Anatomic basis of sequence-dependent predictability exhibited by nigral dopamine neuron firing patterns

Interspike intervals (ISIs) of dopamine (DA) neurons recorded in the substa ntia nigra are predicted partially by their immediate prior history. This s tudy was designed to assess neuroanatomic origins of these sequential relat ionships. ISI data recorded from three groups of nigral DA neurons were stu died: 1) 16 neurons recorded in unlesioned animals, 2) 14 neurons recorded after forebrain hemisection, 3) 12 neurons recorded after partial forebrain hemisection that reproduced nonspecific effects of the surgical lesion whi le leaving forebrain connections intact. As predicted, DA neurons recorded after full forebrain hemisection yielded statistically significant reductio ns in sequential predictability relative to control neurons and neurons rec orded following partial hemisection. These data support the hypothesis that the sequence-dependent behavior of DA neurons arise in part from interacti ons with forebrain structures. ISI sequences recorded from unlesioned rats demonstrated maximum predictability when an average of 3.7 prior events wer e incorporated into the forecasting algorithm, thereby suggesting a physiol ogical process whose "depth" of history-dependence is approximately 600-800 msec. Additional studies examining the functional significance of sequence -dependent ISI structure exhibited by nigral DA neurons are indicated. (C) 2001 Wiley-Liss, Inc.