Relationships between the Prefrontal Cortex and the Basal Ganglia in the Rat: Physiology of the Cortico-Nigral Circuits

Effects of PL/MO stimulation on the activity of SNR cells

Responses evoked by electrical stimulation of PL/MO areas were investigated in 156 SNR cells recorded in 11 rats. As shown in Figure1 and Table1, PL/MO stimulation induced, in 80 cells, an inhibition preceded or not by an early excitation and followed or not by a late excitation. In 30 cells (37.5%), responses consisted of an inhibition [latency (L), 20.2 ± 0.5 msec; duration (D), 14.5 ± 1.0 msec] preceded by an early excitation (L, 8.6 ± 0.4 msec) and followed by a late excitation (L, 37.8 ± 1.1 msec). In eight cells (10%), the inhibition (L, 19.3 ± 2.0 msec; D, 16.1 ± 2.5 msec) was not preceded by an early excitation but was followed by a late excitation (L, 36.6 ± 2.2 msec). In 32 cells (40%), the inhibition (L, 23.0 ± 0.6 msec; D, 24.9 ± 2.0 msec) was preceded by an early excitation (L, 8.7 ± 0.4 msec) but was not followed by a late excitation. An inhibition without early or late excitations was observed in 10 cells (12.5%; L, 22.9 ± 1.1 msec; D, 23.0 ± 4.5 msec). Interestingly, the inhibition was significantly shorter (40.2%;p < 0.001) in cells with a late excitation (D, 14.6 ± 0.8 msec) than in cells without a late excitation (D, 24.4 ± 1.8 msec).

• Download figure
• Open in new tab
• Download powerpoint

Fig. 1.

Patterns of responses evoked by PL/MO stimulation within SNR cells. The inhibition is preceded by an early excitation and is followed (A) or not (B) by a late excitation; the inhibition is not preceded by an early excitation and is followed (D) or not (C) by a late excitation.

All these responding cells were located in the dorsomedial part of the SNR (Fig. 2). Within this SNR region, no obvious topographical distribution of the cells with distinct types of responses could be observed. In addition, it should be noted that in 6 of the 156 cells tested, an excitation with a mean latency of 25.4 ± 1.7 msec was observed instead of an inhibition. Finally, cells with no response to PL/MO stimulation (70 of the 156 cells tested) were mainly located more laterally and ventrally in the SNR.

• Download figure
• Open in new tab
• Download powerpoint

Fig. 2.

Localization of SNR cells that responded to PL/MO stimulation. Note that responding cells are located in the medial part of the SNR, whereas more laterally located cells do not respond to PL/MO stimulation (Not responding cells). Eachdot represents a tested cell. Numbersindicate the distance, in millimeters, from the interaural line.cp, Cerebral peduncle; ml, medial lemniscus; SNC, substantia nigra pars compacta.

Effect of CNQX application into the NAcc on nigral responses evoked by PL/MO stimulation

The effect of a blockade of the glutamatergic cortico-striatal transmission by application of CNQX into the NAcc was examined in 13 SNR cells (eight rats) exhibiting a response to PL/MO stimulation. In control conditions, these cells presented the following patterns of responses: excitation–inhibition–excitation (four cells), excitation–inhibition (six cells), inhibition only (two cells), and inhibition–excitation (one cell).

In all these cells, the inhibition disappeared 8–25 min after the beginning of CNQX application (Fig. 3). In eight cells held long enough, the recovery of the inhibition occurred 25–65 min after the cessation of CNQX application. In addition, the late excitatory response observed in five cells in control conditions was markedly reduced under CNQX, with the maximal effect (63–97% decrease; p < 0.001) being observed 10–25 min after the beginning of the drug application (Fig. 3). In the three cells held long enough, the recovery of the late excitatory response was observed 40–65 min after the cessation of drug application. In contrast (Fig. 3), the CNQX treatment did not modify the early excitation that preceded the inhibition (10 cells).

• Download figure
• Open in new tab
• Download powerpoint

Fig. 3.

Effect of CNQX application into the NAcc on the response evoked by PL/MO stimulation in an SNR cell. Fromtop to bottom, The response exhibited a triphasic excitation–inhibition–excitation sequence in control conditions; under CNQX application into the NAcc, the early excitation was not affected, the inhibition disappeared, and the late excitation was markedly decreased. The maximal effect was observed 10 min after the beginning of CNQX application, and the recovery of the inhibition and of the late excitation occurred 40 min after the cessation of CNQX application. Each poststimulus time histogram represents 100 superimposed sweeps. Arrow indicates the stimulation artifact.

Effect of bicuculline application into the VP on nigral responses evoked by PL/MO stimulation

Bicuculline was applied into the VP to block the GABAergic transmission of the NAcc–VP pathway. The effect of bicuculline on the responses evoked by PL/MO stimulation was examined in 10 cells (eight rats). In control conditions, PL/MO stimulation induced an inhibition preceded by an early excitation and followed by a late excitation. In 9 of these 10 cells, bicuculline markedly reduced the late excitation, with the maximal effect (70–100% decrease; p < 0.001) being observed 8–35 min after the beginning of the drug application (Fig. 4). In eight cells held long enough, the recovery of the late excitation occurred 40–75 min after the cessation of bicuculline application. Interestingly, the disappearance of the late excitation was associated with a significant increase in the duration of the inhibition (control conditions: D, 15.8 ± 0.5 msec; bicuculline treatment: D, 24.4 ± 2.8 msec; mean increase, 54%; p < 0.01) without a significant change of the latency (control: L, 20.7 ± 0.6 msec; bicuculline: L, 21.3 ± 0.6 msec). Finally, bicuculline treatment did not affect the early excitatory response (Fig. 4).

• Download figure
• Open in new tab
• Download powerpoint

Fig. 4.

Effect of bicuculline application into the VP on the response evoked by PL/MO stimulation in an SNR cell. Fromtop to bottom, The response exhibited a triphasic excitation–inhibition–excitation sequence in control conditions; under bicuculline application into the VP, the early excitation was not modified, the duration of the inhibition was slightly increased, and the late excitation was markedly decreased. The maximal effect was observed 10 min after the beginning of bicuculline application, and the recovery of the late excitation occurred 60 min after the cessation of bicuculline application. Each poststimulus time histogram represents 100 superimposed sweeps. Arrowindicates the stimulation artifact.

Effect of CNQX application into the VP on nigral responses evoked by PL/MO stimulation

CNQX was applied into the VP to block the glutamatergic input from the STN and thus to investigate the possible influence of the subthalamo-pallidal loop on nigral responses evoked by PL/MO stimulation. In control conditions, the patterns of responses observed in the six SNR cells tested (four rats) were as follows: excitation–inhibition–excitation (five cells) and inhibition–excitation (one cell). In all cells, CNQX treatment markedly enhanced the late excitatory response (Fig.5). The duration of the late excitation increased from 13.0 ± 1.7 msec (control conditions) to 31.2 ± 2.8 msec (under CNQX; p < 0.001), although its latency was slightly decreased (control: L, 37.5 ± 1.1 msec; CNQX: L, 34.0 ± 1.0 msec; p < 0.05). The maximal effect (67–410% increase in the number of spikes during the excitatory period) occurred 15–35 min after the beginning of CNQX application, and a recovery of the response was observed 70–85 min after the cessation of the drug application in the three cells held long enough. Interestingly, this CNQX treatment slightly reduced the duration of the inhibition (control: D, 13.8 ± 1.2 msec; CNQX: D, 10.5 ± 1.0 msec; p < 0.05). Finally, the CNQX application did not significantly modify the early excitatory response recorded in control conditions (five cells).

• Download figure
• Open in new tab
• Download powerpoint

Fig. 5.

Effect of CNQX application into the VP on the response evoked by PL/MO stimulation in an SNR cell. Fromtop to bottom, The response exhibited a triphasic excitation–inhibition–excitation sequence in control conditions; under CNQX application into the VP, the early excitation was not significantly modified, the inhibition was still observed, and the late excitation was markedly increased. The maximal effect was observed 20 min after the beginning of CNQX application, and the recovery of the late excitation occurred 75 min after the cessation of CNQX application. Note that the duration of the inhibition and the latency of the late excitation were decreased under CNQX. Each poststimulus time histogram represents 100 superimposed sweeps.Arrow indicates the stimulation artifact.

Effect of CNQX application into the STN on nigral responses evoked by PL/MO stimulation

The effect of blockade of the glutamatergic cortico-subthalamic transmission by CNQX injection into the STN was examined in eight SNR cells (six rats) exhibiting a response to PL/MO stimulation. In control conditions, the responses evoked by PL/MO stimulation consisted of an inhibition preceded by an early excitation (eight cells) and followed (six cells) or not (two cells) by a late excitation. In all cases, the CNQX injection markedly decreased the early excitation (Fig.6). The maximal effect (52–91%) was observed 5–10 min after the drug application, and the recovery of the response occurred 35–50 min later in the five cells held long enough (Fig. 6). In addition, CNQX increased the duration of the inhibition (control: D, 20.0 ± 3.8 msec; CNQX: D, 44.5 ± 9.8;p < 0.05) and induced a disappearance of the late excitation (six cells).

• Download figure
• Open in new tab
• Download powerpoint

Fig. 6.

Effect of CNQX application into the STN on the response evoked by PL/MO stimulation in an SNR cell. Fromtop to bottom, The response exhibited a triphasic excitation–inhibition–excitation sequence in control conditions; under CNQX application into the STN, the early excitation was markedly decreased, the duration of the inhibition was increased, and the late excitation disappeared. The maximal effect was observed 10 min after the CNQX application, and the recovery occurred 50 min after CNQX application. Each poststimulus time histogram represents 100 superimposed sweeps. Arrow indicates the stimulation artifact.

Role of the disynaptic cortico-subthalamo-nigral pathway

The cortico-subthalamic and subthalamo-nigral projections are glutamatergic and form asymmetrical synaptic contacts with their target neurons in the STN and the SNR (Smith et al., 1998). Based on the conduction time of these projections, it has been proposed that short latency excitations evoked in SNR cells by stimulation of the sensorimotor cortex are mediated through the disynaptic cortico-subthalamo-nigral pathway (Kita, 1994). Accordingly, the early excitations evoked in the SNR by frontal agranular cortex stimulation are no longer observed after excitotoxic lesion of the STN (Ryan and Sanders, 1994). Our data indicate the existence of a similar functional link between PL/MO areas and the medial SNR. Indeed, the latency of the early excitatory responses evoked by PL/MO stimulation in SNR cells is in the range of the conduction time of the disynaptic cortico-subthalamo-nigral pathway, and PL/MO stimulation induces a short latency excitation in STN cells projecting to the SNR (Maurice et al., 1998b). In addition, the early nigral excitatory responses evoked by PL/MO stimulation disappeared after CNQX application into the STN but persisted after the blockade of the cortico-NAcc transmission. This effect cannot be attributed to a diffusion of CNQX into the SNR because, in contrast to the reduced firing of nigral cells reported after local application of glutamatergic antagonists into the SNR (Robledo and Féger, 1990), the spontaneous activity of SNR cells was not modified (20.6 ± 3.4 Hz, before CNQX; 19.2 ± 2.2 Hz, under CNQX).

Role of the direct NAcc-SNR pathway

We have previously shown the existence of a functional link between the PL/MO areas and the NAcc neurons that innervate the SNR. Indeed, stimulation of PL/MO areas induces excitatory responses in NAcc neurons projecting to the SNR (Montaron et al., 1996), and stimulation of the NAcc inhibits the activity of SNR cells (Deniau et al., 1994). The present data demonstrate that the inhibitory responses observed in SNR cells after PL/MO stimulation result from the activation of the direct NAcc–SNR pathway: (1) the inhibitory responses have a latency compatible with the conduction time of the PL/MO–NAcc–SNR pathway and a duration similar to that observed after NAcc stimulation (Deniau et al., 1994); (2) the inhibitory responses were observed in cells located in the dorsomedial part of the SNR, in agreement with the distribution of NAcc projections (Montaron et al., 1996); and (3) finally, the inhibitory responses disappeared after acute blockade of the glutamatergic cortico-striatal neurotransmission by CNQX application into the NAcc, although they persisted after blockade of the striato-pallidal or cortico-subthalamic transmissions. It should be noted that the duration of CNQX application required to obtain a maximal effect in the SNR presented some variability. This is likely to be related to the diffusion time necessary for CNQX to produce a blockade of the synaptic transmission in the whole NAcc territory involved in the PL/MO–NAcc–SNR circuit.

Role of the indirect NAcc–SNR pathway

The indirect striato-nigral pathway, involving the external pallidum and the STN, has been proposed to exert an excitatory influence on the SNR (Alexander et al., 1986). Activation of striato-pallidal neurons inhibits the tonic discharge of the GABAergic pallidal neurons that project to the STN and consequently should lead to a disinhibition of STN. In turn, disinhibition of STN would result in an increased firing of SNR cells because STN–SNR projections are glutamatergic and excitatory.

The present data, along with our previous study (Maurice et al., 1998a), demonstrate that the late excitatory responses that follow the inhibition evoked in SNR cells by PL/MO stimulation are a result of the activation of the indirect NAcc–SNR pathway and result from the disinhibition of the STN. Indeed, the late nigral excitatory responses disappeared after blockade of either cortico-NAcc transmission by CNQX application into the NAcc or NAcc–VP transmission by bicuculline application into the VP. These treatments have also been shown to block the disinhibition of STN cells evoked by PL/MO stimulation (Maurice et al., 1998a). In addition, the latency of the late excitatory responses is in the range of the conduction time of the multisynaptic circuit linking the PL/MO areas to the SNR via the indirect basal ganglia pathway (Montaron et al., 1996; Maurice et al., 1997, 1998a,b).

Role of the STN–VP–STN loop

The STN and pallidum are two tightly interconnected structures, and pallido-subthalamic connections are topographically organized such that the GP and VP project to the STN region from which they receive an input (Groenewegen and Berendse, 1990; Smith et al., 1998). Electrophysiological data suggest that activation of STN cells projecting to the pallidum by cortical inputs leads to a feedback inhibition of the STN (Ryan and Clarke, 1991; Fujimoto and Kita, 1993). Confirming this hypothesis, we have recently shown that disinhibition of STN cells by PL/MO stimulation is markedly increased after blockade of the STN–VP glutamatergic transmission by CNQX application into the VP (Maurice et al., 1998a). The present data show that the late excitatory responses induced by PL/MO stimulation in SNR cells are also markedly increased after CNQX application into the VP. Together, these data indicate that the VP–STN feedback circuit modulates the late excitatory responses of SNR cells to PL/MO stimulation and further confirm that these late excitations result from the disinhibition of the STN.

Functional considerations

The terminals from the striatal and subthalamic afferent pathways form convergent synaptic contacts with individual neurons in the SNR (Smith et al., 1998). Accordingly, the present electrophysiological study shows that the trans-striatal and trans-subthalamic pathways related to PL/MO areas of the prefrontal cortex exert a converging synaptic influence on medial SNR cells. In addition, our data indicate that the inhibition exerted by the direct NAcc–SNR pathway on nigral cells is under the control of the trans-subthalamic pathways. The duration of this inhibition was decreased after CNQX application into the VP, a treatment that blocks the feedback STN–VP–STN inhibitory loop and thus enhances the disinhibition of the STN. In contrast, the duration of the inhibition was increased after bicuculline application into the VP, which blocks the disinhibition of the STN and consequently the late excitatory responses in the SNR. A more prolonged lengthening of the inhibition was observed after CNQX application into the STN, which blocks the cortico-subthalamic pathway. The marked lengthening of the inhibition cannot just be explained by the concomitant disappearance of the late excitatory responses but is likely to result from an increased inhibitory influence of the direct NAcc–SNR pathway. Because the VP receives excitatory inputs from the STN and sends GABAergic projections to the NAcc (Groenewegen et al., 1993), the blockade of the glutamatergic excitatory inputs to STN might lead to a disinhibition of the NAcc and thus would facilitate the activation of the NAcc–SNR pathway by PL/MO stimulation. On the other hand, the presence of metabotropic glutamate receptors of the group III (mGluR7) on GABAergic terminals in the SNR have recently been described (Kosinski et al., 1998). Assuming a presynaptic inhibitory influence of mGluR7 on transmitter release (Conn and Pin, 1997), it can be proposed that blockade of the activation of the glutamatergic subthalamo-nigral pathway by CNQX application into the STN could have reduced the inhibitory effect of mGluR7 receptors on GABA release, resulting in an increased inhibitory influence of the direct NAcc–SNR pathway.

In motor circuits of the basal ganglia, the activation of the direct striato-nigral GABAergic pathway, by inhibiting the tonically active GABAergic projection neurons of the SNR, leads to a disinhibition of their target nuclei in thalamus and brainstem (Chevalier and Deniau, 1990). This disinhibitory process very likely increases the excitability of the frontal cortex and is central to the physiology of the basal ganglia. It has been proposed that the trans-subthalamic pathways, by their excitatory influence on the SNR, participate in the spatio-temporal shaping of this disinhibitory process and thus contributes to the scaling of movements and inhibition of competing motor programs (Mink and Thach, 1993). An imbalance between the direct striato-nigral and the trans-subthalamic pathways is considered to be responsible for motor disorders, such as akinesia and dyskinesia (Albin et al., 1989; Chesselet and Delfs, 1996; Obeso et al., 1997). Similarly, in the PL/MO circuits of the basal ganglia, the present data show that SNR cells receive a direct inhibitory input from the NAcc and that the trans-subthalamic excitatory pathways participate in the shaping of the discharge of nigral output neurons. By analogy with motor circuits, it can be proposed that an imbalance between these pathways could be responsible for perturbation in prefrontal functions, such as perseveration and alterations in attentional and emotional processes.