The percentage of interneurons in the dorsal striatum of the rat, cat, monkey and human: A critique of the evidence

In the published literature on mammals, there are various answers to the question: What percentage of the total population of neurons within the dorsal striatum is represented by interneurons? The percentage of interneurons is reported to be 26% for the human, 23% or 4–6% for the monkey, 5–6% or <3% for the rodent, 4–5% for the cat and 5–10% for the mammal. Based on a critique of the literature, including the use of stereological approaches, caution is needed when citing historical non-stereological data on the percentage of interneurons in the dorsal striatum of the cat, monkey and human. In these species, the vast majority of neurons in the dorsal striatum are spiny projection neurons and a smaller number are interneurons, yet the precise percentage is currently unknown. Hence, in the absence of stereological data on the absolute number of all neuronal subtypes for the cat, monkey and human, it is currently unknown if there is a species difference, or not, for the percentage of interneurons in the dorsal striatum of the human and monkey versus the cat and rat. Modern stereological data indicate that the percentage of interneurons in the rat dorsal striatum is <3%. More specifically, in the Sprague-Dawley rat, 0.8% of all neurons are somatostatin/neuropeptide Y/GABA interneurons, 0.4% are cholinergic interneurons, ∼0.6% are GABA/parvalbumin interneurons and 0.5% are GABA/calretinin interneurons. Current stereological data also indicate that the percentage of interneurons in the dorsal striatum is <1% for cholinergic interneurons in the Sprague-Dawley rat and human.