Preoptic-anterior Hypothalamic Area Research Articles

Hypothalamic interaction between macrophage inflammatory protein-1 alpha (MIP-1 alpha) and MIP-1 beta in rats: a new level for fever control?

1. The microinjection of macrophage inflammatory protein-1 (MIP-1 alpha; 5.0 and 25 pg) into the anterior hypothalamic, preoptic area (AHPOA) induced a slow onset; monophasic fever in rats that persisted for a long period. Microinjection of 25 pg MIP-1 beta into the AHPOA induced a fever of rapid onset, whereas 5.0 pg MIP-1 beta did not alter body temperature (Tb) significantly. When either MIP-1 alpha or MIP-1 beta was heated to 70 degrees C for 30 min prior to their injection, no pyrexic response was produced. 2. The concurrent microinjection of 25 pg MIP-1 alpha and 25 pg MIP-1 beta into the AHPOA attenuated the effects on Tb of either cytokine alone. However, pretreatment with either 5.0 pg MIP-1 beta or 5.0 pg MIP-1 alpha suppressed the febrile response induced by 25 pg MIP-1 alpha or 25 pg MIP-1 beta, given at the same site, respectively. 3. The present experiments show that MIP-1 alpha and MIP-1 beta are active individually and possess distinct differences in their evocation of a febrile response. Further, our results suggest a functional antagonism between MIP-1 alpha and MIP-1 beta that could represent a new level in the development of fever.

Macrophage inflammatory protein-1beta (MIP-1beta) produced endogenously in brain during E. coli fever in rats.

Macrophage inflammatory protein-1 (MIP-1) evokes an intense fever, independent of a prostaglandin mechanism, and is now thought to play an important role in the defence response to bacterial pyrogens. The purpose of this study was 2-fold: (i) to determine whether the potent doublet of this cytokine, MIP-1beta, is actually produced in the brain in response to a pyrogenic dose of a lipopolysaccharide of Escherichia coli and (ii) to determine the anatomical site of synthesis of this cytokine in the brain. Following the intense fever produced by intraperitoneal administration of lipopolysaccharide in the unrestrained rat, MIP-1beta immunoreactivity was identified post mortem in two regions of the brain implicated in fever: the organum vasculosum laminae terminalis (OVLT) and the anterior hypothalamic, preoptic area (AH/POA). Microinjection of goat anti-mouse MIP-1beta antibody (anti-MIP-1beta) directly int the AH/POA markedly suppressed fever in rats in response to lipopolysaccharide. Further anti-MIP-1beta administered 180 min after the injection of lipopolysaccharide acted as an antipyretic and reversed the fever induced by the endotoxin. anti-MIP-1beta or control immunoglobulin G antibody microinjected into the hypothalamus immediately before the intraperitoneal injection of the control saline did not alter the temperature of the rats. Taken together, the present results demonstrate that MIP-1beta is produced in the brain in response to a bacterial endotoxin. These observations, in the light of earlier data on fever induced by MIP-1beta, further support the hypothesis that endogenously synthesized MIP-1beta acts as an intermediary factor in the evocation of fever by acting on the thermosensitive cells of the brain.

1659 Involvement of histamine h1 receptors on the polypnea induced by rising body temperature

A role of central histamine in the preoptic area/anterior hypothalamus (POA/AH) for the regulation of hyperthermia-induced polypnea was examined in anesthetized, paralyzed, vagotomized and artificially ventilated rabbits. Phrenic nerve activities were recorded to monitor respiratory neuronal output. Hyperthermia increased respiratory frequency by reductions of inspiratory time (TI) and expiratory time (TE). Pyrilamine, an H1 receptor antagonist, which was applied to the POA/AH reduced polypnea under hyperthermia. The effect of S(+)α-fluoromethylhistidine, a specific inhibitor of histidine decarboxylase, applied in a lateral ventricle was comparable to the effect of pyrilamine on polypnea. Moreover, histamine dihydrochloride applied into the POA/AH at a normal body temperature produced polypnea by reductions of TI and TE. The results suggest that central histamine in the POA/AH contributes to the generation of polypnea in hyperthermia through H1 receptors.

1905 Capsaicin attenuates the noradrenaline-induced temperature increase in brown adipose tissue in the rat

E series prostaglandins (PGE) are known to elicit potent hyperthermia when injected into the anterior hypothalamic preoptic area (POAH) in rats, but the effector mechanisms mediating the rise in temperature are not well defined. In the present study, microinjection of PGE2 into the POAH dose-dependently increased non-shivering thermogenesis in brown adipose tissue (BAT) in urethananesthetized rats, bringing about a marked and sustained rise in interscapular BAT (IBAT) and core temperatures. The effect of intra-POAH PGE2 injection on IBAT and core temperatures could be blocked by systemic pretreatment with the sympathetic ganglionic blocker chlorisondamine chloride or the β-adrenergic receptor blocker propranolol, thus implicating the involvement of the sympathetic system. Furthermore, the increase in IBAT and core temperatures induced by intra-POAH PGE2 could be blocked by prior injection of the local anesthetic procaine or the GABA receptor agonist muscimol into the ipsilateral ventromedial hypothalamic nucleus (VMH). Taken together, the results suggest that PGE2 increases body temperature by acting in the POAH to stimulate heat production in BAT via a sympathetic efferent mechanism located in the VMH.

Fever induced in rats by intrahypothalamic macrophage inflammatory protein (MIP)‐1β: Role of protein synthesis

The effect of macrophage inflammatory protein-1 beta (MIP-1 beta) on body temperature, following its injection into the anterior hypothalamic pre-optic area (AH/POA), was examined by a radiotelemetry system in the freely moving rat. The purpose of this study was to examine the action of an inhibitor of protein synthesis, anisomycin, on the pyrexia which follows intrahypothalamic injection of MIP-1 beta. The micro-injection of 10 to 20 pg MIP-1 beta into the AH/POA induced a dose-dependent monophasic increase in body temperature, whereas a higher dose of 25 pg of the cytokine caused a biphasic febrile response. When MIP-1 beta was heated at 70 degrees C for 30 min prior to its administration, the pyrogenic response was abolished. Pretreatment of the micro-injection site in the AH/POA with 10 micrograms anisomycin did not alter the febrile response to 25 pg MIP-1 beta given at the same site in the AH/POA. When 10 mg/kg anisomycin was administered subcutaneously, the febrile response to 25 pg MIP-1 beta injected in the AH/POA was significantly suppressed. The present results suggest that fever caused by MIP-1 beta within the cells of the AH/POA may not require the synthesis of a new protein factor; however, the de novo synthesis of a protein outside of the AH/POA presumably plays a functional role, at least in part, in the intense fever produced by this cytokine in the hypothalamus.

Effects of pinealectomy and melatonin treatments on serotonin uptake and release from synaptosomes of rat hypothalamic regions.

This study examined the effects induced by long-term pinealectomy, daily melatonin treatment to pinealectomized and intact rats, and a single melatonin injection on [14C]-serotonin (5-HT) uptake and release from synaptosomes obtained of hypothalamic regions. Pinealectomy inhibited the accumulation of labeled 5-HT by synaptosomes of the preoptic area-anterior hypothalamus (POA-AH), but it failed to alter the [K+]-evoked 5-HT release. Melatonin treatment for 10 consecutive days to pinealectomized rats restored 5-HT uptake in POA-AH, and also increased 5-HT release in medial and posterior hypothalamus. These results suggest that pineal melatonin plays a stimulatory role on the serotoninergic terminals of the hypothalamus. Moreover, when daily melatonin treatment was administered to intact rats a significant increase in 5-HT uptake activity by synaptosomes of all the hypothalamic regions was observed, but 5-HT release was unaffected. In contrast, a single melatonin injection induced a significant decrease in 5-HT release from synaptosomes of the POA-AH was observed. The results suggest the existence of a differential sensitivity in the mechanisms mediating melatonin actions on 5-HT uptake/release, which depends on the presence of the pineal gland in the animals and on the frequency of the treatments with the pineal hormone.

Pups removal enhances thyrotropin-releasing hormone mRNA in the hypothalamic paraventricular nucleus

Previous studies have shown that lactation and suckling alter thyrotropin-releasing hormone (TRH) biosynthesis in hypothalamic paraventricular neurons. The amounts of paraventricular TRH mRNA and mediobasal hypothalamus (MBH) TRH were determined following removal of the pups to examine whether paraventricular TRH neuron activity is altered during the transition from lactation to estrous cycle. Paraventricular TRH mRNA and MBH TRH levels were determined by Northern blot analysis and radioimmunoassay, respectively. We had shown previously that after an 8-h withdrawal of the pups at mid-lactation the MBH TRH and paraventricular TRH mRNA levels are not modified. This condition was compared to one where pups were removed for 56 h, finding a significant decrease (46%, p < 0.005) of MBH TRH and a significant increase (156%, p < 0.02) of paraventricular TRH mRNA. The effect observed in the paraventricular TRH mRNA was correlated negatively with the serum corticosterone levels, a potential negative regulator of paraventricular TRH mRNA. The results were similar if a 1-h suckling period was introduced 8 h after withdrawal of the pups to induce a transient increase of corticosterone levels. The pattern of TRH mRNA was specific to the paraventricular nucleus because there was no enhancement in the preoptic area-anterior hypothalamus. In summary, our data suggest that TRH biosynthesis in paraventricular neurons is slowly adjusted after withdrawal of the pups, possibly to prepare TRH neurons to the new secretory demands of the estrous cycle.

Characterization of progesterone-3-[ 125I-BSA] binding sites in the medial preoptic area and anterior hypothalamus

In this study we utilized radiolabeled progesterone (P) conjugated to bovine serum albumin (BSA) at position 3 (P-3-[ 125I-BSA]) to examine steroid receptors in membrane fractions from the medial preoptic area-anterior hypothalamus (MPOA-AH) of ovariectomized (OVXed) rats. In the MPOA-AH binding of P-3-[ 125I-BSA] was linear across a tissue concentration range of 0.005 to 0.02 mg protein/0.1 ml of membrane suspension. Kinetic experiments revealed an association t 1/2 of 51.4 min and a dissociation t 1/2 of 122.5 min for P-3-[ 125I-BSA] at 0°C. Analysis of data from competition binding experiments using P-3-BSA revealed high- and low-affinity binding sites in the MPOA-AH. Involvement of MPOA-AH binding sites with a G-protein was suggested by a reduction of P-3-[ 125I-BSA] binding in the presence of the non-hydrolyzable GTP analog GTPγS but not ATPγS. In addition, if homogenates from the MPOA-AH were preincubated with 10 −5 M of the G-protein antagonist cholera toxin for 30 min at 37°C, competition binding data indicated only high-affinity binding sites. Once daily injections of OVXed rats with 4 mg P for 12 days significantly increased the density of P-3-[ 125I-BSA] binding sites in the MPOA-AH. This treatment did not affect P-3-[ 125I-BSA] binding in the dorsal tectum, medial basal hypothalamus, ventral tegmental area or the thymus.

Effects of the unilateral implant of haloperidol at the preoptic-anterior hypothalamic area, on ovulation

In order to analyse whether the participation of the dopaminergic system of the preoptic-anterior hypothalamic area (POA-AHA) in the regulation of ovulation is asymmetric and varies during the estrous cycle of the rat, as it occurs with the cholinergic system, the effects of a unilateral implant of haloperidol were studied. Cyclic fourth-day rats with a permanent cannula directed to the right or left side of the POA-AHA, at 13:00 h of estrous (E), diestrous 1 or 2 (D1, D2) or proestrus (P), received an implant of haloperidol (10.0±3.0 μg) or cholesterol (8.0±2.0 μg). The animals were killed at the morning of the next expected day of estrous. In comparison with cholesterolimplanted animals, the ovulation rate was reduced by haloperidol implantation made on E or D1 on either side of POA-AHA (Cholesterol: 26/50vs Haloperidol: 1/35,P<0.05). The implantation of haloperidol on D2 or P did not affect ovulation (Cholesterol: 30/37vs Haloperidol: 23/34, ns). The injection of gonadotropin-releasing hormone (3.7 μg/kg) sc on the afternoon of P to rats with an implant of haloperidol made on E or D1 on either side of POA-AHA, restored ovulation (19/22vs 1/35,P<0.01). The injection of estradiol benzoate (10 μg) at 13:00 h on D2, to D1-haloperidol-implanted rats restored ovulation. The same treatment to E-haloperidol-implanted animals restored ovulation when the haloperidol was placed into the left side of POA-AHA (5/6), and it was ineffective when haloperidol was in the right side (0/8). These results suggest that the participation of the POA-AHA-dopaminergic system on the neuroendocrine mechanisms controlling the release of GnRH on the afternoon of proestrus and ovulation, varies during the estrous cycle, and is necessary at the beginning of the estrous cycle for normal ovulation. The positive feedback of estrogens on D2 needs the integrity of the right side of POA-AHA.

Regulation of gonadotropin-releasing hormone gene expression in the rat during the luteinizing hormone surge.

In the present study we characterized GnRH gene expression in intact female rats across the estrous cycle and during a steroid-induced LH surge in ovariectomized (OVX) rats, using the quantitative ribonuclease protection assay. We measured cytoplasmic messenger RNA (mRNA) levels and nuclear primary transcript levels as an index of transcription. In Exp I, cycling rats were killed at 1100 or 1800 h on estrus, diestrus day 1, or diestrus day 2 (D2) or at 1100, 1500, 1800, or 2100 h on proestrus (P). In Exp II, proestrous rats were killed at the same time points or injected with pentobarbital (Pb) at the onset of the LH surge and killed on that day or the following day. In Exp III, a LH surge was induced in OVX rats treated with estradiol benzoate plus progesterone. Rats were killed at 1200, 1500, 1600, 1700, or 2100 h on the day of the surge. For all experiments, blood samples were collected and frozen for quantitation of LH by RIA. The preoptic area-anterior hypothalamus was dissected, and cytoplasmic and nuclear RNA were extracted and assayed separately by ribonuclease protection assay. In Exp I, cytoplasmic mRNA levels exhibited two significant peaks, one on D2 and another at 1500h on P. Primary transcript levels were significantly elevated only at 1500 h on P. In Exp II, proestrous rats and rats given Pb and killed the next day had a peak in cytoplasmic mRNA levels at 1500 h, which was blocked in rats given Pb and killed the same day. In Exp III with OVX rats, no significant changes in mRNA or primary transcript levels were observed between steroid or control groups. We hypothesize that the increase in cytoplasmic mRNA levels in cycling rats on D2 is probably due to a posttranscriptional mechanism, because it was not paralleled by changes in primary transcript levels, which would be expected if a transcriptional mechanism were involved. On P, both cytoplasmic mRNA and primary transcript levels changed, suggesting a transcriptional mechanism at this time.

Telencephalic and diencephalic origin of radial glial processes in the developing preoptic area/anterior hypothalamus.

Neuronal birth-dating studies using [3H] thymidine have indicated that neurons in the preoptic area/anterior hypothalamus (POA/AH) are derived primarily from progenitors in proliferative zones surrounding the third ventricle. Radial glial processes are potential guides for neuronal migration, and their presence and orientation during development may provide further information about the origin of cells in the POA/AH. In addition to determining the orientation of radial glial fibers, we examined the relationship of neurons with identified birth dates to radial glial processes in the developing POA/AH of ferrets. Neuronal birth dates were determined by injecting ferret fetuses with bromodeoxyuridine (BrdU) at several different gestational ages; brains were taken from ferret kits at subsequent prenatal ages. Sections were processed for immunocytochemistry to reveal vimentin or glial fibrillary acidic protein in radial glial, or BrdU-labeled cell nuclei. Numerous radial glial processes extended from the lateral ventricles through ventral portions of the septal region to the pial surface of the POA/AH. These fibers both encapsulated and coursed ventrally through and around the anterior commissure of ferret, rat, and mouse fetuses. These ventrally directed fibers were less evident at older ages. In double-labeled sections from ferrets, BrdU-labeled cells in the dorsal POA/AH were often aligned in the same dorsal-ventral orientation as adjacent radial glial fibers. We suggest that a subset of neurons, originating in telencephalic proliferative zones, migrates ventrally along radial glial guides into the dorsal POA/AH.

Cytokines and Thermoregulation: Interleukin-9 Injected in preoptic area fails to evoke fever in rats

A number of the members of the family of cytokines including IL-1, IL-2, IL-6, and IL-11 act directly in the brain to induce a febrile response in the rat and other species. The purpose of this study was to examine the effect of interleukin-9 (IL9) when this cytokine is applied directly to the thermosensitive and pyrogen reactive region of the anterior hypothalamic, preoptic area (AH/POA). In male Sprague-Dawley rats, guide cannulae for microinjection into the AH/POA were implanted stereotaxically, and radio transmitters for monitoring body temperature (T b) were placed intraperitoneally. Following postoperative recovery, recombinant murine macrophage inflammatory protein (MIP)-1β was microinjected in the AH/POA of each rat in a dose of 28 pg/1μl to identify pyrogen reactive sites in the AH/POA. Then recombinant human IL-9 was suspended in pyrogen-free CSF vehicle and microinjected in the same sites in concentrations of 2.4, 24, and 240 U/μl. In contrast to the pyrexic action of MIP-1β, IL-9 failed to elicit a significant alteration in the T b of the rats at any of the doses tested. IL-9 was also without effect on the intakes of either water or food. These results demonstrate that IL-9 applied to the region of the diencephalon in which other cytokines act to evoke fever may not play a direct role in the thermogenic component underlying the acute phase response. However, as demonstrated in several different cell systems, IL-9 may require a cofactor related to pyrogen for a febrile response to develop.

Preoptic rather than mediobasal hypothalamic amino acid neurotransmitter release regulates GnRH secretion during the estrogen-induced LH surge in the ovariectomized rat.

Inhibitory and excitatory amino acid neurotransmitters have been suggested to participate in the feedback actions of estradiol (E2) on LH secretion. In the rat estrogen-receptive neurons have been demonstrated in the preoptic/anterior hypothalamic area (POA) and mediobasal hypothalamus/median eminence (MBH) and many of these neurons utilize gamma-aminobutyric acid (GABA) as neurotransmitter. The actions of excitatory amino acids (EAA) differ in ovariectomized (ovx) and ovx E2-substituted rats indicating that EAAs also participate in the positive feedback action of E2 on LH release. However, little information is available as to whether in vivo these transmitters exert their effects in the POA, where most of the GnRH perikarya are located, or in the MBH, i.e. at the nerve terminals. Therefore we conducted push pull cannula perfusions to compare the release rates of GABA, aspartate (ASP) and glutamate (GLU) in the MBH and POA. A subcutaneous implant of a silastic tube containing E2 resulted in LH surges in the afternoon of all treated animals. Prior to and during this LH surge the MBH release rates of neither GABA nor ASP nor GLU were significantly altered. In contrast, a conspicuous drop in preoptic GABA release occurred prior to and during the time of estrogen-induced LH surges and this was accompanied by enhanced preoptic secretion of ASP and GLU. In conclusion, we present the first data about amino acid release in the MBH during the E2-induced LH surge. Since only in the POA the LH surge is associated with changes in amino acid release, it appears that both inhibitory and excitatory amino acids act at the level of the GnRH cell bodies and/or dendrites and not on GnRH nerve terminals to mediate the feedback mechanism of E2 on LH release.

Sex differences in estrogen and progesterone receptor messenger ribonucleic acid regulation in the brain of little striped whiptail lizards.

Sex differences in the regulation of steroid hormone receptors in brain areas controlling female- and male-typical sexual behavior may be important in determining sex differences in the display of these behaviors. This study examined sex differences in estrogenic effects on the relative abundance of messenger RNA for estrogen receptor (ER) and progesterone receptor (PR) in discrete brain areas of whiptail lizards, Cnemidophorus inornatus, by in situ hybridization with radiolabeled riboprobes. Gonadectomized females and males received an estradiol benzoate (EB) injection (0.5 microgram) which effectively induces receptive behavior in females; controls received vehicle alone. Sex and regional differences in estrogenic effects on ER- and PR-mRNA abundance were found. Females responded to EB treatment with increases in ER- and PR-mRNA relative abundance in the ventromedial nucleus of the hypothalamus (VMH). Males had similar relative mRNA abundances to females in gonadectomized controls, but did not exhibit increases with EB treatment. EB treatment increased ER-mRNA abundance in the dorsal hypothalamus of females, but not males. ER-mRNA decreases in the lateral septum and PR-mRNA increases in the posterior hypothalamus with hormone treatment were also found, but did not differ by sex. Neither sex nor treatment effects were definitively shown for ER- or PR-mRNA abundance in the anterior hypothalamus-preoptic area. The VMH controls female-typical receptive behavior in this species. Sex differences in the response to estrogen in this nucleus may therefore underlie sex differences in the display of receptive behavior.(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of testosterone on the behavioral response to arginine vasopressin microinjected into the central gray and septum

Arginine vasopressin (AVP) plays an important role in the control of a gonadal hormone-dependent communicative behavior in the Syrian hamster ( Mesocricetus auratus) called flank marking. Previous studies have shown that gonadal hormones alter the amount of flank marking stimulated by the microinjection of AVP into the medial preoptic area-anterior hypothalamus (MPOA-AH). The purpose of the present study was to determine if testicular hormones alter the amount of flank marking stimulated by the microinjection of AVP into two other sites involved in the control of flank marking, the lateral septum-bed nucleus of the stria terminalis (LS-BNST) and the central gray. The data of the present study indicate that testicular hormones may influence the amount of AVP-stimulated marking in the central gray and LS-BNST; however, these effects are subtle and appear to occur primarily at high concentrations of AVP. When taken together with previous studies, these data indicate that gonadal hormones have greater effects on AVP-stimulated marking in the MPOA-AH than in the LS-BNST or central gray.

Can Gonadal Steroids Influence Cell Position in the Developing Brain?

The preoptic area/anterior hypothalamus (POA/AH) is a site where hormones dramatically influence development. The POA/AH is comprised of multiple subgroups, but little is known about the derivation of these subgroups during development. Results from several laboratories suggest that some cells in the POA/AH originate from progenitor cells in other regions of the developing nervous system. We are exploring pathways for migration in the developing POA/AH in two ways. First, we are examining the distribution of radial glial processes as potential migratory guides using immunocytochemistry. We have identified a transient pattern of radial glial processes from the lateral ventricles to the pial surface at the base of the POA/AH. Additionally, the expression of a molecule in radial glial processes originating in the third ventricle was decreased by prenatal treatment with testosterone. Second, we are utilizing time-lapse video microscopy in vitro to assess the extent and direction of movements of fluorescent dye-labeled cells at different ages in brain slice preparations from the POA/AH of developing rats. Data from these studies indicate that cell migration in the POA/AH includes movements along dorsal-ventral routes and from lateral to medial positions, in addition to the predicted medial to lateral pathway away from the third ventricle. Several researchers have examined effects of gonadal steroids on neurite outgrowth, cell differentiation, cell death, and synaptogenesis. The determination of cell position, however, may be a key event influenced by gonadal steroids earlier in development. The characterization of migratory pathways that contribute to permanent changes in brain structure and ultimately function is essential for unraveling the process of sexual differentiation.

High-frequency ultrasonic vocalization induced by intracerebral glutamate in rats

Direct injection of glutamate, a neuroexcitatory agent, into the anterior hypothalamic-preoptic area of the rat brain induced ultrasonic vocalization. This volcalization was characterized by short-duration calls (below 60 ms) of high sound frequency (pitch), mostly above 40 kHz, and was similar to the known 50-kHz vocalization observed in natural situations. The glutamate-induced vocalization was dose dependent within the dose range of 16.9–67.6 μg and was antagonized by local pretreatment with MK-801, an NMDA antagonist. The increasing dosage of glutamate induced more calls and had a significant influence on frequency and intensity of emitted ultrasound. The average sound frequency increased whereas the mean sound intensity decreased with the dosage of glutamate. On the other hand, the mean duration of a single call and the bandwidth did not significantly change with doses of glutamate. Injection of carbachol, a muscarinic cholinomimetic agent, into the same brain sites as glutamate, induced a different type of ultrasonic vocalization with low sound frequency and long call duration, known as 22-kHz calls. The results suggest that high sound frequency, short-duration calls (50 kHz) and low sound frequency, long-duration calls (22 kHz) have different neurophysiological and neurochemical mechanisms.

Effects of single doses and daily melatonin treatments on serotonin metabolism in rat brain regions.

The acute effects of two doses (0.5 and 1 mg/kg) of melatonin on the levels of tryptophan, serotonin, and 5-hydroxyindoleacetic acid in several rat brain regions were studied. Tryptophan content in the brain regions was unchanged by the treatments. Melatonin at a dosage of 0.5 mg/kg increased medial hypothalamic serotonin levels at 60 and 90 min after the injection. However, the dose of 1 mg/kg increased the levels of this amine or its metabolite in the preoptic area-anterior hypothalamus, medial and posterior hypothalamus, amygdala, and midbrain. These results suggest a specific regional sensitivity to melatonin as well as a dose-dependent response. The stimulatory melatonin effect on the serotoninergic system was also observed after a daily treatment with this hormone (0.5 mg/kg, twice daily during 10 days) in both intact or pinealectomized rats. In intact rats, melatonin treatment increased the levels of 5-hydroxyindole-3-acetic acid in the preoptic area-anterior hypothalamus and medial hypothalamus, while in pinealectomized rats melatonin increased the serotonin content in the medial hypothalamic region. The data support the idea that melatonin has a selective action on serotonin metabolism in regions that contain serotoninergic terminals, especially at medial hypothalamic level.

Variations in angiotensin-II release from the rat brain during the estrous cycle.

To investigate the hypothesis that the release of angiotensin-II (AII) in the rat brain increases on the day of proestrus, samples of cerebrospinal fluid (CSF) and interstitial fluid from the general region of the bed nucleus of the stria terminalis pars ventralis in the preoptic-anterior hypothalamic area were monitored for AII-immunoreactive material (AII-ir) using push-pull cannulas. Samples of CSF were obtained on the day of proestrus and diestrus day 1 at 30-min intervals from 1200-1600 h. Samples of interstitial fluid were obtained at 25-min intervals from 0930-1600 h. The rate of release of AII-ir into CSF was significantly greater on proestrus compared to diestrus day 1, and in the early afternoon of proestrus compared to the late afternoon. In five of seven rats and in the overall comparison of AII-ir release from the preoptic-anterior hypothalamic area, significantly more AII-ir was released on the day of proestrus vs. diestrus day 1. These observations are consistent with previous studies suggesting that brain AII may play a role in the regulation of LH release on the day of proestrus.

Thyrotropin-releasing hormone: Inhibitory function on growth hormone through both somatostatin and growth hormone-releasing factor neurons

Double labelling immunohistochemistry using antibodies to thyrotropin releasing hormone (TRH) and somatostatin (SS) was undertaken in the anterior hypothalamus in 6 rats. Light microscopic quantitation revealed that 94.5% of SS immunopositive perikarya in the preoptic anterior hypothalamic area ( PO AHA ) and 97.5% in the paraventricular nucleus appeared to be contacted by one or more TRH immunopositive terminals. In the chronically cannulated unanaesthetised male rat, unilateral microinjections of a range of doses of TRH were made in the PO AHA , where SS neurons are located, or in the medial basal hypothalamus, where growth hormone (GH)-releasing factor (GRF) neurons are located. Transient reductions in GH plasma levels occurred only after injections of the highest (10 nmol) dose of TRH in both sites. The function of TRH inputs to both somatostatin and GRF neurons appears to be inhibitory for GH. The physiological conditions in which these inputs function remain to be defined.