How glutamate regulates dopamine (DA) release in striatum has been a controversial issue. Here, we resolve this by showing that glutamate, acting at AMPA receptors, inhibits DA release by a nonclassic mechanism mediated by hydrogen peroxide (H(2)O(2)). Moreover, we show that GABA(A)-receptor activation opposes this process, thereby enhancing DA release. The influence of glutamate and GABA on DA release was assessed in striatal slices using carbon-fiber microelectrodes and fast-scan cyclic voltammetry. Modulation by both transmitters was prevented by H(2)O(2)-metabolizing enzymes. In addition, the influence of GABA(A)-receptor activation was lost when AMPA receptors were blocked with GYKI-52466. Together, these data show that modulation of DA release by glutamate and GABA depends on H(2)O(2) generated downstream from AMPA receptors. This is the first evidence that endogenous glutamate can lead to the generation of reactive oxygen species under physiological conditions. We also show that inhibition of DA release by H(2)O(2) is mediated by sulfonylurea-sensitive K(+) channels: tolbutamide blocked DA modulation by glutamate and by GABA. The absence of ionotropic glutamate or GABA receptors on DA terminals indicates that modulatory H(2)O(2) is generated in non-DA cells. Thus, in addition to its known excitatory actions in striatum, glutamate mediates inhibition by generating H(2)O(2) that must diffuse from postsynaptic sites to inhibit presynaptic DA release via K(+)-channel opening. These findings have significant implications not only for normal striatal function but also for understanding disease states that involve DA and oxidative stress, including disorders as diverse as Parkinson's disease and schizophrenia.

In contrast to the uniform anatomy of the cerebellar cortex, molecular and physiological studies indicate that significant differences exist between cortical regions, suggesting that the spiking activity of Purkinje cells (PCs) in different regions could also show distinct characteristics. To investigate this possibility we obtained extracellular recordings from PCs in different zebrin bands in crus IIa and vermis lobules VIII and IX in anesthetized rats in order to compare PC firing characteristics between zebrin positive (Z+) and negative (Z−) bands. In addition, we analyzed recordings from PCs in the A2 and C1 zones of several lobules in the posterior lobe, which largely contain Z+ and Z− PCs, respectively. In both datasets significant differences in simple spike (SS) activity were observed between cortical regions. Specifically, Z− and C1 PCs had higher SS firing rates than Z+ and A2 PCs, respectively. The irregularity of SS firing (as assessed by measures of interspike interval distribution) was greater in Z+ bands in both absolute and relative terms. The results regarding systematic variations in complex spike (CS) activity were less consistent, suggesting that while real differences can exist, they may be sensitive to other factors than the cortical location of the PC. However, differences in the interactions between SSs and CSs, including the post-CS pause in SSs and post-pause modulation of SSs, were also consistently observed between bands. Similar, though less strong trends were observed in the zonal recordings. These systematic variations in spontaneous firing characteristics of PCs between zebrin bands in vivo, raises the possibility that fundamental differences in information encoding exist between cerebellar cortical regions.

Synchronous complex spike (CS) activity occurs most often among cerebellar Purkinje cells located in a narrow longitudinal (parasagittal) strip of cortex (synchrony band). The relationship of the anatomical organization of the olivocerebellar projection to these synchrony bands has not been investigated in detail. Thus, we studied this relationship by using the aldolase C (zebrin II) expression pattern, another landmark for the cerebellar longitudinal organization, as a reference frame in rat crus IIa. Crus IIa consists of 10 aldolase C-positive and -negative longitudinal compartments. Aldolase C labeling after multiple-electrode recording of CSs indicated that in lateral crus IIa (compartments 5+ to 7+) synchrony bands were generally constrained to single compartments. In contrast, in medial crus IIa (compartments 4a- to 5a-) the synchrony within and across the compartments was much higher than in lateral crus IIa, resulting in wide synchrony bands covering multiple compartments. Retrograde labeling of olivary neurons by injections of biotinylated dextran amine into aldolase C compartments in crus IIa showed that compartments in medial crus IIa were all innervated by the caudal part of the medial accessory olive. On the other hand, each aldolase C compartment in the lateral crus IIa was innervated by a region in a different subnucleus in the rostral inferior olive. These regions in different subnuclei were located close to each other. These results suggest that CS synchrony bands reflect the olivocerebellar compartmental projection pattern and neuronal coupling within a particular olivary subnucleus, and that medial and lateral crus IIa may be functionally distinct.

Complex spike (CS) synchrony patterns are modulated by the release of GABA within the inferior olive (IO). The GABAergic projection to most of the IO arises from the cerebellar nuclei, which are themselves subject to strong inhibitory control by Purkinje cells in the overlying cortex. Moreover, the connections between the IO and cerebellum are precisely aligned, raising the possibility that each cortical region controls its own CS synchrony distribution. This possibility was tested using multielectrode recordings of CSs and simple spikes (SSs) in crus 2a of anesthetized rats. Picrotoxin or muscimol was applied to the cerebellar cortex at the borders of the recording array. These drugs induced significant changes in CS synchrony and in CS and SS firing rates and changes in post-CS pauses and modulation of SS activity. The level of CS synchrony was correlated with SS firing rate in control, and application of picrotoxin increased both. In contrast, muscimol decreased CS synchrony. Furthermore, when picrotoxin was applied only at the lateral edge of the array, changes in CS synchrony occurred sequentially across the recording array, with cells located in the lateral half of the array having earlier and larger changes in CS synchrony than cells in the medial half. The results indicate that a double-inhibitory feedback circuit from Purkinje cells to the IO provides a mechanism by which SS activity may regulate CS synchrony. Thus, CS synchrony may be a physiologically controlled parameter of cerebellar activity, with the cerebellum and IO comprising a series of self-updating circuits.

The inferior olive (IO) has among the highest densities of neuronal gap junctions in the nervous system. These gap junctions are proposed to be the underlying mechanism for generating synchronous Purkinje cell complex spike (CS) activity. Gap junctions between neurons are formed mostly by connexin36 proteins. Thus, the connexin36 knockout (Cx36KO) mouse provides an opportunity to test whether gap junction coupling between IO neurons is the basis of CS synchrony. Multiple electrode recordings of crus 2 CSs were obtained from wildtype (Wt) and Cx36KO mice. Wts showed statistically significant levels of CS synchrony, with the same spatial distribution as has been reported for other species: high CS synchrony levels occurred mostly among Purkinje cells within the same parasagittally-oriented cortical strip. In contrast, in Cx36KOs, synchrony was at chance levels and had no preferential spatial orientation, supporting the gap junction hypothesis. CS firing rates for Cx36KOs were significantly lower than for Wts, suggesting that electrical coupling is an important determinant of IO excitability. Rhythmic CS activity was present in both Wt and Cx36KOs, suggesting that individual IO cells can act as intrinsic oscillators. In addition, the climbing fiber reflex was absent in the Cx36KOs, validating its use as a tool for assessing electrical coupling of IO neurons. Zebrin II staining and anterograde tracing showed that cerebellar cortical organization and the topography of the olivocerebellar projection are normal in the Cx36KO. Thus, the differences in CS activity between Wts and Cx36KOs likely reflect the loss of electrical coupling of IO cells.