Action of rilmenidine on locus ceruleus and dorsal raphe cells in vivo and in vitro

Agmatine does not have activity at alpha 2-adrenoceptors which modulate the firing rate of locus coeruleus neurones: an electrophysiological study in rat

Agmatine (decarboxylated arginine) has been proposed as an endogenous ligand for non-adrenoceptor, imidazoline binding sites, but also binds to alpha 2-adrenoceptors. The interaction of agmatine with alpha 2-adrenoceptors was evaluated by studying the effect of agmatine on the firing rate of locus coeruleus (LC) neurones using extracellular recordings in anesthetized rats and rat brain slices. In vivo, local application of agmatine into the LC caused a slight and short-lasting increase in cell firing rate (P < 0.005). In vitro, agmatine failed to change the firing rate of LC neurones nor did it antagonize the inhibitory effect of noradrenaline on these cells. Since alpha 2-adrenoceptors are known to inhibit the firing of LC cells, we conclude that agmatine does not have agonist or antagonist properties at alpha 2-adrenoceptors of these neurones.

Quantitative autoradiographic localization in rat brain of alpha 2-adrenergic and non-adrenergic I-receptor binding sites labelled by [3H]rilmenidine

alpha 2A-Adrenergic receptor (AR) and non-adrenergic imidazoline receptor (I-R) binding sites have been previously characterized in rat cerebral cortex membranes using the N-substituted oxazoline, [3H]rilmenidine ([3H]Ril) [King, P.R. et al., Eur. J. Pharmacol., 218 (1992) 101-108]. In the present study, in vitro autoradiography was used to quantify the regional distribution of these receptors throughout the rat neuroaxis. The distribution and relative density (fmol/mg tissue) of I-Rs was examined in the presence of 1 microM adrenaline to block the adrenergic component of 40 nM [3H]Ril binding and non-specific binding was measured in the presence of another oxazoline, Bay a6781 (10 microM). Both alpha 2A-ARs and I-Rs were broadly, but heterogeneously, distributed. In forebrain, high levels of [3H]Ril-labelled alpha 2A-AR sites were observed in the anterior olfactory nucleus, the piriform, entorhinal and perirhinal cortices, lateral septum, bed nucleus of the stria terminalis, several thalamic nuclei, the amygdala and the arcuate, dorsomedial and posterior hypothalamic nuclei. In hindbrain, alpha 2A-AR sites were concentrated in locus coeruleus, lateral parabrachial nucleus, nucleus of the solitary tract and area postrema. I-R sites accounted for 50% or more of specific [3H]Ril binding (40 nM) in most cortical and hypothalamic nuclei, nucleus of the solitary tract, cranial motor nuclei and most spinal cord layers. The highest densities of I-Rs were found in the arcuate, dorsomedial and posterior hypothalamic nuclei, the locus coeruleus, the area postrema, the cranial motor nuclei and associated with spinal motor neurones. A very high concentration of I-Rs was also detected in the pineal gland. The distribution of alpha 2-AR sites determined resembled that reported with [3H]p-aminoclonidine which appears to specifically label alpha 2-ARs and not I1-R sites in rat brain sections, and [3H]methoxyidazoxan which is a selective alpha 2-AR antagonist. The regional and cellular distribution of I-R binding sites was unlike the distribution of putative I1-R sites labelled by [3H]clonidine in human brain, although comparable autoradiographic mapping studies in rat brain have not been done using this ligand. The regional and cellular distribution of [3H]-labelled I-R binding sites had both similarities and differences to that reported using the imidazoline ligand, [3H]idazoxan, with common labelling of areas such as area postrema, arcuate and interpeduncular nuclei and pineal gland with the two ligands, and differential relative binding levels ([3H]Ril > [3H]idazoxan) associated with hippocampal pyramidal cells and brainstem and spinal motor neurones.(ABSTRACT TRUNCATED AT 400 WORDS)

Reduction in focal cerebral ischemia by agents acting at imidazole receptors

Treatment with the alpha 2-adrenergic antagonist idazoxan (IDA) can provide protection from global cerebral ischemia. However, IDA also recognizes another class of receptors, termed imidazole (IM) receptors, which differ from alpha 2-adrenergic receptors and are responsible for the hypotensive actions of some centrally acting agents such as the oxazole rilmenidine (RIL). We therefore sought to determine whether RIL, an agent highly selective for IM receptors, offered protection from focal cerebral ischemia elicited in rat by ligation of the middle cerebral artery (MCA). We compared the effects of RIL with the effects of IDA and the selective non-IM alpha 2-antagonist SKF 86466 (SKF). In addition, we examined whether the neuroprotective effects of RIL and IDA could be attributed to changes in local CBF (LCBF). The MCA was occluded and animals either received immediate administration of drug while arterial pressure was maintained for 1 h or had local CBF increased to 200% of control for 1 h by hypercapnia or hypertension. RIL elicited a significant dose-dependent preservation of tissue to 33% of control at optimal dose (0.75 mg/kg). IDA (3 mg/kg) significantly reduced the size of ischemic infarction by 22%. In contrast, SKF (15 mg/kg) as well as doubling of LCBF did not preserve ischemic tissue. We conclude that both RIL and IDA can reduce focal ischemic infarction but that the mechanism does not appear secondary to antagonism of alpha 2-adrenergic receptors or elevation of LCBF. Occupation of IM receptors, either in the ischemic zone or at remote brain sites, may be responsible for neuroprotection of RIL and IDA.