Posterior Hypothalamus Of Rats Research Articles

Avian Orexin: Feed Intake Regulator or Something Else?

Originally named for its expression in the posterior hypothalamus in rats and after the Greek word for “appetite”, hypocretin, or orexin, as it is known today, gained notoriety as a neuropeptide regulating feeding behavior, energy homeostasis, and sleep. Orexin has been proven to be involved in both central and peripheral control of neuroendocrine functions, energy balance, and metabolism. Since its discovery, its ability to increase appetite as well as regulate feeding behavior has been widely explored in mammalian food production animals such as cattle, pigs, and sheep. It is also linked to neurological disorders, leading to its intensive investigation in humans regarding narcolepsy, depression, and Alzheimer’s disease. However, in non-mammalian species, research is limited. In the case of avian species, orexin has been shown to have no central effect on feed-intake, however it was found to be involved in muscle energy metabolism and hepatic lipogenesis. This review provides current knowledge and summarizes orexin’s physiological roles in livestock and pinpoints the present lacuna to facilitate further investigations.

Nitric oxide-mediated defensive and antinociceptive responses organised at the anterior hypothalamus of mice are modulated by glutamatergic inputs from area 24b of the cingulate cortex.

Previous studies suggested that Cg1 area of the cingulate cortex of rats controls glutamate-mediated fear-induced defensive behaviour and antinociception organised at the posterior hypothalamus. In turn, microinjection of the nitric oxide donor SIN-1 into the anterior hypothalamus of mice produced defensive behaviours and fear-induced antinociception. However, it remains unknown whether Cg1 also modulates the latter mechanisms in mice. The present study examined the influence of Cg1 on SIN1-evoked fear-induced defensive behaviour and antinociception organised at the anterior hypothalamus of mice. The fear-like behavioural and antinociceptive responses to the microinjection of SIN-1 (300 nmol) into the anterior hypothalamus were evaluated after the microinjection of either N-methyl-D-aspartic acid receptor agonist (0.1, 1 and 10 nmol) or physiological saline into the cingulate cortex of C57BL/6 male mice. In addition, neurotracing and immunohistochemistry were used to characterise Cg1-anterior hypothalamus glutamatergic pathways. The data showed that activation of Cg1 N-methyl-D-aspartic acid receptors increased escape while reducing freezing and antinociceptive responses to SIN-1 microinjections into the anterior hypothalamus. Anterograde neural tract tracer co-localised with VGLUT2-labelled fibres suggests these responses are mediated by glutamatergic synapses at the anterior hypothalamus. In contrast with previous studies showing that Cg1 facilitates both escape and antinociception to chemical stimulation of the posterior hypothalamus in rats, the present data suggest that Cg1 facilitates escape while inhibiting defensive antinociception produced by the microinjection of SIN-1 in the anterior hypothalamus of mice. Accordingly, Cg1 may have opposite effects on antinociceptive responses organised in the anterior and posterior hypothalamus of mice and rats, respectively.

The features of summary background electric activity of the hypothalamus of rats under conditions of chronic caffeine alimentation

One of the factors of the environment which essentially shifts homeostasis is diets which contain caffeine. The aim of the study was to find out the basic characteristics of background electrical activity of trophotrophic and ergotrophic zones of the hypothalamus in conditions of chronic caffeine alimentation. Experiments were carried out on non-linear white male rats. The first group consisted of control animals (n = 22). The second group (n = 24) was represented by the animals that were given pure caffeine in an amount of 150 mg/kg/day with their meal. The registration on a electrohypothalamogram was carried out in conditions of acute experiment, every 2 weeks for 12 weeks. The spectral (mkV2) and the normalized power (%) of electrohypothalamogram waves were analyzed within the common frequency band. The analysis of the results allowed us to establish a certain specificity of the reaction of the neuronal system of the trophotropic and ergotropic zones of the rat hypothalamus to the effect of chronic caffeine alimentation. The main difference in the reactive state of electrophysiological indices in the trophotrophic zone of rats is the lack of a typical desynchronization from the 4th to the 8th week of the study and the hypersynchronization after 12 weeks of the experiment. The most probable mechanism that explains the results obtained is the ultra-powerful GABA-ergic modulation of this zone, the main energy-accumulating center. Perhaps, this powerful inhibitory resource in this cerebral locus is the main stress-limiting factor that makes this zone of the central nervous system of rats less sensitive to caffeine exposure. Instead, under the influence of chronic caffeine load in the ergotropic zone of the hypothalamus, after 6 weeks of the experiment desynchronous high-frequency rhythms dominated. During the subsequent time of the experiment, we observed a decrease in both low-frequency and high-frequency components of the electrohypothalamogram of this zone. This gives reason to assume that the key component of the neurophysiological response of the posterior hypothalamus of rats to the caffeine ration is the powerful glutamatergic effects on the pre-synaptic and post-synaptic neurons under conditions of reactive exhaustion of local neurosynthetics. Caffeine depletion of the hypothalamic neurotransmission at the end of the experiment is replaced by an effective adaptive ergotropic restoration of neurosynthetic activity in this locus of the central nervous system of rats. Thus, caffeine has a powerful activating effect on the ergotropic function of the posterior hypothalamus of rats. Such a difference in the chronic effect of caffeine on the trophotropic and ergotropic zone of the rat hypothalamus is primarily due to the different mediator support of these zones underlying their physiological purpose. GABA is the main mediator of the trophotropic zone and the main neurotransmitter of its synchronous activity. At the same time, neurotransmitter support of the ergotropic zone is represented by glutamate, which, along with other agents, implements its desynchronous activity. Since caffeine stimulates excitation, activating the pathways traditionally associated with motivational and motor reactions in the brain, it can be assumed that this explains the fact of a more powerful influence of caffeine precisely on the ergotrophic zone of the hypothalamus.

Involvement of endothelins in deoxycorticosterone acetate-salt hypertension through the modulation of noradrenergic transmission in the rat posterior hypothalamus.

What is the central question of this study? Does ex vivo administration of endothelin-1 and endothelin-3 regulate noradrenergic transmission in the posterior hypothalamus of deoxycorticosterone acetate-salt hypertensive rats compared with normotensive rats? What is the main finding and its importance? Endothelin-1 and endothelin-3 enhanced diverse mechanisms leading to increased noradrenergic transmission in the posterior hypothalamus of deoxycorticosterone acetate-salt hypertensive rats. Unveiling the role of brain endothelins in hypertension would probably favour the development of new therapeutic targets for the treatment of essential hypertension, which still represents a challenging disease with high mortality. Brain catecholamines participate in diverse biological functions regulated by the hypothalamus. We have previously reported that endothelin-1 and endothelin-3 (ET-1 and ET-3) modulate catecholaminergic activity in the anterior and posterior hypothalamus of normotensive rats. The aim of the present study was to evaluate the interaction between endothelins and noradrenergic transmission in the posterior hypothalamus of deoxycorticosterone acetate (DOCA)-salt hypertensive rats. We assessed the effects of ET-1 and ET-3 on tyrosine hydroxylase activity and expression, neuronal noradrenaline (NA) release, neuronal NA transporter (NAT) activity and expression, monoamine oxidase activity and NA endogenous content and utilization (as a marker of turnover) in the posterior hypothalamus of DOCA-salt hypertensive rats. In addition, levels of ETA and ETB receptors were assayed in normotensive and hypertensive rats. Results showed that tyrosine hydroxylase activity and total and phosphorylated levels, NAT activity and content, NA release, monoamine oxidase activity and NA utilization were increased in DOCA-salt rats. Both ET-1 and ET-3 further enhanced all noradrenergic parameters except for total tyrosine hydroxylase level and NA endogenous content and utilization. The expression of ETA receptors was increased in the posterior hypothalamus of DOCA-salt rats, but ETB receptors showed no changes. These results show that ET-1 and ET-3 upregulate noradrenergic activity in the posterior hypothalamus of DOCA-salt hypertensive rats. Our findings suggest that the interaction between noradrenergic transmission and the endothelinergic system in the posterior hypothalamus may be involved in the development and/or maintenance of hypertension in this animal model.

Activation of the central histaminergic system mediates arachidonic-acid-induced cardiovascular effects.

The aim of this study was to explain the involvement of the central histaminergic system in arachidonic acid (AA)-induced cardiovascular effects in normotensive rats using hemodynamic, immunohistochemistry, and microdialysis studies. Intracerebroventricularly (i.c.v.) administered AA (0.25, 0.5, and 1.0 μmol) induced dose- and time-dependent increases in mean arterial pressure and decreased heart rate in conscious normotensive Sprague-Dawley rats. Central injection of AA (0.5 μmol) also increased posterior hypothalamic extracellular histamine levels and produced strong COX-1 but not COX-2 immunoreactivity in the posterior hypothalamus of rats. Moreover, the cardiovascular effects and COX-1 immunoreactivity in the posterior hypothalamus induced by AA (0.5 μmol; i.c.v.) were almost completely blocked by the H2 receptor antagonist ranitidine (50 and 100 nmol; i.c.v.) and partially blocked by the H1 receptor blocker chlorpheniramine (100 nmol; i.c.v.) and the H3-H4 receptor antagonist thioperamide (50 and 100 nmol; i.c.v.). In conclusion, these results indicate that centrally administered AA induces pressor and bradycardic responses in conscious rats. Moreover, we suggest that AA may activate histaminergic neurons and increase extracellular histamine levels, particularly in the posterior hypothalamus. Acting as a neurotransmitter, histamine is potentially involved in AA-induced cardiovascular effects under normotensive conditions.

Intracellular signaling mechanisms underlying the activity and internalization of the neuronal norepinephrine transporter by endothelin‐3 in the rat posterior hypothalamus

Brain norepinephrine (NE) is involved in relevant physiological functions regulated by the hypothalamus. We previously reported that endothelins (ET) participate in the short term regulation of NE uptake in the anterior and posterior hypothalamus by changing the kinetic and internalization of the neuronal NE transporter (NET). In the present work we studied the mechanisms involved in the regulation of NET by ET‐3 in the rat posterior hypothalamic region (PHR). Experiments were performed in vitro and neuronal NE uptake and nisoxetine binding were used as indexes of activity and internalization of NET, respectively. Results showed that ET‐3 reduced neuronal NE uptake through the activation of a non conventional ET receptor involving PKC, PKA and PKG pathways. Furthermore ET‐3 enhanced NET internalization. However incubation with inhibitors of PKC (GF‐109203x), PKA (H‐89) or PKG (KT5823) inhibited ET‐3 response on NET internalization. Present findings suggest that the effect of ET‐3 on NET in the rat PHR is mediated by a non‐conventional ET receptor coupled to various intracellular pathways including PKC, PKA and PKG.(UBACyT B079, CONICET‐PIP 5929, ANPCyT PICT‐38333)

Behavior and Cellular Evidence for Propofol-induced Hypnosis Involving Brain Glycine Receptors

It is well documented that several general anesthetics, including propofol, potentiate glycine receptor function. Furthermore, glycine receptors exist throughout the central nervous system, including areas of the brain thought to be involved in sleep. However, the role of glycine receptors in anesthetic-induced hypnosis has not been determined. Experiments were conducted in rats where the loss of righting reflex (LORR) was used as a marker of the hypnotic state. Propofol-induced LORR was examined in the presence and absence of strychnine (a glycine receptor antagonist), GABAzine (a gamma-aminobutyric acid A receptor antagonist), as well as ketamine (an antagonist of N-methyl-D-aspartic acid subtype of glutamate receptors). Furthermore, the effects of propofol on the currents elicited by glycine and gamma-aminobutyric acid were analyzed in neurons isolated from the posterior hypothalamus of rats. The effects of strychnine and GABAzine on propofol-induced currents were also evaluated. Strychnine and GABAzine dose-dependently reduced the percentage of rats exhibiting LORR induced by propofol. Furthermore, strychnine significantly increased the onset time and reduced the duration of LORR induced by propofol. In contrast, strychnine did not affect the LORR induced by ketamine. In addition, propofol markedly increased the currents elicited by glycine and GABA of hypothalamic neurons. Conversely, strychnine and GABAzine both profoundly attenuated the current induced by propofol. Strychnine, the glycine receptor antagonist, dose-dependently reduced propofol-induced LORR in rats and propofol-induced current of rat hypothalamic neurons. These results suggest that neuronal glycine receptors partially contribute to propofol-induced hypnosis.

Involvement of NO and KATP Channel in Adenosine A2B Receptors Induced Cardiovascular Regulation in the Posterior Hypothalamus of Rats

Previous reports have suggested that the posterior hypothalamic adenosine A2 receptors may play a role in central cardiovascular regulation. In this study, we examined the influence of posterior hypothalamic adenosine A2B receptors on the regulation of blood pressure and heart rate. Drugs were injected into the posterior hypothalamus of anesthetized, artificially ventilated, male Sprague-Dawley rats. Four nanomoles of 5'-N-ethylcarboxamidoadenosine (NECA), an adenosine A 2A receptor agonist, decreased arterial blood pressure and heart rate, whereas 5 nmol of alloxazine, an adenosine A2B receptor antagonist, blocked the depressor and bradycardiac effects of 4 nmol NECA. We examined the role of nitric oxide (NO) and K+ channels on cardiovascular regulation by adenosine A2B receptors in the posterior hypothalamus. Pretreatment with 40 nmol of NG-nitro-L-arginine methyl ester, a NO synthase inhibitor, significantly attenuated the effects of NECA, and 10 nmol of sodium nitroprusside, a NO releaser, strengthened the action of drug. In addition, posterior hypothalamic administration of 20 nmol of glipizide, an K ATP blocker, blocked the cardiovascular depression elicited by NECA. These results suggest that NO mediates cardiovascular regulation by activation of A2B receptors in the posterior hypothalamus. Additionally, ATP-sensitive K+ channels modulate the action of adenosine A2B receptors.

Short-term regulation of tyrosine hydroxylase activity and expression by endothelin-1 and endothelin-3 in the rat posterior hypothalamus

Brain catecholamines are involved in several biological functions regulated by the hypothalamus. We have previously reported that endothelin-1 and -3 (ET-1 and ET-3) modulate norepinephrine release in the anterior and posterior hypothalamus. As tyrosine hydroxylase (TH) is the rate-limiting enzyme in catecholamine biosynthesis, the aim of the present work was to investigate the effects of ET-1 and ET-3 on TH activity, total enzyme level and the phosphorylated forms of TH in the rat posterior hypothalamus. Results showed that ET-1 and ET-3 diminished TH activity but the response was abolished by both selective ET A and ET B antagonists (BQ-610 and BQ-788, respectively). In addition ET A and ET B selective agonists (sarafotoxin S6b and IRL-1620, respectively) failed to affect TH activity. In order to investigate the intracellular signaling coupled to endothelins (ETs) response, nitric oxide (NO), phosphoinositide, cAMP/PKA and CaMK-II pathways were studied. Results showed that N ω-nitro- l-arginine methyl ester and 7-nitroindazole (NO synthase and neuronal NO synthase inhibitors, respectively), 1H-[1,2,4]-oxadiazolo[4,3-alpha]quinozalin-1-one and KT-5823 (soluble guanylyl cyclase, and PKG inhibitors, respectively) inhibited ETs effect on TH activity. Further, sodium nitroprusside and 8-bromoguanosine-3′,5′-cyclic monophosphate (NO donor and cGMP analog, respectively) mimicked ETs response. ETs-induced reduction of TH activity was not affected by a PKA inhibitor but it was abolished by PLC, PKC and CaMK-II inhibitors as well as by an IP 3 receptor antagonist. On the other hand, both ETs did not modify TH total level but reduced the phosphorylation of serine residues of the enzyme at positions 19, 31 and 40. Present results suggest that ET-1 and ET-3 diminished TH activity through an atypical ET or ET C receptor coupled to the NO/cGMP/PKG, phosphoinositide and CaMK-II pathways. Furthermore, TH diminished activity may result from the reduction of the phosphorylated sites of the enzyme without changes in its total level. Taken jointly present and previous results support that ET-1 and ET-3 may play a relevant role in the modulation of catecholaminergic neurotransmission in the posterior hypothalamus of the rat.

Modulatory effect of endothelin-1 and -3 on neuronal norepinephrine release in the rat posterior hypothalamus

Based upon the existence of high density of ET-receptors on catecholaminergic neurons of the hypothalamus, we studied the effects of endothelin-1 (ET-1) and endothelin-3 (ET-3) on neuronal norepinephrine (NE) release in the rat posterior hypothalamus. The intracellular pathways and receptors involved were also investigated. Neuronal NE release was enhanced by ET-1 and ET-3 (10 ηM). The selective antagonists of subtype A and B ET receptors (ET A, ET B) (100 ηM BQ-610 and 100 ηM BQ-788, respectively) abolished the increase induced by ET-1 but not by ET-3. The PLC inhibitor, U73122 (10 μM), abolished ET-1 and ET-3 response. GF-109203X (100 ηM) (PKC inhibitor) blocked the increase in NE release produced by ET-3 and partially blocked ET-1 response. The inositol 1,4,5-trisphosphate-induced calcium release inhibitor, 42 μM 2-APB, inhibited the stimulatory effect induced by ET-3 but not by ET-1. The PKA inhibitor, 500 ηM H-89, blocked the increase in neuronal NE release evoked by ET-1 but not by ET-3. Our results showed that ET-1 as well as ET-3 displayed an excitatory neuromodulatory effect on neuronal NE release in the rat posterior hypothalamus. ET-1 through an atypical ET A or ET B receptor activated the PLC/PKC signalling pathway as well as the cAMP pathway, whereas ET-3 through a non-ET A/non-ET B receptor activated the phosphoinositide pathway. Both ETs would enhance the sympathoexcitatory response elicited by the posterior hypothalamus and thus participate in cardiovascular regulation.

The involvement of nitric oxide on the adenosine A 2 receptor-induced cardiovascular inhibitory responses in the posterior hypothalamus of rats

This study was performed to investigate the putative relationship between nitric oxide (NO) and adenosine A 2 receptors on central cardiovascular regulation in the posterior hypothalamus of rats. Posterior hypothalamic injection of drugs was performed in anesthetized, artificially ventilated male Sprague–Dawley rats. Injection of adenosine A 2 receptor agonist 5′-( N-cyclopropyl)-carboxamidoadenosine (CPCA; 1, 2 and 5 nmol) produced a dose-dependent decrease of blood pressure and heart rate. Pretreatment with adenosine A 2 receptor antagonist 3,7-dimethyl-1-propargylxanthine (10 nmol) blocked the depressor and bradycardiac effects of CPCA (5 nmol). Pretreatment with soluble guanylate cyclase inhibitor LY-83,583 (5 nmol) attenuated the depressor and bradycardiac effects of CPCA (5 nmol). In addition, pretreatment with NO synthase inhibitor N G -nitro- l-arginine methyl ester (40 nmol) attenuated the depressor and bradycardiac responses of CPCA (5 nmol). These results suggest that adenosine A 2 receptor in the posterior hypothalamus plays an inhibitory role in central cardiovascular regulation and that NO participates in the inhibitory response induced by adenosine A 2 receptor stimulation in the posterior hypothalamus.

Modification of cardiovascular responses to adenosine A1 receptor stimulation in the posterior hypothalamus of anaesthetized rats by cAMP and by GABA(B) receptor blockade.

1 Injection of N(6)-cyclohexyladenosine (CHA; 1, 5 and 10 nmol), an adenosine A1 receptor agonist, into the posterior hypothalamus of rats produced a dose-dependent decrease in blood pressure (BP) and heart rate (HR). 2 Pretreatment with 8-cyclopentyl-1,3-dimethylxanthine (CPDX; 50 nmol), an adenosine A1 receptor antagonist, blocked the depressor and bradycardic effects of CHA (10 nmol). 3 Pretreatment with 8-bromo-cyclic adenosine monophosphate (AMP) (10 nmol), a cAMP analogue, attenuated the depressor and bradycardic effect of CHA (10 nmol); 8-bromo-cyclic guanosine monophosphate (GMP) (10 nmol), a cGMP analogue, did not modify those effects of CHA. 4 In addition, pretreatment with 5-aminovaleric acid (25 nmol), a gamma-aminobutyric acid (GABA)(B) receptor antagonist, attenuated the depressor and bradycardic effects of CHA (10 nmol). 5 These results suggest that adenosine A1 receptors in the posterior hypothalamus have an inhibitory role in the central cardiovascular regulation and that these vasodepressive and bradycardic actions are modified by raised cAMP and by GABA(B) receptor inhibition.

Histamine monoclonal antibody for brain immunocytochemistry.

Among five monoclonal antibodies (AHA-1 to 5 mAbs) prepared against glutaraldehyde (GA)-conjugated histamine (HA) in our previous study, only mAb AHA-2 was found to detect HA specifically in rat brain neurons by an immunocytochemistry method (ICC) using GA as a tissue fixative. All the other mAbs, except for AHA-5, reacted with HA in the enterochromaffin-like cells (ECL cells) of rat stomach [Fujiwara et al. (1997) Histochem. Cell Biol. 107, 39-45]. Enzyme-linked immunosorbent assay (ELISA) binding and inhibition tests demonstrated that AHA-2 is specific for HA, with almost no detectable cross-reaction with any other established or putative amino acid neurotransmitters, LH-RH, TRH, or peptides with N-terminal histidines. ELISA assays also suggested that the AHA-2 mAb recognizes a HA epitope structure different from the one recognized by the AHA-1 mAb. The immunostaining patterns with AHA-2 mAb, as seen in the five subgroups of the tuberomammillary nuclei in the rat posterior hypothalamus, were very similar to those described by Inagaki et al. [(1988) Brain Res. 439, 402-405; (1990) Exp. Brain Res. 80, 374-380] and Panula et al. [(1984) Proc. Natl. Acad. Sci. USA 81, 2572-2576; (1988) J. Histochem. Cytochem. 36, 259-269] using polyclonal anti-HA serum. However, it was also noted that moderate numbers of immunoreactive nerve fibers projected into the median eminence. The present HA ICC method using AHA-2 mAb allows highly sensitive HA detection in brain, and thus might permit detailed studies of HA localization hitherto impossible using previously available anti-HA polyclonal antibodies produced against carbodiimide-conjugated HA.

Electrical stimulation of the mammillary nuclei increases seizure threshold to pentylenetetrazol in rats.

High-frequency electrical stimulation of mammillary nuclei (MN) of rat posterior hypothalamus resulted in a significant increase in seizure threshold induced by pentylenetetrazol (PTZ). The anticonvulsant effect was frequency and intensity specific. Stimulation at 100 Hz (1-5 V, 30-200 microA) afforded protection against EEG and behavioral manifestations of PTZ seizures. Stimulation of either low frequency (5 Hz), high intensities (8-20 V, 300-800 microA), or outside the histologically verified MN target region did not increase seizure threshold. In some instances, high-intensity stimulation of MN alone elicited spike-wave epileptiform EEG activity accompanied by either arrest of behavior or myoclonic seizures. In animals with ongoing seizure activity, electrical stimulation of MN disrupted the high-voltage synchronous wave forms on cortical EEG. These data support the concept that electrical perturbation of MN in hypothalamus may functionally inhibit generalization of paroxysmal activity required for expression of the EEG and, in particular, the behavioral component of PTZ seizures. These studies provide additional insight into forebrain-brainstem interactions mediating generalized seizure expression.

Antagonism of central pressor response to angiotensin II by α-human atrial natriuretic polypeptide at the preoptic area and posterior hypothalamus in rats

Effects of α-human atrial natriuretic polypeptides (α-hANP) on pressor responses to angiotensin II (AII) were assessed at the preoptic area, posterior hypothalamus and central amygdaloid nucleus (ACE) in spontaneously hypertensive (SHR) and control normotensive Wistar-Kyoto (WKY) rats. Angiotensin II, administered intracerebroventricularly, at a dose of 100 ng produced a marked pressor response in hypertensive, as well as in normotensive rats and the response was potentiated in hypertensive rats. The response was antagonized in a dose-dependent manner by administration of α-hANP into the preoptic area and posterior hypothalamus but not to the amygdaloid nucleus. The antagonism was more marked in hypertensive than in normotensive rats. Angiotensin II, when injective directly to the preoptic area at a small dose of 10 ng similarly evoked a marked pressor response, which was augmented in hypertensive rats. This response was also antagonized by coadministration of α-hANP to the preoptic area in hypertensive but not in normotensive rats. The results suggest that the antagonistic relationship between ANP and AII exists at the preoptic area and posterior hypothalamus, probably implying that the activity of the ANP and AII systems in brain play a role in centrally controlling the cardiovascular system and is altered at these areas in genetically hypertensive rats.

Regulation of histamine release in rat hypothalamus and hippocampus by presynaptic galanin receptors

The effect of galanin, a peptide present in a subpopulation of histaminergic neurons emanating from the rat posterior hypothalamus, was investigated on K +-evoked [ 3H]histamine release in slices and synaptosomes from rat cerebral cortex, striatum, hippocampus and hypothalamus. Porcine galanin (0.3 μM) significantly inhibited histamine release induced by 25 mM K + in slices from hypothalamus and hippocampus, but not from cerebral cortex and striatum, i.e., only in regions in which a colocalization of histamine and galanin has been described. The inhibitory effect of galanin was concentration dependent, with an EC 50 value of 5.8±1.9 nM. The maximal inhibition was of 30–40% in hypothalamic and hippocampal slices depolarized with 25 mM K +. The galanin-induced inhibition observed in hypothalamic slices was not prevented in the presence of 0.6 μM tetrodotoxin and also occurred in hippocampal and hypothalamic synaptosomes, strongly suggesting the activation by galanin of presynaptic receptors located upon histaminergic nerve endings. The maximal inhibitory effect of galanin in slices or synaptosomes was lower than that previously reported for histamine acting at H 3-autoreceptors, possibly suggesting that not all histaminergic axon terminals, even in the hypothalamus and hippocampus, are endowed with galanin receptors. It increased progressively in hypothalamic and hippocampal synaptosomes as the strength of the depolarizing stimulus was reduced. It is concluded that galanin modulates histamine release via presynaptic receptors, presumably autoreceptors located upon nerve terminals of a subpopulation of cerebral histaminergic neurons.

Amphetamine affects the extinction of self-stimulation differently in prefrontal cortex and posterior hypothalamus of rats

The effects of amphetamine on the extinction of intracranial self-stimulation (ICSS) and on postextinction ICSS performance were examined in rats implanted with electrodes either in medial prefrontal cortex (mPFC) or in the posterior hypothalamus-ventral tegmental area (PH-VTA). Lever-pressing for ICSS was allowed to stabilize in daily 15-minute sessions before each animal was exposed to 5 minutes of extinction (responding without reward). Animals were administered either 0.25 mg/kg d-amphetamine or saline before baseline, extinction and lower in PH-VTA animals compared with saline-treated controls. Rates did not change immediately after extinction but, one day later, postextinction sessions. After amphetamine treatment, the number of lever presses during extinction was higher in mPFC animals and rates had increased in all saline-treated animals (both PH-VTA and mPFC animals) and had decreased in all amphetamine-treated animals. These findings demonstrated that the effects of amphetamine on the extinction of ICSS were different in cortical and hypothalamic sites, possibly because of regional differences in stimulus-evoked reinforcement and inhibitory processes.

GABA receptors in the posterior hypothalamus regulate experimental anxiety in rats

Blockade of γ-aminobutyric acid (GABA) function in the posterior hypothalamus of rats elicits a pattern of physiological and behavioral arousal consisting of increases in heart rate, respiration and blood pressure as well as intense locomotor stimulation and a selective enhancement of avoidance responding. The present study was conducted to assess the possibility that GABA-mediated neurotransmission in the posterior hypothalamus of the rat may regulate anxiety. Male rats were trained in a ‘conflict’ schedule consisting of a high and a low intensity of punishment (‘high’ and ‘low’ conflict) capable of measuring decreases and increases in the level of ‘anxiety’, respectively. Guide cannulae were stereotaxically implanted bilaterally in the posterior hypothalamus of these rats at sites where microinjection of bicuculline methiodide (BMI) 25 ng caused increases in heart rate under anesthesia. After recovery, they were tested: (1) in the high conflict schedule after microinjection of saline and two doses of the GABA A receptor agonist muscimol; and (2) in the low conflict schedule after injecting saline, the GABA A receptor antagonists, BMI and picrotoxin, and the glycine antagonist, strychnine. Injection of muscimol caused a significant and selective anti-conflict effect while both BMI and, at appropriate doses, picrotoxin produced pro-conflict effects. Microinjection of strychnine into the posterior hypothalamus or muscimol and picrotoxin into the lateral hypothalamus did not influence conflict responding. These results suggest that endogenous GABA acts on GABA A receptors in a discrete area of the posterior hypothalamus to regulate the level of experimental anxiety in rats.

Injection of L-allylglycine into the posterior hypothalamus in rats causes decreases in local gaba which correlate with increases in heart rate

Injection of the GABA antagonist, bicuculline methiodide into the posterior hypothalamus of rats has been shown to cause marked increases in heart rate and lesser elevations in blood pressure. Allylglycine is a potent inhibitor of the synthetic enzyme for GABA, glutamic acid decarboxylase, only after in vivo biotransformation into its active form, 2-keto-4-pentenoic acid, through a stereospecific amino acid oxidase. The posterior hypothalamus is thought to contain substantial activity only of L-amino acid oxidase. In this study, the stereoisomers of allylglycine were injected into the posterior hypothalamus at a site also shown to be reactive to bicuculline. Injection of L-allylglycine but not D-allylglycine caused substantial increases in heart rate but only slight increases in blood pressure. Injection of the GABA agonist muscimol prior to treatment with L-allylglycine prevented these cardiovascular changes. In another series of experiments, levels of GABA in the posterior hypothalamus and adjacent areas were measured 90 min after unilateral injection of L-allylglycine (12.5 or 25 μg), D-allylglycine (25 μg) or saline into the posterior hypothalamus. Only L-allylglycine caused increases in heart rate and blood pressure and decreases in levels of GABA. Quantitatively, the increases in heart rate at sacrifice were strongly correlated with the decreases in levels of GABA in the injected posterior hypothalamus ( r = −0.94; P < 0.002) but not in other regions. These findings (1) support the hypothesis that GABAergic synapses in the posterior hypothalamus suppress the activity of a sympathoexcitatory mechanism capable of increasing heart rate and blood pressure in the rat and (2) suggest that L-allylglycine-induced decreases in the levels of GABA in local tissue are quantitatively related to the degree to which local GABAergic inhibition is impaired.

Hypothalamic GABA suppresses sympathetic outflow to the cardiovascular system.

We studied the cardiovascular effects of altering GABA-ergic tone in the posterior hypothalamus in rats. Animals were equipped with chronic guide cannulas placed in the posterior hypothalamus, arterial and venous catheters, and a bipolar electrode on the splanchnic nerve. Microinjected bilaterally into the posterior hypothalamus in conscious rats, the postsynaptic gamma-aminobutyric acid (GABA) antagonists bicuculline methiodide and picrotoxin rapidly increased heart rate, blood pressure, and sympathetic nerve activity. Microinjection of the GABA agonist muscimol into this same region decreased heart rate, blood pressure, and sympathetic nerve activity in conscious rats. In contrast, muscimol infused into the posterior hypothalamus of anesthetized rats failed to alter heart rate or blood pressure. We conclude that 1) the posterior hypothalamus contains a sympathoexcitatory system that is modulated by changes in GABA-ergic tone and 2) tonic GABA-ergic inhibition is sufficient to completely suppress the activity of this hypothalamic sympathoexcitatory system in anesthetized animals but not in conscious rats.