V. De Novellis

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 GABAA 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.

Involvement of nitric oxide in cardiorespiratory regulation in the nucleus tractus solitarii

The aim of this study was to investigate whether nitric oxide (NO) is involved in cardiorespiratory regulation in the nucleus tractus solitarii (NTS). Unilateral microinjections (50 nl) of the NO-donor, sodium nitroprusside (SNP, 40-100-200 mM), into the NTS of anaesthetized rats elicited dose-dependent apnea (7.3 +/- 2.3 sec; 28.6 +/- 5.7 sec; 35.6 +/- 6.4 sec, respectively; n = 6) and a decrease in arterial blood pressure (8.4 +/- 3.1 mmHg; 18.2 +/- 5.8 mmHg; 25.8 +/- 6.7 mmHg, respectively; n = 6). Similarly, unilateral micro-injections (50 nl) of another NO-donor, 3-morpholinosydnonimine (SIN-1, 20-40-100 mM), also induced apnea (5.1 +/- 2.4 sec; 8.7 +/- 4.3 sec; 26.3 +/- 6.4 sec, respectively; n = 6) and a decrease in arterial blood pressure (6.2 +/- 2.3 mmHg; 11.1 +/- 3.3 mmHg; 18.3 +/- 6.1 mmHg, respectively; n = 6). The SNP- and SIN-1-induced apnea and arterial blood pressure decrease were significantly (p < 0.01) blocked by a 3 min pretreatment with two calcium-channel blockers, diltiazem (0.1 mM) and cobalt (10 mM), while lower doses (diltiazem 0.01 and cobalt 1) were ineffective. Microinjections of diltiazem (0.01 mM) and cobalt (1 mM) alone did not induce any change in basal cardiorespiratory values like diltiazem (0.1 mM) and cobalt (10 mM). These data suggest that NO may be involved in NTS cardiorespiratory regulation via calcium-channel activation.

Endothelin-1 in periaqueductal gray area of mice induces analgesia via glutamatergic receptors

&NA; The aim of the study was to examine whether endothelin‐1 (ET‐1) injected into dorsolateral periaqueductal gray (PAG) area of mice produces antinociception. ET‐1, from 1 to 4 pmol/mouse, induced antinociceptive effect in a dose‐dependent manner. This antinociceptive effect was prevented by NMDA receptor antagonists (2‐APV and MK‐801) injected in the same area (2‐APV) or by intraperitoneal route (MK‐801). CNQX, a non‐NMDA receptor antagonist, did not inhibit the ET‐1 effects. Prazosin, an &agr;1‐adrenergic blocking agent, also prevented the ET‐1 antinociceptive effect. We suggest that the activation of NMDA glutamatergic receptors in the PAG area may be a necessary step for ET‐1 induced antinociception.

Opposing effects on blood pressure following the activation of metabotropic and ionotropic glutamate receptors in raphe obscurus in the anaesthetized rat

The microinjection of l-glutamate (1–6 nmol/rat) and N-methyl-d-aspartate (NMDA 1–10 nmol/rat), ionotropic glutamate receptor (iGluR) agonists, into the nucleus raphe obscurus caused a concentration-dependent increase of arterial blood pressure. In contrast, (±)-1-aminocyclopentane-trans-1, 3-dicarboxylic acid (t-ACPD, 14–42 nmol/rat), a metabotropic glutamate receptor (mGluRs) agonist, caused a concentration-dependent decrease in blood pressure. Pretreatment with D, L-2-amino-phosphono valeric acid (2-APV, 5 nmol/rat) a selective NMDA iGluR antagonist, and (+)-5-methyl-10, 11-dihydro-5H-dibenzo[a, b] cyclohepten-5,10-imine hydrogen maleate (MK801, 0.9 nmol/rat), a noncompetitive NMDA iGluR antagonist, blocked both the glutamate and NMDA pressor responses, while pretreatment with (+)-α-methyl-4-carboxyphenylglycine (MCPG, 0.05 nmol/rat), a mGluR1 antagonist, increased the glutamate-induced pressor effects and blocked the fall in blood pressure induced by t-ACPD. 6-Cyano-7-nitroquinoxaline-2,3dione-(CNQX, 0.4 nmol/rat) a non-NMDA iGluR antagonist, did not affected the glutamate-induced hypertension. These observations indicate opposing roles for ionotropic and metabotropic receptors in the glutamate-induced blood pressure changes elicited from the nucleus raphe obscurus. Moreover, we suggest that the glutamate-induced hypertension may be due to the activation of NMDA ionotropic receptor subtypes and the metabotropic receptors may influence this activaction through a reduction of excitability at level of synapses.

Behavioural effects induced by microinjection of L-BOAA into the ventrolateral PAG matter of the mouse

L-BOAA (1 microgram/mouse), microinjected into the ventrolateral periaqueductal gray (PAG) matter, induced a strong reaction of forward avoidance (running) for 20-30 s in 18% of the mice and immobility for 7 +/- 2 min in 72% of the mice from a total of 68 treated animals. Effects were also observed for grooming and clonus in 6% and 4% of the mice, respectively. Duration of L-BOAA-induced immobility was significantly (p < 0.05) reduced by a pretreatment with CNQX (0.5 microgram/mouse), a selective antagonist of AMPA glutamergic subtype receptors, but not by a pretreatment with 2-APV (0.5 microgram/mouse), a selective antagonist of NMDA glutamergic subtype receptors, nor by 2-AP3 (0.5 microgram/mouse), a weak antagonist of metabotropic glutamergic subtype receptors. AMPA (0.05 microgram/mouse), also microinjected into ventrolateral PAG, induced the same pattern of behavioural effects as L-BOAA. Forward avoidance, grooming, and clonus induced by L-BOAA or AMPA were also significantly antagonised by a pretreatment with CNQX (data not shown).

The failure of pyridostigmine to influence the coronary artery spasms induced by vasopressin in conscious rabbits

Abstract In conscious rabbits the intravenous (IV) injection of vasopressin (1 IU/kg IV) induced a coronarocostrictor effect and an increase in S-T segment and T wave amplitude. These changes were accompanied by sinus bradycardia and cardiac arrythmias. The injection of pyridostigmina (250 μg/kg), a reversible cholinesterase inhibitor with predominant muscarinic effects, failed to antagonize the coronarocostrictor effect of vasopressin, as judged by the changes in electrocardiographic recordings.