G. La Grutta

Effects of caudate nucleus on paroxysmal activity in hippocampus of cat

Electrically induced afterdischarges (ADs) were evoked in cats dorsal hippocampus. The effect of the conditioning prestimulation of the caudate head on AD duration was tested. A strong inhibitory action was observed when conditioning caudate stimulation immediately preceded hippocampal test stimulation. An increase in threshold values following caudate stimulation was also noted. The time course of the inhibitory phenomenon showed a decrease of the caudate conditioning effect when the interval between the two stimuli increased: complete disappearance of such effect was found to occur at about 600 msec. When caudate head stimulation followed the onset of hippocampal AD, this was inhibited only during caudate stimulation while presenting facilitation after caudate stimulation terminated. The results are discussed in terms of a possible explanation of the contrasting action of the pallidum, while previous findings attesting the facilitatory role of these structures are considered, as are the effects of caudate stimulation on hippocampal AD.

Contrasting effects of centrifugal olivo-cochlear inhibition and of middle ear muscle contraction on the response characteristics of the cat's auditory nerve

Summary The auditory nerve response to clicks over 40 dB range from threshold was recorded in cats with middle ear muscles cut and Flaxedil paralysis. Response characteristics were expressed by intensity functions and by latency-voltage scatter diagrams of the N1 potential. Stereotaxic activation of the crossed olivo-cochlear bundle (OCB) inhibited N1 and produced a unique transformation of its latency-voltage characteristic (Figs. 2, 3). The effect was proportional to the potency of OCB activation. The effect was not seen when the N1 response was reduced by middle ear muscle contraction, the latter affecting the response characteristics in similar manner to that of a simple attenuation of the sound intensity outside the head. The N1 potential represents the modal activity of high CF auditory units from the basal turn of the cochlea, and the above effects on latency characteristics are of teh right order of magnitude to exert a profound influence on binaural interactions at higher order neurons. The membrane mechanism involved in the OCB effect at the inner ear is briefly discussed.

Striatonigral suppression of focal hippocampal epilepsy.

Both caudate nucleus (CN) and substantia nigra (SN) appear to be involved in the control of epileptogenic events. Previous investigations had demonstrated that both CN and SN stimulations are able to induce hippocampal theta (theta) rhythm and an inhibition of epileptiform spikes. Since the two structures are reciprocally linked by fibre pathways, experiments were carried out to test the possibility that CN influences the hippocampus via SN or vice versa. To this end, changes in penicillin-induced hippocampal spikes by CN or SN stimulation were studied before and after destruction of SN and CN respectively. Steady interictal activity was induced in the hippocampus of encéphale isolé cats by local injection of penicillin. Stimulations of both CN and SN induced statistically significant reduction of hippocampal spike frequency, and in some cases a clear and regular theta-rhythm. These effects were unchanged by the destruction of either CN or SN. The results add further information to the role played by the basal ganglia and SN in the control of epilepsy, and underline the possibility that caudate and nigral influences on the hippocampus are mediated by different pathways.

Relations between basal ganglia and hippocampus: Action of substantia nigra and pallidum

Several interrelationships exist between basal ganglia and hippocampus. The ventral striatum appears to be involved in the control of the dopaminergic nigro-striatal pathway. The caudate, in turn, seems to influence the hippocampal theta rhythm and to inhibit hippocampal spikes. In the present work the role played by globus pallidus pars interna and substantia nigra pars compacta on hippocampal bioelectrical activity is studied. Injection of sodium penicillin i.v. produces steady interictal spikes in the hippocampus. Substantia nigra stimulation induces regular theta rhythm and inhibits the spikes. Pallidal stimulation, on the contrary, appears to strongly enhance epileptiform activity, proceeding to generalised seizure activity. The results are discussed in the light of a putative feedback loop from basal ganglia to hippocampus, probably underlying co-participation of the two subcortical structures in the control of motor behaviour.

Modulation of paroxysmal activity in the hippocampus by caudate stimulation in the chronic cat

Afterdischarges in the dorsal hippocampus (HADs) were studied in freely-moving cats with implanted electrodes following threshold stimulation of the mirror-image point on the contralateral side. Marked inhibition, similar so that seen in acute animals, was observed when the test stimulation was immediately preceded by a conditioning stimulus applied to the caudate nucleus. The inhibitory effect appeared to be larger in these chronic animals than in the acute preparations previously studied, probably because of the total absence of anaesthesia during the recording session. When the HAD is preceded by caudate stimulation, its duration can be graduated by the intensity of the hippocampal test stimulation. The results are discussed in terms of a possible modulation induced directly or indirectly by the caudate nucleus in the hippocampus, which reacts in a gradual manner to the excitatory volley.

Study of spindle-spike interactions: features of basal ganglia control

Changes in cortical spindle distribution following penicillin (PCN) injections were studied in feline generalized PCN epilepsy. PCN activation caused no substantial changes in spindle duration, frequency and intraburst frequency, while significant reductions in the amplitude of the negative waves were noted. At the same time combinations of spindle waves and epileptic complexes were recorded with one or more spikes randomly occurring at the beginning, in the middle, or at the end of a spindle envelope. Low frequency stimulation of the caudate nucleus induced a certain degree of enhancement in cortical precruciate spike frequency while high frequency activation of the entopeduncular nucleus caused significant inhibition of cortical spike frequency. The results are discussed in the light of the reciprocal interrelationship between spindles and spikes. Furthermore, the role played by the caudate and the entopeduncular nucleus in the control of the cortico-thalamo-cortical circuit is also emphasized.