Table 5.

Comparing the average neuronal activity during cocaine and heroin self-administration in the same session

Type of responsesCocaine vs controlHeroin vs controlCocaine vs heroin (n = 58)
% changes in firing raten% of total% Changes in firing raten% of total% Changes in firing raten% of total
mPFC
 Heroin first, cocaine second n = 49Excitatory115.2  ± 24.97*14.3167.3  ± 47.01428.692.3  ± 18.89a15.5
Inhibitory−62.6  ± 3.528*57.1−55.1  ± 3.71735.3−56.2  ± 3.93051.7
 Cocaine first, heroin second n = 40Excitatory171.0  ± 63.4717.5187.5  ± 38.91332.5164.5  ± 101.61025.0
Inhibitory−51.7  ± 3.72152.5−50.3  ± 4.21640.0−47.2  ± 5.61947.5
NAc
 Heroin first, cocaine secondn = 67Excitatory158.5  ± 24.315*22.4103.9  ± 20.92232.8220.2  ± 54.617b25.4
Inhibitory−64.7  ± 3.142*62.7−49.5  ± 3.32334.3−64.7  ± 3.14262.7
 Cocaine first, heroin second n = 52Excitatory414.1  ± 77.3917.3294.5  ± 88.61732.6265.9  ± 59.21732.6
Inhibitory−62.0  ± 4.32955.7−66.7  ± 3.62650.0−58.3  ± 3.82650.0
  • Comparison of overall firing rates during cocaine and heroin self-administration within the same session. The neuronal activity during the entire period of either cocaine or heroin self-administration was compared with each other and with the control period (200 sec at the beginning of the session). The comparisons were made in heroin-first, cocaine-second and cocaine-first, heroin-second sessions for the mPFC (top panel) and the NAc (bottom panel). The comparison between cocaine and heroin self-administration trials is expressed as percentage change as follows: [(cocaine firing rate-heroin firing rate)/heroin firing rate] × 100. χ22 × 2 contingency table tests were used to test whether different numbers of neurons showed excitatory and inhibitory responses under conditions of cocaine and heroin self-administration. Significant differences were observed in heroin first, cocaine second sessions in both the mPFC and NAc (*significantly different number of neurons having excitatory and inhibitory responses as compared to heroin self-administration phase, p < 0.05). χ2goodness of fit test was used to determine whether significantly different numbers of neurons showed excitatory and inhibitory responses when compared between drug conditions. More neurons were inhibited than excited during cocaine self-administration in comparison with heroin self-administration phase (ap < 0.05, bp < 0.01).