Methylphenidate modulates dorsal raphe neuronal activity: Behavioral and neuronal recordings from adolescent rats

Abstract

Methylphenidate (MPD) is a widely prescribed psychostimulants used for the treatment of attention deficit hyperactive disorder (ADHD). Unlike the psychostimulants cocaine and amphetamine, MPD does not exhibit direct actions on the serotonin transporter, however there is evidence suggesting that the therapeutic effects of MPD may be mediated in part by alterations in serotonin transmission. This study aimed to investigate the role of the dorsal raphe (DR) nucleus, one of the major sources of serotonergic innervation in the mammalian brain, in the response to MPD exposure. Freely behaving adolescent rats previously implanted bilaterally with permanent electrodes were used. An open field assay and a wireless neuronal recording system were used to concomitantly record behavioral and DR electrophysiological activity following acute and chronic MPD exposure. Four groups were used: one control (saline) and three experimental groups treated with 0.6, 2.5, and 10.0mg/kg MPD respectively. Animals received daily MPD or saline injections on experimental days 1-6, followed by 3 washout days and MPD rechallenge dose on experimental day (ED)10. The same chronic dose of MPD resulted in either behavioral sensitization or tolerance, and we found that neuronal activity recorded from the DR neuronal units of rats expressing behavioral sensitization to chronic MPD exposure responded significantly differently to MPD rechallenge on ED10 compared to the DR unit activity recorded from animals that expressed behavioral tolerance. This correlation between behavioral response and DR neuronal activity following chronic MPD exposure provides evidence that the DR is involved in the acute effects as well as the chronic effects of MPD in adolescent rats.

Keywords: Adolescent; Behavior; Dorsal raphe; Methylphenidate; Neuronal activity; Rats.

Figure 1. DR neuronal activity histogram

Figure 1 is a series of sequential frequency histograms of the neuronal activity of two dorsal raphe (DR) units recorded after saline injection (baseline) and after 2.5 mg/kg methylphenidate (MPD) exposure on experimental day 1 (ED1) and ED10. A is a representative DR unit in which the initial (acute) MPD exposure elicited significant (p < 0.05) excitation, and B represents a unit that responded with attenuation when comparing the total neuronal activity post MPD exposure (ED1 MPD) to ED1 baseline (ED1 saline). On the right side are the recordings obtained at ED10 after 6 daily doses of 2.5 mg/kg MPD and three washout days before (ED10 saline) and following 2.5 mg/kg MPD rechallenge (ED10 MPD). In both DR units the ED10 saline (baseline) exhibit similar activity to ED1 saline. On ED10 following MPD challenge unit A exhibits further significant (p < 0.05) acceleration following 2.5 mg/kg MPD compared to the initial effect of MPD at ED1, while unit B exhibited the opposite effect. At ED1 the initial effect of MPD was attenuation compared to ED1 saline (baseline) and MPD at ED10 elicits further significant (p < 0.05) attenuation of this DR firing rate compared to the initial effect of MPD on ED1. The insert at the top of each frequency histograms is 20 superimposed spikes showing that the same spike pattern and amplitude was counted during all the experimental sessions. n= is the total number of spikes recorded in a 60 minute session on ED1 and ED10 following saline and 2.5 mg/kg exposure respectively.

Figure 2. Behavioral recordings from control group

Summarizes acute and chronic effects of 8ml 0.9% NaCl (control) injection on horizontal locomotor activity (n = 15). No significant changes in activity were observed following the first or second saline injections on ED1 and ED10.

Figure 3. Behavioral recordings following acute and chronic 0.6, 2.5, and 10.0 mg/kg MPD

Figure 3 summarizes the behavioral activity of the experimental groups in response to acute and chronic 0.6, 2.5, or 10.0 mg/kg MPD. For each dose, the leftmost histogram summarizes the horizontal locomotor activity (HA) of all animals as one group. The middle histogram summarizes the HA of the individual animals that express behavioral sensitization and the rightmost histogram summarizes the individual animals that expressed behavioral tolerance. ED1 saline–the activity at baseline (ED1 BL); ED1 MPD–HA following MPD administration on ED1; ED10 MPD–the activity following rechallenge MPD administration. indicates significant (p < 0.05) change in activity following acute MPD administration on ED1 compared to ED1 baseline (saline) activity. indicates a significant (p < 0.05) difference in the effect of initial MPD exposure on locomotor activity at ED10 compared to activity following acute MPD administration on ED1.

Figure 4. ED1 neuronal recordings following saline and 2.5 mg/kg MPD

Figure 4 is two tracings of raw analog data recorded from DR units before (upper tracing) and after (lower tracing) 2.5 mg/kg MPD on ED1. Recordings were obtained 17 minutes post saline and MPD injection.