In vivo voltammetric monitoring of catecholamine release in subterritories of the nucleus accumbens shell

Abstract

Fast-scan cyclic voltammetry (FSCV) at carbon-fiber microelectrodes has been used to demonstrate that sub-second changes in catecholamine concentration occur within the nucleus accumbens (NAc) shell during motivated behaviors, and these fluctuations have been attributed to rapid dopamine signaling. However, FSCV cannot distinguish between dopamine and norepinephrine, and caudal regions of the NAc shell receive noradrenergic projections. Therefore, in the present study, we examined the degree to which norepinephrine contributes to catecholamine release within rostral and caudal portion of NAc shell. Analysis of tissue content revealed that dopamine was the major catecholamine detectable in the rostral NAc shell, whereas both dopamine and norepinephrine were found in the caudal subregion. To examine releasable catecholamines, electrical stimulation was used to evoke release in anesthetized rats with either stimulation of the medial forebrain bundle, a pathway containing both dopaminergic and noradrenergic projections to the NAc, or the ventral tegmental area/substantia nigra, the origin of dopaminergic projections. The catecholamines were distinguished by their responses to different pharmacological agents. The dopamine autoreceptor blocker, raclopride, as well as the monoamine and dopamine transporter blockers, cocaine and GBR 12909, increased evoked catecholamine overflow in both the rostral and caudal NAc shell. The norepinephrine autoreceptor blocker, yohimbine, and the norepinephrine transporter blocker, desipramine, increased catecholamine overflow in the caudal NAc shell without significant alteration of evoked responses in the rostral NAc shell. Thus, the neurochemical and pharmacological results show that norepinephrine signaling is restricted to caudal portions of the NAc shell. Following raclopride and cocaine or raclopride and GBR 12909, robust catecholamine transients were observed within the rostral shell but these were far less apparent in the caudal NAc shell, and they did not occur following yohimbine and desipramine. Taken together, the data demonstrate that catecholamine signals in the rostral NAc shell detected by FSCV are due to change in dopamine transmission.

Figure 1

Electrically evoked catecholamine responses measured in the rostral NAc shell. (A) Representative maximal catecholamine responses in the rostral NAc shell were evoked by MFB (solid line) or VTA/SN (dotted line) stimulation (60 Hz, 24 pulses). The bars under the current traces denote the period of electrical stimulation. Insets: background-subtracted cyclic voltammograms measured during the evoked responses. (B) Solid line in the schematic diagram illustrate the approximate path of the carbon-fiber microelectrodes through the rostral NAc shell (left). The location of the carbon-fiber microelectrode tip in the rostral NAc shell (right) was visualized by the electrolytic lesion (black arrow). The coronal section is adapted from the atlas of Paxinos and Watson (2007). (C) Maximal stimulated release during electrical stimulation measured in the rostral NAc shell as the carbon-fiber microelectrode was lowered in small increments through the regions shown in B. The relative response is the concentration at particular depth (C_d^x) divided by the maximum concentration (C_d^max). The catecholamine response in the rostral NAc shell was evoked by MFB or VTA/SN stimulation (60 Hz, 24 pulses). Abbreviations used: CPu, caudate-putamen.

Figure 2

Electrically evoked catecholamine responses measured in the caudal NAc shell. (A) Representative maximal catecholamine responses at the peak current for catecholamines in the caudal NAc shell. The bars under the current traces denote the period of electrical stimulation (60 Hz, 24 pulses). Insets: background-subtracted cyclic voltammograms measured during the evoked responses. (B) Solid line in the schematic diagram illustrate the approximate path of the carbon-fiber microelectrodes in the caudal NAc shell (left). The placement of the carbon-fiber microelectrode tip in the caudal NAc shell (right) was visualized by the electrolytic lesion (black arrow). The coronal section is adapted from the atlas of Paxinos and Watson (2007). (C) Maximal stimulated release during electrical stimulation measured in the caudal NAc shell as the carbon-fiber microelectrode was lowered in small increments through the regions shown in B. The relative response is the response at particular depth (C_d^x) divided by the maximum concentration (C_d^max). The catecholamine response in the caudal NAc shell was evoked by MFB or VTA/SN stimulation (60 Hz, 24 pulses). Abbreviations used: CPu, caudate-putamen.

Figure 3

Effect of yohimbine (Yo, 5 mg/kg), desipramine (DMI, 15 mg/kg), raclopride (Ra, 2 mg/kg) and cocaine (Co, 15 mg/kg) on catecholamine overflow in the rostral NAc shell. Representative recordings of extracellular catecholamine in the rostral NAc shell evoked by MFB (left) or VTA/SN stimulation (right) in the presence of (A) Yo (---) and Yo + DMI (…), (B) Ra (---) and Ra + Co (…) at 60 Hz with 24 pulses. The bars under the current traces indicate the period of electrical stimulation.

Figure 4

Effect of yohimbine (Yo, 5 mg/kg), desipramine (DMI, 15 mg/kg), raclopride (Ra, 2 mg/kg) and cocaine (Co, 15 mg/kg) on catecholamine overflow in the caudal NAc shell. Representative recordings of extracellular catecholamine in the rostral shell evoked by MFB stimulation (left) or VTA/SN (right) in the presence of (A) Yo (---) and Yo + DMI (…), (B) Ra (---) and Ra + Co (…) at 60 Hz with 24 pulses. The bars under the current traces indicate the period of electrical stimulation.

Figure 5

Drug induced catecholamine concentration transients in the rostral and caudal NAc shell after raclopride (2 mg/kg, i.p.) and cocaine (15 mg/kg, i.p.). Two-dimensional color plot representation of the background-subtracted cyclic voltammograms collected over 30 sec (A) before and (B) following administration of drugs in the rostral and caudal NAc shell. Catecholamine concentration changes are apparent in the color plots (B) at the potential for its oxidation (~ 0.65 V) and its reduction (−0.2 V). Principal component regression was used to extract the time course of the catecholamine concentration transients (lower traces in A and B). Times are indicated by the vertical bars. Inset: Cyclic voltammograms recorded at the time indicated by the arrows were identical to those for a catecholamine.