An immunocytochemical study using the PAP method for tyrosine hydroxylase and serotonin in alternate sections, and in situ hybridization to detect tryptophan hydroxylase mRNA in the rat's locus ceruleus

Selectivity of Neuromodulatory Projections from the Basal Forebrain and Locus Ceruleus to Primary Sensory Cortices

Acetylcholine and noradrenaline are major neuromodulators that affect sensory processing in the cortex. Modality-specific sensory information is processed in defined areas of the cortex, but it is unclear whether cholinergic neurons in the basal forebrain (BF) and noradrenergic neurons in the locus ceruleus (LC) project to and modulate these areas in a sensory modality-selective manner. Here, we mapped BF and LC projections to different sensory cortices of the mouse using dual retrograde tracing. We found that while the innervation of cholinergic neurons into sensory cortices is predominantly modality specific, the projections of noradrenergic neurons diverge onto multiple sensory cortices. Consistent with this anatomy, optogenetic activation of cholinergic neurons in BF subnuclei induces modality-selective desynchronization in specific sensory cortices, whereas activation of noradrenergic LC neurons induces broad desynchronization throughout multiple sensory cortices. Thus, we demonstrate a clear distinction in the organization and function of cholinergic BF and noradrenergic LC projections into primary sensory cortices: cholinergic BF neurons are highly selective in their projections and modulation of specific sensory cortices, whereas noradrenergic LC neurons broadly innervate and modulate multiple sensory cortices.

SIGNIFICANCE STATEMENT

Neuromodulatory inputs from the basal forebrain (BF) and locus ceruleus (LC) are widespread in the mammalian cerebral cortex and are known to play important roles in attention and arousal, but little is known about the selectivity of their cortical projections. Using a dual retrobead tracing technique along with optogenetic stimulation, we have identified anatomic and functional differences in the way cholinergic BF neurons and noradrenergic LC neurons project into primary sensory cortices. While BF projections are highly selective to individual sensory cortices, LC projections diverge into multiple sensory cortices. To our knowledge, this is the first definitive proof that BF and LC projections to primary sensory cortices show both anatomic and functional differences in selectivity for modulating cortical activity.

Induction of the arginine vasopressin-enhanced green fluorescent protein fusion transgene in the rat locus coeruleus

We examined the effects of intracerebroventricular (i.c.v.) administration of colchicine on the expression of the arginine vasopressin (AVP)-enhanced green fluorescent protein (eGFP) fusion gene in rats. In rats administered i.c.v. vehicle (control), eGFP fluorescence was observed in the supraoptic nucleus (SON), the magnocellular division of the paraventricular nucleus (PVN), the suprachiasmatic nucleus (SCN), the median eminence (ME) and the posterior pituitary. Two days after i.c.v. administration of colchicine, eGFP fluorescence was markedly increased in the SON, the magnocellular and parvocellular divisions of the PVN, the SCN, the ME and the locus coeruleus (LC). Immunohistochemical staining for eGFP confirmed the distribution of fluorescence in both groups. In the colchicines-administered groups, immunohistochemistry for tyrosine hydroxylase (TH) revealed that the eGFP fluorescence was co-localised with TH-immunoreactivity in the LC. Similarly, in situ hybridization histochemistry for eGFP mRNA revealed a significant increase in gene expression in the LC, the SON and the PVN 12-48 h after administration of colchicine. Our results indicate that the synthesis of AVP-eGFP is upregulated in noradrenergic neurones in the LC after colchicine administration. This implies that AVP and noradrenaline, originating from LC neurones, might play a role in response to chronic stress.

Inducible expression of tryptophan hydroxylase without serotonin synthesis in hypothalamic dopaminergic neurons

In the present study we have further studied the previous findings that rat hypothalamic dopaminergic neuronal cell groups may express tryptophan hydroxylase (TpH), the serotonin synthesizing enzyme, without a detectable serotonin synthesis. Chemical and mechanical neuronal injuries, namely colchicine treatment and axonal transection, respectively, were performed, and distributions of neurons exhibiting immunoreactivity for TpH and/or tyrosine hydroxylase (TH), the dopamine synthesizing enzyme, were analyzed throughout the hypothalamic periventricular and arcuate nuclei. After colchicine treatment there was a statistically significant 87% (P = 0,01) increase in the number of TpH expressing neurons, while TH expression remained essentially similar. Axonal transection resulted also in a statistically significant 131% (P < 0,01) increase in the number of TpH expressing neurons, while TH expression was not significantly altered. All TpH expression coexisted with TH expression, and the induction of TpH expression by neuronal injuries occurred evenly throughout the rostrocaudal length of the territory studied. A possible serotonin synthesis by TpH was examined by giving drugs that increase brain serotonin synthesis, but no immunohistochemically detectable serotonin synthesis could be found in any of the TpH expressing neurons. Finally the possibility was studied that the relative shortage of the cofactor tetrahydrobiopterin would limit serotonin synthesis. However, an administration of tetrahydrobiopterin did not result in detectable serotonin synthesis in these neurons. Taken together these results suggest that dopaminergic neurons in the hypothalamic periventricular and arcuate nuclei are able to express TpH, this expression is induced after neuronal injury, and this induction occurs similarly throughout the territories studied. TpH expression occurs independently of TH expression, and the newly expressed TpH appears not to synthesize serotonin, regardless of pharmacological pretreatments. Thus, our findings (i) support the idea that neurons may possess inducible expression of nonfunctional transmitter-synthesizing enzymes, in this case TpH, and (ii) suggest that expression of an enzyme synthesizing a certain transmitter may not necessarily imply the corresponding transmitter phenotype.

Developmental expression of tryptophan hydroxylase mRNAs in the rat pineal gland: an in situ hybridization study

The expression of messenger RNAs encoding for tryptophan hydroxylase (TPOH), the first enzyme involved in serotonin and melatonin synthesis, has been investigated by in situ hybridization during the development of the rat pineal gland. TPOH mRNAs were detected as early as the twentieth day of gestation (E20) in the rat embryo before any nerve ending was observed in the pineal gland. After birth, their expression increased strongly, and attained a plateau during the second week. This coincides with the setting up of sympathetic innervation. From day 17 (D17), the TPOH mRNA expression diminished. These results indicate that noradrenergic innervation is not involved in the initiation of rat pinealocyte differentiation, but might modulate cell maturation. This study showed the existence of three types of cells arranged in patches in the young rat pineal gland (D6): regions in which cells expressed TPOH mRNAs, regions in which cells expressed vimentin, an intermediate filament protein present in the cytoskeleton of immature cells, and regions in which both TPOH mRNAs and vimentin are expressed. In older rat pineal gland (D20), almost all cells express TPOH mRNAs, and some cells still express vimentin. This suggests that all cells do not reach the same level of differentiation at the same time in the rat pineal gland.

Anatomical evidence for the presence of glutamate or enkephalin in noradrenergic projection neurons of the locus coeruleus

This paper reviews the anatomical evidence for the presence of glutamate (GLU) in noradrenergic neurons of the nucleus locus coeruleus (LC) and adjacent nuclei in the dorsolateral pontine tegmentum (DLPT) that project to the spinal cord, cerebellum, or cerebral cortex. Additionally, the evidence for the existence of methionine-enkephalin (ENK) in noradrenergic neurons of the DLPT that project to the spinal cord of the cat is reviewed. In these studies, we have combined the retrograde transport of either Fast Blue (FB), rhodamine labeled latex microspheres (MS), or rhodamine labeled dextran and indirect immunofluorescence histochemistry to determine whether the neurons that contain tyrosine hydroxylase (TH) and project to these terminal fields also contain GLU or ENK. The neurons of the cat that project to the spinal cord, cerebellum, and neocortex were observed in the nucleus LC and Kölliker-Fuse (KF) nucleus. They were also present, to a lesser extent, in the nucleus subcoeruleus (SC) and nuclei parabrachialis medialis (PBM) and lateralis (PBL). In the rat the majority of the neurons that project to the neocortex and hippocampus were located in the nucleus LC. Our data revealed a major proportion of these neurons to be immunostained for both GLU and TH (cat, rat), or ENK and TH (cat). Functional implications of such colocalized neurochemicals within individual LC projection neurons are discussed.

Immunocytochemical and in situ hybridization evidence for the coexistence of GABA and tyrosine hydroxylase in the rat locus ceruieus

We have demonstrated the coexistence of GABA-like and tyrosine hydroxylase-like immunoreactivities (GABA-LI and TH-LI, respectively) in the same neurons of the rat locus ceruleus (LC). The profiles of these cells were labeled by alternately immunostaining adjacent sections for GABA-LI or TH-LI by the avidin-biotin-peroxidase complex method or the peroxidase-anti-peroxidase method after perfusion (either Zamboni's fixative or PPG), and observation at light and electron microscopic levels. For light microscopy, pairs of adjacent sections of more than 590 (Zamboni's) and 260 (PPG), and for electron microscopy, 40 ultrathin sections cut from adjacent semithin plastic sections (Zamboni's), were examined. GABA-LI was found in 80% (1,309/1,642 in total) of small and medium-sized neurons, uniformly scattered throughout the LC. Observations unequivocally show that the majority of GABA-ergic neurons are also noradrenergic. Several neurons are neither noradrenergic nor GABA-ergic, while other noradrenergic neurons do not show GABA-LI. It is shown that astrocytes, but not oligodendrocytes, contain GABA. In situ hybridization using a probe DNA fragment of the glutamic acid decarboxylase (GAD) cDNA, amplified by the polymerase chain reaction, detected GAD mRNA signals in many neurons throughout the LC, supporting the presence of a GAD/GABA system in the LC. Multiple "classical" transmitters, including GABA, serotonin, and noradrenaline, coexist in many LC neurons and may contribute to its widely diverging projections throughout the entire CNS.