Interaction of formamidine pesticides with the presynaptic alpha(2)-adrenoceptor regulating.

The effects of the formamidine pesticides amitraz and chlordimeform on the alpha(2)-adrenergic receptor subtype that mediates the release of [(3)H]noradrenaline by synaptosomes from rat hypothalami were studied. We initially characterized the presynaptic autoreceptor on noradrenergic nerve endings using selective antagonists. Yohimbine (a nonselective alpha(2) antagonist) and BRL 44408 (selective for subtypes alpha(2A)/alpha(2D)) diminished the inhibitory effect of xylazine on K(+)-evoked release of [(3)H]noradrenaline; the K(B) values were 481 and 154 nM, respectively. In contrast, prazosin (a selective alpha(2B)/alpha(2C) antagonist) did not modify the inhibitory effect of xylazine. These results indicate that the release of noradrenaline by noradrenergic nerve endings in the rat hypothalamus is regulated by alpha(2D)-adrenoceptors, a species variation of the human alpha(2A) subtype. We then assessed the effects of the two pesticides on the K(+)-evoked release of [(3)H]noradrenaline. Amitraz reduced release in a dose-dependent manner; the effect observed at the maximal concentration tested (10 microM) was 13.0 +/- 2.0% and it was reversed by yohimbine. Amitraz also diminished the inhibitory effects of the alpha(2)-adrenergic agonists clonidine and xylazine. Chlordimeform displayed no effects, possibly because the true active compound of this insecticide is its demethylated metabolite. Based on these findings we conclude that the formamidine pesticides act as partial agonists of presynaptic alpha(2D)-adrenergic receptors in the rat hypothalamus. This interaction may be responsible for the in vivo alterations in catecholaminergic regulation of cyclic variations in gonadotropin-releasing hormone (GnRH) secretion, which can have grave functional repercussions on the reproductive system of mammals exposed to these xenobiotics.

K(+)-Evoked [(3)H]D-aspartate release in rat spinal cord synaptosomes: modulation by neuropeptide Y and calcium channel antagonists.

This study was conducted to investigate mechanisms regulating the release of [(3)H]D-aspartate (or endogenous glutamate) in the rat spinal cord. Presynaptic modulation of glutamate release was studied in superfused synaptosomes depolarized with 20 mM KCl. Calcium-channel antagonists, omega-conotoxin GVIA (omega-CgTx GVIA; N-type), nifedipine (L-type), and omega-conotoxin MVIIC (omega-CmTx MVIIC; P/Q type), were used to characterize the voltage-operated Ca(2+) channels (VOCCs) involved in this release. Nifedipine had no significant effect on the K(+)-evoked release of [(3)H]D-aspartate, but the omega-conotoxins GVIA and MVIIC produced dose-dependent inhibitory effects that were additive. The most substantial reduction (54.30% +/- 4.40%) was seen with omega-CgTx GVIA, indicating that N-type channels play a major role in the release of glutamate in this tissue. We investigated the effects of neuropeptide Y (NPY), NPY(13-36), and [Leu(31)][Pro(34)]NPY on Ca(2+)-dependent, K(+)-evoked [(3)H]D-aspartate release. NPY and NPY(13-36) equipotently inhibited the release of glutamate in a concentration-dependent manner. The half-maximal response was observed at about 12 nM; maximal inhibition of 44.22% +/- 4.60% was achieved with 0.3 microM. The selective GABA(B) agonist (-)baclofen inhibited K(+)-evoked [(3)H]D-aspartate release from superfused spinal cord synaptosomes by 50.00% +/- 4.80% at 10 microM. When NPY(13-36) and (-)baclofen were used together at maximal doses, their release-inhibiting effects were not additive. In addition, neither of the agonists was able to enhance the inhibition produced by pretreating the synaptosomes with the selective inhibitor of N-type VOCCs omega-CgTx GVIA. These results are consistent with the hypothesis that presynaptic Y(2)-like and GABA(B) receptors regulate glutamate release by blocking Ca(2+) currents through N-type VOCCs. Characterization of the receptors that can inhibit the release of glutamate may provide useful information for treatment of conditions characterized by excessive glutamatergic transmission in the spinal cord.