Burst firing in identified rat geniculate interneurons
Section snippets
Slice preparation
Sprague–Dawley rats (Harlan, Indianapolis, IN; four to 10 weeks old; 100–300 g) were deeply anesthetized using halothane. A minimum number of animals were used to obtain the necessary results and the animals were not allowed to suffer in any way. After decapitation, the brain was quickly removed into cold (6–8°C) physiological solution containing (in mM): NaCl 126, KCl 2.5, NaH2PO4 1.25, NaHCO3 26, MgSO4 1, dextrose 20, CaCl2 2 (pH 7.35). The solution was continuously bubbled with 95% O2/5% CO2.
Neuronal identification
We recorded from 102 interneurons in the rat LGN using whole-cell patch recording technique. Of these, 40 were assessed for the burst firing reported in this study. Thalamic interneurons were identified physiologically and morphologically. Physiological identification was based on previously described criteria,52., 59., 84. with some modification resulting from our use of the whole-cell technique. All cells were successfully labeled with biocytin, so that their identity could be confirmed
Discussion
In this study, all geniculate interneurons recorded produced burst firing. The bursts were generally of prolonged duration and low frequency, distinct from the bursts found in TC cells, but somewhat similar to those of RE cells. The pattern of bursting seen was also similar to that seen in previous recordings of thalamic interneurons both in vivo and in vitro.1., 4., 11., 13., 15., 21., 59., 63., 77., 84. However, in previous in vitro studies, bursts were not elicited consistently.
Conclusion
Inhibitory mechanisms play an important role in thalamic function, due in part to the existence of low-threshold bursting mechanisms in the other major cell types of the thalamus. The presence of similar bursting mechanisms in the inhibitory interneurons suggests additional complexity for the functional thalamic circuit. Although the depolarized shift in the threshold for activation of low-threshold bursts in interneurons makes them somewhat less dependent on hyperpolarization preceding a
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