Sexual differences in the diurnal changes of tuberoinfundibular dopaminergic neuron activity in the rat: role of cholinergic control

A diurnal change of the tuberoinfundibular dopaminergic (TIDA) neuron activity in ovariectomized rats treated with or without estrogen was recently reported, and the endogenous cholinergic system may be responsible for its induction. Whether a similar rhythm exists in intact female or male rats was the focus of this study. TIDA neuron activity was assessed by measurement of the precursor or metabolite of dopamine in the median eminence by HPLC with electrochemical detection. In intact female Sprague-Dawley rats, diurnal changes in TIDA neuron activity were observed during all stages of the estrous cycle, i.e., proestrus, estrus, and diestrus 1, and they were nearly identical. No such rhythm, however, was observed in intact male rats. Castration alone decreased and increased basal TIDA neuron activity in female and male rats, respectively, and estrogen treatment increased the activity in both sexes. The diurnal changes in TIDA neuron activity, however, were observed only in the female rats, not the male rats, regardless of castration or estrogen treatment. Serum prolactin levels in the male rats exhibited no diurnal changes either, irrespective of whether the animals had been castrated or treated with estrogen. Central administration of mecamylamine (1 micrograms/3 microliters per rat, i.c.v.), a nicotinic receptor antagonist, in the morning (at 1000 h) or afternoon (at 1500 and 1700 h) induced a differential effect on the TIDA neuron activity of intact female rats, i.e., no effect in the morning and stimulation in the afternoon; it had no effect on the TIDA neuron activity in the male rats regardless of the injection time. In castrated male rats treated with estrogen, however, mecamylamine treatment further increased TIDA neuron activity, but still with no diurnal difference. In summary, a sexual difference in the diurnal changes of TIDA neuron activity was observed, and these changes may be under differential control by the cholinergic system.

Central administration of 8-OH-DPAT and mCPP stimulates prolactin secretion in ovariectomized, estrogen-treated rats: lack of an effect on tuberoinfundibular dopaminergic neuron activity

The effects of central administration of two serotonin receptor agonists, 8-OH-DPAT and mCPP, on tuberoinfundibular dopaminergic (TIDA) neuron activity and serum prolactin (PRL) levels in ovariectomized, estrogen-treated rats were determined. 8-OH-DPAT dose-dependently (0.1-10 microgram/rat, icv) stimulated serum PRL levels, and depressed serotonergic neuron activity in 30 min. However, the TIDA neuron activity was not affected at all. Similar treatment of mCPP was less effective than 8-OH-DPAT: only the highest dose of mCPP (10 microgram) stimulated PRL secretion and inhibited serotonergic neuron activity. No change in TIDA neuron activity was observed either. We conclude that central serotonin acts on 5-HT1 receptors to stimulate the PRL secretion, which may not involve the TIDA neurons.

Cancer of the gingiva, buccal mucosa, and palate

While most cases of intraoral cancer arise in the cancer-prone horseshoe comprised of the floor of the mouth, the posterolateral borders and ventral surface of the tongue, and the retromolar region, no intraoral site is immune. This article reviews the clinical features of squamous cell carcinoma of the gingiva, buccal mucosa and palate, as well as the less common oral malignancies that can arise at these intraoral sites.

Bombesin acts in the suprachiasmatic nucleus to affect circadian changes in tuberoinfundibular dopaminergic neuron activity and prolactin secretion

Central administration of bombesin has been shown to block the afternoon PRL surge. A circadian change in the activity of tuberoinfundibular dopaminergic (TIDA) neurons that coincides with the onset of the PRL surge has also been found. Whether bombesin acts on the TIDA neuron to affect the PRL surge and where bombesin acts were the focuses of this study. Bombesin (0.75 microgram/3 microliters.rat) was first injected into the lateral cerebroventricle of estrogen-primed ovariectomized rats at 1200 h through preimplanted cannulas. Both the PRL surge and the circadian changes in TIDA neuron activity (using both 3,4-dihydroxyphenylacetic acid concentration and 3,4-dihydroxyphenylalanine accumulation in the median eminence as indexes) were blocked by the treatment. In suprachiasmatic nucleus-lesioned rats, however, both rhythms were absent, and the injection of bombesin exhibited no significant effect. Using microinjection of bombesin (50 ng/0.2 microliter) bilaterally into either the suprachiasmatic or arcuate nuclei at 1200 h, only the former injection effectively blocked both TIDA neuron activity and the PRL surge. We conclude that bombesin may act on the rhythm generation center, the suprachiasmatic nucleus, to exert its effect to disrupt TIDA neuron activity and the PRL surge.

An endogenous cholinergic rhythm may be involved in the circadian changes of tuberoinfundibular dopaminergic neuron activity in ovariectomized rats treated with or without estrogen

We recently reported that a circadian change in the activities of hypothalamic tuberoinfundibular dopaminergic (TIDA) neurons exists in ovariectomized (OVX) rats treated with or without estrogen. The involvement of an endogenous cholinergic control mechanism was the focus of this study. Adult female Sprague-Dawley rats OVX for 2 weeks and treated with or without a long-acting estrogen (poly-estradiol phosphate, 0.1 mg/rat, sc) were used in the study. An intracerebroventricular cannula for drug injection was implanted into the lateral cerebroventricle of each rat 6 days before experiment. TIDA neuron activity was determined by measuring the concentrations of 3,4-dihydroxyphenylacetic acid and 3,4-dihydroxyphenylalanine in the median eminence by HPLC plus electrochemical detection. Serum PRL levels were determined by RIA. Neither atropine nor mecamylamine, two cholinergic receptor antagonists, had any effect on TIDA neuron activity in the morning before 1200 h, when endogenous TIDA neuron activity is high. Both drugs, however, exhibited a dose-related stimulating effect on the TIDA neuron activity in the afternoon, when endogenous activity is low. The estrogen-induced afternoon PRL surge was also blocked by a single injection of atropine or mecamylamine at 1300 h. The rhythmic changes in endogenous TIDA neuron activity and their responses to atropine were also observed in OVX rats with no estrogen replacement. In conclusion, a daily change in endogenous cholinergic neuron activity may be responsible for the change in TIDA neuron activity in female rats, which is a prerequisite for the PRL surge.

Effects of endogenous opioid peptides and their analogs on the activities of hypothalamic arcuate neurons in brain slices from diestrous and ovariectomized rats

Various endogenous opioid peptides and some of their analogs were used in this study to test their effects on the membrane activities of hypothalamic arcuate neurons in brain slices. Both ovariectomized and diestrous rats were used in the study, and freshly prepared brain slices from these animals were used for extracellular single-unit recording studies. All of the opioids exhibited potent inhibitory effects on the firing of arcuate neurons, viz., beta-endorphin inhibited 55% (n = 33), DAGO 62% (n = 21), dynorphin A 55% (n = 11), U50,488 36% (n = 39), Met-enkephalin 35% (n = 54), and DPDPE 50% (n = 8) of tested arcuate neurons from ovariectomized rats. Significantly higher percentage of inhibition was observed in slice preparations from diestrous rats for DAGO 86% (n = 22), and slightly higher for dynorphin A 59% (n = 22) and U50,488 53% (n = 15). Pretreatment with naloxone prevented most of the actions by beta-endorphin and DAGO, and nor-binaltorphimine prevented those by dynorphin A and U50,488. Most of the effects of Met-enkephalin could also be blocked by nor-binaltorphimine (67%, n = 6), but less by naltrindole (25%, n = 8). Naltrindole, however, seemed to be more effective in blocking the action of [D-Pen2,5]-enkephalin (100%, n = 2). In summary, all opioids tested exerted potent inhibitory effects upon the firing of arcuate neurons possibly through multiple opioid receptors, and the presence of ovarian hormones may have an effect on the neuron's responsiveness to opioid acting on mu type receptors.

Differential effects of angiotensin II on the activities of suprachiasmatic neurons in rat brain slices

The effects of angiotensin II (AII) on the firing rates of suprachiasmatic neurons were determined in rat brain slices. AII in pmol ranges stimulated 25% and inhibited another 25% of 52 irregular firing neurons, while it stimulated 23% and inhibited 4% of 30 regular firing neurons. Three "oscillating" neurons whose firing rates oscillated with rather constant amplitudes and periods were recorded. AII induced the occurrence of oscillation in one unit and modulated the oscillation amplitude of the other two. Pretreatment with saralasin, an AII antagonist, effectively blocked (100%) the actions of AII (n = 5). The present findings suggest that AII may act as an important mediator in the suprachiasmatic nucleus and its mode of action may be variable in different neurons.

Circadian changes of serum prolactin levels and tuberoinfundibular dopaminergic neuron activities in ovariectomized rats treated with or without estrogen: the role of the suprachiasmatic nuclei

Variations of serum prolactin (PRL) levels and activities of tuberoinfundibular dopaminergic (TIDA) neurons during the afternoon of ovariectomized (OVX) rats treated with or without estrogen were determined in this study. Long-term OVX rats treated with or without polyestradiol phosphate (0.1 mg/rat, s.c.) were decapitated every hour from 10.00 to 19.00 h (except 11.00 and 13.00 h). Serum PRL and median eminence (ME) dihydroxyphenylacetic acid (DOPAC) or dihydroxyphenylalanine (DOPA) levels were determined by radioimmunoassay and high performance liquid chromatography plus electrochemical detection, respectively. A prominent PRL surge started and peaked around 14.00-15.00 h, and remained significantly higher than levels of 10.00 and 12.00 h throughout the afternoon. Significant decreases of ME DOPAC and DOPA concentrations were also observed between 14.00 and 19.00 h. In OVX rats with no estrogen replacement, no PRL surge was observed and the changes of ME DOPAC concentrations during the afternoon were not significant except for that at 17.00 h. The ME DOPA accumulation, however, exhibited significantly lower levels from 14.00 to 19.00 h than that at 12.00 h, indicating that an endogenous rhythm for DA synthesis existed in OVX rats. In estrogen-treated OVX rats bearing bilateral lesions of the suprachiasmatic nuclei, both changes in serum PRL level and TIDA neuron activity were abolished. We conclude that an endogenous rhythm of the activities of TIDA neurons may exist in both OVX and OVX plus estrogen-treated rats. The rhythm is regulated by the suprachiasmatic nuclei and may be amplified by estrogen for the induction of PRL surge.

Enhanced tuberoinfundibular dopaminergic neuron activity in thyroidectomized, ovariectomized and estrogen-treated rats with hyperprolactinemia

The involvement of the tuberoinfundibular dopaminergic (TIDA) neurons in the genesis of hyperprolactinemia in female thyroidectomized rats treated with estrogen was the focus of this study. Rats that were ovariectomized (3 weeks), thyroidectomized (2 weeks) and treated with estrogen for 6 days had a 5- to 10-fold increase in serum prolactin (PRL) levels, while the rats receiving the same treatments but without estrogen had lower PRL levels. The activity of TIDA neurons, using dihydroxyphenylacetic acid (DOPAC) concentration or the DOPAC/dopamine (DA) ratio in the median eminence (ME) as an index, was increased and decreased in estrogen-treated and untreated rats, respectively. The increases in serum PRL level and the activity of TIDA neurons were dependent on the duration of thyroidectomy and could both be lowered by daily injection of thyroid hormone (20 or 100 micrograms/kg b.w. thyroxine, i.p.) for 12 days in a dose-dependent manner. Using dihydroxyphenylalanine (DOPA) accumulation in the ME as another index, we also found an increased DOPA/DA ratio in estrogen-treated hypothyroid rats, which effect could be reversed by replacement of thyroid hormone. Furthermore, treatments with bromocryptine (3 mg/kg b.w./day, s.c.) for 3 days or with cysteamine (100 mg/kg b.w., s.c.) at 26, 15 and 2 h before the rats were sacrificed lowered both serum PRL level and TIDA neuron activity. Since PRL can exert a short-loop feedback control on its own secretion, these results indicate that the increased serum PRL levels in estrogen-treated hypothyroid female rats should be the cause, but not the result, of the increased activity of TIDA neurons.

Differential neuronal responses to angiotensin III from the subfornical organ of normotensive and spontaneously hypertensive rats

We previously reported that chronic central administration of angiotensin III (AIII) fails to produce sustained drinking behavior in spontaneously hypertensive rats (SHR), possibly because of the development of early desensitization of the angiotensin receptors. The present study extended these findings to the cellular level, using brain-slice preparation from Wistar-Kyoto rats (WKY) and SHR, in conjunction with single-neuron recording in the subfornical organ (SFO), a target site for angiotensin II-induced drinking. We found that a majority of the SFO neurons studied (13/18 in WKY, 20/28 in SHR) responded in a dose-related manner to AIII, given in the range of 10(-6)-10(-5) M. This excitation was receptor-specific, since it was reversed by Ile7-AIII (10(-4)-10(-3) M), the selective AIII antagonist. Bestatin (10(-5)-10(-4) M), an aminopeptidase B inhibitor, did not discernibly affect basal spike frequency when delivered alone. Nevertheless, given in combination with the heptapeptide, bestatin reduced the intensity and duration of SFO neuronal response in WKY to the higher dose (10(-5) M), and in SHR to both doses (10(-6) or 10(-5) M), of AIII. These data suggest that the SFO may also be a central site of action for AIII. Moreover, prolonging the action of AIII by protecting it from being metabolized with bestatin may produce desensitization of the angiotensin receptors on SFO neurons. This was particularly so in the SHR, which are thought to be defective in the degradation of the heptapeptide in the brain.

Stimulatory effects of bombesin-like peptides on hypothalamic arcuate neurons in rat brain slices

The effects of bombesin, gastrin-releasing peptide, neuromedin C, ranatensin, and neuromedin B on hypothalamic arcuate neurons were tested in this study using extracellular single-unit recording in fresh brain tissue slices. Adult ovariectomized Sprague-Dawley rats were used for preparation of brain slices. All bombesin-like peptides in pmol ranges exhibited potent stimulatory effects on the firing of arcuate neurons, i.e., gastrin-releasing peptide stimulated 90.9% (n = 22), bombesin 78.0% (n = 41), neuromedin C 63.2% (n = 19), ranatensin 58.0% (n = 22), and neuromedin B 50.0% (n = 6) of arcuate neurons tested. Pretreatments with either [Leu13-psi(CH2NH)-Leu14]-bombesin or [D-Phe6,Des-Met14]-bombesin6-14 ethylamide, two bombesin antagonists, significantly blocked most of the actions of bombesin-like peptides tested. The present results further support the notion that bombesin-like peptides may play a significant role in the arcuate nucleus.

Correlation between prolactin secretion and Gs protein expression during sustained cholera-toxin stimulation

We have studied the chronic effect of cholera toxin (CTX) on prolactin synthesis and secretion in GH3 pituitary-tumour cells. Time-course analysis showed that prolactin secretion increased with time of CTX exposure, reached a peak at 3 h, and decreased thereafter. Prolactin synthesis was also shown to be stimulated by CTX. The basic and forskolin-stimulated cyclic AMP levels of the CTX-treated cells followed a biphasic time response similar to that of prolactin secretion. Exposure of cells to CTX for more than 3 h abolished the subsequent CTX-catalysed ADP-ribosylation in vitro. Moreover, a significant decrease in the pertussis-toxin-catalysed ADP-ribosylation was found after cells were exposed to CTX for longer than 6 h. Western-blot analysis indicated that the amount of Gs alpha (alpha-subunit of Gs) protein increased within 3 h, followed by a gradual decrease to 50% of the control level at 24 h. The accumulation of Gs alpha mRNA increased within 6 h of CTX exposure, and decreased thereafter to 40% of the basal level at 48 h. Our findings that prolonged treatment of CTX induced similar patterns of time responses in Gs alpha protein expression, cyclic AMP production and prolactin secretion indicate that CTX-induced changes in Gs alpha protein levels may be responsible for the cellular response leading to prolactin secretion.

Comparison of the effects of losulazine and reserpine on central aminergic neurons

The purpose of this study was to examine the effects of both acute and chronic administration of the peripheral sympatholytic antihypertensive agent losulazine on central dopaminergic, noradrenergic, and 5-hydroxytryptaminergic neurons in the rat. For comparison, the acute effects of reserpine were also examined. Acute systemic administration of losulazine produced marked dose- and time-dependent decreases in dopamine and norepinephrine concentrations in regions outside the blood-brain barrier (i.e., the median eminence, intermediate lobe, and neural lobe), that were accompanied by an increase in plasma concentrations of prolactin and alpha-melanocyte-stimulating hormone. By comparison, losulazine caused a relatively modest, transient depletion of dopamine and norepinephrine (but not 5-hydroxytryptamine) in regions of the brain protected by the blood-brain barrier (i.e., the striatum, nucleus accumbens, and dorsomedial nucleus of the hypothalamus). In contrast to losulazine, acute systemic administration of reserpine caused a prolonged depletion of dopamine, norepinephrine, and 5-hydroxytryptamine in all brain and pituitary regions examined. These results suggest that regional differences in the response of aminergic neurons to acute administration of losulazine and reserpine reflect differences in the ability of these drugs to penetrate the blood-brain barrier. Chronic systemic administration of losulazine produced a similar decrease in dopamine and norepinephrine in the median eminence, intermediate lobe, and neural lobe, suggesting that tolerance does not develop to the ability of losulazine to deplete catecholamines in these regions. Chronic losulazine administration also decreased dopamine concentrations in the striatum, and norepinephrine concentrations in the dorsomedial nucleus, suggesting that losulazine may have cumulative effects on central catecholamine neurons terminating in these brain regions.

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.

Potentiating effects of serotonin and vasoactive intestinal peptide on the action of glutamate on suprachiasmatic neurons in brain slices

Using extracellular single-unit recording in brain slices, we have tested the effects of serotonin and vasoactive intestinal peptide (VIP) on the action of glutamate on hypothalamic suprachiasmatic neurons. We hypothesized that serotonin and VIP may interact with glutamate so that a modulation of the circadian rhythm entrainment to the light-dark may be possible. Given individually, glutamate excited 32% and inhibited 1% (n = 65), serotonin excited 8% and inhibited 27% (n = 73), and VIP excited 7% and inhibited 7% (n = 41) of suprachiasmatic neurons. When administered together, however, serotonin or VIP not only increased the percentage of neurons (up to 50%) that responded to glutamate, but also potentiated the neuronal responses to glutamate. These findings substantiated our hypothesis that both serotonin and VIP may modulate the action of glutamate in the suprachiasmatic nucleus.

Stimulatory effects of bombesin-like peptides on suprachiasmatic neurons in brain slices

The effects of bombesin on hypothalamic suprachiasmatic (SCN) neurons were tested in this study using extracellular single-unit recording in brain tissue slices. Fresh slices containing the SCN were obtained from adult ovariectomized Sprague-Dawley rats. Bombesin in pmol ranges stimulated 75% of irregular firing SCN neurons (n = 113), while it stimulated and inhibited 17% and 34% of regular firing units, respectively. Half of the regular firing SCN units, however, were not responsive to bombesin (49% of 61 units). A dose-dependent (from 5 to 500 pmol) excitatory effect of bombesin on SCN neurons was also observed. Pretreatment with [Leu13-psi(CH2NH)-Leu14]-bombesin, a bombesin receptor antagonist, blocked the action of bombesin in 67% of 18 units responded to bombesin, indicating a specific receptor is involved in the action. Gastrin-releasing peptide, a well-recognized bombesin-like peptide in mammals, behaved almost the same as bombesin did in most SCN neurons tested (same responses in 24 of 25 units). The present finding indicates that bomesin-like peptides may play a significant role in the SCN for the rhythmic control mechanism.

Central administration of bombesin blocks the estrogen-induced afternoon prolactin surge

The effect of central administration of bombesin (BB), a neuropeptide originally isolated from the skin of frogs, on the estrogen-induced afternoon prolactin (PRL) surge was studied. Two doses of BB, 0.15 and 0.75 micrograms/rat, were injected into the lateral cerebroventricle of estrogen-primed ovariectomized rats at 12.00 h through preimplanted cannulae. Serial blood samples were obtained through preimplanted intra-atrial catheters during the whole afternoon hours. The small dose of BB delayed the afternoon PRL surge for an hour, but did not prevent it from occurring. The large dose of BB, however, blocked the surge completely. When the same effective dose of BB was co-administered with a BB antagonist, [Leu13-psi(CH2NH) Leu14]-BB (L psi L-BB), it no longer inhibited the PRL surge. The afternoon PRL surge was also reinstated in BB-treated rats by giving a single injection of sulpiride (1 microgram/rat, i.v.), a dopamine antagonist, at 14.00 h in the same afternoon. These results suggest that BB, by acting through specific BB receptors, can inhibit the PRL surge possibly through activating the dopaminergic systems.

Effects of various neuroactive substances on single-unit activities of hypothalamic arcuate neurons in brain slices

Extracellular single-unit activities of 288 dorsomedial/ventrolateral hypothalamic arcuate (ARC) neurons were studied electrophysiologically in brain slices in vitro. We tested the effects of several neuroactive substances plus some of their analogs in this study. Among them, baclofen, a GABAb-receptor agonist, inhibited 95.6% of ARC neurons tested, although GABA itself was much less effective (23.8%). About half of baclofen's effect was blocked by phaclofen, a GABAB antagonist. Serotonin and dopamine also exhibited mostly inhibitory effects on responsive ARC neurons (80 and 78.4%, respectively), although more than half ARC neurons tested (53.3% of 169) were not responsive to dopamine. Neither ketanserin, a 5-HT2, nor domperidone, a D2 receptor antagonist, had much effect on blocking the actions of 5-HT or DA. Histamine and carbachol excited 67.4% and 52.2% of ARC neurons tested, respectively. Moreover, their effects were completely blocked by pyrilamine, a H1, and atropine, a muscarinic receptor antagonist, respectively. Ranitidine, a H2 receptor antagonist, however, was less effective. Norepinephrine had about equal number of excitation (33.3%) and inhibition (38.5%) on ARC neurons. Cholecystokinin-octapeptide sulphate (CCK-8S), a neuropeptide tested exerted potent excitatory effects on ARC neurons (62.8% of 137). In summary, ARC neurons in a more localized region aiming at the tuberoinfundibular dopaminergic neurons were selected in this study. Their responses to many agents and the implications on the regulation of prolactin secretion were discussed.

Bombesin and neurotensin excite neurons in hypothalamic arcuate nucleus in brain slices: an extracellular single-unit study

Using extracellular single-unit recording in brain tissue slices, the effects of bombesin (Bomb), neurotensin (NT), and their analogs on hypothalamic arcuate (ARC) neurons were tested in this study. Adult ovariectomized Sprague-Dawley rats were used for obtaining the brain slices. Both Bomb and NT in pmol ranges stimulated nearly 70% of ARC neurons tested and the effects were all very significant. [D-Trp11]-NT, a NT analog, behaved more like an analog of NT instead of an antagonist, although [Leu13-psi(CH2NH)-Leu14]-Bomb, a Bomb receptor antagonist, could not block most of Bomb's effects on ARC neurons. The significant effects of Bomb and NT shown in this report indicate that both peptides may play significant roles in the neuroendocrine control system.

Low calcium/high magnesium medium increases activities of hypothalamic arcuate and suprachiasmatic neurons in brain tissue slices

Extracellular single-unit recording was conducted in hypothalamic arcuate (ARC) and suprachiasmatic nucleus (SCN) in rat brain slices. The perifusion medium was switched from normal artificial cerebrospinal fluid (ACSF) to ACSF containing low Ca2+ and high Mg2+ to test the effect of extracellular calcium on the spontaneous activities of these neurons and their responses to some test agents. It was found that almost all of the ARC and SCN units tested increased their firing rates in low Ca2+, high Mg2+ ACSF, and exhibited more significant responses to excitatory test agents. The effects of low Ca2+, high Mg2+ ACSF were repeatable and reversible. The results suggest that extracellular Ca2+ is essential in maintaining the membrane stability of most hypothalamic neurons and most agents tested are acting postsynaptically on the unit recorded.

Effects of ketanserin on DOI-, MCPP- and TRH-induced prolactin secretion in estrogen-treated rats

The effects of ketanserin (Ket), a serotonin (5-HT2) receptor antagonist, on DOI- and mCPP-, two 5-HT agonists, and TRH-induced PRL secretion were studied. Adult female Sprague-Dawley rats ovariectomized for two weeks and treated with a long-acting estrogen, polyestradiol phosphate for one week were used. Drug administration and serial blood sampling were accomplished through indwelling intraatrial catheters which were implanted two days before the experiment. Both DOI (0.5 mg/kg BW) and mCPP (1 mg/kg BW) stimulated prolactin secretion within 10 min after iv injection and the effects were diminished by 30 min. In animals pretreated with Ket (5 mg/kg BW, sc), the effect of DOI was blocked, while that of mCPP was augmented. Co-administration of Ket (1 mg/kg BW, iv) with DOI or mCPP produced similar effect. Pretreatment with Ket, similar to sulpiride (Sulp), a dopamine antagonist, potentiated the TRH-induced prolactin secretion. Co-administration of Ket and Sulp further potentiated the TRH action. It is concluded that Ket not only acts as a 5-HT2 receptor antagonist that blocks the action of DOI, but may also act on dopamine receptor(s) with lower sensitivity to Sulp.

Neurotensin-induced activation of hypothalamic dopaminergic neurons is accompanied by a decrease in pituitary secretion of prolactin and alpha-melanocyte-stimulating hormone

The effects of neurotensin on the activity of hypothalamic tuberoinfundibular and periventricular-hypophysial dopaminergic (DA) neurons, and on the secretion of pituitary hormones that are tonically regulated by these neurons (i.e. prolactin and alpha-melanocyte-stimulating hormone [alpha MSH], respectively) were examined in estrogen-primed ovariectomized rats. The activity of tuberoinfundibular and periventricular-hypophysial DA neurons was estimated by measuring concentrations of the dopamine metabolite 3,4-dihydroxyphenylacetic acid (DOPAC) in the terminals of these neurons in the median eminence and intermediate lobe of the posterior pituitary, respectively. Intracerebroventricular administration of neurotensin caused a dose- and time-related increase in DOPAC concentrations in both the median eminence and intermediate lobe, and a concurrent decrease in plasma levels of prolactin and alpha MSH. These results suggest that neurotensin-induced inhibition of secretion of prolactin and alpha MSH from the pituitary may be due to the stimulatory action of this neuropeptide on the release of dopamine from tuberoinfundibular and periventricular-hypophysial neurons.

The study of serum apoprotein levels as indicators for the severity of angiographically assessed coronary artery disease

Serum apoprotein A-I (Apo A-I) and B (Apo-B) concentrations were determined in 40 subjects undergoing coronary angiography for past myocardial infarction and angina pectoris, and the authors studied the relationship between the apoprotein concentrations and the severity of coronary artery disease (CAD). During this study, serum total cholesterol, triglyceride, and high-density lipoprotein, low-density lipoprotein, and very low density lipoprotein cholesterol concentrations were determined to control analysis. The results showed that the decrease in serum Apo A-I levels was the best indicator distinguishing CAD from non-coronary artery disease; the Apo B/Apo A-I ratio had the most consistent association with the severity of CAD as assessed by angiography; Apo B/Apo A-I values ranging from 0.98 to 1.00 might be considered critical values for early CAD.

Increased plasma prolactin levels in ovariectomized thyroidectomized rats treated with estrogen

It is well established that TRH exerts a stimulatory effect on the secretion of both TSH and PRL. Clinically, hyperprolactinemia is usually present in hypothyroid women, but not men. In experimental studies, results vary because of the sexes, and treatments of animals differ. The purpose of this study was to further investigate the physiological control of PRL secretion in hypothyroid female rats. Adult female Sprague-Dawley rats that were surgically ovariectomized (OVX) and/or thyroidectomized (Tx) for 2 weeks were used. Serial blood samples were collected through indwelling intraatrial catheters, and plasma PRL and TSH levels were measured by RIA. We found that OVX + Tx and polyestradiol phosphate (PEP; 0.1 mg/rat, sc)-treated rats exhibited significantly higher basal PRL and TSH levels and afternoon surge PRL levels than sham Tx rats with the same treatments. On the other hand, if OVX + Tx rats were not treated with estrogen, their plasma PRL levels were not significantly different from those in sham Tx controls. If challenged with TRH (1 microgram/rat, iv), significantly higher PRL responses were found in OVX + Tx + PEP rats than in sham Tx rats. The contents of TRH in the median eminence of Tx rats, however, were not different from those in sham Tx rats. When challenged with domperidone (10 micrograms/rat, iv), a dopamine antagonist, no difference in PRL increments was found in the two groups of animals. Treatment with CB154, a potent dopamine agonist, did not eliminate the difference in basal PRL levels between the two groups. Pretreatment with a smaller dose of domperidone (1 microgram/rat), however, enhanced the PRL-releasing effect of TRH more in Tx than in sham Tx rats. When T4 (2 or 10 micrograms/100 g BW.day for 21 days) was replaced in Tx rats starting the second day after Tx, both basal and TRH-stimulated PRL secretion were significantly decreased in a dose-dependent manner. In conclusion, the increased PRL levels in OVX + Tx + PEP rats may be due to increased responsiveness of the anterior pituitary gland to TRH, and not to a decreased responsiveness to dopamine. In addition, the elevation of plasma PRL in OVX + Tx + PEP rats is negatively correlated with plasma levels of thyroid hormone.

Paradoxical effects of oxytocin and vasopressin on basal prolactin secretion and the estrogen-induced prolactin surge

The roles of oxytocin (OT) and vasopressin (AVP) on both basal and estrogen-induced prolactin (PRL) secretion were examined. Adult female Sprague-Dawley rats that were ovariectomized for 3 weeks and received estrogen treatment for 1 week were used. Intravenous administration of hormones and serial blood sampling were accomplished through indwelling intraatrial catheters which were implanted two days before. Plasma PRL levels were measured by radioimmunoassay. Oxytocin at a dose of 20 micrograms/rat stimulated a moderate PRL release in the morning and lower doses (5 and 10 micrograms) were without effect. Vasopressin was most effective at a dose of 5 micrograms/rat in stimulating PRL release, while consecutive injections of higher doses (10 and 20 micrograms) were less effective. In contrast, TRH, ranging from 1 to 8 micrograms/rat, induced a dose-dependent increases in PRL secretion. Using the effective dosages determined from the morning studies, repeated injections of either OT, AVP or their specific antagonists MPOMeOVT [( 1-(beta-mercapto-beta, beta-cyclopentamethylene propanoic acid), 2-(O-methyl)tyrosine, 8-ornithine]-vasotocin) and d (CH2)5Tyr(Me)AVP ([1-(beta-mercapto-beta, beta-cyclo-pentamethylene propionic acid), 2-(O-methyl)tyrosine, 8-arginine]-vasopressin), were given hourly between 1300 to 1800 h and blood samples were obtained hourly from 1100 to 1900 h. It was found that either OT or AVP significantly reduced the afternoon PRL surge, while their antagonists were not as effective. When OT or AVP were administered together with their specific antagonists, the inhibitory effects of either hormone on PRL surge were reversed. Thus it is concluded that both OT and AVP assume a non-specific stress-like effect on PRL release, in which basal secretion is stimulated and surge secretion is inhibited.