ARE THE INTERLAMINAR ZONES OF THE FERRET DORSAL LATERAL GENICULATE-NUCLEUS ACTUALLY PART OF THE PERIGENICULATE NUCLEUS

The ferret dorsal lateral geniculate nucleus (LGNd) contains interneur ons within the interlaminar zones situated between the laminae corresp onding to the ipsi- and contralateral eyes. We found that a subset of these neurons exhibits electrophysiological properties similar to thos e previously reported for perigeniculate (PGN) neurons, including the generation of rhythmic sequences of rebound low-threshold Ca2+ spikes at a frequency of 1-4 Hz after the intracellular injection of a hyperp olarizing current pulse. These ''PGN-like'' interlaminar interneurons innervated restricted regions of the A-laminae, inhibited thalamocorti cal cells through GABA(A), and perhaps GABA(B), receptors, and were ex cited by axon collaterals from thalamocortical cells. This reciprocal relationship is identical to that formed by PGN cells and allowed the PGN-like interlaminar neurons to participate in the generation of spin dle waves and other network oscillations. Pharmacologically, PGN-like interlaminar interneurons were also similar to PGN neurons: both gener ated a prolonged depolarization in response to the local application o f serotonin, 1S,3R-ACPD, and CCK8S, and a rapid depolarization followe d by a more prolonged hyperpolarization in response to acetylcholine. Examination of parvalbumin and calbindin staining in the ferret LGNd r evealed that both PGN and a subset of interlaminar neurons were parval bumin-positive. In contrast, calbindin-positive cells were relatively absent in the PGN and sparsely present in the interlaminar zones, but were numerous in the A and C laminae. Our results indicate that the in terlaminar zone in between laminae A and Al and Al and C in the ferret LGNd possesses a cell type that is electrophysiologically, pharmacolo gically, anatomically, immunocytochemically, and functionally similar to neurons in the PGN.