Local circuitry in the IMHV of the domestic chick ( Gallus domesticus )

Abstract

The responses to local stimulation have been recorded from neurons in the intermediate part of the medial hyperstriatum ventrale (IMHV) of the domestic chick, by using an in vitro slice preparation. When the slice is bathed in gassed Krebs’ solution, a single stimulus evokes a short-lasting diphasic response. The first phase is negative and lasts some 3 ms, whereas the second, positive phase is often of lower amplitude and usually persists for about 15 ms. The first phase is little altered by perfusion with either Ca 2+ -free Krebs’ solution or Krebs’ solution containing a high concentration of Mg 2+ . In contrast, the second phase is abolished by these procedures. The post-synaptic phase is positive when it is recorded anywhere between 0.1-1.25 mm from the stimulated point; however, in the immediate vicinity (0.0- 0.1 mm) of the stimulating electrodes, the post-synaptic response is strongly negative. A pair of stimuli has to be separated by at least 10 s to guarantee complete recovery of excitability of the post-synaptic response. The recovery curve for this response shows a refractory period of some 5 ms, a peak of excitability at an interval of about 20 ms, and then a sharp trough of relative inexcitability at about 200 ms. The post-synaptic response is considerably reduced in magnitude and duration by the addition of AP-5 to the perfusion fluid; the remaining post-synaptic response is completely abolished by kynurenic acid. The addition of bicuculline methiodide in concentrations of at least 1 x 10 -6 M increases both the magnitude and duration of the second, positive phase of the response to single stimuli. This extended positive response (which may last from 500-800 ms) is abolished by perfusion with bicuculline dissolved in Ca 2+ -free Krebs’ solution. For the entire duration of the extended post-synaptic positive response produced by bicuculline, the irregular discharge of single neurons can be recorded. Like the post-synaptic positive response in Krebs’ solution, the much larger response produced by bicuculline shows a very localized negativity beneath the stimulating electrodes and displays an almost identical timecourse for the recovery of excitability following a single stimulus. The bicuculline induced positive response is also considerably reduced by the presence of AP-5; the addition of kynurenic acid abolishes the remaining post-synaptic response completely. A post-synaptic response, similar to that produced under bicuculline, can be produced by the addition of a maximally effective dose of d-tubocurarine. This response has the same pharmacological and electrophysiological properties as that produced under bicuculline, only differing in that a maximal response evoked in the presence of curare, can be increased in amplitude and duration by the addition of bicuculline, whereas a maximal bicuculline response is unaffected by the addition of curare.