The pineal complex: luminance detection, melatonin release and shadow response

The pineal complex of vertebrates is able to measure environmental illuminance.

This was demonstrated with electrophysiological recordings from pineal photore-ceptor cells, which showed that the changes in membrane potential are linearly

cor-convergence of photoreceptors on ganglion cells is almost absent (in contrast to the retina), meaning that each projection neuron receives inputs from one, or sometimes only few photoreceptors.

The pineal complex has a dual output. The most famous, the hormonal output, consists in the release of melatonin during darkness. A second output is purely neural, it is also implicated in circadian entrainment and locomotor behaviour, but has been less intensely studied.

The humoral output: melatonin release.

Melatonin is an indoleamine synthesized in two steps from serotonin or 5-hydroxytryptamine (5-HT). Briefly, serotonin is produced from tryptophan directly in the pinealocyte by an enzymatic cascade, which involves the specific enzyme Tryptophan Hydroxylase (Tph). Serotonin is then acetylated by the enzyme Ary-lalkylamine N-Acetyltransferase (Aanat); the resulting product, N-acetylserotonin, is converted to melatonin with a methylation reaction catalyzed by the enzyme Hydroxyindole O-Methyltransferase (Hiomt), also called Asmt). While Aanat can acetylate many different substrates, Hiomt instead is very specific of the melatonin pathway.

Melatonin synthesis is directly coupled to darkness. In mammals, pineal activity is controlled by the SCN. In contrast, in anamniotes, melatonin synthesis is regu-lated by an endogenous circadian clock (Cahill and Besharse, 1993), and can also be suppressed directly by illumination during the dark phase. The circadian clock in the pineal photoreceptors controls the transcription of Aanat and/or Hiomt, while light suppression involves the post-transcriptional control of the stability and/or the enzymatic activity of these enzymes (Ribelayga et al., 1999; Falcón et al., 2010a). In zebrafish, the light spectrum able to induce melatonin suppression is broad, indicat-ing that light suppression is triggered by multiple photopigments (Ziv et al., 2007).

Although the details haven’t been elucidated, the acute effects of light on melatonin suppression are likely to involve the full phototransduction cascade, since they are correlated with the decrease of intracellular Ca²⁺concentration, which is controlled by voltage-gated L-type Ca²⁺channels (Bégay et al., 1994; Meissl et al., 1996; Kroeber et al., 2000).

Being directly coupled to darkness, melatonin is a perfect signal for daily but also seasonal rhythms (Reiter, 1993). Indeed, pineal melatonin has been implicated in a range of processes, like seasonal control of sexual maturation, induction of sleep,

regulation of pigmentation, metabolism and hypothalamic activity (Pandi-Perumal et al., 2006; Falcón et al., 2007, 2010b; Zhdanova et al., 2001; Appelbaum et al., 2009;

Morgan and Hazlerigg, 2008; Ekström and Meissl, 1997; Burgess and Granato, 2007;

Zhdanova et al., 2001).

This pleiotropic role as a modulator of physiology and nervous system activity is also indicated by the distribution of melatonin receptors, which are expressed in several parts of the brain, including the SCN, and areas involved in sensory process-ing (like the optic tectum, but also the retina itself) and in neurosecretion (like the pituitary). Thus, it has been proposed that three functions - systemic signalling of darkness, modulation of visual processing and control of neuroendocrine secretion - were associated with melatonin signalling at the beginning of vertebrate evolution.

Unfortunately, there are not so many studies of melatonin function in fishes or lampreys, which would be more telling on the ancestral functions of melatonin.

Moreover, many investigators analyzed pineal function used pinealectomy, which doesn’t distinguish between humoral and neural outputs.

The pineal and parapineal neural outputs.

The analysis of the pineal efferent projections suggests that the photic input from the pineal has a broad impact on brain activity. The pineal projections of anamniotes overlap considerably with the projections coming from the retinal ganglion cells. The most conserved projection sites are the optic tectum, the SCN/preoptic area and the mesencephalic tegmentum, with variations between different species (Pombal et al., 1999; Mandado et al., 2001; Yáñez et al., 2009). The innervation of pre-motor areas from the pineal indicates that the photic input from this organ can modulate the overall activity patterns and behavioural state. But as discussed before, the relative contribution of humoral and neural outputs is not clear.

The shadow response is the best characterized function of the pineal efferent path-ways. In this respect, the pineal is more correctly described as a “semivisual” organ (Vígh et al., 2002); indeed, the presence of pigment cells provides some discrimina-tion of light direcdiscrimina-tionality.

The shadow response has been well studied in Xenopus tadpoles, where light dim-ming induces upward swimdim-ming; this behaviour depends on the OFF-response of the pineal organ. A pineal-dependent response to light dimming has been reported also in surface fishes and cavefish (Foster and Roberts, 1982; Yoshizawa and Jef-fery, 2008; Wales, 1975). In Xenopus, this response is mediated by a direct neuronal pathway from the pineal to diencephalic-mesencephalic interneurons, and then the

tryptophan hydroxylase (TPH)

decarboxylase

arylalkylamine N-acetyltrasferase (AANAT)

hydroxyindol-O-methyltransferase (HIOMT)

tryptophan

5-hydroxytryptophan

serotonin (5-hydroxytryptamine)

N-acetylserotonin

melatonin (N-acetyl-5-methoxytryptamine)

Figure 2.5: The melatonin synthesis pathway. Serotonin is produced from tryptophan with two en-zymatic reactions. Then, serotonin is converted in melatonin after one acetylation step, catalyzed by Aanat, and an hydroxylation reaction, catalyzed by Hiomt.

hindbrain (Jamieson and Roberts, 1999, 2000).

The function and the connectivity of the parapineal are quite different. The fish parapineal and its homolog in lizard, the parietal eye, have also been implicated in the shadow response, but also in sun-compass navigation. The parapineal tributes to the so called dorsal diencephalic conduction system (DDC), a very con-served pathway in the vertebrate brain, that integrates sensory information to mod-ulate locomotion (Bianco and Wilson, 2009; Hikosaka, 2010). The DDC is formed by the habenulae, dorsal epithalamic nuclei that project to different locomotor con-trol centers in the brainstem; projections from the habenula reach the serotoninergic raphe nuclei either directly or via the interpeduncular nucleus (IPN) of the hindbrain (Stephenson-Jones et al., 2011; Lorente-Cánovas et al., 2012). Neurons of the parap-ineal were found to project to the lateral habenula and to the IPN directly, meaning

that photic information controls directly the locomotor centers, as also shown by behavioural studies.