Dichotomous Dopaminergic Control of Ventral Pallidum Neurons

Abstract

The ventral pallidum (VP) is crucially involved in reward processing. Dopaminergic afferents reach the VP from the ventral tegmental area (VTA). Recent in vivo studies suggest dopamine application increase the firing in the VP. However, little is known about the cellular effects of dopamine within the VP. We aimed to address this paucity of data using brain slices containing the VP and multi-electrode array recordings. Dopamine significantly affected firing in 86% of spontaneously active VP neurons. Among the affected neurons, 84% were excited, while 16% were inhibited. The selective D1-like receptor agonist SKF81297 also had modulatory effects on the majority of VP neurons, but its effects were universally excitatory. On the other hand, the D2-like receptor agonist quinpirole had modulatory effects on 87% of VP neurons studied. It caused significant inhibitory effects in 33% of the cases and excitatory effects in the remaining 67%. The effects of D1-like receptor activation were presynaptic as blocking synaptic transmission with low Ca²⁺ abolished the effects of SKF81297 application. Furthermore, SKF81297 effects were abolished by blocking ionotropic glutamate receptors, suggesting that D1-like receptors boost glutamate release, which in turn excites VP neurons through postsynaptic glutamate receptors. Effects caused by D2-like receptor activation were found to involve pre and postsynaptic mechanisms, as low Ca²⁺ abolished the excitatory effects of quinpirole but not the inhibitory ones. Increases in firing frequency (ff) to quinpirole application were abolished by a group 2/3 mGluR antagonist, suggesting that D2-like receptors cause presynaptic inhibition of glutamate release, resulting in reduced postsynaptic activation of inhibitory mGluRs. Conversely, the inhibitory effects of quinpirole persisted in low Ca²⁺ and therefore can be attributed to postsynaptic D2-like receptor activation. VP neurons excited by dopamine had shorter spike half-widths and are excited by D1-like receptors (presynaptically) and by D2-like receptors (postsynaptically). VP neurons inhibited by dopamine have longer spike half-widths and while D1-like receptor activation has a presynaptic excitatory influence on them, D2-like receptor activation has a postsynaptic inhibitory effect that prevails, on balance. These data provide novel insights into the cellular mechanisms by which dopamine controls information processing within the VP.

Keywords: basal ganglia; dopamine; multielectrode; reward; ventral pallidum.

Figure 1

Spike half-width calculation. (A) A raw data trace of tonic neuronal activity measured in the ventral pallidum (VP). (B) An example of a typical spike recorded in the VP with the reference point used for calculation of its spike half-width, halfway between baseline and maximum negative amplitude.

Figure 2

Dopamine application has dual effects on VP neurons. (A) Excitatory response to dopamine application in a VP neuron. (B) Changes in firing frequency (ff) for 30 VP neurons in experiments similar to that illustrated in panel (A). Firing frequency was measured before and during the application of dopamine. (C) Inhibitory response to dopamine application in a VP neuron. (D) Changes in firing frequency for five VP neurons in experiments similar to those illustrated in panel (C). Firing frequency was measured before and during the application of dopamine. (E) Distribution of excitatory (yellow) and inhibitory (black) responses (% change compared to baseline) to dopamine application in VP neurons with their corresponding average values. (F) Spike half-widths (ms) for each neuron compared to % change in firing frequency in response to dopamine application. (G) Firing frequency for VP neurons excited by dopamine (yellow) and those inhibited (black). (H) Significant differences in spike half-width (ms) for VP neurons excited bydopamine application (yellow) and those inhibited (black). In this and followingfigures, *P < 0.05 and **P < 0.01.

Figure 3

D1-like receptor agonists excite while D2-like receptor agonists excite and inhibit VP neurons. (A) Excitatory response to the application of quinpirole and SKF81297 (separated by complete washout out). (B) Changes in firing frequency for 14 VP neurons induced by quinpirole application and SKF81297 application similar to that illustrated in panel (A). Firing frequency was measured; before the application of quinpirole, after the application of quinpirole and subsequently, after a period of wash out, for the application of SKF81297. Responses characterized by significant (P < 0.05) changes in firing frequency are in gray, the other ones in yellow. (C) Inhibitory response to the application of quinpirole and excitatory response to the application of SKF81297 (separated by complete washout). (D) Changes in firing frequency for seven VP neurons induced by quinpirole application and SKF81297 application similar to that illustrated in panel (C). Firing frequency was measured; before the application quinpirole, after the application of quinpirole and subsequently, after a period of wash out, for the application of SKF81297. Responses characterized by significant (P < 0.05) changes in firing frequency are in gray, the other ones in yellow. (E) The majority of neurons analyzed responded with increases in firing frequency in response to both quinpirole and SKF81297 (yellow), but a minority decreased their firing frequency in response to quinpirole and increased their firing frequency in response to SKF81297 (black). (F) Firing frequency for those neurons excited by both quinpirole and SKF81297 (yellow) and those neurons that were inhibited by quinpirole (black). (G) Coefficient of variation (CoV) for those neurons excited by both quinpirole and SKF81297 (yellow) and those inhibited by quinpirole (black). (H) Significant differences in spike half-width (ms) for those neurons excited by both quinpirole and SKF81297 (yellow) and those that were inhibited by quinpirole (black).

Figure 4

Repeated application of dopamine, D1-like receptor agonists and D2-like receptor agonists produce no clear sensitization effects in VP neurons. (A) Excitatory responses to two similar dopamine applications (separated by complete washout) in a VP neuron. (B) Inhibitory responses to two dopamine applications in a different VP neuron. (C) Changes in firing frequency for 10 VP neurons induced by repeated dopamine applications similar to that illustrated in panels (A,B). Firing frequency was measured before the first application of dopamine, during the first application of dopamine, after dopamine wash out and finally during the second application of dopamine. Responses characterized by significant (P < 0.05) changes in firing frequency are in gray, the other ones in yellow. (D) Firing frequency during first and second dopamine application for neurons excited by dopamine. (E) Firing frequency during first and second dopamine application for neurons inhibited by dopamine. (F) Excitatory responses to two quinpirole applications (separated by complete washout) in a VP neuron. (G) Inhibitory response to two quinpirole applications in a different neuron. (H) Changes in firing frequency for nine VP neurons to the repeated application of quinpirole in experiments similar to that illustrated in panels (A,B). Firing frequency was measured before the first application of quinpirole, during the application of quinpirole, after wash out, before the second application of quinpirole and finally during the second application of quinpirole. Responses characterized by significant (P < 0.05) changes in firing frequency are in gray, the other ones in yellow. (I) Firing frequency changes during first and second application of quinpirole in neurons excited by quinpirole. (J) Firing frequency changes during first and second application of quinpirole in neurons inhibited by quinpirole application. (K) Excitatory responses to two applications of SKF81297 (separated by complete washout) in a VP neuron. (L) Changes in firing frequency for seven VP neurons in response to repeated SKF81297 application in experiments similar to that illustrated in panel (A). Firing frequency was measured; before the first application of SKF81297, during the application of SKF81297, after wash out, before the second application of SKF81297 and finally during the second application of SKF81297. Responses characterized by significant (P < 0.05) changes in firing frequency are in gray, the other ones in yellow. (M) Firing frequency changes during first and second application of SKF81297 for neurons excited by SKF81297 application.

Figure 5

D1-like and D2-like receptor antagonists modulate the effect of dopamine in the VP. (A) For some neurons, when dopamine was applied in the presence of sulpiride, it produced a larger increase in firing than when it was applied in the absence of sulpiride. (B) Firing frequency changes to dopamine application in control and in the presence of sulpiride, for neurons similar to that illustrated in panel (A). (C) For some neurons, when dopamine was applied in the presence of sulpiride, it produced a smaller increase in firing than when it was applied in the absence of sulpiride. (D) Firing frequency changes to dopamine application in control and in the presence of sulpiride, for neurons similar to that illustrated in panel (C). (E) Firing frequency changes to dopamine application for neurons that showed a larger increase in firing to dopamine application (black) in the presence of sulpiride and neurons that showed a smaller increase in firing to dopamine application in the presence of sulpiride (yellow). (F) CoV for neurons that showed a larger increase in firing to dopamine in the presence of sulpiride (black) and those that showed a smaller increase in firing to dopamine in the presence of sulpiride (yellow). (G) Significant differences in spike half-width (ms) for neurons that showed a larger increase in firing to dopamine in sulpiride (black) compared to those that showed a smaller increase in firing to dopamine in sulpiride (yellow). (H) For some neurons, when dopamine was applied in the presence of SCH39166, it produced a smaller increase in firing than when it was applied in the absence of SCH39166. (I) Firing frequency changes during dopamine application and dopamine in the presence of SCH39166, for neurons similar to that illustrated in panel (H). (J) Excitatory responses to dopamine application followed (after complete washout of dopamine) by inhibitory responses to dopamine application in the presence of SCH39166. (K) Firing frequency changes during application of dopamine and dopamine in the presence of SCH39166, for neurons similar to those illustrated in panel (J). (L) Firing frequency changes to dopamine application for those neurons that showed a smaller increase in firing to the application of dopamine in the presence of SCH39166 (yellow), and those that showed a decrease in firing frequency to dopamine in the presence SCH39166 (black). (M) CoV for neurons that showed a smaller increase in firing to dopamine in the presence of SCH39166 (yellow), and those that showed a decrease in firing frequency to dopamine in the presence of SCH39166 (black). (N) Significant differences in spike half-width (ms) for those neurons that showed a smaller increase in firing to dopamine in the presence of SCH39166 (yellow), compared to those that showed a decrease in firing frequency to dopamine in the presence of SCH39166 (black).

Figure 6

Low Ca²⁺ aCSF blocks the excitatory effects of D1-like and D2-like receptor agonists but not the inhibitory effects of D2-like receptor agonists. (A) Excitatory response to SKF81297 application in standard aCSF, which is not present in low Ca²⁺. (B) Significant differences in firing frequency for SKF81297 application in low Ca²⁺ compared to the application of SKF81297 in standard aCSF. (C) Excitatory responses to quinpirole application alone were occluded in response to quinpirole application in low Ca²⁺ aCSF. (D) Significant difference in firing frequency for quinpirole application alone compared to quinpirole application in low Ca²⁺ aCSF. (E) Inhibitory responses to quinpirole application persevere to quinpirole application in the presence of low Ca²⁺ aCSF. (F) Firing frequency responses to quinpirole alone compared to quinpirole application in low Ca²⁺ aCSF.

Figure 7

D1-like receptor mediated excitation requires ionotropic glutamate receptors. (A) Excitatory responses to SKF81297 application alone that are no longer present in response to SKF81297 in the presence of NBQX and AP5. (B) Firing frequency for (ff) seven VP neurons in response to SKF81297 application and SKF81297 application in the presence of NBQX and AP5, similar to that illustrated in panel (A). Firing frequency was measured before the application of SKF81297, during the application of SKF81297, after a period of wash out, in response to the application of NBQX and AP5 and finally in response to SKF81297 in the presence of NBQX and AP5. Responses characterized by significant (P < 0.05) changes in firing frequency are in gray, the other ones in yellow. (C) Significant differences in firing frequency (ff) to SKF81297 alone compared to the application of SKF81297 in the presence of NBQX and AP5.

Figure 8

Excitatory effects of D2-like receptor agonists require metabotropic glutamate receptors. (A) Excitatory responses to quinpirole that are not present for the same neuron in response to quinpirole in the presence of MCPG. (B) Changes in firing frequency for 13 VP neurons to application of quinpirole and quinpirole in the presence of MCPG, similar to that illustrated in panel (A). Firing frequency was measured before the application of quinpirole, during the application of quinpirole, after wash out of quinpirole, during application of MCPG and finally during the application of quinpirole in the presence of MCPG. Responses characterized by significant (P < 0.05) changes in firing frequency are in gray, the other ones in yellow. (C) Significant differences in firing frequency in response to quinpirole compared to quinpirole in the presence of MCPG. (D) Excitatory responses to quinpirole that are not present for the same neuron in response to quinpirole in the presence of MSPG. (E) Changes in firing frequency for 11 VP neurons to the application of quinpirole and quinpirole in the presence of MSPG, similar to that illustrated in panel (A). Firing frequency was measured; before the application of quinpirole, during the application of quinpirole, after wash out of quinpirole, during application of MSPG and finally during the application of quinpirole in the presence of MSPG. Responses characterized by significant (P < 0.05) changes in firing frequency are in gray, the other ones in yellow. (F) Significant (P < 0.05) differences in firing frequency in response to quinpirole compared to quinpirole in the presence of MSPG.

Figure 9

Inhibitory effects of D2-like receptor agonists continue in the presence of mGluR antagonists and low Ca²⁺ aCSF. (A) Inhibitory responses to quinpirole application alone and in the presence of MCPG. (B) Changes in firing frequency for five VP neurons to application of quinpirole and quinpirole in the presence of MCPG, similar to that illustrated in panel (A). Firing frequency was measured before the application of quinpirole, during the application of quinpirole, after wash out of quinpirole, during application of MCPG and finally during the application of quinpirole in the presence of MCPG. Responses characterized by significant (P < 0.05) changes in firing frequency are in gray, the other ones in yellow. (C) Firing frequency in response to quinpirole compared to quinpirole in the presence of MCPG. (D) Inhibitory responses to quinpirole application in the presence of low Ca²⁺ aCSF are repeated in the presence of low Ca²⁺ aCSF and MCPG. (E) Changes in firing frequency for three VP neurons to application of quinpirole and quinpirole in the presence of MCPG, similar to that illustrated in panel (A). Firing frequency was measured; before the application of quinpirole, during the application of quinpirole, after wash out of quinpirole, during application of MCPG and finally during the application of quinpirole in the presence of MCPG. Responses characterized by significant (P < 0.05) changes in firing frequency are in gray, the other ones in yellow. (F) Firing frequency in response to quinpirole in low Ca²⁺ aCSF compared to quinpirole in the presence of low Ca²⁺ aCSF and MCPG.

Figure 10

D1-like receptor agonists continue to excite VP neurons in the presence of MCPG. (A) Excitatory responses to SKF81297 application alone and in the presence of MCPG. (B) Changes in firing frequency for 11 VP neurons to application of SKF81297 and SKF81297 in the presence of MCPG, similar to that illustrated in panel (A). Firing frequency was measured; before the application of SKF81297, during the application of SKF81297, after wash out of SKF81297, during application of MCPG and finally during the application of SKF81297 in the presence of MCPG. Responses characterized by significant (P < 0.05) changes in firing frequency are in gray, the other ones in yellow. (C) Firing frequency in response to SKF81297 compared to SKF81297 in the presence of MCPG.

Figure 11

A minimal model explaining the differential effects of D1-like and D2-like receptor activation on type I and type II VP neurons in the VP. (A) Presynaptic D1-like receptors facilitate glutamate release and excite type I VP neurons through increased activation of ionotropic glutamate receptors. Another glutamatergic input to type I neurons activates inhibitory metabotropic glutamate receptors. Presynaptic D2-like receptors have an inhibitory effect on this glutamate input, reducing glutamate release and disinhibiting type I neurons. Thus, both D1-like and D2-like receptor have excitatory influences on type I neurons. (B) Presynaptic D1-like receptors facilitate glutamate release and excite type II VP neurons by increasing activation of ionotropic glutamate receptors. Postsynaptic D2-like receptors have direct inhibitory effects on type II VP neurons. The net influence of dopamine on type II neurons results from the balance of excitatory D1-like receptor effects and inhibitory D2-like receptor effects.