Lack of a high affinity transport system for dopamine in the median eminence and posterior pituitary

Acute Suppression of LH Secretion by Prolactin in Female Mice Is Mediated by Kisspeptin Neurons in the Arcuate Nucleus

Hyperprolactinemia causes infertility, but the specific mechanism is unknown. It is clear that elevated prolactin levels suppress pulsatile release of GnRH from the hypothalamus, with a consequent reduction in pulsatile LH secretion from the pituitary. Only a few GnRH neurons express prolactin receptors (Prlrs), however, and thus prolactin must act indirectly in the underlying neural circuitry. Here, we have tested the hypothesis that prolactin-induced inhibition of LH secretion is mediated by kisspeptin neurons, which provide major excitatory inputs to GnRH neurons. To evaluate pulsatile LH secretion, we collected serial blood samples from diestrous mice and measured LH levels by ultrasensitive ELISA. Acute prolactin administration decreased LH pulses in wild-type mice. Kisspeptin neurons in the arcuate nucleus and in the rostral periventricular area of the third ventricle (RP3V) acutely responded to prolactin, but prolactin-induced signaling in kisspeptin neurons was up to fourfold higher in the arcuate nucleus when compared with the RP3V. Consistent with this, conditional knockout of Prlr specifically in arcuate nucleus kisspeptin neurons prevented prolactin-induced suppression of LH secretion. Our data establish that during hyperprolactinemia, suppression of pulsatile LH secretion is mediated by Prlr on arcuate kisspeptin neurons.

Dopamine Release Dynamics in the Tuberoinfundibular Dopamine System

The relationship between neuronal impulse activity and neurotransmitter release remains elusive. This issue is especially poorly understood in the neuroendocrine system, with its particular demands on periodically voluminous release of neurohormones at the interface of axon terminals and vasculature. A shortage of techniques with sufficient temporal resolution has hindered real-time monitoring of the secretion of the peptides that dominate among the neurohormones. The lactotropic axis provides an important exception in neurochemical identity, however, as pituitary prolactin secretion is primarily under monoaminergic control, via tuberoinfundibular dopamine (TIDA) neurons projecting to the median eminence (ME). Here, we combined electrical or optogenetic stimulation and fast-scan cyclic voltammetry to address dopamine release dynamics in the male mouse TIDA system. Imposing different discharge frequencies during brief (3 s) stimulation of TIDA terminals in the ME revealed that dopamine output is maximal at 10 Hz, which was found to parallel the TIDA neuron action potential frequency distribution during phasic discharge. Over more sustained stimulation periods (150 s), maximal output occurred at 5 Hz, similar to the average action potential firing frequency of tonically active TIDA neurons. Application of the dopamine transporter blocker, methylphenidate, significantly increased dopamine levels in the ME, supporting a functional role of the transporter at the neurons' terminals. Lastly, TIDA neuron stimulation at the cell body yielded perisomatic release of dopamine, which may contribute to an ultrafast negative feedback mechanism to constrain TIDA electrical activity. Together, these data shed light on how spiking patterns in the neuroendocrine system translate to vesicular release toward the pituitary and identify how dopamine dynamics are controlled in the TIDA system at different cellular compartments.SIGNIFICANCE STATEMENT A central question in neuroscience is the complex relationship between neuronal discharge activity and transmitter release. By combining optogenetic stimulation and voltammetry, we address this issue in dopamine neurons of the neuroendocrine system, which faces particular spatiotemporal demands on exocytotic release; large amounts of neurohormone need to be secreted into the portal capillaries with precise timing to adapt to physiological requirements. Our data show that release is maximal around the neurons' default firing frequency. We further provide support for functional dopamine transport at the neurovascular terminals, shedding light on a long-standing controversy about the existence of neuroendocrine transmitter reuptake. Finally, we show that dopamine release occurs also at the somatodendritic level, providing a substrate for an ultrashort autoregulatory feedback loop.

Incomplete concordance of dopamine transporter Cre (DATIREScre)-mediated recombination and tyrosine hydroxylase immunoreactivity in the mouse forebrain

Co-localization of the expression of the dopamine transporter (DAT) with the catecholamine synthesising enzyme tyrosine hydroxylase (TH) has been investigated using transgenic mice expressing Cre recombinase (Cre) dependent green fluorescent protein (GFP) under the control of the DAT promoter (DAT^IREScre/GFP). Brain sections from adult female mice were stained for Cre-induced GFP and TH using immunohistochemistry, revealing a high degree of co-expression in the midbrain dopaminergic neurons (A8-10) with the exception of the periaqueductal and dorsal raphe nuclei where dual-labelling was notably lower. In contrast, most of the rostral groups of TH-expressing neurons in the forebrain (A11, A13 - A15) showed little or no co-localization with Cre-induced GFP. Interestingly, a subpopulation of about 30% of the TH-immunoreactive neurons in the arcuate nucleus (A12) also express GFP staining. This observation supports the proposal that this hypothalamic cluster of dopaminergic neurons is neurochemically, and thus potentially functionally, heterogeneous. This study extends earlier literature focusing primarily on DAT expression in midbrain structures to demonstrate a heterogeneity of DAT and TH co-localization in forebrain neurons, particularly those in the hypothalamus. It also highlights the importance of carefully selecting and validating transgenic mouse lines when studying dopaminergic neurons.

Dopamine Autoreceptor Regulation of a Hypothalamic Dopaminergic Network

How autoreceptors contribute to maintaining a stable output of rhythmically active neuronal circuits is poorly understood. Here, we examine this issue in a dopamine population, spontaneously oscillating hypothalamic rat (TIDA) neurons, that underlie neuroendocrine control of reproduction and neuroleptic side effects. Activation of dopamine receptors of the type 2 family (D2Rs) at the cell-body level slowed TIDA oscillations through two mechanisms. First, they prolonged the depolarizing phase through a combination of presynaptic increases in inhibition and postsynaptic hyperpolarization. Second, they extended the discharge phase through presynaptic attenuation of calcium currents and decreased synaptic inhibition. Dopamine reuptake blockade similarly reconfigured the oscillation, indicating that ambient somatodendritic transmitter concentration determines electrical behavior. In the absence of D2R feedback, however, discharge was abolished by depolarization block. These results indicate the existence of an ultra-short feedback loop whereby neuroendocrine dopamine neurons tune network behavior to echoes of their own activity, reflected in ambient somatodendritic dopamine, and also suggest a mechanism for antipsychotic side effects.

TIDAL WAVES: Network mechanisms in the neuroendocrine control of prolactin release

Neuroendocrine tuberoinfundibular dopamine (TIDA) neurons tonically inhibit pituitary release of the hormone, prolactin. Through the powerful actions of prolactin in promoting lactation and maternal behaviour while suppressing sexual drive and fertility, TIDA neurons play a key role in reproduction. We summarize insights from recent in vitro studies into the membrane properties and network behaviour of TIDA neurons including the observations that TIDA neurons exhibit a robust oscillation that is synchronized between cells and depends on intact gap junction communication. Comparisons are made with phasic firing patterns in other neuronal populations. Modulators involved in the control of lactation - including serotonin, thyrotropin-releasing hormone and prolactin itself - have been shown to change the electrical behaviour of TIDA cells. We propose that TIDA discharge mode may play a central role in tuning the amount of dopamine delivered to the pituitary and hence circulating prolactin concentrations in different reproductive states and pathological conditions.

Sustained resistance to acute MPTP toxicity by hypothalamic dopamine neurons following chronic neurotoxicant exposure is associated with sustained up-regulation of parkin protein

Hypothalamic tuberoinfundibular dopamine (TIDA) neurons remain unaffected in Parkinson disease (PD) while there is significant degeneration of midbrain nigrostriatal dopamine (NSDA) neurons. A similar pattern of susceptibility is observed following acute exposure to the neurotoxicant 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP), and the resistance of TIDA neurons to MPTP is associated with increased expression of parkin and ubiquitin carboxy-terminal hydrolase L-1 (UCHL-1). In the present study, the response of TIDA and NSDA neurons to acute MPTP administration following chronic MPTP exposure was examined. Mice were treated with ten injections of either MPTP (20mg/kg; s.c.; every 3.5 days) or saline vehicle (10 ml/kg; s.c.; every 3.5 days). Following a 21 day recovery period, chronic saline- and MPTP-treated mice received an additional injection of either saline (10 ml/kg; s.c.) or MPTP (20mg/kg; s.c.) and were sacrificed 24h later. NSDA neurons displayed significant axon terminal degeneration (as reflected by decreases in DA, tyrosine hydroxylase (TH) and DA transporter concentrations in the striatum) as well as loss of TH-immunoreactive (IR) neurons in the substantia nigra (SN) following MPTP, whereas TIDA neurons revealed no overt axon terminal pathology or loss of TH-IR cell bodies. NSDA neuronal pathology was associated with transient decreases in concentrations of parkin and UCHL-1 protein in the SN, which returned to normal levels by 21 days following cessation of chronic neurotoxicant exposure. Resistance of TIDA neurons to MPTP toxicity was correlated with a transient increase in UCHL-1 and a sustained elevation in parkin in the arcuate nucleus. TIDA neurons represent a DA neuron population with a unique and inherent ability to adapt to acute and chronic toxicant administration with a sustained elevation of the neuroprotective protein parkin. The correlation between the ability to increase parkin and UCHL-1 expression and the resistance of DA neurons to neurotoxicant exposure is consistent with a functional link between these features and an underlying differential susceptibility to toxicant-associated neurodegeneration.

Recovery of hypothalamic tuberoinfundibular dopamine neurons from acute toxicant exposure is dependent upon protein synthesis and associated with an increase in parkin and ubiquitin carboxy-terminal hydrolase-L1 expression

Hypothalamic tuberoinfundibular dopamine (TIDA) neurons remain unaffected in Parkinson disease (PD) while there is significant degeneration of midbrain nigrostriatal dopamine (NSDA) neurons. A similar pattern of susceptibility is observed in acute and chronic 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) mouse and rotenone rat models of degeneration. It is not known if the resistance of TIDA neurons is a constitutive or induced cell-autonomous phenotype for this unique subset of DA neurons. In the present study, treatment with a single injection of MPTP (20 mg/kg; s.c.) was employed to examine the response of TIDA versus NSDA neurons to acute injury. An acute single dose of MPTP caused an initial loss of DA from axon terminals of both TIDA and NSDA neurons, with recovery occurring solely in TIDA neurons by 16 h post-treatment. Initial loss of DA from axon terminals was dependent on a functional dopamine transporter (DAT) in NSDA neurons but DAT-independent in TIDA neurons. The active metabolite of MPTP, 1-methyl, 4-phenylpyradinium (MPP+), reached higher concentration and was eliminated slower in TIDA compared to NSDA neurons, which indicates that impaired toxicant bioactivation or distribution is an unlikely explanation for the observed resistance of TIDA neurons to MPTP exposure. Inhibition of protein synthesis prevented TIDA neuron recovery, suggesting that the ability to recover from injury was dependent on an induced, rather than a constitutive cellular mechanism. Further, there were no changes in total tyrosine hydroxylase (TH) expression following MPTP, indicating that up-regulation of the rate-limiting enzyme in DA synthesis does not account for TIDA neuronal recovery. Differential candidate gene expression analysis revealed a time-dependent increase in parkin and ubiquitin carboxyl-terminal hydrolase-L1 (UCH-L1) expression (mRNA and protein) in TIDA neurons during recovery from injury. Parkin expression was also found to increase with incremental doses of MPTP. The increase in parkin expression occurred specifically within TIDA neurons, suggesting that these neurons have an intrinsic ability to up-regulate parkin in response to MPTP-induced injury. These data suggest that TIDA neurons have a compensatory mechanism to deal with toxicant exposure and increased oxidative stress, and this unique TIDA neuron phenotype provides a platform for dissecting the mechanisms involved in the natural resistance of central DA neurons following toxic insult.

Anatomical and functional characterization of clock gene expression in neuroendocrine dopaminergic neurons

Oscillations of gene expression and physiological activity in suprachiasmatic nucleus (SCN) neurons result from autoregulatory feedback loops of circadian clock gene transcription factors. In the present experiment, we have determined the pattern of PERIOD1 (PER1), PERIOD2 (PER2), and CLOCK expression within neuroendocrine dopaminergic (DAergic) neurons (NDNs) of ovariectomized (OVX) rats. We have also determined the effects of per1, per2, and clock mRNA knockdown in the SCN with antisense deoxyoligonucleotides (AS-ODN) on DA release from NDNs. Diurnal rhythms of PER1 and PER2 expression in tuberoinfundibular DAergic (TIDA) and periventricular hypophyseal DAergic (PHDA) neurons, peaked at circadian time (CT)18 and CT12, respectively. Rhythms of PER1 expression in tuberhypophyseal neuroendocrine DAergic (THDA) neurons were undetectable. Rhythms of PER2 expression were found in all three populations of NDNs, with greater levels of PER2 expression between CT6 and CT12. AS-ODN injections differentially affected DA turnover in the axon terminals of the median eminence (ME), neural lobe (NL) and intermediate lobe (IL) of the pituitary gland, resulting in a significant decrease in DA release in the early subjective night in the ME (TIDA), a significant increase in DA release at the beginning of the day in the IL (PHDA), and no effect in the NL (THDA). AS-ODN-treatment induced a rhythm of DA concentration in the anterior lobe, with greater DA levels in the middle of the day. These data suggest that clock gene expression, particularly PER1 and PER2, within NDNs may act to modulate diurnal rhythms of DA release from NDNs in the OVX rat.

Ovarian steroid hormones modulate circadian rhythms of neuroendocrine dopaminergic neuronal activity

Dopamine (DA) is the primary inhibitor of prolactin (PRL) secretion. Three populations of neuroendocrine dopaminergic neurons (NDNs) designated tuberoinfundibular (TIDA), tuberohypophyseal (THDA) and periventricular hypophyseal DAergic (PHDA) neurons regulate PRL secretion. Given that ovarian steroids modulate both DA release and PRL secretion independently, we characterized the role of steroid hormones in coupling rhythmic NDN activity and PRL secretion. OVX rats under a standard 12:12 L:D cycle (L:D), constant dark (DD), or a 6-h phase-delayed L:D cycle (pdL:D) were treated with Estradiol-17beta (E) or E and Progesterone (E+P). NDN activity, defined by DA:DOPAC ratio in nerve terminals, was determined by HPLC-EC. E or E+P stimulated PRL surges in L:D that persisted under DD. In TIDA neurons, E or E+P treatment reduced the amount of DA released under L:D and DD and advanced the rhythm of DA turnover. E and E+P treatment reduced THDA and PHDA neuron activity under L:D, but did not affect these rhythms under DD. Circadian rhythms of PRL, corticosterone and DA turnover in NDN terminals from steroid treated rats entrained to a pdL:D cycle within 7 days. Therefore, ovarian steroids differentially adjust the timing and magnitude of NDN activity to facilitate coupling of DA release and PRL secretion.

Dopamine synthesis by non-dopaminergic neurons expressing individual complementary enzymes of the dopamine synthetic pathway in the arcuate nucleus of fetal rats

This study was aimed to test our hypothesis about dopamine (DA) synthesis by non-DAergic neurons expressing individual complementary enzymes of the DA synthetic pathway in cooperation, i.e. L-dihydroxyphenylalanine (L-DOPA) synthesized in tyrosine hydroxylase (TH)-expressing neurons is transported to aromatic L-amino acid decarboxylase (AADC)-expressing neurons for conversion to DA. The mediobasal hypothalamus of rats at the 21st embryonic day was used as an experimental model because it contains mainly monoenzymatic TH neurons and AADC neurons (>99%) whereas the fraction of bienzymatic (DAergic) neurons does not exceed 1%. The fetal substantia nigra containing DAergic neurons served as a control. DA and L-DOPA were measured by high performance liquid chromatography in: (1) cell extracts of the cell suspension prepared ex tempora; (2) cell extracts and incubation medium after the static incubation of the cell suspension with, or without exogenous L-tyrosine; (3) effluents of the incubation medium during perifusion of the cell suspension in the presence, or the absence of L-tyrosine. Total amounts of DA and L-DOPA in the incubation medium and cell extracts after the static incubation were considered as the indexes of the rates of their syntheses. L-Tyrosine administration caused the increased L-DOPA synthesis in the mediobasal hypothalamus and substantia nigra. Moreover, L-tyrosine provoked an increase of DA synthesis in the substantia nigra and its decrease in the mediobasal hypothalamus. This contradiction is most probably explained by the L-tyrosine-induced competitive inhibition of the L-DOPA transport to the monoenzymatic AADC-neurons after its release from the monoenzymatic TH neurons. Thus, this study provides convincing evidence of cooperative DA synthesis by non-DAergic neurons expressing TH or AADC in fetal rats at the end of the intrauterine development.

Circadian rhythms of neuroendocrine dopaminergic neuronal activity in ovariectomized rats

Prolactin (PRL) secretion is inhibited by dopamine (DA) released from hypothalamic neuroendocrine neurons designated tuberoinfundibular (TIDA), tuberohypophyseal (THDA) and periventricular hypophyseal (PHDA) dopaminergic (DAergic) neurons. Since PRL is secreted in many physiological states with a circadian rhythm, the purpose of these experiments was to determine if patterns of neuroendocrine DAergic neuronal activity in rats are also circadian. The activity of neuroendocrine DAergic neurons, defined as DA turnover rate in nerve terminals and quantitated as the ratio of DOPAC (a primary DA metabolite) to DA content, was measured by high-performance liquid chromatography with electrochemical detection (HPLC-EC) in these populations of DA neurons of OVX rats. TIDA neurons exhibit a rhythm of activity in a light:dark cycle which free-runs in constant dark (DD) and is entrained by light, indicating that TIDA neuronal activity is circadian. THDA and PHDA neurons also display daily rhythms entrained to a photoperiod and PHDA neuronal activity free-runs in DD with a period of approximately 24 h. However, a significant rhythm of THDA neuronal activity was not detected under DD. In the OVX rat, the activities of TIDA and PHDA neurons, but not THDA neurons, describe all the characteristics of a circadian rhythm as they are both entrained by light, but only TIDA and PHDA neurons maintain a significant rhythm of activity under DD.

Physiological role of salsolinol: its hypophysiotrophic function in the regulation of pituitary prolactin secretion

We have recently observed that 1-methyl-6,7-dihydroxy-1,2,3,4-tetrahydroisoquinoline (salsolinol) produced by hypothalamic neurons can selectively release prolactin from the anterior lobe (AL) of the pituitary gland. Moreover, high affinity binding sites for SAL have been detected in areas, like median eminence (ME) and the neuro-intermediate lobe (NIL) that are known terminal fields of the tuberoinfundibular DAergic (TIDA) and tuberohypophysial (THDA)/periventricular (PHDA) DAergic systems of the hypothalamus, respectively. However, the in situ biosynthesis and the mechanism of action of SAL are still enigmatic, these observations clearly suggest that sites other than the AL might be targets of SAL action. Based on our recent observations it may be relevant to postulate that an "autosynaptocrine" regulatory mechanism functioning at the level of the DAergic terminals localized in both the ME and NIL, may play a role in the hypophyseotrophic regulation of PRL secretion. Furthermore, SAL may be a key player in these processes. The complete and precise mapping of these intra-terminal mechanisms should help us to understand the tonic DAerg regulation of PRL secretion. Moreover, it may also give insight into the role of pre-synaptic processes that most likely have distinct and significant functional as well as pathological roles in other brain areas using DAergic neurotransmission, like striatonigral and mesolimbic systems.

Determination of release and uptake parameters from electrically evoked dopamine dynamics measured by real-time voltammetry

Quantifying mechanisms underlying extracellular signaling by the neurotransmitter dopamine (DA) is a difficult task, particularly in the complex extracellular microenvironment of the intact brain. In this study, two methods for evaluating release and uptake from DA dynamics monitored by real-time voltammetry are described. Both are based on a neurochemical model characterizing electrically evoked levels of DA as a balance between these opposing mechanisms. The theoretical basis of what is called here nonlinear regression and single curve analyses is given. Fitting simulated data tests the reliability of the methods. The two analyses are also compared with an experimental data set describing the effects of pharmacologically inhibiting the DA transporter in the caudate-putamen (CP) and nucleus accumbens (NAc). The results indicate that nonlinear regression and single curve analyses are suitable for quantifying release and uptake mechanisms underlying DA neurotransmission. Additionally, the most important experimental finding of this technical study was the independent confirmation of high affinity (approximately 0.2 microM) DA uptake in the intact striatum.

Dopamine as a prolactin (PRL) inhibitor

Dopamine is a small and relatively simple molecule that fulfills diverse functions. Within the brain, it acts as a classical neurotransmitter whose attenuation or overactivity can result in disorders such as Parkinson's disease and schizophrenia. Major advances in the cloning and characterization of biosynthetic enzymes, transporters, and receptors have increased our knowledge regarding the metabolism, release, reuptake, and mechanism of action of dopamine. Dopamine reaches the pituitary via hypophysial portal blood from several hypothalamic nerve tracts that are regulated by PRL itself, estrogens, and several neuropeptides and neurotransmitters. Dopamine binds to type-2 dopamine receptors that are functionally linked to membrane channels and G proteins and suppresses the high intrinsic secretory activity of the pituitary lactotrophs. In addition to inhibiting PRL release by controlling calcium fluxes, dopamine activates several interacting intracellular signaling pathways and suppresses PRL gene expression and lactotroph proliferation. Thus, PRL homeostasis should be viewed in the context of a fine balance between the action of dopamine as an inhibitor and the many hypothalamic, systemic, and local factors acting as stimulators, none of which has yet emerged as a primary PRL releasing factor. The generation of transgenic animals with overexpressed or mutated genes expanded our understanding of dopamine-PRL interactions and the physiological consequences of their perturbations. PRL release in humans, which differs in many respects from that in laboratory animals, is affected by several drugs used in clinical practice. Hyperprolactinemia is a major neuroendocrine-related cause of reproductive disturbances in both men and women. The treatment of hyperprolactinemia has greatly benefited from the generation of progressively more effective and selective dopaminergic drugs.

Effects of chronic bromocriptine treatment on tyrosine hydroxylase (TH) mRNA expression, TH activity and median eminence dopamine concentrations in ageing rats

The purpose of this study was to investigate the age-related changes in the responsiveness of tuberoinfundibular dopamine (TIDA) neurones to chronic hypoprolactinemia induced by treatment with bromocriptine, a dopamine receptor agonist. In one experiment, TIDA neuronal activity after acute hypoprolactinemia or exogenous prolactin was monitored by measuring tyrosine hydroxylase (TH) activity in the stalk median eminence of middle-aged cycling female rats (10-12 months), old constant oestrous rats (18-20 months) and old pseudopregnant rats (22-24 months). In another experiment, middle-aged cycling (10-12 months) rats were treated with bromocriptine for 6 or 12 months. TH activity was measured in the stalk median eminence, TH mRNA levels were measured in the arcuate nucleus and dopamine concentrations were measured in the arcuate nucleus and median eminence. Responsiveness of TIDA neurones to exogenous prolactin and to the withdrawal of bromocriptine in these rats was also tested. While the TIDA neurones in all three age groups responded to acute hypoprolactinemia by showing a reduction in TH activity, older rats failed to respond to exogenous prolactin administration. In contrast, chronic hypoprolactinemia for 12 months enabled the rats to retain TIDA neuronal responsiveness to exogenous prolactin. It also decreased TIDA neuronal function as measured by dopamine concentrations in the median eminence, TH activity in the stalk median eminence and TH mRNA in the arcuate nucleus of ageing rats. The restoration of the responsiveness of these neurones to prolactin stimulation in older rats demonstrates for the first time that hypoprolactinemia produced by chronic bromocriptine treatment indeed provides a neuroprotective effect on TIDA neurones. These results indicate that maintaining a low level of neuronal activity by lowering prolactin levels may be a contributing factor in retaining the plasticity of TIDA neurones.

Anti-tumor effect of L-deprenyl is associated with enhanced central and peripheral neurotransmission and immune reactivity in rats with carcinogen-induced mammary tumors

L-Deprenyl, a monoamine oxidase-B (MAO-B) inhibitor, has previously been shown to improve immune responses and restore noradrenergic (NA) nerve fibers in the spleen of old rats. In tumor-bearing rats, L-deprenyl inhibited tumor incidence and enhanced tuberoinfundibular dopaminergic (TIDA) neurotransmission in the hypothalamus. The aim of the present study was to investigate whether alterations in sympathetic NA activity and cellular immune responses in the spleen, and TIDA activity in the hypothalamus, accompany deprenyl-induced regression of 9,10-dimethyl-1,2-benzanthracene (DMBA)-induced mammary tumors. Rats with DMBA-induced mammary tumors were treated with 0, 2.5 mg, or 5.0 mg/kg body weight of deprenyl daily for 13 weeks. Saline-treated tumor-bearing rats exhibited reduced splenic IL-2 and IFN-gamma levels, and lowered splenic norepinephrine (NE) concentration and hypothalamic dopaminergic activity, compared to rats without tumors. In contrast, treatment with 2.5 mg/kg and 5.0 mg/kg of deprenyl reduced the number and size of mammary tumors. Deprenyl-induced tumor regression was accompanied by increased immune measures in the spleen, including enhanced IL-2 and IFN-gamma production, and NK cell activity. Neural measures enhanced by deprenyl included NE concentration in the spleen and TIDA neuronal activity in the hypothalamus. These results suggest that (1) mammary tumorigenesis is associated with the inhibition of sympathetic NA activity in the spleen, TIDA activity in the hypothalamus, and cell-mediated immunity, and (2) reversal of the inhibition of catecholaminergic neuronal activities of the central nervous system and peripheral nervous system by deprenyl may enhance anti-tumor immunity.

Non-amine dopamine transporter probe [(3)H]tropoxene distributes to dopamine-rich regions of monkey brain

Drug development in psychopharmacology has adhered to the unwritten precept that compounds targeting monoamine transporters must contain an amine nitrogen in the molecular structure. A series of non-amine-bearing aryloxatropanes that are potent inhibitors of the dopamine transporter (DAT) challenged this precept. In the present study, we investigated the brain distribution of a selective, high-affinity DAT non-amine, [(3)H]tropoxene (2-carbomethoxy-3, 4dichloro-3-aryl-8-oxabicyclo[3.2.1] octene), which binds to the DAT in monkey striatum. The autoradiographic distribution of [(3)H]tropoxene was conducted in tissue sections of rhesus (Macaca mulatta) monkey brain. Highest accumulation of the radioligand was detected in the putamen and caudate nucleus, with significant levels also observed in the nucleus accumbens and substantia nigra. Moderate to low levels of [(3)H]tropoxene binding were noted in the hypothalamus, amygdala, ventral tegmental area, and thalamus. The distribution of [(3)H]tropoxene was restricted to brain regions previously identified as expressing DAT, and the relative densities of [(3)H]tropoxene binding sites in various brain regions corresponded to those observed with other selective monoamine radioligands for the DAT. This is the first report to demonstrate that transporter-selective compounds that bear no amine nitrogen in their structure bind selectively to brain regions rich in the transporter. The results support our conclusion that an amine nitrogen is not necessary for compounds to bind to monoamine transporters and distribute in brain according to the known distribution of transporters. The findings provide further incentives to investigate the pharmacological potential of transport inhibitors lacking an amine nitrogen in the molecular structure.

Region-specific alterations in the concentrations of catecholamines and indoleamines in the brains of young and old F344 rats after L-deprenyl treatment

The effects of L-deprenyl, a monoamine oxidase-B (MAO-B) inhibitor, on the concentrations of norepinephrine (NE), dopamine (DA), 3,4-dihydroxyphenylacetic acid (DOPAC), homovanillic acid (HVA), serotonin (5-hydroxytryptamine), and 5-hydroxyindoleacetic acid (5-HIAA) in medial basal hypothalamus (MBH), substantia nigra (SN), striatum (Str), and nucleus accumbens (NAc) of young (3 month) and old (21 month) male F344 rats were examined after a 7-day wash-out period following 1, 15, or 30 days of deprenyl treatment in young rats and a 9-day wash-out period after a 10-week deprenyl treatment in old rats. The brain areas were microdissected and the concentrations of neurotransmitters were measured by High Performance liquid chromatography with electrochemical detection (HPLC-EC). Deprenyl administration following the drug wash-out period increased the concentrations of DOPAC in the SN, Str, and in the NAc of young rats but it was decreased in the NAc of old rats. The concentration of HVA was lower in the Str of young deprenyl-treated rats, and in the Str and NAc of old deprenyl-treated rats, but it was higher in the SN of young deprenyl-treated rats. The concentration of 5-HIAA was increased in the MBH, SN, and in the NAc of young deprenyl-treated rats, but it was decreased in the Str and NAc of old deprenyl-treated rats. The concentration of NE was increased in the MBH, SN, Str, and in the NAc of young rats treated with deprenyl and in the MBH of old deprenyl-treated rats. The concentration of 5-HT was increased in the SN of young deprenyl-treated rats. The concentration of DA increased in the Str of both young and old deprenyl-treated rats. We concluded that a drug wash-out period after deprenyl treatment differentially affects the metabolism of catecholamines and indoleamine depending on the region of the brain and that this effect may be due to variation in the kinetics of MAO inhibition.

Regional differences in striatal dopamine uptake and release associated with recovery from MPTP-induced parkinsonism: an in vivo electrochemical study

This study directly assessed striatal dopamine (DA) uptake rates and peak release in response to KCl in normal, symptomatic, and recovered 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP)-treated cats using in vivo electrochemistry. DA uptake rates measured after direct application of known concentrations of DA to the striatum were slowed significantly in both dorsal and ventral striatum in symptomatic cats compared with rates recorded in normal animals. DA uptake rates remained significantly slowed in recovered cats and were not significantly different from the rates recorded in symptomatic animals. In symptomatic cats, both DA uptake rates and the signal recorded in response to KCl stimulation were significantly decreased from normal in all dorsal and ventral striatal regions sampled. Reduction/oxidation (redox) ratios recorded in response to KCl stimulation suggested DA to be the predominant electroactive species. In spontaneously recovered MPTP-treated cats, recordings in the ventral striatum subsequent to KCl stimulation again suggested DA to be the predominant electroactive species released, and peak levels were significantly higher than those recorded in symptomatic animals. In the dorsal striatum of recovered cats, redox ratios recorded subsequent to KCl stimulation suggested serotonin rather than DA to be the predominant electroactive species released. Peak levels of release in the dorsal striatum were not significantly greater than those recorded in symptomatic animals. These results suggest that in spontaneously recovered MPTP-treated cats, there is partial recovery of ventral striatal DAergic terminals, persistent loss of dorsal striatal DAergic terminals, and a down-regulation of DA transporter number/function throughout the striatum. These processes may contribute to volume transmission of DA in the striatum and promote functional recovery.

Intrahypothalamic Serotonergic Neurons

Serotonin's role as a neuronal transmitter was established already forty years ago, and the anatomy and many of the functions of the major serotonergic systems have been carefully mapped. The intimate association of serotonergic mechanisms with central control of food intake has also been extensively studied. While the present concepts of serotonergic functions rely on the ascending, raphe nuclei-originating serotonergic pathways, there is an accumulating evidence to support that hypothalamic neurons may also exhibit many features normally attributed to serotonergic neurons only. Neurons in the hypothalamic arcuate and periventricular nuclei express tryptophan hydroxylase, the serotonin synthesizing enzyme, while they do not transport or synthesize serotonin. On the other hand, dorsomedial nucleus contains a select population of neurons that do actively accumulate serotonin, while they do not express tryptophan hydroxylase. These and some other serotonin-associated features of the hypothalamic neuronal groups are discussed. Finally the present data is projected against the prevailing concept of hypothalamic regulation of food intake.

The effect of neurointermediate lobe denervation on hypothalamic neuroendocrine dopaminergic neurons

The contribution of tuberohypophyseal and periventricular-hypophyseal dopaminergic neurons to the regulation of the secretion of prolactin (PRL) has yet to be clarified. In this study, we used pituitary stalk compression to disrupt hypothalamic neural input to the neurointermediate lobe (NIL). Neurointermediate lobe denervation (NIL-D) selectively disrupts the axons of tuberohypophyseal and periventricular-hypophyseal dopaminergic neurons, while leaving tuberoinfundibular dopaminergic neurons and the vascular supply of the pituitary gland intact. NIL-D was performed in ovariectomized (OVX) rats. The concentration of DA and 3,4-dihydroxyphenylacetic acid (DOPAC) in the median eminence (ME) and various regions of the pituitary gland of OVX and OVX+NIL-D rats were measured by HPLC-EC. The concentration of PRL, alpha-melanocyte stimulating hormone (alpha-MSH), and luteinizing hormone (LH) in serum were determined by radioimmunoassay. Successful NIL-D was confirmed by increased water intake. One week after NIL-D, serum PRL and alpha-MSH were elevated, but there was no change in the concentration of LH in serum. The concentration of DA was increased in the median eminence (ME), decreased in the outer zone of the anterior lobe (AL-OZ), as well as the intermediate (IL) and neural lobes (NL), and remained unchanged in the inner zone of the anterior lobe (AL-IZ). The concentration of DOPAC was increased in the ME and NL, decreased in the IL, and remained unchanged in both the AL-IZ and AL-OZ. These data confirm that pituitary stalk compression denervates the NIL. Moreover, decreases in the concentration of DA in the IL and AL-OZ, coupled with elevation of serum PRL and alpha-MSH indicate that DA from the NIL contributes to the increased inhibition of the secretion of PRL and alpha-MSH in OVX rats.

Characterization of the dopaminergic input to the pituitary gland throughout the estrous cycle of the rat

Changes in the concentrations of dopamine (DA) and its major metabolite, 3,4-dihydroxyphenylacetic acid (DOPAC), were characterized in the pituitary gland throughout the 4-day estrous cycle of the rat. Female rats were sacrificed at 2- to 3-hour intervals throughout each day of the 4-day estrous cycle. Pituitary glands were removed, and the concentrations of DA and DOPAC were determined by high-performance liquid chromatography coupled to electrochemical detection. The concentration of prolactin (PRL) in serum from these same animals was determined by radioimmunoassay. The concentration of DA in the anterior lobe was constant throughout most of the 4-day estrous cycle. Prior to initiation of the proestrous surge of PRL, there were significant (p < 0.05) decreases in the concentrations of both DA and DOPAC in the anterior lobe which returned to elevated baseline levels just prior to the termination of the proestrous surge of PRL. The concentrations of DA and DOPAC in the intermediate lobe exhibited a daily rhythm. However, in the intermediate as well as in the anterior lobe, there were significant (p < 0.001) decreases in the concentrations of both DA and DOPAC, coincident with the initiation of the proestrous surge of PRL. Similarly, coincident with the peak of the proestrous surge of PRL, there were significant (p < 0.001) increases in the concentrations of DA and DOPAC in the intermediate lobe, followed by a return to basal levels and resumption of the daily rhythm. The pattern of the concentrations of DA and DOPAC in the neural lobe was also daily in nature, with peaks occurring between 13.00 and 15.00 h each day of the 4-day estrous cycle. These data, taken together: (1) confirm that a decrease of the concentrations of DA and DOPAC occurs in the anterior lobe prior to the proestrous surge of PRL; (2) reveal that DA is released in a daily pattern at intermediate and neural lobes, and (3) suggest an apparent role for DA released to the intermediate lobe in the regulation of the proestrous surge of PRL.

Antitumor effect of L-deprenyl in rats with carcinogen-induced mammary tumors

Deprenyl, a monoamine oxidase-B (MAO-B) inhibitor, has a wide range of pharmacological properties that are beneficial therapeutically in the treatment of human neurodegenerative diseases. Recent studies have demonstrated that deprenyl possesses a neuroprotective function that is not dependent on its MAO-B inhibitory activity. The focus of the present study was to investigate whether prolonged treatment of young Sprague-Dawley female rats with deprenyl before and after 9,10-dimethyl-1,2-benzanthracene (DMBA) administration would inhibit the development of mammary tumors by exerting a neuroprotective effect on the tuberoinfundibular dopaminergic (TIDA) neurons in the medial basal hypothalamus (MBH). For this purpose, the concentrations of catecholamines, indoleamine and their metabolites were measured in the MBH by high-performance liquid chromatography (HPLC) at the end of the treatment period. Female Sprague-Dawley rats (28-29 days old) were treated intraperitoneally with saline, or 0.25 or 2.5 mg of deprenyl/kg b.w. daily for 4 weeks prior to the administration of DMBA. Following the administration of DMBA, the rats were treated with saline or deprenyl daily for 27 weeks. At the end of the treatment period, there was a significant reduction in the tumor incidence and tumor number in rats that received 2.5 mg/kg deprenyl before and after the administration of DMBA and also in rats that were treated with 2.5 mg/ kg deprenyl following DMBA. There also was a significant decrease in tumor number in rats that were treated with 0.25 mg/kg deprenyl during the entire treatment period of 31 weeks. Body weight increased throughout the treatment period with no significant differences between the groups. Treatment of rats with 2.5 mg of deprenyl following the administration of DMBA and also during the entire treatment period resulted in a significant decrease in the concentrations of the metabolites of norepinephrine (NE), dopamine (DA) and serotonin (5-HT) in the MBH, but there were no significant alterations in the concentrations of NE, DA and 5-HT in the MBH. These results suggest that the administration of deprenyl blocked the development of mammary tumors in part by inhibiting the metabolism of catecholamines and indoleamine and possibly by conferring a neuroprotective effect on the TIDA neurons in the MBH, especially at 0.25 mg/kg of deprenyl.

Inhibitory effect of dopamine on dorsomedial arcuate neurons in rat brain slices: potentiation by coadministration of cocaine

Whether dopamine (DA) can have a direct effect on the tuberoinfundibular dopaminergic neurons has been a controversial issue. The present report used single-unit recording of neurons in dorsomedial region of the arcuate nucleus, where most tuberoinfundibular dopaminergic neurons are located, to study this question. By focusing our recording in this region, we found that DA in 25-250 nmol ranges inhibited a significant number of arcuate neurons tested (74.2% of 182 units). The inhibitory effect of DA was not only prominent in most cases, it also persisted in low Ca2+, high Mg2+ solution in several trials. Cocaine, a drug of abuse whose main effect is due to its inhibition of DA transporters and increasing the DA concentration in synaptic clefts, also inhibited a significant number of arcuate neurons by itself (51.5% of 97 units), although its effects were lesser than those of DA. Nevertheless, when coadministered with DA, cocaine significantly potentiated the inhibitory effect of DA in 82% of DA-responsive units (n = 39). These results clearly demonstrate that DA exhibits a predominantly inhibitory effect on presumed DA neurons in dorsomedial arcuate nucleus. The effects of cocaine further support this notion.

Dopamine transporter immunohistochemistry in median eminence, amygdala, and other areas of the rat brain

In an extension of our previous work, an antibody directed against a peptide from the N-terminal region of DAT was used to localize specific dopamine transporter immunoreactivity (DAT-IR) in several regions of rat brain. Apparent axons and varicosities were found in the zona incerta, external layer of the median eminence, various nuclei of the amygdala, the cortex-amygdala transition zone, and in periglomerular regions in the olfactory bulb. Apparent stained neuronal perikarya and dendrites were observed in the arcuate nucleus and olfactory bulb. These regions are known to have dopaminergic neurons and innervations, although there was not a perfect correspondence between DAT-IR and the known distribution of dopaminergic neurons. A possible explanation is that different dopamine containing cell groups express different levels of DAT mRNA and protein, as we have previously shown. Also in the tuberoinfundibular neurons, for example, DAT-IR was preferentially localized to distal axons in the median eminence, suggesting intracellular compartmentation.

Regional quantification of dopamine transporter mRNA in rat brain using a ribonuclease protection assay

We describe the regional distribution of dopamine transporter (DAT) mRNA in selected regions of rat brain using a highly sensitive and specific nuclease protection assay. This method determines the absolute quantity of mRNA expressed in the brain regions surveyed. DAT mRNA level varied widely between brain regions, and was detected only in cell body regions of the major dopaminergic pathways. Highest expression was seen in substantia nigra/ventral tegmentum (SN/VTA). Lower but detectable expression of a protected mRNA of the expected size was observed within hypothalamus. Expression could not be detected by this method in other brain regions studied. Our results indicate that this method is sufficiently sensitive to allow study of mRNA expression in individual animals.

Effects of dopamine D1 and D2 receptor agonists and antagonists on basal and ethanol-modulated beta-endorphin secretion from hypothalamic neurons in primary cultures

In the present study, we determined the effects of dopamine receptor agonists and antagonists on basal and ethanol-modulated beta-endorphin (beta-EP) secretion from hypothalamic neurons in primary cultures. Treatment with various concentrations of dopamine D1 agonist SKF 38393 and D1 antagonist SCH 23390 did not affect basal IR-beta-EP release. However, dopamine D2 receptor agonist LY 141865 reduced basal immunoreactive (IR)-beta-EP release in a concentration dependent manner. D2 receptor antagonist, sulpiride, on the other hand, stimulated basal IR-beta-EP release and blocked LY 141865-induced inhibition of IR-beta-EP release in a concentration dependent manner. When the actions of these DA receptor agents on ethanol-modulated IR-beta-EP release were studied, both D1 and D2 receptor agents failed to affect ethanol-modulated IR-beta-EP release. These data suggest that the endogenous secretion of beta-EP from hypothalamic neurons is under the influence of an inhibitory dopaminergic system involving the D2 receptor. Furthermore, ethanol's effects on beta-EP secretion are not mediated by dopamine.

Dopaminergic system mediation of stimulatory effect of melatonin on secretion of prolactin

OBJECTIVE

The purpose of the present study was to clarify the mechanism of the stimulatory effect of melatonin on the secretion of prolactin.

METHODS

Determination was made of the levels of dopamine (DA), 3,4-dihydroxyphenlacetic acid (DOPAC), serotonin (5-HT), and 5-hydroxyindoleacetic acid (5-HIAA) in the median eminence of rats following intravenous melatonin (100 micrograms) administration.

RESULTS

Administration of 100 micrograms of melatonin to rats resulted in a significant increase in prolactin secretion (p < 0.01, analysis of variance). Administration of melatonin also resulted in significant decreases in the DOPAC content (p < 0.05) and in the DOPAC/DA ratio (p , 0.05) in the median eminence 1 hour after melatonin administration, at which time serum prolactin was maximal. However, no significant changes were detected in the content of 5-HT or 5-HIAA or in the 5-HIAA/5-HT ratio in the median eminence.

CONCLUSION

These results suggest that the stimulatory effect of melatonin on the secretion of prolactin might be mediated by dopaminergic mechanisms.

In vivo voltammetric measurement of evoked extracellular dopamine in the rat basolateral amygdaloid nucleus

1. The in vivo measurement of evoked extracellular dopamine was established in the basolateral amygdaloid nucleus (BAN) using fast-scan cyclic voltammetry at carbon-fibre microelectrodes. 2. The identification of evoked extracellular dopamine in the BAN was based on anatomical, electrochemical and pharmacological criteria. Electrochemical and pharmacological evidence indicated that the species was a catecholamine. Mesencephalic sites eliciting overflow and amygdaloid sites supporting overflow correlated well with the mesoamygdaloid dopamine innervation. 3. Marked differences in the dynamics and magnitude of evoked dopamine overflow were observed in the BAN, caudate-putamen and amygdalo-striatal transition area. The results underscore the importance of making spatially resolved measurements of extracellular dopamine in the amygdala. 4. Mesoamygdaloid dopamine neurons have similar release characteristics as mesostriatal dopamine neurons but share with mesoprefrontal cortical dopamine neurons the ability to use a greater percentage of intraneuronal dopamine stores for release.

Effects of systemic administration of 6-hydroxydopamine, 6-hydroxydopa and 1-methyl-4-phenyl-1,2,3,6-tetrahydroxypyridine (MPTP) on tuberoinfundibular dopaminergic neurons in the rat

Using systemic route of administration, the effects of several neurotoxins on hypothalamic tuberoinfundibular dopaminergic neurons were focused in this study. 6-Hydroxydopamine (6-OHDA, 10 or 100 mg/kg b.wt., i.v. or ip) produced a dose (37 vs. 50%)- and time (41 to 29% from day 4 to day 9)-dependent depletion of hypothalamic median eminence dopamine concentrations, and increases of serum prolactin levels in ovariectomized rats. Other central dopaminergic neurons, however, were not significantly affected. Similar treatments with 6-hydroxydopa (6-OHDOPA) were less effective. On the other hand, treatments of 1-methyl-4-phenyl-1,2,3,6-tetrahydroxypyridine (MPTP, 10 mg/kg b.wt./day, ip) for 7 or 14 days produced significant decreases of dihydroxyphenylacetic acid (DOPAC) levels in the median eminence and periventricular regions, and increases in serum prolactin levels. Other central dopaminergic neurons were not significantly affected, though. These results suggest that systemic administration of 6-OHDA, 6-OHDOPA, or MPTP, can produce specific destructive effects on the tuberoinfundibular dopaminergic neurons.

GBR12909 stimulates hypothalamo-pituitary-adrenal activity by inhibition of uptake at hypothalamic dopamine neurons

We have previously demonstrated that the inhibition of neurotransmitter uptake at dopamine (DA) terminals stimulates the hypothalamo-pituitary-adrenal axis. In the present study we investigated the role of central DA neuronal systems in the regulation of this axis. Administration of the DA uptake inhibitor GBR12909 (3-30 micrograms) into the third ventricle dose-dependently elevated serum adrenocorticotropin hormone (ACTH) levels in rats. GBR12909 (10 micrograms) elevated serum ACTH levels following administration into the paraventricular nucleus of the hypothalamus but not into the lateral ventricle. The administration of 6-OHDA into the third ventricle significantly decreased DA content in the hypothalamus and striatum and significantly attenuated the ACTH response to GBR12909 (10 micrograms, third ventricle or 10 mg/kg, i.p.). Conversely, 6-OHDA lesions of the medial forebrain bundle, which depleted 99% of DA in the caudate but did not decrease DA content in the hypothalamus, and did not attenuate the ACTH response to i.p. GBR12909. Measurement of GBR12909-induced inhibition of [3H]DA uptake into synaptosomal preparations indicates the presence of GBR12909-sensitive DA transporters in the region of the hypothalamus surrounding the third ventricle. The present findings suggest that an endogenous DA neuronal system terminating in the hypothalamus mediates the effects of stimulants on hypothalamo-pituitary-adrenal function and might play a role in the ongoing regulation of hypothalamo-pituitary-adrenal activity.

Effects of acute and chronic cocaine on the activity of tuberoinfundibular dopamine neurons in the rat

We investigated the effects of acute and chronic cocaine on dopamine (DA) synthesis in tuberoinfundibular dopamine (TIDA) neurons by measuring the accumulation of 3,4-dihydroxyphenylalanine (DOPA) in the median eminence (ME). Cocaine-induced prolactin (PRL) responses were evaluated for comparison. Acute cocaine inhibited DA synthesis in the ME and decreased circulating PRL. Chronic cocaine did not alter basal DOPA levels in TIDA nerve terminals or PRL levels in plasma. After repeated cocaine injections, cocaine's inhibitory effect on DA synthesis was abolished in the ME while the PRL response to drug was unchanged. These results suggest that TIDA neurons become tolerant to cocaine-induced suppression of DA synthesis after chronic cocaine exposure, but the altered sensitivity of TIDA neurons is not revealed by plasma PRL measures.

Dopamine transporter mRNA expression is intense in rat midbrain neurons and modest outside midbrain

Dopamine transporter mRNA expression in individual neurons from the substantia nigra pars compacta. 'All' area, arcuate nucleus of the hypothalamus, retina, and olfactory bulb was assessed by in situ hybridization. High levels of expression were noted over individual neurons in midbrain nuclei; much lower expression was found in cells of the inner nuclear layer of the retina, glomerular cell layer of the olfactory bulb, and medial aspect of the arcuate nucleus of the hypothalamus. The low levels of expression in the latter nuclei are consistent with the paucity of effects of cocaine in visual and olfactory systems, failure to detect photoaffinity-labelled transporter protein in hypothalamus or olfactory bulb, and observations that little or no damage is found in dopaminergic neurons outside the basal midbrain in idiopathic Parkinsonism.

Resistance of hypothalamic dopaminergic neurons to neonatal 6-hydroxydopamine toxicity

We studied the effects of neonatal intracisternal administration of the 6-hydroxydopamine (6-OHDA) following desipramine pretreatment on dopaminergic (DA) neurons in the rat hypothalamus and substantia nigra by immunocytochemistry with an antiserum against tyrosine hydroxylase (TH). Neonatal intracisternal 6-OHDA injection induced almost complete loss of the TH-immunoreactivity in the substantia nigra and the caudate-putamen when examined at final (adult) stage. However, in this stage, no difference of TH-immunoreactivity was observed in hypothalamic DA neurons in the arcuate nucleus (A12), periventricular area (A14), zona incerta (A13), and posterior hypothalamic area (A11). In the initial (neonatal) stage after the 6-OHDA injection, nigral DA neurons started to degenerate in 12 h and were almost completely destructed in 96 h, but hypothalamic DA neurons did not show any degenerative change at any time examined. The route of the injection (cistern, third ventricle or lateral ventricle) of the toxin did not influence the distribution of damage. These data show that 6-OHDA is not equally toxic to all brain DA neurons in neonates, and that all hypothalamic DA neuronal groups resist the toxicity of 6-OHDA, despite their anatomical and functional differences.

Dopamine transporter mRNA: dense expression in ventral midbrain neurons

Oligonucleotides and a full-length cDNA encoding a functional dopamine transporter (DAT1) hybridize to a 3.7 kb mRNA that is concentrated in mRNA prepared from midbrain and absent in specimens from cerebellum or cerebral cortex. In situ hybridization reveals substantial hybridization densities overlying neurons of the substantia nigra, pars compacta, and the parabrachialis pigmentosus region of the ventral tegmental area (VTA). Neurons in the linear and paranigral VTA regions display lower levels of expression. Preliminary studies in arcuate neurons suggest modest hybridization. Different dopaminergic cell groups display different levels of DAT1 dopamine transporter expression.

Effects of reuptake inhibitors on dopamine release from the stalk-median eminence and posterior pituitary in vitro

Similar to other dopaminergic systems, the dopaminergic neurons innervating the stalk-median eminence (SME) and posterior pituitary (PP) possess an uptake mechanism for dopamine (DA). However, the extent of DA reuptake in these tissues and its physiological significance are debated since much of the released DA is removed by the hypophysial portal vasculature before recapture. The objectives of this study were: (1) to establish in vitro conditions for examining the effects of reuptake inhibitors on DA release from the PP and SME; (2) to compare the effects of nomifensine, diclofensine and amphetamine on DA release from the SME and PP; and (3) to distinguish between reuptake and releasing properties of these drugs. Individual SME and PP were dissected from ovariectomized rats and incubated in either a static or perifusion system. Media DA was extracted with alumina and quantitated by high performance liquid chromatography with electrochemical detection. The reuptake inhibitors, nomifensine, dichlofensine and amphetamine, in the presence of pargyline, a monoamine oxidase inhibitor, stimulated both basal and K(+)-evoked release of DA from the SME and PP under static incubation conditions. The drugs elicited a 2-3-fold higher increase in basal DA release from the SME as compared to the PP. Only amphetamine stimulated DA release in the perifusion system whereas nomifensine and diclofensine were without effects. We concluded that: (1) a mechanism for the reuptake of DA is operable in both the SME and PP; (2) the reuptake of DA appears to be more active in the SME than the PP; and (3) unlike amphetamine, nomifensine and diclofensine are pure reuptake inhibitors devoid of direct DA releasing activities.

Monoamine content of the stalk-median eminence and hypothalamus in adult female sheep as affected by daylength

Abstract In the ewe, plasma luteinizing hormone and prolactin concentrations exhibit seasonal variations. During long days, inhibition of pulsatile luteinizing hormone secretion is mediated by monoamines. In a model of ovariectomized ewes bearing a subcutaneous oestradiol implant, we previously showed that the steroid-dependent inhibition of luteinizing hormone involves the A15 dopaminergic nucleus of the retrochiasmatic area. In the present work, we compared the aminergic activities of tele-diencephalic structures in groups of ovariectomized ewes under artificial illumination for short versus long days (8 versus 16 h/day of light, respectively). Half the animals in each group were bearing a subcutaneous oestradiol implant. Using high-performance liquid chromatography and electrochemical detection, we measured the levels of amines and amine metabolites in 'punches' of tissues from regions containing luteinizing hormone-releasing hormone axon terminals or cell bodies and catecholaminergic structures. Concurrently, we checked the pulsatile luteinizing hormone release and plasma prolactin concentration to assess the ability of our model to mimic seasonal changes in the hormonal status. As expected, ovariectomized ewes with a subcutaneous oestradiol implant showed an inhibition of the pulsatile luteinizing hormone release under long days. A higher concentration of plasma prolactin was also observed under long days, without any effect of the steroid treatment. Under this light regimen, statistically significant higher contents of dopamine than under short days were found in the stalk-median eminence. Larger contents of homovanillic acid, a dopamine metabolite, and 4-hydroxy-3-methoxyphenylethyleneglycol (MHPG), a noradrenaline metabolite were observed in the infundibular nucleus, while the catechola-mines themselves remained unchanged. Furthermore, oestradiol also significantly increased the content of MHPG in the latter structure. During long days, animals without oestradiol treatment exhibited a significant lower content of noradrenaline in the A15 nucleus, without any alteration of the dopamine content. Daylength or oestradiol treatment had no significant effects on the levels of amines or amine metabolites in the preoptic or septal areas. Thus, our results in the ewe underline the role played by the medial basal hypothalamus in the catecholaminergic regulation of seasonal changes in hormone release and suggest modifications in the turnover of the neurotransmitters in some structures.

Contribution of noradrenergic neurons to 3,4-dihydroxyphenylacetic acid concentrations in the regions of the rat brain containing incertohypothalamic dopaminergic neurons

The purpose of this study was to determine the source of 3,4-dihydroxyphenylacetic acid (DOPAC) in medial zona incerta (MZI) and dorsomedial nucleus (DMN), with the overall aim of ascertaining whether alterations in the concentration of this dopamine (DA) metabolite reflect the activity of incertohypothalamic dopaminergic neurons. In both the MZI and DMN, the concentration of norepinephrine (NE) exceeds that of DA, reflecting a higher density of noradrenergic vs. dopaminergic neurons in these brain regions. The ratio of DOPAC to DA was greater than the ratio of 3-methoxy-4-hydroxyphenylethyleneglycol (MHPG) to NE indicating that the activity of dopaminergic neurons in the MZI and DMN is greater than that of noradrenergic neurons. Destruction of noradrenergic neuronal axons in the ventral bundle following bilateral injections of the neurotoxin 5-amino-2,4-dihydroxy-alpha-methylphenylethylamine (5-ADMP) decreased NE concentrations in the MZI and DMN, but had no effect on the concentrations of DA or DOPAC, revealing that under basal conditions noradrenergic neurons contribute little to DOPAC concentrations in these brain regions. The DA receptor antagonist haloperidol increased, while the DA receptor agonist apomorphine decreased DOPAC concentrations in the MZI and DMN, indicating that alterations in the activity of incertohypothalamic dopaminergic neurons are accompanied by corresponding changes in the concentration of this DA metabolite. On the other hand, activation of noradrenergic neurons following administration of the alpha 2-adrenergic receptor antagonist idazoxan increased DOPAC concentrations in both the MZI and DMN in intact, but not in ventral bundle-lesioned rats.(ABSTRACT TRUNCATED AT 250 WORDS)

Chronic cocaine administration sensitizes behavioral but not neuroendocrine responses

Many behavioral responses to cocaine become progressively exaggerated with chronic treatment. Most studies have focused on cocaine-induced locomotor stimulation and changes in the dopamine projection systems which mediate these actions. However, it is not clear if a uniform 'sensitization' develops in all dopamine systems during chronic stimulant administration. To test this possibility the neuroendocrine actions of cocaine which are mediated, in part, by hypothalamic dopamine neurons were evaluated after chronic cocaine administration. Treatment of rats with cocaine (15 mg/kg, 2x/day) markedly enhanced the locomotor response to cocaine after 3 and 7 days of chronic administration. In contrast, neither the stimulation of adrenocorticotropic hormone (ACTH)/corticosterone (CS) secretion nor the slight inhibition of prolactin caused by acute cocaine administration changed, although a small elevation of basal corticosterone secretion was observed after 7 days. These findings suggest that the marked sensitization observed in other dopamine systems does not occur in the hypothalamic dopamine neurons thought to mediate the neuroendocrine responses to cocaine during chronic administration, despite recently described commonalities in uptake and autoreceptor functions in these dopamine cell populations. This contrast suggests a role for long loop feedback systems unique to telencephalic dopamine systems or region-specific neurochemical adaptations in cocaine sensitization.

Dopamine transporter: deglycosylation with exo- and endoglycosidases

The dopamine transporter from rat caudate-putamen was photolabeled with [125I]DEEP as previously described. Treatment of photolabeled membranes with neuraminidase and N-glycanase reduced the molecular weight of the [125I]DEEP photolabeled dopamine transporter complex, whereas treatment with alpha-mannosidase had no effect. The solubilized [125I]DEEP photolabeled dopamine transporter complex readily bound to wheat-germ agglutinin but not to concanavalin-A sepharose columns. These results suggest that the carbohydrate moiety of the dopamine transporter is N-linked and contains significant quantities of sialic acid but not high mannose residues. A DEEP binding protein was readily detectable in other brain regions including the nucleus accumbens and olfactory tubercle, but not in the prefrontal cortex, olfactory bulb or hypothalamus under similar conditions. The DEEP binding protein in the other brain regions was similar to that in the striatum.

Enhanced haloperidol-induced prolactin stimulation with chronic neuroleptic treatment in the rat

Animals were treated either acutely, or chronically for 21 days, with a low dose (0.1 mg/kg) of haloperidol, then sacrificed to obtain trunk blood for radioimmunoassay of prolactin (PRL) level. PRL concentrations on day 21 of chronic treatment were greater than two-fold those produced by acute neuroleptic. Challenge with apomorphine to rats withdrawn for 48 hours revealed similar PRL reductions as a group withdrawn from chronic vehicle injections.

Modification of gonadectomy-induced increases in brain monoamine metabolism by steroid hormones in male and female rats

Concentrations of monoamines (dopamine, DA; serotonin, 5-HT) and their major metabolites (homovanillic acid--HVA; dihydroxyphenylacetic acid--DOPAC; 5-hydroxyindolacetic acid--5-HIAA) were measured in selected brain areas of chronically gonadectomized, steroid- or oil-treated male and female rats. Concentrations of DOPAC and HVA were markedly increased in the hypothalamus (male, female), striatum (male, female) and brainstem (male) following gonadectomy, whereas the levels of DA remained unaltered in most of the brain areas examined. Most of the changes were reversed or attenuated by chronic estradiol (EB) substitution. In contrast, chronic treatment with physiological concentrations of testosterone (TP) reduced indexes of DA turnover only in the striatum of ovariectomized (OVX) and brainstem of orchidectomized (ORDX) rats. ORDX-related increases in striatal levels of DOPAC and HVA were not reversed by either EB or TP. ORDX increased the levels of 5-HIAA (hypothalamus, striatum) and decreased those of 5-HT (hypothalamus, hippocampus). These changes were reversed by chronic treatment with either TP or EB. Brain metabolism of 5-HT remained unaltered following OVX. Gonadectomy and chronic steroid replacement therapy appear to alter brain monoamine metabolism in a brain region and sex-dependent manner. Our data demonstrate that gonadectomy-related increases in the activity of brain monoaminergic neurons in both male and female rats was attenuated more effectively with physiological concentrations of estradiol than with testosterone. Insensitivity of monoaminergic neurons in a number of brain areas (e.g., hypothalamus, striatum) to the action of testosterone was evident in both sexes.

Neuroendocrine measures of dopaminergic function in chronic cocaine users

Plasma prolactin (PRL) and growth hormone (GH) levels are determined, in part, by the effects of dopamine (DA) at pituitary and hypothalamic DA receptors, respectively. To determine if chronic cocaine abuse alters dopaminergic activity, basal PRL and GH concentrations were measured in 16 male patients meeting DSM-III-R criteria for cocaine dependence (8 cocaine users and 8 cocaine + alcohol users) and 8 normal controls. In addition, the functional responsivity of DA receptors was assessed in the same group of patients by measuring the change in plasma PRL and GH concentrations following the administration of the direct-acting DA agonist, apomorphine (0.01 mg/kg, s.c.) or saline. No difference in basal plasma PRL and GH levels or plasma PRL and GH responses to apomorphine administration was found between the entire group of cocaine patients and normal controls. However, three of the cocaine patients had basal plasma PRL levels that were more than 2.5 SD greater than that of the normal controls, suggesting that some interference of dopaminergic inhibition of PRL secretion might be present in at least some cocaine users. Although baseline plasma PRL levels were elevated in a subgroup of cocaine users, these data do not support the hypothesis that chronic cocaine abuse produces consistent abnormalities in dopaminergic function at the pituitary or hypothalamus.

Multiple, but not acute, infusions of cocaine alter the release of prolactin in male rats

Hypothalamic dopamine tonically inhibits the release of prolactin (PRL) from the anterior pituitary gland. Cocaine, in turn, alters dopaminergic transmission. We compared the effects of acute and repeated injections of cocaine on the release of PRL in male rats to assess whether cocaine could affect dopaminergically mediated hormonal responses. We found that the concentration of PRL in plasma was not affected by single i.v. injections of 1, 3 or 10 mg/kg of cocaine. However, in rats infused repeatedly with 1 mg/kg of cocaine for 5 s every 12 min for 2 h over 10 days, the pre-infusion concentrations of PRL increased in a time-dependent manner whereas cocaine uniformly decreased post-infusion levels of PRL. Repeated administration of cocaine may produce long-term changes in either the tuberoinfundibular dopaminergic neurons or the adenohypophysial dopamine D2-receptors, or both. Changes in the peripheral concentration of PRL after multiple injections of cocaine and during cocaine withdrawal may reflect dopaminergic activity in the hypothalamus. In contrast, single injections of cocaine increased adrenocorticotropin (ACTH) in a dose-dependent manner whereas repeated infusions did not increase peripheral concentrations of ACTH or corticosterone. It seems that repeated injections of cocaine do not result in persistent changes in the hypothalamo-pituitary-adrenal axis.

3,4-Dihydroxyphenylacetic acid concentrations in the intermediate lobe and neural lobe of the posterior pituitary gland as an index of tuberohypophysial dopaminergic neuronal activity

Tuberohypophysial dopamine (DA) neurons terminate in the intermediate and neural lobes of the posterior pituitary. The objective of this study was to determine if concentrations of 3,4-dihydroxyphenylacetic acid (DOPAC), a major metabolite of DA in these regions, reflect the activity of tuberohypophysial DA neurons. The concentrations of DOPAC and DA in the intermediate lobe were approximately twice those in the neural lobe, so that the ratios of DOPAC/DA were similar between lobes. The administration of a monoamine oxidase inhibitor pargyline produced a rapid decline (by 5 min) of DOPAC concentrations in both the intermediate and neural lobes. The administration of nomifensine, an inhibitor of DA uptake at the nerve terminal, produced a modest 33% decline in DOPAC concentrations in the intermediate lobe, but was without effect in the neural lobe. Activation of tuberohypophysial DA neurons by electrical stimulation of the pituitary stalk increased both the rate of DA synthesis (accumulation of dihydroxyphenylalanine (DOPA) after administration of the decarboxylase inhibitor NSD 1015) and the concentrations of DOPAC in the intermediate and neural lobes. Administration of the DA antagonist haloperidol increased, and the DA agonist apomorphine decreased both the rate of DOPA accumulation and DOPAC concentrations in the intermediate lobe but not the neural lobe. The results of the present study demonstrate that: (1) elimination of DOPAC from the intermediate lobe and neural lobe is rapid and alterations in DOPAC concentrations reflect dynamic changes in metabolism of DA; (2) DA which is released and recaptured is a minor contributor to DOPAC concentrations; and (3) alterations in the activity of tuberohypophysial DA neurons are accompanied by corresponding changes in DOPAC concentrations.(ABSTRACT TRUNCATED AT 250 WORDS)

Pure uptake blockers of dopamine can reduce prolactin secretion: studies with diclofensine

The effects of diclofensine, a pure dopamine (DA) uptake inhibitor on 1) 3H-DA uptake in rat arcuate-periventricular nucleus-median eminence synaptosomes, 2) basal and K+-evoked endogenous DA release from tuberoinfundibular dopaminergic (TIDA) neurons and 3) in vivo prolactin (PRL) secretion were studied. Diclofensine, in concentrations of 0.01, 0.1 and 1 microM caused a marked decrease of 3H-DA uptake. In addition, it was unable to stimulate basal endogenous DA release which, on the contrary, was elicited by d-amphetamine in the same concentration (50 microM). On the other hand, diclofensine (50 microM) caused a 3 fold enhancement of K+-evoked DA release. Finally, the compound, when administered in vivo to male rats, significantly reduced basal serum PRL levels. The results of the present study seem to indicate that the pharmacological blockade of DA uptake in TIDA neurons is a condition sufficient to cause a reduction of PRL release.

Electrical stimulation of the arcuate nucleus increases the metabolism of dopamine in terminals of tuberoinfundibular neurons in the median eminence

The purpose of the present study was to examine the effects of procedures that alter impulse flow in tuberoinfundibular dopamine (DA) neurons on the metabolism of DA in the median eminence and on the secretion of prolactin from the anterior pituitary. Twenty min following the administration of gamma-butyrolactone (GBL, 1000 mg/kg, i.p.) there was a marked decrease in 3,4-dihydroxyphenylacetic acid (DOPAC) concentrations in the median eminence and an increase in prolactin concentrations in the serum, indicating that a decrease in activity of tuberoinfundibular DA neurons is accompanied by a decrease in DA metabolism in the median eminence and a loss of tonic inhibition of pituitary prolactin secretion. Activation of tuberoinfundibular DA neurons by bilateral stimulation of the arcuate nucleus in GBL-treated rats produced a rapid increase in median eminence DOPAC concentrations and a time-dependent decrease in serum prolactin concentrations. Nomifensine (25 mg/kg, i.p., 30 min), a DA uptake inhibitor, had no effect on median eminence DOPAC concentrations in sham- or arcuate nucleus-stimulated rats, indicating that regardless of the level of activity of tuberoinfundibular neurons, very little DA is recaptured and metabolized in terminals of these neurons in the median eminence. Taken together these results reveal that alterations in impulse flow in tuberoinfundibular DA neurons are accompanied by corresponding changes in the metabolism of DA in the median eminence.