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Given the evidence that the inferior colliculus (IC) and superior colliculus (SC) seem to play key roles in connecting auditory pathways and seizure output pathways in the neuronal network for audiogenic seizures (AS) in rats, we examined Fos activation in GABAergic cells and cells immunopositive for glutamate N-methyl-D-aspartate (NMDA) receptors in the IC and SC following AS using the double-labeling procedure. Generalized tonic-clonic seizures (GTCS), which developed as an advanced form of AS in some of the susceptible rats, induced an increase in Fos expression in three IC substructures-the dorsal cortex of IC (DCIC), central nucleus of IC (CIC), and external cortex of IC (ECIC)-and in one SC substructure, the deep gray layer of SC (DpG). Compared with the rats showing GTCS, rats exhibiting wild running (WR) without proceeding to GTCS showed a different pattern of AS-induced Fos expression. The DpG in the WR animals showed no significant increase in the levels of Fos-like immunoreactivity. The degrees of Fos activation that occurred in GABAergic cells and cells immunopositive for NMDA receptors were similar in the DCIC, CIC, ECIC, and DpG following AS. These results suggest that Fos activation in the DpG is involved in the development from WR to GTCS in AS-susceptible rats. They also provide some evidence that some GABAergic neurons in the IC and SC and glutamatergic afferents (via NMDA receptors) to these structures are activated by AS.
Responses to stimuli moving with velocities of 3-180 degrees X sec-1 were studied in each of four axes (eight directions) separated by 45 degrees in neurons of the cat superior colliculus. 69% of units were directionally selective. In 55% of all the neurons the directional selectivity depended on stimulus velocity and was the largest for high velocities (over 10-30 degrees X sec-1). The preferred direction for some neurons was reversed with velocity increasing. The directional selectivity-velocity relationships were compared for the cat superior colliculus and visual cortex and discussed in regard to probable role of this phenomenon in the oculomotor system.
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The inferior colliculus (IC) is the main ascending auditory relay station prior to the superior colliculus (SC). The morphology and origin of the connection from inferior to superior colliculus (I-SC) was analyzed both by anterograde and retrograde tracing. Irrespective of the subregion of the IC in which they originate, the terminal fields of these connections formed two main tiers in the SC. While the dorsal one primarily involved the stratum opticum and the stratum griseum intermediale, the ventral one innervated the deep strata, although some fibers did connect these tiers. While the dorsal tier occupied almost the whole extension of the SC, the ventral one was mostly confined to its caudomedial quadrant. The fiber density in these tiers decreased gradually in a rostral gradient and the terminal fields became denser as the anterograde tracer at the injection site was distributed more externally in the cortex of the IC. Retrograde tracing confirmed this result, although it did not reveal any topographic ordering for the I-SC pathway. Most presynaptic boutons of the I-SC terminal field were located either inside or close to the patches of acetylcholinesterase activity. Together with previous anatomical and physiological studies, our results indicate that the I-SC connection relays behaviorally relevant information for sensory-motor processing. Our observation that this pathway terminates in regions of the superior colliculus, where neurons involved in fear-like responses are located, reinforce previous suggestions of a role for the IC in generating motor stereotypes that occur during audiogenic seizures.
Distribution of fibrillar terminals, taking origin from the anterior colliculi of the midbrain tectum, in the central visual field 17 has been studied electron microscopically. Degenerating axonal terminals are detected in layers IV, V, VI and I. They are most numerous in layer IV. Most of them form contacts on small dendritic branches, at the same time, in layer IV other forms of degenerated interneuronal contacts occur rather often: on spiculae, on dendrites of larger diameter, in some cases,--on their main trunks. A suggestion is made that in the visual projection zone distribution and termination of the fibers taking origin from the anterior colliculi of the midbrain tectum are similar to the projection organization in other specific visual structure--the lateral geniculate body.
The brain preparations have been obtained from 20 cats after vital injection of their vessels with Indian ink and gelatin and a subsequent additional staining of paraffin sections after Nissle. The cytoarchitectonic stratification of the structural formation studied is demonstrated to correspond to its division into four microvascular layers. Topical relations of the capillaries and neurons are characterized. Owing to a quantitative analysis performed on vascularization of the cells, a suggestion is made that large neurons of the superficial grey layer belong to the cells participating most actively in the visual information treatment.
In chronic experiments on alert cats the preceding stimulation of the lateral geniculate body (LGB) facilitates the response in the visual cortex to testing stimulation of the anterior colliculi (AC) at all studied delays between stimuli in contrast to the associative cortex where this response has been depressed in intervals of 10-40 ms. In reverse order of stimulation, facilitation of response to testing LGB stimulation is observed at all studied delays in the associative cortex, while in the visual cortex this response is slightly depressed. The obtained data point to the importance of the information coming in the accessory zones of LGB and AC projections and to different informational value of AC inputs to the associative and visual cortices and reciprocal relations between inputs to these cortical areas from LGB and AC.
In acute experiments on rats it was shown that activation of SC by bilateral microinjection of penicillin (5-15 IU) or bicuculline (25-50 ng) resulted in the increased latency and decreased severity of i.p. picrotoxin-induced seizures (2 mg/kg). The suppression of behavioral convulsions and the decreased epileptic activity in the hippocampus and cerebral cortex were followed by occurrence of spike discharges in SC with an amplitude of 200-500 mcV and frequency of 5-12/sec which testifies to the formation of penicillin- or bicuculline-induced generator excitation in SC. The lesions of SC by kainic acid administration resulted in the decreased seizure threshold and, also, facilitated the development of seizure under conditions of picrotoxin kindling. The conclusion is made that SC activation plays an important role in the realization of functional integrative activity of the antiepileptic system.
A previous study demonstrated that developmental monocular enucleation resulted in bilateral disruption of the formation of the auditory space map in both superior collculi, even though only one colliculus had been deprived of visual input. The present study found similar map disruption, when monocular enucleation was carried out following the initial emergence of the map at 32 days after birth. The study also investigated the effect of monocular enucleation plus simultaneous transection of the intercollicular commissure (a recognised pathway for communication between the colliculi). Multi-unit responses to free-field auditory stimuli were recorded from both colliculi of the two experimental groups. In the group which had later monocular enucleation alone, the auditory responses were disrupted in a manner very similar to those recorded from animals which had undergone early monocular enucleation. Similarly, in the group in which the collicular commissure had been severed at the time of monocular enucleation, the auditory responses were also disrupted in the colliculus contralateral to the enucleation. In contrast, the colliculus ipsilateral to the enucleation exhibited normal, topographically organised, auditory responses. Thus, severing the intercollicular commissure at the time of monocular enucleation protected the map of auditory space in the ipsilateral colliculus.
The trigemino-tectal projections were investigated with anterograde and retrograde tracing techniques in the Madagascan lesser hedgehog tenrec, Echinops telfairi. There were prominent contralateral projections to the inferior colliculus (CoI) and the superior colliculus (CoS), each showing its own characteristic pattern of terminations. While the projections to the CoI were confined consistently to a circumscribed region in its ventrolateral, external portion, the projections from particularly the rostral trigeminal subdivision to the CoS were distributed inhomogenously across almost the entire rostro-caudal and mediolateral extents. Comparing these data with the spino-tectal projections published previously, it demonstrates that the somatotopic organization of ascending tectal afferents is more distinct in the CoI than in the CoS. There were roughly twice as many trigeminal neurones projecting to the CoS than to the CoI. This difference might be due to the fact that the cells projecting to CoS were distributed extensively across the trigeminal nuclear complex (peak densities in the principal and interpolar subdivisions), while the neurones projecting to the CoI were largely confined to the interpolar and caudal trigeminal subdivisions. The latter cells were located adjacent to the spinal trigeminal tract; the neurones projecting to the CoS occupied preferentially the ventral trigeminal regions at rostral levels, while from the interpolar subdivision caudalward the labelled cells shifted dorsolaterally. In comparison to other mammals the trigeminal projection to the tenrec's CoI is unique. There is evidence for such a projection in other species too, but it is poorly documented, presumably due to technical reasons.
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