydroxyphenylacetic acid (DOPAC) and homovanillic acid (HVA) using microdialysis techniques. NSD-1015 (50-200 mg/kg i.p.) inhibited AADC activity both in the liver and striatum of normal rats. Administration of NSD-1015 (50-200 mg/kg i.p.), delayed the onset of circling produced by administration of L-DOPA (25 mg/kg i.p.) and carbidopa (12.5 mg/kg i. p.), suggesting blockade of central AADC activity. However, the duration of the L-DOPA-induced circling was prolonged and overall no inhibition of circling behaviour occurred. L-DOPA (25 mg/kg i.p.) plus carbidopa (12.5 mg/kg i.p.) increased extracellular levels of L-DOPA, 3-OMD, dopamine, DOPAC and HVA in the 6-OHDA-lesioned striatum. Pretreatment of rats with the central AADC inhibitor, NSD-1015 (100 mg/kg i.p.), potentiated the increase in dialysate levels of L-DOPA and 3-OMD. However, it did not reduce striatal dopamine levels in the 6-OHDA-lesioned hemisphere, which were elevated following L-DOPA administration. The increases in DOPAC and HVA levels were abolished by NSD-1015 pretreatment. These results suggest that, while NSD-1015 blocks central AADC activity, it also acts as a monoamine oxidase inhibitor so maintaining striatal dopamine concentration by reducing dopamine metabolism. NSD-1015, therefore, may not be an appropriate tool for the study of brain AADC activity and for assessing the neuromodulatory role of L-DOPA.     

Tryptophan hydroxylase and 5-HTP-decar☐ylase activity in cockroach brain and the effects of p-chlorophenylalanine and 3-hydroxybenzylhydrazine (NSD-1015)

The activities of two enzymes of the serotonin biosynthetic pathway, namely tryptophan hydroxylase (TPH) and 5-hydroxytryptophan-decar☐ylase (5-HTP-D), were determined in the cockroach, Periplaneta americana, brain using radiometric techniques, significant levels of both enzymes were found, p-Chlorophenylalanine (PCPA) inhibited brain TPH activity strongly both in vitro and in vivo. In vitro, 10 mM PCPA reduced the TPH activity by 76% and in vivo either 15 μg/g or 30 μ/g of PCPA inhibited brain TPH activity by 80–85%. The activity of 5-HTP-D enzyme was inhibited significantly in vivo by 3-hydroxybenzylhydrazine, commonly known as NSD-1015. A recovery in the activity of 5-HTP-D enzyme was observed 12 h after 50 μg/g of NSD-1015 but 100 μ/g caused an inhibition lasting for the 24 h period studied.     

Evidence that corticotropin-releasing factor within the extended amygdala mediates the activation of tryptophan hydroxylase produced by sound stress in the rat

Non-endocrine corticotropin-releasing factor (CRF) is believed to be involved in mediating stress behaviors in rats. The present study investigated the role of CRF in mediating the activation of tryptophan hydroxylase, the rate-limiting enzyme in serotonin synthesis, produced in response to sound stress. Bilateral injections of 0.5–3.0 μg of CRF directed towards the central nucleus of the amygdala increased tryptophan hydroxylase activity measured ex vivo when compared to vehicle-injected controls. This increase in enzyme activity, like that due to sound stress, was reversed in vitro by alkaline phosphatase. Intra-amygdala CRF (0.5 μg) also enhanced the in vivo accumulation of 5-hydroxytryptophan (5-HTP) following the administration ofm-hydroxylbenzylamine (NSD-1015, 200 mg/kg). The activation of tryptophan hydroxylase, produced by intra-amygdala CRF, was blocked by the CRF receptor antagonist α-helical CRF_9–41 (10 μg). Additionally, the 5-HT_1A agonist, gepirone, given either systemically (10 mg/kg) or intracerebrally into the region of the dorsal raphe (14 μg), blocked the tryptophan hydroxylase response to CRF. CRF did not increase tissue levels of 5-hydroxyindole acetic acid (5-HIAA) or the ratio of 5-HIAA to serotonin (5-HT) within the striatum of the same animals in which tryptophan hydroxylase activity was quantified, an effect produced by sound stress. Thus, while intra-amygdala CRF failed to mimic the sound stress response in its entirety, these data suggest that CRF is involved in mediating the activation of tryptophan hydroxylase produced by sound stress within the midbrain serotonin neurons.     

Dopamine synthesis in inbred mouse strains which differ in numbers of dopamine neurons

BALB/cJ and CBA/J mice have been shown to have different numbers of dopamine (DA) neurons in the central nervous system, with BALB/cJ mice having 20–50% more DA neurons in each dopaminergic cell group which is reflected in a difference in tyrosine hydroxylase activity in these cell groups. The present study compared the levels of DA and the rate of DA synthesis between these two inbred mouse strains. Three measures were used to reflect the rate of DA synthesis: the levels of DA metabolites (DOPAC and HVA) in the striatum, the rate of disappearance of DA following inhibition of tyrosine hydroxylase withα-methyl-P-tyrosine, and the rate of accumulation of DOPA following inhibition of aromatic amino acid decar☐ylase with NSD-1015. Striatal DA levels were slightly higher in CBA/J mice than BALB/cJ mice. The rate of DA synthesis in the striatum, as estimated from the accumulation of DOPA following NSD-1015 injection or from the decline of DA levels followingα-methyl-p-tyrosineinjection, was from 30–50% greater in the BALB/cJ mice compared to the CBA/J mice. In striatum, DOPAC levels were higher, HVA levels lower, and DOPAC plus HVA levels equal in CBA/J mice compared to BALB/cJ mice. The results show that BALB/cJ mice, with more DA neurons than CBA mice, also synthesize more DA. In addition, the data suggest that DA levels do not necessarily reflect numbers of DA neurons, and that catecholamine metabolite levels are not a good measure for comparing catecholamine synthesis between inbred animal strains.     

Simultaneous determination of in vivo hydroxylation of tyrosine and tryptophan in rat striatum by microdialysis-HPLC: relationship between dopamine and serotonin biosynthesis

Summary Using a microdialysis technique, the rat striatum was perfused with NSD-1015, an inhibitor of aromatic L-amino acid decarboxylase, and the amount of L-3,4-dihydroxyphenylalanine (L-DOPA) and 5-hydroxytrytophan (5-HTP) accumulating in dialysate was measured as an index of in vivo activities of tyrosine hydroxylase and tryptophan hydroxylase. NSD-1015 increased the concentration of L-DOPA much higher than that of 5-HTP in a dose-related manner (1–100 mol/L). In order to examine the relationship between dopaminergic and serotonergic neurons in the striatum, either 5-HTP or L-DOPA was injected intraperitoneally to rats pretreated with benserazide, an inhibitor of peripheral decarboxylase. 5-HTP administration increased 5-HTP, but decreased L-DOPA in a dose-dependent manner. Conversely, 5-HTP concentration decreased in an association with the increased content of L-DOPA following L-DOPA administration. The decrease of 5-HTP caused by L-DOPA administration was not as remarkable as that of L-DOPA by 5-HTP injection. These results suggest that L-DOPA and 5-HTP, the precursor amino acids for catecholamines and indoleamines, could affect mutually each other neuronal activity through the inhibition of their rate-limiting enzymes.Abbreviations AADC aromatic L-aminoacid decarboxylase - ANOVA analysis of variance - DA dopamine - DOPAC 3,4-dihydroxyphenylacetic acid - ECD electrochemicaldetection - 5-HIAA 5-hydroxyindoleacetic acid - HPLC high performance liquid chromatography - 5-HT 5-hydroxytryptamine (serotonin) - 5-HTP 5-hydroxytryptophan - HVA homovanillic acid - Km Michaelis constant - L-DOPA L-3,4-dihydroxyphenylalanine - (6 R)BH₄ (6R)-L-erythro-tetrahydrobiopterin - TH tyrosine hydroxylase - TRH tryptophan hydroxylase - TRP tryptophan - TYR tyrosine     

Electrical stimulation of the nucleus raphe magnus in the rat. Effects on 5-HT metabolism in the spinal cord

The direct electrical stimulation (with biphasic pulses of 1 msec, 10 pulses/sec, 200 microA, for 30 min) of the nucleus raphe magnus in chloral hydrate anaesthesized rats produced a significant acceleration (+50%) of 5-HT synthesis in the spinal cord as revealed by the increased rate of 5-HTP accumulation occurring at this level after the blockade of central 5-HTP decarboxylase with benserazid. In contrast, no change was detected in 5-HT metabolism in the forebrain of stimulated rats. The acceleration of 5-HT synthesis was likely not due to an increased availability of tryptophan for the rate-limiting enzyme, tryptophan hydroxylase, since the concentration of this amino acid was changed neither in the spinal cord, nor in the forebrain of stimulated rats. The measurement of tryptophan hydroxylase activity in soluble extracts from the spinal cord of control and stimulated rats revealed that the acceleration in 5-HT synthesis produced by the electrical stimulation of the nucleus raphe magnus was not associated with a persisting activation of this enzyme. Although one cannot completely exclude that a short-lasting activation of tryptophan hydroxylase, no longer detectable in soluble extracts, has occurred in the spinal cord of stimulated rats, the present findings rather suggest that the rate of 5-HT synthesis can be controlled by factors other than only the concentration of tryptophan and the intrinsic activity of tryptophan hydroxylase in serotoninergic neurons. The demonstration of an acceleration of 5-HT synthesis in bulbospinal serotoninergic neurons under stimulating conditions close to those producing analgesia in rats further supports the role of these neuronal systems in the physiological mechanisms of pain control.     

Stress-induced increase in extracellular dopamine in striatum: role of glutamatergic action via N-methyl-D-aspartate receptors in substantia nigra

There is considerable support for an influence of excitatory amino acids released from corticofugal neurons on dopaminergic activity in the basal ganglia. However, the relative importance of cortico-striatal and cortico-mesencephalic projections remains unclear, particularly with respect to the nigro-neostriatal pathway. We have therefore examined the influence of endogenous excitatory amino acids in substantia nigra on stress-induced dopaminergic activity in neostriatum. Microdialysis probes were implanted unilaterally into substantia nigra and ipsilateral neostriatum, and dopamine release in neostriatum was monitored by measuring changes in extracellular dopamine. In separate animals, neostriatal dopamine synthesis was assessed by measuring extracellular DOPA in the presence of 3-hydroxylbenzylhydrazine (NSD-1015; 100 microM), an inhibitor of aromatic amino acid decarboxylase. Thirty minutes of intermittent foot shock increased both dopamine release (+41%) and synthesis (+37%) in neostriatum. Infusion of 2-amino-5-phosphonovalerate (APV; 100 microM), an inhibitor of N-methyl-D-aspartate (NMDA) receptors, into substantia nigra greatly attenuated the stress-induced increase in neostriatal dopamine release, while having no effect on the apparent increase in stress-induced dopamine synthesis. These data suggest that excitatory amino acids such as glutamate act on NMDA receptors in substantia nigra to increase striatal dopamine release produced by exposure to stress, but that the increase in dopamine synthesis is mediated through a separate mechanism.     

Commonly used L-amino acid decarboxylase inhibitors block monoamine oxidase activity in the rat

Summary. The effects of the peripheral aromatic amino acid decarboxylase (AADC) inhibitors, carbidopa and benserazide, and the central AADC inhibitor, 3-hydroxybenzylhydrazine (NSD-1015) on peripheral and brain monoamine oxidase (MAO) A and B activity were investigated in the rat. In vitro, carbidopa, benserazide and NSD-1015 all potently inhibited hepatic MAO A and B activity (IC₅₀ 10–50 μM). In ex vivo studies following systemic drug administration, NSD-1015 (100 mg/kg ip) produced 88% and 96% inhibition of hepatic and striatal MAO A and B activity respectively. Carbidopa (12.5 mg/kg i.p.) and benserazide (50 mg/kg i.p.) had no effect on striatal MAO A activity or hepatic MAO B activity. However, they inhibited striatal MAO B activity by 45 ± 10% and 36 ± 10% respectively. In conclusion, carbidopa and benserazide may not only protect L-DOPA from peripheral decarboxylation, but also increase striatal dopamine content through MAO inhibition. NSD-1015 should not be used to investigate the neuromodulatory role of L-DOPA as it potently inhibits rat striatal MAO. Received March 8, 2002; accepted July 15, 2002 Published online November 22, 2002 Authors' address: Prof. P. Jenner, Neurodegenerative Disease Research Centre, Guy's, King's and St Thomas' School of Biomedical Sciences, King's College, London SE1 1UL, United Kingdom, e-mail: div.pharm@kcl.ac.uk     

Fos expression following self-stimulation of the medial prefrontal cortex

Self-stimulation of the medial prefrontal cortex and medial forebrain bundle appears to be mediated by different directly activated fibers. However, reward signals from the medial prefrontal cortex do summate with signals from the medial forebrain bundle, suggesting some overlap in the underlying neural circuitry. We have previously used Fos immunohistochemistry to visualize neurons activated by rewarding stimulation of the medial forebrain bundle. In this study, we assessed Fos immunolabeling after self-stimulation of the medial prefrontal cortex. Among the structures showing a greater density of labeled neurons in the stimulated hemisphere were the prelimbic and cingulate cortex, nucleus accumbens, lateral preoptic area, substantia innominata, lateral hypothalamus, anterior ventral tegmental area, and pontine nuclei. Surprisingly, little or no labeling was seen in the mediodorsal thalamic nucleus or the locus coeruleus. Double immunohistochemistry for tyrosine hydroxylase and Fos showed that within the ventral tegmental area, a substantial proportion of dopaminergic neurons did not express Fos. Despite previous suggestions to the contrary, comparison of the present findings with those of our previous Fos studies reveals a number of structures activated by rewarding stimulation of both the medial prefrontal cortex and the medial forebrain bundle. Some subset of activated cells in the common regions showing Fos-like immunoreactivity may contribute to the rewarding effect produced by stimulating either site.     

Stress-induced increase in extracellular dopamine in striatum: role of glutamatergic action via N-methyl- -aspartate receptors in substantia nigra

There is considerable support for an influence of excitatory amino acids released from corticofugal neurons on dopaminergic activity in the basal ganglia. However, the relative importance of cortico-striatal and cortico-mesencephalic projections remains unclear, particularly with respect to the nigro-neostriatal pathway. We have therefore examined the influence of endogenous excitatory amino acids in substantia nigra on stress-induced dopaminergic activity in neostriatum. Microdialysis probes were implanted unilaterally into substantia nigra and ipsilateral neostriatum, and dopamine release in neostriatum was monitored by measuring changes in extracellular dopamine. In separate animals, neostriatal dopamine synthesis was assessed by measuring extracellular DOPA in the presence of 3-hydroxylbenzylhydrazine (NSD-1015; 100 μM), an inhibitor of aromatic amino acid decarboxylase. Thirty minutes of intermittent foot shock increased both dopamine release (+41%) and synthesis (+37%) in neostriatum. Infusion of 2-amino-5-phosphonovalerate (APV; 100 μM), an inhibitor of N-methyl- -aspartate (NMDA) receptors, into substantia nigra greatly attenuated the stress-induced increase in neostriatal dopamine release, while having no effect on the apparent increase in stress-induced dopamine synthesis. These data suggest that excitatory amino acids such as glutamate act on NMDA receptors in substantia nigra to increase striatal dopamine release produced by exposure to stress, but that the increase in dopamine synthesis is mediated through a separate mechanism.     

The significance of increase in striatal D(2) receptors in epileptic EL mice

The present study systematically and quantitatively analyzed the immunohistochemical distribution of various substances involved in synthesis, binding, and transport of dopamine in the forebrain of epileptic mice (EL mouse strain) using a brain mapping analyzer. A reduction in serum calcium levels decreases calcium/calmodulin-dependent-dopamine synthesis in the brain and subsequently increases susceptibility to epileptic convulsions and induces abnormal behavior in EL mice. The immunohistochemical levels of D(2) receptors in the medial area of the neostriatum were significantly higher in EL mice than in ddY mice (mother strain of EL mice), while there were no differences in the levels of tyrosine hydroxylase, calcium/calmodulin-dependent protein kinase II, calmodulin, D(1) receptors, and dopamine transporters. Together with our previous findings, the results suggest that the decrease in serum calcium levels and subsequent decrease in brain dopamine synthesis comprise the primary physiologic disorder in EL mice, and convulsions or increased D(2) receptors are secondarily-induced phenomena to improve or compensate for the principal disorder.