Relationships between oxygen changes in the brain and periphery following physiological activation and the actions of heroin and cocaine

Abstract

Using two-sensor electrochemical recordings in freely moving rats, we examined the relationship between physiological and drug-induced oxygen fluctuations in the brain and periphery. Animals chronically implanted with oxygen sensors in the nucleus accumbens (NAc) and subcutaneous (SC) space were subjected to several mildly arousing stimuli (sound, tail-pinch and social interaction) and intravenous injections of cocaine and heroin. Arousing stimuli induced rapid increases in NAc oxygen levels followed by and correlated with oxygen decreases in the SC space. Therefore, cerebral vasodilation that increases cerebral blood flow and oxygen entry into brain tissue results from both direct neuronal activation and peripheral vasoconstriction, which redistributes arterial blood from periphery to the brain. The latter factor could also explain a similar pattern of oxygen responses found in the substantia nigra reticulata, suggesting hyperoxia as a global phenomenon with minor structural differences during early time intervals following the stimulus onset. While arousing stimuli and cocaine induced similar oxygen responses in the brain and SC space, heroin induced a biphasic down-up brain oxygen fluctuation associated with a monophasic oxygen decrease in the SC space. Oxygen decreases occurred more rapidly and stronger in the SC space, reflecting a drop in blood oxygen levels due to respiratory depression.

Figure 1

Relationship between changes in oxygen levels in the nucleus accumbens (NAc) and subcutaneous (SC) space induced by auditory stimulus (Sound), Tail-pinch and Social Interaction in awake, freely moving rats. Slow time-course analysis. (A) Mean (± SEM) changes in oxygen levels relative to baseline (100%) calculated with 1-min time bins. Vertical hatched lines show onset and offset of stimulus presentation. Filled symbols mark values significantly different from pre-stimulus baseline (P < 0.05). (B) Rate of oxygen changes. (C) Correlative relationships between oxygen changes in two recording locations. r = coefficient of correlation calculated for the time interval of significant increase in NAc oxygen levels. Asterisks show significance of coefficients of correlation (** and ***, P < 0.01 and 0.001, respectively). Data were obtained in 6 rats; the numbers of averaged responses in each group are 9–16 (see Table for ANOVA values and n in each group in Supplementary materials).

Figure 2

Rapid time-course analysis of relationships between changes in oxygen levels in the nucleus accumbens (NAc) and subcutaneous (SC) space induced by arousing stimuli in awake, freely moving rats. (A) Mean (± SEM) changes in oxygen levels relative to baseline (100%) calculated with 4-s time bins. Vertical hatched lines show onset and offset of stimulus presentation. Filled symbols mark values significantly different from pre-stimulus baseline (P < 0.05). (B) Correlative relationships between oxygen changes in two recording locations shown for the time interval between the stimulus onset and maximal oxygen increase (duration of this interval is shown in brackets). Data were obtained in 6 rats; the numbers of averaged responses and F values of one-way repeated measure ANOVAs are shown in Supplementary materials.

Figure 3

Relationships between changes in oxygen levels in the nucleus accumbens (NAc) and subcutaneous (SC) space induced by intravenous cocaine and heroin in awake, freely moving rats. Slow time-course analysis. (A) Mean (± SEM) changes in oxygen levels relative to baseline (100%) calculated with 1-min time bins. Vertical hatched lines show onset and offset of stimulus presentation. Filled symbols mark values significantly different from pre-stimulus baseline (P < 0.05). (B) Rate of oxygen changes. (C) Correlative relationships between oxygen changes in two recording locations shown for time intervals of brain oxygen increases. Coefficients of correlation for heroin (r) are shown for two time intervals: brain oxygen increase from its lowest point to peak and subsequent return to baseline (timing of these intervals are shown in brackets). Asterisks show significance of coefficients of correlation (** and ***, P < 0.01 and 0.001, respectively). Data were obtained in 6 rats; the numbers of averaged responses are 11–14 (see Table in Supplementary materials for ANOVA values and n in each group).

Figure 4

Rapid time-course analysis of relationships between changes in oxygen levels in the nucleus accumbens (NAc) and subcutaneous (SC) space induced by intravenous cocaine and heroin in awake, freely moving rats. (A) Mean (± SEM) changes in oxygen levels relative to baseline (100%) calculated with 4-s time bins. Vertical hatched lines show onset and offset of stimulus presentation. Filled symbols mark values significantly different from pre-stimulus baseline (P < 0.05). (B) Correlative relationships between oxygen changes in two recording locations shown separately for oxygen decreases and subsequent oxygen increases (timing of these intervals is shown in brackets). Asterisks show significance of coefficients of correlation (***, P < 0.001 and 0.001, respectively).

Figure 5

Changes in oxygen levels in substantia nigra, pars reticulata (SNr) induced by auditory stimulus (Sound), Tail-pinch and Social Interaction in awake, freely moving rats. Comparison with nucleus accumbens (NAc) responses. (A) Mean (± SEM) changes in oxygen levels relative to baseline (100%) calculated with 1-min time bins. Vertical hatched lines show onset and offset of stimulus presentation. Filled symbols mark values significantly different from pre-stimulus baseline (P < 0.05). (B) Correlative relationships between oxygen changes in two recording locations. r = coefficient of correlation calculated for the time interval of significant increase in NAc oxygen levels (values in brackets). (C) Mean (± SEM) changes in oxygen levels relative to baseline (100%) calculated with 4-s time bins. SNr data were obtained in 4 rats; 12, 9 and 9 responses were averaged for cocaine and heroin at two doses, respectively) (see Supplementary materials for ANOVA values and n in each group). Asterisks show significance of coefficients of correlation (*, ** and ***, P < 0.05, P < 0.01 and 0.001, respectively). Green line with asterisk shows significant between-structure differences in oxygen curves for the initial period following stimulus presentation.

Figure 6

Changes in oxygen levels in substantia nigra pars reticulata (SNr) induced by intravenous cocaine (1.0 mg/kg) and heroin (0.1 and 0.4 mg/kg) in awake, freely moving rats. Comparison with nucleus accumbens data. (A) Mean (± SEM) changes in oxygen levels relative to baseline (100%) calculated with 1-min time bins. Vertical hatched lines show onset and offset of stimulus presentation. Filled symbols mark values significantly different from pre-stimulus baseline. r = coefficients of correlations calculated for the time interval to the peak of NAc increase. (B) Mean (± SEM) changes in oxygen levels relative to baseline (100%) calculated with 4-s time bins. Green line with asterisk shows significant between-structure differences in oxygen responses. Asterisks show significance of coefficients of correlation (* and ***, P < 0.05, and 0.001, respectively). See Supplementary materials for quantitative results of statistical comparisons (group sizes, ANOVAs F values).