1. Tonically active neurons (TANs) in the primate striatum develop transient responses to sensory conditioning stimuli during behavioral training in classical conditioning tasks. In this study we examined the temporal characteristics of such TAN responses and mapped the sites of TANs responding to auditory and visual conditioned stimuli in the striatum in macaque monkeys, We further mapped the locations of TANs recorded acutely in the squirrel monkey striatum in relation to the neurochemically distinguished striosome and matrix compartments of the striatum, and made quantitative comparisons between the densities and compartmental distributions of TANs and those of four major types of striatal interneuron identified by histochemical and immunohistochemical staining. 2. We made recordings from 858 TANs at different sites in the striatum in two behaving macaque monkeys at different times during training with auditory (click) and visual (light-emitting diode flash) conditioning stimuli. TANs distributed across large parts of the striatum developed responses to the conditioning stimuli, The responses comprised a decrement of tonic Firing (pause) followed by a rebound excitation, Measurements were made of the onsets, offsets, and durations of the pauses of individual TANs and of the interspike intervals (ISIs) of the same cells. 3. The mean duration of the pause responses (268.3 ms) was greater than the mean ISI of the same neurons (181 ms), suggesting that the pause represents on active suppression of TAN firing. The coefficient of variation (CV! for the pause responses was 0.28, compared with a CV of 0.63 for the same cells' ISIs. The population CV for the pauses was 0.16, compared with a population CV of 0.20 for the ISIs. These data, together with temporal analysis of the responses and population histograms, suggest that the pauses became temporally aligned across large parts of the striatum after ]earning. Analyses of variance (ANOVAs) were carried out to determine whether there were differences in the onset and offset latencies of the pause response or in the durations of the pause responses for TANs at different sites. These analyses suggested that, with rare exceptions, there was no difference in the timing of the TAN responses across large (>10 mm(3)) parts of the striatum. 4. Comparisons of TAN responses in different regions of the striatum showed that, for responses to a given modality of conditioned stimulus, there were no significant differences in pause offset times for TANs recorded in the caudate nucleus or putamen, or for TANs recorded in more anterior or more posterior parts of these nuclei. The only regional difference found was for pause onset rimes, which were earlier in the caudate nucleus than in the putamen for the responses to visual stimuli. Comparisons of responses to auditory and visual conditioned stimuli showed that the responses were generally similar, but that the offset times and pause durations were greater for clicks than for lights. 5. Comparisons of TAN responses recorded at different times during behavioral learning suggested that the main difference in TAN firing was an increase in the number of responsive neurons !from 16.7% to 62.0%). In addition, the responses tended to last slightly longer. 6. Responsive TANs could be divided into three categories: those that responded only to the auditory conditioned stimuli, those that responded only to the visual conditioned stimuli, and those that responded to both. These types were encountered in roughly equal numbers. TANs of the different types were not obviously clustered, but runs of responders or nonresponders, or of cells sharing a modality selectivity profile, were occasionally found. 7. Acute mapping of TANs in squirrel monkeys, made by depositing Chicago sky blue dye marks al 82 sites of TAN activity and subsequent staining for striosome-matrix compartmentation, showed that fully half of the TAN sites lay at striosomal borders, as defined by immunostaining For enkephalin-like peptide, or within similar to 100 mu m of those borders, The remaining TAN sites were ail in the matrix beyond the borders. These values for TAN distributions differed significantly (P < 0.001) from those estimated from the proportional areas For striosomes (similar to 16%), 100 mu m rims around striosomes (similar to 8%), and matrix beyond (similar to 76%). 8. Distribution maps of cholinergic (choline acetyltransferase-immunoreactive), somatostatinergic (NADPH diaphorase-positive), strongly and weakly parvalbumin-immunoreactive, and calretinin-immunoreactive neurons in the squirrel monkey striatum established that, with the exception of the weakly stained subgroup of parvalbumin-immunoreactive neurons, all four types of interneuron shared some aspects of the compartmental asymmetries found in the TAN maps, The somatostatin and calretinin interneurons, like TANs, were rarified in striosomes. All but the calretinin-immunoreactive interneurons (and weakly stained parvalbumin cells) showed some differential concentration at striosome-matrix borders. 9. There were sharply different density distributions for the different interneuronal types. The calretinin-immunoreactive and darkly stained parvalbumin-immunoreactive neurons were the most numerous (65.3 and 59.5 cells/mm(2)), somatostatinergic neurons were intermediate in density (29.2 cells/mm(2)), and the cholinergic interneurons (10.1 cells/mm(2)) and the weakly stained parvalbumin-immunoreactive cells (7.9 cells/mm(2)) were very sparse, Some of the interneuronal distributions showed marked gradients in density across one or more of the cardinal planes of the striatum. 10. Simulation recording analysis of the interneuron distributions suggested that, in a single electrode penetration, approximately 12 cholinergic neurons, 10 weakly stained parvalbumin neurons, 54 somatostatinergic neurons, 80 darkly stained parvalbumin neurons, 108 total parvalbumin neurons, and 113 calretinin neurons would be recorded. The TANs actually recorded in the squirrel monkeys and macaque monkeys were found at a density of 3-5 in a penetration. Thus the compartmental distributions of the interneurons appeared to exclude the weakly parvalbumin-positive subtype as a probable candidate phenotype for the electrophysiologically defined TANs, and the density distributions suggested that of the remaining cell types, the cholinergic interneurons are most likely to correspond to the TANs. The tonic firing property of the TANs strongly supports this view, because the cholinergic interneurons definitively identified in rodents are unique among known striatal interneurons in having such tonic activity. 11. These temporospatial characteristics of TANs in the primate striatum, taken together, suggest that the responses of these neurons become temporally coordinated during sensorimotor learning. A working hypothesis suggested by these findings is that such conditioned, temporally coordinated activity of the TANs could participate in mechanisms reorganizing striatal activity during sensorimotor learning.
