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Interneurons immunoreactive for vasoactive intestinal polypeptide (VIP) are integral elements of columnar organization patterns in the rat cerebral cortex. By application of the sensitive mirror technique, the co-localization of VIP with the classical inhibitory neurotransmitter gamma-aminobutyric acid (GABA) and the acetylcholine-synthesizing enzyme, choline acetyltransferase (ChAT), was investigated in neocortical neurons. Furthermore, the frequency of co-localization of ChAT with GABA was determined. In a sample of 118 VIP-immunoreactive neurons, mostly from the primary somatosensory cortex, it was demonstrated that virtually all of them reveal immunoreactivity for GABA and, therefore, are to be GABAergic. Moreover, 34% of mostly bipolar, VIP-positive neurons contained ChAT and are, thus, supposedly cholinergic as well. Co-localization of VIP and ChAT varied according to cortical laminae. Finally, 88% of a total of 60 ChAT-immunoreactive neurons were also immunostained for GABA. It is concluded that almost all VIP-immunoreactive neurons and most of the cholinergic neurons in rat neocortex represent partly overlapping subpopulations of inhibitory interneurons utilizing GABA.

The convergence and segregation of medial septal and median raphe afferents in the innervation of different subpopulations of GABAergic interneurons was investigated in the rat hippocampal formation. Following local injections of 5,7-dihydroxytryptamine into the median raphe nucleus destroying all serotonergic neurons, iontophoretic injections of Phaseolus vulgaris leucoagglutinin (PHAL) into the medial septum resulted in anterograde labelling of axons in the hippocampus. The labelled varicose fibres made multiple contacts with calbindin D28K-, parvalbumin-, and cholecystokinin-immunoreactive interneurons. These results disproved the possibility that PHAL-labelled afferents innervating hippocampal interneurons following septal PHAL injections would have been raphe axons passing through the injection site. In the second set of experiments a double anterograde tracing technique (PHAL from the septum and biotinylated-PHAL from the median raphe) and a triple or double immunostaining procedure was used to determine the types of interneurons (calbindin D28K-, parvalbumin-, or cholecystokinin-immunoreactive) innervated by one, or the other, or both pathways. The results showed that parvalbumin-containing neurons were innervated by septal afferents but avoided by raphe axons, whereas calbindin D28K-containing cells, and to a smaller extent cholecystokinin-containing cells served as targets for both pathways. In some cases the same individual calbindin D28K- or cholecystokinin-containing neurons received multiple contacts from afferents of both septal and raphe origin. Thus, our results indicate that different subcortical nuclei modulate largely different inhibitory circuits in the hippocampal formation. However, considering the occasional convergence of the two subcortical nuclei not only onto the same type, but even onto the same individual calbindin D28K-containing interneurons, we propose that a particular inhibitory function, most probably feed-forward inhibition in the distal dendritic region, is under the control of both pathways.

Sustained stimulation of the perforant path has been shown to damage the CA1 area and impair spatial memory in rats. The pattern of cell death is similar in human epileptics, who also exhibit memory deficits. In this study we demonstrate that the learning/memory impairment in water maze test and the development of interictal spikes that also followed stimulation-induced damage were antagonized by CGP 39551. Pretreatment with this NMDA receptor antagonist also slightly diminished somatostatin cell loss in the hilus but not CA1 pyramidal cell damage. These results indicate that the impairment of spatial learning/memory seems to be dependent not only on the degree of cell degeneration in the CA1 subfield of the hippocampus but also on the frequency of interictal spikes, at least in this model of epilepsy.

The non-pyramidal cells of the hippocampus are heterogeneous with respect to their morphology, peptide content, physiological properties, and postsynaptic targets. Here we demonstrate that the content of peptides (cholecystokinin, somatostatin) and calcium-binding proteins (parvalbumin and calbindin) of non-pyramidal cells is not related to a characteristic fine structure or synaptic input. Varying numbers of GABA-negative and GABA-positive input synapses of non-pyramidal cells indicate that these neurons are differently integrated in inhibitory and disinhibitory circuits.

Septal axons were visualized in the hippocampus by anterograde transport of Phaseolus vulgaris leucoagglutinin (PHAL) injected into the medial septal region, which contains large numbers of parvalbumin (PV)-immunoreactive somata. A proportion of the PHAL-labelled afferents in the hippocampus were shown to be immunoreactive for PV by immunostaining alternate sections. This population of septohippocampal axons alone was found to be immunoreactive for gamma-aminobutyric acid (GABA) following post-embedding immunogold staining of ultrathin sections cut from the same material.

Using a monoclonal antibody to choline acetyltransferase, immunoreactive synaptic boutons were identified in the neostriatum, cingulate cortex, basolateral nucleus of the amygdala, hippocampus and interpeduncular nucleus of the rat. The synapses were generally symmetrical although some asymmetrical membrane specializations were observed. Postsynaptic targets included perikarya, dendritic shafts and dendritic spines.

Chandelier or axo-axonic cells (AACs) are specialized interneurons terminating on the axon initial segments of pyramidal neurons. Two AACs have been localized by Golgi impregnation, one in the CA1 region of the hippocampus and one in the visual cortex of cat, for structural analysis and for the identification of their transmitter. They had 323 and 268 terminal bouton rows, respectively, probably making synapses with an equal number of initial segments. The distribution of the dendrites of the hippocampal cell was strikingly similar to that of pyramidal cells suggesting a similar input. Using an antiserum to GABA and postembedding GABA-immunocytochemistry, developed for Golgi-impregnated neurons, both cells were found to be GABA-immunoreactive. The strategic location of their synapses and the presence of GABA in AACs suggest that in normal cortical tissue they play a major role in GABA-mediated inhibition.

[3H]GABA was injected into the visual cortex of rats in vivo. The labeled amino acid was demonstrated by autoradiography using semithin sections of Golgi material. Selective accumulation was seen in the perikarya of Golgi-stained, gold-toned, aspinous stellate neurons. Spine-laden pyramidal-like cells did not show labeling. This method gives direct information about the dendritic arborization of a neuron, and its putative transmitter, and allows the identification of its synaptic connections.