Ultrastructural examination of the corticocollicular pathway in the guinea pig: a study using electron microscopy, neural tracers, and GABA immunocytochemistry

Abstract

Projections from auditory cortex (AC) can alter the responses of cells in the inferior colliculus (IC) to sounds. Most IC cells show excitation and inhibition after stimulation of the AC. AC axons release glutamate and excite their targets, so inhibition is presumed to result from cortical activation of GABAergic IC cells that inhibit other IC cells via local projections. However, it is not known whether cortical axons contact GABAergic IC cells directly. We labeled corticocollicular axons by injecting fluorescent dextrans into the AC in guinea pigs. We visualized the tracer with diaminobenzidine and processed the tissue for electron microscopy. We identified presumptive GABAergic profiles with post-embedding anti-GABA immunogold histochemistry on ultrathin sections. We identified dextran-labeled cortical boutons in the IC and identified their postsynaptic targets according to morphology (e.g., spine, dendrite) and GABA-reactivity. Cortical synapses were observed in all IC subdivisions, but were comparatively rare in the central nucleus. Cortical boutons contain round vesicles and few mitochondria. They form asymmetric synapses with spines (most frequently), dendritic shafts and, least often, with cell bodies. Excitatory boutons in the IC can be classified as large, medium or small; most cortical boutons belong to the small excitatory class, while a minority (~14%) belong to the medium excitatory class. Approximately 4% of the cortical targets were GABA-positive; these included dendritic shafts, spines, and cell bodies. We conclude that the majority of cortical boutons contact non-GABAergic (i.e., excitatory) IC cells and a small proportion (4%) contact GABAergic cells. Given that most IC cells show inhibition (as well as excitation) after cortical stimulation, it is likely that the majority of cortically-driven inhibition in the IC results from cortical activation of a relatively small number of IC GABAergic cells that have extensive local axons.

The auditory corticocollicular system: Molecular and circuit-level considerations

“…An important proviso to this interpretation is that it is difficult to distinguish between terminals and fibers using this technique, raising the possibility that the anterograde signals seen in the central nucleus were passing fibers. Since then, multiple studies using modern tracers and electron microscopy have found clear evidence for CC terminals in the CNIC in rats, cats, gerbils, guinea pig and ferrets (Saldaña, Feliciano and Mugnaini 1996; Winer et al 1998; Bajo and Moore 2005; Bajo et al 2007; Nakamoto et al 2013), though generally in lower numbers than in other regions of the IC. In cats, the strongest projections to the central nucleus, come intriguingly from nontonotopic areas of dorsal and intermediate, part of the posterior ectosylvian gyrus.…”

“…This was based on physiological findings that CC inputs inhibited sound-induced IC responses (Massopust Jr and Ordy 1962; Amato, La Grutta and Enia 1970). However, electron microscopy of labeled CC synapses in the DC, LC and CNIC all showed asymmetric terminals, and the presence of small round vesicles in CC synaptic boutons that are indicative of excitatory synapses (Saldaña, Feliciano and Mugnaini 1996; Nakamoto et al 2013). In addition, lesioning of the CC system depressed synaptic release of labeled aspartate, further suggesting that these projections are glutamatergic (Feliciano and Potashner 1995).…”

“…One possibility is that the cellular targets of the CC system are inhibitory. However, only approximately 4% of the neurons targeted by the CC pathway are GABAergic (Nakamoto et al 2013). As discussed by Malmierca and Ryugo (2011), this implies that alternative inhibitory pathways may be involved, such as longer polysynaptic pathways in the IC, non-GABAergic (e.g.…”

“…glycinergic) pathways, longer-range pathways (e.g. corticobulbar, or cascading projections to the olivocochlear system), or, alternatively that the GABAergic cells by the CC pathway are very highly branched, producing substantial inhibition out of proportion to their cellular numbers (Nakamoto et al 2013). …”

“…There are data suggesting that both commissural and tectothalamic cells receive axosomatic input from the AC (Coomes Peterson and Schofield 2007; Nakamoto, Sowick and Schofield 2013). However, most of the inputs from the CC system terminate in the neuropil, primarily on distal dendrites and spines (Nakamoto et al 2013), suggesting that the majority of the cell types receiving CC input and their projection targets have not yet been characterized. Further, the cells that have been shown to receive AC input have not had any physiological characterization.…”