L. Stella

The antinociceptive effect of 2-chloro-2′-C-methyl-N6-cyclopentyladenosine (2′-Me-CCPA), a highly selective adenosine A1 receptor agonist, in the rat

Abstract This study was undertaken in order to investigate the effect of 2‐chloro‐2′‐C‐methyl‐N6‐cyclopentyladenosine (2′‐Me‐CCPA), a potent and highly selective adenosine A1 receptor agonist, on nociceptive responses and on the ongoing or tail flick‐related changes of rostral ventromedial medulla (RVM) ON‐ and OFF‐cell activities. Systemic administrations of 2′‐Me‐CCPA (2.5–5 mg/kg, i.p.) reduced the nociceptive response in the plantar and formalin tests, in a way prevented by DPCPX (3 mg/kg, i.p.), a selective A1 receptor antagonist. Similarly, intra‐periaqueductal grey (PAG) 2′‐Me‐CCPA (0.5–1–2 nmol/rat) reduced pain behaviour in the plantar and formalin tests, in a way inhibited by DPCPX (0.5 nmol/rat). Moreover, when administered systemically (2.5–5 mg/kg, i.p.) or intra‐PAG (0.5–1 nmol/rat) 2′‐Me‐CCPA increased the tail flick latencies, delayed the tail flick‐related onset of the ON‐cell burst and decreased the duration of the OFF‐cell pause in a dose dependent manner. Furthermore, it decreased RVM ON‐cell and increased OFF‐cell ongoing activities. The in vivo electrophysiological effects were all prevented by DPCPX (0.5 nmol/rat). This study confirms the role of adenosine A1 receptors in modulating pain and suggests a critical involvement of these receptors within PAG–RVM descending pathway for the processing of pain.

Periaqueductal gray matter glutamate and GABA decrease following subcutaneous formalin injection in rat.

Glutamate and GABA are important nociception modulating transmitters in specific brain regions, i.e. the spinal cord, the thalamic nuclei and the periaqueductal gray (PAG). However, quantitative and topographical changes in glutamate and GABA release in these brain regions during peripheral inflammation episodes have not been characterized in awake animals. To address this issue, an in vivo microdialysis study was carried out in freely moving rats in order to analyze PAG extracellular glutamate and GABA concentrations following unilateral formalin injection into the dorsal skin of the right hind-paw. Both glutamate and GABA release decreased after the injection of formalin during phase I and phase II of hyperalgesia. Because naloxone prevented the decrease of GABA and glutamate release induced by formalin, this study shows that, in vivo, a nociceptive stimulation may activate opioidergic fibres into the PAG. The increased release of endogenous opioids may, in turn, inhibit the activity of the GABAergic neurons (i.e. opioid disinhibition). Formalin injection also decreased extracellular glutamate concentration. However, we found that intra-PAG perfusion with tetrodotoxin only decreased GABA, but not glutamate dialysate values. Although it should be reasonable to speculate that opioids also inhibit glutamate fibres, further investigation is needed to clarify whether or not the dialysate glutamate we measured reflects change in the metabolism or neurotransmitter pool of this amino acid.

Nitric oxide participates in the hypotensive effect induced by adenosine A2 subtype receptor stimulation.

In a previous study, we demonstrated that adenosine plays an important role in the central control of the cardiovascular system with involvement of adenosine A2 rather than A1 subtype receptors. In the present study, we investigated the putative relationship between nitric oxide (NO) and adenosine in the central and peripheral control of the cardiovascular system. Adult male nor-motensive anesthetized rats were treated with N6-cyclohexyladenosine (CHA), an A1-punnoceptor agonist, and 5‘-N-cyclopropyl-carboxamidoadenosine (CPCA), an A2-purinoceptor agonist intracerebroventricularly (i.c.v. 3rd ventricle; 0.05–0.1–0.5 μg/rat) and by intravenous injection (0.5–1-5 μg kg−1 i.v.). CPCA and CHA induced a significant and dose-dependent decrease in arterial blood pressure (BP). CHA effects were less marked than CPA. Rats were pretreated with xanthine amine congener (XAC), and A1 adenosine antagonist, with 3,7-dimethyl-1-propargylxanthine (DMPX), an A2 adenosine antagonist (both administered at doses of 0.05 μg/rat i.c.v. or 0.5 μg kg−1 i.v.) and with N+-nitro-L-arginine methyl ester, an NO synthase inhibitor, (L-NAME, 90 μg/rat i.c.v. and 0.3 mg kg−1 i.v.). The intracerebroventricular and intravenous pretreatment with DMPX or L-NAME inhibited CPCA-induced hypotension; the effect of L-NAME was weaker than that of DMPX. The L-NAME inhibitory effect was reversed both in the central nervous system (CNS) and at the peripheral level by pretreatment with L-arginine (L-Arg; 90 mg kg−1 i.v.), a precursor of NO synthesis. Pretreatment with XAC, but not with L-NAME, reduced the hypotensive effect of CHA. Moreover, intracerebroventricular pretreatment with L-Arg (174 μg/rat) increased the hypotensive effect of CPCA. These results suggest that NO partially mediates the hypotensive effect induced by A2 subtype receptors stimulation in the CNS and at a peripheral level.