Spontaneous Depolarizing Synaptic Potentials in the Neostriatum

Abstract

Although several “in vivo” and “ in vitro” studies have previously shown that neostriatal neurons have a very low frequency of spontaneous firing activity, a common finding of some of these reports was the presence of large spontaneous depolarizing potentials (SDPs) during intracellular recordings (Hull et al. 1970; Buchwald et al. 1973; Bernardi et al. 1976; Sugimori et al. 1978; Wilson and Groves 1981; Bishop et al. 1982; Calabresi et al. 1987b). These SDPs were very frequent “in vivo”, but their amplitude was insufficient to trigger high frequencies of firing activity. The finding that these potentials are present also “in vitro”, where they are smaller and less frequent, seems to suggest that SDPs are synaptically mediated (Calabresi et al. 1990a; 1990b). In fact, the surgical manipulations carried out in the slicing procedures could at least in part interrupt synaptic projections originating from the afferent structures to the neostriatum. In several neurons the SDPs cause brief bursts of action potentials followed by relatively long pauses of firing activity (Wilson and Groves 1981; Calabresi et al. 1990a). The contribution of these potentials to the regulation of the neuronal excitability of neostriatal cells may be of major importance in controlling of the functions of the basal ganglia during movements (De Long, 1973; Groves, 1981). In addition, the impairment of the mechanisms from which these potentials originate may significantly alter the complex balance between intrinsic neuronal properties and afferent synaptic inputs within the neostriatum thus generating some extrapyramidal syndromes (Bird and Iversen, 1974; Groves, 1983; Calabresi et al. 1989). For these reasons in the present paper we have studied some physiological and pharmacological characteristics of the SDPs recorded both “ in vivo ” and “ in vitro ”. In this report we also review previously published data concerning these potentials (Calabresi 1987a; 1987b; 1988a; 1988b; 1990a; 1990b).