Hypothalamic neuronal histamine modulates ad libitum feeding by rats

Manipulating histamine endogenously, its effects on brain functions were assessed in rats. alpha-Fluoromethylhistidine (FMH), an inhibitor of histamine synthesis, elicited feeding (P less than 0.01) after intra-third cerebroventricular infusion at the early light phase when hypothalamic histamine was normally highest. No periprandial drinking was observed. The effect of FMH was attenuated, and thioperamide, an antagonist of auto-inhibitory effects on both histamine synthesis and release at presynaptic H3-receptor, conversely suppressed food intake (P less than 0.05), when these probes were carried out during the minimum histamine level early in the dark period. Bilateral microinfusion of FMH into the ventromedial hypothalamus (VMH) and the paraventricular nucleus (PVN) selectively induced feeding, but the infusion into the remaining sites of the hypothalamus had no effect. These data show that neuronal histamine plays a physiological role in feeding suppression through the VMH and the PVN in the rat.     

Histaminergic control of energy balance in rats

Manipulating neuronal histamine in the hypothalamus, its effects on brain functions were assessed in nonobese normal rats and Zucker rats. Alpha-fluoromethylhistidine (FMH), an inhibitor of histamine synthesis, induced feeding dose-dependently after 2.24 mumol infusion at 1100 h, when hypothalamic histamine was normally high. This dose of FMH selectively decreased hypothalamic histamine, but not other neurotransmitters. Thioperamide, an antagonist of autoinhibitory H3-receptors, decreased food intake after infusion at 1940 h, when hypothalamic histamine was normally low. Bilateral microinfusion of 224 nmol FMH or 26 nmol chlorpheniramine, an H1-antagonist, into the ventromedial hypothalamus (VMH) and the paraventricular nucleus (PVN), elicited feeding. However, Zucker obese rats showed no significant responses to chlorpheniramine, thioperamide or histamine. Concentration of their hypothalamic histamine was excessively lower than that of the nonobese. Contents of hypothalamic histamine were lowered at 4 degrees C and raised at 31 degrees C. FMH attenuated increase in histamine, and then disrupted adaptive behavior. These findings indicate that neuronal histamine may convey the suppressive signal of food intake through H1-receptors in the VMH and/or the PVN, and play critical roles in homeostatic control of adaptive behavior.     

Hypothalamic sites of neuronal histamine action on food intake by rats

To identify sites of histaminergic modulation of food intake, histamine H₁-receptor antagonist was microinfused into the rat hypothalamus, the ventromedial hypothalamus (VMH), the lateral hypothalamus (LHA), the paraventricular nucleus (PVN), the dorsomedial hypothalamus (DMH), or the preoptic anterior hypothalamus (POAH), during the early light period. Feeding, but not drinking, was elicited in 100% of the rats (P<0.01) that were bilaterally microinfused with 26 nmol chlorpheniramine into the VMH. Unilateral infusion into the VMH did not affect food intake at doses of 26 or 52 nmol. Feeding was also induced by bilateral microinfusion into the PVN, but only the 52 nmol dose was effective. Bilateral infusions into the LHA, the DMH or the POAH did not affect ingestive behavior. Feeding induced by an H₁-antagonist was completely abolished in all 7 rats tested when endogenous neuronal histamine was decreased by pretreatment with α-fluoromethylhistidine (100 mg/kg). The findings suggest that H₁-receptors in the VMH and the PVN, but not in the LHA, the DMH or the POAH, may be involved in histaminergic suppression of foof intake.     

Aminoglucose-induced feeding suppression is regulated by hypothalamic neuronal histamine in rats

Central mechanisms involved in feeding suppression produced by 1-deoxy-d-glucosamine (1-DGlcN) and1-deoxy-N-acetylglucosamine (1- DGlcNAc) are unclear. To clarify the mechanisms, we investigated the role of hypothalamic neuronal histamine (HA) in feeding suppression induced by 1-DGlcN and 1-DGlcNAc in rats. Food intake was suppressed for 3 days after a single infusion of 24 μmol 1-DGlcN into the third cerebroventricle (i.c.v.) Depletion of presynaptic HA due to intraperitoneal infusion (i.p.) of α-fluoromethylhistidine (FMH), a specific inhibitor of the HA synthesizing enzyme histidine decar☐ylase (HDC), abolished feeding suppression completely. Blockade of postsynaptic H₁-receptors by i.p. injection of 26 μmol chlorpheniramine also abolished the suppression. Oral administration of 2.4 mmol 1-DGlcNAc suppressed food intake. However, depletion of neuronal HA due to FMH did not affect the suppression. I.c.v. infusion of 24 μmol 1-DGlcN increased turnover rate of HA at 1 h after the infusion. Hypothalamic HA concentration, but not that of tele-methylhistamine (t-MH), increased at 24 h after i.c.v. infusion of 1-DGlcN, which suggests a correlation between HA concentration and the behavioral response. These results indicate that 1-DGlcN, but not 1-DGlcNAc, modulates feeding suppression through HA neurons in the hypothalamus. Differences in mechanisms of feeding suppression by these aminoglucoses may depend on the principal sites of acton in the brain and/or peripheral organs.     

Effects of intracerebroventricularly infused histamine and selective H1, H2 and H3 agonists on food and water intake and urine flow in Wistar rats

The actions of intracerebroventricularly infused histamine and selective histamine H₁, H₂ and H₃ receptor agonists on food and water intake and urine flow were studied in rats. It was found that 100–800 nmoles of histamine significantly suppressed feeding. The H₁ agonist 2-(3-trifluoromethylphenyl)histamine (FMPH) decreased food intake, whereas the H₂ agonist dimaprit was without effect. Histamine- and FMPH-induced suppressions of feeding were attenuated by blockade of H₁ but not by H₂ receptors. The results clearly demonstrate that activation of brain H₁ receptors decreases food intake. In subsequent studies, we found that both metoprine and thioperamide, which increase histaminergic activity through different mechanisms, also reduced food intake. This finding indicates that the brain histaminergic system is associated with feeding behavior. The same is true with body water homeostasis. Histamine caused a long-lasting diuresis. Also dimaprit and metoprine increased urine flow and the blockade of H₂ receptors abolished the diuretic responses to histamine and dimaprit. On the other hand, the H₃ agonist (R)-α-methylhistamine elicited drinking and this effect could be prevented by thioperamide pretreatment. The results imply that activation of H₃ receptors predominantly provokes drinking, whereas central H₂ receptors mediate the diuretic effect of histamine.     

Anorexic action of a new potential neuropeptide Y antagonist [ -Tyr, -Thr]-NPY (27–36) infused into the hypothalamus of the rat

Neuropeptide Y (NPY) produces a vigorous feeding response in several species when it is injected into hypothalamic structures involved in eating behavior. The purpose of this study was to determine whether a unique carboxy terminal fragment of NPY would alter the pattern of eating induced in the rat either by NPY injected into the hypothalamus or by a 24-h period of food deprivation. In this case, two -tyrosine residues and one t.-threonine residue of the NPY_27–36 fragment were transformed to their D-conformation to produce [ -Tyr^27,36, -Thr³²]-NPY (27–36), i.e., D-NPY_27–36. Guide cannulae for microinjection were implanted stereotaxically just dorsal to the paraventricular nucleus (PVN) or ventromedial hypothalamus (VMH) of 24 adult male Sprague-Dawley rats. Following postoperative recovery, a microinjection of artificial CSF or 1.1 jig or 3.3 μg of a peptide was made directly into the PVN or VMH as follows; native NPY; D-NPY_27–36; or [L-Tyr^27,36 L-Thr³²]-NPY (27–36), i.e., L-NPY_27–36. Food intakes were measured at intervals of 0.25, 0.5, 1.1, 2.0, 4.0, and 24 h. When D-NPY_27–36 was microinjected at NPY reactive sites in the PVN or VMH of the rat 15 min before a similar microinjection of NPY, the intense eating response induced by the peptide was reduced significantly. Not only was the effect dose dependent, but D-NPY_27–36 also augmented the latency to feed. A mixture of the two doses of NPY and DNPY_27–36 injected at the same hypothalamic loci did not attenuate the intake of food but tended to enhance the feeding response in the rats. After the rats were deprived of food for 24 h, D-NPY_27–36 microinjected in the same hypothalamic sites similarly caused a dose-dependent suppression of normal feeding behavior. However, the CSF control vehicle and L-NPY_27–36 microinjected in the PVN or VMH were without effect on the pattern of eating. Further, D-NPY_27–38 injected in the same hypothalamic sites affected neither body temperature nor water intakes of the rats significantly. These results demonstrate that the D substitution of this C-fragment of the NPY molecule, i.e., D-NPY_27–36, serves to inhibit feeding evoked in the rat by hypothalamic NPY as well as the natural eating response to food deprivation. Thus, the D-NPY_27–36 molecule may act as an antagonist at one or more subtypes of the NPY receptor in the brain of the rat.     

GABA-related feeding control in genetically obese rats

Feeding in response to glucoprivation induced by 2-deoxy-D-glucose (2-DG) is impaired in genetically obese (Zucker) rats. Muscimol, a GABAA-agonist (0.5 nmol/0.5 microliter in each area) increased food intake in lean rats over 3 h but in fatty rats only at 30 min after infusion into the VMH. Injection of muscimol into the DMH and PVN increased feeding of both phenotypes. Picrotoxin, a non-competitive GABAA-antagonist (0.1 nmol/0.5 microliter) increased food intake after infusion into the LH of both phenotypes and decreased food intake over a 3 h period when infused into the VMH. DMH and PVN of fatty rats. In the lean littermates, picrotoxin was only effective in reducing food intake at 30 min after infusion into the VMH and PVN but not the DMH. The present results suggest that the fatty Zucker rat has a disturbance in the GABA-related regulatory mechanism of feeding behavior in the ventromedial hypothalamus, which may be responsible for the impaired response to glucoprivation found in these rats.     

α1-Adrenergic agonists act on the ventromedial hypothalamus to cause neuronal excitation and lordosis facilitation: electrophysiological and behavioral evidence

To see if activation of central α₁-adrenergic receptors can cause facilitation of lordosis in rats, the behavioral effects of centrally administered α₁-agonist, methoxamine (MA) and phenylephrine (PhE), and related agents were studied. In ovariectomized rats treated with estrogen, infusion of MA, PhE, or a β-agonist isoproterenol, into the lateral ventricle, or bilateral infusions of MA or PhE into the ventromedial hypothalamus (VMH) facilitated lordosis. Conversely, intra-VMH infusion of the α₁-antagonist prazosin (PZ) inhibited lordosis. Intra-VMH infusion of isoproterenol or an α₂-agonist clonidine, had no effect. Neither was the intra-VMH infusion of MA effective if: (i) the rats were not primed with estrogen; (ii) the tips of the cannulae were outside the VMH; or (iii) it was preceded by an intra-VMH infusion of the α_1b-antagonist, chloroethylclonidine (CEC). These results not only verify implications from recent studies that α₁-receptors in the hypothalamus are important for lordosis facilitation, but further show that the adrenergic facilitatory effect are: (i) mediated specifically by α_1b-subtype of the α₁-receptor, (ii) estrogen-dependent, and (iii) site-specific to VMH. To investigate neural mechanisms potentially underlying the lordosis-facilitating effect of α₁-activation, the actions of MA and PhE on the electrical activity of single neurons of the ventromedial nucleus of the hypothalamus (VMN) in vitro were studied. As in the behavioral study, the neuronal actions of the agonists are similar to each other: (i) both are excitatory — MA affected 78% of the 65 units tested, all by excitation, and PhE affected 74% of 65 VMN units, predominantly with excitation; (ii) the excitatory actions of MA and PhE affected the same population of VMN neurons: and (iii) the excitatory actions of both agonists were blocked by both PZ and CEC, indicative of mediation by α_1b-receptors. Behavioral/electrophysiological parallels suggest that act α₁-agonists through α_1b-receptors to excite VMN neurons and thereby facilitate lordosis. Since α_1b-receptors are known to be coupled to phosphoinositide (PI) second messenger system, and since the behavioral effect is estrogen-dependent, the facilitatory effect of α₁-agonists may require estrogen modulation of α_1b-receptors and/or the coupled PI system in the VMN.     

The effects of histamine H3-receptor antagonists on amygdaloid kindled seizures in rats

The effects of histamine H₃-receptor antagonists, thioperamide, and clobenpropit on amygdaloid kindled seizures were investigated in rats. Both intracerebroventricular (i.c.v.) and intraperitoneal (i.p.) injections of H₃-antagonists resulted in a dose-related inhibition of amygdaloid kindled seizures. An inhibition induced by thioperamide was antagonized by an H₃-agonist [(R)-α-methylhistamine] and H₁-antagonists (diphenhydramine and chlorpheniramine). On the other hand, an H₂-antagonist (cimetidine and ranitidine) caused no antagonistic effect. Metoprine, an inhibitor of N-methyltransferase was also effective in inhibiting amygdaloid kindled seizure, and this effect was augmented by thioperamide treatment.     

Pharmacological characterisation of the histamine H3 receptor in the rat hippocampus

The purpose of this report was to pharmacologically characterise the histamine H₃ in the rat hippocampus using radioligand binding studies with the H₃ receptor antagonist [¹²⁵I]iodophenpropit and the H₃ receptor mediated inhibition of [³H]noradrenaline release. A dissociation constant of 0.33 nM and a maximal number of binding sites of 125 fmol/mg protein were found for [¹²⁵I]iodophenpropit. Competition studies showed stereoselectivity for the (R) and (S) enantiomers of α-methylhistamine and 10 μM of GTP_γS shifted the curve of (R)-α-methylhistamine rightwards. Up to 1 μM, (R)-α-methylhistamine displaced only 30% whereas the tested H₃-antagonists displaced 50–60% of the total [¹²⁵I]iodophenpropit bound. This indicates the presence of an additional non-H₃ receptor binding site(s) for [¹²⁵I]iodophenpropit in the rat hippocampus. This secondary site shows low affinity for H₃ agonists, but high affinity for the tested H₃ antagonists. Electrically evoked [³H]acetylcholine release was shown in slices of rat hippocampus. No H₃ receptor modulation of [³H]acetylcholine release from hippocampal slices was detectable. However, H₃ receptor activation inhibited 42% of the electrically-evoked [³H]noradrenaline release in rat hippocampal slices. The inhibition of [³H]noradrenaline release was effectively antagonized by the H₃ antagonists thioperamide and burimamide. We describe the pharmacological identification of the histamine H₃ receptor in the rat hippocampus and its similarities and differences from the cortical H₃ receptor. These studies enable us to investigate changes in density and functionality of the hippocampal H₃ receptor under (patho)physiological conditions.     

Progesterone has rapid and membrane effects in the facilitation of female mouse sexual behavior

Ovariectomized (ovx) mice require both estradiol (E₂) and progesterone (P) administration to reinstate feminine sexual behavior (lordosis). The importance of P's actions at E₂-induced intracellular progestin receptors (PRs) to facilitate lordosis was investigated in PR knockout (PRKO) mice, PRKO's wild type littermates (C57X129), and wild type C57BL/6J (C57) mice. Subjects were ovx, E₂-primed (0.5 μg) and tested following intravenous (i.v.) and intercereberal P. Intravenous P (200 μg) significantly increased lordosis of all mice within 10 min of P, but vehicle infusion did not (Experiment 1). Intravenous P significantly increased the amount and duration and reduced the latency of lordosis, over that seen with vehicle infusion, in PRKO and wild type mice. Whole brain concentrations of P and its 5α-reduced metabolite, 5α-pregnan-3α-ol-20-one (3α,5α-THP), which has low affinity for intracellular PRs, were also increased following P compared to vehicle infusion. Progesterone, but not vehicle infusions, significantly increased the number of PR-immunoreactive (PR-IR) cells in the ventromedial hypothalamus (VMH) of C57 and C57X129 mice and increased number of 3α,5α-THP-immunoreactive (3α,5α-THP-IR) cells in the ventral tegmental area (VTA) of all mice. In Experiment 2, P conjugated to bovine serum albumin (P:BSA) increased lordosis when applied bilaterally to both the VMH and VTA of E₂-primed mice more than BSA implants. Progesterone implants increased the number of PR-IR cells in the VMH of C57 and C57X129 mice and the number of 3α,5α-THP-IR cells in the VTA of all mice. The rapid facilitation of lordosis with i.v. P infusion and increases in lordosis when P's effects are relegated to the membrane in the VMH and VTA of PRKO and wild type mice suggest that P may facilitate lordosis through actions at substrates other than intracellular PRs. The present findings suggest a role of 3α,5α-THP.     

Zucker obese rats: defect in brain histamine control of feeding

Manipulation of hypothalamic histamine produced different effects on feeding between the Zucker obese (fa/fa) and their lean littermate rats (Fa/−). Infusion of a histamine H₁-receptor antagonist into the third cerebroventricle elicited feeding in the lean and Wistar King A rats, but it did not affect feeding in the obese rats. To enhance hypothalamic neuronal histamine, thioperamide, an H₃-receptor antagonist, was similarly infused. The lean and Wistar rats decreased their food intake after the infusion, but thioperamide produced no significant effect on feeding in the obese rats. Infusion of histamine into the third cerebroventricle mimicked the effects of thioperamide on feeding: reduction of food intake in the lean and Wistar rats, but no significant change in the obese rats. Hypothalamic histamine of the obese rats (0.430 nmol/g) was significantly lower than the lean (1.209 nmol/g) and Wistar rats (4.838 nmol/g). The histamine concentration of the cerebral cortex in the obese rats was also lower than the non-obese animals. The results indicate that the feeding abnormality of Zucker obese rats may be at least due to disturbance of histamine suppressive signals both at presynaptic and postsynaptic levels.     

Effects of auricular stimulation on feeding-related hypothalamic neuronal activity in normal and obese rats

It is known that auriculotherapy occasionally affects dramatic body weight reduction for obese patients, although the physiological and anorexigenic functions are not clear. Effects of auricular stimulation on feeding-related lateral (LHA) and ventromedial (VMH) hypothalamic neuronal activity in normal and experimental (hypothalamic and dietary) obese rats were investigated. The LHA and/or VMH neuronal activity were recorded from feeding-related regions in Wistar SPFNAF male and experimental (hypothalamic and dietary) obese rats, anesthetized with urethane-chloralose, under stereotaxic coordination. Recording was through 3 M KCI glass microelectrodes, while stimulating the ipsilateral vagal innervated region of the auricle. This is equivalent to the cavum conchae in the human, and was identified by resistance less than 10–50 kπ The stimulating electrode was a stainless steel ear acupuncture (0.12 x 2.0 mm). The latency of potentials evoked in the LHA by unilateral stimulation of a specific site in the ear was 28.1 ± 3.3 ms (8–92, n = 41). LHA neuronal activity was depressed 45.6% (n = 12, p < 0.01, and VMH activity was excited (60.5%, n = 18, p < 0.01. The auricular acupuncture stimulation clearly modulates feeding-related hypothalamic neuronal activity of experimental (both hypothalamic and dietary) obese rats. These auricle acupuncture stimulation effects were correlated to the degree of obesity. In conclusion, the results suggest that auricular acupuncture stimulation may not reduce appetite, but is more likely concerned with satiation formation and preservation. Thus, auricular acupuncture should be more effective on obese rats than on normal rats.     

Effects of Cholecystokinin in the Supraoptic Nucleus and Paraventricular Nucleus are Negatively Modulated by Leptin in 24‐h Fasted Lean Male Rats

Cholecystokinin (CCK) and leptin are two important satiety factors that are considered to act in synergy to reduce meal size. Peripheral injection of CCK activates neurones in several hypothalamic nuclei, including the supraoptic (SON) and paraventricular (PVN) nuclei and neurones in the brainstem of fed rats. We investigated whether peripheral leptin would modulate the effects of CCK on neuronal activity in the hypothalamus and brainstem of fasted rats by investigating Fos expression in the PVN, SON, arcuate nucleus, ventromedial hypothalamus (VMH), dorsomedial hypothalamus (DMH), area postrema (AP) and the nucleus tractus solitarii (NTS). Male rats, fasted for 24 h, received either one i.p. injection of vehicle, leptin or CCK‐8 alone, or received one injection of vehicle or leptin before an i.p. injection of CCK‐8. We found that CCK increased Fos expression in the PVN and SON as well as in the NTS and AP, but had no effect on Fos expression in the arcuate nucleus, VMH or DMH compared to vehicle. Leptin injected alone significantly increased Fos expression in the arcuate nucleus but had no effect on Fos expression in the VMH, DMH, SON, PVN, AP or NTS compared to vehicle. Fos expression was significantly increased in the AP in rats injected with both leptin and CCK compared to rats injected with vehicle and CCK. Unexpectedly, there was significantly less Fos expression in the PVN and SON of fasted rats injected with leptin and CCK than in rats injected with vehicle and CCK, suggesting that leptin attenuated CCK‐induced Fos expression in the SON and PVN. However, Fos expression in the NTS was similar in fasted rats injected with vehicle and CCK or with leptin and CCK. Taken together, these results suggest that leptin dampens the effects of CCK on Fos expression in the SON and PVN, independently from NTS pathways, and this may reflect a direct action on magnocellular neurones.     

Cardiovascular effects of central administration of clonidine in conscious cats

The effects on arterial blood pressure and heart rate after an intracerebroventricular (i.c.v.) administration of clonidine were investigated using conscious normotensive cats. Injection of clonidine (5–10 μg; 5 μl; i.c.v.) elicited a decrease in mean arterial pressure (MAP) and heart rate (HR) in a dose-dependent manner. The highest dose of 10 μg of clonidine decreased MAP and HR by 39 ± 3 mmHg and 74 ± 5 b.p.m., respectively (n = 7). Pretreatment with yohimbine, the α₂-adrenoceptor antagonist (8 μg; 5 μl; i.c.v.) blocked the cardiovascular responses to a subsequent i.c.v. injection of 10 μg clonidine (n = 7). Furthermore, preadministration of cimetidine (100 μg; 5 μl; i.c.v.), the H₂ histamine receptor antagonist with imidazoline receptor activating properties, prevented the decreases in MAP and HR to a subsequent i.c.v. injection of 10 μg clonidine (n = 7). By contrast, pretreatment with the specific I₁ imidazoline receptor blocker, efaroxan (100–500 μg; 5 μl; i.c.v.), failed to inhibit the cardiovascular effects of an i.c.v. administration of 10 μg clonidine (n = 7). These results suggest that the effects of centrally administered clonidine on MAP and HR are probably not mediated through activation of the I₁ subtype of imidazoline receptors in conscious cats. However, the cardiovascular effects elicited by i.c.v. administration of clonidine appear to result from stimulation of central α₂-adrenergic or the H₂ histaminergic-like receptors.     

Reversible hyperphagia and obesity following intracerebral microinjections of colchicine into the ventromedial hypothalamus of the rat

Colchicine, a drug which produces a reversible inhibition of intraaxonal transport and synaptic transmission, was used as a reversible neural blocker to investigate the role of the ventromedial hypothalamus (VMH) in the control of ingestive behavior and body weight regulation. Male Sprague-Dawley rats received intracranial microinjections of colchicine into the VMH. Volume and concentration of the colchicine solution were varied to assess specificity of action and dose-response relationship. When colchicine (2 and 4 μg) was microinjected bilaterally into the VMH, there was a dose-dependent increase in food and water intakes and body weight gain which lasted several days. The acute period of hyperphagia was followed by a marked depression in feeding which persisted until body weight was lowered to control levels. This suppression of feeding appeared to be a consequence of the preceding period of hyperphagia and obesity, since colchicine-treated rats which were pair-fed with controls to prevent obesity continued to maintain normal food intake and body weight gain when later fed ad libitum. The results of this study confirm the importance of the VMH in the long term regulation of feeding, and indicate that reversible neuronal blocking with colchicine is a useful technique for investigating the neural substrates of feeding and other behaviors.     

Heterogeneity of histaminergic neurons in the tuberomammillary nucleus of the rat

Histaminergic neurons of the hypothalamic tuberomammillary nuclei (TMN) send projections to the whole brain. Early anatomical studies described histaminergic neurons as a homogeneous cell group, but recent evidence indicates that histaminergic neurons are heterogeneous and organized into distinct circuits. We addressed this issue using the double‐probe microdialysis in freely moving rats to investigate if two compounds acting directly onto histaminergic neurons to augment cell firing [thioperamide and bicuculline, histamine H₃‐ and γ‐aminobutyric acid (GABA)_A‐receptor (R) antagonists, respectively] may discriminate groups of histaminergic neurons impinging on different brain regions. Intra‐hypothalamic perfusion of either drug increased histamine release from the TMN and cortex, but not from the striatum. Thioperamide, but not bicuculline, increased histamine release from the nucleus basalis magnocellularis (NBM), bicuculline but not thioperamide increased histamine release from the nucleus accumbens (NAcc). Intra‐hypothalamic perfusion with thioperamide increased the time spent in wakefulness. To explore the local effects of H₃‐R blockade in the histaminergic projection areas, each rat was implanted with a single probe to simultaneously administer thioperamide and monitor local changes in histamine release. Thioperamide increased histamine release from the NBM and cortex significantly, but not from the NAcc or striatum. The presence of H₃‐Rs on histaminergic neurons was assessed using double‐immunofluorescence with anti‐histidine decarboxylase antibodies to identify histaminergic cells and anti‐H₃‐R antibodies. Confocal analysis revealed that all histaminergic somata were immunopositive for the H₃‐R. This is the first evidence that histaminergic neurons are organized into functionally distinct circuits that influence different brain regions, and display selective control mechanisms.     

Anorexic action of a new potential neuropeptide Y antagonist [d-Tyr, d-Thr]-NPY (27–36) infused into the hypothalamus of the rat

Neuropeptide Y (NPY) produces a vigorous feeding response in several species when it is injected into hypothalamic structures involved in eating behavior. The purpose of this study was to determine whether a unique carboxy terminal fragment of NPY would alter the pattern of eating induced in the rat either by NPY injected into the hypothalamus or by a 24-h period of food deprivation. In this case, two l-tyrosine residues and one t.-threonine residue of the NPY_27–36 fragment were transformed to their D-conformation to produce [d-Tyr^27,36,d-Thr³²]-NPY (27–36), i.e., D-NPY_27–36. Guide cannulae for microinjection were implanted stereotaxically just dorsal to the paraventricular nucleus (PVN) or ventromedial hypothalamus (VMH) of 24 adult male Sprague-Dawley rats. Following postoperative recovery, a microinjection of artificial CSF or 1.1 jig or 3.3 μg of a peptide was made directly into the PVN or VMH as follows; native NPY; D-NPY_27–36; or [L-Tyr^27,36 L-Thr³²]-NPY (27–36), i.e., L-NPY_27–36. Food intakes were measured at intervals of 0.25, 0.5, 1.1, 2.0, 4.0, and 24 h. When D-NPY_27–36 was microinjected at NPY reactive sites in the PVN or VMH of the rat 15 min before a similar microinjection of NPY, the intense eating response induced by the peptide was reduced significantly. Not only was the effect dose dependent, but D-NPY_27–36 also augmented the latency to feed. A mixture of the two doses of NPY and DNPY_27–36 injected at the same hypothalamic loci did not attenuate the intake of food but tended to enhance the feeding response in the rats. After the rats were deprived of food for 24 h, D-NPY_27–36 microinjected in the same hypothalamic sites similarly caused a dose-dependent suppression of normal feeding behavior. However, the CSF control vehicle and L-NPY_27–36 microinjected in the PVN or VMH were without effect on the pattern of eating. Further, D-NPY_27–38 injected in the same hypothalamic sites affected neither body temperature nor water intakes of the rats significantly. These results demonstrate that the D substitution of this C-fragment of the NPY molecule, i.e., D-NPY_27–36, serves to inhibit feeding evoked in the rat by hypothalamic NPY as well as the natural eating response to food deprivation. Thus, the D-NPY_27–36 molecule may act as an antagonist at one or more subtypes of the NPY receptor in the brain of the rat.     

γ-Aminobutyric acid receptor subtype antagonists differentially alter opioid-induced feeding in the shell region of the nucleus accumbens in rats

Food intake is significantly increased by administration of μ-selective opioid agonists into the nucleus accumbens, particularly its shell region. Pretreatment with either opioid (μ, δ₁, δ₂ or κ₁) or dopaminergic (D₁) receptor antagonists in the nucleus accumbens shell reduce μ opioid agonist-induced feeding. Selective GABA_A (muscimol) and GABA_B (baclofen) agonists administered into the nucleus accumbens shell each stimulate feeding which is respectively and selectively blocked by GABA_A (bicuculline) and GABA_B (saclofen) antagonists. The present study investigated whether feeding elicited by the μ-selective opioid agonist, [ -Ala²,NMe⁴,Gly-ol⁵]-enkephalin in the nucleus accumbens shell was decreased by intra-accumbens pretreatment with an equimolar dose range of either GABA_A or GABA_B antagonists, and further, whether general opioid or selective GABA antagonists decreased feeding elicited by GABA_A or GABA_B agonists in the nucleus accumbens shell. Feeding elicited by the μ-selective opioid agonist was dose-dependently increased following intra-accumbens pretreatment with GABA_A (bicuculline) antagonism; this enhancement was significantly blocked by pretreatment with general or μ-selective opioid antagonists. In contrast, μ opioid agonist-induced feeding elicited from the nucleus accumbens shell was dose-dependently decreased by GABA_B (saclofen) antagonism. Neither bicuculline nor saclofen in the nucleus accumbens shell altered baseline food intake. Whereas muscimol-induced feeding elicited from the nucleus accumbens shell was reduced by bicuculline and naltrexone, but not saclofen pretreatment, baclofen-induced feeding elicited from the nucleus accumbens shell was reduced by saclofen, but not by bicuculline or naltrexone. These data indicate that GABA_A and GABA_B receptor subtype antagonists differentially affect feeding elicited by μ opioid receptor agonists within the nucleus accumbens shell in rats.     

Brain penetration of the histamine H3 receptor antagonists thioperamide and clobenpropit in rat and mouse, determined with ex vivo [I]iodophenpropit binding

We investigated the brain penetration of the histamine H₃ receptor antagonists thioperamide and clobenpropit using ex vivo [¹²⁵I]iodophenpropit binding. Homogenates of the rat cortex, striatum and mouse whole brain were prepared 1 h after subcutaneous injection of the H₃ antagonists and incubated with [¹²⁵I]iodophenpropit, a radiolabeled H₃ receptor antagonist, to determine the H₃ receptor occupancy. Specific [¹²⁵I]iodophenpropit binding to the rat cortex and striatum was inhibited by thioperamide with IC₃₀ values of 1.0 and 1.5 mg/kg, respectively. Clobenpropit also inhibited [¹²⁵I]iodophenpropit binding, but was less potent (IC₃₀: 18 and 19 mg/kg in the rat cortex and striatum, respectively) than thioperamide. Similar results were obtained in experiments with mouse whole brain (3.5 and 13 mg/kg for thioperamide and clobenpropit), indicating that there is no important species differences in the brain penetration of these drugs between rats and mice. These findings suggest that after peripheral injection both in rat and mouse thioperamide penetrates the blood-brain barrier more efficiently compared to clobenpropit.