GABA iontophoretic application

Storm-Mathisen I would like to refer to the Purkinje cells of the cerebellum. The axons of these cells are distributed to the cerebellar nuclei and to the dorsal half of the lateral vestibular (Deiters ) nucleus. Here they produce inhibitory postsynaptic potentials that can be mimicked by iontophoretic application of GABA. Further, GABA, but not other amino acids, is released into the 4th ventricle during activation of the Purkinje cells, so it seems that GABA is the transmitter of these cells. We found 2 to 3 times more GAD in the dorsal half of Deiters nucleus which receives Purkinje terminals than in the ventral half which is devoid of such terminals. We also made lesions of the specific regions in the cerebellar cortex to destroy the Purkinje cells sending their axons to the cerebellar and Deiters nuclei, and measured the GAD activity. This dropped by a factor of 3 in the locations deprived of their Purkinje terminals, but stayed constant elsewhere. The Purkinje terminals and axons constitute only a small fraction of the tissue, probably no more than 10%. The concentration of GAD in these structures must therefore be enormous (Fonnum, Storm-Mathisen and Walberg, 1970). 

Kerkut, G. A., Pitman, R. M. and Walker, R. J. (1969) Iontophoretic application of ACh and GABA onto insect central neurones. Comp. Biochem. Physiol., 31,611-633. 

Thallmann, R. H. and Hershkowitz, N. (1985) Some factors that influence the decrement in the response to GABA during its continuous iontophoretic application to hippocampal neurons Brain Res 342,219-233. 

Thus the iontophoretic application of GABA in vitro appears consistently to be accompanied by a substantial depolarization and a decrease in membrane resistance, which invariably leads to a decrease in excitability (Scholfield, 1978fl,ft Brown and Scholfield, 1979). However, recent findings have shown GABA to have two independent actions on the same hippocampal pyramidal cell (Langmoen et aL, 1978 Andersen et aL, 1978a, 1980 Thalmann et at., 1979 Jahnsen and Laursen, 1981). 

Fig. 27. Intracellular recordings from a granule cell in the hippocampal slice to show the response to the iontophoretic application of GABA. (A) Voltage record made on moving film to show the way in which an intracellular injection of a depolarizing ramp of current is used to test the cell s excitability. Every alternate second a hyperpolarizing current pulse is used to measure the input resistance. As shown more clearly in the single shots in (B) and (C), the application of GABA inhibited the spike discharge evoked by the ramp, caused a substantial decrease in the input resistance, and produced a small depolarization with respect to resting membrane potential [dotted line in (B)]. (From Assaf et ai, 1981.)...

Fig. 30. Regression analysis to show the linear way in which the depolarizations (AV) evoked by both GABA and 5-HT are correlated with an associated decrease in membrane resistance Rm IRm)- The slope of the line is a measure of the difference between equilibrium potential of the putative neurotransmitter ( pni) and membrane potential (Tm). In this instance, the difference is greater than 30 mV in a depolarizing direction. The similarity of the results obtained during iontophoretic applications of GABA and 5-HT raises at least two, equally likely, possibilities. Either the action of 5-HT is mediated by the opening of ionic channels similar to those activated by GABA, or a presynaptic action of 5-HT which causes GABA release. (From Assaf et al, 1980.)...

Figure 4.1 Effect of an iontophoretic injection of bicuculline, a GABAa antagonist, on a locus coeruleus noradrenergic neuron. With reduced activity during slow wave sleep, this neuron then becomes silent during paradoxical sleep (PS). The application of bicuculline reversibly restores tonic discharge, indicating the role of GABA in the cessation of activity of this neuron during PS.

Tersigni TJ, Rosenberg HC (1996) Local pressure application of cannabinoid agonists increases spontaneous activity of rat substantia nigra pars reticulata neurons without affecting response to iontophoretically-applied GABA. Brain Res 733 184-192... 

Figure 7. Effect of prolonged (25 min) bath application of 22,23-dihydroavermectin Bla (DHAVM, 10 ug/ml) on the amplitude of iontophoretically evoked GABA hyperpolarizations a) Examples of GABA responses recorded before and after DHAVM application b) Plot of GABA response amplitude versus time DHAVM was first dissolved in dimethylsulfoxide (DMSO) and then diluted with saline the final DMSO concentration in saline of 1% had no detectable effect in control experiments ...

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