Neurons maintained in dispersed primary culture offer a number of advantages as a model system and are particularly well-suited for studies of the intrinsic electrical properties of neurons by patch-clamp. We have characterized the immunocytochemical and electrophysiological properties of cultured rat striatal neurons as they develop in vitro in order to compare this model system to the known properties found in vivo. We found a high abundance of cells in vitro corresponding to the principal striatal output neuron, the medium spiny neuron.Immunocytochemical studies indicate that these cells have both dopamine-1 and dopamine-2 receptors and that there is overlap in their expression within the population of neurons. Semiquantitative analysis revealed bimodal distributions of dopamine receptor expression among the population of neurons. The principal peptide neurotransmitters substance P and enkephalin were present but at reduced levels compared to adult preparations. Other striatal markers such as calbindin, calretinin and the cannabinoid-1 receptor were abundant. An immunocytochemical survey of voltage-gated K + channel subunits characteristic of adult tissue demonstrated the presence in vitro of Kv1.1, Kv1.4, Kv4.2, Kv4.3 and Kvβ1.1 which have been associated with the rapidly inactivating currents. Electrophysiological studies employing voltage clamp revealed that outward currents had a large inactivating (A-type) component characteristic of mature basal ganglia. Current clamp studies reveal complex spontaneous firing patterns in a subset of neurons including bursting behaviors superimposed on a slow depolarization. The inward rectifying channels Kir2.1 and Kir2.3 which are specific to particular compartments in adult striatum were present in culture. KeywordsStriatal neurons; Dopamine receptors; Potassium channels; substance P; Enkephalin; Calcium binding proteins Neurons derived from fetal rodent brain and maintained in primary culture have been widely employed as model systems to study developmental processes and physiology at the singlecell level (Mandel and Banker, 1996). Significant progress has been made in the study of neuronal polarity (Dotti and Banker, 1991), G protein-coupled receptors (Vilchis et al., 2002;Morris et al., 2000), ion channel physiology (Sucher et al., 2000;Parak et al., 2001;Centonze et al., 2002) and inhibitory synaptic transmission (Kowalski et al., 1995) by using primary neuronal culture. Striatal neurons maintained in primary culture have been employed to study physiology of the NMDA receptors (Popp et al., 1998), opioid receptors (Vaysse et al., 1990), metabotropic glutamate receptors (Paolillo et al., 1998), and acetylcholine receptors (Tence et al., 1995 These preparations are particularly well suited for gene-transfer studies intended to manipulate electrical activity, since transduced neurons can be identified in the living state by the use of fluorescent reporter genes and their electrophysiological properties may be studied individually by patch clamp methods (Falk...