Interactive responses of a thalamic neuron to formalin induced lasting pain in behaving mice

Abstract

Thalamocortical (TC) neurons are known to relay incoming sensory information to the cortex via firing in tonic or burst mode. However, it is still unclear how respective firing modes of a single thalamic relay neuron contribute to pain perception under consciousness. Some studies report that bursting could increase pain in hyperalgesic conditions while others suggest the contrary. However, since previous studies were done under either neuropathic pain conditions or often under anesthesia, the mechanism of thalamic pain modulation under awake conditions is not well understood. We therefore characterized the thalamic firing patterns of behaving mice in response to nociceptive pain induced by inflammation. Our results demonstrated that nociceptive pain responses were positively correlated with tonic firing and negatively correlated with burst firing of individual TC neurons. Furthermore, burst properties such as intra-burst-interval (IntraBI) also turned out to be reliably correlated with the changes of nociceptive pain responses. In addition, brain stimulation experiments revealed that only bursts with specific bursting patterns could significantly abolish behavioral nociceptive responses. The results indicate that specific patterns of bursting activity in thalamocortical relay neurons play a critical role in controlling long-lasting inflammatory pain in awake and behaving mice.

Figure 1. Behavioral nociceptive responses and temporal changes in VB neuronal firing patterns induced by formalin in behaving mice.

(A) Behavioral pain responses to formalin analyzed in 5 min segments (F = 14.42, p<0.01). All data points are mean±SEM. n = 9 mice. ANOVA with Repeated measures were used for statistical analysis over time. (B) Spike sorting sample from a tetrode. (C) Normalized overall VB neuronal firing rate changes to formalin over time in 5 min segments. (D) Normalized tonic firing and burst firing rate changes to formalin over time in 5 min segments. (C and D) n = 48 neurons, 7 mice. All data points are mean±SEM. Dotted line is the behavioral nociceptive responses superimposed for comparison with the VB neuronal firing responses. Student's t-test was used to compare each data points with the baseline. *indicates significant differences at p<0.05.

Figure 2. Histology and schematic drawing indicating all recording locations.

Numbers on the left corner of each drawing indicates millimeter distance from the bregma.

Figure 3. Temporal changes of burst properties before and after formalin injection.

(A) Contour maps for JPD of the 1^st and the 2^nd IntraBI for baseline and different pain response phases after formalin injection. (B) Mean of all IntraBIs to formalin over time. (C) Number of burst spikes within a burst changes to formalin over time. (D) Sum of pre- or post-silent periods per cell changes to formalin over time. (E) Pre- or post-silence per burst changes to formalin over time. (B–E) Vertical grey stripes indicate the formalin injection point. All data points are mean±SEM. To compare each data point with the baseline, student's t-test was used. * indicates significant difference at p<0.05. n = 48 neurons, 7 mice.

Figure 4. Alterations in pain responses by different electrical stimulation conditions during the formalin test.

(A) Schematic drawing and histology sample of stimulation sites. (B) Behavioral pain responses to burst (3 ms IntraBI) or low frequency burst (5 ms and 15 ms IntraBI) stimulations compared with the sham control. All stimulation conditions were composed of 5 burst spikes and the total stimulation frequency was set to be ∼8 Hz by modifying inter-burst-intervals. Kolmogorov-Smirnov Z test showed that only burst stimulation condition significantly reduced pain responses compared to that of the sham control. (* p<0.01).