Normal adult neurogenesis is known to occur in the brain, specifically in the hippocampus presented by the proliferation of cells. Production of new neurons in the dentate gyrus (DG) of hippocampus is related to the learning and memory functions (Shors et al. 2002). Introduction of ∆9-THC or other cannabinoid drugs affect the adult neurogenesis and later influence the cognitive function (Jiang et al. 2005).

Neurogenesis mechanism had been illustrated in six developmental milestone, as described by Kempermann et al. (2004). Meanwhile, Von Bohlen und Halbach (2007) had simplified the mechanism into five stages, namely proliferation (nestin), differentiation (nestin, Pax6), migration (NeuroD, DCX, PSA-NCAM), axonal and dendritic targeting (PSA-NCAM, DCX, TUC-4, Calretinin), and synaptic integration (NeuN, TuJ-1, Calbindin). As proposed by von Bohlen und Halbach (2007), the expression of markers is observed in different time point to describe the different stages of neurogenesis. However, due to limited information of altered cognitive function by ∆9-THC-induced neurogenesis, the expression of markers was studied in one time point.

Throughout the study, expression of BrdU, GFAP, nestin, DCX and TuJ-1 in ∆9-THC-treated rats were increased as compared to that in control, supported by Kaplan and Bell (1984). Kaplan and Bell (1984) had demonstrating an adult neurogenesis in the hippocampus. Reflecting the review by von Bohlen und Halbach (2007), studied markers are responsible for neurogenesis.

BrdU is a thymidine analogue that was suggesting to incorporate with the deoxyribonucleic acid (DNA) of dividing cells during the S-phase of the cell cycle as it is used in monitoring the cell proliferation occurs in the tissues (Philippe 2006). BrdU is used to confirm the division of neuronal progenitor cell. The BrdU-positive cell can be observed at the border between hilus and the granule cell layer (GCL) of DG (Scott et al. 1998). As BrdU labelled all S-phase cells, the labelled cells were indifferent between newly formed glia cells and neurons. Without the use of other neurogenesis markers, BrdU labelling demonstrating the general cell genesis occurs in the brain (von Bohlen und Halbach 2007).

Present data showed the upregulation of nestin at all doses of ∆9-THC. It was suggested these markers expressed on the proliferative neuron stage (von Bohlen und Halbach 2007). In differentiation phase, double-stained cell was expressed with nestin-positive and GFAP-negative. Later, the cell will stop expressing nestin while start to express doublecortin (DCX) and polysialylated embryonic form of the neural cell adhesion molecule (PSA-NCAM) (Fukuda et al. 2003). Increased DCX after the acute and chronic treatments of ∆9-THC in the study was corresponding to differentiation and migration stages of neuron. Meanwhile, elevated expression of TuJ-1 upon the treatment of ∆9-THC was reflecting the survival or differentiation of neuron (von Bohlen und Halbach 2007).

Throughout the study, there are consistent findings for all the markers involved in the neurogenesis process in the rat treated with ∆9-THC. Although the present research was only used one time point, the mechanism of neurogenesis was unclear, there is illustrated process extracted from this study. The consistent expression of all markers as simplified in Table 1 has exemplified the effect of ∆9-THC on the respected markers as illustrated in Fig. 9.

Fig. 9 Illustrated mechanism of neurogenesis (DCX, doublecortin; TuJ-1, class III β-tubulin; GFAP, glial fibrillary acidic protein). The figure showed the stages of adult neurogenesis in the dentate gyrus and the expression pattern of the specific markers. (Source: von Bohlen und Halbach 2007). Full size image

As reported by Nyffeler et al. (2010), the activity of neurogenesis has improved the cognitive performance. To observe the interaction of interaction of curiosity and memory, NOD test was used to represent the declarative memory as it gave a few exposures to the animals as cues to learn. In natural exploratory behaviour, normal animals tend to increase the exploratory behaviour to novel stimuli and decrease the exploratory behaviour on familiar stimuli (Shors et al. 2002). The ∆9-THC-treated rat noticeably distinguished the familiar and novel object by spending lesser time on the familiar object compared to control-treated rat. The acute and chronic treatments of 1.5 mg/kg of ∆9-THC showed significant improvement studied by the discrimination index, D2. The rat treated with ∆9-THC spent more time on the novel object, postulating the rat was able to recognise the familiar object introduced before. The ability of rat to differentiate the objects reflecting the ability to learn and memorise the previous experiences (Bevins and Besheer 2006) and the similar pattern of data were observed in short- and long-term memory tests for acute and chronic treatments.

Involvement of DCX in the cognitive perspective was related to the neurogenesis (Nyffeler et al. 2010). Neurogenesis activity was reported to increase after the cognitive task demonstrated by the up-regulation of DCX expression. Beside cognitive tasks, enrichment of the environment and exercise were suggested to cause increment in the expression of DCX that was explaining the neurogenesis activity. Expressions of DCX at DG strengthen the postulation of learning and memory function (Barnea and Nottebohm 1994). Present data showed the increased expression of DCX in the ∆9-THC groups. The positive value of mean relative density was reflecting the increased expression of the protein as compared to that in control (Long et al. 2010). In response to cognitive test done before, the learning and memory activities had increase neuron plasticity expressed by the expression of DCX (Nyffeler et al. 2010). Although cognitive test did not outline the significant different between the acute and chronic treatments, expression of DCX showed significant increased level of DCX in chronic treatment as compared to that in acute. The chronic treatment of 1.5 mg/kg of ∆9-THC was elevating the level of DCX significantly as compared to acute treatment.

Expression of BDNF in this study proposed its roles in memory and learning leading to the process of hippocampal long-term potential (LTP) (Tyler 2002). BDNF is a member of the neurotrophin family that plays a role in neuronal survival, differentiation and synaptic plasticity (Lu et al. 2008). Previous experience on the learning and memory had increased the expression of BDNF. Present data showed the acute and chronic treatments of ∆9-THC increased the level of BDNF as compared to control. Increased plasticity activity through behavioural experience had increased the plasticity markers, BDNF and DCX supported by the Gooney’s report (Gooney et al. 2002).

The presented data were contrast to the report by Kim and Thayer (2001). Instead of using animal, Kim and Thayer (2001) used cultured hippocampus derived from rat. They demonstrated the inhibition of synapses and newly generated cells by exposing the forskolin-induced cell with ∆9-THC. The present study observed the increased neurogenesis by inducing the plasticity activity of the neuron. The flaw of this study was lacking study on functional synaptic boutons. As reported by Gooney et al. (2002) and Lu et al. (2008), learning and memory activities leading to plasticity of the neuron, thus leading to adult neurogenesis to take place.

Brain plasticity is modulated by the endocannabinoid system, involving CB1 and CB2 receptor. The consumption of ∆9-THC as synthetic cannabinoid has described on the neuronal progenitor cells. Wolf et al. (2010) has elucidated the involvement of CB1 in the adult neurogenesis. However, the study has reported the cognitive impairment accompanied with the treatment of ∆9-THC, which was contrast with present study. Taken together by other reports, ∆9-THC cannot be plainly categorised into impairing or enhancing (Abush and Akirav 2010), indeed it requires more research considering the route of administration, region of brain affected (Lorivel and Hilber 2007), and behavioural test used (Wolf et al. 2010). Although the present study did not elaborate on the receptor involved, many reports have elucidate the involvement of CB1 in the neurogenesis and cognition functions, taking into account the abundant volume of CB1 in the brain as compared to CB2 (Jin et al. 2004). The inhibition of CB1 had been reported to decrease the rate of neurogenesis process (Jin et al. 2004; Kim et al. 2006).

The study had demonstrated the influenced of ∆9-THC on the cognitive performances of the rat by increasing the learning and memory functions accompanied by high expression of plasticity markers, DCX and BDNF. The adult neurogenesis was demonstrated by the upregulation of BrdU, nestin, TuJ-1 and DCX. The acute and chronic treatments of ∆9-THC had increased the cognitive function of Sprague Dawley, demonstrated by behavioural and molecular perspectives. Table 1 has simplified the data for neurogenesis and cognitive perspectives. The treatment of 1.5 mg/kg of ∆9-THC has increase all the markers for neurogenesis and cognition function while improve the cognitive performance.

Neuron plasticity by the learning and memory activities had stimulate the adult neurogenesis to take place in the brain of the rat. Administration of ∆9-THC as synthetic cannabinoid was observed to enrich the neurogenesis process while influence the cognitive performance of rat dose-dependently. It was consistent by Jiang et al. (2005), reported the role of cannabis in cognitive functions.