An electrode was stereotaxically implanted in the unilateral dentate nucleus (DN), the contralateral ventrolateral nucleus of the thalamus (VL) and on the surface of the contralateral anterior sigmoid gyrus (ASG) in thirty-five cats. The evoked responses from the contralateral VL and ASG following DN stimulation were studied using an averaging computor technique. Electrical stimulation of DN was commenced in an unanesthetised state 4 to 5 days after the implantation. Square pulse current, 0.2 msec in duration was applied at a frequency of 1 Hz. Modification of the evoked responses was studied following parenteral administration of various centrally acting agents. The results were as follows. 1) Single stimulation of the DN demonstrated evoked responses in the contralateral VL with N(1) (first negative), P(1) (first positive), N(2) (second negative), followed by P(2) (second positive), and N(3) (third negative). Peak latencies of these waves were estimated to be 0.8±0.1 msec in N(1), 1.1±0.2 msec in P(1), 1.5±0.3 msec in N(2), 2.4±0.4 msec in P(2), and 5.3±1.2 msec in N(3). N(1) and N(2) corresponded to presynaptic tract response and postsynaptic relayed response respectively. From the latencies of N(1) and the distance between VL and DN (21mm), the conduction velocity in the dentato-thalamic pathway was roughly calculated as 23-30m/sec. 2) The evoked responses on the contralateral ASG following a single stimulation of the DN demonstrated a wave form composed of P(1), N(1), P(2), and N(2). Peak latencies of these waves were estimated to be 2.1±0.4 msec in P(1), 3.3±0.5 msec in N(1), 5.8±0.7 msec in P(2), 13.2±1.9 msec in N(2). P(2) and N(2) is corresponding to Creutzfeldt's primary EPSP and secondary EPSP respectively. 3) The effects of various centrally acting agents on the above mentioned evoked responses were observed (See table 3). 1 L-5HTP markedly decreased the amplitudes of both P(2) and N(2) of ASG responses. PCPA caused no paticular changes. 2 AOAA also caused effects upon ASG responses similar to L-5HTP. A subconvulsive dose of picrotoxin caused no particular changes. 3 Atropine decreased the amplitudes of both P(2) and N(2) in ASG. Physostigmine caused no particular changes. 4 L-Dopa and ET495 tended to stabilize ASG responses especially when it was in the suppressed state. Reserpine and haloperidol caused no particular changes. These data suggest that 5-hydroxytryptamine (5-HT), γ.-aminobutyric acid (GABA), acetylcholine (ACh), and dopamine (DA) do not play a primary role in the dentato-thalamocortical pathway but work indirectly through other neuronal pathways. This method using evoked responses seems to be useful to know the site and the time course of drug action in relation to the function of a pathway.
Anterior sigmoid grus, Evoked response
Centrally acting agents