Expression of cholecystokinin mRNA in corticothalamic projecting neurons: a combined fluorescence in situ hybridization and retrograde tracing study in the ventrolateral thalamus of the rat

Structure and function of neocortical layer 6b

Cortical layer 6b is considered by many to be a remnant of the subplate that forms during early stages of neocortical development, but its role in the adult is not well understood. Its neuronal complement has only recently become the subject of systematic studies, and its axonal projections and synaptic input structures have remained largely unexplored despite decades of research into neocortical function. In recent years, however, layer 6b (L6b) has attracted increasing attention and its functional role is beginning to be elucidated. In this review, I will attempt to provide an overview of what is currently known about the excitatory and inhibitory neurons in this layer, their pre- and postsynaptic connectivity, and their functional implications. Similarities and differences between different cortical areas will be highlighted. Finally, layer 6b neurons are highly responsive to several neuropeptides such as orexin/hypocretin, neurotensin and cholecystokinin, in some cases exclusively. They are also strongly controlled by neurotransmitters such as acetylcholine and norepinephrine. The interaction of these neuromodulators with L6b microcircuitry and its functional consequences will also be discussed.

Area-specific substratification of deep layer neurons in the rat cortex

Gene markers are useful tools to identify cell types for fine mapping of neuronal circuits. Here we report area-specific sublamina structure of the rat cerebral cortex using cholecystokinin (cck) and purkinje cell protein4 (pcp4) mRNAs as the markers for excitatory neuron subtypes in layers 5 and 6. We found a segregated expression, especially pronounced in layer 6, where corticothalamic and corticocortical projecting neurons reside. To examine the relationship between gene expression and projection target, we injected retrograde tracers into several thalamic subnuclei, ventral posterior (VP), posterior (PO), mediodorsal (MD), medial and lateral geniculate nuclei (MGN and LGN); as well as into two cortical areas (M1 and V1). This combination of tracer-in situ hybridization (ISH) experiments revealed that corticocortical neurons predominantly express cck and corticothalamic neurons predominantly express pcp4 mRNAs in all areas tested. In general, cck(+) and pcp4(+) cells occupied the upper and lower compartment of layer 6a, respectively. However, the sublaminar distribution and the relative abundance of cck(+) and pcp4(+) cells were quite distinctive across areas. For example, layer 6 of the prelimbic cortex was almost devoid of cck(+) neurons, and was occupied instead by corticothalamic pcp4(+) neurons. In the lateral areas, such as S2, there was an additional layer of cck(+) cells positioned below the pcp4(+) compartment. The claustrum, which has a tight relationship with the cortex, mostly consisted of cck(+)/pcp4(-) cells. In summary, the combination of gene markers and retrograde tracers revealed a distinct sublaminar organization, with conspicuous cross-area variation in the arrangement and relative density of corticothalamic connections.

Neuronal network of panic disorder: the role of the neuropeptide cholecystokinin

Panic disorder (PD) is characterized by panic attacks, anticipatory anxiety and avoidance behavior. Its pathogenesis is complex and includes both neurobiological and psychological factors. With regard to neurobiological underpinnings, anxiety in humans seems to be mediated through a neuronal network, which involves several distinct brain regions, neuronal circuits and projections as well as neurotransmitters. A large body of evidence suggests that the neuropeptide cholecystokinin (CCK) might be an important modulator of this neuronal network. Key regions of the fear network, such as amygdala, hypothalamus, peraqueductal grey, or cortical regions seem to be connected by CCKergic pathways. CCK interacts with several anxiety-relevant neurotransmitters such as the serotonergic, GABA-ergic and noradrenergic system as well as with endocannabinoids, NPY and NPS. In humans, administration of CCK-4 reliably provokes panic attacks, which can be blocked by antipanic medication. Also, there is some support for a role of the CCK system in the genetic pathomechanism of PD with particularly strong evidence for the CCK gene itself and the CCK-2R (CCKBR) gene. Thus, it is hypothesized that genetic variants in the CCK system might contribute to the biological basis for the postulated CCK dysfunction in the fear network underlying PD. Taken together, a large body of evidence suggests a possible role for the neuropeptide CCK in PD with regard to neuroanatomical circuits, neurotransmitters and genetic factors. This review article proposes an extended hypothetical model for human PD, which integrates preclinical and clinical findings on CCK in addition to existing theories of the pathogenesis of PD.

Activation of cholecystokinin neurons in the dorsal pallium of the telencephalon is indispensable for the acquisition of chick imprinting behavior

Chick imprinting behavior is a good model for the study of learning and memory. Imprinting object is recognized and processed in the visual wulst, and the memory is stored in the intermediate medial mesopallium in the dorsal pallium of the telencephalon. We identified chicken cholecystokinin (CCK)-expressing cells localized in these area. The number of CCK mRNA-positive cells increased in chicks underwent imprinting training, and these cells expressed nuclear Fos immunoreactivity at high frequency in these regions. Most of these CCK-positive cells were glutamatergic and negative for parvalbumin immunoreactivity. Semi-quantitative PCR analysis revealed that the CCK mRNA levels were significantly increased in the trained chicks compared with untrained chicks. In contrast, the increase in CCK- and c-Fos-double-positive cells associated with the training was not observed after closure of the critical period. These results indicate that CCK cells in the dorsal pallium are activated acutely by visual training that can elicit imprinting. In addition, the CCK receptor antagonist significantly suppressed the acquisition of memory. These results suggest that the activation of CCK cells in the visual wulst as well as in the intermediate medial mesopallium by visual stimuli is indispensable for the acquisition of visual imprinting.

Cortical sources of CRF, NKB, and CCK and their effects on pyramidal cells in the neocortex

In order to investigate how neuropeptide transmission can modulate the neocortical network, we mapped the expression of neurokinin (NK) B, cholecystokinin (CCK), and corticotropin-releasing factor (CRF) and their receptors to neuronal types using patch-clamp and single-cell reverse transcription-polymerase chain reaction in acute slices of rat neocortex. Classification of neurons by unsupervised clustering based on the analysis of multiple electrophysiological and molecular properties disclosed 3 GABAergic interneuron clusters and 1 pyramidal cell cluster. The 3 neuropeptides were expressed in a cluster of interneurons characteristically expressing vasoactive intestinal peptide. CRF was additionally found in a cluster containing almost exclusively somatostatin-expressing interneurons, whereas CCK was present in all clusters. The respective receptors of these peptides, NK-3, CCK-B, and CRF-1, were essentially expressed in pyramidal cells. At -60 mV, pyramidal cells were weakly depolarized by each of these peptides. When pyramidal neurons were maintained to about 5 mV below spike threshold, depolarization induced by each peptide resulted in a long-lasting action potential discharge. Neuropeptide effects were prevented by selective antagonists of NK-3, CCK-B, and CRF-1 receptors. These results suggest that pyramidal neurons are the primary target of NKB, CCK, and CRF in the neocortex. They further indicate that specific interneuron types coordinate the release of these peptides and can induce a long-lasting increase of the excitability of the neocortical network.

Unidirectional axonal transport in in vitro adult rat brain explants

The present study examined the properties of anterograde and retrograde transport in central axonal pathways maintained in vitro. The commonly-used tracers biocytin, dextran rhodamine B, FluoroGold, True Blue or rhodamine latex microspheres were injected into the medial geniculate body or the inferior colliculus of the adult rat brain explant. Injection of biocytin into the inferior colliculus consistently resulted in extensive anterograde labelling of axonal trunks and terminals in the ipsilateral medial geniculate body and in the contralateral inferior colliculus. Labelled axons were obtained 2-3 h after the injection at a site 3-4 mm away from the injection site and could be found up to 1.5 mm below the explant surface. Despite massive anterograde labelling with biocytin, all the tracers applied in the gray or white matter failed to show retrograde transport. These results suggest that axonal transport can occur in an anterograde-selective fashion in adult brain explants in vitro.

Expression of cholecystokinin messenger RNA in reciprocally-connected auditory thalamus and cortex in the rat

Cholecystokinin exerts a potent antiepileptic action in mammalian auditory system and undergoes seizure-mediated up-regulation. The present study investigated cholecystokinin messenger RNA expression in the reciprocally-connected auditory thalamus and cortex in the rat. Immunofluorescence in situ hybridization was performed using a 24-base cholecystokinin-messenger RNA oligonucleotide probe. Corticothalamic projection neurons were identified by means of the retrograde fluorescent tracer rhodamine latex microspheres injected into the medial geniculate body. In our experiments, cholecystokinin messenger RNA transcripts were found in about 80% of neurons located within the reciprocally-connected regions of the medial geniculate body and the auditory cortices. These observations provide evidence of cholecystokinin production in the reciprocally-connected regions of the auditory thalamus and cortex, the structures which jointly create the thalamo-corticothalamic circuit which has been implicated in seizure genesis.

Two distinct subgroups of cholecystokinin-immunoreactive cortical interneurons

Examination of cholecystokinin-immunoreactive cells in the rat frontal cortex revealed the presence in layers I-VI of a non-uniform population ranging in size from small to large. All were also immunoreactive for GABA. The most commonly observed dendritic form of the small cells were bipolar or bitufted although some were multipolar and demonstrated vasoactive intestinal polypeptide and in a few case calretinin immunoreactivity. The large cells were multipolar or bitufted and lacked expression of vasoactive intestinal polypeptide and calretinin immunoreactivity but occasionally showed calbindin D28k immunoreactivity. Therefore, the cholecystokinin-immunoreactive cells could be divided into two distinct subpopulations depending on their chemistry and morphology. Our previous studies showed that GABAergic cells in the neocortex could be classified into at least three chemically different subgroups: (1) parvalbumin-containing cells; (2) somatostatin-containing cells (most of them also contain calbindin D28k); and (3) vasoactive intestinal polypeptide- and/or calretinin-containing cells. The present results indicated that the small cholecystokinin-immunoreactive non-pyramidal cells constitute a subset of the vasoactive intestinal polypeptide- and/or calretinin-containing cortical GABAergic cells. The large cells remain to be categorized.

Peptidergic modulation of intrathalamic circuit activity in vitro: actions of cholecystokinin

Cholecystokinin (CCK)-mediated actions on intrathalamic rhythmic activities were examined in an in vitro rat thalamic slice preparation. Single electrical stimuli in the thalamic reticular nucleus (nRt) evoked rhythmic activity (1-15 sec duration) in nRt and the adjacent ventrobasal nucleus (VB). Low CCK concentrations (20-50 nM) suppressed rhythmic oscillations in 43% of experiments but prolonged such activities in the remaining slices. Higher CCK concentrations (100-400 nM) had a predominantly antioscillatory effect. Suppression of oscillations was associated with a relatively large membrane depolarization of nRt neurons that changed their firing mode from phasic (burst) to tonic (single-spike) output. This decreased burst discharge of nRt neurons during CCK application reduced inhibitory drive onto VB neurons from multiple peaked inhibitory postsynaptic currents (IPSCs) to single peaked inhibitory events. We hypothesize that suppression of inhibitory drive onto VB neurons decreases their probability of burst output, which, together with a reduction of nRt burst output, dampens the oscillatory activity. Low CCK concentrations, which produced little or no depolarization of nRt neurons, did not alter the firing mode of the nRt neurons. However, the probability of burst output from nRt neurons in response to subthreshold stimuli was increased in low CCK concentrations, presumably leading to an increase in the number of nRt neurons participating in the rhythmic activity. Our findings suggest that the neuropeptide CCK, by altering the firing characteristics of nRt neurons, has powerful modulatory effects on intrathalamic rhythms; the ultimate action was dependent on CCK concentration and resting state of these cells.

Immunofluorescence in situ hybridization (IFISH) in neurones retrogradely labelled with rhodamine latex microspheres

The method of non-radioactive in situ hybridization was developed as an alternative to radioactive assay because of the difficulties and disadvantages of the safety measures required, extensive time required for autoradiography (especially with 3H-labelled probes) and limited cellular resolution obtained using 32P- and 35S-labelled probes. This method holds great potential for studying functional anatomy of specific neuronal pathways if it can be used in conjunction with conventional tract tracing techniques. In this article we describe a simple method by which immunofluorescence in situ hybridization (IFISH) was jointly used with rhodamine latex microspheres (RLM) to trace the origin of the thalamic cholecystokininergic input in rat. RLM is a widely used retrograde fluorescence tracer and seems ideal for IFISH because: (1) it lacks aversive effect on the hybridization and immunocytochemical reactions, (2) it is resistant to the rather harsh tissue treatment required for IFISH, and (3) both the RLM and mRNA hybrids give fluorescence signals; therefore, the extent of signal co-localization can be conveniently and more accurately verified under an epifluorescence microscope. Success of the IFISH-RLM combination is chiefly limited by the quantity and availability of mRNA signals in the tissue. In our case, we used a digoxigenin (DIG)-labelled oligonucleotide probe, which through immunological amplification significantly enhanced the sensitivity of mRNA detection.