INTRANEURONAL ACCUMULATION AND PERSISTENCE OF RADIOLABEL IN RAT-BRAINFOLLOWING IN-VIVO ADMINISTRATION OF [H-3] CHLORISONDAMINE

1 Chlorisondamine (CHL), a bisquaternary amine, produces a remarkably long-lasting blockade of central responses to nicotine. The mechanism underlying this blockade is not known. The main aim of this study was to test for possible accumulation of [H-3]-CHL in rat brain during the period of chronic blockade. 2 Rats received CHL, either systemically (10 mg kg(-1)) or centrally (10 mu g i.c.v.). Seven days later, striat al synaptosomes prepared from these animals were tested for nicotine-i nduced [H-3]-dopamine release. This experiment showed that i.c.v. admi nistration of CHL was as effective as systemic administration in produ cing ex vivo blockade of central nicotinic receptors. 3 Rats received bilateral i.c.v. infusions of [H-3]-CHL (10 pg) and radioactivity was subsequently quantified in dissected cerebral cortex, striatum, hippoc ampus, midbrain and cerebellum. Radiolabel was detected at all three s urvival times (1, 7, and 21 days). Regional heterogeneity was apparent at 7 and 21 days survival. Radiolabel was almost exclusively confined to the insoluble subcellular fraction in all areas sampled. 4 The ana tomical distribution of radiolabel was also visualized in brain sectio ns. Rats received bilateral i.c.v. infusions of [H-3]-CHL (10 mu g) an d were killed at 1, 7, 21 or 84 days. Immediately before they were kil led, all rats were tested behaviourally, and central nicotinic blockad e was demonstrated at 1, 7 and 21 days; partial recovery was observed at 84 days. Particularly at longer survival times, tritium was found t o be heavily concentrated in the substantia nigra pars compacta, ventr al tegmental area, dorsal raphe nucleus, and the granular layer of the cerebellum. 5 The possibility of retrograde axonal transport of radio label was then examined. Rats received a unilateral intrastriatal infu sion of [H-3]-CHL (0.34 or 0.034 mu g) one week before they were kille d. Autoradiographic labelling was largely confined to the site of infu sion and to the ipsilateral substantia nigra pars compacta and dorsal raphe nucleus. 6 Thus, after i.c.v. administration, CHL (and/or centra lly-formed derivatives) is initially widely distributed within the bra in and is then selectively retained within a few brain areas. A persis tent accumulation occurs within putative dopaminergic and 5-hydroxytry ptaminergic neurones, at least partly through uptake by terminals and/ or axons followed by retrograde transport. This persistent and anatomi cally-selective intraneuronal accumulation possibly underlies the long -term central nicotinic blockade associated with chlorisondamine.