Quantitative and ultrastructural study of serotonin innervation of the globus pallidus in squirrel monkeys

The present immunohistochemical study was aimed at characterizing the serotonin (5‐HT) innervation of the internal (GPi) and external (GPe) pallidal segments in the squirrel monkey (Saimiri sciureus) with an antibody against the 5‐HT transporter (SERT). At the light microscopic level, unbiased count...

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Published in:The European journal of neuroscience 2013-05, Vol.37 (10), p.1659-1668
Main Authors: Eid, Lara, Champigny, Marie-France, Parent, André, Parent, Martin
Format: Article
Language:English
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Summary:The present immunohistochemical study was aimed at characterizing the serotonin (5‐HT) innervation of the internal (GPi) and external (GPe) pallidal segments in the squirrel monkey (Saimiri sciureus) with an antibody against the 5‐HT transporter (SERT). At the light microscopic level, unbiased counts of SERT+ axon varicosities showed that the density of innervation is similar in the GPi (0.57 ± 0.03 × 106 varicosities/mm3 of tissue) and the GPe (0.60 ± 0.04 × 106), with the anterior half of both segments being more densely innervated than the posterior half. Dorsoventral and mediolateral decreasing gradients of SERT varicosities occur in both pallidal segments, but are statistically significant only in the GPi. The neuronal density being significantly greater in the GPe (3.41 ± 0.23 × 103 neurons/mm3) than in the GPi (2.90 ± 0.11 × 103), the number of 5‐HT axon varicosities per pallidal neuron was found to be superior in the GPi (201 ± 27) than in the GPe (156 ± 26). At the electron microscopic level, SERT+ axon varicosities are comparable in size and vesicular content in GPi and GPe, where they establish mainly asynaptic contacts with unlabeled profiles. Less than 25% of SERT+ varicosities display a synaptic specialization, which is of the symmetrical or asymmetrical type and occurs exclusively on pallidal dendrites. No SERT+ axo‐axonic synapses are present, suggesting that 5‐HT exerts its well‐established modulatory action upon various pallidal afferents mainly through diffuse transmission, whereas its direct control of pallidal neurons results from both volumic and synaptic release of the transmitter. Unbiased examination of 5‐HT axon varicosities in GPi and GPe of squirrel monkey indicates a decreasing anteroposterior gradient. Less than 25% of 5‐HT varicosities display a synaptic contact, exclusively found on pallidal dendrites, indicating that 5‐HT exerts its modulatory effect upon pallidal afferents mainly through diffuse transmission, whereas its direct control of pallidal neurons results from both synaptic and diffuse transmission.
ISSN:0953-816X
1460-9568
DOI:10.1111/ejn.12164