Somatostatin inhibits gabaergic transmission in the sensory thalamus via presynaptic receptors

The action of somatostatin on GABA-mediated transmission was investigated i n cat and rat thalamocortical neurons of the dorsal lateral geniculate nucl eus and ventrobasal thalamus in vitro. In the cat thalamus, somatostatin (1 0 mu M) had no effect on the passive membrane properties of thalamocortical neurons and on the postsynaptic response elicited in these cells by bath o r iontophoretic application of (+/-)baclofen (5-10 mu M) or GABA, respectiv ely. However, somatostatin (1-10 mu M) decreased by a similar amount (45-55 %) the amplitude of electrically evoked GABA(A) and GABA(B) inhibitory post synaptic potentials in 71 and 50% of neurons in the lateral geniculate and ventrobasal nucleus, respectively. In addition, the neuropeptide abolished spontaneous bursts of GABA(A) inhibitory postsynaptic potentials in 85% of kitten lateral geniculate neurons, and decreased (40%) the amplitude of sin gle spontaneous GABA(A) inhibitory postsynaptic potentials in 87% of neuron s in the cat lateral geniculate nucleus. Similar results were obtained in t he rat thalamus. Somatostatin (10 mu M) had no effect on the passive membra ne properties of thalamocortical neurons in this species, or on the outward current elicited by puff-application of (+/-)baclofen (5-10 mu M). However , in 57 and 22% of neurons in the rat lateral geniculate and ventrobasal nu clei, respectively, somatostatin (1 mu M) reduced the frequency, but not th e amplitude, of miniature GABA(A) inhibitory postsynaptic currents by 31 an d 37%, respectively. In addition, the neuropeptide (1 mu M) decreased the a mplitude of evoked GABA(A) inhibitory postsynaptic currents in 20 and 55% o f rat ventrobasal neurons recorded in normal conditions and during enhanced excitability, respectively: this effect was stronger on bursts of inhibito ry postsynaptic currents(100% decrease) than on single inhibitory postsynap tic currents (41% decrease). These results demonstrate that in the sensory thalamus somatostatin inhibit s GABA(A)- and GABA(B)-mediated transmission via a presynaptic mechanism, a nd its action is more prominent on bursts of GABAergic synaptic currents/po tentials. (C) 2000 IBRO. Published by Elsevier Science Ltd. All rights rese rved.