Dynamics of synaptic transmission between fast-spiking interneurons and striatal projection neurons of the direct and indirect pathways

Abstract

The intrastriatal microcircuit is a predominantly inhibitory GABAergic network comprised of a majority of projection neurons [medium spiny neurons (MSNs)] and a minority of interneurons. The connectivity within this microcircuit is divided into two main categories: lateral connectivity between MSNs, and inhibition mediated by interneurons, in particular fast spiking (FS) cells. To understand the operation of striatum, it is essential to have a good description of the dynamic properties of these respective pathways and how they affect different types of striatal projection neurons. We recorded from neuronal pairs, triplets, and quadruplets in slices of rat and mouse striatum and analyzed the dynamics of synaptic transmission between MSNs and FS cells. Retrograde fluorescent labeling and transgenic EGFP (enhanced green fluorescent protein) mice were used to distinguish between MSNs of the direct (striatonigral) and indirect (striatopallidal) pathways. Presynaptic neurons were stimulated with trains of action potentials, and activity-dependent depression and facilitation of synaptic efficacy was recorded from postsynaptic neurons. We found that FS cells provide a strong and homogeneously depressing inhibition of both striatonigral and striatopallidal MSN types. Moreover, individual FS cells are connected to MSNs of both types. In contrast, both MSN types receive sparse and variable, depressing and facilitating synaptic transmission from nearby MSNs. The connection probability was higher for pairs with presynaptic striatopallidal MSNs; however, the variability in synaptic dynamics did not depend on the types of interconnected MSNs. The differences between the two inhibitory pathways were clear in both species and at different developmental stages. Our findings show that the two intrastriatal inhibitory pathways have fundamentally different dynamic properties that are, however, similarly applied to both direct and indirect striatal projections.

Figure 1.

Patch recordings from MSN and FS striatal neurons. A, Infrared microscopy image of a multineuron patch experiment. B, Biocytin-filled pair of MSNs (rat; 18 d of age) stained for light microscopy. The enlarged image (B*) shows dendritic spines characteristic of MSNs. C, FS interneuron, biocytin loaded and stained for fluorescent microscopy with aspiny beaded dendrites (rat; 15 d of age). D, Example of the current–voltage relationship obtained from an MSN (black squares) and FS cell (red triangles). Note the rectification in the MSN, apparent from the change in curve slope. E, Response of an MSN to increasing step current injections showing the rectification in hyperpolarized steps, as well as characteristic ramp and delay preceding the action potential. F, A typical response of an FS cell to the same stimulation protocol as in E, showing high discharge rate, nonaccommodating discharge pattern, and the fast and deep afterhyperpolarization. Scale bars, 25 μm.

Figure 2.

Synaptic connections formed by MSNs and FS cells onto MSNs. A, Two examples of synaptic connectivity between MSNs. The presynaptic MSN was stimulated with a 20 Hz train of action potentials (top trace; black) and postsynaptic responses (bottom traces; blue) were recorded in postsynaptic MSNs. The middle trace shows a facilitating synaptic response, whereas the bottom one is of a depressing synapse, recorded from a different MSN. B, Synaptic connectivity from FS to MSN. An example of divergent connection from an FS interneuron (green) onto two target MSNs (blue and black). C, Synaptic amplitude of the two types of synapses, measured at the first synaptic response in the train (MSN–MSN connections in black, n = 31; FS–MSN in gray, n = 23). The difference was significant, with p = 0.02, Student's t test. D, Paired-pulse depression in the respective pathways, as calculated from the amplitudes of the first and second responses (p = 0.03, t test). The average paired-pulse ratio of MSN–MSN pairs was larger because of the occurrence of depressing and facilitating synapses, which were absent in FS–MSN connections. *p < 0.05, Student's t test. Error bars indicate SEM.

Figure 3.

Differential synaptic dynamics of feedback and feedforward connections. A, An example of converging MSN–MSN connectivity. A postsynaptic MSN (M3) received direct connections from two neighboring MSNs (M1 and M2). The connection was tested in different train frequencies (10, 20, 40 Hz trains of 8 action potentials), showing frequency-dependent depression and facilitation. Note the increase in amplitude of the recovery test response (denoted by blue arrows), revealing the underlying facilitatory component. B, Examples of divergent synaptic connectivity from FS to two neighboring MSNs as recorded in voltage clamp (red traces, bottom). Note the depression of the recovery test response after 0.55 s after a 20 Hz train. The top traces are of the same connections as in A, acquired in voltage-clamp mode. C, MSNs receive different types of input from neighboring MSNs and FS cells. Two examples of synaptic responses to 20 Hz trains (green) and a recovery response are depicted. Note the facilitation of the recovery response in the MSN→MSN connection (black) compared with the depression of the FS→MSN response (red). D, Average responses of all analyzed connections normalized to the amplitude of the first PSP (FS–MSN connections in red, n = 23; MSN–MSN in black, n = 31). E, Facilitation and depression time constants of all analyzed connections are plotted against each other in a logarithmic plot. The dashed line represents the F = D curve, showing that all FS→MSN, but not all MSN→MSN connections, were depressing. F, The synaptic dynamics of the two connection types are significantly different, as seen by the values of the time constants for facilitation (F) and depression (D), and the ratio (right bar graph). ***p < 0.001, Student's t test. Error bars indicate SEM.

Figure 4.

Fluorescent identification of direct-pathway MSNs. A, Retrograde labeling of rat striatonigral MSNs by injection of fluorescent beads into the substantia nigra pars reticulata. The beads are transported by the axons to the cell bodies and can then be visualized under fluorescence microscopy. B, Individual striatal neurons after retrograde labeling. The red arrows designate retrogradely labeled striatonigral MSNs, and the white arrows show the position of unlabeled neurons, which are MSNs or interneurons. C, The same neurons as in B under infrared microscopy. D, Confocal image of the striatum and substantia nigra pars reticulata of a BAC transgenic D₁-EGFP mouse (image by courtesy of Emmanuel Valjent and Gilberto Fisone, Karolinska Institute, Stockholm, Sweden). E, Individual striatal neurons in slices of EGFP mouse. The labeled neurons are visible under epifluorescence microscopy and can be selected for recording. F, The same neurons as in E, under IR microscopy. The red arrows designate the fluorescent neurons selected to be recorded under IR optics. Scale bars, 50 μm. SNr, Substantia nigra pars reticulata; Str, striatum; Cx, cortex.

Figure 5.

Synaptic connections between EGFP-identified MSNs. A, A network of neighboring MSNs in ventral striatum displaying divergent and convergent synaptic connections. Cell 3 is a direct-pathway MSN (dM) receiving convergent input from two indirect MSNs (iM) with different dynamic properties. Note the facilitation in the 1→3 connection (top trace) compared with the depressing connection received by cell 3, and the facilitating connections onto both targets of cell 1. B, Average amplitudes, normalized to the amplitude of the first PSP, for connections from iMSNs onto dMSNs (in green; n = 10) and iMSNs (in red; n = 7). Note the facilitation of the recovery test response, which is absent in FS→MSN connections (Figs. 3, 6). C, Facilitation and depression time constants of connections between identified MSNs are plotted against each other in a logarithmic plot. iMSN→iMSN connections are marked with red triangles, iMSN→dMSN in green squares, presynaptic dMSN connections in gray circles, and FS→MSN connections in black “X.” D, Synaptic properties of connections from presynaptic iMSNs did not significantly differ according to the postsynaptic MSN type and were not significantly different from connections between MSNs in which presynaptic neurons were dMSNs. Error bars indicate SEM.

Figure 6.

Synaptic connections from FS cells onto both types of MSNs. A, An example of a divergent connection from a single FS cell onto three target MSNs of different projection types. Note the similarity in the dynamics of the responses on the different MSN types. B, Average amplitudes normalized to the amplitude of the first PSP, for connections from FS onto iMSNs (in red; n = 6) and dMSNs (in green; n = 9). C, Synaptic properties of connected FS cells did not differ significantly according to the postsynaptic MSN type. Both B and C show the high degree in homogeneity in the FS→MSN pathway onto both projection types of MSNs. Error bars indicate SEM.