MULTIPLE CELL-TYPES DISTINGUISHED BY PHYSIOLOGICAL, PHARMACOLOGICAL, AND ANATOMIC PROPERTIES IN NUCLEUS HVC OF THE ADULT ZEBRA PINCH

Nucleus HVc of the song bird is a distinct forebrain region that is es sential for song production and shows selective responses to complex a uditory stimuli. Two neuronal populations within HVc give rise to its efferent projections. One projection, to the robust nucleus of the arc histriatum (RA), serves as the primary motor pathway for song producti on. and can also carry auditory information to RA. The other projectio n of HVc begins a pathway through the anterior forebrain, (area X --> medial portion of the dorsolateral nucleus of the thalamus (DLM) --> l ateral portion of the magnocellular nucleus of the anterior neostriatu m (L-MAN) --> RA) that is crucial for song learning but, although acti ve during singing, is not essential for adult song production. To test whether these different projection neuron classes have different func tional properties, we recorded intracellularly from neurons in nucleus HVc in brain slices. We observed at least three classes of neuron bas ed on intrinsic physiological and pharmacological properties as well a s on synaptic inputs. We also examined the morphological proper ties o f the cells by filling recorded neurons with neurobiotin. The differen t physiological cell types correspond to separate populations based on their soma size, dendritic extent, and axonal projection. Thus HVc ne urons projecting to area X have large somata, show little spike-freque ncy adaptation, a hyperpolarizing response to the metabotropic glutama te receptor (mGluR) agonist (1S,3R)-trans-1-aminocyclopentane-1,3-dica rboxylic acid (ACPD), and exhibit a slow inhibitory postsynaptic poten tial (IPSP) following tetanic stimulation. Those HVc neurons projectin g to motor nucleus RA have smaller somata, show strong accommodation, are not consistently hyperpolarized by ACPD, and exhibit no slow IPSP. A third, rarely recorded class of neurons fire in a sustained fashion at very high-frequency and may be interneurons. Thus the neuronal cla sses within HVc have different functional properties, which may be imp ortant for carrying specific information to their postsynaptic targets .