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1 The adenosine A3 receptor agonist, N6-2-(4-aminophenyl)ethyladenosine (APNEA) induces hypotension in the anaesthetized rat

We have investigated the role of somatostatin receptor subtypes sst2 and sst4 in limbic seizures and glutamate-mediated neurotransmission in mouse hippocampus. As compared to wild-type littermates, homozygous mice lacking sst2 receptors showed a 52% reduction in EEG ictal activity induced by intrahippocampal injection of 30 ng kainic acid (P < 0.05). The number of behavioural tonic-clonic seizures was reduced by 50% (P < 0.01) and the time to onset of seizures was doubled on average (P < 0.05). Seizure-associated neurodegeneration was found in the injected hippocampus (CA1, CA3 and hilar interneurons) and sporadically in the ipsilateral latero-dorsal thalamus. This occurred to a similar extent in wild-type and sst2 knock-out mice. Intrahippocampal injection of three selective sst2 receptor agonists in wild-type mice (Octreotide, BIM 23120 and L-779976, 1.5-6.0 nmol) did not affect kainate seizures while the same compounds significantly reduced seizures in rats. L-803087 (5 nmol), a selective sst4 receptor agonist, doubled seizure activity in wild-type mice on average. Interestingly, this effect was blocked by 3 nmol octreotide. It was determined, in both radioligand binding and cAMP accumulation, that octreotide had no direct agonist or antagonist action at mouse sst4 receptors expressed in CCl39 cells, up to micromolar concentrations. In hippocampal slices from wild-type mice, octreotide (2 micro m) did not modify AMPA-mediated synaptic responses while facilitation occurred with L-803087 (2 micro m). Similarly to what was observed in seizures, the effect of L-803087 was reduced by octreotide. In hippocampal slices from sst2 knock-out mice, both octreotide and L-803087 were ineffective on synaptic responses. Our findings show that, unlike in rats, sst2 receptors in mice do not mediate anticonvulsant effects. Moreover, stimulation of sst4 receptors in the hippocampus of wild-type mice induced excitatory effects which appeared to depend on the presence of sst2 subtypes, suggesting these receptors are functionally coupled.

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1 We have explored the role of allergen sensitization and challenge in de®ning the response of the airways of the Brown Norway (BN) rat to adenosine. 2 In naõÈ ve animals or in rats sensitized to ovalbumin (OA) adenosine induced only weak bronchoconstrictor responses. Challenge of sensitized animals with OA induced a marked airway hyperresponsiveness to adenosine which was not seen with methacholine or bradykinin. 3 The augmented bronchoconstrictor response to adenosine was not a ected by acute bivagotomy or atropine nor mimicked by an i.v. injection of capsaicin. It was, however, blocked selectively by disodium cromoglycate methysergide or ketanserin and reduced in animals treated sub-chronically with compound 48/80. 4 The augmented response to adenosine was associated with increases in the plasma concentrations of both histamine and 5-hydroxytryptamine (5-HT), which were attenuated by pretreatment with disodium cromoglycate, and degranulation of mast cells in the lung. 5 Parenchymal strips from lungs removed from sensitized rats challenged with OA gave augmented bronchoconstrictor responses to adenosine relative to strips from sensitized animals challenged with saline. Responses were inhibited by methysergide and disodium cromoglycate. 6 These data demonstrate a marked augmentation of the bronchoconstrictor response to adenosine in actively sensitized BN rats challenged with OA. The augmented response is primarily a consequence of mast cell activation, leading to the release of 5-HT, which in turn induces bronchoconstriction. Our data further suggest the involvement of a discrete lung-based population of mast cells containing and releasing mainly 5-HT and brought into play by prior exposure to allergen.

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