The Amygdalostriatal Projection. In: Ohye C, Kimura M, Mckenzie JS, editors. The Basal Ganglia V. Boston: Springer US; 1996. pp. 33-42. [DOI: 10.1007/978-1-4899-0194-1_5]

The anatomy of co-morbid neuropsychiatric disorders based on cortico-limbic synaptic interactions

Many brain disorders appear to involve dysfunctions of aminergic systems. Alterations in dopamine activity may underpin both schizophrenia and the establishment and maintenance of drug dependence while disruption of serotonergic signalling may be crucial in both depression and schizophrenia. The co-existence of nicotine and alcohol abuse with depression and schizophrenia is well-documented as is the particular vulnerability of adolescents. At the same time, a common group of brain structures is increasingly implicated in neuropathological studies. For example, depression may involve a lack of serotonin signalling, particularly in the prefrontal cortex, while in schizophrenia there is evidence for reduced dopamine signalling in the same brain region, co-existing with hyperactivity in the mesolimbic dopamine pathway. Increased dopamine release from the mesolimbic dopamine pathway is also a common factor of drugs of abuse. Furthermore, the control of motivational behaviour and dopamine release is apparently modified by hippocampal and amygdala activity, both brain regions showing pathological changes in schizophrenia and depression. Our work has focused on the intricate synaptic interactions of aminergic terminals and cortical and subcortical neurons in order to unravel the anatomical basis for these disorders and their treatments. We show convergence of dopamine and cortical inputs onto single neurons in the nucleus accumbens, and between different cortical inputs to individual neurons, providing a basis for the gating mechanisms attributed to these interactions. We have also examined local and extrinsic connections in the prefrontal cortex and the basis for regulation of both cortical neurons and midbrain dopamine neurons by serotonin from the raph é nucleus. Together with data concerning subcellular receptor distributions, this information provides a detailed synaptic framework for interpreting behavioural, pharmacological and physiological data and enhances our understanding of possible circuitry underlying comorbidity of disorders such as schizophrenia and depression with drug abuse, information invaluable in the introduction of enhanced therapies.

Thalamic input to parvalbumin-immunoreactive GABAergic interneurons: organization in normal striatum and effect of neonatal decortication

The neocortex and thalamus send dense glutaminergic projections to the neostriatum. The neocortex makes synaptic contact with spines of striatal projection neurons, and also targets a distinct class of GABAergic interneurons immunoreactive for the calcium-binding protein parvalbumin. We determined whether the parafascicular thalamic nucleus also targets striatal parvalbumin-immunoreactive interneurons. The anterograde tracer biotinylated dextranamine was injected into the parafascicular nucleus of adult rats. Double-labeled histochemistry/immunohistochemistry revealed overlapping thalamic fibers and parvalbumin-immunoreactive neurons in the neostriatum. Areas of overlap within the sensorimotor striatum were analysed by electron microscopy. Of 311 synaptic boutons originating from the parafascicular nucleus, 75.9% synapsed with unlabeled dendrites, 22.5% with unlabeled spines, and 1.3% had parvalbumin-immunoreactive dendrites as a postsynaptic target. Only 4% of all asymmetric synapses on parvalbumin-immunoreactive dendrites were derived from the parafascicular nucleus. A separate group of animals underwent bilateral neocortical deafferentation on the third postnatal day, prior to injection of anterograde tracer into the parafascicular nucleus of adult animals. These experiments were performed with the dual purpose of (i) reducing the possibility that thalamic inputs to parvalbumin-immunoreactive neurons are the result of transsynaptic uptake of tracer by a thalamo-cortico-striatal route, and (ii) determining whether competitive interactions between developing corticostriatal and thalamostriatal fibers may account for the relatively sparse thalamic input onto parvalbumin-immunoreactive interneurons. In decorticates, 219 striatal synaptic contacts derived from the parafascicular nucleus, out of which 77.2% were on unlabeled dendrites, 20.9% were upon unlabeled spines, and 0.9% targeted parvalbumin-immunoreactive dendrites. We conclude that the thalamic parafascicular nucleus indeed sends synaptic input to parvalbumin-immunoreactive striatal neurons. Parafascicular nucleus inputs to striatal parvalbumin-immunoreactive interneurons are sparse in comparison to other asymmetric inputs, most of which are likely to be of cortical origin. The synaptic profile of thalamostriatal inputs to parvalbumin-immunoreactive neurons and unlabeled elements is unchanged following neonatal decortication. This suggests that competitive interaction between developing thalamostriatal and corticostriatal projections is not a major mechanism determining synaptic input to striatal subpopulations.