Striatal extracellular dopamine levels in rats with haloperidol-induced depolarization block of substantia nigra dopamine neurons

Abstract

Correlations between substantia nigra (SN) dopamine (DA) cell activity and striatal extracellular DA were examined using simultaneous extracellular single-unit recordings and in vivo microdialysis performed in drug-naive rats and in rats treated repeatedly with haloperidol (HAL). Intact rats treated with HAL for 21-28 d exhibited significantly fewer active DA cells, indicating the presence of depolarization block (DB) in these cells. However, in rats that received surgical implantation of the microdialysis probe followed by a 24 hr recovery period, HAL-induced DA cell DB was reversed, as evidenced by a number of active DA neurons that was significantly higher than that in HAL-treated intact rats and similar to that of drug-naive rats. In contrast, using a modified probe implantation procedure that did not reverse SN DA neuron DB, we found striatal DA efflux to be significantly lower than in controls and significantly correlated with the reduction in DA neuron spike activity. Furthermore, although basal striatal DA efflux was independent of SN DA cell burst-firing activity in control rats, these variables were significantly correlated in rats with HAL-induced DA cell DB. Therefore, HAL-induced DB of SN DA neurons is disrupted by implantation of a microdialysis probe into the striatum using standard procedures. However, a modified microdialysis method that allowed reinstatement of DA neuron DB revealed that the HAL-induced inactivation of SN DA neurons was associated with significantly lower extracellular DA levels in the striatum. Moreover, the residual extracellular DA maintained in the presence of DB may, in part, depend on the burst-firing pattern of the noninactivated DA neurons in the SN.

Fig. 1.

Microdialysis probe placements and location of recording electrode track. Top, Photomicrograph of a Nissl-stained sagittal section showing the recording electrode track terminating in the substantia nigra pars compacta and the guide cannula track terminating just ventral to the corpus callosum. Extending ventrally from the guide cannula track is the dorsal extent of the smaller track made by the dialysis probe. The probe terminated medial to this plane at ∼1.9 mm lateral to the midline and 5–6 mm ventral to the skull surface (represented in the bottom panel). The relatively intensely stained cells near the termination of the electrode track indicate that the electrode tip was in the region of the SN DA cells. Bottom, Schematic drawings of coronal sections (Paxinos and Watson, 1997) showing the placements of the microdialysis probes in the striatum. Vertical bars represent probe tips, with HAL-treated and drug-naive rats represented in the left and righthemispheres, respectively. Probe tips used in the 24 hr probe condition are represented by the narrower, longer bars, whereas the wider, shorter bars represent probes in the chronic guide cannula condition.Hatched bars show placements in subjects in which extracellular DA levels were measured.

Fig. 2.

Effects of microdialysis probe or guide cannula implantation on the number of spontaneously firing DA cells in drug-naive and chronic HAL-treated rats. Top, The number of spontaneously active cells per electrode track in drug-naive and HAL-treated rats in the intact (open bars), 24 hr probe (solid bars), and chronic guide cannula (hatched bars) groups. Values represent between-animal means and SEMs. HAL-treated intact rats exhibited significantly fewer active DA cells per track than did the drug-naive group (*p < 0.01). In the 24 hr probe group, the number of active DA cells was significantly higher than that in the HAL-treated intact group and was not different than that in drug-naive animals (†p< 0.01). In contrast, HAL-treated animals in the chronic guide cannula group showed a lower number of active DA cells in the SN compared with that in drug-naive intact rats (‡p < 0.001) but were not different than the HAL-treated, intact animals.Bottom, The acute effect of microdialysis probe insertion on SN DA cell activity in rats with HAL-induced DA cell DB. In a subset of the intact HAL-treated group, SN DA cell activity was measured before and after insertion of a microdialysis probe into the striatum. As shown in the top, in the intact hemisphere of HAL-treated rats, few SN DA cells were found to be spontaneously active (left bar). Within 2 hr of striatal probe implantation into the opposite hemisphere, a significantly greater number of DA cells in the SN ipsilateral to the probe were spontaneously active (right bar), relative to the control hemisphere (§p < 0.05).

Fig. 3.

Effects of microdialysis probe or guide cannula implantation on firing rate and burst firing in drug-naive and HAL-treated rats. Presented are means and SEMs from drug-naive and HAL-treated rats that were in the intact (open bars), 24 hr probe (solid bars) or chronic guide cannula (open hatched bars) groups and from HAL-treated rats in which the SN was sampled during the first 2 hr after striatal probe implantation (shaded hatched bar; from Fig. 2,bottom). Top, Average firing rate of all active DA cells sampled in six to nine tracks. Neither drug treatment nor microdialysis probe implantation significantly affected the average firing rate of DA cells in the SN. Bottom, Burst firing. The percentage of spikes fired in bursts in the HAL-treated 24 hr probe group was significantly higher than that in the HAL-treated intact group (*p < 0.05).

Fig. 4.

Striatal DA efflux measured in untreated and HAL-treated rats using the chronic guide cannula procedure.Top, Basal striatal DA efflux in drug-naive (control) and HAL-treated rats. HAL-treated rats showed significantly lower levels of extracellular DA in the striatum (*p < 0.05, relative to drug-naive and chronic guide cannula group).Bottom, Effects of systemic baclofen on basal striatal DA efflux in drug-naive and chronic HAL-treated rats. There was a trend (p = 0.09) for systemic administration of a low dose of the GABA_B agonist to affect differentially DA efflux in the two groups. This effect is consistent with a baclofen-induced reversal of SN DA cell DB in the HAL-treated rats.

Fig. 5.

Correlations (indicated by regression lines) between basal striatal DA efflux and nigral DA cell electrophysiological activity measured simultaneously in drug-naive (triangles) and HAL-treated (circles) rats. Top, Inset, Population firing rate in drug-naive (open bar) and HAL-treated (solid bar) rats (*p < 0.05, relative to drug-naive group). Main, Correlation between striatal DA efflux and population firing rate in all animals. DA efflux was significantly correlated with population firing rate (Pearson’sr = 0.50; p < 0.05).Bottom, Inset, Population burst firing in drug-naive (open bar) and HAL-treated (solid bar) rats. Main, Correlations between striatal DA efflux and population burst firing in drug-naive (triangles and dotted regression line) and HAL-treated (circles and solid regression line) rats. The correlation in drug-naive rats was not significant (Pearson’s r = 0.30; p= 0.31), whereas it was significant (Pearson’s r = 0.64; p < 0.05) for the HAL-treated rats. Thus, basal striatal DA efflux appeared to depend on firing rate in both untreated and HAL-treated rats; however, only in HAL-treated rats did basal DA efflux appear also to depend on burst-firing activity.