Juxtacellular labeling of tonically active neurons and phasically active neurons in the rat striatum

Tonically active neurons (TANs) and phasically active neurons (PANs) are widely believed to be the cholinergic interneurons and GABAergic projection neurons, respectively, in the striatum based on in vivo intracellular recordings coupled with morphological examinations of anesthetized rats, and on histochemical, electrophysiological, and labeling studies of in vitro slice preparations. TANs of alert behaving animals exhibit prolonged pause responses to behaviorally significant events. PANs, on the other hand, are mostly inactive when subjects are quiet and not performing any actions, but exhibit burst discharges in response to external stimuli and/or voluntary actions. Several other types of interneurons have also been identified in the striatum, such as parvalbumin-containing GABAergic interneurons (fast-spiking cells), somatostatin-containing interneurons, and calretinin-containing interneurons. To identify the neurochemical and morphological characteristics of TANs and PANs in a more direct manner, we conducted juxtacellular labeling, combining electrophysiology with immunohistochemistry and morphology in anesthetized rats. All of the juxtacellularly labeled TANs (n=3) among those recorded (n=10) were ChAT-positive and had large cell somata with aspiny dendrites. Thus, although our observations are based on a limited number of neurons, our findings provide the most convincing evidence to date that TANs in the striatum are cholinergic neurons. We also found that the majority of PANs are GABA-immunoreactive (46 of 48 tested) and approximately two-thirds had spiny dendrites (30 of 48 tested), indicating that the majority are medium-sized, spiny, GABAergic projection neurons, consistent with general beliefs. Conversely, the remaining one-third of PANs had aspiny dendrites (n=18), indicating that they were interneurons. Therefore, the present study reveals that TANs are cholinergic neurons and that the majority of PANs are medium-sized, spiny, GABAergic projection neurons, while a smaller number are GABAergic interneurons.