J. Brendan Ritchie Joanna Szczepanik

RESPONSES OF RAT TRIGEMINAL GANGLON NEURONS TO LONGITUDINAL WHISKER STIMULATION

Maik C. Stüttgen, Stephanie Kullmann and Cornelius Schwarz

http://www.youtube.com/watch?v=K9nJ87CpTuY&feature=user

Whisking in the Dark!

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are needed to see this picture.

•Early interest - Vincent, in Watson’s lab, noticed that rats whose whiskers were clipped showed behavioral impairments, for example on maze navigation. She concluded that whiskers are ‘delicate tactile organs, which function in equilibrium, locomotion, and the discrimination of surfaces’•Shiffman et al, 1970’s studied rats with a depth perception task, and found that rats rely on whiskers and hence tactile information rather than vision when descending from a platform.• Animals with unilaterally clipped whiskers are cautious about exploring their environment, and walk with the intact whisker side along the wall•Bilateral vibrissotomy results in impairment of locomotion and swimming, and behavioral ‘frustration’•Studies in the 90’s determined that rats are capable of fine texture discrimination (a task where rats were rewarded for jumping onto either rough or smooth surface platform). Carvell and Simons concluded that rat’s whiskers are a tactile organ as good as primate’s fingertips!•New findings in this decade- by whisking, rats can differentiate spacing between walls, even when a difference is only 3mm. They are good at discriminating horizontal object locations. More evidence of how much information rats get from whisking-it is 10x faster than previously thought, and allow rats to move confidently in the dark-detect objects, determine texture, size and shape of objects.•Barrels, barreloids and barrelettes reflect organization cortical level – but what about the first level of processing, from moving whisker to trigeminal ganglion?

What’s so interesting about rat’s whiskers?

What’s so interesting about rat’s whiskers?

The Relevance of Stüttgen, Kullman & Schwarz (2008)

Looked at the quantitative relationship between longitudinal whisker stimulation, and activation in the TG.

Interesting conclusion: Longitudinal stimulation of TG cells provides a good method for classification into: (1) amplitude responsive, slow adapting cells, and (2) velocity responsive, fast adapting cells.

Stüttgen, Kullman & Schwarz: Methods

10 anesthetized adult female rats To access TG, left hemisphere sucked out. Whiskers probed for receptive field of cells in TG, then

trimmed to ~5mm and attached to actuator. Stimulation was at the same longitudinal axis as whisker. All whiskers were caudal: whiskers 1-4 in rows A-E, and

straddlers. Stimuli: fast half cosine, 500ms plateau, then slow cosine

return. 15 stimuli based on 3 amplitudes (95, 155, 285 μm) and 5 velocities (5, 22, 43, 87, 130mm/s).

Two Analyses for separating the effects of variation in peak velocity and amplitude in relation to variation in cell spike responses: (1) η2 as a measure of effect size and (2) for each neuron a multiple regression equation with number of spikes as criterion variable.

Analysis was insensitive to time windows of spike counting (which TG neurons are sensitive to; Stüttgen, et al., 2006).

Results!

33 of 38 neurons sampled responded to longitudinal stimulation (note: all 33 were responsive to the lowest amplitude used in the study, 95 μm…with enough velocity)

Example PSTHsY axis = amplitudes X axis = velocitiesA: Representative slow adapting (SA) cell with high-amplitude and low-velocity responsiveness. B: Representative rapidly adapting (RA) cell with low-amplitude and high-velocity responsiveness. C: Normalized PSTHs for all neurons superimposed. Orange is amplitude-responsive neurons, green is velocity-responsive neurons

Figure 1

Quantitative analysis of spike responsesA: spike counts of individual units as a function of deflection velocity averaged over amplitudes. B: spike counts of individual units as a function deflection amplitude, averaged over velocities. C: scatter plot of individual units η2 for amplitude and velocity.D: scatter plot of individual units' β weights from multiple regression analysis. E: Spike responses of all neurons to a slow high-amplitude longitudinal stimulus (285 µm, 5 mm/s).Asterisks = RA neurons.

Figure 2

Three Other Characteristics That Differentiate the Two Categories

1. The two classifications exhibit different adaptation profiles along the same lines as SA and RA cells in latitudinal studies.

2. Average spike counts of AR cells (53, n = 22) were much higher than those for VR cells (2.7, n = 11).

VR and AR cells had different velocity thresholds (Fig. 2E)

Question:

Does longitudinal classification predict latitudinal classification for TG cells?

Answer: yes…eventually.

AR/SA VR/RA Row totals

Class membership using latitudinal stimulation-adaptation in hold phase only (55% overlap)

SA 6 1 7

RA 11 7 18

Column totals 17 8 25

Class membership using adaptation in hold and return phases (68%)

SA 10 1 11

RA 7 7 14

Column totals 17 8 25

Class membership using adaptation in hold and return phases and velocity thresholds (Overlap 88%)

SA 15 1 16

RA 2 7 9

Column totals 17 8 25

Class Membership Using Longitudinal Stimulation

Table 1

What Explains the Remaining Discrepancies?

1. Missing responses of SA cells that require a precise direction preference.

2. SA cells are extremely sensitive to latitudinal position of stimulation (see Fig.3)…

PSTHs for cell from Fig. 1A. A: responses to rostrocaudal whisker stimulation (rostral first). Stimulus waveforms are truncated at 1 s due to fixed recording duration during calibration. B: Offset of absolute starting position by  1 mm.

Figure 3

Conclusion

TG cells are highly sensitive to longitudinal stimulation.

In fact, because of this, they can be categorized into AR/SA and VR/RA cell types.

(Question: what do you think is the functional significance of this division?)