Anti-nociception Induced by Systemic or PAG-microinjected Lysine-acetylsalicylate in Rats. Effects on Tail-flick Related Activity of Medullary Off- and On-cells

Previous experiments using metamizol have shown that this non-steroidal anti-inflammatory drug (NSAID) produces a central anti-nociceptive effect probably through neural substrates that also support the analgesic effects of opiates, such as the periaqueductal grey matter (PAG) and the off- and on-cells of the rostral ventromedial medulla (RVM). Off- and on-cells have been postulated to respectively inhibit and facilitate nociceptive transmission, since the heat-elicited tail flick reflex (TF) occurs only after off-cells have decreased (pause), and on-cells have increased (burst) their activity. The aim of the present study was to examine whether the effect of metamizol upon TF and off- and on-cells responses could be generalized to other NSAIDs such as, in this case, lysine-acetylsalicylate (LASA). Fifty-nine off- and on-cells of the RVM were recorded in lightly anaesthetized rats. Systemic administration (200 and 300 mg/kg) or PAG microinjection (30, 50 and 100 μg) of LASA caused retardation of the heat-elicited off-cell pause, on-cell burst and the corresponding TF. Neuronal responses and TF retained their mutual time relationship but shifted simultaneously toward longer latencies. This anti-nociceptive effect of LASA was dose-dependent, present 5 min after administration and reached a maximum in 30 min for both administration methods. These data confirm that analgesics typically defined as peripherally-acting, such as metamizol and LASA in this study, may also have an anti-nociceptive effect by acting directly upon PAG, and suggest that this central effect involves the RVM off- and on-cells.     

Metabotropic glutamate receptors modulate serotonin release in the rat periaqueductal gray matter

The role of metabotropic (mGluRs) and N-methyl-D-aspartate (NMDA) glutamate receptors on 5-hydroxytryptamine (5-HT) release has been studied in rat periaqueductal gray (PAG) matter by using in vivo microdialysis. (1S,3R)-aminocyclopentane-1,3-dicarboxylic acid [(1S,3R)-ACPD; 0.5 or 1 mM], a group I/group II mGluRs agonist, increased the dialysate 5-HT concentration. (2S)-α-ethylglutamic acid (EGlu; 1 mM), an antagonist of group II mGluRs, but not (RS)-1-aminoindan-1,5-dicarboxylic acid (AIDA; 1 mM), an antagonist of group I mGluRs, antagonized the 1S,3R-ACPD-induced effect. (S)-3,5-dihydroxyphenylglycine (DHPG; 0.5 and 1 mM), an agonist of group I mGluRs, did not modify dialysate 5-HT. (2S, 3S, 4S)-α-(carboxycyclopropyl)-glycine (L-CCG-I; 0.5 and 1 mM), an agonist of group II mGluRs, increased extracellular 5-HT. This effect was antagonized by EGlu. Similarly, L-serine-O-phosphate (L-SOP; 1 and 10 mM), an agonist of group III mGluRs, increased extracellular 5-HT and this effect was antagonized by (RS)-α-methylserine O-phosphate (M-SOP; 1 mM), an antagonist of group III mGluRs. Out of the several N-methyl-D-aspartate concentrations used (NMDA; 10, 50, 100, 500 and 1000 μM) only the 50 μM infusion significantly decreased dialysate 5-HT. The GABA_A receptor agonist, bicuculline (30 μM), increased 5-HT release on its own and antagonized the decrease caused by the opiate antagonist, naloxone (2 mM), as well as the increases caused by CCG-I or L-SOP. These data show that stimulation of PAG’s group II/group III mGluRs increases 5-HT release, while stimulation of NMDA glutamate receptors may decrease it. We speculate that glutamate does not modulate 5-HT release in the PAG directly, but via activation of tonically active GABAergic interneurons. Received: 15 January 1998 / Accepted: 22 July 1998     

Dorsal column input to inferior raphe centralis neurons

In anesthetized decerebellate cats with additional decerebration or decortication and with one of two types of cervical spinal lesions which either eliminated the dorsal half of the spinal cord or spared the dorsal funiculi, peripheral electric and localized natural stimuli activated neurons in the inferior raphe nuclear complex. Medial lemniscal as well as direct dorsal funicular stimulation was also effective. The majority of raphe neurons activated by stimulation of the dorsal funiculi were also discharged by ventrolateral funicular stimulation.     

Modulation of the jaw-opening reflex by peripheral electrical stimulation

Modulation of the jaw-opening reflex (JOR) by peripheral electrical stimulation was studied in the rat. The JOR was evoked by electrical stimuli delivered to the tongue, infraorbital nerve, or tooth pulp chamber, and single-pulse conditioning stimuli were delivered to the forelimb, hind limb, or tail. Threshold current for eliciting the JOR was modulated in a biphasic manner with facilitation when the delay between conditioning and test stimuli was short (peaking at 10 to 15 ms) and inhibition at longer intervals (peaking at 40 to 60 ms). Modulation was similar for all peripheral conditioning sites and was not affected by Fentanyl, naloxone, or picrotoxin. Thus, the modulation of the JOR by single-pulse peripheral electrical stimulation is a widespread, nonsegmental phenomenon, and is probably not associated with the endogenous opiate system. Data collected during the course of this study call into question the usefulness of the JOR elicited by electrical stimulation in the rat incisor tooth pulp chamber as a pain model.     

The benefits of magnetic resonance imaging methods to extend the knowledge of the anatomical organisation of the periaqueductal gray in mammals

The periaqueductal gray (PAG) is a mesencephalic brain structure involved in the expression of numerous behaviours such as maternal, sexual and emotional. Histological approaches showed the PAG is composed by subdivisions with specific cell organisation, neurochemical composition and connections with the rest of the brain. The comparison of studies performed in rodents and cats as the most often examined species, suggests that PAG organisation differs between mammals. However, we should also consider the plurality of the methods used in these studies that makes difficult the comparison of the PAG organisation between species. Therefore, to study the PAG in all mammals including human, the most relevant in vivo imaging method seems to be the magnetic resonance imaging (MRI). The purpose of this review was to summarize the knowledge of the anatomical organisation of the PAG in mammals and highlights the benefits of MRI methods to extend this knowledge. Results obtained by MRI so far support the conclusions of ex vivo studies, especially to describe the subdivisions and the connections of the PAG. In these latter, diffusion-weighted MRI and functional connectivity seem the most appropriate methods. In conclusion firstly, the MRI seems to be the best judicious method to compare species and improve the comprehension of the role of the PAG. Secondly, MRI is an in vivo method aimed to manage repeated measures in the same cohort of subjects allowing to study the impact of aging and the development on the anatomical organisation of the PAG.     

Fluoxetine attenuates the effects of pentylenetetrazol on rat freezing behavior and c-Fos expression in the dorsomedial periaqueductal gray

The aim of the study was to investigate the role of the periaqueductal gray (PAG) in anxiolytic-like actions of fluoxetine in animals treated with an anxiogenic drug, pentylenetetrazol (PTZ), and subjected to fear conditioning procedure. The data showed that PTZ given at the dose of 30 mg/kg 15 min before a retention trial significantly decreased freezing reaction (p<0.01), and potently enhanced rat locomotor activity (p<0.01), in comparison to the control group. These effects were reversed by prior (60 min) administration of fluoxetine (20 mg/kg). Simultaneously, PTZ significantly increased c-Fos expression in the dorsomedial periaqueductal gray (DMPAG), examined 2h after the retention trial, in comparison to the control group (p<0.01). Fluoxetine (20 mg/kg) administered 60 min before PTZ reversed this effect. PTZ given at the same dose and time interval in the open field test did not affect rat locomotor behavior. Importantly, fluoxetine pretreatment did not change PTZ concentration in brain tissue. Our experiment based on PTZ-enhanced aversive conditioning revealed that acutely administered fluoxetine antagonized PTZ-induced panic-like behavior, and this phenomenon was accompanied by inhibition of activity of DMPAG.     

¹H nuclear magnetic resonance based metabolic urinary profiling of patients with ischemic heart failure

Objectives

We sought to identify metabolic pathways characterizing human heart failure (HF) using ¹NMR based urinary metabolomic analysis in conjunction with multivariate statistics.

Design and methods

Patients with systolic HF of ischemic origin (n = 15) and healthy controls (n = 20) participated in this study. Patients with type 2 diabetes mellitus were excluded.

Results

The results showed that the urine of the HF patients had higher levels of metabolites for acetate (p < 0.05) and acetone (p < 0.01) compared to the healthy controls. In addition, there was a perturbation in methylmalonate metabolism as shown by increased urinary levels of methylmalonic acid (p < 0.001) in the HF patients. HF patients also had increased urinary levels of cytosine (p < 0.01) and phenylacetylglycine (p < 0.01) and decreased 1-methylnicotinamide (p < 0.05) compared to healthy controls.

Conclusions

TCA cycle metabolites and fatty acid metabolism were modified in the HF patients, indicating altered energy metabolism. Moreover, perturbations of metabolism in nucleotide and methylmalonate were observed.     

Distribution of PDE8A in the nervous system of the Sprague-Dawley rat

Phosphodiesterases (PDEs) are essential regulators of cyclic nucleotide signaling. Little is known of the distribution and function of the cyclic adenosine monophosphate (cAMP) hydrolyzing PDE8A family.Employing immunohistochemistry and Western blots this study maps the distribution of PDE8A in the brain of adult male Sprague-Dawley rats and in the trigeminal ganglion.PDE8A was confined to neuronal perikaryal cytoplasm and to processes extending from those perikarya. The neurons exhibiting PDE8A-immunoreactivity were widely distributed in the forebrain, brain stem, and cerebellum. Strongly immunoreactive neurons were located in the olfactory bulb, the septal area, zona incerta, and reticular nucleus of the thalamus. Less immunoreactivity was seen in the hippocampus and cerebral cortex. Intense staining was detected in both the substantia nigra and the sensory trigeminal nucleus. In cerebellum PDE8A immunoreactivity was located not only in the Purkinje cells, but also in the granular cells as well as the parallel fibres in the molecular layer. PDE8A immunoreactivity was represented in the epithelial lining of the choroids plexus, the dura mater, and the neurons of the trigeminal ganglion.The localization of the cAMP degrading PDE8A may indicate a role for PDE8A in cAMP signaling related to pain transmission, motor function, cognition and olfaction.     

Effects induced by cannabinoids on monoaminergic systems in the brain and their implications for psychiatric disorders

The endocannabinoid system and CB(1) receptors participate in the control of emotional behavior and mood through a functional coupling with the classic monoaminergic systems. In general, the acute stimulation of CB(1) receptors increases the activity (spontaneous firing rate) of noradrenergic (NE), serotonergic (5-HT) and dopaminergic (DA) neurons as well as the synthesis and/or release of the corresponding neurotransmitter in specific brain regions. Notably, the antagonist/inverse agonist rimonabant (SR141617A) can decrease the basal activity of NE and 5-HT neurons, suggesting a tonic/constitutive regulation of these neuronal systems by endocannabinoids acting at CB(1) receptors. Monoaminergic systems are modulated via CB(1) receptors by direct or indirect effects depending on the localization of this inhibitory receptor, which can be present on monoaminergic neurons themselves and/or inhibitory (GABAergic) and/or excitatory (glutamatergic) regulatory neurons. The repeated stimulation of CB(1) receptors is not associated with the induction of tolerance (receptor desensitization) on the activity of NE, 5-HT and DA neurons, in contrast to chronic agonist effects on neurotransmitter synthesis and/or release in some brain regions. CB(1) receptor desensitization may alter the direct and/or indirect effects of cannabinoid drugs modulating the functionality of monoaminergic systems. The sustained activation of monoaminergic neurons by cannabinoid drugs can also be related to changes in the function of presynaptic inhibitory α(2)-adrenoceptors or 5-HT(1A) receptors (autoreceptors and heteroreceptors), whose sensitivity is downregulated or upregulated upon chronic CB(1) agonist exposure. The functional interactions between endocannabinoids and monoaminergic systems in the brain indicate a potential role for CB(1) receptor signaling in the neurobiology of various psychiatric disorders, including major depression and schizophrenia as the major syndromes.     

Potassium channels underlie postsynaptic but not presynaptic GABA(B) receptor-mediated inhibition on ventrolateral periaqueductal gray neurons

γ-Aminobutyric acid (GABA) is the principle inhibitory neurotransmitter in adult mammalian brain. GABA receptors B subtype (GABA(B)Rs) are abundantly expressed at presynaptic and postsynaptic neuronal structures in the rat ventrolateral periaqueductal gray (PAG), an area related to pain regulation. Activation of GABA(B)Rs by baclofen, a selective agonist, induces presynaptic inhibition by decreasing presynaptic glutamate release. At the same time, baclofen induces a postsynaptic inhibitory membrane current or potential. We here report that in the ventrolateral PAG, the postsynaptic inhibition is mediated by activation of G protein-coupled inwardly rectifying K(+) (GIRK) channels. Blockade of K(+) channels largely prevents postsynaptic action of baclofen. In contrast, presynaptic inhibition of baclofen is insensitive to K(+) channel blockade. The data indicate that potassium channels play different roles in GABA(B)R-mediated presynaptic and postsynaptic inhibition on PAG neurons.