Elsevier

Neuroscience

Volume 80, Issue 2, 14 July 1997, Pages 545-557
Neuroscience

Electrophysiological and immunocytochemical characterization of GABA and dopamine neurons in the substantia nigra of the rat

https://doi.org/10.1016/S0306-4522(97)00093-6Get rights and content

Abstract

Neurons in the substantia nigra pars reticulata and pars compacta of the rat were studied using a combination of intracellular electrophysiological recording in in vitro and subsequent immunocytochemical double and triple labelling techniques. The neurons recorded in the pars reticulata were identified as either GABA or dopamine neurons: neurons were considered to be GABA neurons if they were immunopositive for glutamate decarboxylase, whereas those neurons which were immunopositive for tyrosine hydroxylase were considered to be dopaminergic. The GABA neurons had short duration action potentials (0.45±0.03 ms halfwidth), no apparent rectifying currents, no low threshold calcium spikes, were spontaneously active (7.4±3.7 Hz), and could maintain high firing rates. The dopamine neurons had long duration action potentials (1.49±0.10 ms), displayed both anomalous inward and transient outward rectifying currents, and more than half (12/17 neurons) displayed a low threshold calcium spike. Their spontaneous firing rate was lower than that of the GABA neurons (2.3±1.0 Hz), and they displayed strong frequency adaptation. Morphological reconstruction of neurobiotin-filled neurons revealed that the pars reticulata GABA neurons had more extensive local dendritic arborization than the dopamine neurons from either the pars reticulata or the pars compacta. All of the neurons recorded from the pars compacta were dopamine neurons; they were found not to be different either electrophysiologically or morphologically from pars reticulata dopamine neurons.

The electrophysiology of the GABA neurons suggests that input activity is translated linearly to spike frequency. These GABA neurons probably represent the projection neurons of the pars reticulata, and it is thus likely that this basal ganglia output is frequency coded. The close similarity between the dopamine neurons in the pars compacta, which give rise to the nigrostriatal pathway, and those in the pars reticulata supports the notion that the dopamine neurons in these two regions are part of the same neuronal population.

Section snippets

Electrophysiology

Slices were prepared from male Sprague–Dawley rats (Harlan: 60–150 g) using previously described methods.19, 81Briefly, the rats were anaesthetized with methoxyflurane and decapitated. The brain was removed and placed in ice-cold artificial cerebrospinal fluid (ACSF), the composition of which was (mM): NaCl 124, KCl 2.5, NaHCO3 26, NaH2PO4 1.24, MgSO4 1.3, CaCl2 2.4, glucose 10. Sagittal sections (400–500 μm) were cut on a Lancer Series 2000 Vibratome, and were maintained in ACSF at room

Results

A total of 34 neurons recorded from the SN were selected for analysis, of which 22 were from the SNr and 11 from the SNc.

Discussion

The aim of this study was to provide an electrophysiological characterization of neurons within the SNr and the SNc based upon a combination of electrophysiological, morphological and immunocytochemical data. We found two classes of neurons within the SNr: GABA and dopamine neurons. The GABA neurons have a very different electrophysiological profile from the dopamine neurons: the dopamine neurons from the SNr, on the other hand, appear to form an electrophysiologically homogeneous population

Conclusion

In sum, projection neurons within the SNr could be divided into two classes, i.e. GABA or dopamine neurons, based upon electrophysiology, immunocytochemistry and morphology; the SNc contained only dopamine neurons. The dopamine neurons in the two regions of the SN were found not to be different in any of the parameters studied. On the other hand, the GABA neurons were different in their morphology from the dopamine neurons, and additionally displayed very different electrophysiological

Acknowledgements

The authors wish to express their gratitude to Dr Jang-Yen Wu for the kind gift of the anti-GAD antibody. We would also like to thank Dr Dietmar Plenz for his helpful comments and criticisms. This study was supported by USPHS grants NS 20702, NS 26473, and the National Parkinson Foundation.

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