ACTIVITY-DEPENDENT CHANGES IN A NEURONAL CIRCUIT IMPORTANT FOR SOUND LOCALIZATION

Aside from recognizing and distinguishing sound patterns, the
ability to localize sounds in the horizontal plane is an essential
component of the mammalian auditory system. It facilitates
approaching potential mating partners and allows avoiding
predators. The superior olivary complex (SOC) within the auditory
brainstem is the first site of binaural interaction and its major
projections and inputs are well investigated. The adult input
pattern, however, is not set from the beginning but changes over
the period of development. Mammals including humans experience
different stages and conditions of hearing during auditory
development. The human brain for instance has to perform a
transition after birth from the perception of sound waves
transmitted in amniotic fluid to the perception of airborne sounds.
Furthermore, small mammals like rodents, which are common model
organisms for auditory research, perceive airborne sounds for the
first time some days after birth, when their ear canals open. The
basic neuronal projections and the intrinsic properties of neurons,
such as the expression of specific ion channels, are already
established and adjusted in the SOC during the perinatal period of
partial deafness. An additional refinement of inputs and further
adaptations of intrinsic characteristics occur with the onset of
hearing in response to the new acoustic environment. It is likely
that with ongoing maturation well-established inputs within the
sound localization network need these adaptations to balance
anatomical changes such as an increasing head size. In addition,
short-term adjustments of synaptic inputs in the adult auditory
system are equally necessary for a faithful representation of
auditory space. A recent study suggests that these short-term
adaptations are partially represented at the auditory brainstem
level. The question of how intrinsic properties change during
auditory development, to what extent auditory experience is
involved in these changes and the functional implications of these
changes on the sound localization circuitry is only partially
answered. I used the hyperpolarization-activated and cyclic
nucleotide-gated cation channels (HCN channels), which are a key
determinant of the intrinsic properties of auditory brainstem
neurons, as a target to study the influence of auditory experience
on the intrinsic properties of neurons in the auditory brainstem.
Another important question still under discussion is how neurons in
the auditory brainstem might fine-tune their firing behavior to
cope optimally with an altered acoustic environment. Recent data
suggest that auditory processing is also affected by modulatory
mechanisms at the brainstem level, which for instance change the
input strength and thus alter the spike output of these neurons.
One possible candidate is the metabotropic GABAB receptor (GABABR)
which has been shown to be abundant in the adult auditory
brainstem, although GABAergic projections are scarce in the mature
auditory brainstem. These questions were investigated by performing
whole-cell patch-clamp recordings of SOC neurons from Mongolian
gerbils at different developmental stages in the acute brain slice
preparation. Specific currents and receptors were isolated using
pharmacological means. Immmunohistochemical results additionally
supported physiological findings. In the first study, I
investigated the developmental regulation of HCN channels in the
SOC and their underlying depolarizing current Ih, which has been
shown to regulate the excitability of neurons and to enhance the
temporally precise analysis of binaural acoustic cues. I
characterized the developmental changes of Ih in neurons of the
lateral superior olive (LSO) and the medial nucleus of the
trapezoid body (MNTB), which in the adult animals show different
HCN subunit composition. I showed that right after hearing onset
there was a strong increase of Ih in the LSO and just a minor
increase in the MNTB. In addition, the open probability of HCN
channels was shifted towards more positive voltages in both nuclei
and the activation time constants accelerated during the first days
of auditory experience. These results implicate that Ih is actively
regulated by sensory input activity. I tested this hypothesis by
inducing auditory deprivation which was achieved by surgically
removing the cochlea in gerbils before hearing onset. The effect
was opposite in neurons of the MNTB and the LSO. Whereas in LSO
neurons auditory deprivation resulted in increased Ih amplitude,
MNTB neurons displayed a moderate decrease in Ih. These results
suggest that auditory experience differentially changes the amount
of HCN channels dependent on the subunit composition or possibly
alters intracellular cAMP levels, thereby shifting the voltage
dependence of Ih. This regulatory mechanism might thus maintain
adequate excitability levels within the SOC. A second study was
carried out to investigate the role of GABABRs in the medial
superior olive (MSO). Upon activation, these metabotropic receptors
are known to decrease the release probability of neurotransmitters
at the presynapse thereby altering excitatory and inhibitory
currents at the postsynaptic site. Neurons in the MSO analyze
interaural time differences (ITDs) by comparing the relative timing
of the excitatory inputs from the two ears using a coincidence
mechanism. In addition, these neurons receive a precisely timed
inhibitory input from each ear which shifts ITDs in the
physiological relevant range. Since the major inhibitory input
changes its transmitter type from mixed GABA/glycinergic to only
glycinergic after hearing onset it was now interesting to examine
the mediated effects of GABABRs, which have been shown to be
abundant in the prehearing and adult MSO of gerbils. Furthermore,
revealing the precise expression pattern of GABABRs and their
influence on excitatory and inhibitory currents in the MSO during
auditory development should provide further evidence of their
functional relevance. Performing pharmacological experiments I
could now demonstrate that the activation of GABABRs before hearing
onset decreases the current of excitatory inputs stronger than that
of inhibitory inputs whereas a switch is performed after hearing
onset and inhibitory currents are stronger decreasedcompared to
excitatory currents. In a similar way, also the expression pattern
of GABABRs changes before and after hearing onset as revealed by
immunohistochemistry. Since the main inhibitory inputs to the adult
MSO are purely glycinergic, it was commonly assumed that GABABRs
occupy only a minor role in the mature auditory brainstem.
Contradictory to this, it was possible to activate presynaptic
GABABRs by synaptic stimulation even in adult animals and to
observe a profound decrease of inhibitory current in MSO neurons.
These results suggest GABAergic projections of yet unknown origin
targeting the MSO. It is therefore quite likely that GABABRs
modulate and possibly improve the localization of low frequency
sounds even in adult mammals. Summarized, the outcome of this
thesis contributes to a better understanding of the developmental
adaptation in the auditory system and demonstrates that the orderly
specification of intrinsic properties within the SOC is dependent
on auditory experience. Moreover, I show that even in mature
animals the synaptic strength of MSO inputs can be modulated by
synaptic GABA release. This should emphasize the importance of
modulatory mechanisms and could be the basis for future studies
concerning the field of sound localization.