I. NO and Parkinson's disease
A considerable body of data indicates that corticostriatal glutamatergic and nigrostriatal dopaminergic systems are critically involved in the integration of motor information by medium spiny projection neurons (MSNs). Dysfunctional neurotransmission within these striatal networks is believed to underlie the pathophysiology of several neurological disorders including Parkinson's disease (PD) and schizophrenia. Recently, evidence has accumulated suggesting that striatal nitric oxide (NO) producing interneurons may play an important role in mediating a component of corticostriatal neurotransmission. In addition to their corticostriatal inputs, these interneurons receive synaptic contacts from midbrain dopamine (DA) neurons. Indirect measures of NO synthase (NOS) activity also suggest that NO interneurons may be differentially regulated by DA D1 and D2 receptor activation. Additionally glutamatergic and/or dopaminergic activation of striatal NOS-containing interneurons may play an important role in integrating motor information and synchronizing the activity of functionally related striatal output pathways. This is supported by studies demonstrating that corticostriatal activation of NO signaling mediates the induction of electrotonic coupling and long-term depression of synaptic activity in MSNs. Similarly to D1 agonists, NO has been shown to activate DA and cAMP-regulated phosphoprotein (DARPP-32) via cyclic nucleotide-dependent pathways. These studies suggest that the activation of NO-dependent signaling pathways may represent an additional means by which higher cortical centers coordinate the neural activity of striatal MSNs. Thus, NO may play a key role in the integration of corticostriatal information and the generation of normal motor activity. In support of this, multiple behavioral studies have demonstrated that pharmacological blockade of NO signaling decreases basal locomotor activity and activity induced by substance P and DA agonists. Moreover, NOS activity is depressed in 6-hydroxydopamine (6-OHDA) lesioned animals and NOS interneuron numbers and mRNA are reduced significantly in post-mortem Parkinsonian brains. The above findings indicate that the characterization of the functional effects of striatal afferents on NOS activity and the role of NO effector pathways in modulating the activity of MSNs will be relevant for understanding information integration in the normal striatum and in pathophysiological conditions such as PD and schizophrenia.
Specific ongoing projects include the following:
1) ) Examine how glutamatergic and dopaminergic afferent systems regulate striatal NOS activity in vivo. Pharmacological studies have shown that striatal NOS activity is stimulated by NMDA and DA D1 receptor activation. However, the role of different afferent systems in regulating NOS activity has not been examined directly. Additionally, the role of DA receptor activation in modulating glutamatergic activation of striatal NOS has not been investigated. Thus our ongoing studies utilize electrical and chemical stimulation procedures to activate specific striatal afferents involved in regulating the activity of NOS. Striatal NO synthesis is routinely assessed in the intact animal and in brain slice preparations using electrochemical microsensor measures of extracellular NO levels. Our systems-oriented approach has already provided novel information as to the role of specific striatal afferent systems and their interactions in regulating NO neurotransmission in the intact animal.
2) Examine how the activation of striatal NO signaling pathways affects activity states and responsiveness of MSNs to DA. It is known that the nigrostriatal dopaminergic projection innervate both striatal MSNs and NOS interneurons. The influence of NO interneurons on the membrane activity of MSNs and the signaling mechanisms involved in mediating NO neurotransmission remain to be characterized thoroughly. Thus, our ongoing studies are aimed examining the impact of NO signaling on MSN activity in the intact and 6-OHDA lesioned (parkinsonian) animal using combined in vivo intracellular recordings and microdialysis (for local drug manipulations to selectively activate or inhibit nitrergic and dopaminergic effector pathways). Intracellular recordings also will be performed in brain slice preparations in order to elucidate the signaling mechanisms involved in NO neurotransmission. These different yet complementary approaches should provide valuable information regarding the interaction between striatal dopaminergic and nitrergic signaling, and their influence on the membrane activity of electrophysiologically and morphologically identified MSNs.
II. NO, glutamate and dopamine interactions in schizophrenia
Dysfunction of prefrontal cortical and temporal lobe inputs to the limbic striatum may play an important role in the pathophysiology of schizophrenia. Although the precise nature of this dysfunction remains to be characterized, recent studies indicate that inappropriate gating of synaptic signals transmitted through prefrontal-limbic circuits at the level of the nucleus accumbens may be involved. The nucleus accumbens is densely innervated by glutamatergic (GLUergic) projections from the prefrontal cortex (PFC), hippocampus, and amygdala. These GLUergic afferents primarily target the dendritic spines of medium-sized projection neurons (MSNs), but also make synaptic contacts on the dendrites of nitric oxide synthase (NOS)-containing interneurons. A role for nitric oxide (NO) in modulating nucleus accumbens function is supported by recent studies demonstrating that electrical stimulation of the fimbria/fornix increased local GLU and GABA efflux via stimulation of NOS activity. Additionally, NO generated via the activation of NOS stimulates DA and GLU efflux and MSN firing activity. This NO-dependent facilitation of DA and GLU release has been shown to be critically involved in the induction of synaptic plasticity in corticostriatal pathways.
Given the important role of NO signaling in synaptogenesis and synaptic plasticity, it is likely that a dysfunction within prefrontal or temporal corticostriatal circuits may cause alterations in NO signaling and disrupt the integration of information within ventral striatal neuronal networks. The possibility that abnormal NO signaling may play a role in the neurodevelopmental pathogenesis of schizophrenia is supported by studies showing alterations in the density of NOS-containing interneurons in multiple brain regions of patients with schizophrenia. Additionally, recent genetic studies examining a large population of schizophrenic patients have revealed that a polymorphism in the NOS-1 gene may confer increased susceptibility to schizophrenia. Abnormal NOS activity has also been observed in the striatum in a rodent model of the developmental prefrontal-temporolimbic pathology of schizophrenia. Moreover, animals treated with NOS inhibitors on postnatal days 3-5 exhibit enhanced sensitivity to amphetamine and PCP, as well as deficits in prepulse inhibition and social interaction in adulthood. Taken together, the above findings indicate that the characterization of NO signaling in the nucleus accumbens will be relevant for the development of novel pharmacotherapies for the treatment of schizophrenia.
Thus, the goal of this project is to utilize the combined techniques of in vivo intracellular recordings and microdialysis to study the role of nitrergic systems in modulating the interactions between afferent inputs involved in regulating the excitability of MSNs in the nucleus accumbens of normal animals and animals exposed to the neuronal NOS inhibitor 7-nitroindazole (7-NI) on postnatal days 3-5. It is anticipated that these studies characterizing the impact of these systems on neuronal activity in the nucleus accumbens of control and 7-NI treated rats will further our understanding of information integration within nucleus accumbens networks involved in normal and pathophysiological states such as schizophrenia.