Neurons within the trigeminal mesencephalic nucleus encode for the kinematic parameters of the whisker pad macrovibrissae

Abstract

It has been recently shown in rats that spontaneous movements of whisker pad macrovibrissae elicited evoked responses in the trigeminal mesencephalic nucleus (Me5). In the present study, electrophysiological and neuroanatomical experiments were performed in anesthetized rats to evaluate whether, besides the whisker displacement per se, the Me5 neurons are also involved in encoding the kinematic properties of macrovibrissae movements, and also whether, as reported for the trigeminal ganglion, even within the Me5 nucleus exists a neuroanatomical representation of the whisker pad macrovibrissae. Extracellular electrical activity of single Me5 neurons was recorded before, during, and after mechanical deflection of the ipsilateral whisker pad macrovibrissae in different directions, and with different velocities and amplitudes. In several groups of animals, single or multiple injections of the tracer Dil were performed into the whisker pad of one side, in close proximity to the vibrissae follicles, in order to label the peripheral terminals of the Me5 neurons innervating the macrovibrissae (whisking-neurons), and therefore, the respective perikaria within the nucleus. Results showed that: (1) the whisker pad macrovibrissae were represented in the medial-caudal part of the Me5 nucleus by a single cluster of cells whose number seemed to match that of the macrovibrissae; (2) macrovibrissae mechanical deflection elicited significant responses in the Me5 whisking-neurons, which were related to the direction, amplitude, and frequency of the applied deflection. The specific functional role of Me5 neurons involved in encoding proprioceptive information arising from the macrovibrissae movements is discussed within the framework of the whole trigeminal nuclei activities.

Keywords: Me5 proprioception of macrovibrissae movements; Me5 whisking‐neurons; rat; trigeminal mesencephalic nucleus.

Figure 1

Injection sites of the tracer DIL into the whisker pad in animals of Group 2. (A–C) Rat muzzle showing the macrovibrissae arrangement in the whisker pad (black spots). The arrows indicate the tracer injection sites in animals of subgroups 2.1, 2.2, and 2.3 respectively. (D) Histological hematoxylin‐eosin stained section of the rat follicle‐sinus complexes. (E) Histological section showing the diffusion of the tracer (white arrow) injected in animals of subgroup 2.1, in which one follicle‐sinus complex was labeled. (F) Fluorescence detection of the tracer injected in animals of subgroups 2.2 and 2.3. This figure shows an example of the labeling extent.

Figure 2

Positive control of the retrograde diffusion of the tracer. (A) Fluorescence detection of a trigeminal Gasser ganglion (GG) longitudinal section labeled by the tracer Dil injected into the ipsilateral whisker pad (from an animal of subgroup 2.4). Arrows indicate some labeled neurons and the insets a₁ and a₂ their zoomed views. (B) Histological longitudinal section of the same ganglion stained with cresyl violet showing its structure and (b) a zoomed view of its neurons.

Figure 3

Electrical activity of a Me5 neuron and its response to macrovibrissae deflection in backward/forward direction. (A) Functional identification of the neuron location into the Me5 nucleus elicited by masseter muscle stretch‐test (see method). (B (a)) the Me5 neuron electrical activity recorded in basal conditions, during macrovibrissae deflection in backward/forward direction (mode run, V = 2 mm/sec, 45°), during its return to resting position and after the end of the deflection; (b) schematic drawing of the time course of the four tested phases, i.e.: basal conditions (1), macrovibrissae deflection (2), return to the resting position (3) and after the end of the deflection (4). (C) Peri‐Stimulus‐Time‐Histogram (PSTH) of the same neuron activity before, during, and after mechanical deflection of the macrovibrissae in backward/forward direction (mode run, V = 2 mm/sec, 45°). (D) Cumulative Frequency Distribution (CFD) of the same neuron constructed from its firing before, during, and after macrovibrissae deflection using the mode run at different velocities. The CFD diagram was obtained adding each frequency value to all of the previous values. In all recordings arrows (ON) and (OFF) indicate the beginning and the end of stimulus application. Calibrations: Horizontal 0.5 sec for A–B, 4 sec/interval for PSTH in C, 50 intervals/division (62.43 ms/bin) for CFD in D; Vertical 0.5 mV for A–B, 10 counts/division for PSTH in C, 500 counts/division for both CFDs in D.

Figure 4

Histograms summarizing the effect elicited in all tested neurons by macrovibrissae deflection in backward‐forward direction at different velocities. Each column refers to a single neuron and indicates the total counts of its firing obtained from the respective CFD (calculated as for Fig. 3) constructed at 62.43 ms/bin during 8 seconds recordings, and with macrovibrissae deflection at 45° angle in backward‐forward direction. Me5 neuron responses were tested at increasing velocities (1–4 mm/sec). Horizontal axes: Neurons analyzed (n = 34); Vertical axes: Total counts for each recorded neuron.

Figure 5

Electrical activity of a Me5 neuron during macrovibrissae deflection performed at different inclination angles. (A) Functional identification of the neuron location into the Me5 nucleus by the masseter muscle stretch‐test. Calibrations, (ON) and (OFF) as for Fig. 3A. (B) Peri‐stimulus‐time‐histogram (PSTH) of the Me5 neuron activity before, during, and after mechanical deflection of the macrovibrissae in backward/forward direction (mode run, V = 2 mm/sec, 45°).ON‐OFF interval: 30 sec; Vertical calibration: 10 counts. (C) Recordings of the spontaneous firing of the same neuron, performed at different inclination angles of macrovibrissae deflection (mode burst, V: 600 μm/sec) from 0° up to 70°: (a) basal conditions, i.e. at 0°, (b) at 35°, and (c) at 70°. Trace d shows the neuron activity at 25° during macrovibrissae return to their resting position, and trace (e) at 0°. Traces show the peak of neuron firing at precise angles. For each step, the macrovibrissae deflection was maintained steady for about 20 sec to verify the presence of the adaptation phenomenon. The schematic drawing refers to the time course of the tested phases (1–4). Calibrations: Horizontal 1 sec, Vertical 1 mV.

Figure 6

Histograms summarizing the effects elicited in the Me5 neurons by macrovibrissae deflection performed at constant velocity in backward‐forward direction, with different inclination angles. In all histograms, each column refers to a single neuron and indicates the total counts of its firing obtained from the respective CFD (calculated as for Fig. 3) constructed at 62.43 ms/bin and 8 sec analysis, during macrovibrissae deflection at different angles (25–70°) and performed at the constant velocity of 650 μm/sec in backward‐forward direction. Horizontal axes: The neurons analyzed (n = 34); Vertical axes: The total counts for each recorded neuron.

Figure 7

Me5 nucleus of the left‐hand side labeled by the tracer Dil injected into the ipsilateral whisker pad. (A) Fluorescence detection of the Me5 nucleus labeled by Dil. a₁: rat muzzle showing the macrovibrissae arrangement in the whisker pad (black spots). a₂: magnification of a Me5 labeled neuron. (B) Longitudinal histological section of the rat brain stained with cresyl violet showing the Me5 nucleus, and (b1) its zoomed view. Scale bar: 27 micron (A, a2), 250 micron (B), 150 micron (b1). Me5: trigeminal mesencephalic nucleus; Cb: cerebellum.

Figure 8

Axons and neurons of the Me5 nucleus labeled by Dil injected into the ipsilateral whisker pad. (A) Fluorescence detection of the Me5 nucleus showing axons (arrows) retrogradely labeled by the tracer injected into the ipsilateral whisker pad, and magnifications of some labeled Me5 neurons (a₁–a₃). Scale bar: 27 micron (A, a1, a3), 33 micron (a2).