Atypical location of cannabinoid receptors in white matter areas during rat brain development

Abstract

Previous evidence suggests that the endogenous cannabinoid system could emerge and be operative early during brain development. In the present study, we have explored the distribution of specific binding for cannabinoid receptors in rat brain at gestational day 21 (GD21), postnatal days 5 (PND5) and 30 (PND30), and at adult age (> 70 days after birth) by using autoradiography with [3H]CP-55,940. Our results indicated that specific binding for cannabinoid receptors can be detected in the brain of rat fetuses at GD21 in the classic areas that contain these receptors in adulthood-in particular, in the cerebellum and the hippocampus and, to a lesser extent, in the basal ganglia, several limbic structures, and cerebral cortex. The density of cannabinoid receptors in all these structures increased progressively at all postnatal ages studied until reaching the classical adult values in 70-day-old animals. Interestingly, cannabinoid receptor binding can also be detected at GD21 in regions, in which they are scarcely distributed or not located in the adult brain and that have the particularity of all being enriched in neuronal fibers. Among these were the corpus callosum, anterior commissure, stria terminalis, fornix, white matter areas of brainstem, and others. This atypical location was quantitatively high at GD21, tended to wane at PND5, and practically disappeared at PND30 and in adulthood, with the only exception being the anterior commissure, which exhibited a moderate density for cannabinoid receptors. Moreover, the binding of [3H]CP-55,940 to cannabinoid receptors in the white matter regions at GD21 seems to be functional and involves a GTP-binding protein-mediated mechanism. Thus, the activation of these receptors with an agonist such as WIN-55,212-2 increased the binding of [35S]-guanylyl-5'-O-(gamma-thio)-triphosphate, measured by autoradiography, in the corpus callosum and white matter areas of brainstem of fetuses at GD21. This increase was reversed by coincubation of WIN-55,212-2 with SR141716, a cannabinoid receptor antagonist. As this antagonist is specific for the cerebral cannabinoid receptor subtype, called CB1, we can assert that the signal found for cannabinoid receptor binding in the fetal and early postnatal brain likely corresponds to this receptor subtype. Collectively, all these data suggest the existence of a transient period of the brain development in the rat, around the last days of the fetal period and the first days of postnatal life, in which CB1 receptors appear located in neuronal fiber-enriched areas. During this period, CB1 receptors would be already functional acting through a GTP-binding protein-mediated mechanism. After this transient period, they progressively acquire the pattern of adult distribution. All this accounts for a specific role of the endogenous cannabinoid system in brain development.

A transient expression of functional α2-adrenergic receptors in white matter of the developing brain

“…It is interesting to note that our findings on a 2 -AR are similar to the developmental white matter expression of another GPCR, the cannabinoid receptor (CB1) (Berrendero et al, 1998;Romero et al, 1997). Both a 2 -AR and CB1 receptors are present in rat ges- tational white matter.…”

Cannabinoids and gene expression during brain development

“…This (these) function(s) would be specific of the period of development and significantly different than their classic role(s) in adult brain, such as control of movement or nociception. This is concordant with the fact that CB 1 receptors, although present in the fetal and early postnatal brain (Romero et al, 1997;Berrendero et al, 1998;, only reach their adult distribution and density after several weeks after birth. These authors reported that immature mice were unable to exhibit hypokinetic and analgesic responses, which were not fully evident until adulthood (Fride and Mechoulam, 1996).…”

“…However, several comments deserve to be made as regards to these effects. However, we also found CB 1 receptors in the fornix and the fasciculus retroflexus (Romero et al, 1997;Berrendero et al, 1998) and L1-mRNA remained unchanged in these two tracts, thus indicating the existence of a certain selectivity and region-dependency for the effects of ∆ 9 -THC on L1 gene expression, whose explanation will require additional studies. For instance, most of the grey matter regions analyzed, such as several cortical structures, basal ganglia, hippocampus, diencephalic regions and limbic regions, did not exhibit any changes in their levels of L1 gene expression after the prenatal exposure to ∆ 9 -THC, with the only two exceptions of the habenula and the septum nuclei, which exhibited modest increases.…”

Differential anxiogenic, aversive, and locomotor effects of THC in adolescent and adult rats

“…It is also possible that CB-1 receptor mechanisms responsible for the anxiogenic effects of THC may mature later than those regulating memory. Studies have shown that CB-1 receptor mechanisms in the hippocampus that are responsible for effects on learning are generally at adult levels by adolescence (Belue et al 1995;Morozov and Freund 2003;Romero et al 1997). However, sites in the amygdala, which are thought to mediate anxiogenic effects, have not been examined.…”