Stress, prolactin and hypothalamic dopaminergic neurons

Most of what is known about dopamine (DA)-containing neurons in the brain has been learned from studies on the nigrostriatal system, although it is misleading to consider a neuron in this system as a "model" for other DA neurons in the brain. In this presentation the properties of nigrostriatal DA neurons are compared with those hypothalamic DA neurons that comprise the tuberoinfundibular system. These latter neurons, which have cell bodies in the arcuate nucleus and short axons which terminate in the median eminence, function to inhibit the release of prolactin from the anterior pituitary. The activities of the nigrostriatal and tuberoinfundibular DA neurons were estimated biochemically by measuring the rates of synthesis (accumulation of DOPA after the administration of a decarboxylase inhibitor, NSD 1015), turnover (decline of DA after the administration of a tyrosine hydroxylase inhibitor, alpha-methyltyrosine) and metabolism (concentrations of dihydroxyphenylacetic acid) of DA in regions of the brain that contain the terminals of these neurons (striatum and median eminence, respectively). Tuberoinfundibular DA neurons differ from nigrostriatal DA neurons in that the former neurons: (1) are not directly regulated by DA receptor-mediated mechanisms, (2) are stimulated by prolactin, (3) exhibit a sexual difference with activity being 2-3 times greater in females and (4) are inhibited by afferent neuronal circuits that are activated by suckling and restraint stress.

A small hydrophobic domain anchors leader peptidase to the cytoplasmic membrane of Escherichia coli

Leader peptidase is an enzyme of the Escherichia coli cytoplasmic membrane which removes amino-terminal leader sequences from many secreted and membrane proteins. Three potential membrane-spanning segments exist in the first 98 amino acids of leader peptidase. We have characterized the topology of leader peptidase based on its sensitivity to protease digestion. Proteinase K and trypsin treatment of right-side-out inner membrane vesicles and spheroplasts yields protected fragments of approximately 80 and 105 amino acid residues, respectively. We have shown that both fragments are derived from the amino terminus of the protein and that the smaller protected peptide can be derived from the larger. Removal of the third potential membrane-spanning segment (residues 82-98) does not affect the size of the proteinase K-protected fragment but does reduce the size of the trypsin-protected peptide. Because the proteinase K-protected fragment is about 9000 daltons, is derived from the amino terminus of leader peptidase, and its size is not affected when amino acids 82-98 are removed from the protein, it must extend from the amino terminus to approximately residue 80. Likewise, the trypsin-protected fragment must extend from the amino terminus to about residue 105. These data suggest a model for the orientation of leader peptidase in which the second hydrophobic stretch (residues 62-76) spans the cytoplasmic membrane and the third hydrophobic stretch resides in the periplasmic space.

Restraint stress decreases the neurosecretory activity of tuberoinfundibular dopaminergic neurons in young but not in aged female rats

The regulation of prolactin secretion by tuberoinfundibular dopamine (DA) neurons appears to be altered in the aged rat: the concentration of prolactin in the serum increases and the activity of the tuberoinfundibular DA neurons decreases. In the young female rat a brief period of stress reduces the tuberoinfundibular DA neurosecretory activity and increases the secretion of prolactin. The purpose of the present study was to determine if the responsiveness of tuberoinfundibular DA neurons to restraint stress is altered in the aged female rat. The activity of these neurons was estimated from the rate of DA synthesis in their terminals in the median eminence, as measured by the rate of accumulation of dihydroxyphenylalanine (DOPA) after the administration of a decarboxylase inhibitor. Thirty minutes of restraint stress increased serum prolactin concentrations in both young (3 months) and aged (26 months) constant estrous rats, but reduced the rate of DOPA accumulation in the median eminence of only the young rats. Restraint also decreased the rate of DOPA accumulation in the median eminence of intermediate-aged rats (14 months) independently of whether the rats were exhibiting normal ovarian cycles (measured on the day of estrus) or were in a constant estrus. This suggests that the loss of ovarian cyclicity per se is not associated with the age-related change in the response of tuberoinfundibular DA neurons to restraint stress.(ABSTRACT TRUNCATED AT 250 WORDS)

Differential action of bromocriptine on nigrostriatal versus mesolimbic dopaminergic neurons

The present study was undertaken to compare the abilities of the dopaminergic agonists apomorphine, bromocriptine, and lergotrile to inhibit the synthesis of dopamine (DA) in terminals of nigrostriatal and mesolimbic DA neurons. The in vivo synthesis of DA was estimated by measuring the rate of accumulation of dihydroxyphenylalanine (DOPA) in terminals of nigrostriatal (striatum) and mesolimbic (nucleus accumbens, olfactory tubercle) neurons 30 min after the administration of NSD 1015, a decarboxylase inhibitor. The activation of DA autoreceptors in these regions was evaluated by measuring the abilities of the DA agonists to inhibit DA synthesis in brain regions of rats pretreated with gamma-butyrolactone (GBL). Apomorphine (0.03-1.0 mg/kg for 45 min) and bromocriptine (0.1-10 mg/kg for 90 min) produced dose-dependent decreases in the rate of DA synthesis in all three brain regions of both vehicle- and GBL-treated rats. A time course of the effects of the highest dose of bromocriptine (10 mg/kg), however, demonstrated dramatic regional differences in the ability of this drug to inhibit DA synthesis in saline-versus GBL-pretreated rats. Bromocriptine inhibited the GBL-induced increase in DA synthesis for 6 hours in all regions examined. In the striatum of saline-treated rats the decrease in DA synthesis was evident only at 1.5 hours after bromocriptine administration, while in the nucleus accumbens and olfactory tubercle DA synthesis remained inhibited for 6 hours. By contrast, lergotrile reduced DA synthesis to a similar extent in all three regions for at least 6 hours in both vehicle- and GBL-treated rats. These results suggest that there is no regional difference in the ability of bromocriptine to inhibit DA synthesis via DA autoreceptor mechanisms, but there appear to be differences in postsynaptic DA receptor-mediated mechanisms which regulate nigrostriatal versus mesolimbic DA neurons.

Responsiveness of tuberoinfundibular dopamine neurons in the aged female rat to the stimulatory actions of prolactin

The autoregulatory feedback control of prolactin, which is mediated by tuberoinfundibular dopamine (DA) neurons, is altered in the aged rat; this is evidenced by increased circulating concentrations of prolactin and decreased activity of these neurons. In the present study the action of prolactin on tuberoinfundibular DA neurons in young and aged female rats was estimated by measuring the rate of DA synthesis (dopa accumulation following the administration of a decarboxylase inhibitor) in the median eminence. The rate of dopa accumulation in the median eminence of the aged (26 months) rat was reduced to 50-60% of that in the young (3 months) rat. The acute systemic administration of haloperidol, a DA antagonist which increases serum concentrations of prolactin or intracerebroventricular infusions of prolactin increased the rate of dopa accumulation in the median eminence of both young and aged rats by the same relative amount. The administration of haloperidol and prolactin increased the rate of DA synthesis to a greater extent in young than in aged rats. The administration of bromocriptine, a DA agonist which reduces serum concentrations of prolactin, decreased the rate of dopa accumulation in the median eminence of both young and aged rats. In young animals the intracerebroventricular administration of prolactin reversed the bromocriptine-induced decrease in DA synthesis in the median eminence after 4 h and caused a further increase after 12 h. Qualitatively similar effects were seen in the aged rats; however, prolactin-treated young rats had much higher levels of DA synthesis than aged rats.(ABSTRACT TRUNCATED AT 250 WORDS)

5-Hydroxytryptamine is synthesized in neurons terminating in the neural and intermediate lobes of the rat pituitary gland

Immunocytochemistry has revealed that nerve fibers within the neural and intermediate lobes of the rat pituitary gland contain 5-hydroxytryptamine (5-HT). Recent anatomical evidence suggests that the content of this amine in the intermediate but not the neural lobe of the pituitary gland may represent 5-HT that has been taken up from the blood rather than synthesized intraneuronally. The purpose of this study was to determine if 5-HT is synthesized in neurons of the neurointermediate lobe of the pituitary gland. 5-HT synthesis was estimated by measuring the accumulation of the 5-HT precursor, 5-hydroxytryptophan (5-HTP), in the neurointermediate lobe of male Long-Evans rats following the administration of NSD 1015, an inhibitor of aromatic L-amino acid decarboxylase. Thirty min following the injection of NSD 1015 (100 mg/kg, i.p.), 5-HTP accumulated in the neurointermediate lobe and the rate of this accumulation was increased by the administration of the 5-HTP precursor, tryptophan, and by electrical stimulation of the pituitary stalk. In addition, repeated injections of the 5-HT uptake inhibitor, fluoxetine (10 mg/kg, i.p., every 12 h for a total of 7 injections), induced a marked depletion of platelet 5-HT but did not alter the concentration of 5-HT in either the neural or intermediate lobes of the pituitary gland. Taken together these results indicate that much of the 5-HT in the neurointermediate lobe of the pituitary gland does not represent 5-HT taken up from the blood, but rather the amine is synthesized in neurons projecting to this region.(ABSTRACT TRUNCATED AT 250 WORDS)

Interactions between prolactin and dopaminergic neurons

The secretion of prolactin from the adenohypophysis is tonically inhibited by dopamine that is released into the hypophysial portal blood from terminals of tuberoinfundibular neurons located in the external layer of the median eminence. These tuberoinfundibular neurons are unique among other dopaminergic neurons in the brain (including the well-characterized nigrostriatal neurons) in that they are not directly regulated by dopaminergic receptor-mediated mechanisms, but instead are selectively responsive to changes in prolactin concentrations in blood and cerebrospinal fluid. In the rat, the activity of the tuberoinfundibular dopaminergic neurons is higher in the female than in the male, exhibits a characteristic cyclical pattern during the first half of pregnancy and is constantly high as a result of stimulation by placental lactogen during the last 9 days of pregnancy, and is reduced in lactating animals and acutely inhibited during suckling.

Neurochemical characterization of the actions of 5-amino-2,4- dihydroxy-alpha-methylphenylethylamine (5-ADMP): a selective neurotoxin to central noradrenergic neurons

The concentrations of amine neurotransmitters [dopamine (DA), norepinephrine (NE) and 5-hydroxytryptamine] and some of their deaminated metabolites [dihydroxyphenylacetic acid and 5-hydroxyindoleacetic acid] were determined in selected regions of the rat brain, including several hypothalamic nuclei, at various times after the i.c.v. injection of various doses of 5-amino-2,4-dihydroxy-alpha-methylphenylethylamine (5-ADMP). Seven days after a single i.c.v. injection of 100 micrograms of 5-ADMP the concentration of NE in all brain regions examined was reduced markedly, as was the concentration of DA in the median eminence. The concentrations of DA and dihydroxyphenylacetic acid in regions other than the median eminence, and the concentrations of 5-hydroxytryptamine and 5-hydroxyindoleacetic acid in all regions analyzed were unaltered. Multiple injections of 5-ADMP (100 micrograms i.c.v. every 48 hr X 3) did not increase the NE depletion, but caused slight decreases in DA and 5-hydroxytryptamine in some regions. The NE concentrations in hypothalamic nuclei were reduced significantly 4 hr after 100 micrograms i.c.v. administration of 5-ADMP and this depletion was maintained for at least 28 days. The 5-ADMP-induced decline of NE in all brain region, but not the decline of DA in the median eminence, was prevented partially by pretreatment with desipramine. These results indicate that 5-ADMP is a relatively specific neurotoxin for NE neurons, being particularly effective in destroying NE nerve terminals in the hypothalamus.

Effects of gonadal steroids on tuberoinfundibular and tuberohypophysial dopaminergic neuronal activity in male and female rats

The activities of tuberoinfundibular and tuberohypophysial dopamine (DA) neurons were estimated by measuring the turnover of DA in terminals of these neurons in the median eminence and in the neural and intermediate lobes of the pituitary, respectively. The rate of DA turnover (alpha-methyltyrosine-induced decline of DA) in the median eminence was two to three times faster in females than in males, but no sexual differences in DA turnover rates were noted in the neural and intermediate lobes. Two weeks following gonadectomy the rate of DA turnover in the median eminence was increased in the male but decreased in the female. These effects were reversed by testosterone and estrogen replacement in gonadectomized males and females, respectively. Neither gonadectomy nor steroid replacement altered DA turnover in the neural or intermediate lobes of either males or females. These results indicate that estrogen stimulates and testosterone inhibits tuberoinfundibular DA neuronal activity while neither steroid affects tuberohypophysial DA neuronal activity.

A comparison of biochemical indices of 5-hydroxytryptaminergic neuronal activity following electrical stimulation of the dorsal raphe nucleus

The activity of 5-hydroxytryptaminergic neurons has been estimated from measurements of: concentrations of 5-hydroxyindoleacetic acid; the ratio of the concentrations of 5-hydroxyindoleacetic acid to 5-hydroxytryptamine; the rate of accumulation of 5-hydroxytryptophan following the administration of an aromatic L-amino acid decarboxylase inhibitor (e.g., NSD 1015); the rate of accumulation of 5-hydroxytryptamine, and the rate of decline of 5-hydroxyindoleacetic acid following the administration of a monoamine oxidase inhibitor (e.g., pargyline). The purpose of the present study was to compare these different methods under conditions of changing neuronal impulse traffic produced by electrical stimulation of 5-hydroxytryptaminergic neurons. Male rats anesthetized with chloral hydrate were killed following 0, 15, or 30 min of electrical stimulation of the dorsal raphe nucleus at a frequency of 0, 5, or 10 Hz. The concentrations of 5-hydroxytryptamine, 5-hydroxyindoleacetic acid, and 5-hydroxytryptophan in nucleus accumbens, amygdala, suprachiasmatic nucleus, and dorsomedial nucleus were measured using HPLC coupled to an electrochemical detector. In each brain region, stimulation elicited an increase in the concentration of 5-hydroxyindoleacetic acid and the 5-hydroxyindoleacetic acid/5-hydroxytryptamine concentration ratio in saline-treated animals and an increase in 5-hydroxytryptophan accumulation in NSD 1015-treated animals, but did not alter the concentration of 5-hydroxytryptamine or 5-hydroxyindoleacetic acid in pargyline-treated rats. The results o f this study indicate that although the first three methods serve as valid indices of 5-hydroxytryptaminergic neuronal activity, the pargyline-dependent techniques are not responsive to changes in the rate of 5-hydroxytryptamine nerve firing.

Exogenous tryptophan increases synthesis, storage, and intraneuronal metabolism of 5-hydroxytryptamine in the rat hypothalamus

The effects of tryptophan administration on neurochemical estimates of synthesis [5-hydroxytryptophan (5-HTP) accumulation following administration of a decarboxylase inhibitor], storage [5-hydroxytryptamine (5-HT) concentrations], and metabolism [5-hydroxyindoleacetic acid (5-HIAA) concentrations] of 5-HT in selected regions of the hypothalamus were determined using HPLC coupled to an electrochemical detector. Tryptophan methyl ester HCl (30-300 mg/kg i.p.) produced a dose-dependent increase in the rate of 5-HTP accumulation throughout the hypothalamus but had no effect on the rate of accumulation of 3,4-dihydroxyphenylalanine. Peak 5-HTP levels were attained by 30 min following administration of tryptophan (100 mg/kg i.p.) and were maintained for an additional 60 min. Tryptophan also produced concomitant dose-dependent increases in 5-HT and 5-HIAA concentrations in these same regions without changes in the 5-HIAA/5-HT ratio. These results indicate that exogenous tryptophan administration selectively increases the synthesis, storage, and metabolism of 5-HT in the hypothalamus without altering the synthesis of catecholamines. Inhibition of 5-HT uptake with chlorimipramine or fluoxetine produced modest (10-40%) reductions in 5-HIAA concentrations throughout the hypothalamus, revealing that only a minor portion of 5-HIAA is derived from released and recaptured 5-HT, whereas the major portion of this metabolite reflects intraneuronal metabolism of unreleased 5-HT. In both chlorimipramine- and fluoxetine-treated rats, 5-HIAA concentrations were significantly increased by tryptophan administration, indicating that the increase in synthesis of 5-HT following precursor loading is accompanied by an increase in the intraneuronal metabolism of 5-HT.

Effects of acute and chronic bupropion on locomotor activity and dopaminergic neurons

Acute administration of bupropion (10 or 30 mg/kg) to rats increased locomotor activity in a dose-related manner. The highest dose increased the dopamine (DA) concentration while both doses reduced the concentration of dihydroxyphenylacetic acid (DOPAC) in the striatum. The enhancement of locomotor activity and the decrease of striatal DOPAC concentrations were increased with chronic administration (up to 40 days) of bupropion. The rate of DA synthesis in the striatum was increased by the acute administration of d-amphetamine but was not altered by acute or chronic administration of bupropion.

Comparative effects of the neurotoxins N-chloroethyl-N-ethyl-2-bromobenzylamine hydrochloride (DSP4) and 6-hydroxydopamine on hypothalamic noradrenergic, dopaminergic and 5-hydroxytryptaminergic neurons in the male rat

The intracerebroventricular (i.c.v.) administration of 6-hydroxydopamine (6-OHDA; 50 micrograms X 3) and the systemic administration of DSP4 (50 mg/kg X 2; i.p.), alone and in combination, were compared for their abilities to alter the concentrations of norepinephrine (NE), dopamine (DA), dihydroxyphenylacetic acid (DOPAC) and 5-hydroxytryptamine (5-HT) in selected hypothalamic and extra-hypothalamic (striatum, frontal cortex, hippocampus) regions of the male rat brain. DSP4 markedly lowered NE concentrations in extrahypothalamic regions, and within the hypothalamus produced a mild and variable reduction of NE without altering concentrations of DA, DOPAC or 5-HT. 6-OHDA markedly lowered NE concentrations in all brain regions, but was without effect on DA, DOPAC and 5-HT concentrations in any region analyzed. Combined treatment with DSP4 and 6-OHDA did not produce additional effects on levels of NE, DA and DOPAC over either drug alone, but did cause a mild reduction of 5-HT in several brain regions. These results indicate that systemic treatments with DSP4 per se are not as effective as i.c.v. 6-OHDA in depleting NE in the hypothalamus, and that when the two neurotoxins are administered there appears to be some destruction of 5-HT neurons.

Comparison of the effects of castration and steroid replacement on incertohypothalamic dopaminergic neurons in male and female rats

The activities of incertohypothalamic (IH) and tuberoinfundibular (TI) dopamine (DA) neurons were compared in selected brain regions of male and female rats by measuring the rate of DA turnover (alpha-methyltyrosine-induced decline in brain DA concentrations). The rates of DA turnover in regions containing TIDA (median eminence) and rostral IHDA (rostral periventricular and medial preoptic nuclei) neurons were greater in diestrous females than in intact males. In contrast, the rate of DA turnover in the caudal IHDA neurons (medial zona incerta), was greater in intact males than diestrous females. These results indicate that the activities of IHDA neurons, like those of TIDA neurons, differ between the sexes but that the sexual differentiation of IHDA neurons is not homogeneous. Two weeks following orchidectomy, the rates of DA turnover were increased in the median eminence and decreased in the medial preoptic nucleus. Testosterone replacement in orchidectomized males produced opposite effects, causing a decrease in DA turnover in the median eminence and an increase in the medial preoptic nucleus. In female rats, the rates of DA turnover were decreased in the median eminence and medial zona incerta and increased in the medial preoptic nucleus 2 weeks following ovariectomy. Only in the median eminence did 2 days of estrogen replacement in ovariectomized rats produce effects opposite those seen after ovariectomy alone. These data show that the activities of IHDA neurons, as estimated from measurements of DA turnover, can be altered by the removal and replacement of the gonadal steroids.

The rapid 'tonic' and the delayed 'induction' components of the prolactin-induced activation of tuberoinfundibular dopaminergic neurons following the systemic administration of prolactin

The present study was designed to characterize the time and dose relationships of the response of tuberoinfundibular dopaminergic (TIDA) neurons in the rat to systemically administered prolactin (PRL). The activity of TIDA neurons was estimated by measuring the rate of dopamine (DA) synthesis in the median eminence (DOPA accumulation following the administration of a decarboxylase inhibitor). Rats were pretreated with bromocriptine, a dopaminergic agonist, so as to inhibit the release of endogenous PRL from the anterior pituitary, and thereby reduce the activity of TIDA neurons to a 'basal' level from which it could be increased subsequently by exogenously administered PRL. In control animals an intraperitoneal injection of ovine PRL (oPRL) increased DOPA accumulation at 16 h, but not before, whereas in bromocriptine-pretreated animals an intraperitoneal injection of oPRL increased DOPA accumulation after 4 h ('tonic' component) and caused a further increase after 16 h ('induction' component). Continuous intravenous infusions of oPRL into bromocriptine-pretreated rats increased DOPA accumulation in the median eminence by 4 h, and when infused into control rats oPRL reduced serum concentrations of endogenous rat PRL (rPRL) by 2 and 4 h. Continuous intravenous infusions of rPRL increased DOPA accumulation in the median eminence after 2 h; this effect exhibited a very steep dose-response relationship (possibly an 'all-or-none' response). TIDA neurons were very sensitive to changes in circulating concentrations of PRL; their activity was increased if serum PRL concentrations were merely doubled by infusing a low concentration of rPRL for 4 h. Three daily injections of haloperidol elevated circulating rPRL concentrations and increased the rate of DOPA accumulation in the median eminence.(ABSTRACT TRUNCATED AT 250 WORDS)

Acute effects of morphine on neurochemical estimates of activity of incertohypothalamic dopaminergic neurons in the male rat

The effects of an acute injection of morphine on the activities of mesotelencephalic, tuberoinfundibular and incertohypothalamic dopamine (DA) neurons was estimated by measuring: the rate of turnover of DA (decline after alpha-methyltyrosine); and the concentration of the DA metabolite, dihydroxyphenylacetic acid (DOPAC), in brain regions containing cell bodies or terminals of these neurons (i.e. nucleus accumbens, striatum, median eminence and various hypothalamic nuclei). The rate of turnover of DA and the concentration of DOPAC were increased in nucleus accumbens and striatum and decreased in the median eminence 60 min after the administration of morphine (10 mg/kg, s.c.). Morphine increased the rate of turnover of DA and the concentration of DOPAC in brain regions containing both cell bodies (periventricular nucleus and medial zona incerta) and terminals (medial preoptic, preopticosuprachiasmatic and dorsomedial nuclei) of incertohypothalamic DA neurons. The effects of morphine in all brain regions were blocked by pretreatment with naltrexone. These results indicate that incertohypothalamic DA neurons are stimulated by the acute administration of morphine, and in this respect they resemble the extrahypothalamic mesotelencephalic DA neurons rather than hypothalamic tuberoinfundibular DA neurons.

Acute restraint stress decreases tuberoinfundibular dopaminergic neuronal activity: evidence for a differential response in male versus female rats

The basal activity o f tuberoinfundibular dopaminergic (TIDA) neurons is higher and the response of these neurons to the stimulatory actions of prolactin is greater in the female than in the male rat. In the female rat, the restraint-stress-induced increase in serum prolactin concentrations is accompanied by a concurrent decrease in the activity of TIDA neurons. The purpose of the present study was to compare these effects of restraint in male and female rats. TIDA neuronal activity was estimated by measuring the rate of dopamine (DA) synthesis (DOPA accumulation after the administration of a decarboxylase inhibitor, NSD 1015) and the rate of DA turnover (decline of DA after administration of a tyrosine hydroxylase inhibitor; alpha-methyltyrosine) in the median eminence. Thirty minutes of restraint increased serum prolactin concentrations in both male and female rats, but a greater response was observed in the females. Restraint also decreased the rates of synthesis and turnover of DA in the median eminence of the female but not the male rat. The difference in the response of TIDA neurons in male and female rats to restraint is not the consequence of neuronal differentiation resulting from neonatal androgen exposure, because restraint aso decreased the activity of TIDA neurons in androgen-sterilized female rats. The inability of restraint stress to reduce TIDA neuronal activity in the male rat appears to be the consequence of testosterone, since TIDA neurons were responsive to restraint following castration of the males.(ABSTRACT TRUNCATED AT 250 WORDS)

Acute restraint stress decreases dopamine synthesis and turnover in the median eminence: a model for the study of the inhibitory neuronal influences on tuberoinfundibular dopaminergic neurons

The effects of acute stress on serum prolactin concentrations and tuberoinfundibular dopaminergic (TIDA) neuronal activity were studied in female rats. TIDA neuronal activity was estimated by measuring the rate of dihydroxyphenylalanine (DOPA) accumulation after the administration of a decarboxylase inhibitor (NSD 1015) and the rate of decline of dopamine (DA) after the administration of a tyrosine hydroxylase inhibitor (alpha-methyltyrosine) in the median eminence. Serum prolactin concentrations were increased following 30 min of supine immobilization (restraint stress), but returned to control levels by 2, 8, and 16 h after the onset of this stress. The rate of DOPA accumulation was decreased during the 30 min of restraint; it was still further reduced 2 h later but had returned to control levels 8 and 16 h later. No change in the rate of DOPA accumulation was observed in the striatum or neurointermediate lobe of the pituitary at any time after the start of restraint. Restraint stress also decreased the rate of DA turnover in the median eminence, but was without effect on the rates of DA turnover in the striatum or neurointermediate lobe. These results suggest that restraint stress activates an inhibitory neuronal pathway which decreases the activity of TIDA neurons and may be responsible, at least in part, for the increase in serum prolactin concentrations. The responsiveness of TIDA neurons to the stress-induced decrease in activity was not influenced by the time of day or the stage of the estrous cycle. Not all stressful manipulations decreased TIDA neuronal activity.(ABSTRACT TRUNCATED AT 250 WORDS)

Differential effects of morphine on the rates of dopamine turnover in the neural and intermediate lobes of the rat pituitary gland

The acute administration of morphine to male rats decreased the rate of dopamine turnover in the median eminence and in the neural lobe of the pituitary, but was without effect in the intermediate lobe of the pituitary. Pretreatment with the opiate antagonist, naltrexone, reduced the effects of morphine. These results indicate that morphine, by acting on opiate receptors, inhibits the activity of tuberoinfundibular dopaminergic neurons that terminate in the median eminence and those tuberohypophysial dopaminergic neurons that terminate in the neural lobe of the pituitary.

Hypoprolactinemia induced by hypophysectomy and long-term bromocriptine treatment decreases tuberoinfundibular dopaminergic neuronal activity and the responsiveness of these neurons to prolactin

The effect of long-term decreases in circulating concentrations of prolactin was determined on the responsiveness of tuberoinfundibular dopamine (DA) neurons to this hormone. The activity of these neurons in ovariectomized rats was estimated by measuring the rate of DA synthesis (DOPA accumulation after the administration of a decarboxylase inhibitor) in the median eminence at various times after serum concentrations of prolactin had been reduced by hypophysectomy or the chronic administration of a DA agonist (bromocriptine, 3 mg/kg/day). The concentration of DA in the median eminence, but not in striatum, declined progressively up to 12 days after hypophysectomy, but did not change at any time during bromocriptine treatment. On the other hand, norepinephrine concentrations in the median eminence were increased 12 days after both treatments. Within 24 h after hypophysectomy or the first injection of bromocriptine the rate of DA synthesis in the median eminence was decreased; this decrease was maintained for at least 12 days suggesting that tuberoinfundibular DA neuronal activity is normally maintained by endogenous prolactin. Intracerebroventricular (ICV) injections of prolactin (10 micrograms, 12 h prior to sacrifice) increased the rate of DA synthesis in the median eminence of control, 24-hour hypophysectomized and 24-hour bromocriptine-treated rats. After longer periods (6-12 days) of bromocriptine treatment or after hypophysectomy the responsiveness of tuberoinfundibular DA neurons to prolactin was reduced. Dose-response studies revealed that the sensitivity and magnitude of response to ICV prolactin was markedly reduced in 12-day hypophysectomized rats.(ABSTRACT TRUNCATED AT 250 WORDS)

Adenohypophysial dopamine content and prolactin secretion in the aged male and female rat

In male rats the concentrations of dopamine (DA) in the adenohypophysis and of PRL in serum increase progressively with age. The increase in the DA content is not a consequence of reduced metabolism of DA, since the activities of the enzymes that deaminate (monoamine oxidase) and O-methylate (catechol-O-methyltransferase) this amine are not reduced in the anterior pituitary of aged male rats; indeed, both type A and B monoamine oxidase activities are increased in the pituitaries of aged rats. The increased DA present in the aged adenohypophysis is not in a static pool but, as in young rats, is rapidly decreased by pharmacological treatments that reduce the activity of tuberoinfundibular DA neurons (gamma-butyrolactone), reduce the synthesis of DA in these neurons (alpha-methyltyrosine), or block DA receptors (haloperidol) in the adenohypophysis. All of these treatments increased serum PRL levels and reduced DA concentrations in the adenohypophysis of both young and aged rats. Similarly, in female rats, where age-related differences in BW and body composition are minimized, serum PRL concentrations and DA concentrations in the adenohypophysis of both young and aged rats changed in the same way after the administration of a DA agonist (apomorphine) or a DA antagonist (haloperidol). Apomorphine reversed the alpha-methyltyrosine-induced increase in serum PRL concentrations and the decrease in anterior pituitary DA content in both young and aged rats, although the latter animals appear to be more responsive to this drug. Similarly, aged rats were more sensitive to the increase in serum PRL concentrations and the decrease in anterior pituitary DA content caused by haloperidol. The actions of acute and chronically administered haloperidol are reversible, with the reduced DA content of the adenohypophysis and the elevated serum concentrations of PRL returning to respective pretreatment levels in both young and aged rats once treatment stops. The time course for recovery of serum PRL levels and anterior pituitary DA contents after both the acute and chronic haloperidol treatment is of longer duration in the aged rat, which is consistent with a decreased rate of clearance of haloperidol in these animals. Twelve daily injections of bromocriptine, a DA agonist, reduced serum PRL and anterior pituitary DA concentrations in both young and aged rats, and these effects persisted for up to 7 days after the injections were stopped.(ABSTRACT TRUNCATED AT 400 WORDS)

Lick-trading by rats: on the substitutability of dry, water, and saccharin tubes

Thirsty rats licked two metal tubes: a water tube paired with another water tube, with saccharin, or with a dry tube. For each pair, a multipoint baseline function was measured by offering free access to one tube throughout each session, and free or restricted access to the other. The three resulting baseline functions showed the members of each pair to be mutual substitutes: When access to either tube was restricted, the rats made more licks at the other. A linear function identified the two water tubes as perfect substitutes. Convex functions identified the members of the saccharin-water and the dry-water pair as imperfect substitutes. Each pair was also tested under several reciprocal fixed-ratio schedules that required instrumental licking of either tube for contingent access to the other. The resulting schedule functions showed the members of each pair to be perfect substitutes: Water licks decreased linearly as licks at the other water tube, the saccharin, or the dry tube increased, in agreement with a conservation model of instrumental performance. Baseline and schedule functions, indistinguishable in the water-water pair, indicated a schedule facilitation of dry-tube licking in the dry-water pair and of water-tube licking in the saccharin-water pair.

Differential regulation of tuberohypophysial dopaminergic neurons terminating in the intermediate lobe and in the neural lobe of the rat pituitary gland

In order to characterize the properties of tuberohypophysial dopaminergic neurons which terminate in the intermediate (IL) and neural (NL) lobes of the pituitary gland a technique was developed which permitted the selective dissection of the rat pituitary into its three distinct lobes (NL, IL and anterior lobe, AL). The success of the dissection was evaluated histologically and biochemically by measuring the distribution of peptide hormones characteristic of the dissected regions. As would be predicted, prolactin was found almost exclusively in the AL, arginine-vasopressin in the NL and alpha-melanotropin in the IL. Over two-thirds of total immunoreactive beta-endorphin was located in the IL and less than 30% was found in the AL. The concentration of dopamine (DA) was greater in the IL than in the NL, but the rate of turnover of the amine was approximately the same suggesting that the basal activity of tuberohypophysial DA neurons is similar in both regions. On the other hand, the turnover of DA in the IL, but not NL, was increased following the administration of a DA antagonist (haloperidol) and decreased following a DA agonist (bromocriptine). Thus, the activity of DA neurons terminating in the IL is regulated, at least in part, by DA receptor-mediated mechanisms and in this regard these neurons resemble DA neurons terminating in the nucleus accumbens and striatum. Since DA turnover in NL was not altered by the administration of haloperidol or bromocriptine it is proposed that these neurons lack DA receptor-mediated regulatory mechanisms and thus resemble tuberoinfundibular DA neurons terminating in the median eminence.

Simultaneous determination of 5-hydroxytryptophan and 3,4-dihydroxyphenylalanine in rat brain by HPLC with electrochemical detection following electrical stimulation of the dorsal raphe nucleus

HPLC coupled with electrochemical detection was used to make concurrent measurements of the rate of accumulation of 5-hydroxytryptophan and 3,4-dihydroxyphenylalanine in selected brain regions (striatum, nucleus accumbens, septum, medial periventricular hypothalamus) and thoracic spinal cords of rats treated with NSD 1015, an inhibitor of aromatic-L-amino-acid decarboxylase. 5-Hydroxytryptophan and 3,4-dihydroxyphenylalanine accumulated in all brain regions 30 min after the intravenous infusion of various doses of NSD 1015; there were no significant differences in the responses to 12.5, 25, 50, and 100 mg/kg. After the intravenous administration of 25 mg/kg NSD 1015 the concentrations of 5-hydroxytryptophan and 3,4-dihydroxyphenylalanine increased linearly with time in all brain regions for at least 30 min. Electrical stimulation of 5-hydroxytryptamine neurons in the dorsal raphe nucleus for 30 min at 5 or 10 Hz increased 5-hydroxytryptophan accumulation in all brain regions but not in the spinal cord. Unexpectedly, this stimulation also increased the accumulation of 3,4-dihydroxyphenylalanine in the hypothalamus and spinal cord. These results suggest that 5-hydroxytryptophan accumulation following the administration of NSD 1015 is a valid index of 5-hydroxytryptamine neuronal activity in the brain.

Effects of estradiol and prolactin on incertohypothalamic dopaminergic neurons in the male rat

The activity of incertohypothalamic, tuberoinfundibular, mesolimbic and nigrostriatal dopamine (DA) neurons was estimated by measuring the rate of turnover of DA (decline of DA after alpha-methyltyrosine) in brain regions containing the cell bodies and terminals of these neurons. Estradiol benzoate (25 micrograms/kg/day X 3; s.c.), markedly increased the concentration of prolactin in plasma and increased the rate of turnover of DA in median eminence, but failed to alter DA neuronal activity in the striatum or nucleus accumbens. An intracerebroventricular injection of prolactin (10 micrograms/rat, 12 h) mimicked the effect of estradiol on the turnover of DA in the median an eminence. Within the incertohypothalamic DA system, both estradiol and prolactin increased DA turnover in the dorsomedial nucleus but decreased DA turnover in the rostral periventricular, medial preoptic and preoptico-suprachiasmatic nuclei. Hypophysectomy decreased the tonic activity of tuberoinfundibular DA neurons, but failed to alter the activity of incertohypothalamic DA neurons. The stimulatory effects of estrogen on DA turnover in the median eminence and dorsomedial nucleus, and the inhibitory effects of this hormone on rostral hypothalamus DA neurons were blocked by hypophysectomy. Taken together these results suggest that estradiol induced changes in intact male rats are mediated by prolactin. Estradiol increased incertohypothalamic DA neuronal activity in the medial preoptic and preoptic-suprachiasmatic nuclei of hypophysectomized rats suggesting that in the absence of circulating prolactin, estradiol has a direct stimulatory effect on incertohypothalamic DA neurons in the rostral, periventricular hypothalamus.(ABSTRACT TRUNCATED AT 250 WORDS)

Adenohypophysial dopamine content during physiological changes in prolactin secretion

Studies were undertaken to characterize the relationship between anterior pituitary dopamine (DA) content and changes in PRL secretion that occur during physiological states in which changes in tuberoinfundibular DA neuronal activity have been previously characterized. The results of these studies are summarized as follows. Serum PRL concentrations and the anterior pituitary DA content were not different between male and diestrous female rats, an unexpected result considering that tuberoinfundibular DA neuronal activity in female rats is 2- to 3-fold greater than that in male rats. The surge of PRL secretion during the afternoon of proestrus was accompanied by a significant decrease in the anterior pituitary content of DA. Increases in PRL secretion that occur early in pregnancy and during suckling were accompanied by a decrease in tuberoinfundibular DA neuronal activity and a decrease in the DA content in the anterior pituitary. Restraint stress increased PRL secretion and reduced the anterior pituitary DA content in both male and female rats, although only female rats respond to immobilization with a decrease in tuberoinfundibular DA neuronal activity. In both male and female aged rats, there were increases in serum PRL concentrations and the anterior pituitary DA content, despite the fact that tuberoinfundibular DA neuronal activity is reduced in the aged rat. Dehydration induced by water deprivation was associated with a small increase in serum PRL concentrations and a large increase in anterior pituitary DA content; similar water deprivation regimens have induced no change in indices of tuberoinfundibular DA neuronal activity. These results demonstrate that there is not a simple relationship among physiologically induced changes in PRL secretion, anterior pituitary DA content, and tuberoinfundibular DA neuronal activity. The results also indicate that the content of DA in the anterior pituitary cannot be used as a reliable index of tuberoinfundibular DA neuronal activity.

Long-term treatment with estradiol induces reversible alterations in tuberoinfundibular dopaminergic neurons: a decreased responsiveness to prolactin

Previous studies have demonstrated that short-term (3-5 days) treatment with estradiol increases the rate of turnover and synthesis of dopamine (DA) in terminals of tuberoinfundibular (TI) neurons in the median eminence by virtue of the ability of this hormone to increase circulating concentrations of prolactin. The present studies were undertaken to examine the long-term effects of estradiol on serum prolactin concentrations and TIDA neuronal activity (estimated by the rate of DOPA accumulation in the median eminence after the administration of a decarboxylase inhibitor). Female rats, ovariectomized for 2 weeks, were implanted subcutaneously with silastic capsules containing estradiol benzoate and sacrificed 6, 12 and 18 days after capsule implantation. Serum prolactin concentrations were markedly increased at 6, 12 and 18 days whereas the rate of DOPA accumulation was increased at 6 days but not at 12 days, and was decreased at 18 days. The concentration of DA in the median eminence was reduced at 6 days and further reduced at 12 and 18 days. The low rate of DOPA accumulation in the median eminence despite the high circulating concentrations of prolactin suggests that long-term estradiol treatment reduces the ability of TIDA neurons to respond to prolactin. This was confirmed by the finding that direct intracerebroventricular (icv) injections of prolactin increased the rate of DOPA accumulation in the median eminence of sham-implanted rats but not in 18 day estradiol-treated rats. To determine if the effects of estradiol were reversible, ovariectomized rats were implanted with estradiol-containing capsules for 18 days.(ABSTRACT TRUNCATED AT 250 WORDS)

Pharmacological manipulation of anterior pituitary dopamine content in the male rat: relationship to serum prolactin concentration and lysosomal enzyme activity

The content of dopamine (DA) and the activity of beta-glucuronidase (a marker for lysosomal enzymes) in the anterior pituitary and the concentration of PRL in serum were determined in male rats that were treated with a variety of drugs that influence the release, storage, or actions of DA. Drugs that increase DA in hypophysial portal blood (amphetamine, methylphenidate and L-dopa) increased DA content, decreased PRL secretion, and had no effect on lysosomal enzyme activity. Drugs that activate DA receptors in the anterior pituitary (apomorphine and bromocriptine) decreased serum PRL but had no effect on DA content or lysosomal enzyme activity. Drugs that decrease DA in the hypophysial portal blood (alpha-methyltyrosine, gamma-butyrolactone, and reserpine) or block DA receptors (haloperidol) increased PRL secretion and decreased DA content and lysosomal enzyme activity. These results suggest that there is no obvious relationship between anterior pituitary DA content and PRL secretion. In addition, lysosomal enzyme activity is not stimulated by increasing the concentration of DA or activating DA receptors in the anterior pituitary, but lysosomal enzyme activity does appear to be tonically stimulated by DA in the control animals since decreasing DA concentrations or receptor activation in the anterior pituitary decreases beta-glucuronidase activity. This latter proposal was confirmed by the demonstration that apomorphine reduced the alpha-methyltyrosine-induced increase in serum PRL and prevented the decrease in anterior pituitary DA content and beta-glucuronidase activity. Furthermore, pretreatment with bromocriptine and L-dopa blocked both the increase in serum PRL concentration and the decrease in anterior pituitary DA content induced by alpha-methyltyrosine and gamma-butyrolactone. On the other hand, bromocriptine and L-dopa blocked the increase in serum PRL concentration but not the reduction in anterior pituitary DA content caused by reserpine, indicating that reserpine has a direct action on DA storage mechanisms in the anterior pituitary. These data suggest that the ability of DA to inhibit PRL secretion and the incorporation of DA into the anterior pituitary are not causally related.

Thyrotropin-releasing hormone and its analog MK-771 increase the cerebroventricular perfusate content of dihydroxyphenylacetic acid

The effects of thyrotropin-releasing hormone (TRH) and its synthetic analog, pyro-2-aminoadipyl-histidyl-thiazolidine-4-carboxamide (MK-771), were determined on the efflux of dihydroxyphenylacetic acid (DOPAC) collected from push-pull cannulae chronically implanted into the lateral cerebral ventricles of rats. Intracerebroventricular and intraperitoneal injections of both peptides increased the efflux of DOPAC. These results suggest that TRH and MK-771 increase the activity of dopaminergic neurons that terminate in periventricular regions.

Effects of behaviorally active doses of thyrotropin-releasing hormone and its analog MK-771 on dopaminergic neuronal systems in the brain of the rat

The effects of thyrotropin-releasing hormone (TRH) and an analog of this hormone, MK-771, were determined on body shaking behavior and on biochemical estimates of the activity of dopamine (DA) neurons in the rat. Both compounds elicited dose-related episodes of "wet-dog shakes". A dose of TRH (20 mg/kg, i.p.) which caused marked shaking behavior did not alter the steady-state concentration of DA in any brain region, but, after an injection of alpha-methyltyrosine did enhance the rate of decline of DA in the nucleus accumbens, but not in the striatum of olfactory tubercle. The same dose of TRH increased the concentration of dihydroxyphenylacetic acid selectively in the nucleus accumbens, and caused a marked increase in the rate of synthesis of DA (accumulation of DOPA after the administration of a decarboxylase inhibitor) in the nucleus accumbens, and a modest and inconsistent increase in the striatum and olfactory tubercle. A single injection of MK-771 (3-100 mg/kg, i.p.) failed to change the rate of synthesis of DA in any brain region, while two injections of this compound (20 mg/kg, i.p.) slightly increased the rate of synthesis of DA in the striatum. These results suggest that TRH selectively increases the activity of DA neurons which terminate in the nucleus accumbens whereas its synthetic analog, MK-771, lacks this property. Since both compounds elicit similar body shaking behavior, it would appear that this behavior is not causally related to the actions of TRH on mesolimbic DA neurons which terminate in the nucleus accumbens.

Dopamine receptor-mediated regulation of incertohypothalamic dopaminergic neurons in the male rat

The incertohypothalamic dopaminergic (DA) neuronal system has been divided into a rostral component of neurons originating in the rostral periventricular nucleus and projecting to the preopticosuprachiasmatic and medial preoptic nuclei and a caudal component originating in the medial zona incerta and projecting to the dorsomedial and anterior hypothalamic nuclei. The purpose of the present study was to determine if the activity of these intrahypothalmic DA neurons is regulated by DA receptor-mediated mechanisms, as are those in the major ascending nigrostriatal and mesolimbic neurons, or if they resemble another group of intrahypothalamic DA neurons, those that comprise the tuberoinfundibular system, which are not responsive to the acute actions of DA agonists or antagonists. The rate of DA turnover (decline after alpha-methyltyrosine) in micropunched regions of the striatum (ST), nucleus accumbens (NA) and hypothalamic regions which contain cell bodies or terminals of incertohypothalamic DA neurons was increased after administration of a DA antagonist (haloperidol) and decreased after administration of a DA agonist (bromocriptine). gamma-Butyrolactone increased DA concentrations in the ST, NA and hypothalamic brain regions containing incertohypothalamic DA neurons, and this effect was blocked by the DA agonist apomorphine. In contrast, none of these treatments affected the concentration or rate of turnover of DA in the median eminence (terminal region of tuberoinfundibular neurons). Injections of either gamma-hydroxybutyric acid or baclofen into the substantia nigra/ventral tegmental region of the midbrain increased DA concentrations in the NA and/or ST but failed to alter DA concentrations in any hypothalamic region. These results suggest that the incertohypothalamic DA system is composed of neurons whose activity can be rapidly modulated by DA receptor-mediated mechanisms and thus resemble the DA neurons in the major ascending nigrostriatal and mesolimbic systems rather than the hypothalamic neurons which comprise the tuberoinfundibular DA system.

Prolactin-induced activation of tuberoinfundibular dopaminergic neurons: evidence for both a rapid 'tonic' and a delayed 'induction' component

Results of previous studies have revealed that prolactin causes a delayed (12-16 h) increase in the rate of synthesis and turnover of dopamine (DA) in terminals of tuberoinfundibular (TI) neurons in the median eminence. Attempts to demonstrate a rapid in vivo action of prolactin on these neurons has been frustrated because pharmacological manipulations needed to make the biochemical measurements of TIDA neuronal activity (i.e., administration of alpha-methyltyrosine or NSD 1015) inhibit DA synthesis and thereby remove the tonic inhibitory control of DA on prolactin secretion. Thus, 'control' rates of synthesis and turnover of DA in terminals of TIDA neurons are actually values obtained in the presence of high circulating concentrations of prolactin. Results of the present in vivo studies demonstrate that there are two components to the activation of TIDA neurons by prolactin: a rapid 'tonic' component, which is responsive to acute changes in prolactin concentrations, and a delayed 'induction' component, which is activated by long-term changes in prolactin concentrations. Experimental observations which support this proposal are described below. Hypophysectomy or treatment with bromocriptine (a DA agonist) reduce circulating levels of prolactin and reduce the rate of DA synthesis in the median eminence. Intracerebroventricular (i.c.v.) administration of prolactin to these animals increases the rate of DA synthesis in the median eminence within 4 h (rapid 'tonic' component) and then causes a further increase after 12 h (delayed 'induction' component); only the latter component is blocked by treatment with cycloheximide, indicating the involvement in protein synthesis.(ABSTRACT TRUNCATED AT 250 WORDS)

Self-administration of central stimulants by rats: a comparison of the effects of d-amphetamine, methylphenidate and McNeil 4612

Rats were trained to press a lever for the intravenous administration of d-amphetamine. The rate of responding was decreased in a dose-dependent and time-related manner by non-contingent injections of d-amphetamine, methylphenidate or McNeil 4612. Methylphenidate and McNeil 4612 also maintained self-administration behavior when they were substituted for d-amphetamine, whereas substitution of saline for d-amphetamine resulted in extinction of the self-administration behavior. These data suggest that methylphenidate and McNeil 4612, like d-amphetamine, can act as reinforcers in rats.

5-Hydroxytryptamine synthesis and metabolism in discrete nuclei of the rat brain during surges of prolactin associated with restraint stress or suckling

5-hydroxytryptamine (5-HT) synthesis [accumulation of 5-hydroxytryptophan (5-HTP) following inhibition of aromatic L-amino acid decarboxylase with NSD 1015] and metabolism [concentration of the primary metabolite of 5-HT, 5-hydroxyindole-3-acetic acid (5-HIAA)] were measured in the suprachiasmatic (SCN), medial preoptic (MPO), and arcuate (AN) nuclei as well as the median eminence (ME) and striatum (ST) of the rat brain and correlated with changes in serum prolactin resulting from restraint stress (30 min) or the stimulus of acute suckling (30 min, following 4 h pup deprivation). Restraint stress increased serum increased serum prolactin and 5-HT synthesis and metabolism in the SCN and MPO. Acute suckling increased serum prolactin and 5-HT synthesis and metabolism only in the MPO, although 5-HT neuronal activity in the ME of lactating rats was stimulated versus diestrous control values regardless of whether the animals were suckled. These results demonstrate a differential effect on 5-HT synthesis and metabolism in discrete nuclei of the rat brain during two paradigms where the secretion of several hormones, including prolactin, is undergoing a dynamic change.

Differential effects of d-amphetamine, beta-phenylethylamine, cocaine and methylphenidate on the rate of dopamine synthesis in terminals of nigrostriatal and mesolimbic neurons and on the efflux of dopamine metabolites into cerebroventricular perfusates of rats

The in vivo effects of four psychomotor stimulants (d-amphetamine, beta-phenylethylamine, cocaine and methylphenidate) were determined on: 1) the rate of dopamine (DA) synthesis, as measured by the accumulation of dihydroxyphenylalanine (DOPA) after aromatic L-amino acid decarboxylase inhibition, in the striatum (terminals of nigrostriatal neurons) and in the nucleus accumbens and olfactory tubercle (terminals of mesolimbic neurons) and 2) the efflux of the DA metabolite 3,4-dihydroxyphenylacetic acid (DOPAC) into cerebroventricular perfusates of conscious, freely-moving rats. d-Amphetamine and beta-phenylethylamine produced biphasic responses with lower doses of each drug increasing both the efflux of DOPAC and the rate of DA synthesis in the striatum. Higher doses of each drug either had no effect or actually decreased the efflux of DOPAC and also decreased the rate of DA synthesis in the striatum. On the other hand, cocaine and methylphenidate only decreased the efflux of DOPAC and the rate of DA synthesis in the striatum. The effects of the drugs on the rate of DA synthesis in the nucleus accumbens and olfactory tubercle were similar to, but less pronounced than those seen in the striatum. These results are consistent with the following suggestions: 1) low doses of d-amphetamine and beta-phenylethylamine facilitate the neuronal release of DA while higher doses of both drugs facilitate release and inhibit neuronal reuptake of the amine, and 2) cocaine and methylphenidate preferentially block the neuronal reuptake of DA.

6-Hydroxydopamine and 5,7-dihydroxytryptamine selectively reduce dopamine and 5-hydroxytryptamine metabolites in cerebroventricular perfusates of rats

The efflux into the lateral cerebral ventricles of metabolites of dopamine (DA) and 5-hydroxytryptamine (5HT) was determined in unanesthetized rats bearing chronically implanted push-pull cannulae. Pretreatment with 6-hydroxydopamine (6-OHDA) reduced the basal efflux of 3,4-dihydroxyphenylacetic acid (DOPAC) and homovanillic acid (HVA), but not of 5-hydroxyindoleacetic acid (5HIAA). The haloperidol-induced increase in the efflux of DOPAC and HVA was markedly attenuated in the 6-OHDA-pretreated rats. In rats treated with 5,7-hydroxytryptamine (5,7-DHT) the basal efflux of DOPAC and HVA was unaffected, while that of 5HIAA was markedly reduced; in these animals the ability of L-tryptophan to increase the perfusate content of 5HIAA was abolished. These results indicate that metabolites of DA and 5HT appearing in cerebroventricular perfusates of rats originate from DA and 5HT neuronal terminals in the brain.

A comparison of domperidone and haloperidol effects on different dopaminergic neurons in the rat brain

Domperidone, a dopamine (DA) receptor antagonist with reportedly preferential actions outside of the blood-brain barrier, and haloperidol, a centrally active DA antagonist, were compared with respect to their abilities to increase the activity of dopaminergic neurons in the rat brain. The activity of nigrostriatal, mesolimbic, tuberohypophyseal and tuberoinfundibular dopamine nerves was estimated by measuring the in vivo rate of DA synthesis (dihydroxyphenylalanine accumulation following administration of an inhibitor of aromatic L-amino acid decarboxylase) in the striatum, olfactory tubercle, posterior pituitary and median eminence, respectively. In an initial study, the rates of DA synthesis in striatum, olfactory tubercle, and posterior pituitary were determined at 2, 8, and 16 h after subcutaneous administration of 0.25, 2.5, or 25 mg/kg domperidone. At the lowest dose of domperidone, DA synthesis was increased only in the posterior pituitary at 8 and 16 h; at the intermediate dose, DA synthesis increased in the posterior pituitary at 8 and 16 h and in the olfactory tubercle at 8 h. Only at 8 h after the highest dose of domperidone was DA synthesis increased in the striatum. When 2.5 mg/kg of domperidone or haloperidol were administered, DA synthesis in posterior pituitary and median eminence was increased in a similar fashion (in the latter region only at 16 h). In contrast, domperidone promoted only modest and delayed increases in DA synthesis in the olfactory tubercle and had no effect in the striatum. These results indicate that systemically administered domperidone preferentially increases DA synthesis in neurons terminating outside the blood-brain barrier, but after a pronounced delay, high doses of the drug can also activate DA neurons which project to the forebrain.

Increased self-administration of d-amphetamine by rats pretreated with metergoline

Rats trained to self-administer d-amphetamine were pretreated with metergoline, a long-acting 5-hydroxytryptaminergic antagonist, and immediately placed in self-administration cages for 8 hours. During the first 3 hours after metergoline the normal pattern of d-amphetamine self-administration was unaltered, but thereafter the rate of self-injections was increased. Between 4 and 6 hr the self-injections in metergoline-treated rats were increased in a regular fashion and subsequently (7-8 hr) in a stereotyped manner, i.e., rapid bursts of lever presses with little space between injections. In amphetamine-naive rats the levels of striatal and nucleus accumbens dopamine metabolites, dihydroxyphenylacetic acid and homovanillic acid, rose with time after metergoline injection. Levels of 5-hydroxyindoleacetic acid remained unchanged in these brain areas. The neurochemical results suggest a correlation between the dopaminergic actions of metergoline and d-amphetamine self-administration.

Measurement of 5-hydroxytryptamine synthesis and metabolism in selected discrete regions of the rat brain using high performance liquid chromatography and electrochemical detection: pharmacological manipulations

High performance liquid chromatography coupled with electrochemical detection (LCEC) was employed to measure 5-hydroxytryptamine (5-HT), 5-hydroxyindole-3-acetic acid (5-HIAA) and 5-hydroxytryptophan (5-HTP) in the suprachiasmatic (SCN), medial preoptic (MPO) and arcuate (AN) nuclei as well as the median eminence (ME) and striatum (ST) of individual rat brains. Biochemical estimations of changes in 5-HT neuronal activity were made by measuring: (1) concentrations of 5-HT and 5-HIAA and (2) the rate of 5-HT synthesis (5-HTP accumulation following the administration of NSD 1015, an inhibitor of aromatic L-amino acid decarboxylase) after the administration of pharmacological agents known to influence these neurons. Pargyline increased the concentration of 5-HT and decreased the concentration of 5-HIAA while probenecid increased the concentration of 5-HIAA in all 5 brain regions. At both 2 and 24 hours after reserpine the concentration of 5-HT decreased, 5-HIAA increased or did not change, and the rate of 5-HT synthesis increased. In most of the brain regions blockers of 5-HT neuronal uptake (fluoxetine, chlorimipramine) did not influence 5-HT or 5-HIAA concentrations dramatically, but increased the rate of 5-HT synthesis. L-tryptophan generally increased the concentrations of 5-HT and 5-HIAA as well as the rate of accumulation of 5-HTP in all regions except the ME where 5-HIAA and 5-HTP concentrations both were unaffected. These results reveal that the method using LCEC is sensitive enough to detect pharmacologically-induced changes in 5-HT metabolism and synthesis in discrete regions of rat brain. The drugs examined in the present study generally caused similar changes in 5-HT dynamics in all 5 brain regions examined.

The effect of morphine on 5-hydroxytryptamine synthesis and metabolism in the striatum, and several discrete hypothalamic regions of the rat brain

The effects of morphine on 5-hydroxytryptamine (5-HT) synthesis (accumulation of 5-hydroxytryptophan following inhibition of aromatic L-amino acid decarboxylase) and metabolism (concentration of 5-HT and its primary metabolite, 5-hydroxyindole-3-acetic acid [5-HIAA]) were determined in discrete nuclei of the rat brain using high performance liquid chromatography coupled with electrochemical detection (LCEC). Morphine (10 mg/kg, s.c.) increased 5-HT synthesis in the medial preoptic (MPO), suprachiasmatic (SCN) and arcuate (AN) nuclei as well as the striatum (ST) 1 hour following its administration. 5-HT synthesis in the median eminence (ME) was not affected at any time examined. A lower dose of morphine (5 mg/kg) also stimulated 5-HT synthesis in the AN. Although steady state concentrations of 5-HT were not greatly affected by morphine administration, the concentration of 5-HIAA in the AN, MPO, and ST increased following morphine (10 mg/kg, s.c., 1 hour). The increase in 5-HT synthesis observed in the MPO, SCN, AN, and ST 1 hour following morphine involved the activation of opiate receptors as administration of an opiate receptor antagonist, naloxone, blocked this effect. These results, indicate that morphine causes an increase in 5-HT synthesis and metabolism via an opiate receptor-mediated mechanism in the AN, MPO, SCN, and ST but not in the ME.

Injection of hypertonic saline or mannitol accelerates the dehydration-induced activation of dopamine synthesis in the neurointermediate lobe of the rat hypophysis

The rate of dopamine (DA) synthesis (DOPA accumulation after the administration of an inhibitor of aromatic L-amino acid decarboxylase) was determined in terminals of tuberohypophyseal DA neurons in the neurointermediate lobe (NIL) of the rat hypophysis at various times after the presentation of osmotic stimuli. DA synthesis in the NIL, but not in median eminence or striatum, was increased 24 h but not 4 h after an injection of hypertonic saline (5 ml 15% NaCl/kg, s.c.), provided the animals were not permitted access to water after the injection; 24 h of water deprivation per se was without effect on the rate of DA synthesis. DOPA accumulation in the NIL was also increased after an intravenous infusion of mannitol (25 ml 20% mannitol/kg) plus 24 h of water deprivation. These results suggest that the dehydration-induced activation of tuberohypophyseal DA neurons which is normally seen after 2-3 days of water deprivation can be accelerated if water deprivation is preceeded by injections of hypertonic saline or mannitol.

Lack of effect of cerebellectomy on ascending dopaminergic neurons in the rat brain

The dopamine content in striatum and substantia nigra of the rat was not altered 3 weeks after total or hemicerebellectomy. Furthermore, 3 days after hemicerebellectomy basal and haloperidol-stimulated activity of ascending mesolimbic and nigrostriatal dopaminergic neurons, as estimated from the in vivo rate of dopamine synthesis in striatum and nucleus accumbens, was not altered. Cupric silver degeneration staining following hemicerebellectomy failed to identify a direct anatomical pathway between cerebellum and substantia nigra in the rat.

Tolerance develops to the haloperidol-induced increase in the efflux of dopamine metabolites from the brains of unanesthetized, freely-moving rats

The lateral cerebral ventricles of freely moving rats were perfused by means of chronically implanted push-pull cannulae every second day for 2 weeks. Perfusates were analyzed for metabolites of dopamine [dihydroxyphenylacetic acid (DOPAC) and homovanillic acid (HVA)] and of 5-hydroxytryptamine [5-hydroxyindoleacetic acid (5HIAA)] using high performance liquid chromatography and an amperometric detector. Rats received daily subcutaneous injections of haloperidol (1 mg/kg) or its vehicle. After the first injection of haloperidol the concentrations of DOPAC and HVA were markedly increased while that of 5HIAA was unchanged. Complete tolerance developed to the haloperidol-induced increased efflux of dopamine metabolites by day 9, although a higher dose of haloperidol (2 mgf/kg) on day 15 was still capable of eliciting a modest increase in the efflux of DOPAC and HVA.