Specific tricyclic antidepressant binding sites in rat brain.

The discovery of high-affinity binding sites for psychoactive drugs such as benzodiazepines, opiates and neuroleptics has opened up new approaches to the study of these drugs and their mechanisms of action. Although most tricyclic antidepressants inhibit neuronal uptake of noradrenaline and serotonin, their mechanism of action remains unclear. Changes in the sensitivity of the beta-receptor after chronic tricyclic antidepressant treatment suggest that they modulate noradrenergic neurotransmission. Tricyclic antidepressants also act directly on cholinergic, histaminergic, alpha-adrenergic and serotonergic receptors. It is not clear, however, which, if any, of these effects are related to the primary antidepressant effect or whether they are simply responsible for some of the side effects. We have thus investigated the possibility that specific binding sites for tricyclic antidepressants exist in the central nervous system. So far, binding studies using 3H-labelled tricyclic antidepressant drugs have only detected binding to histaminergic H2 and cholinergic muscarinic receptors and low-affinity binding. We demonstrate here a population of specific high-affinity binding sites for 3H-imipramine on brain membranes which may be responsible for the antidepressant effects of these drugs.     

Neurotransmitter turnover in rat striatum is correlated with morphine self-administration

Drugs of abuse probably exert their reinforcing effects through 'reward' pathways in the central nervous system (CNS). Neuronal systems mediating opiate reinforcement have been investigated using pharmacological and electrolytic lesion procedures. Drugs that interfere with catecholaminergic and cholinergic neuronal activity decrease intravenous (i.v.) morphine self-administration in monkeys and rats. Electrolytic lesion procedures in rats have demonstrated that the medial forebrain bundle and caudate nucleus are important in maintaining i.v. morphine self-administration. We have now carried out a direct investigation of striatal (caudate nucleus, putamen and globus pallidus) neuronal systems. We show here that striatal catecholaminergic systems are important in mediating opiate reinforcement, and present direct evidence for the involvement of neurotransmitter systems in morphine reward.     

Identification of two related pentapeptides from the brain with potent opiate agonist activity.

Enkephalin, a natural ligand for opiate receptors is composed of the pentapepides H-Tyr-Gly-Gly-Phe-Met-OH and H-Tyr-Gly-Gly-Phe-Leu-OH. The evidence is based on the determination of the amino acid sequence of natural enkephalin by the dansyl-Edman procedure and by mass spectrometry followed by synthesis and comparison of the natural and synthetic peptides.     

3H-baclofen and 3H-GABA bind to bicuculline-insensitive GABA B sites in rat brain

The presence of a novel receptor for the neurotransmitter gamma-aminobutyric acid (GABA) on peripheral autonomic nerve terminals and in mammalian brain slices has been described recently. This receptor differs from the classical GABA site as it is unaffected by recognized GABA antagonists such as bicuculline and is not sensitive to the majority of accepted GABA-mimetics such as 3-aminopropanesulphonic acid (3-APS) or isoguvacine. We propose to designate the classical site as the GABA A and the novel site as the GABA B receptor. The beta-p-chlorophenyl derivative of GABA, baclofen, is stereospecifically active at the GABA B site whereas it is devoid of activity at the classical GABA A3 site. We now report that high-affinity saturable binding of 3H-baclofen and 3H-GABA to the GABA B site can be detected in fragments of crude synaptic membranes prepared from rat brain. The results support the concept of a novel GABA receptor within the mammalian brain and show that GABA and baclofen can compete for the same recognition site.