The effects of choline on body temperature in conscious rats

Tonic inhibition of brown adipose tissue sympathetic nerve activity via muscarinic acetylcholine receptors in the rostral raphe pallidus

KEY POINTS

A tonically active, muscarinic cholinergic inhibition of rostral raphe pallidus (rRPa) neurons influences thermogenesis of brown adipose tissue (BAT) independent of ambient temperature conditions. The tonically active cholinergic input to rRPa originates caudal to the hypothalamus. Muscarinic acetylcholine receptor (mAChR) activation in rRPa contributes to the inhibition of BAT sympathetic nerve activity (SNA) evoked by activation of neurons in the rostral ventrolateral medulla (RVLM). The RVLM is not the sole source of the muscarinic cholinergic input to rRPa. Activation of GABA receptors in rRPa does not mediate the cholinergic inhibition of BAT SNA.

ABSTRACT

We sought to determine if body temperature and energy expenditure are influenced by a cholinergic input to neurons in the rostral raphe pallidus (rRPa), the site of sympathetic premotor neurons controlling thermogenesis of brown adipose tissue (BAT). Nanoinjections of the muscarinic acetylcholine receptor (mAChR) agonist, oxotremorine, or the cholinesterase inhibitor, neostigmine (NEOS), in the rRPa of anaesthetized rats decreased cold-evoked BAT sympathetic nerve activity (SNA, nadirs: -72 and -95%), BAT temperature (Tbat, -0.5 and -0.6°C), expired CO₂ (Exp. CO₂ , -0.3 and -0.5%) and heart rate (HR, -22 and -41 bpm). NEOS into rRPa reversed the increase in BAT SNA evoked by blockade of GABA receptors in rRPa. Nanoinjections of the mAChR antagonist, scopolamine (SCOP), in the rRPa of warm rats increased BAT SNA (peak: +1087%), Tbat (+1.8°C), Exp. CO₂ (+0.7%), core temperature (Tcore, +0.5°C) and HR (+54 bpm). SCOP nanoinjections in rRPa produced similar activations of BAT during cold exposure, following a brain transection caudal to the hypothalamus, and during the blockade of glutamate receptors in rRPa. We conclude that a tonically active cholinergic input to the rRPa inhibits BAT SNA via activation of local mAChR. The inhibition of BAT SNA mediated by mAChR in rRPa does not depend on activation of GABA receptors in rRPa. The increase in BAT SNA following mAChR blockade in rRPa does not depend on the activity of neurons in the hypothalamus or on glutamate receptor activation in rRPa.

Cholinergic neurons in the dorsomedial hypothalamus regulate mouse brown adipose tissue metabolism

Objective

Brown adipose tissue (BAT) thermogenesis is critical in maintaining body temperature. The dorsomedial hypothalamus (DMH) integrates cutaneous thermosensory signals and regulates adaptive thermogenesis. Here, we study the function and synaptic connectivity of input from DMH cholinergic neurons to sympathetic premotor neurons in the raphe pallidus (Rpa).

Methods

In order to selectively manipulate DMH cholinergic neuron activity, we generated transgenic mice expressing channelrhodopsin fused to yellow fluorescent protein (YFP) in cholinergic neurons (choline acetyltransferase (ChAT)-Cre::ChR2-YFP) with the Cre-LoxP technique. In addition, we used an adeno-associated virus carrying the Cre recombinase gene to delete the floxed Chat gene in the DMH. Physiological studies in response to optogenetic stimulation of DMH cholinergic neurons were combined with gene expression and immunocytochemical analyses.

Results

A subset of DMH neurons are ChAT-immunopositive neurons. The activity of these neurons is elevated by warm ambient temperature. A phenotype-specific neuronal tracing shows that DMH cholinergic neurons directly project to serotonergic neurons in the Rpa. Optical stimulation of DMH cholinergic neurons decreases BAT activity, which is associated with reduced body core temperature. Furthermore, elevated DMH cholinergic neuron activity decreases the expression of BAT uncoupling protein 1 (Ucp1) and peroxisome proliferator-activated receptor γ coactivator 1 α (Pgc1α) mRNAs, markers of BAT activity. Injection of M2-selective muscarinic receptor antagonists into the 4th ventricle abolishes the effect of optical stimulation. Single cell qRT-PCR analysis of retrogradely identified BAT-projecting neurons in the Rpa shows that all M2 receptor-expressing neurons contain tryptophan hydroxylase 2. In animals lacking the Chat gene in the DMH, exposure to warm temperature reduces neither BAT Ucp1 nor Pgc1α mRNA expression.

Conclusion

DMH cholinergic neurons directly send efferent signals to sympathetic premotor neurons in the Rpa. Elevated cholinergic input to this area reduces BAT activity through activation of M2 mAChRs on serotonergic neurons. Therefore, the direct DMH^ACh–Rpa^5-HT pathway may mediate physiological heat-defense responses to elevated environmental temperature.

Dynamic expression of the suppressor of cytokine signaling-3 and cytokines in the cerebral basilar artery of rats with subarachnoid hemorrhage, and the effect of acetylcholine

BACKGROUND

There are complex interactions between acetylcholine (ACh), the suppressor of cytokine signaling-3 (SOCS-3), and cytokines, however, little is known about their dynamic expression or their effects on cerebral vasospasm (CVS) after subarachnoid hemorrhage (SAH). Therefore, we aimed to describe and clarify the dynamic expression of SOCS-3 and cytokines after SAH, as well as the relationships between the levels of SOCS-3, cytokines, and ACh.

METHODS

The rat model of single cisterna magna injection was used to mimic acute SAH. The degree of CVS was indicated by lumen diameter and artery wall thickness under H&E staining. A semi-quantitative immunohistochemical analysis method was used to clarify the role of SOCS-3 in the CVS after SAH. We also measured the content of IL-6 and IL-10 in cerebrospinal fluid.

RESULTS

We found that SOCS-3 expression levels increased rapidly within 12 h after SAH, more slowly after 12 h, and did not reach a peak within 48 h. Interleukin 6 (IL-6) levels rapidly increased within 24 h after SAH, reached a peak 24 h after SAH, and decreased slightly at 48 h. IL-10 levels increased during the first 6 h after SAH, after which this increase tapered off. ACh treatment reduced IL-6 levels and resulted in elevated levels of SOCS-3, but had no effect on IL-10 expression. Furthermore, ACh treatment relieved basilar arterial vasospasm, whereas mecamylamine pretreatment counteracted the activity of ACh.

CONCLUSIONS

Taken together, these data indicate that SOCS-3 was involved in vasospasm via an IL-6- and IL-10-related mechanism, and that CVS following SAH could be reversed by the intraventricular injection of ACh.

Neuroprotective effect of oral choline administration after global brain ischemia in rats

Choline - now recognized as an essential nutrient - is the most common polar group found in the outer leaflet of the plasma membrane bilayer. Brain ischemia-reperfusion causes lipid peroxidation triggering multiple cell death pathways involving necrosis and apoptosis. Membrane breakdown is, therefore, a major pathophysiologic event in brain ischemia. The ability to achieve membrane repair is a critical step for survival of ischemic neurons following reperfusion injury. The availability of choline is a rate-limiting factor in phospholipid synthesis and, therefore, may be important for timely membrane repair and cell survival. This work aimed at verifying the effects of 7-day oral administration with different doses of choline on survival of CA1 hippocampal neurons following transient global forebrain ischemia in rats. The administration of 400 mg/kg/day divided into two daily doses for 7 consecutive days significantly improved CA1 pyramidal cell survival, indicating that the local availability of this essential nutrient may limit postischemic neuronal survival.

Magnetic resonance spectroscopy of the brain under mild hypothermia indicates changes in neuroprotection-related metabolites

Brain hypothermia has demonstrated pronounced neuroprotective effect in patients with cardiac arrest, ischemia and acute liver failure. However, its underlying neuroprotective mechanisms remain to be elucidated in order to improve therapeutic outcomes. Single voxel proton magnetic resonance spectroscopy ((1)H-MRS) was performed using a 7 Tesla MRI scanner on normal Sprague-Dawley rats (N=8) in the same voxel under normothermia (36.5 degrees C) and 30min mild hypothermia (33.5 degrees C). Levels of various brain proton metabolites were compared. The level of lactate (Lac) and myo-inositol (mI) increased in the cortex during hypothermia. In the thalamus, taurine (Tau), a cryogen in brain, increased and choline (Cho) decreased. These metabolic alterations indicated the onset of a number of neuroprotective processes that include attenuation of energy metabolism, excitotoxic pathways, brain osmolytes and thermoregulation, thus protecting neuronal cells from damage. These experimental findings demonstrated that (1)H-MRS can be applied to investigate the changes of specific metabolites and corresponding neuroprotection mechanisms in vivo noninvasively, and ultimately improve our basic understanding of hypothermia and ability to optimize its therapeutic efficacy.

Central choline suppresses plasma renin response to graded haemorrhage in rats

Central administration of choline increases blood pressure in normotensive and hypotensive states by increasing plasma concentrations of vasopressin and catecholamines. We hypothesized that choline could also modulate the renin-angiotensin pathway, the third main pressor system in the body. Plasma renin activity (PRA), which serves as an index of the function of the peripheral renin-angiotensin system, was determined in rats subjected to graded haemorrhage following central choline administration. Intracerebroventricular (i.c.v.) injection of choline (12.5-150 microg), a precursor of the neurotransmitter acetylcholine (ACh), inhibited the increase in PRA in rats subjected to graded haemorrhage by sequential removal of 0.55 mL blood/100 g bodyweight. Choline, in the range 50-150 microg, increased blood pressure. Intraperitoneal (i.p.) administration of 150 microg choline failed to alter blood pressure and plasma renin responses to graded haemorrhage. Administration of a higher dose (90 mg/kg, i.p.) of choline decreased blood pressure and enhanced PRA in the first two blood samples obtained during the graded haemorrhage. Physostigmine (10 microg, i.c.v.), ACh (10 microg, i.c.v.), carbamylcholine (10 microg, i.c.v.) and cytidine 5'-diphosphocholine (CDP-choline; 250 microg, i.c.v.) increased blood pressure and attenuated plasma renin responses to graded haemorrhage. Inhibition of PRA by i.c.v. choline was abolished by i.c.v. pretreatment with mecamylamine (50 microg), but not atropine (10 microg). Blood pressure responses to choline (150 microg) were attenuated by pretreatment with both mecamylamine and atropine. Inhibition of PRA in response to central choline administration was associated with enhanced plasma vasopressin and catecholamine responses to graded haemorrhage. Pretreatment of rats with a vasopressin antagonist reversed central choline-induced inhibition of plasma renin responses to graded haemorrhage without altering the blood pressure response. In conclusion, central administration of choline inhibits the plasma renin response to graded haemorrhage. Nicotinic receptor activation and an increase in plasma vasopressin appear to be involved in this effect.

Peripheral administration of CDP-choline, phosphocholine or choline increases plasma adrenaline and noradrenaline concentrations

1 Intraperitoneal (i.p.) injection of 200-600 mumol/kg of cytidine-5'-diphosphocholine (CDP-choline) increased plasma adrenaline and noradrenaline concentrations dose- and time-dependently. 2 CDP-choline treatment caused several-fold increases in plasma concentrations of CDP-choline and its metabolites phosphocholine, choline, cytidine monophosphate (CMP) and cytidine. 3 Equivalent doses (200-600 mumol/kg; i.p.) of phosphocholine or choline, but not CMP or cytidine, increased plasma adrenaline and noradrenaline dose-dependently. 4 CDP-choline, phosphocholine and choline (600 mumol/kg; i.p.) augmented the increases in plasma adrenaline and noradrenaline in response to graded haemorrhage. 5 The increases in plasma adrenaline and noradrenaline induced by i.p. 600 mumol/kg of CDP-choline, phosphocholine or choline were abolished by pre-treatment with hexamethonium (15 mg/kg; i.p.), but not atropine (2 mg/kg; i.p.). 6 At 320-32 000 mum concentrations, choline, but not CDP-choline or phosphocholine, evoked catecholamine secretion from perfused adrenal gland. Choline (3200 mum)-induced catecholamine secretion was attenuated by the presence of 1 mum of hexamethonium or mecamylamine, but not atropine, in the perfusion medium. 7 Intracerebroventricular (i.c.v.) injection of choline (0.5-1.5 mumol) also increased plasma adrenaline and noradrenaline dose- and time-dependently. Pre-treatment with mecamylamine (50 mug; i.c.v.) or hexamethonium (15 mg/kg; i.p.), but not atropine (10 mug; i.c.v.), prevented i.c.v. choline (1.5 mumol)-induced elevations in plasma adrenaline and noradrenaline. 8 It is concluded that i.p. administration of CDP-choline or its cholinergic metabolites phosphocholine and choline increases plasma adrenaline and noradrenaline concentrations by enhancing nicotinic cholinergic neurotransmission in the sympatho-adrenal system. Central choline also activates the sympatho-adrenal system by increasing central nicotinic cholinergic neurotransmission.

Activation of the central cholinergic pathway increases post-exercise tail heat loss in rats

The aim of this study was to evaluate the effects of stimulation of the central cholinergic pathway on the regulation of post-exercise tail heat loss in rats. Either 2.0microL of 25x10(-3)M physostigmine (Phy) or 0.15M NaCl solution (Sal) were injected into the right lateral cerebral ventricle of both resting (n=8) and post-exercising rats (n=6; 24mmin(-1); 25min; 5% inclination). Tail temperature (Ttail) was measured using a thermistor taped to the tail, and intraperitoneal temperature, an index of core temperature (Tc), was recorded using a telemetry sensor implanted into the peritoneal cavity. In resting rats, Phy induced an increase in both Ttail (26.8+/-0.3 degrees C Phy versus 25.2+/-0.6 degrees C Sal; P<0.05) and in heat loss index (0.26+/-0.03 Phy versus 0.14+/-0.05 Sal; P<0.05; 30min after injection), and a decrease in Tc compared to the Sal injection group (36.6+/-0.2 degrees C Phy versus 37.0+/-0.2 degrees C Sal; P<0.05). In post-exercising rats, Phy injection attenuated the decrease in both T(tail) (28.3+/-0.8 degrees C Phy versus 26.4+/-0.6 degrees C Sal; P<0.05) and heat loss index (0.37+/-0.07 Phy versus 0.19+/-0.02 Sal; P<0.05) without altering Tc. We conclude that activation of the central cholinergic pathway increases post-exercise tail heat loss in rats.

Differential sensitivity to the motor and hypothermic effects of the GABA B receptor agonist baclofen in various mouse strains

RATIONALE

Comparison of different mouse strains can provide valuable information about the genetic control of behavioural and molecular phenotypes. Recent evidence has demonstrated the importance of GABA B receptors in anxiety and depression. Investigation of the phamacogenetics of GABA B receptor activation may aid in the understanding of mechanisms underlying the role of GABA B in affect.

OBJECTIVES

The aim of current study was to determine the relative sensitivity of different mouse strains to GABA B receptor agonism in two models of GABA B receptor function, namely hypothermia and motor incoordination.

METHODS

Mice each from 11 strains (BALB/cByJIco, DBA/2JIco, OF1, FVB/NIco, CD1, C3H/HeOuJIco, 129/SvPasIco, NMRI, C57BL/6JIco, A/JOlaHsd and Swiss) were trained to walk on a rotarod for 300 s. On the following day, mice received 0, 3, 6 or 12 mg/kg of L: -baclofen PO. Rectal temperature and rotarod performance were measured at 0, 1, 2 and 4 h after drug application.

RESULTS

L: -Baclofen produced a significant dose-dependent hypothermia and ataxia in most, but not all, mouse strains examined. The magnitude and duration of response was influenced by strain, with mice of the 129/SvPasIco strain showing largest hypothermic response to 12 mg/kg l-baclofen and C3H/HeOuJIco the lowest, whereas the BALB/cByJIco strain demonstrated greatest ataxic response on the rotarod, and NMRI the least. Interestingly, some strains (notably C3H/HeOuJIco) had marked differential hypothermic and ataxic responses, with minimal body temperature responses to L: -baclofen but significant ataxia on the rotarod observed.

CONCLUSION

There is differential genetic control on specific GABA B receptor populations that mediate hypothermia and ataxia. Further, these studies demonstrate that background strain is an important determinant of GABA B receptor mediated responses, and that hypothermic and ataxic responses may be influenced by independent genetic loci.

Choline increases serum insulin in rat when injected intraperitoneally and augments basal and stimulated aceylcholine release from the rat minced pancreas in vitro

Intraperitoneal injection of choline (30-90 mg.kg-1) produced a dose-dependent increase in serum insulin, glucose and choline levels in rats. The increase in serum insulin induced by choline (90 mg.kg-1) was blocked by pretreatment with the muscarinic acetylcholine receptor antagonists, atropine (2 mg.kg-1), pirenzepine (2 mg.kg-1) and 4-diphenylacetoxy-N-methylpiperidine (2 mg.kg-1) or the ganglionic nicotinic receptor antagonist, hexamethonium (15 mg.kg-1). The effect of choline on serum insulin and glucose was enhanced by oral glucose administration (3 g.kg-1). Choline administration was associated with a significant (P < 0.001) increase in the acetylcholine content of pancreatic tissue. Choline (10-130 microm) increased basal and stimulated acetylcholine release but failed to evoke insulin release from the minced pancreas at considerably higher concentrations (0.1-10 mm). Hemicholium-3, a choline uptake inhibitor, attenuated the increase in acetylcholine release induced by choline augmentation. Choline (1-32 mm) inhibited [3H]quinuclidinyl benzilate binding to the muscarinic receptors in the pancreatic homogenates. These data show that choline, a precursor of the neurotransmitter acetylcholine, increases serum insulin by indirectly stimulating peripheral acetylcholine receptors through the enhancement of acetylcholine synthesis and release.

Intracerebroventricular choline increases plasma vasopressin and augments plasma vasopressin response to osmotic stimulation and hemorrhage

Intracerebroventricular (i.c.v.) injection of choline (50-150 microg), a precursor of the neurotransmitter acetylcholine, produced a time-and dose-dependent increase in plasma vasopressin levels in conscious, freely moving rats. The increase in plasma vasopressin in response to i.c.v. choline (150 microg) was inhibited by pretreatment with the nicotinic receptor antagonist, mecamylamine (50 microg; i.c.v.), but not by the muscarinic receptor antagonist, atropine (10 microg; i.c.v). The choline-induced rise in plasma vasopressin levels was greatly attenuated by hemicholinium-3 (HC-3; 20 microg; i.c.v.), a neuronal choline uptake inhibitor. Choline (50 or 150 microg; i.c.v.) produced a much greater increase in plasma vasopressin levels in osmotically stimulated or hemorrhaged rats than in normal rats. Choline (150 microg; i.c.v.) also enhanced plasma vasopressin response to graded hemorrhage; the enhancing effect of choline was also attenuated by HC-3 (20 microg; i.c.v.). Choline and acetylcholine concentrations in hypothalamic dialysates increased significantly following i.c.v. injection of choline (150 microg). It is concluded that choline increases plasma vasopressin levels by stimulating central nicotinic receptors indirectly, through the enhancement of acetylcholine synthesis and release, and augments the ability of osmotic stimulations or hemorrhage to stimulate vasopressin release.

Hyperglycemia induced by intracerebroventricular choline: involvement of the sympatho-adrenal system

Intracerebroventricular (i.c.v.) injection of choline (75-300 microg) produced a dose-dependent increase in blood glucose levels. Pre-treatment with the nicotinic acetylcholine receptor antagonist, mecamylamine (50 microg, i.c.v.) blocked the hyperglycemia induced by choline (150 microg, i.c.v.), but the response was not affected by pre-treatment with the muscarinic acetylcholine receptor antagonist, atropine (10 microg, i.c.v.). Pre-treatment with the neuronal choline uptake inhibitor, hemicholinium-3 (20 microg, i.c.v.), attenuated the hyperglycemia induced by choline. The hyperglycemic response to choline was associated increased plasma levels of adrenaline and noradrenaline. The hyperglycemia elicited by choline was greatly attenuated by bilateral adrenalectomy, and entirely blocked by either surgical transection of the splanchnic nerves or by pre-treatment with the alpha-adrenoceptor antagonist, phentolamine. These data show that choline, a precursor of acetylcholine, increases blood glucose and this effect is mediated by central nicotinic acetylcholine receptor activation. An increase in sympatho-adrenal activity appears to be involved in the hyperglycemic effect of choline.

Opposite regulation of body temperature by cholinergic input to the paraventricular nucleus and supraoptic nucleus in rats

Hypothalamic cholinergic system plays an important role in the regulation of body temperature and fluid balance. We have previously shown that cholinergic stimulation of the anterior hypothalamus and preoptic area was accompanied by a fall in body temperature, increased water intake, and increased Fos protein in the paraventricular nucleus (PVN) and supraoptic nucleus (SON). In the present study, to estimate the role played by cholinergic input to the PVN and SON in thermoregulation and water intake, we used microdialysis for cholinergic stimulation with neostigmine and analysis of the nucleus, and also investigated immunoreactivity for c-Fos protein in the brain. This stimulation increased extracellular concentration of acetylcholine in these nuclei. Stimulation of the PVN decreased body temperature and increased water intake. On the other hand, stimulation of the SON increased body temperature. Both in PVN-stimulated and SON-stimulated rats, c-Fos-like immunoreactivity (Fos-IR) was evident in the PVN, SON and certain regions including locus coeruleus (LC), area postrema and nucleus of the solitary tract (NTS). Addition of atropine to the dialysis medium attenuated the increase of Fos-IR and suppressed the cholinergic stimulation-induced responses in body temperature and water intake. These results suggest that cholinergic muscarinic mechanisms in PVN and SON play an opposite function in the regulation of body temperature. The same neuronal pathway including LC and NTS may participate in an advance both in hypothermia and in hyperthermia.