Colocalization of γ‐aminobutyric acid with vasopressin, vasoactive intestinal peptide, and somatostatin in the rat suprachiasmatic nucleus

Abstract

The seemingly contradictory observations in previous publications that gamma-aminobutyric acid (GABA) is detected in all cell bodies of the suprachiasmatic nucleus (SCN) and that terminals originating from the SCN are only 20-30% GABA positive prompted us to investigate whether this might be explained by a preference of colocalization in terminals of certain peptidergic neurons in the SCN or by a day/night rhythm in GABA synthesis. At three different circadian times, animals were perfusion fixed, and their SCNs were stained for vasopressin (VP), somatostatin (SOM), or vasoactive intestinal polypeptide (VIP). Subsequently, the number of GABA peptide-positive terminals was determined using GABA postembedding staining in ultrathin sections. It appeared that the highest percentage of colocalization with GABA was detected in VIP terminals (38%) and the lowest in VP terminals (15%). No differences in colocalization percentages could be observed in any parameter at any circadian time. In the dorsomedial hypothalamus, one of the target areas of the VP and VIP fibers from the SCN, a colocalization of GABA within VP and VIP terminals was found similar to that in the SCN. In the region of the somatostatin-containing neurons in the SCN, a number of axoaxonal contacts could be observed that sometimes exhibited synaptic specializations. In nearly all cases, the axoaxonic terminals contained GABA and/or SOM. The conclusion is that the high level of intrinsic GABAergic connections in the SCN represents a putatively powerful mechanism to synchronize or shut down the activity of the SCN. We discuss the possibility that, depending on the firing frequency of the neurons, the colocalization of GABA with all peptides under investigation allows for the selection of which transmitter is released, the peptidergic one or the amino acid.

Morphological heterogeneity of the GABAergic network in the suprachiasmatic nucleus, the brain's circadian pacemaker

“…6 D). In SCNs of Sprague-Dawley rats, Buijs et al (1995) also found that GABA\VIP colocalisation in boutons was about twice as frequent as GABA\VP colocalisation, al- though the overall occurrence of colocalisations reported for the rat was lower than that in the mouse. On the other hand, in SCNs of Wistar rats Tanaka et al (1997) reported that GABA colocalisation occurred in about 90 % of both VIP and VP-containing neurons.…”

“…2 B). The nature of the nuclear labelling in GABAergic neurons in the SCN is unclear, both in terms of the identity of the immunoreactive epitopes and the functional implications, as noted also by Buijs et al (1994Buijs et al ( , 1995. Nevertheless, similar marking of GABAergic nuclei in other parts of the brain is regarded as not understood rather than nonspecific (P. Somogyi, personal communication).…”

“…A number of immunocytochemical studies have shown that GABA appears to be the most ubiquitous neurotransmitter in the SCN (Okamura et al 1989 ;Castel et al 1990 a, b ;Decavel & Van den Pol, 1990 ;Moore & Speh, 1993 ;Buijs et al 1994Buijs et al , 1995Belenky et al 1996). However, less attention has been paid to the heterogeneity of the GABA-containing elements in the SCN, as a possible basis for the multiple roles played by this neurotransmitter in circadian regulation.…”

“…In many SCN neurons, GABA or its synthesising enzyme glutamic acid decarboxylase (GAD), are coexpressed with these neuropeptides (Castel & Cohen, 1991 ;Buijs et al 1994Buijs et al , 1995Tanaka et al 1997 ;Pennartz et al 1998 b). Although colocalisation of peptides with GABA in the central nervous system is widespread (Somogyi et al 1985), the robust peptide\GABA association in the SCN may have a special significance for circadian physiology (Tanaka et al 1997).…”

Effects of melatonin on the ovarian response to pinealectomy or continuous light in female rats: similarity with polycystic ovary syndrome

“…Buijs et al (28) be an endocrine code of the environmental light-dark cycle conveying photic information that is used by an organism for both circadian and seasonal temporal organization (29-31) and it seems that there is no regular fluctuation in the pineal metabolism leading to melatonin synthesis and release throughout the estrous cycle in female rats (32,33). The SCN is also fundamental in the induction of renewed positive estrogen control of the preovulatory wave of LH, and melatonin may act together with the hypothalamus in altering the secretion of GnRH and the consequent production of gonadotropins (16).…”