5.1 Introduction
So far, the pattern of distribution of mechanisms and the functional consequences of that pattern was discussed. It was shown that adaptation acts at different places (Chap-ter 4) and that the pat(Chap-tern of distribution of the mechanisms can be explained by the function a neuron serves in the processing of sensory signals (Chapter 4). Thus,where adaptation occurs is of central functional relevance, but what abouthowadaptation is realized, by which actual mechanism?
The modelling of adaptation in Chapter 4 provides examples for the consequences of adaptation on the coding of subsequent stimuli. Generally, how a neuron trans-forms its inputs into the output spike frequency is not fixed but changes in response to sensory environment, performed task and context. These adaptations to a specific behavioural or sensory background will change the input-response curve of a sensory neuron. These changes can be seen as modulatory operations between background and acute signal parameters. In this context, adaptational changes of response curves can be classified into two types: subtractive and divisive modulations (Fig. 5.1): by subtraction, the threshold of the curve shifts, but the slope of the curve remains un-changed. Divisive alterations, also referred to as gain control, change the slope of the curve. Both operations are known to be of central importance to the nervous system (Barlow and Levick, 1965; Carandini and Heeger, 1994; Peña and Konishi, 2001; Gab-biani et al., 2002), since they provide basic building blocks of computation: subtraction and division.
- ÷
stimulus intensity
response
subtractive divisive
Figure 5.1:Subtractive and divisive adaptation effects on response curves.
The biophysical machinery behind subtractive changes is well documented, includ-ing both hyperpolarizinclud-ing and shuntinclud-ing inhibition (Holt and Koch, 1997; Chance et al., 2002) as well as spike driven adaptation currents (Benda and Herz, 2003). However
proposed mechanisms of divisive gain control have shown to be elusive and often only the interplay of several mechanisms was observed to act divisively. These include con-ductance changes controlled by large populations of balanced inhibitory and excitatory inputs (Chance et al., 2002; Baca et al., 2008), shunting inhibition in combination with synaptic noise (Prescott and Koninck, 2003; Ly and Doiron, 2009) and combinations of pre- and postsynaptic nonlinearities (Gabbiani et al., 2002). Recently, short-term synap-tic plassynap-ticity also has been shown to provide means for gain control (Abbott et al., 1997;
Rothman et al., 2009).
With the exception of synaptic plasticity, these mechanisms demand large number of inputs that provide a synaptic background, which may not be available in the sensory periphery in both invertebrates and vertebrates. A candidate for gain modulation in peripheral sensory and motor circuits is presynaptic inhibition, a mechanism that is widely termed ‘gain control’ (Burrows and Matheson, 1994; Root et al., 2008), but its divisive nature has not been thoroughly tested. Presynaptic inhibition results from an axo-axonal connection, in which the target neurons terminal is inhibited by GABAergic ionotropic Cl−-channels, which have a shunting effect on the membrane potential of the target cell and reduce the amplitude of incoming spikes (Clarac and Cattaert, 1996;
Rudomin, 2009).
Here, I want to test the effect of different mechanisms that alter the response curve of an auditory interneuron in crickets (AN2). The AN2 receives direct input from two populations of receptor neurons (Hennig, 1988) that are sensitive in two different fre-quency ranges (Imaizumi and Pollack, 1999). These carrier frequencies are relevant in different behavioural contexts: the low frequency channel (3 kHz) is mainly devoted to processing of conspecific songs, while at frequencies higher than 15 kHz, receptors mainly detect echolocation sounds of predating bats (Moiseff et al., 1978). In addition to the excitatory inputs to AN2 by afferents, it is also the target of direct postsynaptic inhibition (Nolen and Hoy, 1987; Pollack, 2005), but presynaptic GABAergic connec-tions at the afferent terminals are also reported (Hardt and Watson, 1999). At the level of the receptors, the two frequency channels are well separated, but they converge cen-trally, where processing has to comply with different computational demands for the two frequencies (Marsat and Pollack, 2004). It is thus likely that the input-response curves for the two frequencies in AN2 have different shapes, reflecting these demands.
Possibly, the modulatory effect of adaptation on the response curve is also different for low and high frequency sound.
The first goal of the experiments described below was to test if (1) subtractive and di-visive effects of a modulatory background will be different in the two carrier frequency domains. In a second step (2), cross-adaptation paradigms with current or the respec-tive other frequency revealed where in the pathway different components of divisive and subtractive modulation of the response curves take place. Finally (3), selective pharmacological blocking of presynaptic inhibition uncovered its specific modulatory effect.
5.2 Methods
5.2 Methods
5.2.1 Stimulus protocols
In all experiments, response curves were obtained by presenting 100 ms square pulses with 2 ms ramps. For the adapted response curves, these pulses were preceded by at least 500 ms of the adapting background intensity (Fig. 5.2). When adaptor and test pulse were both acoustic and of the same frequency, 2 ms ramps lead from the adapting background to the test amplitude. For current and cross-frequency adaptation experi-ments, the latency for each stimulation modus was obtained separately and the onset corrected in such a way that switching off the adaptor and switching on of the test pulse coincided. Latency for current stimuli was shortest (2.4±0.7ms), followed by the high frequency stimuli (9.7±2.7ms) and low frequency (12.4±3.1ms). All stimuli were repeated at least 15 times.
test steps adapting
background
200
500 1000
400 600
0 0
spike frequency [Hz]
time [ms]
Figure 5.2:Stimulus protocol for adapted response curves.In order to construct onset response curves at different adaptation levels, a background stimulus was played and interrupted for short test pulses (upper trace). The neural response for each test inten-sity was quantified as the spike frequency response at the beginning of the test pulses, indicated by the grey circles.
5.2.2 Data analysis
For the parameterization of the unadapted response curves, a squared hyperbolic tan-gent of the form
fon(x) =
fmaxtanh2(k(xthr−x)) ; ifx >xthr
0 ; else ,
was used. Squaring was introduced to match the asymmetric shape of the response curves. x is the input intensity in dB, xthr is the threshold of the neuron, fmax is the maximal spike frequency of the response curve andkis a slope factor and can be used
to calculate the slope at the turning point of the curvesson: son = 4
3√
3fmaxk.
For the quantifications of shift and slope change, the adapted onset response curves were parameterized using the same fmax as for the unadapted curve and onlyk and xthrwere fit to the data.
5.3 Results
5.3.1 Changes of the response curves in the two carrier channels
The AN2 of crickets is sensitive in two frequency ranges that correspond to two differ-ent behavioural contexts, mate selection and predation risk (Moiseff et al., 1978). Since the demands on coding and processing in these two contexts are different (Marsat and Pollack, 2004) , it was first tested whether adaptation changes the response curve of the AN2 in a frequency-specific way. To test for this, stimuli of different intensities were presented with either no adapting background (unadapted response curve) or preceded by a defined adaptation level of the same carrier frequency (Fig. 5.2). Both carrier frequencies were tested separately in this way and the onset response to each test step was quantified by the spike frequency.
Examples of a single cell are shown in Fig. 5.3. For the low frequency the response curves retained their overall shape, but thresholds shifted considerably depending on the adaptation level. This shift was observed over a wide range of intensities, in some cases up to 20 dB (Fig. 5.3A). When the adapting background was subtracted from the absolute stimulation level, all adapted curves matched the unadapted curve, demon-strating a strictly subtractive effect of the adapting background intensity at the low car-rier frequency (Fig. 5.3B). At the high carcar-rier frequency, response curves were shifted as at low frequencies, but also showed a clear decrease of the slope in response two different adapting backgrounds (Fig. 5.3C). Plotting relative rather than absolute in-tensities (Fig. 5.3D), a subtractive change was observed, but this did not account for the entire effect of adaptation. In a second step, the input intensities of the adapted
Figure 5.3(following page): Effect of adaptation on the onset response curves. Dis-played are onset responses curves at two different carrier frequencies, 3 kHz (A and B) and 16 kHz (C, D and E) for a single recorded neuron. Black symbols and lines represent unadapted response curves. Coloured symbols and lines represent re-sponse curves after adaptation to the background level indicated by the dashed lines in (A) and (C), respectively. In order to illustrate a shift of the response curves, in panels (B) and (D) the data in (A) and (C) was re-plotted with the adaptation back-ground subtracted from input intensity for each curve. In (E), input intensity was additionally divided by the adaptation background to show the divisive effect of adaptation at 16 kHz. Error bars indicate standard deviation, solid lines in panels (A) and (C) stem from hyperbolic tangent fits to the data.
5.3 Results
−10 0 10
100 200 300 400 500
relative intensity [dB]
onset spike freq. [Hz]
70 80 90 100
0 200 400 600
intensity [dB SPL]
onset spike freq. [Hz]
0 5 10
200 400 600
scaled intensity
onset spike freq. [Hz]
−5 0 5 10 15
200 400 600
relative intensity [dB]
60 70 80 90
0 200 400 600
intensity [dB SPL]