ELECTROPHYSIOLOGICAL AND PHARMACOLOGICAL PROPERTIES OF INTERNEURONS IN THE CAT DORSAL LATERAL GENICULATE-NUCLEUS

We investigated the electrophysiological and pharmacological propertie s of morphologically identified and putative interneurons within lamin ae A and Al of the cat dorsal lateral geniculate nucleus maintained in vitro. These intralaminar interneurons possess unique electrophysiolo gical characteristics, including (1) action potentials of a short dura tion (average width at half amplitude of 0.34 ms), (2) the ability to generate high-frequency trains of action potentials exceeding 500 Hz, without strong spike frequency adaptation, and (3) a low-threshold reg enerative response with variable magnitude of expression, ranging from a subthreshold depolarization towards the generation of one to severa l action potentials in different cells. The low-threshold regenerative depolarization following a hyperpolarizing current pulse was increase d in size by application of 4-aminopyridine, was reduced by nickel, an d was not influenced by extracellular cesium. These findings indicate that this event is mediated by an underlying Ca2+-dependent mechanism, such as a low-threshold Ca2+ current, that is regulated by the activa tion of opposing transient K+ currents. Every interneuron tested respo nded to glutamate, kainate, quisqualate, or N-methyl-D-aspartate with depolarization and action potential discharge. In contrast, we did not observe a postsynaptic response to activation of the metabotropic rec eptors with S,3R-(+/-)-1-amino-cyclopentane-1,3-dicarboxylate. Applica tion of gamma-amino-butyric acid (GABA) strongly inhibited spike firin g through a biphasic hyperpolarization and increase in membrane conduc tance, a response that reversed close to the presumed chloride equilib rium potential and was imitated by the GABA(A) receptor agonist muscim ol. The GABA(B) receptor agonist baclofen evoked only a weak membrane hyperpolarization from rest and suppression of spontaneous spike activ ity. Application of acetylcholine, or the muscarinic agonist acetyl-be ta-methylcholine, inhibited spontaneous action potential activity thro ugh hyperpolarization of the membrane potential, presumably resulting from an increase in membrane potassium conductance. In contrast, appli cation of serotonin only slightly facilitated tonic activity in a subp opulation of interneurons, histamine induced a small, slow depolarizat ion apparently through activation of presynaptic excitatory pathways, and noradrenaline and adenosine had no detectable effect on the sponta neous firing or resting potential of interneurons. We suggest that int ralaminar interneurons may function in a relatively linear manner to t ransform retinal and cortical inputs into a local field of inhibition in the dorsal lateral geniculate and that the excitability of these ne urons is largely controlled by retinal, cortical, GABAergic, and choli nergic (brainstem) afferents.