1 The release-inhibiting a 2 -adrenoceptors of cerebral serotoninergic axons were studied in mice. Slices of the hippocampus or the occipito-parietal cortex from NMRI mice, from mice lacking the a 2A/D -, the a 2B -, the a 2C -or both the a 2A/D -and the a 2C -adrenoceptor, and from mice sharing the genetic background of the receptor-de®cient animals (WT) were preincubated with [ 3 H]-serotonin and then superfused and stimulated electrically, in most experiments by trains of 8 pulses at 100 Hz. 2 The concentration-response curves of the a 2 -adrenoceptor agonist medetomidine were virtually identical in hippocampal slices from NMRI and WT mice, with maximally 70% inhibition and an EC 50 of about 2 nM. In hippocampal slices from NMRI mice, phentolamine and rauwolscine were equipotent antagonists against medetomidine.3 The e ect of medetomidine was greatly reduced, with maximally 20% inhibition, in hippocampal slices from a 2A/D -adrenoceptor-de®cient mice; was slightly reduced, with maximally 59% inhibition, in hippocampal slices from a 2C -adrenoceptor-de®cient mice; was not changed in hippocampal slices from a 2B -adrenoceptor-de®cient mice; and was abolished in hippocampal slices from mice lacking both the a 2A/D -and the a 2C -adrenoceptor. 4 Similar results were obtained in: (i) occipito-parietal slices from NMRI and a 2A/D -adrenoceptorde®cient mice and (ii) hippocampal slices that were preincubated with [ 3 H]-serotonin in the presence of oxaprotiline to rule out cross-labelling of noradrenergic axons. 5 The serotoninergic axons of the mouse brain possess both a 2A/D -heteroreceptors, which predominate, and a 2C -heteroreceptors but lack a 2B -adrenoceptors. The situation resembles the coexistence of a 2A/D -and a 2C -autoreceptors but lack of a 2B -autoreceptors at the noradrenergic axons of mice.

1 Mammals possess three types of a 2 -adrenoceptor, a 2A , a 2B and a 2C . Our aim was to determine the type of a 2 -adrenoceptor involved in the control of gastrointestinal motility. 2 In transmitter over¯ow experiments, myenteric plexus longitudinal muscle (MPLM) preparations of the ileum were preincubated with [ 3 H]-choline and then superfused. The a 2 -adrenoceptor agonist medetomidine reduced the electrically evoked over¯ow of tritium from preparations taken from wild type but not a 2A -adrenoceptor-knockout mice. 3 In a second series of over¯ow experiments, MPLM preparations were preincubated with [ 3 H]-noradrenaline and then superfused. Again medetomidine reduced the electrically evoked over¯ow of tritium from wild type but not a 2A -knockout preparations. 4 In organ bath experiments, medetomidine reduced electrically evoked contractions of segments of the ileum from wild type but not a 2A -knockout mice. 5 In each of these three series, phentolamine antagonized the e ect of medetomidine in wild-type preparations with greater potency than rauwolscine. 6 In conscious mice, gastrointestinal transit was assessed by means of an intragastric charcoal bolus. In a 2A -knockout mice, the speed of gastrointestinal transit was doubled compared to wildtype. Medetomidine, injected intraperitoneally, slowed gastrointestinal transit in wild type but not a 2A -knockout mice. 7 We conclude that the cholinergic motor neurons of the enteric nervous system of mice possess a 2 -heteroreceptors which mediate inhibition of acetylcholine release, of neurogenic contractions and of gastrointestinal transit. The noradrenergic axons innervating the intestine possess a 2 -autoreceptors. Both hetero-and autoreceptors are exclusively a 2A . It is the a 2A -adrenoceptor which in vivo mediates the inhibition of intestinal motility by the sympathetic nervous system.

The stimulation frequency-noradrenaline release relationship was studied in the vas deferens and the cerebral cortex of NMRI mice, mice in which the alpha2A-, the alpha2B-, the alpha2C- or both the alphaCA- and the alpha2C-adrenoceptor gene had been disrupted (alpha2AKO, alpha2BKO, alpha2CKO and alpha2ACKO), 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 with a constant number of pulses (30 in vas deferens and 50 in brain cortex) at frequencies between 0.03 and 100 Hz. The frequency-evoked tritium overflow curves ascended monophasically in the vas deferens of wildtype and NMRI mice. Disruption of the alpha2B-adrenoceptor gene caused no change. In the vas deferens of alpha2CKO mice, the overflow evoked by low frequencies (0.3 and 1 Hz) was slightly increased. In the vas deferens of alpha2AKO and alpha2ACKO mice, the evoked overflow was increased to a greater extent. Rauwolscine (1 microM) caused a marked increase of the evoked overflow of tritium from the vas deferens of NMRI, wildtype, alpha2BKO and alpha2CKO mice. Rauwolscine also increased the evoked overflow of tritium from the vas deferens of alpha2AKO and alphaC2ACKO mice, but to a smaller extent. The gene disruptions and rauwolscine slightly steepened the slope of the vas deferens frequency-overflow curve. In the brain cortex of wildtype and NMRI mice, the frequency-evoked tritium overflow curves were U-shaped. In the brain cortex of alpha2BKO and alpha2CKO mice, the evoked overflow was slightly reduced. In the brain cortex of alpha2AKO and alpha2AcKO mice, in contrast, the evoked overflow was increased. Rauwolscine (1 microM) caused a marked increase of the evoked overflow of tritium from the brain cortex of NMRI, wildtype, Q2BKO and alpha2CKO mice. Rauwolscine also increased the evoked overflow of tritium from the brain cortex of alpha2AKO and alpha2ACKO mice, but to a smaller extent. The gene disruptions and rauwolscine flattened the U shape of the brain cortex frequency-overflow curve. It is concluded that alpha2-autoinhibition is one factor that shapes the frequency-noradrenaline release relationships in the mouse vas deferens and cerebral cortex. The autoreceptors are mainly alpha2A and to a minor extent, and well detectable in the vas deferens only, alpha2C. When both the alpha2A- and the alpha2C-adrenoceptor have been deleted, alpha2B-adrenoceptors may be expressed as autoreceptors in noradrenergic neurons. It seems possible that alpha2C-autoreceptors depress mainly release at low (around 1 Hz) whereas alpha2A-autoreceptors depress mainly release at high (around 10 Hz) frequencies.

No abstract