Hemispheric lateralization of pain processing by amygdala neurons

Abstract

Recent biochemical and behavioral data suggest right-hemispheric lateralization of amygdala functions in pain. Our previous electrophysiological studies showed pain-related neuroplasticity in the latero-capsular division of the central nucleus of the amygdala (CeLC) in the right brain hemisphere. Here we determined differences in the processing of pain-related signals in right versus left CeLC neurons. Individual CeLC neurons were recorded extracellularly before and after induction of an arthritis pain state in anesthetized rats. Brief innocuous and noxious test stimuli were applied to peripheral tissues ipsi- and contralateral to the recording site. A monoarthritis was induced in the ipsi- or contralateral knee by intraarticular injections of kaolin and carrageenan. Under normal conditions, CeLC neurons in the left amygdala had smaller receptive fields than those in the right, but the magnitude of background and evoked activity was not significantly different. After arthritis induction, neurons in the right, but not left, CeLC developed increased background activity and evoked responses, irrespective of the location of the arthritis (ipsi- or contralateral to the recording site). A protein kinase A (PKA) inhibitor decreased the activity of right CeLC neurons after arthritis induction but had no effect in the left amygdala. Forskolin, however, increased the activity of left and right CeLC neurons under normal conditions. The results show for the first time laterality of pain-related electrophysiological activity changes in individual amygdala neurons. Whereas both left and right amygdala neurons receive nociceptive inputs and can become sensitized in principle, a yet unknown mechanism prevents PKA activation and pain-related changes in the left amygdala.

Fig. 1.

Histologically verified recording sites of 32 neurons in the laterocapsular division of the central nucleus of the amygdala (CeLC) in the left and right brain hemisphere. ■, ●, ▴, the locations of neurons that were recorded before and after arthritis induction; □, ○, ▵, neurons that were recorded only under normal conditions. Symbols also differentiate between the different types of receptive fields (see Fig. 2): ■, □, contralateral hindlimb only; ●, ○, bilateral hindlimbs; ▴, ▵, whole body. Diagrams (adapted from Paxinos and Watson 1998) show coronal brain sections at different levels posterior to bregma (−2.30 to −2.80). Next to each section is shown in detail the central nucleus and its medial (CeM), lateral (CeL), and latero-capsular (CeLC) subdivisions. Calibration bars are 1 mm.

Fig. 2.

Receptive fields of neurons in the right (A) and left (B) CeLC. Receptive fields in the deep tissue are shown before (normal) and 5 h postinduction of arthritis. Changes of receptive field size were observed in right, not left, CeLC neurons. A and B: all neurons showed increasing responses to graded innocuous and noxious stimulation of areas colored black (= multireceptive neurons, see methods). , high-threshold receptive fields, stimulation of which activated the neuron weakly. i.l., ipsilateral; c.l., contralateral to recording site. A: under normal conditions, receptive fields of right CeLC neurons (n = 15) were symmetrical in the deep tissue of both hindlimbs and the tail (n = 7, left) or covered the whole body (n = 8, right). B: receptive fields of left CeLC neurons (n = 17) were either confined to the contralateral hindlimb (n = 9) or included an additional high-threshold receptive field in the ipsilateral hindlimb (n = 8). C: semi-quantitative analysis of the receptive field size in neurons that were recorded continuously before and after arthritis induction. The body map was divided into 21 areas (A and B, - - -). The total number of areas that contained part of the receptive field was calculated for each neuron and averaged for left (n = 11) and right (n = 9) CeLC neurons (see Data analysis). Only neurons that were recorded before and after arthritis induction are included in the analysis. In the graph, each box extends from the 25th to the 75th percentile, with a line at the median (50th percentile). The whiskers extend above and below the box to show the highest and lowest values. ***, P < 0.0005 (receptive field size of right compared with left CeLC neurons; Mann-Whitney U test), #, P < 0.025 (receptive field size after arthritis compared with normal; Wilcoxon signed-rank test), alpha level adjusted for multiple comparisons.

Fig. 3.

Right-hemispheric lateralization of arthritis pain-related changes. A: unchanged background and evoked activity of 1 left CeLC neuron in arthritis. B: increased background and evoked activity of 1 right CeLC neuron after arthritis induction. A and B: line graphs show the time course of extracellularly recorded responses (number of spikes per second) to brief (15 s) innocuous (500 g/30 mm²) and noxious (2,000 g/30 mm²) stimulation of the knee and background activity. Symbols show the mean activity during a 15-s period before stimulation (= background activity) or the difference between mean activity during and before 15-s stimuli (= net activity evoked by noxious or innocuous stimuli; see methods). Peristimulus time histograms (insets) show individual responses (spikes per second) before and 5 h after induction of arthritis. Top traces show recordings of the force (g/30 mm²) applied to the knee joint with a calibrated forceps (see methods). C: comparison of left vs. right CeLC neurons. Under normal conditions before arthritis, there was no significant difference of background and evoked activity between left (n = 11) and right (n = 9) CeLC neurons (P > 0.05, Tukey test). Five hours after arthritis induction, activity of right CeLC neurons was significantly higher than that of left CeLC neurons (P < 0.01–0.05, Tukey test). Bar histograms show background activity and responses to innocuous (500 g/30 mm²) and noxious (2,000 g/30 mm²) stimulation of the knee averaged for the sample of neurons (means ± SE). Single asterisk, P < 0.05; double asterisks, P < 0.01 (Tukey test, comparing values in right vs. left CeLC neurons).

Fig. 4.

Pain-related lateralization is independent of the side of arthritis. A and B: there was no significant change of background and evoked activity of left CeLC neurons after induction of arthritis in the right (contralateral, A; n = 5) or left (ipsilateral, B; n = 6) knee. C and D: background and evoked activity of CeLC neurons in the right hemisphere increased 5 h after arthritis was induced in the left (contralateral, C; n = 5) or right (ipsilateral, D; n = 4) knee. Bar histograms show background activity and responses to brief (15 s) innocuous (500 g/30 mm²) and noxious (2,000 g/30 mm²) stimulation of the knee expressed as percent of prearthritis values (set to 100%). Data were averaged for the sample of neurons (means ± SE). *, P < 0.05 (paired t-test comparing values in arthritis with prearthritis values under normal conditions; statistical analysis was performed on raw data).

Fig. 5.

Lateralized effects of a protein kinase A (PKA) inhibitor in the arthritis pain model. A: administration of KT5720 (100 μM, concentration in the microdialysis probe; 15 min) into the left CeLC had no effect on background and evoked activity of a neuron in the left CeLC. B: normalized data summarize the lack of effects of KT5720 on background activity and evoked responses of left CeLC neurons (n = 5) 5–6 h postinduction of arthritis in the contralateral (right) knee. C: KT5720 (100 μM) administered into the right CeLC inhibited the increased background activity and evoked responses of a neuron in the right CeLC. D: normalized data summarize the significant inhibition of background activity and evoked responses right CeLC neurons (n = 5) 5–6 h postinduction of arthritis in the contralateral (left) knee. A and C: symbols show background activity (mean activity during a 15-s period before stimulation) and responses to innocuous (500 g/30 mm²) and noxious (2,000 g/30 mm²) stimuli before [in arificial cerebrospinal fluid (ACSF)], during and after KT5720 administration (see - - -). Evoked responses are calculated as the difference between mean activity during and before 15-s stimuli (see methods). Insets: individual responses (spike/s, peristimulus-time histograms, bin width: 1 s) and recordings of the force (g/30 mm², top traces) applied to the knee joint with a calibrated forceps (see methods) before, during, and after administration of KT5720 into the CeLC. B and D: bar histograms show averaged values (means ± SE) during drug administration normalized to predrug control values (in ACSF, set to 100% as indicated, - - -). *, P < 0.05; **, P < 0.01 (paired t-test).

Fig. 6.

Forskolin effects are not lateralized. A: administration of forskolin (1 mM, concentration in the microdialysis probe; 15 min) into the left CeLC increased background activity and evoked responses of a neuron in the left CeLC. B: normalized data summarize the significant effects of forskolin in the sample of left CeLC neurons (n = 4). C: forskolin (1 mM) administered into the right CeLC had similar facilitatory effects on a right CeLC neuron. D: normalized data summarize the significant facilitatory effects of forskolin in the right CeLC (n = 4). A and C: symbols show background and evoked net activity before, during, and after forskolin administration (same display as in Fig. 5, A and C). B and D: bar histograms show averaged values (means ± SE) during drug administration normalized to predrug control values (same display as in Fig. 5, B and D). Recordings were made under normal conditions (no arthritis). *, P < 0.05; **, P < 0.01 (paired t-test).