Adenosine analogs mediating depressant effects on synaptic transmission in rat hippocampus: structure-activity relationships for the N6 subregion.

Previous studies have shown that analogs of adenosine with substituents upon the N6-nitrogen (e.g., N6-[1-phenyl-2(R)-propyl]adenosine; R-PIA) are often much more potent than the parent compound in activating adenosine receptors, particularly those of the A1 subtype. The present investigation characterized the potencies of a number of N6-substituted adenosine analogs in depressing excitatory synaptic transmission in slices of rat hippocampus, an electrophysiological response mediated by receptors of the A1 subtype. These potencies correlated well with previously reported affinities of these analogs for A1 receptor sites in brain, but not with coronary vasodilation in the dog heart, an A2 receptor mediated response. Analogs with alkyl or aryl substituents at the N6 position were generally more potent than adenosine, although analogs with a tertiary carbon attached directly to the N6-nitrogen were usually only weakly active. Although it has been suggested that there may be a subregion of the A1 receptor with some specificity for aryl groups, these experiments did not suggest that this was the case. Analogs with chiral centers attached to the N6-nitrogen usually displayed stereoselectivity, with R-isomers more potent than the S-isomers. The mechanism underlying this selectivity appeared to be both a facilitating effect of alkyl substituents in the propyl C1 position of R-PIA, and a hindering effect of substituents in the position normally occupied by the hydrogen attached to propyl C2 of R-PIA. These results indicate that although there are some similarities in terms of requirements for activity at A1 and A2 receptors, differences between the N6 sub-regions of these receptors are sufficient to permit the development of selective analogs for these two receptor sites.

Facilitation of recurrent inhibition in rat hippocampus by barbiturate and related nonbarbiturate depressant drugs.

The effects of anticonvulsant, anesthetic and convulsant barbiturates and of related depressant drugs were characterized on excitatory and inhibitory synaptic transmission in slices of rat hippocampus. The duration of recurrent GABAergic inhibition was increased by all of the drugs tested, including the convulsant barbiturate 5-ethyl-5-[1,3-dimethylbutyl]barbituric acid, anesthetic barbiturates such as pentobarbital and nonbarbiturate anesthetics such as (+)-etomidate. Several barbiturates, including phenobarbital and (+)-mephobarbital facilitated inhibition, but the maximal responses to these agents were significantly less than with pentobarbital. In general, there was a good correspondence between the potencies of these drugs in facilitating inhibition and their previously reported abilities to regulate binding at the gamma-aminobutyric acid/benzodiazepine/barbiturate receptor complex. In addition to facilitating recurrent GABAergic inhibition, at successively higher doses most of these drugs induced direct depression of the population spike response, field excitatory postsynaptic potential and presynaptic fiber spike. 5-Ethyl-5-[1,3-dimethylbutyl]barbituric acid, (+)-mephobarbital and pentobarbital facilitated excitatory synaptic transmission at the Schaffer collateral/commissural synapses on the CA1 pyramidal neurons at low doses, but caused depression at higher doses. The net effects observed with each drug tested (facilitation/depression of excitatory transmission, enhancement of GABAergic inhibition) correlated well with the behavioral effects of these agents in vivo.

DSP4-induced noradrenergic lesions increase beta-adrenergic receptors and hippocampal electrophysiological responsiveness.

Following profound (greater than 90%) depletions of norepinephrine (NE) by the noradrenergic neurotoxin N-(2-chloroethyl)-N-ethyl-2-bromobenzylamine (DSP4), the numbers of beta-adrenergic receptors were significantly increased (20-25%) in rat hippocampal and somatosensory cortical membranes; however, the numbers of alpha 1-adrenergic receptors and the affinities of both types of receptors were unaffected. This selective up-regulation of beta-adrenergic receptors was evident 1 week after DSP4 administration and was maintained for at least 2 more weeks. In electrophysiological experiments in the hippocampal slice preparation, responses to threshold as well as maximal concentrations of isoproterenol were enhanced 150% and 33%, respectively, in the DSP4-lesioned animals. The results demonstrate that nearly complete depletion of brain NE produced by administration of DSP4, like that produced by 6-hydroxydopamine, results in increased numbers of beta- but not alpha-adrenergic receptors, and suggest that the density of the former are regulated by afferent noradrenergic fibers. Furthermore, the functional significance of the increased number of hippocampal beta-adrenergic receptors is directly manifested in a greater electrophysiological responsiveness to an exogenously administered beta-adrenergic receptor agonist.

Modulation of long-term potentiation: effects of adrenergic and neuroleptic drugs.

A variety of drugs which either mimic or antagonize the effects of norepinephrine and dopamine were tested for their ability to modulate long-term potentiation (LTP) in the rat hippocampus in vitro. Neither administration of norepinephrine, amphetamine or adrenergic antagonists, nor pretreatment with reserpine or DSP4 (which selectively destroys noradrenergic afferents to the hippocampus) had any significant effect on the magnitude of LTP. Isoproterenol, a beta-adrenergic receptor agonist, was able to partially block LTP, but this did not appear to be due to a direct action of isoproterenol on LTP. Neuroleptic drugs such as trifluoperazine were able to block LTP almost completely; however, this action was not stereospecific. Other dopamine antagonists such as sulpiride had no effect on LTP. The ability of neuroleptics to antagonize LTP was more closely related to their ability to block calmodulin than to their relative potencies as dopamine antagonists. It would appear that neither norepinephrine nor adrenergic antagonists influence the amount of LTP elicited by repetitive stimulation; however, drugs which have been shown to interfere with calmodulin-mediated cellular processes do antagonize this phenomenon.

Sedative and anticonvulsant effects of adenosine analogs in mouse and rat.

The behavioral and physiological effects of L-phenylisopropyladenosine, cyclohexyladenosine and 2-chloroadenosine were examined in mice and rats. These analogs of adenosine are agonists which bind with high affinity to putative central A1 receptors in vitro. Relatively low doses of these drugs administered i.p. produced marked sedation and hypothermia; higher doses resulted in an almost complete cessation of spontaneous motor activity as well as some ataxia. These analogs also antagonized seizures elicited by a variety of convulsants with different mechanisms of action. The differences observed in the anticonvulsant potencies of the analogs suggest that these effects are not produced by the interaction of these drugs with a single class of adenosine receptor. In particular, 2-chloroadenosine and cyclohexyladenosine appear to be more related to each other pharmacologically than to L-phenylisopropyladenosine. Because some of the anticonvulsant actions of L-phenylisopropyladenosine are not reversed by the adenosine antagonist theophylline, and are not shared by the other analogs, these may reflect actions mediated by other, perhaps nonpurinergic receptors. Although benzodiazepines also have sedative, hypothermic and anticonvulsant properties, responses to benzodiazepines can be clearly dissociated from responses to the adenosine agonists.