A study of presynaptic α 2 -autoreceptors in α 2A/D -, α 2B - and α 2C -adrenoceptor-deficient mice

Abstract

The function of presynaptic alpha2-autoreceptors was studied in the hippocampus, occipito-parietal cortex, atria and vas deferens of NMRI mice, mice in which the alpha2A/D-, the alpha2B- or alpha2c-adrenoceptor gene had been disrupted (alpha2A/DKO, alpha2BKO and alpha2CKO, respectively), and the wildtype mice from which the knockout animals had been generated. Tissue pieces were preincubated with 3H-noradrenaline and then superfused and stimulated electrically. The alpha2-adrenoceptor agonist medetomidine reduced the electrically evoked overflow of tritium in all tissues from all mouse strains (stimulation with single pulses or single high-frequency pulse trains, called POPs, i.e. pulse patterns leading to minimal autoinhibition). The effects of medetomidine did not differ in NMRI, wildtype, alpha2BKO and alpha2CKO mice but were greatly reduced in alpha2A/DKO brain preparations and to a lesser extent in alpha2A/DKO atria and vasa deferentia. Six drugs were tested as antagonists against medetomidine. Their pKd values indicated that the hippocampal and occipito-parietal alpha2-autoreceptors in NMRI and wildtype mice were alpha2D (the rodent variant of the alpha2A/D-adrenoceptor) whereas the atrial and vas deferens alpha2-autoreceptors in NMRI and wildtype mice could not be identified with a single alpha2 subtype. Deletion of the alpha2A/D gene changed the pKd values in all tissues so that they now reflected alpha2C properties, whereas deletion of the alpha2C gene changed the pKd values in atria and vasa deferentia so that they now had alpha2D properties (as they had in NMRI and wildtype brain preparations). Autoinhibition by released noradrenaline was created using trains of up to 64 pulses or up to 4 POPs, and the overflow-enhancing effect of the alpha2 antagonist rauwolscine was determined. Results did not differ, irrespective of whether preparations were obtained from NMRI, wildtype, alpha2BKO or alpha2CKO mice: the overflow of tritium elicited by p pulses or POPs was much smaller than p times the overflow elicited by a single pulse or POP, and rauwolscine greatly increased the evoked overflow. Results differed, however, in tissues taken from alpha2A/DKO mice: in these tissues, the overflow of tritium elicited by p pulses or POPs was close to p times the overflow elicited by a single pulse or POP, and rauwolscine did not increase the evoked overflow of tritiumor increased it only marginally. When a greater degree of autoinhibition was produced in atria and vasa deferentia by stimulation with 120 pulses, both disruption of the alpha2A/D gene and disruption of the alpha2C gene but not disruption of the alpha2B gene attenuated the overflow-enhancing effects of phentolamine and rauwolscine. In NMRI and wildtype atria and vasa deferentia, the relative potencies of phentolamine and rauwolscine at enhancing the evoked overflow were not easily compatible with a single alpha2 subtype. In alpha2A/DKO atria and vasa deferentia, the relative potencies of phentolamine and rauwolscine indicated that the autoinhibition-mediati...

Alpha2A-Adrenoceptors are Important Modulators of the Effects of D-Amphetamine on Startle Reactivity and Brain Monoamines

“…In the absence of a 2A -AR, or during pharmacological blockade of all a 2 -AR subtypes with an antagonist, the NE stores in noradrenergic neurons are susceptible to depletion by D-amph. This corroborates the evidence from a 2 -AR subtype-deficient mice revealing the principal role of the a 2A -AR in regulating the stimulation-induced NE release in cortical and hippocampal slices in vitro Trendelenburg et al, 1999Trendelenburg et al, , 2001Scheibner et al, 2001;Bücheler et al, 2002) and in the prefrontal cortex in vivo . Furthermore, the observed contribution of the a 2 -ARnoradrenergic system to modulation of the effects of D-amph is in good agreement with two earlier extensive studies on genetically modified mice, reporting increased sensitivity to psychostimulants when the brain NE homeostasis is disturbed, that is, in mice lacking the NE transporter (Xu et al, 2000) or incapable of synthesizing NE (Weinshenker et al, 2002).…”

Modulation of α2-Adrenoceptor Functions by Heterotrimeric Gαi Protein Isoforms

“…Pieces of prefrontal cortex were prepared from each animal after cervical dislocation as described previously (Trendelenburg et al, 2001). Tissue specimens from cardiac atria were processed as described previously Hein et al, 1999).…”

“…The overflow elicited by electrical stimulation was calculated as the difference "total tritium outflow during and after stimulation" minus "basal outflow" and was then expressed as a percentage of the tritium content of the tissue at the time of stimulation (Trendelenburg et al, 2001(Trendelenburg et al, , 2003. The calculation yielded the E max value and EC 50 value of medetomidine (concentration causing half-maximal inhibition), and n is the number of superfusion chambers (containing one piece of tissue).…”

Stress and Drug Dependence Differentially Modulate Norepinephrine Signaling in Animals with Varied HPA Axis Function

“…As two subtypes of a 2 receptors are expressed in the BNST (Scheinin et al, 1994), we paired fast-scan cyclic voltammetry measurements with receptorsubtype-specific pharmacology to assay control over norepinephrine release by each subtype. Knock-out mice were used to determine that a 2A acts as the principle autoreceptor (Trendelenburg et al, 2001). In agreement with this, we found the inhibition of a 2A increased norepinephrine overflow in the vBNST to a similar extent in both rat strains.…”