Fever evoked by macrophage inflammatory protein-1 (MIP-1) injected into preoptic or ventral septal area of rats depends on intermediary protein synthesis

Macrophage inflammatory protein-1 (MIP-1), a novel cytokine composed of α/β subunits, is released from macrophages during infection, MIP-1 injected intravenously in the rabbit or into the anterior hypothalamic, preoptic area (AH/POA) of the rat causes an intense fever, which is not blocked by prostaglandin synthesis inhibitors, ibuprofin or indomethacin, respectively. The purpose of this study was to determine the role of de novo protein synthesis on the fever evoked by MIP-1 applied to thermosensitive cells of the AH/POA. Guide cannulae were implanted bilaterally above the AH/POA or ventral septal area (VSA) and medially above the third cerebral ventricle in each of 11 male Sprague-Dawley rats. Following postoperative recovery, body temperature (T_b) was monitored by a colonic thermistor probe. The bilateral microinjection of MIP-1 in a dose of 14 pg per 0.5 μ1 into the AH/POA caused a biphasic elevation in T_b to 0.9 ± 0.2 °C within 3.0 h, and persisted for over 6.0 h. An identical injection of MIP-1 into the VSA increased T_b biphasically to 0.1 ± 0.1 °C within 1.0 h and to 0.8 ± 0.3 °C within 3.0 h. The infusion into the third ventricle of 80 μg/10 μ1 of the inhibitor of protein synthesis, anisomycin, either 10 or 30 min before the microinjection of MIP-1 into the AH/POA, attenuated significantly the rise in T_b for 1.0 to 3.0 h or 2.5 to 3.0 h, respectively. These results coincide with the earlier finding that anisomycin inhibits both endotoxin- and IL-1β-induced fevers. Further, the synthesis of a new protein factor may be required functionally for the initiation and maintenance of a fever whose mechanism of induction apparently is metabolically independent of the cyclooxygenase pathway.     

Fever and feeding in the rat: Actions of intrahypothalamic interleukin-6 compared to macrophage inflammatory protein-1β (MIP-1β)

The chemokines, macrophage inflammatory protein-1 (MIP-1) and its subunit MIP-1β, induce an intense fever in the rat when they are injected directly into the anterior hypothalamic, pre-optic area (AH/POA), a region containing thermosensitive neurons. The purpose of this study was to compare the central action on body temperature (T_b) of MIP-1β with that of interleukin-6 (IL-6), which also has been implicated in the cerebral mechanism underlying the pathogenesis of fever. Following the stereotaxic implantation in the AH/POA of guide cannulae for repeated micro-injections, radio transmitters which monitor T_b continuously were inserted intraperitoneally in each of 15 male Sprague-Dawley rats. Each micro-injection was made in a site in the AH/POA in a volume of 1.0 μl of pyrogen-free artificial CSF, recombinant murine MIP-1β, or recombinant human IL-6. MIP-1β in a dose of 25 pg evoked an intense fever characterized by a short latency, a mean maximum rise in T_b of 2.4 ± 0.21°C reached by 3.7 ± 0.42 hr, and a duration exceeding 6.5 hr. Injected into homologous sites in the AH/POA, IL-6 induced a dose dependent fever of similar latency and a mean maximal increase in T_b of 1.2 ± 0.25°C, 1.8 ± 0.15°C, and 2.1 ± 0.22°C and duration of 6.2 ± 1.28 hr, 6.7 ± 0.49 hr, and 6.8 ± 0.65 hr when given in doses of 25, 50, and 100 ng, respectively. These results show that MIP-1β and the highest dose of IL-6 induce a fever of comparable intensity, but MIP-1β exerts its action in a much lower concentration. Thus, the de novo synthesis and subsequent action of the MIP-1 family of cytokines on neurons of the AH/POA in response to a pyrogen challenge apparently play a functional role in the pathogenesis of fever. Further, the endogenous activity of IL-6 in the hypothalamus which is enhanced in response to a lipopolysaccharide also may reflect its essential part in the acute phase response to a bacterial challenge. Copyright © 1994 Wiley-Liss, Inc.     

Inhibition of interleukin-1β and prostaglandin E2 thermogenesis by glycyl-glutamine, a pro-opiomelanocortin-derived peptide

Interleukin-1β (IL-1β) and other cytokines produce fever by stimulating prostaglandin E₂ (PGE₂) synthesis in thermoregulatory regions of the preoptic area and anterior hypothalamus (POA/AH). Prostaglandin E₂ is thought to raise body temperature, at least in part, by stimulating β-endorphin release from pro-opiomelanocortin neurons that innervate the POA/AH. In this study, we investigated whether glycyl-glutamine (β-endorphin_30–31), an inhibitory dipeptide synthesized from β-endorphin post-translationally, inhibits IL-1β and PGE₂-induced hyperthermia. Hyperthermic sites were identified by microinjecting PGE₂ (3 fmol/1 μl) into the medial preoptic area (mPOA) of conscious, unrestrained rats. Interleukin-1β (1 U) injection into the same PGE₂ responsive thermogenic sites in the mPOA elicited a prolonged rise in colonic temperature (T_c) (+1.02±0.06°C) that persisted for at least 2 h. Glycyl-glutamine (3 nmol) co-injection into the mPOA inhibited IL-1β thermogenesis completely (T_c=−0.18±0.22°C). Glycyl-glutamine had no effect on body temperature when given alone to normothermic rats. Co-injection of individual amino acids, glycine and glutamine (3 nmol each amino acid), failed to influence IL-1β-induced thermogenesis, which indicates that Gly-Gln hydrolysis does not explain its inhibitory activity. Glycyl-glutamine (3 nmol) also prevented the rise in body temperature produced by PGE₂ (PGE₂=0.89±0.05°C; PGE₂ plus Gly-Gln=−0.16±0.14°C), consistent with evidence that PGE₂ mediates IL-1β-induced fever. These findings demonstrate that Gly-Gln inhibits the thermogenic response to endogenous pyrogens.     

The role of protein synthesis in the hypothalamic mechanism mediating pyrogen fever

The effects of inhibition of protein synthesis by anisomycin on the pathogenesis of fever and normal thermoregulatory processes were investigated in the conscious and unrestrained cat. Subcutaneous administraton of 5.0–25.0 mg/kg of anisomycin prevented the fever normally evoked by an intravenous infusion of either 1.0 ml (10⁸ organisms) of a 1:10 dilution of S. typhosa or 1.0–5.0 ml (3.5×10⁵−2.1×10⁷ cells/ml) of endogenous pyrogen. In addition, systemic pre-treatment with anisomycin delayed and/or blocked the fever typically elicited by a direct micro-injection into the anterior hypothalamic, preoptic area (AH/POA) at AP 12.5–16.0 of 1.0 μl of the endotoxin. Anisomycin did not alter the hyperthermic response to an anterior hypothalamic injection of either 1.0–7.0 μg/1.0 μl of serotonin (5-HT) or 100.0 ng/1.0 μl of prostaglandin (PGE). Inhibition of protein synthesis, furthermore, did not prevent the fall in body temperature usually produced by an intrahypothalamic micro-injection of 2.33–14.0 μg/1.0 μl of either norepinephrine (NE) or dopamine (DA). The thermoregulatory capacity of the cat was unaffected by the administration of comparable doses of anisomycin, i.e., the animal was able to maintain normal body temperature (±0.5°C) when exposed to an ambient temperature of either 10°C or 34°C. These results strongly suggest that the synthesis of new protein within the region of the AH/POA is a functional requisite for the development of a pyrogen-induced fever.     

Differential sensitivities of pyrogenic chemokine fevers to cyclooxygenase isozymes antibodies

It has been proposed that prostaglandin (PG)E(2) production via a process catalyzed by the inducible isoform of cyclooxygenase (COX)-2 and activation of specific PGE(2) receptor subtypes within the preoptic/anterior hypothalamus (AH/POA) is the last step and unique pathway in the induction of a fever. However, many data support the existence of a PG-independent pathway. That is, other more rapid mechanisms, which involve the constitutive COX-1 isozyme, may be more critical for a PG-dependent fever. Thus, we examined the role of both COX isoforms in the AH/POA in fevers induced by macrophage inflammatory protein (MIP)-1beta, a PG-independent pyrogen, and RANTES (regulated on activation, normal T-cells expressed and secreted), a PG-dependent pyrogen. In freely moving rats, two independent polyclonal antibodies were used which neutralize COX-1 and COX-2. The microinjection of either MIP-1beta or RANTES into the pyrogen-sensitive region of the AH/POA induced an intense fever of rapid onset. Peripheral pretreatment with an antipyretic dose of dexamethasone which prevents COX-2 expression, or the microinjections into the AH/POA of either anti-COX-1 or anti-COX-2, blocked the febrile response induced by RANTES but not that induced by MIP-1beta. These results provide strong evidence for the existence of rapid mechanisms in the AH/POA which involve both COX isozymes during the fever induced by RANTES, and further support the existence of an alternative PG-independent pathway in the febrile response.     

Hypothalamic prostaglandin E2 during lipopolysaccharide-induced fever in guinea pigs

Prostaglandin E₂ (PGE₂) is postulated to be a central mediator of fever. It is generally believed that it is produced in the preoptic area of the anterior hypothalamus (POA) because, among other evidence, its level increases both in the third ventricle and in the POA in response to pyrogens. However, lately, the question has arisen whether PGE₂ might, in fact, be formed outside of the brain substance and then penetrate it, in particular through the organum vasculosum laminae terminalis. If produced outside the brain substance, the peripheral blockade of its synthesis should prevent lipopolysaccharides (LPS)-induced fever, whereas the intracarotid infusion of PGE₂ should produce an increase in core temperature (T_c) as well as in preoptic PGE₂. To verify this hypothesis, continuous measurements of T_c and preoptic PGE₂ levels were made in conscious guinea pigs administered the PGE₂ synthase inhibitor, indomethacin (10 or 50 mg/kg, im) 30 min before S. enteritidis LPS (2 μg/kg, iv) or before PGE₂ microdialyzed into the POA (1 μg/μl at 2μl/min for 2.5 h) and during PGE₂ infused into a carotid artery (1 μg and 10 μg/μl at 2 μl/min for 1 h). LPS induced a biphasic 1.4°C fever that was consistently associated with an increase in the level of PGE₂ in the POA. Indomethacin at 10 mg/kg attenuated the course of the LPS-induced fever and prevented the associated increase in preoptic PGE₂ for 90 min after fover onset; thereafter, PGE₂ was significantly reduced by comparison with controls. Indomethacin at 50 mg/kg completely abolished both the fever and the increased levels of PGE₂ in the POA; the fever induced by PGE₂ microdialyzed into the POA was not affected by indomethacin pretreatment. The intracarotid infusion of PGE₂ produced T_c falls and no increase in preoptic PGE₂ levels. The indomethacin-induced blockade of fever and inhibition of the associated increase in preoptic PGE₂ levels further substaintiates the presumptive link between PGE₂ in the POA and fever caused by LPS. The failure of exogenous PGE₂ infusion to induce increases in T_c and preoptic PGE₂ levels excludes the possibility that PGE₂ formed outside of the brain penetrates the POA and induces fever. Thus, in guinea pigs, the PGE₂ associated with LPS-induced fever may be synthesized in the POA.     

Antibodies to macrophage inflammatory protein-1β in preoptic area of rats fail to suppress PGE2 hyperthermia

This study determined whether macrophage inflammatory protein-1β (MIP-1β) plays a role in the hyperthermia caused by prostaglandin E₂ (PGE₂) given intracerebroventricularly (i.c.v.) in the rat. In these experiments, anti-murine MIP-1β antibody (anti-MIP-1β) was micro-injected in the anterior hypothalamic, preoptic area (AH/POA) just before i.c.v. PGE₂. The results showed that anti-MIP-1β failed to alter the PGE₂ hyperthermia. However, immunocytochemical studies revealed MIP-1β immunoreactivity detectable in both the organum vasculosum laminae terminalis (OVLT) and AH/POA in the febrile rat. These data thus demonstrate that MIP-1β is sequestered in diencephalic structures underlying thermoregulation even though it is not involved in PGE₂ hyperthermia. This dissociation supports the viewpoint that at least two distinct systems exist in the brain which underlie a febrile response: MIP-1β underlies one component whereas PGE₂ comprises the other.     

Fever induced by macrophage inflammatory protein-1 (MIP-1) in rats: hypothalamic sites of action.

The purpose of this study was to clarify the central site of action as well as functional characteristics of the febrile response of the cytokine, macrophage inflammatory protein-1 (MIP-1). Guide cannulae for microinjection were implanted stereotaxically in the rat just above the pyrogen and thermosensitive area of the anterior hypothalamic, preoptic area (AH/POA). Following postoperative recovery, the body temperature of each rat (Tbo) was monitored during an experiment by a colonic thermistor probe at 0.5-1.0-h intervals. When MIP-1 was microinjected in a 0.5-microliter volume into the AH/POA in one of eight concentrations ranging from 0.0028 nanograms (ng) to 9.0 ng, an intense monophasic or biphasic fever was evoked. The MIP-1-induced increase in the Tbo of the rat was characterized by its short latency of 15 to 30 min and an inverse dose-response curve. Measures of mean latency and maximal rise in Tbo following MIP-1 confirmed the potency of this dose. Although the dose of 0.028 ng produced a fever of over 2.0 degrees C with a latency of only 15 min or less, the hyperthermic response became less intense as the dose of MIP-1 was increased. An anatomical mapping of sites of microinjection which reacted to MIP-1 in mediating fever revealed that the medial portion of the POA of the rat just rostral to the border of the AH was the region of maximum sensitivity to the cytokine.(ABSTRACT TRUNCATED AT 250 WORDS)     

Genesis of biphasic thermal response to intrapreoptically microinjected clonidine

Intrapreoptic (IPO) microinjections of various agents cause unavoidable brain tissue injury, often resulting in prostaglandin (PG)-mediated core temperature (T_c) rises. However, IPO microinjection of the α₂-adrenoreceptor agonist clonidine (Clo) generally evokes a T_c fall, seemingly avoiding the influence of injury due to the microinjection procedure per se. To clarify this, we microinjected bilaterally into the preoptic/anterior hypothalamus of conscious guinea pigs various doses of Clo dissolved in pyrogen-free saline (PFS, 1 μl/side). Clo caused biphasic hypo-/hyperthermic responses. The initial hypothermia was dose dependent: no decrease in T_c for 0.1 μg of Clo, −0.4 ± 0.1°C for 0.5 μg, −0.9 ± 0.1°C for 1.5 μg, and −1.2 ± 0.1°C for 5.0 μg. During the hyperthermic phase, T_c increased to a dose-independent level (1.0–1.5°C), remaining there up to 5 h postinjection. PFS microinjected IPO also induced hyperthermia, but without any initial T_c decrease. This T_c rise was delayed by 100 min when the cyclooxygenase inhibitor indomethacin (Indo, 50 μg/μl) was injected. Nontreated animals (time controls) maintained T_c at baseline levels during the whole experiment. The α₂-antagoni rauwolscine (2 μg/side), microinjected IPO 10 min before Clo (0.5 μg/side), abolished the hypothermic without affecting the hyperthermic response phase; Indo (10 mg/kg), injected intramuscularly 20 min after the IPO microinjection of Clo (0.5 μg), significantly attenuated the hyperthermic phase. These results confirm that an artifactitious, PG-mediated T_c rise consequent to nonspecific brain tissue injury contaminates the thermal response to agents (hyper- or hypothermizing) microinjected IPO. The similarity of the thermal responses to Clo to those to norepinephrine provides additional evidence that the authentic (hypothermie) effect of IPO norepinephrine in guinea pigs is mediated via α₂-adrenergic receptors.     

Prostanoids in the preoptic hypothalamus mediate systemic lipopolysaccharide-induced hyperalgesia in rats

The systemic administration of lipopolysaccharide (LPS), an experimental model of systemic bacterial infection is known to modulate nociception. It increases the prostaglandin E₂ (PGE₂) levels in the preoptic area of the hypothalamus (POA) and the microinjection of PGE₂ into the POA and the neighboring basal forebrain induces hyperalgesia. We, therefore, hypothesized that the PGE₂ synthesized in these regions mediates intravenous (i.v.) LPS-induced hyperalgesia. To test this hypothesis, we microinjected cyclooxygenase (COX) inhibitors into several sites in the rat hypothalamus and observed their effects on the LPS (0.1–100 μg/kg, i.v.)-induced changes in nociceptive behavior as assessed by a plantar test. LPS (10 and 100 μg/kg, i.v.) reduced the paw-withdrawal latency at 90 min and 45–60 min after injection, respectively, both thus indicating a hyperalgesic effect. This hyperalgesia was observed only in the period before the development of fever which started 120–135 min after the LPS injection. The LPS (100 μg/kg, i.v.)-induced hyperalgesia was completely abolished by pretreatment with the microinjection of diclofenac (an inhibitor of COX-1 and 2) at 1.0 ng into the bilateral POA. Furthermore, it was also blocked by the microinjection of NS-398 (a selective COX-2 inhibitor) at 1.0 ng into the bilateral POA and the horizontal limb of the diagonal band of Broca (DBB), but not the lateral hypothalamic area, the paraventricular hypothalamic nucleus, and the ventromedial hypothalamic nucleus. These findings suggest that LPS (i.v.)-induced hyperalgesia is mediated predominantly through a COX-2 induced prostanoids in the POA and the DBB in rats.     

Elevation of plasma ACTH concentration in rabbits made febrile by systemic injection of bacterial endotoxin

This study was carried out to investigate the adrenocorticotrophic hormone (ACTH) response in rabbits made febrile by systemic injection of lipopolysaccharide (LPS,Salmonella typhosa endotoxin). Intravenous (i.v.) injection of LPS (0.1 μg/kg and 1.0 μg/kg) increased rectal temperature (biphasic fever) and the plasma concentration of ACTH (ACTH response) in a dose-related manner. These responses were suppressed by pretreatment with indomethacin (20 mg/kg, subcutaneously). Intracerebroventricular (i.c.v.) administration of indomethacin (400 μg) had no effect on the ACTH response to LPS, although it significantly suppressed febrile response. Small increases in plasma concentration of ACTH and significant fevers followed i.c.v. administration of prostaglandin E₂ (2 μg) or F_2α (2 μg). I.v. administration of corticotropin releasing factor (CRF) antagonist [α-helical CRF (9–41) (200 μg/kg)] partly suppressed the ACTH increased induced in plasma by i.v. LPS. These results suggest that prostagandins synthesized outside the blood-brain barrier play an important role in the ACTH response and that the mechanism for induction of the ACTH response is not exactly the same as that for the febrile response, although prostaglandins are involved in both responses.     

ACh and 5-HT stimulated thermogenesis at different core temperatures in the He-Cold hypothermie hamster

Hamsters in deep experimentally induced hypothermia, at body temperatures between 7°C and 11.5°C, were microinjected with 5-HT and ACh at brain sites in the anterior-preoptic area of the hypothalamus (AH/POA). ACh or 5-HT was injected into an AH/POA site at different starting core temperatures in different groups of hypothermic hamsters. Colonic temperatures (Tc) were maintained, following He-Cold induction, in a temperature controlled environmental chamber and measured with a YSI thermister probe and YSI tele-thermometer. Injections of either 5-HT or ACh at Tc's between 7.0°C and 9.0°C elicited only modest increases in Tc i.e., 0.3°C–0.6°C, respectively. As Tc increased, however, to ranges between 9.1°C–10.0°C and in different animals to greater than 10°C both ACh and 5-HT at the same sites elicited significant increases in Tc, 1.5°C for 5-HT and 2.2°C for ACh compared to saline injections. These data suggest that at the lowest Tc's we are observing a “cold block” of temperature sensitive sites in the AH/POA. Increasing the starting Tc beyond 9.0°C however, evokes significant increases in heat-gain following AH/POA injection of either ACh or 5-HT. These data are consistent with Myers' observations concerning the organization of heat-gain mechanisms at AH/POA sites. In addition, they suggest that both the afferent limb of the heat-gain circuit (5-HT) and the efferent limb of the circuit (ACh) are functionally impaired when Tc is close to the physiological limit in the He-Cold hypothermic hamster.     

Blockade by interleukin-1 receptor antagonist of IL-1β-induced neuronal activity in guinea pig preoptic area slices

Interleukin-1α (IL-1α) and interleukin-1β (IL-1β) are thought to be endogenous pyrogens, i.e., to mediate fever production; warm-sensitive (W) and cold-sensitive (C) neurons in the preoptic area (POA) are presumed to be the ultimate targets of endogenous pyrogens. The recent purification of an IL-1 receptor antagonist (IL-1ra) has provided a means for verifying the presumptive action of IL-1 on these neurons. This study was undertaken, therefore, to investigate whether IL-1ra may block the IL-1α and IL-1β effects on the firing rates (FR) of W and C neurons in guinea pig POA slices. Human recombinant (hr) IL-1β (500 ng/ml) reduced the FR of 26 W neurons and increased those of 3 C neurons recorded; it had no effect on 8 thermally insensitive neurons. hrIL-1α (200–600 ng/ml) did not change the FR of any neuron. IL-1ra (0.01–0.5 mg/ml) had no effect by itself on the FR of all the neurons, but it blocked the hrIL-1β-induced FR changes of 24 of the 26 W and of all 3 C neurons when given before the cytokine. The lowest effective dose was 0.05 mg/ml. These results support the hypothesis, therefore, that POA thermosensitive neurons may be direct targets of IL-1β and that it may be an endogenous pyrogen acting on these units to induce fever production.     

Anorexia and adipsia: dissociation from fever after MIP-1 injection in ventromedial hypothalamus and preoptic area of rats.

Certain cytokines such as tumor necrosis factor (TNF) and interleukin-1 (IL-1) act centrally to affect eating behavior and thermoregulation and may be involved in the physiological mechanisms leading to anorexia, adipsia and loss in body weight. The newly discovered macrophage inflammatory protein-1 (MIP-1) infused into the anterior hypothalamic, preoptic area (AH/POA) evokes an intense hyperthermia. The present experiments were designed to determine whether MIP-1 affects the feeding mechanism in the ventromedial hypothalamus (VMH) independently of the thermoregulatory mechanism in the AH/POA. For the microinjection of MIP-1, guide cannulae were implanted stereotaxically in the rat just above the VMH or AH/POA. Following postoperative recovery, each unrestrained rat was adapted to procedures whereby body temperature and intakes of food and water available ad lib were monitored at predetermined intervals. When an efficacious dose of 5.6 picograms (pg) MIP-1 was microinjected in a volume of 0.5 microliters into the VMH, the intake of food in the rat was reduced significantly in the short term and throughout the following 22 h. Within intervals of 30 min and 4.0 h following MIP-1, the amount of food consumed was 4.0 and 10 g, respectively, below that eaten by control rats given the saline solvent vehicle injected at the same site in the VMH. Over the entire test period, the intake of water was similarly significantly below that of the control rats. Whereas MIP-1 injected into the AH/POA evoked fever accompanied by a transient decline in feeding, the body temperature of the rats was unaffected by the cytokine injected in the VMH.(ABSTRACT TRUNCATED AT 250 WORDS)     

5-HT receptors and hyper- or hypothermia: Elucidation by catecholamine antagonists injected into the cat hypothalamus

The receptor mechanisms in the hypothalamus and preoptic area which mediate thermoregulatory changes produced by serotonin (5-HT) were examined in the unrestrained cat. Stainless steel guide tubes were implanted just above the rostral diencephalon or preoptic area in each of twelve cats. 5-HT in a dose of 1.5 to 10.5 μg was micro-injected in a volume of 0.75 to 1.5 μl into a total of 256 test sites in the hypothalamus. An individual site at which 5-HT evoked a rise or a fall in core temperature of 0.5°C within 30 min was considered to be reactive to the indoleamine. Within such a region of maximum sensitivity, i.e., the cat's anterior hypothalamic, preoptic area (AH/POA), either norepinephrine (NE) or dopamine (DA) micro-injected in a dose of 2.33 to 14.0 μg in similar volumes evoked only a dose-dependent decline in the body temperature of the cat. 5-HT was found to cause either hyper- or hypothermia. The direction of temperature change caused by 5-HT was dependent principally upon the neuroanatomical locus of injection. Each of the 5-HT reactive sites was characterized pharmacologically by their pre-treatment with phentolamine (PHT), d-butaclamol (BUTAC) or methysergide (METHY) in doses of 1.0 to 10.0 μg. At the most rostral sites in POA, the catecholamine receptor antagonists selectively delayed the onset of the 5-HT hypothermia and limited the magnitude of the response. Within sites located in the AH in which 5-HT induced only a rise in body temperature, prior treatment of the site with the catecholamine receptor antagonists PHT or BUTAC failed to modify the response. These results indicate that 5-HT in producing hypothermia not only may saturate the 5-HT receptor sites but also may be taken up by catecholamine receptors which mediate the diencephalic heat loss pathway.     

Temperature regulation and dopaminergic systems in the brain: Does the substantia nigra play a role?

Male Sprague-Dawley rats (250–300 g) were stereotaxically implanted above the substantia nigra (SN) and preoptic/anterior hypothalamus (PO/AH) with 23 gauge stainless-steel guide tubes. Microinjections of apomorphine (APO), pimozide, or 0.9% saline were made bilaterally in 0.5 or 1.0 μl vols. using a Harvard infusion pump. Oxygen consumption, and colonic (T_c), tail-skin, and ambient temperatures were monitored each minute. Microinjections of APO into the SN produced a dose-dependent hypothermia that was antagonized by the central (1.0 μg) or systemic (0.5 mg/kg i.p.) injection of pimozide. This hypothermia was associated with increased heat loss and decreased heat production and occurred at ambient temperatures of 15, 23 and 35°C indicating that APO did not produce a poikilothermic state. Injecting 20 μg APO into the PO/AH or SN produced similar hypothermic responses; T_c fell 0.87± 0.10°C and1.02 ± 0.08°C, respectively. Pimozide injected into the SN failed to alter thermoregulation during exercise or exogenous heating. Moreover, systemic injections of APO before and after electrolytic lesioning of the SN produced similar hypothermic responses. However, when the SN was lesioned, (a) resting T_c was significantly reduced, and (b) during exposure to a 35°C environment for 55 min, T_c lose to39.5 ± 0.68°C before the lesion compared with a rise to only38.5 ± 0.13°C after the lesion. We conclude that the pharmacological data implicate a thermoregulatory role for dopamine receptors in the SN of the rat, but the functional significance of this central location in temperature regulation remains to be elucidated.     

Reduced thermal sensitivity in the rabbit by β-endorphin injection into the preoptic/anterior hypothalamus

Male New Zealand White rabbits,Oryctolagus cuniculus, were stereotaxically implanted with a guide tube above the preoptic/anterior hypothalamus (PO/AH) for the injection of β-endorphin (β-E) or saline at ambient temperatures of 20 and 25 °C. Ear skin and PO/AH temperatures were recorded in loosely restrained control and β-E-pretreated rabbits while radiant heat was applied to the dorsal skin. Without β-E administration the ear skin temperature (T_ear) underwent a rapid increase during back heating. Following β-E administration there was a marked vasoconstriction along with a large reduction in responsiveness of ear skin temperature to radiant 3eat. The time to respond to radiant heat for β-E-pretreated rabbits was significantly longer than that for control rabbits. In control animals, the increase in T_ear in response to radiant heat exposure dependend upon the initial ear temperatures. However, in β-E-pretreated rabbits vasodilatation response to radiant heat exposure was nearly the same regardless of the initial T_ear. These data suggest that there is a significant reduction in passage of temperature information from cutaneous thermal receptors to the PO/AH in β-E-pretreated animals and that β-E-induced reduction in sensitivity of the vasomotor system to radiant heat may account for the effectiveness of this opioid peptide to promote hyperthermia in the rabbit.     

Serum thyroxin levels during hyperthermia evoked in the monkey by intrahypothalamic injection of PGE1 5-HT or pyrogen

Serotonin (5-HT), prostaglandin E₁ (PGE₁) or a bacterial pyrogen (E. coli or S. typhosa) was microinjected in a volume of 1.0–1.5 μl into the hypothalamus of the unanesthetized monkey to evoke a long-term hyperthermia. Samples of venous blood collected every 15 min, before, during and after each fever were analyzed by radioimmunoassay for plasma thyroxin levels, There was no statistically significant correlation between plasma thyroxin values and a given phase of the hyperthermic episode induced by the microinjections of 5-HT, PGE₁ or bacteria. The possibility that an enhanced release of the thyroid hormone serves to sustain a long-term elevation in temperature evoked by a centrally acting pyrogenic substance is not supported.     

Central thermosensitivity during fever produced by intra-PO/AH and intravenous injections of pyrogen

LIPTON, J. M. AND J. I. KENNEDY. Central thermosensitivity during fever produced by intra-PO/AH and intravenousinjections of pyrogen. BRAIN RES. BULL. 4(1) 23–34, 1979.—Squirrel monkeys with thermodes implanted in the preoptic/ anterior hypothalamic (PO/AH) region and the medulla oblongata were used to examine three questions about central thermoresponsiveness in fever: Does thermoresponsiveness of the PO/AH region and medulla change during fevers caused by injection of bacterial endotoxin IV or directly into the PO/AH region? Does thermosensitivity of these brain regions determine the upper fever limit? Is thermoresponsiveness of the PO/AH region affected by local injections of salicylate? Changes in rectal temperature and oxygen consumption in response to heating and cooling the PO/AH region were reduced during fever caused by intra-PO/AH injections of bacterial endotoxin compared with changes produced during afebrile periods. PO/AH thermosensitivity was also reduced during fever caused by IV administration of bacterial pyrogen. Prolonged cooling of the PO/AH region or the medulla oblongata during fever produced by peripheral and central pyrogen injections did not cause rectal temperature (T_re) to rise above 41.1°C although local heating reduced T_re or limited the fever maximum. From the latter result it is concluded that both pools of central thermoreceptors can limit maximal fever by reacting to local high temperature but that lowered temperature in neither region can raise T_re above a level determined by antagonistic input from thermoreceptors in other parts of the body. Injections of sodium salicylate into the PO/AH region had no effect on thermoresponsiveness of the region. This finding reinforces the idea that salicylates do not produce antipyresis by acting directly on thermosensitive cells of the central temperature control system.     

Medullary μ and δ opioid receptors modulate mesencephalic morphine analgesia in rats

Supraspinal opioid analgesia is mediated in part by connections between the midbrain periaqueductal gray (PAG) and rostral ventral medulla (RVM) which includes the nuclei raphe magnus and reticularis gigantocellularis. Serotonergic 5HT₂ and 5HT₃ receptor subtypes appear to participate in this pathway since general and selective serotonergic antagonists microinjected into the RVM significantly reduced morphine analgesia elicited from the PAG. Since both an enkephalinergic pathway between the PAG and RVM and intrinsic enkephalinergic cells in the RVM exist, the present study evaluated the abilities of general (naltrexone), μ-selective (β-funaltrexamine: B-FNA) andδ₂-selective (naltrindole) opioid receptor subtype antagonists microinjected into the RVM to alter morphine (2.5 μg) analgesia elicited from the PAG as measured by the tail-flick and jump tests. Mesencephalic morphine analgesia was significantly reduced after pretreatment in the RVM with naltrexone (1–10 μg), B-FNA (0.5–5 μg) or naltrindole (0.5–5 μg). Naltrexone in the RVM failed to alter basal nociceptive thresholds and none of the opioid antagonists were effective in reducing mesencephalic morphine analgesia when they were microinjected into placements lateral or dorsal to the RVM. These data indicate that μ andδ₂ opioid receptors in the RVM modulate the transmission of opioid pain-inhibitory signals from the PAG.